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Charge Transfer in Molecular Complexes with 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ): A Density Functional Theory Study
Abstract
Molecular doping is a charge-transfer process intended to improve the electrical properties of organic semiconductors and the efficiency of organic electronic devices, by incorporation of a complex-forming, strong molecular electron acceptor or donor. Using density functional theory methods with dispersion corrections, we seek to monitor charge transfer and estimate its amount via calculations of experimental observables. With 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) as a p-dopant (electron acceptor) and an array of π-conjugated molecules as hole-transport materials (donors), the amount of charge transfer is seen to be a non-monotonic function of the offset defined by the donor ionization potential (IP) and the acceptor electron affinity (EA), IP – |EA|. Interestingly, a well-defined, linear relationship between the amount of charge transfer and IP – |EA| is obtained when the IP and EA values are adjusted to reflect intramolecular geometric changes in the final form of the complex. This study offers a straightforward way to match donor–acceptor pairs with desired doping effects and to estimate the resulting charge density in organic semiconductors.
... Physical observables like IE and electron affinity (EA) depend on the different screening properties of the environment, i.e. these values differ for a single molecule in vacuum, for a molecule in a single-component film and for a molecule surrounded by a matrix of a different material, either of crystalline or amorphous nature. [12][13][14] This has to be considered, e.g., for predicting electron or integer charge transfer (ICT) from inspection of energy levels determined by different experimental techniques 15 and should also be taken into account for EDA complexes. Furthermore, geometric changes occur during charge transfer resulting in geometry-induced energy level shifts with a dependence of the CT character on the reorganisation energy. ...
... Furthermore, geometric changes occur during charge transfer resulting in geometry-induced energy level shifts with a dependence of the CT character on the reorganisation energy. 12,16 Based on these findings, the definition of a strongly coupled molecular system is given by the formation of an EDA complex, whereas weakly coupled molecular systems show ICT. 16 The electronic and geometric structures of resulting EDA complexes are based on an interplay between electronic and geometric structure of the separated molecules as well as their electronic and steric interaction. ...
... A more sophisticated and detailed description of EDA complexes can be performed by DFT and applying time-dependent DFT or Bethe-Salpeter calculation for optical transitions. 12,[74][75][76] However, the knowledge of the crystal structure is fundamental for these types of calculations as the charge transfer interaction depends on the molecular arrangement and can strongly differ in different polymorphs. [9][10][11] The study presented here shows a crossing of the neutralionic boundary by chemical variation of donor and acceptor molecules. ...
Electron donor–acceptor (EDA) complexes are of interest as low-band gap molecular semiconductors and as dopants for molecular semiconducting matrices. This contribution establishes a link between optical, structural and vibrational properties of EDA complexes as well as the electrical doping by them. We comprehensively characterise co-deposited films of the donors dibenzotetrathiafulvalene and diindenoperylene and the acceptors tetracyanonaphthoquinodimethane and its hexa-fluorinated derivative. All co-deposited donor:acceptor systems form mixed crystalline structures and the EDA complex is characterised by the complex-related absorption and X-ray scattering features. The absorption energies of the analysed EDA complexes cross the neutral-to-ionic boundary. The degree of charge transfer is determined by vibrational spectroscopy. Here, strong spatial anisotropy is found for the diindenoperylene containing complexes. The electrical transport measurements reveal an exponential relation between electrical conductivity and activation energy of transport for all complex-doped systems. We show with this result that doping via complexes has the same dominant activation process as doping via integer charge transfer, which is the separation of Coulombically bound charges. Our results are put in a broader context and we provide an outlook on future possibilities and research on EDA complexes.
... This is a challenging task especially for ab initio quantum-mechanical simulations, where the chemical composition and the initial geometry of the system are the only input. In spite of the proven success of first-principles methods in unraveling the electronic and optical properties of a variety of organic compounds or composite systems thereof [8][9][10][11][12][13][14][15][16][17][18][19][20], the simplified models that are often adopted hinder the possibility to achieve a comprehensive picture, in which all (or at least most of) the involved degrees of freedom are taken into account. For example, assuming hydrogenated oligomers in vacuo to describe polymers in solution not only fails to include solvation effects but also neglects the role of alkyl chains that enhance the solubility of extended segments [21][22][23]. ...
... The interaction with dopant species leads to supramolecular compounds with unique characteristics that are determined by the hybridization between the donor and the acceptor and by the charge transfer between them [33,35,[37][38][39][40][41]. Also in this scenario, ab initio simulations in vacuo have successfully led to the understanding of the basic mechanisms driving the electronic and optical activity of these complexes [13,18,19,39,40,42]. However, open questions concerning the role of solvents and functional chains have not found an answer yet. ...
... We simulate CT complexes by combining 6T with F4-TCNQ molecules to form π-π stacked structures. Regardless of the presence of alkyl chains bound to the 6T, in the optimized geometry of the complex, the tetrafluorobenzene ring of the acceptor is situated above the link between T 3 and T 4 [13,18,19] [see Figure 1e)]. The four T rings underneath the F4-TCNQ are almost flat. ...
The first-principles simulation of the electronic structure of organic semiconductors in solution poses a number of challenges that are not trivial to address simultaneously. In this work, we investigate the effects and the mutual interplay of alkylization, solvation, and doping on the structural, electronic, and optical properties of sexithiophene, a representative organic semiconductor molecule. To this end, we employ (time-dependent) density functional theory in conjunction with the polarizable-continuum model. We find that the torsion between adjacent monomer units plays a key role, as it strongly influences the electronic structure of the molecule, including energy gap, ionization potential, and band widths. Alkylization promotes delocalization of the molecular orbitals up to the first methyl unit, regardless of the chain length, leading to an overall shift of the energy levels. The altered electronic structure is reflected in the optical absorption, which is additionally affected by dynamical solute-solvent interactions. Taking all these effects into account, solvents decrease the optical gap by an amount that depends on its polarity, and concomitantly increase the oscillator strength of the first excitation. The interaction with a dopant molecule promotes planarization. In such scenario, solvation and alkylization enhance charge transfer both in the ground state and in the excited state.
... However, their distribution is narrowed upon adsorption on the surface. Especially in the case of the C;N stretch, we ascribe this behavior to the larger amount of charge accumulated on the N atoms in the adsorbed molecule compared to the gas-phase one (see also Ref [128].), as discussed in Section 3.2 below. The average CCN angles are reduced by about 4 at 300 K if compared to the optimized structures, which reflects the anharmonicity of the related bending mode. ...
... According to the conventional nomenclature of semiconductor heterojunctions [129], this is a type-III interface with the frontier orbitals of the H-Si(111) cluster being both at higher energy than those of F4TCNQ. This result is qualitatively different from the type-II level alignment resulting in all-organic interfaces when, for example, F4TCNQ acts as a dopant of thiophene oligomers [128,[130][131][132]. Due to this level alignment, when the electron-accepting molecule and the H-Si(111) cluster interact with each other forming the hybrid interface, the latter is pdoped, with both the highest-occupied molecular orbital (HOMO) and the lowest-unoccupied molecular orbital (LUMO) being partially occupied. ...
... However, they exhibit substantially different nature: The HOMO is delocalized across the interface and results from the hybridization between the HOMO of the Si cluster and the LUMO of the molecular acceptor. In contrast, the LUMO of the hybrid corresponds almost identically to the LUMO of the isolated molecule (see, e.g., Refs [128,131].), with only very small contributions coming from the hybridization with the inorganic part of the interface. A gap of approximately 2 eV separates the LUMO to the higher states in the conduction band, which are localized solely on the H-Si(111) cluster in the energy window shown for the PDOS (see Figure 3(b)). ...
Hybrid interfaces formed by inorganic semiconductors and organic molecules are intriguing materials for opto-electronics. Interfacial charge transfer is primarily responsible for their peculiar electronic structure and optical response. Hence, it is essential to gain insight into this fundamental process also beyond the static picture. Ab initio methods based on real-time time-dependent density-functional theory coupled to the Ehrenfest molecular dynamics scheme are ideally suited for this problem. We investigate a laser-excited hybrid inorganic/organic interface formed by the electron acceptor molecule 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4TCNQ) physisorbed on a hydrogenated silicon cluster, and we discuss the fundamental mechanisms of charge transfer in the ultrashort time window following the impulsive excitation. The considered interface is p-doped and exhibits charge transfer in the ground state. When it is excited by a resonant laser pulse, the charge transfer across the interface is additionally increased, but contrary to previous observations in all-organic donor/acceptor complexes, it is not further promoted by vibronic coupling. In the considered time window of 100 fs, the molecular vibrations are coupled to the electron dynamics and enhance intramolecular charge transfer. Our results highlight the complexity of the physics involved and demonstrate the ability of the adopted formalism to achieve a comprehensive understanding of ultrafast charge transfer in hybrid materials.
... However, their distribution is narrowed upon adsorption on the surface. Especially in the case of the C≡N stretch, we ascribe this behavior to the larger amount of charge accumulated on the N atoms in the adsorbed molecule compared to the gas-phase one (see also Ref. [128]), as discussed in Section 3.2 below. The average CCN angles are reduced by about 4 • at 300 K if compared to the optimized structures, which reflects the anharmonicity of the related bending mode. ...
... According to the conventional nomenclature of semiconductor heterojunctions [129], this is a type-III interface with the frontier orbitals of the H-Si(111) cluster being both at higher energy than those of F4TCNQ. This result is qualitatively different from the type-II level alignment resulting in all-organic interfaces when, for example, F4TCNQ acts as a dopant of thiophene oligomers [128,[130][131][132]. Due to this level alignment, when the electron-accepting molecule and the H-Si(111) cluster interact with each other forming the hybrid interface, the latter is p-doped, with both the highest-occupied molecular orbital (HOMO) and the lowest-unoccupied molecular orbital (LUMO) being partially occupied. ...
... In contrast, the LUMO of the hybrid corresponds almost identically to the LUMO of the isolated molecule (see, e.g., Refs. [128,131]), with only very small contributions coming from the hybridization with the inorganic part of the interface. A gap of approximately 2 eV separates the LUMO to the higher states in the conduction band, which are localized solely on the H-Si(111) cluster in the energy window shown for the PDOS (see Fig. 3b). ...
Hybrid interfaces formed by inorganic semiconductors and organic molecules are intriguing materials for opto-electronics. Interfacial charge transfer is primarily responsible for their peculiar electronic structure and optical response. Hence, it is essential to gain insight into this fundamental process also beyond the static picture. Ab initio methods based on real-time time-dependent density-functional theory coupled to the Ehrenfest molecular dynamics scheme are ideally suited for this problem. We investigate a laser-excited hybrid inorganic/organic interface formed by the electron acceptor molecule 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4TCNQ) physisorbed on a hydrogenated silicon cluster, and we discuss the fundamental mechanisms of charge transfer in the ultrashort time window following the impulsive excitation. The considered interface is $p$-doped and exhibits charge transfer in the ground state. When it is excited by a resonant laser pulse, the charge transfer across the interface is additionally increased, but contrary to previous observations in all-organic donor/acceptor complexes, it is not further promoted by vibronic coupling. In the considered time window of 100~fs, the molecular vibrations are coupled to the electron dynamics and enhance intramolecular charge transfer. Our results highlight the complexity of the physics involved and demonstrate the ability of the adopted formalism to achieve a comprehensive understanding of ultrafast charge transfer in hybrid materials.
... Geometries of pristine 8-PNP-O12, F 4 TCNQ and the CTC were firstly optimized by semi-empirical methods with further DFT optimisation on B3LYP/6-31++G(d,p) level with additional empirical dispersion corrections (GD2) for the CTC. This have proven itself to significantly enhance [147] the accuracy of systems which involve intermolecular charge transfer. Optimized geometry had zero imaginary frequencies. ...
... The role of vibrational spectroscopy in the study of charge transfer related to F 4 TCNQ and its homologues is undeniable [155,160,148,147,39,161]. The vibrational analysis of the charge transfer is based on the shift of nitrile band, since this vibrational band is situated in the part of the spectrum which is usually isolated: it does not contain any other vibrational modes of initial products. ...
... It seems that a vast number of research groups favor IR spectroscopy over Raman [65,162,28]. In fact, pristine F 4 TCNQ molecule exhibits 4 vibrational modes in this region of the spectrum (2100/cm to 2300/cm), with two of them being IR active (b 2u and b 1u at 2213/cm and 2227/cm [147], respectively), and the other two -Raman active (b 3g and a g at 2219/cm and 2227/cm [160], respectively). ...
This thesis is dedicated to various aspects of liquid crystalline (LC) organic semiconductors (OSCs) in regard to their applications in the field of organic electronics. The first part of this work deals with a well-known LC OSC based on phenyl-naphtalene. Two major ways of performance improvement are proposed and investigated : stabilization of LC structure by in situ photo-polymerization and introduction of electron acceptor doping impurity. In the first case, the influence of polymer network on mesophase order and charge transport is investigated by conventional experimental techniques and Time-Of-Flight (TOF) mobility measurements. Fot the doped materials, ab initio calculations are employed to predict their spectroscopic properties which is exhaustively compared with the experimental data obtained by optical and vibrational spectroscopy. The charge transport is studied by TOF method in the mesophase, while crystalline phase is investigated via conductive atomic force microscopy. A prototype of organic field effect transistor (OFET) is prepared to obtain an estimate of performance for a relevant real-world application. The second part of this work includes design and synthesis of a novel LC semiconductor based on anthracene, additional attention is made to obtain an easy-to-make and low production cost material. Noval molecule is fully characterized : molecular structure is confirmed by relevant techniques ; frontier molecular energy levels are studied by optical spectroscopy and cyclic voltammetry and confronted to values obtaines via ab initio calculations ; mesophase properties are investigated by optical microscopy and scanning calorimetry. charge transporting properties are characterized by means of an OFET device : it is found that new anthracene-molecule exhibits significant improvement of field-effect hole mobility over previously studied phenyl naphtalene derivative. Finally, photoconductive properties of the novel material are addressed in order to investigate its potential applications to organic phototransistors.
... A common choice in modelling doped OSC is to consider an isolated dimer formed by a donor and an acceptor molecule interacting with each other. 12,[19][20][21] Representing OSC with an isolated dimer implicitly assumes that the system can be reproduced by a single molecular interface. This model turned out to be successful in describing the level alignment of the donor/acceptor interfaces, 19 the spatial distribution of the resulting frontier states, 12 and also in rationalizing the role of the donor conjugation length. ...
... 12,[19][20][21] Representing OSC with an isolated dimer implicitly assumes that the system can be reproduced by a single molecular interface. This model turned out to be successful in describing the level alignment of the donor/acceptor interfaces, 19 the spatial distribution of the resulting frontier states, 12 and also in rationalizing the role of the donor conjugation length. 20 However, in this way, it is not possible to capture in full the behavior of extended donor/acceptor stacks, nor to reproduce the chemical environment of a molecule surrounded by more than one dopant. ...
... Bonding and anti-bonding frontier orbitals are formed only by donor and acceptor molecules interacting with each other through one single interface. This is obviously the case in the dimers 13,19,20 and also in the trimers formed by one acceptor molecule adsorbed on top of two donors. On the other hand, when one donor (acceptor) is sandwiched between two acceptor (donor) molecules, the LUMO (HOMO) of the complex is localized solely on the latter species, which are present in larger amount, with the LUMO+1 (HOMO-1) exhibiting anti-bonding (bonding) character. ...
Doping in organic semiconductors remains a debated issue from both an experimental and ab initio perspective. Due to the complexity of these systems, which exhibit low degree of crystallinity and high level of disorder, modelling doped organic semiconductors from first-principles is not a trivial task, as their electronic and optical properties are sensitive to the choice of the initial geometries. A crucial aspect to take into account in view of rationalizing the electronic structure of these materials through ab initio calculations, is the role of local donor/acceptor interfaces. We address this problem in the framework of state-of-the-art density-functional theory and many-body perturbation theory, investigating the structural, electronic, and optical properties of quaterthiophene and sexithiophene oligomers doped by 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4TCNQ). We consider different model structures ranging from isolated dimers and trimers, to periodic stacks. Our results demonstrate that the choice of the initial geometry critically impacts the resulting electronic structure and the degree of charge transfer in the materials, depending on the amount and on the nature of the local interfaces between donor and acceptor species. The optical spectra appear less sensitive to these parameters at least from a first glance, although a quantitative analysis of the excitations reveals that their Frenkel or charge-transfer character is affected by the characteristics of the donor/acceptor interfaces as well as by the donor length. Our findings represent an important step forward towards an insightful first-principles description of the microscopic properties of doped organic semiconductors complementary to experiments.
... In such a context, molecular semiconductor modeling was devoted to addressing more specific questions related, among others, to doping mechanisms [41][42][43][44][45][46][47], polymorphism [48][49][50][51][52], and singlet fission [53][54][55][56][57][58][59]. Moreover, increasing attention was dedicated to disclosing the excitonic properties of organic materials [52, 60-62], including quantifying resonance energies and clarifying the character of electron-hole pairs. ...
... At the zone edges (high-symmetry points Z, N, M, and R), the wave functions are spatially segregated over the donor (valence states) and acceptor molecules (conduction states), respectively, see figure 5(d). Notably, both valence-band maximum and conduction-band minimum are located at M. Elsewhere in the BZ, the electronic states are hybridized between the donor and the acceptor, in analogy with the HOMO and the LUMO of the corresponding molecular complexes [41,44]. ...
Modeling the electronic and optical properties of organic semiconductors remains a challenge for theory, despite the remarkable progress achieved in the last three decades. The complexity of these systems, including structural (dis)order and the still debated doping mechanisms, has been engaging theorists with different backgrounds. Regardless of the common interest across the various communities active in this field, these efforts have not led so far to a truly interdisciplinary research area. In the attempt to move further in this direction, we present our perspective as solid-state theorists for the study of molecular materials in different states of matter, ranging from gas-phase compounds to crystalline samples. Considering exemplary systems belonging to the well-known families of oligo-acenes and -thiophenes, we provide a quantitative description of electronic properties and optical excitations obtained with state-of-the-art first-principles methods such as density-functional theory and many-body perturbation theory. Simulating the systems as gas-phase molecules, clusters, and periodic lattices, we are able to identify short- and long-range effects in their electronic structure. While the latter are usually dominant in organic crystals, the former play an important role, too, especially in the case of donor/acceptor complexes. To mitigate the numerical complexity of fully atomistic calculations on organic crystals, we demonstrate the viability of implicit schemes to evaluate band gaps of molecules embedded in isotropic and even anisotropic environments, in quantitative agreement with experiments. In the context of doped organic semiconductors, we show how the crystalline packing enhances the favorable characteristics of these systems for opto-electronic applications. The counter-intuitive behavior predicted for their electronic and optical properties is deciphered with the aid of a tight-binding model, which represents a connection to the most common approaches to evaluate transport properties in these materials.
... Moreover, the fact that dopants tend to lie in the crystalline lamallae, far from the polymer's π system, also explains why doped conjugated polymers tend to undergo integer charge transfer rather than the partial charge transfer that is often observed with doped small molecules or oligomers. [51,78,79] Recent work has shown that under special processing conditions, there can be partial charge transfer between P3HT and dopants like F 4 TCNQ, and that partial charge transfer is associated with π-stacking of the dopant with the polymer backbone. [62,63] Creating dopant/polymer π stacks is clearly kinetically very challenging and potentially also thermodynamically unfavorable, which is why extreme measures must be taken to create these species. ...
... Oligomers and small molecules avoid this energetic conundrum by crystallizing directly as stacked donor/acceptor dimers, so that intimate contact and wavefunction overlap lead to partial rather than integer charge transfer. [51,78,79] We believe that for all chemically doped conjugated polymers, the doping process is essentially identical. If the redox potential is strong enough to dope the polymer backbone with a high enough density of doping events to induce crystalline reorganization, dopants will initially enter the crystalline lamellae; the stronger the redox potential of the dopant, the greater the ability to initiate this reorganization. ...
Carrier mobility in doped conjugated polymers is limited by Coulomb interactions with dopant counterions. This complicates studying the effect of the dopant's oxidation potential on carrier generation because different dopants have different Coulomb interactions with polarons on the polymer backbone. Here, dodecaborane (DDB)‐based dopants are used, which electrostatically shield counterions from carriers and have tunable redox potentials at constant size and shape. DDB dopants produce mobile carriers due to spatial separation of the counterion, and those with greater energetic offsets produce more carriers. Neutron reflectometry indicates that dopant infiltration into conjugated polymer films is redox‐potential‐driven. Remarkably, X‐ray scattering shows that despite their large 2‐nm size, DDBs intercalate into the crystalline polymer lamellae like small molecules, indicating that this is the preferred location for dopants of any size. These findings elucidate why doping conjugated polymers usually produces integer, rather than partial charge transfer: dopant counterions effectively intercalate into the lamellae, far from the polarons on the polymer backbone. Finally, it is shown that the IR spectrum provides a simple way to determine polaron mobility. Overall, higher oxidation potentials lead to higher doping efficiencies, with values reaching 100% for driving forces sufficient to dope poorly crystalline regions of the film.
... For completeness, it is worth specifying that in 4T-F4TCNQ, the acceptor molecule is slightly bent, with the N atoms pointing toward the donor. This is a known behavior related to the role of the cyano groups in mediating charge transfer in those organic complexes.55 Additional insight can be gained by contrasting the power spectrum of the complex against the contributions of its components. ...
The ultrafast dynamics of charge carriers in organic donor-acceptor interfaces are of primary importance to understanding the fundamental properties of these systems as active components of optoelectronic devices and solar cells. In this work, we focus on a charge-transfer complex formed by quaterthiophene $p$-doped by tetrafluoro-tetracyanoquinodimethane and investigate the influence of vibronic interactions and temperature effects on the charge-transfer dynamics driven by resonant excitations in the visible region. The adopted ab initio formalism based on real-time time-dependent density-functional theory coupled to the Ehrenfest dynamics enables monitoring the dynamical charge transfer across the interface and assessing the role played by the nuclear motion. Our results show that in a 100-fs time window, the coherence of this process is overestimated when the temperature is neglected. The analysis of the system thermalized at 100, 200, and 300 K confirms that thermal disorder drastically reduces coherence, and, thus, effectively reduces charge transfer with respect to the ground state.
... Contact doping is an effective way to reduce the contact resistance between the semiconductor channel and metal electrodes. Here, we thermally evaporated a 1.5-nm thick 2,3,5,6-tetra uoro-7,7,8,8-tetracyanoquinodimethane (F 4 -TCNQ) as interlayer between Ag S/D electrodes and C 8 -BTBT single crystal to implement the contact doping [33][34][35] . All the transfer curves of the 8 × 8 FET array exhibit a typical p-type device behavior along with a high on/off switching ratio of ~ 10 8 and a 100% device yield (Fig. 4c). ...
Printable organic semiconducting single crystals (OSSCs) offer tantalizing opportunities for next-generation wearable electronics, but their development has been plagued by a long-standing yet inherent problem—spatially uncontrolled and stochastic nucleation events, which usually causes the formation of polycrystalline films and hence limited performance. Here, we report a convenient approach to precisely manipulate the elusive molecule nucleation process for one-step inkjet printing of OSSCs with record-high mobility. By engineering curvature of contact line with a teardrop-shaped micropattern, molecule nucleation is elegantly anchored at the vertex of the topological structure, enabling formation of a single nucleus for the subsequent growth of OSSC. Using this approach, we achieve patterned growth of 2,7-dioctyl[1]benzothieno[3,2-b][1] benzothiophene single crystals, yielding a breakthrough for organic field-effect transistor array with high average mobility of 12.5 cm ² V ⁻¹ s ⁻¹ . These findings not only provide keen insights into controlling molecule nucleation kinetics, but also offer unprecedented opportunities for high-performance printed electronics.
... This polymorphic dependence for the CT degree was also found for quaterthiophene:F 4 TCNQ dimers. [274] 97 ...
Ziel dieser Arbeit war es, den grundlegenden Mechanismus des Ladungstransfers bei molekularer Dotierung an organisch-organischen Grenzflächen besser zu verstehen. Es wurde eine Vielfalt modernster spektroskopischer Methoden eingesetzt, um die elektronische Struktur und neue dotierungsinduzierte CT-Übergänge zu ergründen. Dazu gehören UPS und XPS für Valenzsignaturen und Kernniveauzustände. Absorptionsspektroskopie im UV-vis-NIR und Röntgenbereich wurde zur Bestimmung der Übergangsenergien eingesetzt. Schwingungsspektroskopie wurde eingesetzt, um den CT-Grad in DA-Systemen für gestapelte und gemischte Heteroübergänge zu quantifizieren. Strom-Spannungs-Messungen wurden zur Bestimmung der elektrischen Leitfähigkeit und Rasterkraftmikroskopie zur Charakterisierung der Oberflächenmorphologie eingesetzt. Die in dieser Arbeit behandelten Themen sind: (1) Planare Heteroübergänge aus DIP und F6TCNNQ wurden hergestellt. Sie wurden im Hinblick auf CT-Komplexbildung, Grenzflächendotierung und Exzitonenbindungsenergien an der D|A-Grenzfläche untersucht. (2) DBTTF wurde mit TCNNQ und F6TCNNQ in Lösung und in dünnen Filmen gemischt. Daraus wurde der Zusammenhang zwischen Dotierungsmechanismen, CTC- und IPA-Bildung, mit dem Aggregatzustand hergeleitet. (3.1) Rubren-Einkristalle wurden mit Mo(tfd)3 und CoCp2 p- und n-dotiert. Nach der Dotierung verschiebt sich die Banddispersion entsprechend, wohingegen die effektive Masse der Löcher konstant bleibt. (3.2) DBTTF-Einkristalle wurden mit TCNNQ, F6TCNNQ und Mo(tfd)3 dotiert. Aus den Änderungen der elektronischen Struktur wurden der CT über die D|A-Grenzfläche sowie die Dichte der Oberflächenzustände quantifiziert. (4) Von drei DA-Systemen mit unterschiedlicher GS-Wechselwirkungsstärke, DIP:C60, DIP:PDIR-CN2 und DIP|F6TCNNQ, wurden die Grenzflächenexzitonen charakterisiert. Ein Vergleich verschiedener Modelle, die die optische CTC Absorption aus dem DA-Energieniveauoffset beschreiben und abschätzen können, rundet die Ergebnisse ab.
... The molecular orientation of TCNQ derivatives and their degree of charge-transfer are well-known to be estimated from the wavenumbers of C≡N stretching vibrations in their IR spectra. The magnified section of the spectrum in the wavenumber region around 2200 cm −1 associated with the C≡N stretching vibrations is shown in the inset of Fig. 4. The FT-IR spectra corroborates the presence of a strong peak at 2188 and 2170 cm −1 , which are assigned to the C≡N stretching vibrations from the F4TCNQ anions, whereas peaks associated with the neutral F4TCNQ molecule appearing at 2230and 2220 cm −1 were completely absent 14,59 . This indicates that the F4TCNQ molecules in the doped ftP3HT thin film prepared by dipping completely changed to anions. ...
An ordered arrangement of electron-accepting molecular dopant, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), in three-dimensionally (3D) oriented poly(3-hexylthiophene) (P3HT) film was clarified. The 3D oriented P3HT thin films prepared by the friction-transfer technique were doped with F4TCNQ by dipping into an acetonitrile solution. The presence of F4TCNQ anions in the 3D oriented P3HT thin films was investigated by polarized ultraviolet/visible/near-infrared absorption spectroscopy, grazing incidence X-ray diffractometry, polarized Fourier transform infrared spectroscopy (FT-IR), and infrared p-polarized multiple-angle incidence resolution spectroscopy (pMAIRS). The F4TCNQ-doped 3D oriented P3HT films showed anisotropic properties in all characterizations. In particular, the anisotropic molecular vibrations from polarized FT-IR and pMAIRS have clearly revealed orientations of polymeric chains and molecular dopant molecules. Considering the results from several independent techniques indicated that F4TCNQ anions in the 3D oriented P3HT were orderly arranged in a 3D manner with respect to the 3D oriented P3HT such that their molecular long-axis parallel to the P3HT backbone, with in-plane molecular orientation. Additionally, the direction of the optical transition moment of the F4TCNQ anion was found to be parallel to the molecular short-axis.
... Regarding the F4TCNQ-Au network, theoretical calculation demonstrates a remarkable band gap of 1.57 eV involving the interfacial interaction with Au(111) (Fig. S7 in the ESM), despite the remarkable charge transfer from Au adatoms to F4TCNQ to induce band dispersion in the topmost valence level [44]. The dI/dV spectrum acquired at the aromatic backbone of F4TCNQ displays states derived from highest occupied molecular orbital (HOMO) and HOMO-1 at −1.1 and −1.6 V, and the lowest unoccupied molecular orbital (LUMO) near the Fermi level at −0.3 V, respectively [53]. In addition, the remarkable peak at 1.75 V above the Fermi level originates from the Au adatom state (Fig. 3(c)) [43]. ...
Lateral two-dimensional (2D) heterostructures have great potential for device engineering at the atomistic scale. Their production is hindered by difficulties in obtaining atomically sharp interface free from intermixture. Here we report the continuous construction of a lateral heterostructure using blue phosphorene and tetrafluoro-tetracyanoquinodimethane (F 4 TCNQ) as the building blocks. The lateral heterostructure is achieved by linking the semiconducting F 4 TCNQ-Au metal organic framework and the metallic blue phosphorene-Au network via Au adatoms. The structural and electronic properties of the heterostructure have been investigated by means of scanning tunneling microscopy and spectroscopy (STM/S), complemented by density functional theory (DFT) calculations, demonstrating a structurally and electrically abrupt interface. Our approach offers the possibility of high flexibility and control that can be extended to other metal-organic species and 2D materials, establishing a foundation for the development of atomically thin in-plane superlattice and devices.
The addition of molecular dopants into organic semiconductors (OSCs) is a ubiquitous augmentation strategy to enhance the electrical conductivity of OSCs. Although the importance of optimizing OSC–dopant interactions is well-recognized, chemically generalizable structure–function relationships are difficult to extract due to the sensitivity and dependence of doping efficiency on chemistry, processing conditions, and morphology. Computational modeling for an integrated OSC–dopant design is an attractive approach to systematically isolate fundamental relationships, but requires the challenging simultaneous treatment of molecular reactivity and morphology evolution. We present the first computational study to couple molecular reactivity with morphology evolution in a molecularly doped OSC. Reactive Monte Carlo is employed to examine the evolution of OSC–dopant morphologies and doping efficiency with respect to dielectric, the thermodynamic driving for the doping reaction, and dopant aggregation. We observe that for well-mixed systems with experimentally relevant dielectric constants, doping efficiency is near unity with a very weak dependence on the ionization potential and electron affinity of OSC and dopant, respectively. At experimental dielectric constants, reaction-induced aggregation is observed, corresponding to the well-known insolubility of solution-doped materials. Simulations are qualitatively consistent with a number of experimental studies showing a decrease of doping efficiency with increasing dopant concentration. Finally, we observe that the aggregation of dopants lowers doping efficiency and thus presents a rational design strategy for maximizing doping efficiency in molecularly doped OSCs. This work represents an important first step toward the systematic integration of molecular reactivity and morphology evolution into the characterization of multi-scale structure–function relationships in molecularly doped OSCs.
We theoretically investigated molecular charge populations of one-dimensional (1D) π-stacked multimers consisting of π-conjugated molecules in the neutral and electron oxidation states based on the valence-bond (VB) theory. Qualitative analysis for a π-stacked trimer model based on the VB mixing diagram suggested that the inner monomer site tends to be more positively charged than the outer sites in the monocationic π-stacked trimer. Spatial expansion of each molecular site orbital toward the stacking direction is predicted to enhance the difference of positive charge populations between the inner and outer monomers. In contrast, an opposite tendency for the site charges was expected in the dicationic π-stacked trimer, primarily due to the hole-hole Coulomb repulsions. To generalize the results of the trimer to π-stacked N-mers, 1D N-site VB configuration interaction (VBCI) models were constructed considering the orbital expansion effects between the sites. We examined how the number of monomers (N), stacking distance (R), and characteristic orbital exponent for the monomers (ζ) affect the molecular charge populations in the monocationic and dicationic π-stacked N-mers through the parameters χij characterizing the orbital expansion effect. The results are expected to help establish design strategies for novel electronic functional materials based on discrete stacks of π-conjugated molecules.
Photofunctional co-crystal engineering strategies based on donor-acceptor π-conjugated system facilitates expedient molecular packing, consistent morphology, and switchable optical properties, conferring synergic ‘structure-property relationship’ for optoelectronic and biological functions. In this work, a series of organic co-crystals were formulated using a twisted aromatic hydrocarbon (TAH) donor and three diverse planar acceptors, resulting in color-tunable solid and aggregated state emission via variable packing and through-space charge-transfer interactions. While, adjusting the strength of acceptors, a structural transformation into hybrid stacking modes ultimately results in color-specific polymorphs, a configurational cis-isomer with very high photoluminescence quantum yield. The cis-isomeric co-crystal exhibits triplet-harvesting thermally activated delayed fluorescence (TADF) characteristics, presenting a key discovery in hydrocarbon-based multicomponent systems. Further, 1D-microrod-shaped co-crystal acts as an efficient photon-transducing optical waveguides, and their excellent dispersibility in water endows efficient cellular internalization with bright cell imaging performances. These salient approaches may open more avenues for the design and applications of TAH based co-crystals.
The ultrafast dynamics of charge carriers in organic donor–acceptor interfaces are of primary importance to understanding the fundamental properties of these systems. In this work, we focus on a charge-transfer complex formed by quaterthiophene p-doped by tetrafluoro-tetracyanoquinodimethane and investigate electron dynamics and vibronic interactions also at finite temperatures by applying a femtosecond pulse in resonance with the two lowest energy excitations of the system with perpendicular and parallel polarization with respect to the interface. The adopted ab initio formalism based on real-time time-dependent density-functional theory coupled to Ehrenfest dynamics enables monitoring the dynamical charge transfer across the interface and assessing the role played by the nuclear motion. Our results show that the strong intermolecular interactions binding the complex already in the ground state influence the dynamics, too. The analysis of the nuclear motion involved in these processes reveals the participation of different vibrational modes depending on the electronic states stimulated by the resonant pulse. Coupled donor–acceptor modes mostly influence the excited state polarized across the interface, while intramolecular vibrations in the donor molecule dominate the excitation in the orthogonal direction. The results obtained at finite temperatures are overall consistent with this picture, although thermal disorder contributes to slightly decreasing interfacial charge transfer.
In this work, we sequentially doped poly(3-hexylthiophene) (P3HT) films with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) from multiple good solvent/orthogonal solvent blends with different ratios. Combining UV-visible (UV-vis) spectroscopy, grazing-incidence wide-angle X-ray diffraction (GIXRD),...
Conjugated polyelectrolytes (CPEs) are a rising class of organic mixed ionic-electronic conductors, with applications in bio-interfacing electronics and energy harvesting and storage devices. Here, we employ a quantum mechanically informed...
Common polymers can accumulate surface charges through contact, a phenomenon known since ancient times. This charge accumulation can have detrimental consequences in industry. It causes accidents and yields enormous economic losses. Many empirical methods have been developed to prevent the problems caused by charge accumulation. However, a general chemical approach is still missing in the literature since the charge accumulation and discharging mechanisms have not been completely clarified. The current practice to achieve charge mitigation is to increase materials conductivity by high doping of conductive additives. A recent study showed that using photoexcitation of some organic dyes, charge decay can be started remotely, and the minute amount of additive does not change the material's conductivity. Here, we show the contact charging and charge decay behavior of polydimethylsiloxane doped with a series of organic charge transfer cocrystals (CTC) of TCNQ acceptor and substituted pyrene donors (CTC-PDMS). The results show that the CTC-PDMS are antistatic, and the discharging propensity of the composites follows the calculated charge transfer degree of the complexes. On the other hand, the CTC-PDMS are still insulators, as shown by their high surface resistivities. Kelvin probe force microscopy images of the contact-charged and discharged samples show a quick potential decay in CTC domains upon illumination. Combined with the fast overall decay observed, the antistatic behavior in these insulators can be attributed to an electron transfer between the mechanoions in the polymer and the CTC frontier orbitals. We believe our results will help with the general understanding of the molecular mechanism of contact charging and discharging and help develop insulator antistatics.
Charge transfer cocrystals offer an opportunity to construct high performance organic photothermal materials. However, it is still challenging to modulate the degree of charge transfer of cocrystals to achieve rational...
Metal halide perovskites (MHPs) hold great potential in thermoelectric (TE) applications, thanks to their regular and soft lattice in nature. However, the poor electrical conductivity caused by low charge carrier density (<1014 cm−3 for lead‐based MHPs) strongly impedes its TE development. In this scenario, tin halide perovskites (THPs) emerge as promising TE candidates owing to their high background hole densities (>1019 cm−3). However, further electrical doping remains challenging, originating from the limited capability of accommodating heterogeneous dopants and the heavy compensation in THPs. Herein, a novel diffusion‐mediated doping approach is demonstrated to prominently increase the p‐type doping level of THPs by a sequence of air exposure and 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) surface treatments. In paradigm photovoltaic THP materials—CH(NH2)2SnI3 (namely FASnI3), the electrical conductivity is dramatically increased by 300× from 0.06 to 18 S cm−1 in thin films, leading to a remarkable enhancement of power factor by 25× up to 53 μW m−1 K−2. In contrast, only a slight variation of thermal conductivity is observed after F4TCNQ deposition, which is in accordance with the increase in electrical conductivity, indicating that the lattice structures of FASnI3 remain intact after doping. This study paves an illuminating way to ameliorate TE properties in halide perovskites. A sequential diffusion doping strategy of air exposure and then F4TCNQ surface treatment is demonstrated in CH(NH2)2SnI3 films to achieve a remarkably increased electrical conductivity up to 18 S cm−1, leading to an impressive thermoelectric power factor of 53 μW m−1 K−2. Such diffusion doping enables effective interactions between heterogenous dopants and CH(NH2)2SnI3 lattices, while revealing the underlying distinct doping mechanisms.
The synthesis of dinuclear ruthenium alkenyl complexes with {Ru(CO)(PⁱPr3)2(L)} entities (L=Cl⁻ in complexes Ru2‐3 and Ru2‐7; L=acetylacetonate (acac⁻) in complexes Ru2‐4 and Ru2‐8) and with π‐conjugated 2,7‐divinylphenanthrenediyl (Ru2‐3, Ru2‐4) or 5,8‐divinylquinoxalinediyl (Ru2‐7, Ru2‐8) as bridging ligands are reported. The bridging ligands are laterally π‐extended by anellating a pyrene (Ru2‐7, Ru2‐8) or a 6,7‐benzoquinoxaline (Ru2‐3, Ru2‐4) π‐perimeter. This was done with the hope that the open π‐faces of the electron‐rich complexes will foster association with planar electron acceptors via π‐stacking. The dinuclear complexes were subjected to cyclic and square‐wave voltammetry and were characterized in all accessible redox states by IR, UV/Vis/NIR and, where applicable, by EPR spectroscopy. These studies signified the one‐electron oxidized forms of divinylphenylene‐bridged complexes Ru2‐7, Ru2‐8 as intrinsically delocalized mixed‐valent species, and those of complexes Ru2‐3 and Ru2‐4 with the longer divinylphenanthrenediyl linker as partially localized on the IR, yet delocalized on the EPR timescale. The more electron‐rich acac⁻ congeners formed non‐conductive 1 : 1 charge‐transfer (CT) salts on treatment with the F4TCNQ electron acceptor. All spectroscopic techniques confirmed the presence of pairs of complex radical cations and F4TCNQ.− radical anions in these CT salts, but produced no firm evidence for the relevance of π‐stacking to their formation and properties.
Organic semiconductors are being perceived as promising materials, which will complement or even substitute inorganic semiconductors, for the development of futuristic versatile flexible electronics. The discovery of organic metal TTF-TCNQ...
Conducting polymers have drawn considerable attention in the field of wearable and implantable thermoelectric devices due to their unique advantages, including availability, flexibility, lightweight, and non-toxicity. To date, researchers have made dramatic breakthroughs in achieving high-performance thermoelectrics; however, the figure of merit ZT of conducting polymers is still far below that of the high-performance thermoelectric Bi2Te3-based alloys at room temperature. This challenge lies in the complex interrelation between electrical conductivity, Seebeck coefficient, and thermal conductivity. In this review, we overview the state-of-the-art on conducting polymers and their thermoelectric devices, starting with the summary of the fundamentals as well as several well-accepted theoretical models. Furthermore, this review examines the key factors determining the charge transport mechanisms in this family of materials and previously reported optimization strategies are discussed and classified. Finally, this review further introduces several favourable device fabrication techniques including illustrating and demonstrating the performance of several typical thermoelectric prototypes, which highlights the bright future of polymer-based flexible thermoelectric devices in wearable and implantable electronics.
Applications of low-cost non-perturbative approaches in real time, such as time-dependent density functional theory, for the study of nonlinear optical properties of large and complex systems are gaining increasing popularity. However, their assessment still requires the analysis and understanding of elementary dynamical processes in simple model systems. Motivated by the aim of simulating optical nonlinearities in molecules, here exemplified by the case of the quaterthiophene oligomer, we investigate light absorption in many-electron interacting systems beyond the linear regime by using a single broadband impulse of an electric field; i.e. an electrical impulse in the instantaneous limit. We determine non-pertubatively the absorption cross section from the Fourier transform of the time-dependent induced dipole moment, which can be obtained from the time evolution of the wavefunction. We discuss the dependence of the resulting cross section on the magnitude of the impulse and we highlight the advantages of this method in comparison with perturbation theory by working on a one-dimensional model system for which numerically exact solutions are accessible. Thus, we demonstrate that the considered non-pertubative approach provides us with an effective tool for investigating fluence-dependent nonlinear optical excitations.
Electronic technologies critically rely on the ability to broadly dope the active semiconductor; yet the promising class of halide perovskite semiconductors so far does not allow for significant control over carrier type (p- or n-) and density. The molecular doping approach offers important opportunities for generating free carriers through charge transfer. In this work, we demonstrate effective p-doping of MAPb0.5Sn0.5I3 films using the molecular dopant F4TCNQ as a grain boundary coating, offering a conductivity and hole density tuning range of up to five orders of magnitude, associated with a 190 meV Fermi level down-shift. While charge transfer between MAPb0.5Sn0.5I3 and F4TCNQ appears efficient, dopant ionization decreases with increasing Pb content, highlighting the need for appropriate energy offset between host and dopant molecule. Finally, we show that electrical p-doping impacts the perovskite optoelectronic properties, with a hole recombination lifetime increase of over one order of magnitude, suggesting passivation of deep traps.
The optimized molecular structures, harmonic vibrational wavenumbers and corresponding vibrational assignments, frontier molecular orbitals and UV data of 7,7,8,8-tetracyanoquinodimethane, 2,5-difluoro-7,7,8,8-tetracyanoquinodimethane and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane were computed using quantum mechanical code. Calculations were carried out at Becke-3-Lee-Yang-Parr (B3LYP) functional with density functional theory (DFT) and time-dependent density functional theory (TD-DFT) using the 6-311++G(d,p) basis set. The theoretical results were successfully compared with some of the available experimental data. The influence of fluorine on the structural, vibrational and electronic properties were investigated. The addition of fluorine reduces both the electrical and optical band gaps. The findings of this research can be useful for analogs of the molecules studied which have potential applications in the design of organic semiconductors for electronic devise applications.
Organic charge transfer complexes (CTCs) with near-infrared absorption received growing interest in the past years, but the details of their photophysics, especially in thin films, remain largely unknown. We combined experimental and computational methods to thoroughly investigate and compare CTCs formed by tetracene with 2,2′-(perfluoronaphthalene-2,6-diylidene)dimalononitrile and 2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane, respectively. Using ultrafast transient absorption spectroscopy, the photophysics of these small bandgap CTCs was revealed, which is dominated by a sub-picosecond relaxation of the excitons back to the ground state. In equimolar blends, tetracene singlet fission is suppressed while in blends with excess of tetracene reduced lifetimes of tetracene, singlet and triplet excitons were found.
Fluorine and its impact on structure and selected properties of molecules (both organic and inorganic), with the key role of theoretical calculations for understanding of this impact, will be described in this personalized account. Key atomic and molecular properties, which render fluorine so different from other elements, will be briefly discussed. Theoretical chemistry methods have been widely applied to reproduce the geometric structure and calculate fundamental properties of fluorine-rich systems, and they are nowadays indispensable for understanding and deliberate tuning of desired properties; last but not the least calculations are used to predict novel systems with desirable characteristics, and to guide experimental research. The chapter brings bits and pieces from modern fluorine chemistry to illustrate some of its most spectacular—and often practical—aspects, augmented by the state-of-the-art theoretical modeling.
In charge-transfer complexes, transition from the donor highest occupied molecular orbital (HOMO) to the acceptor lowest unoccupied molecular orbital (LUMO) gives the charge-transfer absorption. However, in tetracyanoquinodimethane (TCNQ) complexes of thienoacenes, comparison of the observed and calculated charge-transfer absorption demonstrates that the HOMO/LUMO transition is absent in the solid state owing to the orbital symmetry, and the first near-infrared band comes from the transition from the donor next HOMO to the TCNQ LUMO. Maps of the oscillator strength in rotated and translated molecular geometries are calculated on the basis of the time-dependent density functional theory, in which the absence of the HOMO/LUMO transition is approximately maintained even in the general molecular geometry.
The peculiar electronic and optical properties of covalent organic frameworks (COFs) are largely determined by protonation, a ubiquitous phenomenon in the solution environment in which they are synthesized. The resulting effects are nontrivial and appear to be crucial for the intriguing functionalities of these materials. In the quantum-mechanical framework of time-dependent density-functional theory, we investigate from first principles the impact of protonation of triazine and amino groups in molecular building blocks of COFs in water solution. In all considered cases, we find that proton uptake leads to a gap reduction and to a reorganization of the electronic structure, driven by the presence of the proton and by the electrostatic attraction between the positively charged protonated species and the negative counterion in its vicinity. Structural distortions induced by protonation are found to play only a minor role. The interplay between band gap renormalization and exciton binding strength determines the energy of the absorption onsets: when the former prevails over the latter, a red shift is observed. Furthermore, the spatial and energetic rearrangement of the molecular orbitals upon protonation induces a splitting of the lowest-energy peaks and a decrease of their oscillator strength in comparison with the pristine counterparts. Our results offer quantitative and microscopic insight into the role of protonation in the electronic and optical properties of triazine derivatives as building blocks of COFs. As such, they contribute to rationalize the relationships between structure, property, and functionality of these materials.
Charge-transfer (CT) crystals show unique transport and emission properties which are substantially different from those of individual electron donor (D) and acceptor (A) molecules constituting them. While the transport properties of CT crystals are well established as to enable the interpretation and prediction of their electrical properties, their photophysical processes - particularly the emission-structure relationship in CT crystals - are much less explored; this is because of the often weak photo-luminescence (PL), attributed to the small oscillator strength (f) of the emitting state and the arbitrary molecular design of CT complexes reported so far. In this work, we demonstrate that the novel isometric design of D-A molecules with appropriate CT interactions in the mixed-stack organic charge-transfer crystals can produce extremely strong CT emission in a predictable way. It was found that isometric D-A interactions in the mixed-stack isostructural CT crystals can generate highly increased oscillator strength within the slipped stack intermolecular arrangement via favorable configuration interaction, effective suppression of the non-radiative processes, and also triplet harvesting via reverse intersystem crossing. Based on the synergy of these effects, our mixed stack CT crystals marks a record high PL quantum yield of 83 %. Notably, four different CT pairs made of isometric D and A molecules all showed the isomorphic/quasi-isostructural intra-stack (π-stack) crystal, enabling us to find the sole effect of electronic CT interaction on their photophysical properties by decoupling the complicated morphological effect. Based on this specifically designed study of emission-structure relationship using the X-ray structure analysis, photophysical properties investigations and time-dependent density functional theory (TD-DFT) calculations, the mechanism of CT luminescence of mixed stack organic charge-transfer crystal was unraveled in terms of the modulated oscillator strength, non-radiative deactivation, and triplet harvesting.
The mechanistic study of molecular doping of organic semiconductors (OSC) requires an improved understanding of the role and formation of integer charge transfer complexes (ICTC) on a microscopic level. In the present work we go one crucial step beyond the simplest scenario of an isolated bi-molecular ICTC and study ICTCs formed of up to two (poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b,3,4-b”]dithiophene)-alt-4,7-(2,1,3- benzothiadiazole)](PCPDT-BT) oligomers and up to two CN6-CP molecules. We find that depending on geometric arrangement, complexes containing two conjugated oligomers and two dopant molecules can show p-type doping with double integer charge transfer, resulting in either two singly doped oligomers or one doubly doped oligomer. Interestingly, compared to an individual oligomer-dopant complex, the resulting in-gap states on the doped oligomers are significantly lowered in energy. Indicating that, already in the relatively small systems studied here, Coulomb binding of the doping-induced positive charge to the counter-ion is reduced which is an elemental step towards generating mobile charge carriers through molecular doping.
Due to the high mobility and Seebeck coefficient, pentacene (PEN) is a promising candidate for organic small molecule thermoelectric (TE) materials. However, the low intrinsic conductivity impedes its application in thermoelectricity. In this work, hexacyano-trimethylene-cyclopropane (CN6-CP) is employed as dopant for PEN via constructing of bilayer structured thin films. The almost intact crystallinity and high charge carrier generation efficiency of these interface doped PEN films ensure their high conductivity. The time of flight secondary ion mass spectrometry (ToF-SIMS) was applied to demonstrate the diffusion of dopant molecules into PEN layer. The UV-vis spectra analysis reveals that integral charge transfer happens between the PEN and CN6-CP molecules. The doping process is further characterized by ESR, UPS and XPS analysis. Under optimized condition, the conductivity of PEN film deposited on SiO2/Si substrate can reach up to 10.1 S cm-1. To the best of our knowledge, this has been the highest conductivity ever reported for doped PEN thin films. The optimal TE performance with a power factor (PF) of 36.4 μW m-1 K-2 can be achieved in PEN/CN6CP thin film with Seebeck coefficient and conductivity to be of 199 μV K-1 and 9.2 S cm-1, respectively. This result shows that interface doping with strong electron acceptor is a promising approach for optimizing the TE performance of small molecular organic semiconductors.
Coper thiocyanate (CuSCN) is generally considered as a very hopeful inorganic hole transport material (HTM) in semitransparent perovskite solar cells (ST-PSCs) because of its low parasitic absorption, high inherent stability, and low cost. However, the poor electrical conductivity and low work function of CuSCN lead to the insufficient hole extraction and large open-circuit voltage loss. Here, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is employed to improve conductivity of CuSCN and band alignment at the CuSCN/perovskite (PVK) interface. As a result, the average power conversion efficiency (PCE) of PSCs is boosted by ≈ 11%. In addition, benefiting from the superior transparency of p-type CuSCN HTMs, the prepared bifacial semitransparent n-i-p planar PSCs demonstrate a maximum efficiency of 14.8% and 12.5% by the illumination from the front side and back side, respectively. We believe that this developed CuSCN-based ST-PSCs will promote practical applications in building integrated photovoltaics and tandem solar cells.
Lewis acids like tris(pentafluorophenyl)borane (BCF) offer promising routes for efficient p-doping of organic semiconductors. The intriguing experimental results achieved so far call for a deeper understanding of the underlying doping mechanisms. In a first-principles work, based on state-of-the-art density-functional theory and many-body perturbation theory, we investigate the electronic and optical properties of donor/acceptor complexes formed by quarterthiophene (4T) doped by BCF. For reference, hexafluorobenzene (C6F6) and \ce{BF3} are also investigated as dopants for 4T. Modelling the adducts as bimolecules \textit{in vacuo}, we find negligible charge transfer in the ground state and frontier orbitals either segregated on opposite sides of the interface (4T:BCF) or localized on the donor (4T:BF3, 4T:C6F6). In the optical spectrum of 4T:BCF, a charge-transfer excitation appears at lowest-energy, corresponding to the transition between the frontier states, which exhibit very small but non-vanishing wave-function overlap. In the other two adducts, the absorption is given by a superposition of the features of the constituents. Our results clarify that the intrinsic electronic interactions between donor and acceptor are not responsible for the doping mechanisms induced by BCF and related Lewis acids. Extrinsic factors, such as solvent-solute interactions, intermolecular couplings, and thermodynamic effects, have to be systematically analyzed for this purpose.
The tetrafluorinated derivative of 7,7,8,8-tetracyanoquinodimethane (TCNQ), 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), is of interest for charge transfer complex formation and as a p-dopant in organic electronic materials. Fourier transform infrared (FTIR) spectroscopy is commonly employed to understand the redox properties of F4TCNQ in the matrix of interest; specifically, the ν(C≡N) region of the F4TCNQ spectrum is exquisitely sensitive to the nature of the charge transfer between F4TCNQ and its matrix. However, little work has been done to understand how these vibrational modes change in the presence of possible acid/base chemistry. Here, FTIR spectroelectrochemistry is coupled with density functional theory spectral simulation for study of the electrochemically generated F4TCNQ radical anion and dianion species and their protonation products with acids. Vibrational modes of HF4TCNQ-, formed by proton-coupled electron transfer, are identified, and we demonstrate that this species is readily formed by strong acids, such as trifluoroacetic acid, and to a lesser extent, by weak acids, such as water. The implications of this chemistry for use of F4TCNQ as a p-dopant in organic electronic materials is discussed.
F4TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) is used widely as a hole-doping agent in photoresponsive organic semiconducting materials, yet relatively little is known about the photoresponses of the F4TCNQ·‾ anion generated via doping. Furthermore, there is still relatively little systematic exploration of how the properties of the local material or chemical environment impacts the driving force for generating these charge-transfer complexes. Here we present spectroscopic and photophysical studies of F4TCNQ in charge-transfer complexes (CTCs) with the electron donor N,N’-Diphenyl-N-N’-di-p-tolylbenzene-1,4-diamine (MPDA) both in dichloroethane solution and polystyrene matrices. Integer charge transfer (ICT) between donor and acceptor occurs readily in dichloroethane solvent to form F4TCNQ·‾:MPDA+ CTCs, due to a ~150 mV difference in MPDA+/MPDA and F4TCNQ/F4TCNQ·‾ reduction potentials. Ultrafast spectroscopic studies of the CTC as well as electrochemically generated F4TCNQ·‾ and MDPA+ reveal that the photoresponses of these CTCs are dominated by that of the dopant anion, including rapid deactivation (800 fs) after excitation to the anion D1 excited state, followed by slower (~10 ps) vibrational cooling in the anion D0 state. Excitation to the higher-lying D2 state results in a rapid relaxation to the D1 state, in contrast to direct D2-D0 relaxation previously observed for F4TCNQ·‾ in gas phase. CTCs embedded in polystyrene (PS) matrices are observed to lose their integer charge-transfer character upon evaporation of solvent, as evidenced by changes to electronic and vibrational absorption features associated with F4TCNQ·‾. This change is attributed to the loss of solvent stabilization of the ion pair formed through the charge-transfer reaction. Ultrafast spectral measurements reveal that the photoresponses of the partial charge-transfer (PCT) species embedded in PS are still highly similar to those of the ICT species and unlike that of neutral F4TCNQ, implying the electronic properties of the PCT state are likewise dominated by properties of the reduced acceptor molecule. We conclude that excitation of ICT or PCT states introduces optical losses for photoresponses of doped organic semiconductor materials due to the large anion absorption cross section and its rapid, dissipative deactivation dynamics.
Two doping mechanisms are known for the well-studied materials poly(3-hexylthiophene) (P3HT) and poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT), namely, integer charge transfer (ICT) and charge transfer complex (CTC) formation. Yet, there is poor understanding of the effect of doping mechanism on thermal stability and the thermoelectric properties. In this work, we present a method to finely adjust the ICT to CTC ratio. Using it, we characterize electrical and thermal conductivities as well as the Seebeck coefficient and the long-term stability under thermal stress of P3HT and PBTTT of different ICT/CTC ratios. We establish that doping through the CTC results in more stable, yet lower conductivity samples compared to ICT doped films. Importantly, moderate CTC fractions of ∼33% are found to improve the long-term stability without a significant sacrifice in electrical conductivity. Through visible and IR spectroscopies, polarized optical microscopy, and grazing-incidence wide-angle X-ray scattering, we find that the CTC dopant molecule access sites within the polymer network are less prone to dedoping upon thermal exposure.
We present density-functional and time-dependent density-functional studies of the ground, ionic, and excited states of a series of oligomers of thiophene. We show that, for the physical properties, the most relevant highest occupied and lowest unoccupied molecular orbitals develop gradually from monomer molecular orbitals into occupied and unoccupied broad bands in the large length limit. We show that band gap and ionization potentials decrease with size, as found experimentally and from empirical calculations. This gives credence to a simple tight-binding model Hamiltonian approach to these systems. We demonstrate that the length dependence of the experimental excitation spectra for both singlet and triplet excitations can be very well explained with an extended Hubbard-like Hamiltonian, with a monomer on-site Coulomb and exchange interaction and a nearest-neighbor Coulomb interaction. We also study the ground and excited-state electronic structures as functions of the torsion angle between the units in a dimer, and find almost equal stabilities for the transoid and cisoid isomers, with a transition energy barrier for isomerization of only 4.3 kcal/mol. Fluctuations in the torsion angle turn out to be very low in energy, and therefore of great importance in describing even the room-temperature properties. At a torsion angle of 90° the hopping integral is switched off for the highest occupied molecular orbital levels because of symmetry, allowing a first-principles estimate of the on-site interaction minus the next-neighbor Coulomb interaction as it enters in a Hubbard-like model Hamiltonian.
We demonstrate the use of a p-doped amorphous starburst amine, 4, 4′, 4″-tris(N, N-diphenyl- amino)triphenylamine (TDATA), doped with a very strong acceptor, tetrafluoro- tetracyano-quinodimethane by controlled coevaporation as an excellent hole injection material for organic light-emitting diodes (OLEDs). Multilayered OLEDs consisting of double hole transport layers of p-doped TDATA and triphenyl-diamine, and an emitting layer of pure 8-tris-hydroxyquinoline aluminum exhibit a very low operating voltage (3.4 V) for obtaining 100 cd/m2 even for a comparatively large (110 nm) total hole transport layer thickness. © 2001 American Institute of Physics.
We investigated the effectiveness of p-dopants to generate holes in a hole transporting material by comparing the absorption in visible-near-infrared and infrared regions and current density-voltage characteristics. CuI, MoO3, and ReO3 having different work functions were doped in a hole transporting organic material, 4,4′,4″-tris(N-(2-naphthyl)-N-phenylamino)-triphenylamine (2TNATA). Formation of charge transfer (CT) complexes increases linearly with increasing doping concentration for all the dopants. Dopants with higher work function (ReO3>MoO3>CuI) are more effective in the formation of CT complexes and in the generation of the charges in the doped films.
The power conversion efficiency of organic photovoltaic cells can be greatly enhanced by chemical doping to control the conductivity of the organic thin films. We demonstrate a nearly twofold improvement in the efficiency of planar heterojunction copper phthalocyanine/fullerene cells by n-doping the electron acceptor layer with decamethylcobaltocene in the vicinity of the fullerene/cathode interface. Doping improves the charge extraction efficiency and decreases the series resistance of the organic films, improving the current density and fill factor, respectively.
Values of charge transfer integrals, spatial overlap integrals and site energies involved in transport of positive charges along columnar stacked triphenylene derivatives are provided. These parameters were calculated directly as the matrix elements of the Kohn–Sham Hamiltonian, defined in terms of the molecular orbitals on individual triphenylene molecules. This was realized by exploiting the unique feature of the Amsterdam density functional theory program that allows one to use molecular orbitals on individual molecules as a basis set in calculations on a system composed of two or more molecules. The charge transfer integrals obtained in this way differ significantly from values estimated from the energy splitting between the highest occupied molecular orbitals in a dimer. The difference is due to the nonzero spatial overlap between the molecular orbitals on adjacent molecules. Calculations were performed on unsubstituted and methoxy- or methylthio-substituted triphenylenes. Charge transfer integrals and site energies were computed as a function of the twist angle, stacking distance and lateral slide distance between adjacent molecules. The variation of the charge transfer integrals and site energies with these conformational degrees of freedom provide a qualitative explanation of the similarities and differences between the experimental charge carrier mobilities in different phases of alkoxy- and alkylthio-substituted triphenylenes. The data obtained from the present work can be used as input in quantitative studies of charge transport in columnar stacked triphenylene derivatives.
The method of dispersion correction as an add-on to standard Kohn-Sham density functional theory (DFT-D) has been refined regarding higher accuracy, broader range of applicability, and less empiricism. The main new ingredients are atom-pairwise specific dispersion coefficients and cutoff radii that are both computed from first principles. The coefficients for new eighth-order dispersion terms are computed using established recursion relations. System (geometry) dependent information is used for the first time in a DFT-D type approach by employing the new concept of fractional coordination numbers (CN). They are used to interpolate between dispersion coefficients of atoms in different chemical environments. The method only requires adjustment of two global parameters for each density functional, is asymptotically exact for a gas of weakly interacting neutral atoms, and easily allows the computation of atomic forces. Three-body nonadditivity terms are considered. The method has been assessed on standard benchmark sets for inter- and intramolecular noncovalent interactions with a particular emphasis on a consistent description of light and heavy element systems. The mean absolute deviations for the S22 benchmark set of noncovalent interactions for 11 standard density functionals decrease by 15%-40% compared to the previous (already accurate) DFT-D version. Spectacular improvements are found for a tripeptide-folding model and all tested metallic systems. The rectification of the long-range behavior and the use of more accurate C(6) coefficients also lead to a much better description of large (infinite) systems as shown for graphene sheets and the adsorption of benzene on an Ag(111) surface. For graphene it is found that the inclusion of three-body terms substantially (by about 10%) weakens the interlayer binding. We propose the revised DFT-D method as a general tool for the computation of the dispersion energy in molecules and solids of any kind with DFT and related (low-cost) electronic structure methods for large systems.
Photoemission measurements reveal energy level shifts toward the Fermi level when a strong electron acceptor (tetrafluoro-tetracyanoquinodimethane, F4-TCNQ) is deposited on pristine layers of 4,4', 4 ''-tris(N,N-diphenyl-amino)triphenylamine (TDATA) or 4,4'-bis(N-carbazolyl)biphenyl (CBP). The shifts of the TDATA and CBP energy levels toward the Fermi level of the Au substrate could, in principle, arise from p-type doping of the intrinsic organic layers. While this indeed takes place in TDATA, doping of CBP by F4-TCNQ, i.e., charge transfer complex formation, does not occur. The shifts observed in CBP arise from the diffusion of F4-TCNQ toward the Au substrate, which modifies the buried metal surface potential, leading to a realignment of the energy levels of the organic overlayer. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3213547]
The performance of the recently introduced X3LYP density functional which was claimed to significantly improve the accuracy for H-bonded and van der Waals complexes was tested for extended H-bonded and stacked complexes (nucleic acid base pairs and amino acid pairs). In the case of planar H-bonded complexes (guanine...cytosine, adenine...thymine) the DFT results nicely agree with accurate correlated ab initio results. For the stacked pairs (uracil dimer, cytosine dimer, adenine...thymine and guanine...cytosine) the DFT fails completely and it was even not able to localize any minimum at the stacked subspace of the potential energy surface. The geometry optimization of all these stacked clusters leads systematically to the planar H-bonded pairs. The amino acid pairs were investigated in the crystal geometry. DFT again strongly underestimates the accurate correlated ab initio stabilization energies and usually it was not able to describe the stabilization of a pair. The X3LYP functional thus behaves similarly to other current functionals. Stacking of nucleic acid bases as well as interaction of amino acids was described satisfactorily by using the tight-binding DFT method, which explicitly covers the London dispersion energy.
Molecular crystals from thiophene molecules can be doped with TCNQ-F4 molecules for use in all-organic optoelectronic and semiconductor devices. The charge transfer and the molecular orbital energy level formation in between these two organic molecules are investigated here by density functional theory calculations. The isolated molecules are calculated nonbonded and bonded together, forming a charge transfer complex (CTC). The relaxed structure of the complex shows essentially coplanar and centered molecules with the alpha-sexithiophene rings tilted alternatingly by 4.8 degrees. The bond formation of these molecules results in a charge transfer of approximately 0.4 e from the alpha-sexithiophene to the TCNQ-F4 molecule. The highest occupied molecular orbital-lowest unoccupied molecular orbital gap width is reduced as compared to the isolated molecules due to the newly formed orbitals in the CTC. Upon adsorption on a Au(111) surface, electrons are transferred onto the molecule complex, thereby causing the molecular levels to align asymmetric with respect to the charge neutrality level. The theoretical results for the single molecule and CTC layer are compared to experimental photoemission and scanning tunneling spectroscopy results.
We investigate recently published methods for extending density functional theory to the description of long-range dispersive interactions. In all schemes an empirical correction consisting of a C6r(-6) term is introduced that is damped at short range. The coefficient C6 is calculated either from average molecular or atomic polarizabilities. We calculate geometry-dependent interaction energy profiles for the water benzene cluster and compare the results with second-order Møller-Plesset calculations. Our results indicate that the use of the B3LYP functional in combination with an appropriate mixing rule and damping function is recommended for the interaction of water with aromatics.
Ab initio MP2/6-31G* interaction energies were calculated for more than 80 geometries of stacked cytosine dimer. Diffuse polarization functions were used to properly cover the dispersion energy. The results of ab initio calculations were compared with those obtained from three electrostatic empirical potential models, constructed as the sum of a Lennard-Jones potential (covering dispersion and repulsion contributions) and the electrostatic term. Point charges and point multipoles of the electrostatic term were also obtained at the MP2/6-31G* level of theory. The point charge MEP model (atomic charges derived from molecular electrostatic potential) satisfactorily reproduced the ab initio data. Addition of π-charges localized below and above the cytosine plane did not affect the calculated energies. The model employing the distributed multipole analysis gave worse agreement with the ab initio data than the MEP approach. The MP2 MEP charges were also derived using larger sets of atomic orbitals: cc-pVDZ, 6-311 + G(2d, p), and aug-cc-pVDZ. Differences between interaction energies calculated using these three sets of point charges and the MP2/6-31G* charges were smaller than 0.8 kcal/mol. The correlated ab initio calculations were also compared with the density functional theory (DFT) method. DFT calculations well reproduced the electrostatic part of interaction energy. They also covered some nonelectrostatic short-range effects which were not reproduced by the empirical potentials. The DFT method does not include the dispersion energy. This energy, approximated by an empirical term, was therefore added to the DFT interaction energy. The resulting interaction energy exhibited an artifact secondary minimum for a 3.9-4.0 vertical separation of bases. This defect is inherent in the DFT functionals, because it is not observed for the Hartree-Fock + dispersion interaction energy.© 1996 John Wiley & Sons, Inc.
The polarization energies of 44 organic solids were determined by ultraviolet photoelectron spectroscopy in the gaseous and solid states. Condensed polycyclic aromatic hydrocarbons with planar molecular structures were found to have a common value, 1.7 eV, independent of their molecular sizes and also their crystal structures. The common value is approximately interpreted by the first-order expression for the polarization energy. A large variation of values in the range 0.9–3.0 eV was obtained for several compounds. Among them, molecules with intricate structures have smaller values and those with large molecular polarizabilities have larger values than the common value. These results indicate that the polarization energy of an organic solid is mainly determined by two factors: the molecular polarizability and the molecular packing in the solid. Intermolecular interactions in the solid, other than the van der Waals force, also contribute to the value.
Recent experiments have reported a vacuum level shift at the interface between organic materials due to the formation of an interface dipole layer. On the basis of quantum-chemical calculations, this paper sheds light on the factors contributing to the formation of an interface dipole between an electron donor and an electron acceptor, considering as model system a complex made of tetrathiafulvalene (TTF) as a donor and tetracyanoquinodimethane (TCNQ) as an acceptor. The results indicate that the interface dipole is governed both by charge-transfer and polarization effects and allow for disentangling of their respective contributions. Two regimes of charge transfer can be distinguished depending on the strength of the electronic coupling: a fractional charge transfer occurs in the strong coupling regime while only integer charges are transferred when the coupling is weak. The polarization contribution can be significant, even in the presence of a pronounced charge transfer between the donor and acceptor molecules. The values of ionization potential and electron affinity of the donor and acceptor molecules may experience shifts as large as several tenths of an eV at the interface with respect to the isolated compounds.
We present a systematic study on p-type doping of zinc-phthalocyanine by tetrafluoro-tetracyano-quinodimethane as an example of controlled doping of thin organic films by cosublimation of matrix and dopant. The zinc-phthalocyanine layers are prepared both in polycrystalline and amorphous phase by variation of the sublimation conditions. The films are electrically characterized in situ by temperature dependent conductivity and Seebeck and field-effect measurements. In addition to previous work, we show that also amorphous phthalocyanine layers can be doped, i.e., their conductivity increases and their Seebeck coefficient decreases indicating a shift of the Fermi level towards the hole transport level. The field-effect mobility of the polycrystalline samples is in the range of 10-4–10-3 cm-2/Vs and increases with increasing dopant concentration. Adapting a percolation model presented by Vissenberg and Matters [Phys. Rev. B, 57, 12 964 (1998)], which assumes hopping transport within a distribution of localized states, we can quantitatively describe the conductivity (in different organic layers) and the field-effect mobility.
A fundamental aspect in the study of the charge–transfer (CT) organic crystals with ionic or partially ionic ground states is the investigation of the spectroscopic effect of the electron–molecular vibration coupling. 7,7,8,8‐tetracyano‐2,3,5,6‐tetrafluoroquinodimethane (TCNQF4), an electron acceptor much stronger than TCNQ, is an outstanding component of many interesting CT systems. A thorough vibrational analysis of the title compound and of its monomeric radical anion is reported. The analysis is based on the Raman depolarization ratio measurements and on infrared data of solutions of both neutral and ionic species as well as on polarized infrared spectra of oriented crystals of the neutral molecule. The vibrational assignment, completed by a normal coordinate analysis (NCA), brings to the identification of the ionization frequency shifts and to the choice of the fundamentals (b1uν19 and b2uν33) whose frequencies are diagnostic of the degree of charge transfer for a TCNQF4 moiety partner of a CT system. Through a combined use of a RHF‐CNDO/S(CT) electronic calculation and of the eigenvectors of the radical anion given by the NCA, the numerical evaluation of the linear e–mv coupling constants is carried out. The relative values of these constants are extracted, by applying the dimer model, from the vibronic features of the infrared spectrum of the low temperature phase of Rb–TCNQF4. This compound is recognized, by comparison with the alkali salts of TCNQ, to display a phase transition at T≂130 K from a regular to a dimerized stack structure. From the analysis of all these data it is possible to characterize the spectroscopic vibronic effects related to e–mv coupling in CT systems of TCNQF4. This is successfully verified by the segregated dimerized stack system of DBTTF–TCNQF4 and applied to the not yet characterized system TSF–TCNQF4 which is recognized here as a fully ionic complex with segregated dimerized stack motif. As a consequence, through the vibrational spectra, TCNQF4 can be fruitfully used as a sensitive probe for structural and electronic characterization of its CT complexes.
We investigate p-type doping of poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) films with tetrafluorotetracyanoquinodimethane (F4-TCNQ) introduced via cosolution. Doped and undoped films are compared using ultraviolet photoelectron spectroscopy (UPS) and current–voltage (I–V) measurement. In spite of the difference between the ionization energy of PFO (5.8 eV) and the electron affinity of F4-TCNQ (5.24 eV), p doping occurs, as seen from the movement of the Fermi level (EF) toward the polymer highest occupied molecular orbital (HOMO). Interface hole barriers are measured for undoped and doped PFO deposited on three substrates with different work functions, indium-tin-oxide (ITO), gold (Au), and poly-3,4-ethylenedioxythiophene∙polystyrenesulfonate (PEDOT∙PSS). Doping leads to the formation of a depletion region at the PFO/ITO and PFO/Au interfaces. The depletion region is believed to be at the origin of the (hole) current enhancement observed on simple metal/PFO/substrate devices.
An empirical method has been designed to account for the van der Waals interactions in practical molecular calculations with density functional theory. For each atom pair separated at a distance R, the method adds to the density functional electronic structure calculations an additional attraction energy EvdW = −fd(R)C6R−6, where fd(R) is the damping function which equals to one at large value of R and zero at small value of R. The coefficients C6 for pair interactions between hydrogen, carbon, nitrogen, and oxygen atoms have been developed in this work by a least-square fitting to the molecular C6 coefficients obtained from the dipole oscillator strength distribution method by Meath and co-workers. Two forms of the damping functions have been studied, with one dropping to zero at short distances much faster than the other. Four density functionals have been examined: Becke’s three parameter hybrid functional with the Lee-Yang-Parr correlation functional, Becke’s 1988 exchange functional with the LYP correlation functional, Becke’s 1988 exchange functional with Perdew and Wang’s 1991 (PW91) correlation functional, and PW91 exchange and correlation functional. The method has been applied to three systems where the van der Waals attractions are known to be important: rare-gas diatomic molecules, stacking of base pairs and polyalanines’ conformation stabilities. The results show that this empirical method, with the damping function dropping to zero smoothly, provides a significant correction to both of the Becke’s hybrid functional and the PW91 exchange and correlation functional. Results are comparable to the corresponding second-order Møller-Plesset calculations in many cases. © 2002 American Institute of Physics.
The absolute electron affinities of pi charge transfer complex acceptors have been examined and the ’’best’’ values have been chosen. All of the results obtained by the magnetron method, including the estimates for hexafluorobenzene and tetracyanoethylene, were accepted. However, the magnetron results for anthraquinone and benzoquinone are not in agreement with charge transfer complex and half‐wave reduction potential data. The half‐wave reduction potential data and the charge transfer complex data for all of the other acceptors for which absolute electron affinities are available were found to be consistent with the usual correlation equations and their associated assumptions. The parameters from these correlations have been used to calculate the absolute electron affinities for about 150 acceptors.
We have studied the behavior of various intrinsic emission zones on the characteristics of organic light-emitting diodes with a p-doped hole-transport layer and an n-doped electron-transport layer based on our previous work [J. S. Huang, M. Pfeiffer, A. Werner, J. Blochwitz, K. Leo, and S. Liu, Appl. Phys. Lett. 80, 139 (2002)]. This configuration is referred to as a PiN structure. Because the p- and n-doped regions occupy nearly 80% of the total thickness in our PiN device, the intrinsic region becomes a narrow layer between two doped regions. This intrinsic region includes the region where the radiative recombination occurs. Thus, the nature of this layer plays an important role in determining the actual device performance. Employing 8-tris-hydroxyquinoline aluminum as an emitter, we investigated the influence of the thickness of the emitter layer on the performance of the device. The optimum thickness of the emitter layer is found to be 20 nm. Combining the fluorescence dye doping method, we have optimized the PiN structure device. Two emitter systems have been used: Alq3 doped with two highly fluorescent laser dyes, Quinacridone or Coumarin 6, respectively. We have demonstrated the influence of the thickness and the doping of the emission zone on the characteristics of a doped emitter device with PiN structure, and obtained higher-efficiency PiN structure devices. The different properties of PiN devices corresponding to two different emitter dopants with different trapping effect are also discussed. © 2003 American Institute of Physics.
P-doping of zinc phthalocyanine (ZnPc) with tetrafluorotetracyanoquinodimethane (F4-TCNQ) is investigated with ultraviolet and x-ray photoemission spectroscopy, inverse photoemission spectroscopy, and in situ current–voltage (I–V) measurements. The electron affinity of F4-TCNQ (5.24 eV) is found to be equal, within experimental error, to the ionization energy of ZnPc (5.28 eV), consistent with efficient host-to-dopant electron transfer. As a result, the Fermi level in doped ZnPc drops from near midgap to 0.18 eV above the leading edge of the highest occupied molecular orbital and a narrow space-charge layer (<32 Å) is formed at the interface with the Au substrate. In situ I–V measurements show a seven orders of magnitude doping-induced increase in hole current. © 2001 American Institute of Physics.
A new approximate ``absolute'' scale of electronegativity, or electroaffinity, is set up. The absolute electroaffinity on this scale is equal to the average of ionization potential and electron affinity. These quantities must, however, in general, be calculated not in the ordinary way, but for suitable ``valence states'' of the positive and negative ion. Also, the electroaffinity of an atom has different values for different values of its valence; in general the electroaffinity as here calculated (in agreement with chemical facts) is larger for higher valences. Electroaffinity values have been calculated here for H, Li, B, C, N, O, F, Cl, Br, I. They show good agreement in known cases with Pauling's electronegativity scale based on thermal data, and with the dipole moment scale. The present electronegativity scale (like the others) is rather largely empirical, especially as to its quantitative validity; and it remains to be seen whether or not the latter will be more than very rough when tested for a wider range of cases. Nevertheless the new scale has a degree of theoretical background and foundation which throws some new light on the physical meaning of the concept of electronegativity (or electro‐affinity). The basis of the present scale, it should be mentioned, is simpler and more certain for univalent than for polyvalent atoms.—The nature of valence states of atoms is briefly discussed. It is hoped that the tabulations of atomic valence state energies and valence state ionization potentials and electron affinities given at the end of this paper may be useful in problems of molecular structure.
Solution and solid state studies of TCNQF4 are reported. The electron affinity of TCNQF4 has been derived from spectral observations of the visible-near IR charge-transfer band for the pyrene complex and compared to that derived from the half-wave potentials used to characterize its reactivity in solution. It is demonstrated that the series, TCNQ, TCNQF to TCNQF4 shows a monotonic increase in reactivity with increasing fluorine substitution. Crystals of TCNQF4, grown from acetonitrile, are orthorhombic; space group Pbca, with the following crystal data: a = 14.678(7)A, b = 9.337(5)A, c = 8.174(2)A, V = 1120.0(6)A, Z = 4 [based on a molecular weight for C12N4F4 = 276.22], Dmeasd = 1.65(1) g cm, Dcalcd = 1.64 g cm. Full matrix least-squares refinement (including anisotropic thermal parameters for all atoms) using 1763 counter-collected Fa's led to a final R value of 0.067 and a final weighted R value of 0.036. The observed molecular geometry of TCNQF4 is compared to that of its parent molecule TCNQ and to its monoanion found in some charge-transfer complexes. Crystal packing of TCNQF4 affords two interesting types of intermolecular acid/base (or donor/acceptor) interactions utilizing the cyano nitrogen atoms and the fluorinated carbon atoms of the quininoid ring. This subtle amphoterism exhibited by TCNQF4 leads to reduced dimensionality for the crystalline motif and is compared to similar solid state interactions found in several other molecules with π-bonded ring systems containing highly-electronegative substituents.
The molecular structures of monomethylsilyl cyanide and dimethylsilyl cyanide in the gas phase have been determined by electron diffraction. The molecules have bond lengths (pm) and angles (degrees): MeSiH2CN: r(SiCN) 184.7(9), r(SiCH) 186.6(9), r(SiH) 147.9(9), r(CH) 108.5(7), r(CN) 116.7(3), ∠HCH 105.4(20), ∠CSiC(N) 109.5(15); Me2SiHCN: r(SiCN) 186.7(9), r(SiCH3) 185.7(5), r(SiH) 151 fixed, r(CH) 110.0(7), r(CN) 116.4(7), ∠HCH 110.2(7), ∠CSiC(N) 111.4(15), ∠CSiC 103.2(20), ∠SiCN 170(4). There are apparent deviations from linearity at the cyanide carbon in each case, but these are believed to be shrinkage effects. A vibrational assignment is proposed for CH3SiH2CN.
We provide evidence for highly localized charge-transfer complex formation between a series of thiophenetetrafluorobenzene donor copolymers and the molecular acceptor tetrafluorotetracyanoquinodimethane (F4TCNQ). Infrared absorption spectra of diagnostic vibrational bands in conjunction with theoretical modeling show that one acceptor molecule undergoes charge transfer with a quaterthiophene segment of the polymers, irrespective of the macroscopic polymer ionization energy and acceptor concentration in thin films. The interaction is thus determined by the “local ionization potential” of quaterthiophene. Consequently, using materials parameters determined on a macroscopic length scale as a guideline for making new charge-transfer complex materials for high electrical conductivity turns out to be oversimplified, and a reliable material design must take into account property variations on the nm scale as well.Keywords (keywords): organic electronics; conductive polymers; molecular doping; polythiophene; charge-transfer complex
Stereospezi- fische syn-Addition des Carbens (II) an die E/Z-isomeren Diene (I) führt zu Gemischen der Vinylcyclopropane (III) und (IV), die bei der Behandlung mit Butyl-Li unter Ether- Spaltung und Umlagerung der gebildeten Li-Salze die Cyclopentenole (V) liefern.
A series of oligothiophenes (from bithiophene to sexithiophene) substituted at the ends of the chain by various electron donor of acceptor groups or atoms (methoxy, nitro, or bromo) have been prepared. Their electrochemical oxidations have been studied using microelectrodes in order to determine accurately the formal redox potential of the radical-cation generation. The stability of the latter can be deduced from the required potential scan rate. Electron donor substituents (methoxy and bromo) stabilize the radical-cation forms, and the dications can also be obtained from these substituted terthiophenes. The optical properties of the compounds have also been investigated. Only the nitro-substituted group induces a significant increase of the fluorescence quantum yield and of the lifetime of the excited state. For solution study, this positive effect is only limited to the short oligomeric chains for which an intramolecular charge transfer is evidenced by solvatochromic effects. 21 refs., 5 figs., 3 tabs.
We demonstrate enhanced hole injection and lowered driving voltage in vacuum-deposited organic light-emitting diodes (OLEDs) with a hole-transport layer using the starburst amine 4,4′,4″-tris(N,N-diphenyl-amino)triphenylamine (TDATA) p-doped with a very strong acceptor, tetrafluoro-tetracyano-quinodimethane (F4-TCNQ) by controlled coevaporation. The doping leads to high conductivity of doped TDATA layers and a high density of equilibrium charge carriers, which facilitates hole injection and transport. Moreover, multilayer OLEDs consisting of double hole-transport layers of thick p-doped TDATA and a thin triphenyl-diamine (TPD) interlayer exhibit very low operating voltages.
In this paper, we discuss recent experiments which prove that evaporated organic films can be efficiently doped by co-evaporation with organic dopant molecules. Key advantages for devices are the high conductivity and the formation of ohmic contacts despite large energetic barriers. For p-type doping, efficient doping is possible for a variety of polycrystalline and amorphous materials. Despite the differences in the microscopic behavior, all basic effects known from doped inorganic semiconductors are found in organics as well. However, efficient n-type doping with stable molecular dopants is still a challenge.Organic light emitting diodes (OLED) with conductivity doped transport layers show significantly improved properties: For instance, we have achieved a brightness of 100 cd/m2 already at a voltage of 2.55 V, well below previous results for undoped devices. The advantages of doping are even more pronounced for top-emitting, inverted OLED structures: Due to the ohmic contacts nearly independent of the contact properties, it is possible to realize inverted top-emitting devices with parameters comparable to standard devices. Our doping technology is thus a significant advantage for active-matrix OLED displays and other displays on opaque substrate.
The highly parametrized, empirical exchange-correlation functionals, M05-2X and M06-2X, developed by Zhao and Truhlar have been shown to describe noncovalent interactions better than density functionals which are currently in common use. However, these methods have yet to be fully benchmarked for the types of interactions important in biomolecules. M05-2X and M06-2X are claimed to capture "medium-range" electron correlation; however, the "long-range" electron correlation neglected by these functionals can also be important in the binding of noncovalent complexes. Here we test M05-2X and M06-2X for the nucleic acid base pairs in the JSCH-2005 database. Using the CCSD(T) binding energies as a benchmark, the performance of these functionals is compared to that of a nonempirical density functional, PBE, and also to that of PBE plus Grimme's empirical dispersion correction, PBE-D. Due to the impor-tance of "long-range" electron correlation in hydrogen-bonded and interstrand base pairs, PBE-D provides more accurate interaction energies on average for the JSCH-2005 database when compared to M05-2X or M06-2X. M06-2X does, however, perform somewhat better than PBE-D for interac-tions between stacked base pairs.
The mixtures of soluble polythiophene variant poly(3-hexylthiophene) (P3HT) and F4TCNQ, which can form charge-transfer complexes (CTCs) with high conductivity in thin films were reported. The local conformation and electronic structure of P3HT/F4TCNQ CTCs in thin films by combining X-ray absorption near edge structure (XANES) measurements with theoretical modeling using density functional theory (DFT) was also investigated. The polymer was approximated by quarterthiophene (4T) because of computational constraints. The optimized structure of the 4T/F4TCNQ model reveals that F4TCNQ is strongly bent compared to its planar neutral form, and the cyano groups bend down toward 4T, which is also nonplanar. It was observed that the CT interaction between F4TCNQ and P3HT was very strong, leading to significant molecular conformational changes. It was also observed that the highest occupied molecular orbital (HOMO) and the LUMO of the 4T/F4TCNQ complex were formed by hybridizing the 4T HOMO and the F4TCNQ LUMO.
The controlling of p-doping of P3HT, PFB, TFB, and F8BT conjugated polymers by co-blending with F4TCNQ in a common organic solvent was investigated. Polymer films were spun on oxygen plasma treated glass substrates with inter-digitated ITO structures. The spacing between the ITO contacts was 10 μm, 15 μm or 20 μm. The current-voltage characteristics of the films were measured in nitrogen environment, biased between -4V and 4V in steps of 1V. Hole-only diodes were fabricated by using ITO as anode, TFB, PFB, F8BT, P3HT as the active layer. and NiCr as cathode. Absorption spectra for polymer thin films were acquired with a Hewlett Packard 8453 diode array spectrometer. It was observed that doping leads to significant increase in the bulk conductivity and hole current of the polymers with gradual disappearance of turn-on voltage.
We use the resolution of the identify (RI) as convenient way to replace the use of four-index-two-electron integrals with linear combinations of three-index integrals. The method is broadly applicable to a wide range of problems in quantum chemistry. We demonstrate the effectivenes of RI for the calculation of MP2 energies. For the water dimer, agreement within 0.1 kcal/mol is obtained with respect to exact MP2 calculations. The RI-MP2 energies require only about 10% of the time required by conventional MP2.
The lowest unoccupied molecular orbital (LUMO) energies of a variety of molecular organic semiconductors have been evaluated using inverse photoelectron spectroscopy (IPES) data and are compared with data determined from the optical energy gaps, electrochemical reduction potentials, and density functional theory (DFT) calculations. A linear fit to the electrochemical reduction potential (relative to an internal ferrocene reference) vs. the LUMO energy determined by IPES gives a slope and intercept of −1.19 ± 0.08 eV/V and −4.78 ± 0.17 eV, respectively, and 0.92 ± 0.04 and −0.44 ± 0.11 eV, respectively, based on the DFT calculated LUMO energies. From these fits, we estimate the LUMO and exciton binding energies of a wide range of organic semiconductors.
Ab initio MP2/6-31G* interaction energies were calculated for more than 80 geometries of stacked cytosine dimer. Diffuse polarization functions were used to properly cover the dispersion energy. The results of ab initio calculations were compared with those obtained from three electrostatic empirical potential models, constructed as the sum of a Lennard-Jones potential (covering dispersion and repulsion contributions) and the electrostatic term. Point charges and point multipoles of the electrostatic term were also obtained at the MP2/6-31G* level of theory. The point charge MEP model (atomic charges derived from molecular electrostatic potential) satisfactorily reproduced the ab initio data. Addition of π-charges localized below and above the cytosine plane did not affect the calculated energies. The model employing the distributed multipole analysis gave worse agreement with the ab initio data than the MEP approach. The MP2 MEP charges were also derived using larger sets of atomic orbitals: cc-pVDZ, 6-311 + G(2d, p), and aug-cc-pVDZ. Differences between interaction energies calculated using these three sets of point charges and the MP2/6-31G* charges were smaller than 0.8 kcal/mol. The correlated ab initio calculations were also compared with the density functional theory (DFT) method. DFT calculations well reproduced the electrostatic part of interaction energy. They also covered some nonelectrostatic short-range effects which were not reproduced by the empirical potentials. The DFT method does not include the dispersion energy. This energy, approximated by an empirical term, was therefore added to the DFT interaction energy. The resulting interaction energy exhibited an artifact secondary minimum for a 3.9-4.0 vertical separation of bases. This defect is inherent in the DFT functionals, because it is not observed for the Hartree-Fock + dispersion interaction energy.© 1996 John Wiley & Sons, Inc.
For neutral and charged species, atomic and molecular, a property called absolute hardness η is defined. Let E(N) be a ground-state electronic energy as a function of the number of electrons N. As is well-known, the derivative of E(N) with respect to N, keeping nuclear charges Z fixed, is the chemical potential μ or the negative of the absolute electronegativity χ: μ = (∂E/∂N)Z = -χ. The corresponding second derivative is hardness: 2η = (∂μ/∂N)Z = -(∂χ/∂N)Z = (∂2E/∂N2)Z. Operational definitions of χ and η are provided by the finite difference formulas (the first due to Mulliken) χ = 1/2(I + A), η = 1/2(I - A), where I and A are the ionization potential and electron affinity of the species in question. Softness is the opposite of hardness: a low value of η means high softness. The principle of hard and soft acids and bases is derived theoretically by making use of the hypothesis that extra stability attends bonding of A to B when the ionization potentials of A and B in the molecule (after charge transfer) are the same. For bases B, hardness is identified as the hardness of the species B+. Tables of absolute hardness are given for a number of free atoms, Lewis acids, and Lewis bases, and the values are found to agree well with chemical facts.
The result of a calculation of excited states, be it via configuration interaction methods or density functional response theory, is a set of coefficients describing the contribution that individual orbital excitations make to the total transition. Often times, there is no dominant amplitude describing the transition, making its qualitative description difficult. Natural transition orbitals dramatically simplify the situation by providing a compact representation of the transition density matrix. This is accomplished using the corresponding orbital transformation of Amos and Hall, which renders the transition density matrix diagonal and provides a unique correspondence between the excited ‘particle’ and empty ‘hole’.
This review provides a perspective on the use of orbital-dependent functionals, which is currently considered one of the most promising avenues in modern density-functional theory. The focus here is on four major themes: the motivation for orbital-dependent functionals in terms of limitations of semilocal functionals; the optimized effective potential as a rigorous approach to incorporating orbital-dependent functionals within the Kohn-Sham framework; the rationale behind and advantages and limitations of four popular classes of orbital-dependent functionals; and the use of orbital-dependent functionals for predicting excited-state properties. For each of these issues, both formal and practical aspects are assessed.
Scaling factors for obtaining fundamental vibrational frequencies, low-frequency vibrations, zero-point vibrational energies (ZPVE), and thermal contributions to enthalpy and entropy from harmonic frequencies determined at 19 levels of theory have been derived through a least-squares approach. Semiempirical methods (AM1 and PM3), conventional uncorrelated and correlated ab initio molecular orbital procedures [Hartree?Fock (HF), M?ller?Plesset (MP2), and quadratic configuration interaction including single and double substitutions (QCISD)], and several variants of density functional theory (DFT:? B-LYP, B-P86, B3-LYP, B3-P86, and B3-PW91) have been examined in conjunction with the 3-21G, 6-31G(d), 6-31+G(d), 6-31G(d,p), 6-311G(d,p), and 6-311G(df,p) basis sets. The scaling factors for the theoretical harmonic vibrational frequencies were determined by a comparison with the corresponding experimental fundamentals utilizing a total of 1066 individual vibrations. Scaling factors suitable for low-frequency vibrations were obtained from least-squares fits of inverse frequencies. ZPVE scaling factors were obtained from a comparison of the computed ZPVEs (derived from theoretically determined harmonic vibrational frequencies) with ZPVEs determined from experimental harmonic frequencies and anharmonicity corrections for a set of 39 molecules. Finally, scaling factors for theoretical frequencies that are applicable for the computation of thermal contributions to enthalpy and entropy have been derived. A complete set of recommended scale factors is presented. The most successful procedures overall are B3-PW91/6-31G(d), B3-LYP/6-31G(d), and HF/6-31G(d).
Here, controlled p-type doping of poly(2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV) deposited from solution using tetrafluorotetracyanoquinodimethane (F4-TCNQ) as a dopant is presented. By using a co-solvent, aggregation in solution can be prevented and doped films can be deposited. Upon doping the current–voltage characteristics of MEH-PPV-based hole-only devices are increased by several orders of magnitude and a clear Ohmic behavior is observed at low bias. Taking the density dependence of the hole mobility into account the free hole concentration due to doping can be derived. It is found that a molar doping ratio of 1 F4-TCNQ dopant per 600 repeat units of MEH-PPV leads to a free carrier density of 4×10^22m-3. Neglecting the density-dependent mobility would lead to an overestimation of the free hole density by an order of magnitude. The free hole densities are further confirmed by impedance measurements on Schottky diodes based on F4-TCNQ doped MEH-PPV and a silver electrode.
The enzyme herpes simplex virus type 1 thymidine kinase (HSV1 TK) salvages thymidine into the DNA metabolism of the virus. In the active site, the thymine ring of the nucleoside binds in a pocket, formed by two residues, Tyr-172 and Met-128, in a sandwich-type orientation. To investigate the nature of the thymine-enzyme pocket interactions, we have carried out density functional theory calculations with gradient-corrected exchange-correlation functionals of models of the thymine-HSV1 TK adduct. Our calculations indicate that the role of Met-128 in the substrate fixation is purely steric and hydrophobic, while the substrate-Tyr-172 interaction is essentially electrostatic in nature. These findings are completely consistent with the available catalytic properties of mutants on the 128 position.
An empirical method to account for van der Waals interactions in practical calculations with the density functional theory (termed DFT-D) is tested for a wide variety of molecular complexes. As in previous schemes, the dispersive energy is described by damped interatomic potentials of the form C6R(-6). The use of pure, gradient-corrected density functionals (BLYP and PBE), together with the resolution-of-the-identity (RI) approximation for the Coulomb operator, allows very efficient computations for large systems. Opposed to previous work, extended AO basis sets of polarized TZV or QZV quality are employed, which reduces the basis set superposition error to a negligible extend. By using a global scaling factor for the atomic C6 coefficients, the functional dependence of the results could be strongly reduced. The "double counting" of correlation effects for strongly bound complexes is found to be insignificant if steep damping functions are employed. The method is applied to a total of 29 complexes of atoms and small molecules (Ne, CH4, NH3, H2O, CH3F, N2, F2, formic acid, ethene, and ethine) with each other and with benzene, to benzene, naphthalene, pyrene, and coronene dimers, the naphthalene trimer, coronene. H2O and four H-bonded and stacked DNA base pairs (AT and GC). In almost all cases, very good agreement with reliable theoretical or experimental results for binding energies and intermolecular distances is obtained. For stacked aromatic systems and the important base pairs, the DFT-D-BLYP model seems to be even superior to standard MP2 treatments that systematically overbind. The good results obtained suggest the approach as a practical tool to describe the properties of many important van der Waals systems in chemistry. Furthermore, the DFT-D data may either be used to calibrate much simpler (e.g., force-field) potentials or the optimized structures can be used as input for more accurate ab initio calculations of the interaction energies.
Correlated ab initio quantum chemical methods based on second-order perturbation theory and density functional theory (DFT) together with large atomic orbital (AO) basis sets are used to calculate the structures of four cyclophanes with two aromatic rings and one sulphur-containing phane with one aromatic ring. The calculated geometrical data for [2.2]paracyclophane, cyclophane (superphane), 8,16-dimethyl[2.2]metacyclophane, 16-methyl[2.2]metaparacyclophane, and 2,6,15-trithia[3(4,10)][7]metacyclophane are compared to experimental data from X-ray crystal structure determinations. In all cases, very accurate theoretical predictions are obtained from the recently developed spin-component-scaled MP2 (SCS-MP2) method, in which the deviations are within the experimental accuracy and expected crystal-packing or vibrational effects. Especially the inter-ring distances, which are determined by a detailed balance between attractive van der Waals (dispersive) and repulsive (Pauli) contributions, are very sensitive to the level of theory employed. While standard MP2 theory in general overestimates the dispersive interactions (pi-pi correlations) between the two aromatic rings leading to too short distances (between 3 and 8 pm), the opposite is observed for DFT methods (errors up to 15 pm). This indicates that an explicit account of dispersive-type electron correlation effects between the clamped aromatic units is essential for a quantitative description of cyclophane structures. In order to distinguish these effects from "normal" van der Waals interactions, the term "overlap-dispersive" interaction may be employed. The popular B3 LYP hybrid density functional offers no advantage over the pure PBE functional that at least qualitatively accounts for some of the dispersive effects. The use of properly polarized AO basis sets of at least valence-triple-zeta quality is strongly recommended to obtain quantitative predictions with traditional wave function methods.
The intermolecular interaction energies of naphthalene dimers have been calculated by using an aromatic intermolecular interaction model (a model chemistry for the evaluation of intermolecular interactions between aromatic molecules). The CCSD(T) (coupled cluster calculations with single and double substitutions with noniterative triple excitations) interaction energy at the basis set limit has been estimated from the second-order Møller-Plesset perturbation interaction energy near saturation and the CCSD(T) correction term obtained using a medium-size basis set. The estimated interaction energies of the set of geometries explored in this work show that two structures emerge as being the lowest energy, and may effectively be considered as isoenergetic on the basis of the errors inherent in out extrapolation procedure. These structures are the slipped-parallel (Ci) structure (-5.73 kcal/mol) and the cross (D2d) structure (-5.28 kcal/mol). The T-shaped (C2v) and sandwich (D2h) dimers are substantially less stable (-4.34 and -3.78 kcal/mol, respectively). The dispersion interaction is found to be the major source of attraction in the naphthalene dimer. The electrostatic interaction is substantially smaller than the dispersion interaction. The large dispersion interaction is the cause of the large binding energies of the cross and slipped-parallel dimers.