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Potential well for the cisoid-transoid isomerization as a function of the dihedral angle. This picture clearly shows a barrier of 4.3 kcal/mol for cisoid-transoid isomerization, and a small difference in stability of 1.16 kcal/mol between both isomers. For the cisoid isomer we find a minimum in energy for a nonplanar conformation. This should also be the case for the transoid isomer, but this would require a further geometry optimization.
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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 unoccup...
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... f polymer is the oscillator strength of the infinite polymer. Figure 8 very clearly shows the dependence on 1/ N of the oscillator strength from our TDDFT calculations. In order to model the effects of rotational disorder and structural defects on the electronic properties of the oligothiophenes, we studied the ground- and excited-state properties of ␣ -2 T as a function of the dihedral angle between both monomer units. Chadwick and Kohler 15 found experimental evidence of the coexistence of cisoid and transoid bithiophenes in a supersonic expansion. The ratio is dependent on the temperature of the expansion, and the enthalpy difference between the two structures was found to be 1.16 Ϯ 0.13 kcal/mol. One should, of course, realize that the dihedral angles in the gas phase are 72° for the cisoid form and 64° for the transoid form, 15 while those molecular structures are flat in the solid phase. 39 Because interaction between the molecules in the crystal are only weak, we expect a rather shallow potential well for torsion. In Fig. 9 we give the potential well for the cis-trans isomerization with the lowest energy for a flat transoid bithiophene structure. For the most stable cisoid structure the dihedral angle is around 70°, which is very close to experiment, the transoid conformation is 0.72 kcal/mol more stable than the cisoid conformation, and the transition state barrier is 4.13 kcal/mol. In Fig. 10 we give the molecular orbital structure of bithiophene for torsion angles from 180° ͑ transoid ͒ to 0° ͑ cisoid ͒ ͑ although we do in fact a calculation of the molecule in vacuum, we take the structural parameters for the molecule in the solid ͒ . In a tight-binding model the HOMO is built from an antibonding combination of the original thiophene 1 a 2 orbitals and the HOMO-3 from the bonding combination, and their splitting is dependent on the transfer or hopping integral. At 90° the two-ring systems are perpendicular, and the two systems do not interact and the transfer integral t ϭ 0, the two orbitals cross, and the splitting is zero. The transfer integral is a function of the torsion angle ⌽ , t ( ⌽ ) ϭ T ϫ cos( ⌽ ). We can describe the molecular orbitals derived from the 3 b thiophene LUMO in a similar way. In a solid-state de- scription this means that, upon torsion, the transfer integral decreases, the valence and conduction bands narrow, and the optical gap increases. For the thiophene 2 b 1 - and 2 a 2 -derived molecular orbitals, almost no dispersion is observed, and this can be attributed to the small transfer integral due to the small electron densities at the ␣ carbon positions. At 90°, where binding and antibinding orbitals become degenerate, the HOMO and LUMO are now both twofold degenerate orbitals, each localized to one of the molecular entities. We have now a system of essentially noninteracting monomers, and we can analyze the optical spectrum in terms of pure intramolecular and intermolecular ͑ charge-transfer ͒ excitations. The intramolecular excited states are expected to be almost degenerate, and the gerade combination will have small oscillator strength because of the corresponding weak monomer transition. The two charge-transfer-excited states are also expected to be close in energy, but will have no oscillator strength because of the two mutual perpendicular systems. The energy splitting between the intramolecular and intermolecular excitations can in a tight-binding approach be interpreted as U ϩ ⌬ U Ϫ V Ϫ 2 K , in which U is the on-site interaction and V the next-neighbor Coulomb interaction, ⌬ U the reduced screening of U due to end effects, and K the exchange integral. Our TDDFT calculations are in excellent agreement with this simple model. At a dihedral angle of 90° we can identify two sets of two nearly degenerate transitions. At the low- energy end of the spectrum there are two almost degenerate intramolecular transitions at 4.65 eV, in which the gerade component has an oscillator strength of 0.295. About 1.0 eV higher in energy, we calculate two almost degenerate charge- transfer transitions at 5.63-eV energy, which as predicted do not carry any oscillator strength. From the splitting of the average intramolecular and intermolecular excitation energies we can estimate a value of 1.0 eV for U ϩ ⌬ U Ϫ V Ϫ 2 K . We have now extracted values for all the relevant parameters from the TDDTF calculations, which we will use as input parameters for a two-band model Hamiltonian calculation on the oligomers of size 1–6. We also can mimic the polymer limit by applying periodic boundary conditions to an oligomer of size 6. In these calculations the ground state and single excited states were included. The doubly excited states are not important for the low-energy features because the relative large HOMO-LUMO splitting. In these calculations the input parameters mentioned above were further optimized until a good fit for the singlet and triplet optical gaps was obtained ͑ Fig. 11 ͒ . In order to obtain this result one has to take into account the reduced screening of the on-site Coulomb interaction on the terminal thiophene rings. With a static polarizability of about 10 Å 3 for thiophene we esti- mated ⌬ at about 0.5 eV. The influence of the polarizability on the on-site coulomb interaction was already described in detail. 40 If this effect is not taken into account we find very low-energy excitonic states, with the electron on the terminal position and the hole next to it, this is due to the influence of the nearest-neighbor Coulomb interaction V , which lowers the energy of the electron hole pair if they are at the end of the chain. The final set of parameters is given in Table I. From this we note the large value found for U of 2.4 eV, which is about half the bandwidth and sufficient to strongly bind even the singlet states into Frenkel-like excitons. In Fig. 12 we plot the singlet optical spectrum of sexithiophene: here for sim- plicity we assume equal transition dipole moments for the on-site and next-neighbor transitions. From this figure it is clear that the magnitude of the Coulomb interaction is sufficient to form a bound excitonic state in which the electron and hole are mainly on the same monomer as in a Frenkel exciton. Of course, for such a finite chain there is no real distinction between an exciton and an electron-hole pair, since they are always highly confined. In a subsequent paper we will show that these parameters indeed lead to Frenkel- like excitons for both the singlet and triplets, with the triplets much more tightly bound than the singlets. 41 In this paper it has been shown that the progression of the electronic properties with size for oligomers of thiophene can be understood in terms of a simple tight-binding model describing a linear system of weakly coupled monomer units, in which the building blocks mainly retain their molecular identity. Because the HOMO and LUMO are very different in character, the first is aromatic with no sulfur character, and the latter is quinoid with significant density on the sulfur, one needs a two-band tight-binding approach in which the next-neighbor interaction is modeled with HOMO-HOMO, LUMO-LUMO, and intermolecular HOMO-LUMO transfer integrals. We have been able to extract a consistent set of tight-binding parameters from the results of the DFT calculations, which describe the experimental available data very well. It is surprising that a simple tight-binding Hamiltonian with only a monomer HOMO and a monomer LUMO gives such a good description of the details of the electronic structure of the oligomers of thiophene. Of course this may not be representative of other systems, especially polyacetylene. Further studies will explore the generality of this approach. From TDDFT calculations on the lowest singlet and triplet excited states we could estimate an effective exchange integral of about 0.9 eV. The nature of the lowest singlet state can be analyzed in valence bond terms in intramolecular and intermolecular contributions. We showed that almost all the oscillator strength originates from intermolecular contributions, while the intramolecular contribution is weak. This finds its foundation in the fact that the electronic structure of the thiophene molecule is very similar to that of the isoelectronic cyclopentadienyl anion. This also explains the 1/ N dependence of the magnitude of the oscillator strength for the larger oligomers. We also found that rotational disorder is important in these systems, and that there is a very shallow potential well for torsion. These fluctuations will introduce an effective conjugation length, and most probably will be important in the localization of polarons and excitons, and may be an important source of traps in these materials. For the dimer we showed that, if we take a torsion angle of 90°, the systems will be perpendicular and the hopping integral will vanish. We are now left with two sets of excited states: one set of almost degenerate on-site excitations and one set of almost degenerate pure charge-transfer excitations. The splitting between both sets of excitations amounts to an effective on-site Coulomb interaction minus the next- neighbor Coulomb interaction U ϩ ⌬ U Ϫ V Ϫ 2 K , and that splitting is about 1 eV. From TDDFT calculations, numerical values could be extracted for the relevant physical quantities, which are the input parameters for the model Hamiltonian defined in this paper. After a full optimization we were able to realize an almost perfect fit for the singlet optical gap, and we predict the positions of triplet states for longer oligomers, for which they have not yet been observed to our knowledge. We show that electron correlation plays an important role in these systems, and that most of the optical spectral weight is carried by a singlet excitoniclike state, with an electron and hole concentrated on the same monomer or near- neighbor ...
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Aiming at designing more efficient multiphotochromes, we investigate with the help of ab initio tools the impact of the substitution on a series of dimers constituted of two dithienylethenes (DTE), strongly coupled to each other through an ethynyl linker. The electronic structure and the optical properties of a large panel of compounds, substituted...
Citations
... The bandgap is one of the most important parameters determining possible applications of such conducting polymers [35][36][37][38][39]. A common strategy to predict the energy gap of infinite-chain conjugated polymers is to extrapolate the value of the HOMO-LUMO energy gap for increasing numbers of monomers in the oligomer chain [40][41][42][43]. The size of bandgaps often shows an approximately linear dependence on 1/n, allowing extrapolation to n → ∞ [44]. ...
Electronic and vibrational structures of pyrrole oligomer and its derivatives were established on the basis of Density Functional Theory (DFT) and Time-dependent DFT (TD-DFT) computations. The influence of substituent groups on optical and electronic properties was investigated for polypyrrole and polypyrrole derivatives. Molecular structure and frontier molecular orbitals of neutral and ionized oligomers were analyzed. The energy gap, ionization potential, electron affinity, electronegativity, and hardness were calculated and discussed.
... The highest occupied molecular orbital (HOMO) of 6T is the fully antibonding superposition of the six HOMOs of the individual T rings. The corresponding fully bonding superposition is not the HOMO-5, as one might expect, but the HOMO-11, as the HOMO-1 of the T units form an intermediate dispersionless band [96] (see Section S1 in the Supplementary Material for detailed information about the frontier energy levels). The splitting between the KS energies of the HOMO and the HOMO-11 (E HOMO − E HOMO-11 ), which can be seen as the finite equivalent of the valence bandwidth, gives an estimate of the electronic coupling between the rings [97]. ...
... The relation between torsion angle and low-lying virtual orbitals is essentially reversed with respect to the scenario delineated above for the occupied states. Decreasing the coupling between the rings by increasing the torsion raises the energy of the lowest unoccupied moleular orbital (LUMO), as it corresponds to the fully bonding superposition of the LUMOs of the T units [96]. This increase adds up to the influence of the alkyl chains, which increase the energy of all levels (occupied and virtual), resulting in a significantly expanded electronic gap Figure 3d)]. ...
... The absorption spectrum of 6T is dominated by a strong peak around 3.0 eV, stemming from the HOMO-LUMO transition [96,117] (details about the orbital transitions of the first five excited states are listed in Section S3 of the Supplementary Material). As seen in Figure 5a), the excitation energy as a function of the backbone conformation and of the alkyl chain length follows the same trend as the electronic gap [ Figure 3c)]. ...
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.
... This striking effect of the decoupling of the HOPS from E f can be seen in the UPS spectra of SAMs of TnC4 ( Figure S11) and oligothiophenes in the gas-phase. 37 The line-shapes of the SAM and gas-phase spectra are nearly identical, meaning there is no hybridization between the HOPS and metal states. Also, just as the gas-phase peaks shift with decreasing E g , the DOS in the TnC4 spectra shifts toward E f with increasing n, which should be affected further by the application of a topcontact. ...
Molecular tunneling junctions should enable the tailoring of charge-transport at the quantum level through synthetic chemistry, but are hindered by the dominance of the electrodes. We show that the frontier orbitals of molecules can be decoupled from the electrodes, preserving their relative energies in self-assembled monolayers even when a top-contact is applied. This decoupling leads to the remarkable observation of tunneling probabilities that increase with distance in a series of oligothiophenes, which we explain using a two-barrier tunneling model. This model is generalizable to any conjugated oligomers for which the frontier orbital gap can be determined and predicts that the molecular orbitals that dominate tunneling charge-transport can be positioned via molecular design rather than being dominated by Fermi-level pinning arising from strong hybridization. The ability to preserve the electronic structure of molecules in tunneling junctions facilitates the application of well-established synthetic design rules to tailor the properties of molecular-electronic devices.
... In all calculation we will adopt the DFT/B3LYP/3-21G* method to study all compounds, this choice being explained also by time consuming reasons. For predicting various types of band gaps of infinite-chain conjugated polymers, a common strategy relies on extrapolation from a series of oligomers with increasing numbers of monomers [48][49][50][51] . ...
A novel theoretical tool for some new low-band-gap copolymers has been developed on the basis of density functional theory (DFT) quantum chemical calculations to model their photophysical properties. These copolymers are constituted of thiophene and benzothiadiazole (PTBT) units essentially as well as their derivatives leading to donor (D)-acceptor (A) structure-types. Firstly, we study the insertion of thiophene as spacer unit into PTBT backbone to reach polydithiophene-alt-benzothiadiazole (PDTBT) copolymer, secondly, bithiophene usual unit was replaced by cyclopentadithiophene entity to obtain poly (cyclopentadithiophene-alt-benzothiadiazole) (PCPDTBT) copolymer and finally, ethynyl and vinylene spacers were added to obtain a novel oligomer models, denoted poly(cyclopentadithiophene-ethynyl-benzothiadiazole) (PCPDTEBT) and poly(cyclopentadithiophene-vinylene-benzothiadiazole) (PCPDTVBT), respectively, and to investigate their effect into the main backbone on various properties by examining structural and electronic properties.PCPDTEBT copolymer presents the optimal properties to be used as an active layer in organic solar cell (OSC). It shows unique combination of properties: a moderate gap (1.55 eV), high extinction coefficients (ϵ = 3.11 × 105 mol−1 L cm−1), a low oxidation potential associated with high thermal stability and environmental sustainability. Thus, this copolymer is then blended with [6,6]-phenyl-C61- butyric acid methyl ester (PCBM) in bulk-heterojunction solar cell. A model band diagram was established for the proposed solar structure, simulating the energy behavior of such active layer.
... For the LNT growth, E I ≈ 5.8 eV results whether measured at LNT or after warming to RT. This value matches reasonably well the value of 5.9 eV derived from E I of the gas phase being of 7.0 eV [61] and corrected for extra-molecular screening of 1.1 eV, which is common in organic solids [53] and suggests a 'frozen' gas phase. The ionization energy of the LNT-grown film is nearly independent on the film thickness, while that of the RT-grown film dramatically drops with the film thickness by I n t e n s i t y ( a r b . ...
Even though the term `organic electronics' evokes rather organic devices, a significant part of its scope deals with physical properties of `active elements' such as organic films and interfaces. Examination of the film growth and the evolution of the interface formation are particularly needful for the understanding a mechanism controlling their final properties. Performing such
experiments in an ultra-high vacuum allows both to `stretch' the time scale for pseudo real-time observations and to control properties of the probed systems on the atomic level. Photoemission technique probes directly electronic and chemical structure and it has thereby established among major tools employed in the field. This review primarily focuses to electronic properties of oligomeric molecular films and their interfaces examined by photoemission. Yet, it does not aspire after a complete overview on the topic; it rather aims to otherwise standard issues encountered at the photoemission characterization and analysis of the organic materials, though requiring to consider particularities of molecular films in terms of the growth, electronic properties, and their characterization and analysis. In particular, the fundamental electronic parameters of molecular films such as the work function, the ionization energy,
and the interfacial energy level alignment, and their interplay, will be pursued with considering often neglected influence of the molecular orientation. Further, the implication on the band bending in molecular films based on photoemission characterization, and a model on the driving mechanism for the interfacial energy level alignment will be addressed.
... Recientemente, las líneas de investigación en cuanto a la fabricación de dispositivos han tomado otras vías como la autoorganización [53,54] o el interés por medir las propiedades de un solo átomo o de una sola molécula de cara a la posible fabricación de dispositivos a partir de los componentes más básicos: la carga de un átomo individual [55], el spin de un sólo átomo [56], la vibración de una molécula [57]. Una línea de investigación claramente en auge son las moléculas orgánicas: Fullerenos (buckyballs y nanotubos), grafeno, perilenos, ptialocianinas, oligotiofenos, etc. por su interés en la fabricación de dispositivos electrónicos y optoelectrónicos [58][59][60][61][62][63][64][65][66][67]. ...
... A pesar de esta rigidez, pueden existir isómeros conformacionales creados por la rotación alrededor del enlace carbono-carbono de grupos tienil adyacentes [66]. Por lo tanto, al tener cinco enlaces entre carbonos, es posible que las moléculas de T6 disipen energía por medio de los grados de libertad de sus rotaciones internas. ...
... Studies have shown 59,62−64 that the values of vertical IP/EA, calculated in a self-consistent way by removing/ adding an electron from/to the molecule, compare well with the measurements of photoelectron spectroscopy but that such an agreement progressively deteriorates with increasing 65 In the particular case of oligothiophenes, the self-interaction errors can become very visible at six rings (6T). 62 On the other hand, the IP and EA values derived from Koopmans' theorem 66 (that is, the negative of the HOMO (highest occupied molecular orbital) energy and the LUMO (lowest unoccupied MO) energy) have been shown to describe a wide variety of molecules in the solid state extremely well (at the B3LYP level); 60 in other words, the −HOMO and LUMO energies can be directly compared in a practical way with the solid-state UPS and IPES values, respectively. This proves particularly useful for F 4 -TCNQ (Koopmans' vs solidstate = −5.54 ...
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.
... Only the absolute energies change in accordance with the difference in the workfunctions of the different substrate systems. Density functional theory calculations 19 suggest a dispersion that is linear for higher wavenumbers but becomes parabolic close to k = 0 (see Supplementary Information). This agrees with the experimental results at higher k, but cannot account for the observed linear dispersion down to small values of k. ...
... Length (thiophene units) 19 10 lowest state should develop continuously from a LUMO character to a LUMO+1 character. The lowest state should be mostly LUMO in nature, but should exhibit a slight deviation from LUMO for very long chains because of the admixture of other states. ...
In molecular electronics individual molecules serve as electronic devices. In these systems, electron–vibron (e–ν) coupling can be expected to lead to new physical phenomena and potential device functions1, 2, 3. In previous studies of molecular wires, the e–ν coupling occurred as a result of the well-known Franck–Condon principle, for which the Born–Oppenheimer approximation holds. This means that after a vibronic excitation, the electrons and the vibrations evolve independently from each other. Here we show that this simple picture changes markedly when two electronic levels in a molecule are coupled by a molecular vibration4, 5. In molecular wires we observe a non-Born–Oppenheimer regime, for which a coherent coupling of electronic and nuclear motion emerges6. This phenomenon should occur in all systems with strong electron–vibration coupling and an electronic level spacing of the order of vibrational energies. The coherent coupling of electronic and nuclear motion could be used to implement mechanical control of electron transport in molecular electronics
... To shed some light on the field-induced fragmentation and dissociation of polythiophene density functional calculations were done for short chains of polythiophene with up to 8 units in electrostatic fields of the order of volts per angstrom [6]. This approach has proven successful in other studies on the intermolecular interactions in bithiophene [13] and for calculating excitation energies of terthiophene and its dioxide derivatives [14,15]. For fields below 1.6 V/Å the structure of neutral polythiophene does not change appreciably except that there is an intramolecular transfer of electrons down the electric field. ...
After reviewing the new physics and chemistry in high electrostatic fields we use density functional theory to show that in fields around 1.5 V/Å the bandgap of polythiophene reduces to zero leading to field-induced metallization. In poly(ethylene glycol), on the other hand, such fields lead to giant electrostriction of over 20% in length. Lastly, we give two examples of field-induced polymerization: (1) the closure of sulfur molecules Sn at n = 8 is suppressed remaining linear up to n ~ 20. (2) This also happens to water which forms linear whiskers up to n ~ 11.
... This crystal structure, slightly contracted in the in-plane lattice parameters and with the molecules standing homeotropic on the substrate, is also observed in monolayer islands at sub-monolayer coverage of silicon oxide up to 6 ML (the so-called thin film phase) [27] . A population of conformers , generated by the rotation of adjacent thienyl groups around the C–C bond, could coexist [28]. As there are five bonds connecting the six thienyl units, it would be possible, in principle, for T6 molecules to dissipate different amounts of energy through its internal rotational degrees of freedom. ...
We perform a combined experimental and theoretical approach to establish the atomistic origin of energy dissipation occurring while imaging a molecular surface with an amplitude modulation atomic force microscope. We show that the energy transferred by a single nano-asperity to a sexithiophene monolayer is about 0.15 eV/cycle. The configuration space sampled by the tip depends on whether it approaches or withdraws from the surface. The asymmetry arises because of the presence of energy barriers among different deformations of the molecular geometry. This is the source of the material contrast provided by the phase-shift images.
