Photoinduced intramolecular electron-transfer processes in [60]fullerene and N,N-Bis(biphenyl)aniline molecular systems in solutions

Photoinduced intramolecular charge-separation and charge-recombination processes in covalently connected C(60)-(spacer)-bis(biphenyl)aniline (C(60)-sp-BBA) and C(60)-((spacer)-bis(biphenyl)aniline)(2) (C(60)-(sp-BBA)(2)) have been studied by time-resolved fluorescence and transient absorption methods. Since a flexible alkylthioacetoamide chain was employed as the spacer, the folded structures in which the BBA moiety approaches the C(60) moiety were obtained as optimized structures by molecular orbital calculations. The observed low fluorescence intensity and the short fluorescence lifetime of the C(60) moiety of these molecular systems indicated that charge separation takes place via the excited singlet state of the C(60) moiety in a quite fast rate and high efficiency even in the nonpolar solvent toluene, which was a quite new observation compared with reported dyads with different spacers. From the absorption bands at 880 and 1000 nm in the nanosecond transient absorption spectra, generations of C(60)(.-)-sp-BBA(.+) and C(60)(.-)-(sp-BBA(.+))(sp-BBA) were confirmed. The rates of charge separation and charge recombination for C(60)-(sp-BBA)(2) are faster than those for C(60)-sp-BBA, suggesting that one of the BBA moieties approaches the C(60) moiety by pushing another BBA moiety because of the flexible spacers.     

Photoinduced electron transfer processes of fullerene rotaxanes containing various electron-donors

The photosensitized electron-transfer processes in the rotaxane hybrids composed with electron-accepting fullerenes and various electron-donors placed in the rotaxanes are revealed with time-resolved fluorescence and absorption spectral methods. Porphyrins are most useful as light-harvesting donors and photosensitizing donors. In addition, aromatic amines and ferrocene act as electron-donor and also hole-shifting reagents in multi-component rotaxanes. In the rotaxanes with spatially placed donor-acceptor molecules, the role of triplet states becomes important compared with the covalently connected donor–acceptor molecular systems, which may be related to the “through-space” and “through-bond” electron transfer, respectively. In the designed multi-component rotaxanes which maintain mechanically or topologically the electron-acceptor, electron-donor, and hole-shifter, the photoinduced electron transfer, hole-shift, electron–hole recombination are established. As a whole, contribution of the triplet excited states is prominent compared with the covalently bonded molecules and supramolecular systems constructed with coordination bonds.     

Energy and electron transfer in polyacetylene-linked zinc-porphyrin-[60]fullerene molecular wires

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.     

Photoinduced intermolecular electron transfer process of fullerene (C60) and amine-substituted fluorenes studied by laser flash photolysis

Photoinduced intermolecular electron transfer process of fullerene (C60) with 9,9-bis(4-triphenylamino)fluorene (BTAF) and 9,9-dimethoxyethyl-2-diphenylaminofluorene (DAF) in toluene and benzonitrile has been investigated by nanosecond laser photolysis technique in the visible/near-IR regions. By the selective excitation of C60 using 532 laser light, it has been proved that the electron transfer takes place from the ground states BTAF and DAF to the triplet excited state of C60 ((3)C60*) by observing the radical anion of C60 and radical cation of BTAF and DAF. It was observed that the electron transfer of BTAF/(3)C60* is more efficient than DAF/(3)C60* reflecting the effect of amine-substitutents of the fluorene moiety on the efficiency of the electron transfer process. On addition of a viologen dication (OV(2+)), the electron of the anion radical of C60 mediates to OV(2+) yielding the OV(+). These results proved that the photosensitized electron-transfer/electron-mediating processes have been confirmed by the transient absorption spectral method.     

Photoinduced processes in a tricomponent molecule consisting of diphenylaminofluorene-dicyanoethylene-methano[60]fullerene

Photoinduced intramolecular processes in a tricomponent molecule C60(>(CN)2-DPAF), consisting of an electron-accepting methano[60]fullerene moiety (C60>) covalently bound to an electron-donating diphenylaminofluorene (DPAF) unit via a bridging dicyanoethylenyl group [(CN)2], were investigated in comparison with (CN)2-DPAF. On the basis of the molecular orbital calculations, the lowest charge-separated state of C60(>(CN)2-DPAF) is suggested to be C60*-(>(CN)2-DPAF*+) with the negative charge localized on the fullerene cage, while the upper state is C60(>(CN)2*--DPAF*+). The excited-state events of C60(>(CN)2-DPAF) were monitored by both time-resolved emission and nanosecond transient absorption techniques. In both nonpolar and polar solvents, the excited charge-transfer state decayed mainly through initial energy-transfer process to the C60 moiety yielding the corresponding 1C60, from which charge separation took place leading to the formation of C60*-(>(CN)2-DPAF*+) in a fast rate and high efficiency. In addition, multistep charge separation from C60(>(CN)2*--DPAF*+) to C60*-(>(CN)2-DPAF*+) may be possible with the excitation of charge-transfer band. The lifetimes of C60*-(>(CN)2-DPAF*+) are longer than the previously reported methano[60]fullerene-diphenylaminofluorene C60(>(C=O)-DPAF) with the C60 and DPAF moieties linked by a methanoketo group. These findings suggest an important role of dicyanoethylenyl group as an electron mediating bridge in C60(>(CN)2-DPAF).     

Photoinduced energy and electron transfer in phenylethynyl-bridged zinc porphyrin-oligothienylenevinylene-C60 ensembles

Donor-bridge-acceptor triad (Por-2TV-C(60)) and tetrad molecules ((Por)(2)-2TV-C(60)), which incorporated C(60) and one or two porphyrin molecules that were covalently linked through a phenylethynyl-oligothienylenevinylene bridge, were synthesized. Their photodynamics were investigated by fluorescence measurements, and by femto- and nanosecond laser flash photolysis. First, photoinduced energy transfer from the porphyrin to the C(60) moiety occurred rather than electron transfer, followed by electron transfer from the oligothienylenevinylene to the singlet excited state of the C(60) moiety to produce the radical cation of oligothienylenevinylene and the radical anion of C(60). Then, back-electron transfer occurred to afford the triplet excited state of the oligothienylenevinylene moiety rather than the ground state. Thus, the porphyrin units in (Por)-2TV-C(60) and (Por)(2)-2TV-C(60) acted as efficient photosensitizers for the charge separation between oligothienylenevinylene and C(60).     

Sequential energy and electron transfer in aggregates of tetrakis[oligo(p-phenylene vinylene)] porphyrins and C60 in water

Chiral aggregation of oligo(p-phenylene vinylene)-functionalized Zn and free-base porphyrins is observed in water. The formation of mixed assemblies containing both porphyrins results in sequential energy transfer from OPV via zinc porphyrin to free-base porphyrin. Furthermore, the incorporation of C60 as electron acceptor yields a charge separated state by ultimate electron transfer.     

Photoinduced electron transfer in a new Bis(C60)-phthalocyanine triad

[Structure: see text] A novel covalently linked bis([60]fullerene)-phthalocyanine triad is reported, exhibiting long-lived photoinduced charge separation both in solution and in the solid state. The first demonstration of a working solar cell using triad 1 as the active material is also presented.     

Energy and electron transfer in beta-alkynyl-linked porphyrin-[60]fullerene dyads

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.     

Photoinduced electron transfer reactivity of aza[60]fullerene: three discrete functionalization pathways with a single substrate

Azafullerenium carbocation, C59N+, shows photoinduced electron transfer (PET) reactivity toward benzyltrimethylsilane. The reaction between (C59N)2 and benzyltrimethylsilane gives three different aza[60]fullerene monoadducts depending on the reaction conditions used.     

Photoinduced electron transfer in photozyme systems

Photozymes are novel water-soluble polymers usually constructed by copolymerization of a mixture of water-soluble and water-insoluble comonomers, some of which contain chromophores capable of absorbing light and transmitting the excitation energy by means of the antenna effect to selected traps. The interactions between the hydrophobic and hydrophilic groups in the polymer with water cause the formation of hypercoiled pseudomicellar conformations of the polymer coil, leading to hydrophobic regions or pockets in the interior of the macromolecular coil. If the water contains hydrophobic organic molecules, they will locate preferentially in these hydrophobic polymer microdomains, and in the presence of light they can be photochemically transformed into useful products with high efficiency and selectivity. This paper reviews some recent results on photochemical reactions initiated by photoinduced electron transfer in these novel systems, and their possible commercial applications to pollution abatement, and solar production of hydrogen from water.     

Photoinduced electron and energy transfer in phenylene oligomers

Phenylene oligomers represent a borderline case between very strongly π-conjugated molecular wires such as oligo-p-phenylene vinylenes and saturated molecular bridges. Even subtle chemical modifications of phenylene oligomers can therefore have a strong impact on charge transfer rates and mechanisms. On the basis of recently published selected case studies, this tutorial review discusses the key factors that affect charge transfer kinetics in phenylene oligomers with particular focus on the role of donor-bridge energy matching. Selected examples of triplet-triplet energy transfer reactions across phenylene oligomers are also discussed.     

Photoinduced electron transfer processes in 1,8-naphthalimide-linker-phenothiazine dyads

Photoinduced electron transfer (PET) processes of 1,8-naphthalimide-linker-phenothiazine (NI-L-PTZ) dyads have been investigated using the nanosecond- and picosecond-transient absorption measurements. Two kinds of linker were introduced, i.e., polymethylene-linked dyad (NI-C8-PTZ and NI-C11-PTZ) and a poly(ethyl ether)-linked one (NI-O-PTZ). The 355 nm pulsed laser excitation of NI-C8-PTZ, NI-C11-PTZ, and NI-O-PTZ in acetonitrile produced NI radical anion (NI*-) and PTZ radical cation (PTZ*+) with the absorption bands around 420 and 520 nm, respectively, through charge transfer from PTZ to NI in the singlet excited state (NI(S1)) as well as in the triplet excited states (NI(T1)) in acetonitrile. On the other hand, the charge transfer process occurred only from NI(S1) in nonpolar solvents. The rates of charge transfer and charge recombination processes largely depended on the solvent polarity and they are affected by the length of linkers and electronic coupling through polyether linker. The PET mechanism is discussed in terms of the free energy change for the charge transfer.     

Photoinduced electron transfer competitive with energy transfer of the excited triplet state of [60]fullerene to ferrocene derivatives revealed by combination of transient absorption and thermal lens measurements

The quenching processes of the exited triplet state of fullerene (3C60) by ferrocene (Fc) derivatives have been observed by the transient absorption spectroscopy and thermal lens methods. Although 3C60 was efficiently quenched by Fc in the rate close to the diffusion controlled limit, the quantum yields (phi(et)) for the generation of the radical anion of C60 (C60*-) via 3C60 were quite low even in polar solvents; nevertheless, the free-energy changes (deltaG(et)) of electron transfer from Fc to 3C60 are sufficiently negative. In benzonitrile (BN), the phi(et) value for unsubstitued Fc was less than 0.1. The thermal lens method indicates that energy transfer from 3C60 to Fc takes place efficiently, suggesting that the excited triplet energy level of Fc was lower than that of 3C60. Therefore, energy transfer from 3C60 to ferrocene decreases the electron-transfer process from ferrocene to 3C60. To increase the participation of electron transfer, introduction of electron-donor substituents to Fc (phi(et) = 0.46 for decamethylferrocene in BN) and an increase in solvent polarity (phi(et) = 0.58 in BN:DMF (1:2) for decamethylferrocene) were effective.     

Photoinduced electron transfer between C70 fullerene and 3,3',5,5'-tetramethylbenzidine studied by electron paramagnetic resonance

Electron paramagnetic resonance (EPR) in situ spectroscopy was applied in the study of photoinduced electron transfer between 3,3',5,5'-tetramethylbenzidine (TMB) and C70 in different solvent systems. The changes found in UV-vis spectra pointed at ground state charge transfer complex formation [C70-TMB] in benzonitrile. Upon selective excitation of C70 using steady-state monochromatic irradiation with a wavelength of 546 nm, two EPR singlets were observed, which were assigned to C70 mono- and di-anion. In the photochemical and cathodic in situ reductions, identical EPR spectra of anion radicals were obtained. C70 mono-anion was investigated also in frozen 1,2-dichlorobenzene solutions within the temperature range from 110 to 210 K, and at 110 K the anisotropic EPR spectrum of C70 mono-anion was simulated assuming an axially symmetric g-matrix with gparallel - gperpendicular = 0.00165.     

Photoinduced charge-separation and charge-recombination processes of fullerene[60] dyads covalently connected with phenothiazine and its trimer

Photoinduced charge-separation and charge-recombination processes of fullerene[60] dyads covalently connected with phenothiazine and its trimer (PTZ n -C 60, n = 1 and 3) with a short amide linkage were investigated. A time-resolved fluorescence study provided evidence of charge separation via the excited singlet state of a C 60 moiety ( (1)C 60*), which displayed high efficiencies in various solvents; Phi (S) CS (quantum yield of charge separation via (1)C 60*) = 0.59 (toluene) to 0.87 (DMF) for PTZ 1-C 60 and 0.78 (toluene) to 0.91 (DMF) for PTZ 3-C 60. The transient absorption measurement with a 6 ns time resolution in the visible and near-IR regions showed evidence of the generation of radical ion pairs in relatively polar solvents for both dyads. In nonpolar toluene, only PTZ 1- (3)C 60* was observed for PTZ 1-C 60, whereas PTZ 3- (3)C 60* as well as the radical ion pair state in equilibrium were observed for PTZ 3-C 60. The radical ion pairs had relatively long lifetimes: 60 (DMF) to 910 ns ( o-dichlorobenzene) for (PTZ) 1 (*+)-C 60 (*-) and 230 (PhCN) to 380 ns ( o-dichlorobenzene) for (PTZ) 3 (*+)-C 60 (*-). The small reorganization energy (lambda) and the electronic coupling element (| V|) were estimated by the temperature dependence of the charge-recombination rates, i.e., lambda = 0.53 eV and | V| = 1.6 cm (-1) for (PTZ) 3 (*+)-C 60 (*-).     

Photoinduced electron transfer from synthetic chlorophyll analogue to fullerene C60 on carbon paste electrode. Preparation of a novel solar cell

Upon a carbon paste electrode, fullerene C60 and successively methyl pyropheophorbide-a (chlorin) were casted to prepare a chlorin-fullerene modified carbon paste electrode (CFE). Photocurrents on the CFE were produced by irradiating of visible lights (> 510 nm) in an aqueous solution of 0.05 M ethylenediaminetetraacetic acid and 0.1 M Na2SO4 at pH 6.7. Larger anodic photocurrent was induced by the CFE than by the carbon paste electrodes modified with either the fullerene or the chlorin. In addition, the photocurrent of the CFE was dependent upon the amount of fullerene casted. The photocurrent action spectra of the CFE (at 300 mV vs. Ag/Ag+) showed that photoinduced electron transfer occurred from the excited state of the chlorin to the fullerene and/or from the chlorin to the photoexcited fullerene, and the electron of the fullerene anion radical produced was then shifted to the carbon paste. Upon irradiation of > 375 nm lights, the anodic photocurrent of the CFE was enhanced by increase in the illuminated light power and reached 0.03 mA cm-2 in the present system.     

Photoinduced electron transfer of fullerenes (C60 and C70) studied by transient absorption measurements in near-IR region

Efficiencies and rates of electron transfer from various electron donors to excited fullerenes (C₆₀ and C₇₀) have been determined by observing the transient absorption bands in the near-IR region, where the anion radicals of fullerenes appear. From the rise of the absorption bands of C₆₀ ⁻⁺ and C₇₀ ⁻⁺ in the near-IR region, electron transfer takes place via the triplet states (^TC₆₀ ^* and ^TC₇₀ ^*) under appropriately low concentrations of electron donors. By analysis of the rise curves C₆₀ ⁻⁺ and C₇₀ ⁻⁺, contribution of the excited singlet states (^SC₆₀ ^* and ^SC₇₀ ^*) in addition to the route of the triplet states (^TC₆₀ ^* and ^TC₇₀ ^*) is confirmed. The quantum yield for electron transfer via the triplet states Φ_ct ^T was evaluated by the ratio of [C₆₀ ⁻⁺]/[^TC₆₀ ^*] (or [C₇₀ ⁻⁺/[^TC₇₀ ^*]). The Φ_ct ^T depends upon the donor-ability, donor concentration, and solvent polarity. The back electron-transfer process, which was evaluated by observing C₆₀ ⁻⁺, also depends upon the solvent polarity.     

Exploring the photoinduced electron transfer reactivity of aza[60]fullerene iminium cation

Photolysis of (C₅₉N)₂ solutions in the presence of neutral π-donors, such as arenes and electron-rich alkenes leads to a series of novel aza[60]fullerene monoadducts. The key step of the reaction involves a photoinduced electron transfer from the donor molecule to the iminium cation of aza[60]fullerene, followed by radical coupling of the resulting aza[60]fullerenyl radical with an intermediate stabilized radical derived from the substrate. This type of reactivity has been proven efficient with arenes having oxidation potential higher than about 1.5 V. Simple olefins, such as tri- and tetra-methylethylene, as well as cyclohexene, can also participate in this kind of photoinduced electron transfer-initiated reaction with C₅₉N⁺, affording the corresponding aza[60]fullerene derivatives. In the case of 2-methoxyprop-1-ene, 2,4-hexadiene, and β,β-dimethylstyrene, [2+2] cycloaddition reactions with the aza[60]fullerene carbon shell dominate, leading to a mixture of unidentified multiadducts.