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Synthesis and Photophysics of a Copper-Porphyrin−Styrene−C60 Hybrid†
Abstract
A new conjugated porphyrin−fullerene dyad featuring a styrene linkage has been prepared in 11% overall yield from previously synthesized starting materials. In contrast to the singlet and triplet excited states commonly observed in analogous systems with zinc and free base porphyrins, the present investigation involves doublet and quartet ground and excited states. The photophysical pathways in the present system include intramolecular electron and energy transfer events, whose outcome depends predominantly on the solvent polarity.
... As with many conjugated polymer systems, photoinduced electron transfer from porphyrins to fullerenes is also well studied area with a great deal of literature focused on structures 22 which have exhibited stabilized charge separated species and long-lived radical ion pairs [72]. In addition, it has been demonstrated that there exists a high binding affinity between the tetraphenyl porphyrin macrocycle and Ceo structures allowing for the observation of efficient fluorescence quenching by fullerene moieties [73] and dyad linked structures [74]. Structures utilizing porphyrin/C6o nanoclusters [75] and self-assembled monolayers [57] organized on ITO arrays have been reported to give high IPCE values up to 17% for extremely thin layered devices (~ 5 nm). ...
Conductive nanostructured films of poly-tetrakis-5,10,15,20-(4-aminophenyl)porphyrin (TAPP) can be grown electrochemically or through interfacial oxidative polymerization. The poly-TAPP nanomorphology is sensitive to the electrochemical solvent, potentiometric method, and the aminophenyl porphyrin monomer utilized. To elucidate the molecular structure of poly-TAPP and to correlate structures with proposed polymerization and conductivity mechanisms, reflectance FT-IR and spectroelectrochemistry were used to detect the presence and electroactivity of dihydrophenazine and phenazine polymer linkages formed during the polymerization. Poly-TAPP nanofiber films were evaluated for use in a bulk heterojunction solar cell (with PCBM) and in an inverse dye-sensitized TiO2 solar cell using the poly-TAPP nanoporous scaffold to control the interfacial contact region between donor (poly-TAPP) and acceptor (PCBM or TiO2) phases. Poly-TAPP/PCBM cells exhibited short-circuit current densities of 140 muA/cm 2 and open-circuit potential values up to 500 mV under simulated full-sun illumination. An inverse dye-sensitized solar cell was developed by incorporating TiO2 into a dye-coated nanoporous poly-TAPP electrode. These cells demonstrated short-circuit current densities up to 46 muA/cm 2 and open-circuit potential values of 232 mV under AM 1.5 solar illumination.
... As with many conjugated polymer systems, photoinduced electron transfer from porphyrins to fullerenes is also well studied area with a great deal of literature focused on structures 22 which have exhibited stabilized charge separated species and long-lived radical ion pairs [72]. In addition, it has been demonstrated that there exists a high binding affinity between the tetraphenyl porphyrin macrocycle and Ceo structures allowing for the observation of efficient fluorescence quenching by fullerene moieties [73] and dyad linked structures [74]. Structures utilizing porphyrin/C6o nanoclusters [75] and self-assembled monolayers [57] organized on ITO arrays have been reported to give high IPCE values up to 17% for extremely thin layered devices (~ 5 nm). ...
Metalloporphyrins are prominent building blocks in the synthetic toolbox of advanced photodriven molecular devices. When the central ion is paramagnetic, the relaxation pathways within the manifold of excited states are highly intricate so that unravelling the intramolecular energy and electron transfer processes is usually a very complex task. This fact is critically hampering the development of applications based on the enhanced coupling offered by the electronic exchange interaction. In this work, the dynamics of charge separation in a copper porphyrin-fullerene are studied with several complementary spectroscopic tools across the electromagnetic spectrum (from near-infrared to X-ray wavelengths), each of them providing specific diagnostics. Correlating the various rates clearly demonstrates that the lifetime of the photoinduced charge-separated state exceeds by about 10-fold that of the isolated photoexcited CuII porphyrin. As revealed by the spectral modifications in the XANES region, this stabilization is accompanied by a transient change in covalency around the CuII center, which is induced by an enhanced interaction with the C60 moiety. This experimental finding is further confirmed by state-of-the art calculations using DFT and TD-DFT including dispersion effects that explain the electrostatic and structural origins of this interaction, as the CuIIP cation becomes ruffled and approaches closer to the fullerene in the charge-separated state. From a methodological point of view, these results exemplify the potential of multielectron excitation features in transient X-ray spectra as future diagnostics of subfemtosecond electronic dynamics. From a practical point of view, this work is paving the way for elucidating out-of-equilibrium electron transfer events coupled to magnetic interaction processes on their intrinsic time-scales.
Photosemiconductor thin films based on two organic porphine derivatives have been investigated. These compounds have different pendent groups; the film morphology, along with the specific fabrication technique, is determined to a great extent by these groups. The films have been fabricated by vacuum sputtering and using the Langmuir−Schaefer method. According to the atomic force microscopy (AFM) data, the Langmuir−Schaefer films are more homogeneous than the sputtered ones. It is shown that the sputtered films based on substituted porphine have a looser stacking than the initial analog. A spectroscopy study revealed a bathochromic shift of the Soret band in the Langmuir−Schaefer films–sputtered films series. This shift is explained by the increase in the concentration and size of molecular aggregates in sputtered films. It is shown that a polycrystalline C60 fullerene film deposited onto an amorphous substituted porphine layer improves the photoelectric characteristics of the latter. Both the time stability of the photodiode structure and its ampere‒watt sensitivity increase (by a factor of 10 in the transition regime). The steady-state current does not change. The effect of polarity reversal of the photovoltaic signal is observed in a planar С60‒substituted metalloporphine heterostructure, which is similar to the pyroelectric effect. The polarity reversal can be explained by the contribution of the trap charge and discharge current at the interface between the amorphous photosemiconductor and crystalline photosemiconductor to the resulting photoelectric current.
A series of zinc porphyrins substituted at adjacent β-positions with a CN group and para-substituted ethenyl/ethynyl-phenyl group have been studied using electronic absorption spectroscopy, resonance Raman spectroscopy and DFT calculations. The oxidative nucleophilic substitution of hydrogen was utilized for the introduction of a cyano substituent on the porphyrin ring. This modification has a remarkable electronic effect on the ring. The resulting porphyrin cyanoaldehyde was further modified in Wittig condensations to give series of arylalkene- and arylalkyne-substituted derivatives. This substitution pattern caused significant redshifting and broadening of the B band, tuning from 433-446 nm. Additionally the Q/B band intensity ratios show much higher values than observed for the parent porphyrin ZnTPP (0.20 vs. 0.03). Careful analysis of the electronic transitions using DFT and resonance Raman spectroscopy reveal that the substituent does not significantly perturb the electronic structure of the porphyrin core, which is still well described by Gouterman's four-orbital model. However, the substituents do play a role in elongating the conjugation length and this results in the observed spectral changes.
A study on intramolecular photo-cyclization reactions with diazo-functionalized olefin-esters is presented. On irradiation of diphenyl diazo methane esters of cinnamic acids, not only the expected addition to the CC double bond is observed, yielding cyclopropanes, but also the addition to the carboxylic CO double bond is postulated as an intermediate. These formed intermediates undergo a rearrangement to cyclobutanones which further rearrange photo-assisted to oxetanes.Both were isolated and characterized. The reaction progress, the impact of the irradiation wavelength and solvent as well as the influence of the electron density of the olefin on the product distribution is described. Also, the detailed reaction mechanism for the photo-reaction cascade is discussed.
Recent examples of C60 dumbbells covalently linked by different electron-donor fragments (porphyrins, π-conjugated oligomers, TTFs, etc.) are summarized and their main properties highlighted. Although, similarly to the related C60–donor dyads, no significant electronic interaction has been observed between the three electroactive components in the ground state, namely two C60 units and the electron-donor fragment, the presence of a second C60 cage provides enhanced photophysical properties to the final triads in comparison to the related dyads. These findings suggest the presence of a cooperative effect. Some of these C60 dumbbell triads have been applied in the construction of photovoltaic devices and the results obtained indicate that they can play a very active role in the emerging field of optoelectronic devices.
This review complements previous reports and is a collection of the highlights revealed by database searches for the year 2003. During the year significant advances have been made towards carbon nanotube functionalisation and solubilisation, though characterization of the reaction products remains a major challenge.
One mono‐adduct and three novel multi‐adduct analogs of C60‐diphenylaminofluorene C60(>DPAF)n derivatives were synthesized and characterized by various spectroscopic methods. Optical absorption of these samples indicated a systematic increase in relative intensity of the fluorene band centered at 415 nm as the number of DPAF addend increases. Fluorescence of DPAF chromophore in all C60(>DPAF)n derivatives 4 (n = 1), 5 (n = 2), 6 (n = 3), and 7 (n = 4) in o‐dichlorobenzene and chloroform was found to be efficiently quenched as the direct covalent bond attachment of DPAF moieties to the fullerene cage facilitates efficient intramolecular electron or energy transfer processes.
The synthesis of a Zn(II) porphyrin-fullerene dyad in which the two chromophore units are tethered by a conformationally-flexible linker, is described. The synthesis is highlighted by the use of a cross metathesis strategy to prepare the linker between the chromophores. Photoexcitation of the Zn(II) porphyrin unit of the dyad in tetrahydrofuran leads to substantial (77%) quenching of porphyrin fluorescence. The multiple exponentials fluorescence decay kinetics observed are attributed to different rates of electron transfer from photoexcited porphyrin to fullerene in the various conformers present. A charge-separated state with a 330 ns lifetime is observed by transient spectroscopy.
This Perspective article traces the career of the author in the area of mechanistic organic photochemistry from the primitive state of this field in the late 1950s and early 1960s until its maturity into a highly sophisticated discipline at the present time. It focuses on studies carried out by the author and his associates to delineate mechanisms of basic types of photochemical reactions of small organic molecules to photophysical studies in recent years involving electron transfer processes occurring in nanoscale artificial photosynthetic systems in ultrashort time domains. The important role that serendipitous events played in directing key career decisions and events is emphasized. The important ways that developments in instrumentation influenced the choices, possibilities and accomplishments of doing research in photochemistry between 1960 and the present time are emphasized. Acknowledgment is given by the author to the many people who contributed directly and indirectly to the course that his career has taken over more than half a century.
Photophysical investigations on a series of (2,4,6)-tris-substituted metalloporphyrin-fullerene conjugates revealed the effects of an electron-rich microenvironment surrounding the electron-donating porphyrin as a function of the metal center. On one hand, for all conjugates-water-soluble and non-water-soluble-ultrafast charge separation was observed upon photoexcitation. On the other hand, when examining the charge recombination dynamics for the non-water-soluble conjugates it becomes obvious that the (2,4,6)-tris-substitution stabilizes the radical-ion-pair state relative to the mono-substitution in the ortho-, meta-, and para-position. The more efficient protection of the electron-donating porphyrin from solvation is thought to be the major cause for this impact. Nevertheless, the situation is slightly different for the water-soluble conjugates. At first glance, the radical-ion-pair state lifetimes are, also in the case of the (2,4,6)-tris-substitution, longer than for the mono-substituted ortho-, meta- and para-conjugates. Upon closer inspection, they fail, however, to exhibit any metal dependence. Competing with the protection from solvation of the dendrons, dipole-charge interactions impact the stabilization in the polar aqueous environment and, in turn, become the dominant force governing the electron-transfer dynamics.
A series of π-extended TTFs (exTTFs) bearing one (8, 16,19) or two (14) dibenzylammonium units have been threaded through a dibenzo-24-crown-8 (DB24C8) ring covalently linked to a C60 sphere. Whereas 8 and 14 were prepared in a multistep synthetic procedure involving Sonogashira cross-coupling reactions affording rigid donors, exTTFs 16 and 19 were prepared by direct esterification reactions leading to more flexible systems. Complexation experiments carried out by 1H NMR titration and fluorescence studies show the formation of stable supramolecular dyads with binding constants ranging from 103 to 104M–1. Although the UV/Vis and CV studies reveal the lack of interaction between the electroactive species (C60 and exTTF) in the ground state, fluorescence data indicate the presence of an electronic interaction in the excited state. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)
The synthesis and photophysical studies of a butadiyne-linked porphyrin–C60 dyad (ZnP–C60) 6 are described. This is the first porphyrin–[60]fullerene dyad in which the two chromophores are conjugatively linked through a ‘molecular wire’. The UV–visible absorption spectrum for dyad 6 is slightly red shifted relative to the porphyrin precursor 5 whose fluorescence is all but quenched by the attached C60. Copyright © 2004 John Wiley & Sons, Ltd.
Two new fullerene[C(60)]-fluorene dyads, N-methyl-2-(9-fluorenyl)-3,4-fulleropyrrolydine and N-methyl-2-(2-fluorenyl)-3,4-fulleropyrrolydine, were synthesized and investigated. Cyclic voltammetry, fluorescence experiments, electronic spectra and Density Functional Theory (DFT) calculations, all indicate ground and excited state interactions when fluorene is linked at carbon 9, whereas linkage through position 2 does not affect the ground and excited state of the fulleropyrrolidine moiety. As a consequence, only changing the position of the fluorene chromophore linkage, opens the way to modulate interactions, choosing between a simple spacer and an active unit.
The supramolecular assembly containing an electron donor, zinc 5,10,15,20-meso-tetraferrocenylporphyrin, and a pyridine-substituted fulleropyrrolidine as an electron acceptor, has been investigated. A comparison of the photophysical properties of the separate components and of the assembled system revealed a photoinduced electron-transfer process from the porphyrin to the fullerene unit. Time-resolved spectroscopy, on the subpicosecond time scale, was performed to determine the rate of the forward and back electron-transfer reactions.
The physicochemical characterization, that is, ground and excited state, of a new series of dendronized porphyrin/fullerene electron donor-acceptor conjugates in nonaqueous and aqueous environments is reported. In contrast to previous work, we detail the charge-separation and charge-recombination dynamics in zinc and copper metalloporphyrins as a function of first- and second-generation dendrons as well as a function of ortho, meta, and para substitution. Both have an appreciable impact on the microenvironments of the redox-active constituents, namely the porphyrins and the fullerenes. As a matter of fact, the resulting charge-transfer dynamics were considerably impacted by the interplay between the associated forces that reach from dendron-induced shielding to dipole-charge interactions.
A series of truly water-soluble C(60)/porphyrin electron donor-acceptor conjugates has been synthesized to serve as powerful mimics of photosynthetic reaction centers. To this end, the overall water-solubility of the conjugates was achieved by adding hydrophilic dendrimers of different generations to the porphyrin moiety. An important variable is the metal center of the porphyrin; we examined zinc(II), copper(II), cobalt(II), nickel(II), iron(III), and manganese(III). The first insights into electronic communication between the electron donors and the electron acceptors came from electrochemical assays, which clearly indicate that the redox processes centered either on C(60) or the porphyrins are mutually affected. Absorption measurements, however, revealed that the electronic communication in terms of, for example, charge-transfer features, remains spectroscopically invisible. The polar environment that water provides is likely to be a cause of the lack of detection. Despite this, transient absorption measurements confirm that intramolecular charge separation processes in the excited state lead to rapid deactivation of the excited states and, in turn, afford the formation of radical ion pair states in all of the investigated cases. Most importantly, the lifetimes of the radical ion pairs were found to depend strongly on several aspects. The nature of the coordinated metal center and the type of dendrimer have a profound impact on the lifetime. It has been revealed that the nature/electronic configuration of the metal centers is decisive in powering a charge recombination that either reinstates the ground state or any given multiplet excited state. Conversely, the equilibrium of two opposing forces in the dendrimers, that is, the interactions between their hydrophilic regions and the solvent and the electronic communication between their hydrophobic regions and the porphyrin and/or fullerene, is the key to tuning the lifetimes.
This review summarizes recent advances in the use of porphyrins, phthalocyanines, and related compounds as components of solar cells, including organic molecular solar cells, polymer cells, and dye-sensitized solar cells. The recent report of a porphyrin dye that achieves 11% power conversion efficiency in a dye-sensitized solar cell indicates that these classes of compounds can be as efficient as the more commonly used ruthenium bipyridyl derivatives.
Coordinating different transition metals--manganese(III), iron(III), nickel(II), and copper(II)--by a dendronized porphyrin afforded a new family of redox-active metalloporphyrins to which C(60) was attached as a ground-state electron acceptor. Such a strategy introduced an additional center of redoxactivity, that is, a change of the oxidation state of the metal. Cyclic voltammetry and absorption/fluorescence measurements provided support for mutual interactions between the redox-active constituents in the ground state. In particular, slightly anodic shifted reduction potentials/cathodic shifted oxidation potentials and the occurrence of new charge transfer features in the 700-900 nm range prompt to sizable electronic coupling in the range of 300 cm(-1). Photophysical means--steady-state/time-resolved fluorescence and transient absorption measurements--shed light on the excited-state interactions. To this end, we have added pulse radiolytic investigations to characterize the radical cation (i.e., metalloporphyrins) and radical anion (i.e., fullerene) characteristics. Pi-pi stacking of the excited state electron donor and the electron acceptor is key to overcome the intrinsically fast deactivation of the excited states in these metalloporphyrins and to power an exothermic charge transfer. The lifetimes of the rapidly and efficiently generated radical ion pair states, which range from 15 to >3000 ps, revealed several important trends. First, they were found to depend on the solvent polarity. Second, the nature of the transition metal plays a similarly decisive role. It is important that the product of charge recombination, namely tripmultiplet excited states versus ground state, had a great impact. Finally, a correlation between the charge transfer rate (i.e., charge separation and charge recombination) and the free energy change for the underlying reaction reveals a parabolic dependence with parameters of the reorganization energy (0.84 eV) and electronic coupling (70 cm(-1)) closely resembling that seen for the zinc(II) and free base analogues.
Energy transfer in a series of hetero-dyads of zinc porphyrin and free-base porphyrin connected at the beta position by pi conjugated bridges has been determined. The dyads have been characterized and compared with the homo dyads, excellent models for the donor and the acceptor porphyrins in the electronically conjugated system. The homo dyads provide reliable parameters for the determination of the energy transfer rate calculated according to the Förster theory. This model was inadequate to account for the experimental findings and an electron exchange mechanism was shown to contribute. A favorable coplanar arrangement of the bridge and the tetrapyrroles facilitates the energy transfer process, which displays a very low distance dependence and an efficiency >98%.
The subject of this paper is a new fullerene building block design with the potential for defined geometry and good electronic communication. The synthesis and characterization of a new pyridinofullerene ligand capable of forming axially symmetric complexes with metalloporphyrins is reported. X-ray structural and molecular modeling studies, (1)H NMR, UV-vis spectroscopy, electrochemistry studies, and fluorescence quenching data support the formation of a strong complex between the new ligand and the metal center of ZnTPP. On the basis of computational studies, the highest occupied molecular orbital (HOMO) of this ligand is significantly different from a model compound with insulating carbons between the pyridine and the fullerene. The N-pyridinium fulleropyrrolidine salts of the new ligand and model compound were also prepared and their spectral and electrochemical properties are reported.
The synthesis and electrochemical and photophysical studies of a series of alkyne-linked zinc-porphyrin-[60]fullerene dyads are described. These dyads represent a new class of fully conjugated donor-acceptor systems. An alkynyl-fullerene synthon was synthesized by a nucleophilic addition reaction, and was then oxidatively coupled with a series of alkynyl tetra-aryl zinc-porphyrins with 1-3 alkyne units. Cyclic and differential pulse voltammetry studies confirmed that the porphyrin and fullerene are electronically coupled and that the degree of electronic interaction decreases with increasing length of the alkyne bridge. In toluene, energy transfer from the excited zinc-porphyrin singlet to the fullerene moiety occurs, affording fullerene triplet quantum yields of greater than 90 %. These dyads exhibit very rapid photoinduced electron transfer in tetrahydrofuran (THF) and benzonitrile (PhCN), which is consistent with normal Marcus behavior. Slower rates for charge recombination in THF versus PhCN clearly indicate that charge-recombination events are occurring in the Marcus inverted region. Exceptionally small attenuation factors (beta) of 0.06+/-0.005 A(-1) demonstrate that the triple bond is an effective mediator of electronic interaction in zinc-porphyrin-alkyne-fullerene molecular wires.
This critical review covers the timely topic of carbon nanostructures-fullerenes and carbon nanotubes-in combination with metalloporphyrins as integrative components for electron-donor-acceptor ensembles. These ensembles are typically probed in condensed media and at semi-transparent electrode surfaces. In particular, we will present a comprehensive survey of a variety of covalent (i.e., nanoconjugates) and non-covalent linkages (i.e., nanohybrids) to demonstrate how to govern/fine-tune the electronic interactions in the resulting electron-donor-acceptor ensembles. In the context of covalent bridges, different spacers will be discussed, which range from pure "insulators" (i.e., amide bonds, etc.) to sophisticated "molecular wires" (i.e., p-phenylenevinylene units, etc.). Furthermore, we will elucidate the fundamental impact that these vastly different spacers may exert on the rate, efficiency, and mechanism of short- and long-range electron transfer reactions. Additionally, a series of non-covalent motifs will be described: hydrogen bonding, complementary electrostatics, pi-pi stacking and metal coordination-to name a few. These motifs have been successfully employed by us and our collaborators en route towards novel architectures (i.e., linear structures, tubular structures, rotaxanes, catenanes, etc.) that exhibit unique and remarkable charge transfer features.
We investigate the photoinduced intramolecular electron-transfer (IET) behavior of a perylenebisimide dimer in a variety of solvents using femtosecond transient absorption spectroscopy. Overlapping photoinduced absorptions and stimulated emission give rise to complicated traces, but they are well fit with a simple kinetic model. IET rates were found to depend heavily on solvent dielectric constant. Good quantitative agreement with rates derived from fluorescence quantum yield and time-resolved fluorescence measurements was found for forward electron transfer and charge recombination rates.
Three porphyrin-fullerene dyads, in which a diyne bridge links C(60) with a beta-position on a tetraarylporphyrin, have been synthesized. The free-base dyad was prepared, as well as the corresponding Zn(II) and Ni(II) materials. These represent the first examples of a new class of conjugatively linked electron donor-acceptor systems in which pi-conjugation extends from the porphyrin ring system directly to the fullerene surface. The processes that occur following photoexcitation of these dyads were examined using fluorescence and transient absorption techniques on the femtosecond, picosecond, and nanosecond time scales. In sharp contrast to the photodynamics associated with singlet excited-state decay of reference tetraphenylporphyrins (ZnTPP, NiTPP, and H(2)TPP), the diyne-linked dyads undergo ultrafast (<10 ps) singlet excited-state deactivation in toluene, tetrahydrofuran (THF), and benzonitrile (PhCN). Transient absorption techniques with the ZnP-C(60) dyad clearly show that in toluene intramolecular energy transfer (EnT) to ultimately generate C(60) triplet excited states is the dominant singlet decay mechanism, while intramolecular electron transfer (ET) dominates in THF and PhCN to give the ZnP(*+)/C(60)(*-) charge-separated radical ion pair (CSRP). Electrochemical studies indicate that there is no significant charge transfer in the ground states of these systems. The lifetime of ZnP(*+)/C(60)(*-) in PhCN was approximately 40 ps, determined by two different types of transient absorption measurement in two different laboratories. Thus, in this system, the ratio of the rates for charge separation (k(CS)) to rates for charge recombination (k(CR)), k(CS)/k(CR), is quite small, approximately 7. The fact that charge separation (CS) rates increase with increasing solvent polarity is consistent with this process occurring in the normal region of the Marcus curve, while the slower charge recombination (CR) rates in less polar solvents indicate that the CR process occurs in the Marcus inverted region. While photoinduced ET occurs on a similar time scale in a related dyad 15 in which a diethynyl bridge connects C(60) to the para position of a meso phenyl moiety of a tetrarylporphyrin, CR occurs much more slowly; i.e., k(CS)/k(CR) approximately equal to 7400. Thus, the position at which the conjugative linker is attached to the porphyrin moiety has a dramatic influence on k(CR) but not on k(CS). On the basis of electron density calculations, we tentatively conclude that unfavorable orbital symmetries inhibit charge recombination in 15 vis a vis the beta-linked dyads.
Two new beta-substituted arylethynyl meso-tetraphenylporphyrins, 2-[(4'-formyl)phenyl]ethynyl-5,10,15,20-tetraphenylporphyrin (system A) and 2-[(4'-methyl)phenyl]ethynyl-5,10,15,20-tetraphenylporphyrin (system B) and their zinc derivatives were synthesized by palladium catalysis, using a synthetic approach that affords high yields of the target systems. Comparative ultraviolet-visible (UV-vis), NMR, and cyclic voltammetry studies of such macrocycles reveal the presence of an extensive conjugation between the tetrapyrrolic ring and the linker, through pi-pi orbital interaction. This interaction was observed in the form of a "push-pull" effect that moves the electronic charge between the porphyrin and the aldehyde group of system A. System B, bearing a methyl group instead of the formyl group, was synthesized in order to evaluate the effect of the substitution on the charge delocalization, which is necessary to corroborate the push-pull mechanism hypothesis. The new porphyrin, system A, was also used as a starting material for the synthesis of new porphyrin-fullerene dyads in which the [60]fullerene is directly linked to the tetrapyrrolic rings by ethynylenephenylene subunits. Fluorescence and transient absorption measurements of the new dyads reveal that ultrafast energy and electron transfer occur, respectively, in nonpolar and polar solvents, with high values of the rate constant. The UV-vis, NMR, and cyclic voltammetry results show that it is possible for both energy and electron transfer between porphyrin and fullerene to take place through the pi-bond interaction. Such results evidence that the coupling between the donor and acceptor moieties is strong enough for possible photovoltaic applications.
The fluorescence decay kinetics from a benzonitrile solution of a dibenzo-18-crown-6 ether bridged porphyrin-fullerene dyad has been studied in the presence of a range of metal ions. Dual-exponential fluorescence decay behaviour has been attributed to conformational flexibility of the molecule influencing quenching interactions between the photo-excited porphyrin and fullerene. Additions of sodium, potassium and lithium ions significantly modulate the observed fluorescence decay processes while the larger tetrabutylammonium ion has only a minor affect. The results are discussed in terms of ion inclusion within the crown ether affecting both the bridge conformational properties and donor-acceptor electronic interactions.
A new group of porphyrin-fullerene dyads with an azobenzene linker was synthesized, and the photochemical and photophysical properties of these materials were investigated using steady-state and time-resolved spectroscopic methods. The electrochemical properties of these compounds were also studied in detail. The synthesis involved oxidative heterocoupling of free base tris-aryl-p-aminophenyl porphyrins with a p-aminophenylacetal, followed by deprotection to give the aldehyde, and finally Prato 1,3-dipolar azomethineylide cycloaddition to C60. The corresponding Zn(II)-porphyrin (ZnP) dyads were made by treating the free base dyads with zinc acetate. The final dyads were characterized by their 1H NMR, mass, and UV-vis spectra. 3He NMR was used to determine if the products are a mixture of cis and trans stereoisomers, or a single isomer. The data are most consistent with the isolation of only a single configurational isomer, assigned to the trans (E) configuration. The ground-state UV-vis spectra are virtually a superimposition of the spectral features of the individual components, indicating there is no interaction of the fullerene (F) and porphyrin (H2P/ZnP) moieties in the ground state. This conclusion is supported by the electrochemical data. The steady-state and time-resolved fluorescence spectra indicate that the porphyrin fluorescence in the dyads is very strongly quenched at room temperature in the three solvents studied: toluene, tetrahydrofuran (THF), and benzonitrile (BzCN). The fluorescence lifetimes of the dyads in all solvents are sharply reduced compared to those of H2P and ZnP standards. In toluene, the lifetimes of the free base dyads are 600-790 ps compared to 10.1 ns for the standard, while in THF and BzCN the dyad lifetimes are less than 100 ps. For the ZnP dyads, the fluorescence lifetimes were 10-170 ps vs 2.1-2.2 ns for the ZnP references. The mechanism of the fluorescence quenching was established using time-resolved transient absorption spectroscopy. In toluene, the quenching process is singlet-singlet energy transfer (k approximately 10(11) s-1) to give C60 singlet excited states which decay with a lifetime of 1.2 ns to give very long-lived C60 triplet states. In THF and BzCN, quenching of porphyrin singlet states occurs at a similar rate, but now by electron transfer, to give charge-separated radical pair (CSRP) states, which show transient absorption spectra very similar to those reported for other H2P-C60 and ZnP-C60 dyad systems. The lifetimes of the CSRP states are in the range 145-435 ns in THF, much shorter than for related systems with amide, alkyne, silyl, and hydrogen-bonded linkers. Thus, both forward and back electron transfer is facilitated by the azobenzene linker. Nonetheless, the charge recombination is 3-4 orders of magnitude slower than charge separation, demonstrating that for these types of donor-acceptor systems back electron transfer is occurring in the Marcus inverted region.
A considerable amount of work concerning systems in which C60 is an electron acceptor has been published in the past three years, representing a substantial advance in knowledge over that summarized in the reviews published by Guldi and Kamat in 2000 [1] and by Martin et al. in 1998 [2]. Other reviews covering specific topics in this area have also appeared in the interim [3–5]. Accordingly, the present chapter will concentrate on new developments in this field, with only occasional reference to work published before 1999. The fundamental principles behind fullerene donor-acceptor systems are revisited and, for the first time, a section summarizing the experimental methods available for the study of these systems is presented. Other chapters in this volume deal with subjects that are very closely interwoven with the present discussion, specifically “Energy Transfer in Functionalized Fullerenes” (Armaroli), “Reorganization Energy in Functionalized Fullerenes” (Guldi), and “Photovoltaic Applications” (Hummelen). Where these subjects arise, as they will repeatedly, the reader will be referred to these chapters for more extensive discussions.
The remarkable electron accepting character of fullerenes make them attractive molecules in several fields of chemistry [1, 2], For instance, this property has been fruitfully exploited in photochemistry [3], electrochemistry [4], as well as in synthetic [5] and materials chemistry [6]. As far as photochemistry is concerned, it has been widely demonstrated that in chemical systems containing fullerenes and suitable electron donors (either in solution or in the solid state), photoinduced electron transfer may occur.
In the context of optimizing charge-separation processes in artificial model systems, meaningful incentives are lent from bacterial photosynthetic reaction centers [1]. Whereas in green or purple bacteria only one photosynthetic unit — PS II — is carrying out the light-to-chemical product conversion, green plants are using two systems — PS I and PS II [2]. Essential to all these systems is a relay of short-range energy/electron transfer reactions, evolving among chlorophyll- and quinone-moieties embedded in a transmembrane protein matrix. Ultimately the product of these cascades is transformation of light into usable chemical energy. The latter governs water cleavage to O2 and reduction of NADP to NADPH, which is used to produce in its final instant sugars from CO2.
Coupling of an unpaired d electron to the porphyrin phosphorescent triplet gives rise to a “tripdoublet” and a quartet state. Luminescence from Cu porphin and Cu octalkylporphin is largely tripdoublet at liquid nitrogen temperature and quartet at liquid hydrogen temperature, while Cu tetraphenylporphin is quartet at both temperatures.
Time-resolved fluorescence and absorption techniques have been used to investigate energy and photoinduced electron transfer in a covalently linked free-base porphyrin-fullerene dyad and its zinc analogue. In toluene, the porphyrin first excited singlet states decay in about 20 ps by singlet-singlet energy transfer to the fullerene. The fullerene first excited singlet state is not quenched and undergoes intersystem crossing to the triplet, which exists in equilibrium with the porphyrin triplet state. In benzonitrile, photoinduced electron transfer from the porphyrin first excited singlet state to the fullerene competes with energy transfer. The fullerene excited singlet state is also quenched by electron transfer from the porphyrin. Overall, the charge-separated state is produced with a quantum yield approaching unity. This state lives for 290 ps in the free-base dyad and 50 ps in the zinc analog. These long lifetimes suggest that such dyads may be useful as components of more complex light-harvesting systems. 32 refs., 12 figs., 1 tab.
Since their first detection and bulk production, the fullerenes have
gained a primary role on the scientific scene, reaching their climax when
the 1996 Nobel Prize for Chemistry was awarded to Kroto, Curl and Smalley
for their seminal discovery. The unique physical and chemical properties
of these new forms of carbon led many scientists to predict several
technological applications. This created a heavy disappointment when it
was clear that fullerene-based materials would not soon be ready for the
market. However, the fullerenes have so far delighted several dozens of
researchers who found that C
60
and its relatives undergo a
variety of chemical reactions. In most cases, the new derivatives retain
the main properties of the original fullerene, and it is now not unlikely
that some functionalized fullerenes may find useful applications in the
field of materials science and technology. In this Article we summarize
the basic principles of the organic chemistry of fullerenes, together with
a description of the physicochemical properties that have made these
carbon cages popular in materials science, and review the most recent
achievements in the functionalization of fullerenes aimed at the
production of new molecular materials.
Computational studies have been performed on a variety of C60–porphyrin dyads, a class of donor–acceptor materials which have been a subject of considerable attention in recent years. Molecular modelling studies were carried out to clarify the relationship between molecular topology and experimentally determined rates of intramolecular electron and energy transfer in these systems. The systems studied include doubly linked cyclophane-like C60–porphyrin dyads, where structural variations were made computationally on the porphyrin and linker portions, as well as dyads with flexible polyether and rigid steroid linkers. The molecular modelling studies involved building and minimising structures of the various fullerene–porphyrin dyads, followed by molecular dynamics to find the equilibrium and lowest energy conformations. The study confirmed that attractive van der Waals interactions between porphyrin and C60 moieties cause
these dyads to adopt unusual conformations in which these groups are in close proximity, often in orientations which are not readily predictable from conventional structural representations. The implications of these computational data for the design of fullerene–porphyrin dyads with specific properties in the context of electron and energy transfer processes are discussed.
This review documents the exceptional range of research avenues in supramolecular fullerene chemistry that have been pursued during the past decade. It illustrates how molecular complexation of pristine fullerenes developed from solid state enclathration by π-electron-rich compounds to inclusion complexation by designed macrocyclic receptors in the liquid phase. Progress in covalent fullerene functionalisation led to the development of spectacular supramolecular architectures including rotaxanes, catenanes, DNA complexes, diads and triads for photoinduced electron and energy transfer and ordered thin films. All of these molecular assemblies and supramolecular arrays feature distinct properties as a consequence of the presence of the fullerene components. Recent investigations hinting at potential technological applications of supramolecular fullerene, such as in sensorics, are highlighted.
This feature article highlights the advantages of employing
[60]fullerene as a viable electron accepting building block in novel donor
acceptor systems. Different strategies that aim towards improving charge
separation in fullerene containing systems are presented. This is
accomplished, for example, via utilisation of additional
stabilisation forces of the radical pair or, alternatively, diffusional
splitting of the last mentioned. Fine-tuning the topology of the electron
donor moiety has been shown to be a powerful means of influencing the
relative energies of the two states involved (e.g. the
charge-separated state vs. the singlet ground state). Remarkable
effects concerning the lifetime of the charge-separated radical pair were
observed, in particular, in systems that upon charge separation led to a
gain of aromaticity and planarity of the oxidised fragment.
Medium dependence of the emission spectra of several meso-substituted copper porphyrins was studied at 300-77 K. Copper porphyrins emit from the lowest excited (porphyrin triplet) states in fluid solution as well as in rigid media. Emission spectra of several copper porphyrins in toluene liquid solution were red-shifted from those in rigid media such as PMMA (poly(methylmethacrylate)) film. The copper porphyrins, which give red-shifted emission in toluene solution, have 2,4b13(a2ueg)] configuration in the lowest excited states and the amount of the shift depends on meso-substituents ranging up to 1300 cm−1. As the emission spectra in toluene rigid glass at 77 K coincide with those in PMMA film, the observed shift in 2,4[b13(a2ueg)]-type porphyrins is attributable to distortion of the excited molecules in fluid solution. T(2,4,6-(MeO)3)PPCu, in which bulky meso-substituents are likely to suppress torsion of the phenyl rings and distortion of the porphyrin plane, was found to show no shift of emission spectra, in spite of the 2,4[b13(a2ueg)] configuration. In the case of TPrPCu, which has no phenyl group, red shift occurs as a result of the medium being changed to become non-rigid. An out-of-plane distorted structure is proposed. Lifetimes of the emission in toluene solution are remarkably shorter in the copper porphyrins that show a larger emission red-shift from the emission in PMMA film. This relation suggests that the distortion modes are connected with enhancement of the radiationless decay.
The synthesis and photophysical properties of three types of porphyrin–fullerene (P-C60) dyads are presented: (a) conformationally flexible dyads with polyether linkers, (b) rigid dyads with steroid linkers, and (c) a parachute-shaped dyad in which the flat porphyrin and spherical fullerene chromophores are in very close proximity. The course of events following photoexcitation of these dyads and (in some cases) corresponding porphyrin–zinc complexes has been probed through studies of fluorescence quenching, measurements of fluorescence lifetimes and quantum yields for singlet molecular oxygen formation, and transient absorption spectroscopy. These data provide insight into the dynamic competition between intramolecular energy transfer and electron transfer in these dyads, and, for the case of the parachute-shaped dyad, the kinetics of back-electron transfer.
Four different kinds of Cââ-linked zincporphyrins have been prepared by changing systematically the linking position at meso-phenyl ring from ortho to para and their photophysical properties have been investigated. Regardless of the linkage between the two chromophores, photoinduced charges separation (CS) and subsequent charge recombination (CR) were observed in a series of zincporphyrin-Cââ dyads by picosecond fluorescence lifetime measurements and time-resolved transient absorption spectroscopy. In THF the CS occurs from both the excited singlet state of the porphyrin and the Cââ moieties, implying that the absorption cross section by both the chromophores results in the efficient formation of the ion pair (IP) state. On the other hand, in benzene the IP state generated by the photoinduced CS from the excited singlet state of the porphyrin to the Cââ produces or energetically equilibrates with the locally excited singlet state of the Cââ. Both the CS and CR rates for the meta isomer are much slower than those for the other porphyrin-linked Cââ. Linkage dependence of the electron transfer (ET) rates can be explained by superexchange mechanism via spacer. These results demonstrate that Cââ is a new promising building block as an acceptor in artificial photosynthetic models. 32 refs., 14 figs., 3 tabs.
The ‘Marcus-inverted’ region for BET in a
π–π stacked fullerene–porphyrin dyad was determined based
on (i) the photoactivation of an intramolecular ET in a variety of solvents
and (ii) the formation of a highly energetic charge-separated state
Heating fullerenes at 650°C under 3000 atmospheres of the noble gases helium, neon, argon, krypton, and xenon introduces these atoms into the fullerene cages in about one in 1000 molecules. A “window” mechanism in which one or more of the carbon-carbon bonds of the cage is broken has been proposed to explain the process. The amount of gas inside the fullerenes can be measured by heating to 1000°C to expel the gases, which can then be measured by mass spectroscopy. Information obtained from the nuclear magnetic resonance spectra of helium-3-labeled fullerenes indicates that the magnetic field inside the cage is altered by aromatic ring current effects. Each higher fullerene isomer and each chemical derivative of a fullerene that has been studied so far has given a distinct helium nuclear magnetic resonance peak.
The pronounced ability of fullerene C60 to act as an electron and energy acceptor has led to the synthesis of a large number of compounds in which C60 is covalently linked to photoactivatable groups which can serve as potential donors. Such compounds are of interest as model systems for photosynthetic reaction centers and also have potential applications in photodynamic therapy because of the highly efficient photosensitization of singlet molecular oxygen formation by C60 and C60 derivatives. By far the largest number of such systems studied to date utilize porphyrins as antennas for efficient light capture in the visible region of the spectrum, and a variety of linkers. Photophysical studies as well as molecular modeling indicate that in conformationally flexible dyads the porphyrin (P) and C60 moieties are in close proximity, due to -stacking interactions, thus facilitating through-space interactions, as demonstrated by quenching of 1P* fluorescence and generation of fullerene-excited states (by energy transfer) or P+-C60- ion-pair states (by electron transfer).2,4a,b,f-h These ion-pair states can be relatively long-lived, due to the small reorganization energy and strong thermodynamic driving force for back-electron transfer, which places this process within the Marcus inverted region.4g, Recently attention has focused on rigidly linked systems in which the porphyrin (P) and (C60) moieties are in enforced close proximity or are forced apart by a saturated norbornylogous or steroid linker. As part of a program to understand the nature of the dialogue between P and C60 chromophores as the topology of P-C60 dyads is systematically varied, we now report photophysical data for the parachute-shaped dyad 1 and the corresponding zinc complex 1-Zn. We have reported previously the synthesis of 1 by Bingel-Hirsch addition of a strapped porphyrin malonate to C60.
We report the synthesis, transient absorption, FT Raman, resonance Raman, time-resolved resonance Raman, and transient resonance Raman spectra of pseudo-D2h symmetric [5,15-bis[(4‘-nitrophenyl)ethynyl]-10,20-diphenylporphinato]copper(II) (I) and electronically asymmetric [5-[4‘-(dimethylamino)phenyl]ethynyl]-15-[(4‘‘-nitrophenyl)ethynyl]-10,20-diphenylporphinato]copper(II) (II), which bears both electron-releasing and electron-withdrawing groups conjugated directly to the porphyrin periphery. The spectroscopic results suggest extensive electronic communication between the 5- and 15-arylethynyl groups and the porphyrin core. Relative to the parent compound, (tetraphenylporphinato)copper(II) (CuTPP), the arylethynyl substituents increase the lifetime of the excited trip-multiplet states. CuTPP, as well as compounds I and II, however, shows similar solvent-dependent dynamics: the trip-multiplet lifetimes are longer in a noncoordinating solvent such as benzene than in a coordinating solvent such as THF. This behavior is consistent with the existence of a quenching state whose effect is more pronounced upon coordination of solvent. The time-resolved resonance Raman spectrum of compound II shows features commonly associated with the relatively long-lived triplet excited states of copper(II) porphyrins. The transient resonance Raman spectrum of a short-lived excited state present in both compounds I and II is characterized by marked shifts in the nitro and porphyrin stretching frequencies relative to that observed for the ground states of both (4-nitrophenyl)ethyne and (tetraphenylporphinato)copper(II). We interpret these results for the compounds I and II as arising from (i) a short-lived excited state present at early time that possesses enhanced porphyrin-to-nitro charge-transfer character with respect to the ground state and (ii) a longer-lived excited state deriving from this initially probed charge-transfer state that is largely porphyrin localized.
Emission spectra of intra- and intermolecular exciplex systems of benzophenone and N,N-dimethylaniline were studied in detail at low temperature. Two new emissions were observed for the first time in the present work in addition to the well-known phosphorescence of the CT complex formed in the ground state. The first emission has a lifetime of a few tens nanoseconds, which is common to intra- and intermolecular systems. The second emission with a longer lifetime than that of the first one is obtained only in the case of the intramolecular system. A possibility that these emissions are due to fluorescent products was examined. Considering solvent, temperature, and concentration dependences of these emissions, it is proposed that the first and the second emissions are due to the singlet and the triplet exciplexes, respectively. Dynamic behaviors and geometrical structures of these exciplexes are discussed.
Excited-state dynamics of copper(II) tetraphenylporphyrin left bracket Cu(TPP) right bracket and copper(II) etioporphyrin left bracket Cu(Etio) right bracket depends dramatically on the coordinating properties of the solvent system. Relaxation of transient absorption and recovery of ground-state bleaching following excitation with 35-ps flashes at 355 nm range from greater than 10 ns in the noncoordinating solvent, toluene, to approx. 100 ps in the coordinating solvent, piperidine. Lifetimes of intermediate duration are observed in toluene/piperidine mixtures; for Cu(TPP) the time constants are 650 ps and 2. 1 ns in 1 and 0. 3 M piperidine, respectively. Analogous results are found for Cu(Etio) and in pyridine. Despite such variation in kinetic behavior, transient difference spectra observed for both Cu porphyrins are essentially independent of solvent.
The absorption features of 1(π,π*), 3(π,π*), 3(d,π*), and (d,d) excited states of metalloporphyrins have been closely examined between 420 and 900 nm with subpicosecond/picosecond transient absorption spectroscopy. The spectra of all of these excited states exhibit strong but not readily distinguishable absorption between the Soret- and Q-band bleachings, a region used extensively and often exclusively in transient absorption studies on porphyrins. The absorption features between 600 and 900 nm are smaller than those observed in the Soret region but have distinctive characteristics that aid in assessing the presence or absence of a particular type of transient state. We discuss the electronic origins of the prominent bands in the excited-state spectra. Our results and discussion provide fundamental information on the optical properties of metalloporphyrin excited states and a much needed framework for better interpreting the results of in vivo and in vitro transient absorption studies on these complexes.
We have investigated the excited-state relaxation dynamics and pathways of copper(II) octaethylporphyrin (CuOEP) and copper(II) tetraphenylporphyrin (CuTPP) in noncoordinating solvents at temperatures between 295 and 77 K. The excited-state deactivation of CuOEP depends markedly on temperature and solvent. For example, the lifetime in methylcyclohexane varies from 270 ns at 295 K to 10 μs at 150 K. The lifetime at 295 K varies from 100 ns to 1 μs with a change in solvent polarity. In contrast, the lifetime of photoexcited CuTPP is 30-40 ns, essentially independent of temperature and solvent. These observations can be explained in terms of a model that includes the participation of a charge-transfer (CT) state, most likely a ring-to-metal (π,d) CT, in the deactivation of the tripdoublet (2T) excited state. Our results suggest that the CT excited state lies 600-800 cm-1 above 2T in CuOEP and between 2T and the quartet (4T) in CuTPP.
A new approach for obtaining information about the electronic structure and, in particular, the relative ordering of the frontier and subfrontier orbitals in metalloporphyrin systems is presented. This treatment involves a combination of electrochemical and spectral data. A series of 2-substituted copper(II) 5,10,15,20-tetraphenylporphyrins (2-11), in which the electronic nature of the substituent has been significantly varied, have been studied. The substituent has a considerable effect on the energies of the two highest occupied molecular orbitals, the a2u and the a1u orbitals, and can even cause the relative order of these orbitals to change. Substantial modulation of the a2u/a1u orbital energies is achieved by variation of the nature of a pyrrolic beta-substituent. The effect is, as expected, felt much more strongly on the a1u orbital, which has significant electron density associated with the pyrrolic beta-position of metalloporphyrins. Indeed, the relative energy of the a1u orbital in the nitroporphyrin 2 and the aminoporphyrin 11 differs by 0.71 eV; the corresponding difference in the energies of the a2u orbitals is 0.14 eV. In cases where there is a good electron-donating 2-substituent (NH2, OCH3, SPh), the "normal" ordering a2u > a1u is reversed. These observations have important consequences in metalloporphyrin systems as their patterns of reactivity, influenced by the electron distribution in the highest filled molecular orbital, will be particularly sensitive to the relative a1u/a2u separation and ordering. A role for pyrrolic beta-substituents in the fine tuning of energy levels in porphyrin-based molecular electronic logic and memory devices is suggested.
Time-resolved resonance Raman (TR3) spectra were obtained with a pump/probe technique using two 7 ns pulsed lasers for Cu(I1) complexes of four meso-substituted porphyrins, including tetraphenylporphin (TPP), tetrakis(3,4,5-trimethoxyphenyl)porphin (T ~ , ~ , ~ o M ~ P P) , tetramesitylporphin (TMP), and tetrakis(pentaflu0-ropheny1)porphin (TF~PP) to investigate why emission spectra of (TPP)Cu, and (T ~ , ~ , ~ o M ~ P P) C U are significantly red-shifted in fluid media compared with in rigid media, but those of (TMP)Cu and (TF~PP)CU are not. Raman bands of TI spectra were assigned on the basis of deuteration shifts for TPP-dg and TPPd20. All porphyrin skeletal bands are similarly broader in the TI state except for a phenyl internal mode. It was unexpected from the empirical rule on the al, and a2, cation radicals that both the v2 and V I I bands were shifted to lower frequencies in the TI state than those of the SO state irrespective of the symmetry property, al, or a2", of the HOMO. The ~ 2 7 mode (C,-phenyl out-of-phase stretching) was resonance enhanced and shifted to higher frequencies in the TI state. The magnitudes of frequency shifts of vz , V I , and v27 bands upon excitation to the T I state changed in the same order as that of the red-shift of the emission spectra in the fluid solution, that is, (T ~ , ~ , ~ o M ~ P P) C U x (TPP)Cu > (TMP)Cu > (TF~PP)CU. Since these vibrations contain C,-C, or C,-phenyl stretching character, the present observation suggests that the red-shift in the fluid solvent is associated with the increase of their coupling term and thus structural distortions at the methine bridges. The phenyl vg, band was strongly enhanced in the TI state for (TPP)Cu but not for (TMP)Cu and (T3,4,5~MePP)C~, although their frequencies remain unaltered, suggesting that the T, -T I excitation involves distortion of the phenyl ring along vg, coordinate for (TPP)Cu but not for others.
We propose a novel strategy using fullerenes for the construction of solar energy conversion systems that mimic the primary electron transfer events in photosynthesis. Redox-active fullerenes such as C60 and C70 were covalently bound to a porphyrin and the photophysical properties of the resulting compounds were investigated. Regardless of solvent and linkage, the charge-separated state is produced efficiently in zincporphyrin–fullerene dyads, showing that fullerenes are good electron acceptors. The most intriguing characteristic of fullerenes in electron transfer is that they accelerate photoinduced charge separation as well as charge shift and slow down charge recombination, properties that are in sharp contrast with those of conventional two-dimensional aromatic acceptors such as quinones and imides. The peculiar electron transfer properties of fullerenes can be explained by the small reorganization energies, which make it possible to optimize artificial photosynthetic multistep charge separation. A combination of the two strategies, multistep electron transfer and small reorganization energy of fullerenes, has been applied to light energy conversion systems as well as the more complex molecular systems such as triads. Highly efficient photosynthetic multistep electron transfer has been realized at gold electrodes modified with self-assembled monolayers of fullerene-containing molecules. These results will provide new principles and concepts to develop artificial photosynthetic materials as well as molecular devices.
Redox-active fullerenes can be covalently bound to a variety of donors, their photophysical properties have been investigated. Their photochemical processes. Including electron transfer and energy transfer, are varied, depending on the donor, linkage between the donor and C60, and solvent. Regardless of the solvent and linkage, the charge-separated state is produced efficiently in zinc porphyrin-C60 systems, showing that C6o is a good electron acceptor. The most intriguing characteristic of C60 in electron transfer is that C60 accelerates photoinduced charge separation and retards charge recombination in the dark. The long-lived charge-transfer state: of the C60–porphyrin dyad was successfully converted to photocurrent using a self-assembled monolayer technique. These findings will provide a new strategy for the design and synthesis of artificial photosynthetic systems and photoactive materials using C60 as a building block.
A series of 1,4-phenylene-bridged ZP-HP hybrid porphyrins (ZP = zinc porphyrin, HP = free-base porphyrin) 1-8 ZH have been prepared in which an electron-donating ZP moiety is kept constant and electron-accepting HP moieties are varied by introducing electron-accepting substituents, so that the energy gap for charge separation, ZP-1HP*--> ZP(+)-HP-, covers a range of about 0.9 eV in DMF. Here selective excitation at the HP moiety was employed to avoid complication in the determination of electron transfer rates derived from energy transfer, 1ZP*-HP --> ZP-1HP*. Definitive evidence for the electron transfer has been obtained in three solvents (benzene, THF, and DMF) through picosecond-femtosecond transient absorption studies, which have allowed the determination of the rates of the photoinduced charge separation, ZP-1HP* --> ZP(+)-HP-, and subsequent thermal charge recombination ZP(+)-HP- --> ZP-HP. Dyad 1ZH in THF exhibits a biphasic fluorescence decay that indicates thermal repopulation of the ZP-1HP* from ZP(+)-HP-; this has been also supported by the transient absorption spectra. On this ground, the energy levels of the ZP(+)-HP- ion pairs have been estimated. Similar biphasic fluorescence decay has been observed for 5 ZH in benzene; this allows furhter estimation of the energy level of the ZP(+)-HP- ion pairs. The free-energy-gap dependence (energy-gap law) has been probed from the normal to the upper limit region for the rate of the charge separation alone, and only the inverted region for the rate of the charge recombination. It was not possible to reproduce both energy-gap dependencies of the charge separation and the charge recombination assuming common parameter values for the reorganization energy and electronic interaction responsible for the electron transfer with the classical Marcus equation. Although both energy-gap dependencies can be approximately reproduced by means of the simplified semiclassical equation, which takes into consideration the effect of the high-frequency vibrations replaced by one mode of averaged frequency, many features, which include the effects of solvent polarity, electron-tunneling matrix element, and so forth on the energy-gap law, are considerably different from those of the previous studied porphyrin-quinone systems with weaker inter-chromophore electronic interactions.
This Account reviews our main achievements in the field of excited-state properties of fullerene derivatives. The photosensitizing and electron-acceptor features of some relevant classes of functionalized fullerene materials are highlighted, considering the impact of functionalization on fullerene characteristics. In addition, the unique optimization in terms of redox potentials, water-solubility, and singlet oxygen generation is presented for several novel fullerene-based materials.
Increased understanding of photosynthetic energy conversion and advances in chemical synthesis and instrumentation have made it possible to create artificial nanoscale devices and semibiological hybrids that carry out many of the functions of the natural process. Artificial light-harvesting antennas can be synthesized and linked to artificial reaction centers that convert excitation energy to chemical potential in the form of long-lived charge separation. Artificial reaction centers can form the basis for molecular-level optoelectronic devices. In addition, they may be incorporated into the lipid bilayer membranes of artificial vesicles, where they function as components of light-driven proton pumps that generate transmembrane proton motive force. The proton gradient may be used to synthesize adenosine triphosphate via an ATP synthase enzyme. The overall energy transduction process in the liposomal system mimics the solar energy conversion system of a photosynthetic bacterium. The results of this research illustrate the advantages of designing functional nanoscale devices based on biological paradigms.
Photoinduced charge separation (CS) and charge recombination (CR) processes have been examined in various porphyrin-fullerene linked systems (i.e., dyads and triads) by means of time-resolved transient absorption spectroscopy and fluorescence lifetime measurements. The investigated compounds comprise a homologous series of rigidly linked, linear donor-acceptor arrays with different donor-acceptor separations and diversified donor strength: freebase porphyrin-C60 dyad (H2P-C60), zincporphyrin-C60 dyad (ZnP-C60), ferrocene-zincporphyrin-C60 triad (Fc-ZnP-C60), ferrocene-freebase porphyrin-C60 triad (Fc-H2P-C60), and zincporphyrin-freebase porphyrin-C60 triad (ZnP-H2P-C60). Most importantly, the lowest lying charge-separated state of all the investigated systems, namely, that of ferrocenium ion (Fc+) and the C60 radical anion (C60.-) pair in the Fc-ZnP-C60 triad, has been generated with the highest quantum yields (close to unity) and reveals a lifetime as long as 16 micros. Determination of CS and CR rate constants, together with the one-electron redox potentials of the donor and acceptor moieties in different solvents, has allowed us to examine the driving force dependence (-DeltaG0ET) of the electron-transfer rate constants (kET). Hereby, the semilogarithmic plots (i.e., log kET versus -DeltaG0ET) lead to the evaluation of the reorganization energy (lambda) and the electronic coupling matrix element (V) in light of the Marcus theory of electron-transfer reactions: lambda = 0.66 eV and V = 3.9 cm(-1) for ZnP-C60 dyad and lambda = 1.09 eV and V = 0.019 cm(-1) for Fc-ZnP-C60, Fc-H2P-C60, and ZnP-H2P-C60 triads. Interestingly, the Marcus plot in Fc-ZnP-C60, Fc-H2P-C60, and ZnP-H2P-C60 has provided clear evidence for intramolecular CR located in both the normal and inverted regions of the Marcus parabola. The coefficient for the distance dependence of V (damping factor: betaCR = 0.58 A(-1) is deduced which depends primarily on the nature of the bridging molecule.
Chem. Rev. 2000, 100, 1075−1120 Discrete Fulleride Anions and Fullerenium Cations Christopher A. Reed* and Robert D. Bolskar Department of Chemistry, University of CaliforniasRiverside, Riverside, California 92521-0403 Received June 22, 1999 Contents I. Introduction, Scope, and Nomenclature II. Electrochemistry A. Reductive Voltammetry B. Oxidative Voltammetry III. Synthesis A. Chemical Reduction of Fullerenes to Fullerides i. Metals as Reducing Agents ii. Coordination and Organometallic Compounds as Reducing Agents iii. Organic/Other Reducing Agents B. Electrosynthesis of Fullerides C. Chemical Oxidation of Fullerenes to Fullerenium Cations IV. Electronic (NIR) Spectroscopy A. Introduction B. C 60 n- Fullerides C. C 70 and Higher Fullerenes D. Fullerenium Cations E. Diffuse Interstellar Bands V. Vibrational Spectroscopy A. Infrared Spectroscopy B. Raman Spectroscopy VI. X-ray Crystallography A. Introduction B. [PPN] 2 [C 60 ] and Related C 602- Structures C. C 60 - Structures D. C 603- Structures E. Comparison of Discrete and Extended Structures VII. Magnetic Susceptibility and Spin States VIII. NMR Spectroscopy A. Introduction B. 13 C NMR Data in Solution C. Interpretation of Solution NMR Data D. 13 C NMR Data in the Solid State E. Knight Shift in A 3 C 60 F. 3 He NMR of Endohedral Fullerides G. 13 C NMR of Derivatized Fullerenes H. 13 C NMR of Fullerenium Cations IX. EPR Spectroscopy A. Introduction B. Features of the C 60 - Spectrum i. The Low g Value ii. Temperature Dependence of the Line Width (∆H pp ) X. XI. XII. XIII. iii. Anisotropy iv. Problem of the Sharp Signals v. Origins of Sharp Signals vi. The C 120 O Impurity Postulate vii. The Dimer Postulate C. Features of the C 602- EPR Spectrum D. Features of the C 603- EPR Spectrum E. Features of C 604- and C 605- EPR Spectra F. EPR Spectra of Higher Fullerides G. EPR Spectra of Fullerenium Cations Chemical Reactivity A. Introduction B. Fulleride Basicity C. Fulleride Nucleophilicity/Electron Transfer D. Fullerides as Intermediates E. Fullerides as Catalysts F. Fullerides as Ligands G. Fullerenium Cations Conclusions and Future Directions Acknowledgments References I. Introduction, Scope, and Nomenclature The electron-accepting ability of C 60 , the archetypal fullerene, is its most characteristic chemical property. It is a natural consequence of electronic structure and was anticipated in early molecular orbital calcula- tions 1 which place a low-lying unoccupied t 1u level about 2 eV above the h u HOMO: 2-4 Early in the gas-phase investigations of fullerenes, the electron affinity of C 60 was measured and found to be high (2.69 eV). 5-7 When the macroscopic era of C 60 chemistry began in 1990, this property was soon found to translate into the solution phase. 8 In a rather remarkable cyclic voltammogram (see Figure 1), the reversible stepwise addition of up to six electrons was soon demonstrated electrochemically. 9,10 10.1021/cr980017o CCC: $35.00 © 2000 American Chemical Society Published on Web 02/16/2000
Fullerenes and porphyrins are molecular architectures ideally suited for devising integrated, multicomponent model systems to transmit and process solar energy. Implementation of C60 as a 3-dimensional electron acceptor holds great expectations on account of their small reorganization energy in electron transfer reactions and has exerted a noteworthy impact on the improvement of light-induced charge-separation. This article describes how the specific compositions of porphyrin chromophores linked to C60--yielding artificial light harvesting antenna and reaction center mimics--have been elegantly utilized to tune the electronic couplings between donor and acceptor sites and the total reorganization energy. Specifically, the effects that these parameters have on the rate, yield and lifetime of the energetic charge-separated states are considered.
- Armaroli N.
- Da Ros T.