Intramolecular Photoinduced Electron-Transfer Processes in Tetrathienylethylene−Quaterthiophene−[60]Fullerene Triad in Solutions

Intramolecular photoinduced electron-transfer processes in buta-1,3-diynyl-benzene-linked porphyrin-fullerene dyad

Intramolecular electron-transfer process of porphyrin-fullerene dyad linked by phenyl buta-1,3-diynyl-phenyl unit ( ZnP - sp - C 60) was studied by laser flash photolysis. Picosecond fluorescence lifetime and transient absorption measurements revealed that photoinduced charge-separation takes place via the excited singlet state (1 ZnP *) with the rate constant of (1-2) × 1010 s −1. For the charge recombination, about a half of the radical-ion pair decayed quickly with 2.9 × 109 s −1 as evaluated from picosecond transient absorption measurements, whereas the remaining half was long-lived with slow decay (1.6 × 106 s −1) as estimated from nanosecond transient absorption measurements. The lifetime of the radical-ion pair of ZnP - sp - C 60 was longer than those of directly connected dyads with a buta-1,3-diynyl bridge and buta-1,3-diynyl-phenyl bridge by the insertion of an extra phenyl group in addition to the pyrrodino ring.

Synthesis and photoinduced electron transfer processes of rotaxanes bearing [60]fullerene and zinc porphyrin: effects of interlocked structure and length of axle with porphyrins

Three rotaxanes, with axles with two zinc porphyrins (ZnPs) at both ends penetrating into a necklace pending a C60 moiety, were synthesized with varying interlocked structures and axle lengths. The intra-rotaxane photoinduced electron transfer processes between the spatially positioned C60 and ZnP in rotaxanes were investigated. Charge-separated (CS) states (ZnP*+, C60*-)rotaxane are formed via the excited singlet state of ZnP (1ZnP*) to the C60 moiety in solvents such as benzonitrile, THF, and toluene. The rate constants and quantum yields of charge separation via 1ZnP decrease with axle length, but they are insensitive to solvent polarity. When the axle becomes long, charge separation takes place via the excited triplet state of ZnP (3ZnP*). The lifetime of the CS state increases with axle length from 180 to 650 ns at room temperature. The small activation energies of charge recombination were evaluated by temperature dependence of electron-transfer rate constants, probably reflecting through-space electron transfer in the rotaxane structures.

Synthesis and Photoinduced Intramolecular Processes of Fulleropyrrolidine–Oligothienylenevinylene–Ferrocene Triads

Two new triads based on N-methylfulleropyrolidine, oligothienylenevinylenes (nTV) and ferrocene (Fc), namely C(60)-nTV-Fc (n=2, 4) have been synthesized. A HOMO-LUMO gap as low as 1.09-1.11 eV was experimentally determined by cyclic voltammetry. In both polar and nonpolar solvents, photoinduced charge-separation (CS) processes in C(60)-nTV-Fc predominantly take place from the singlet excited states of C(60) and nTV; this result was indicated by steady and time-resolved emission spectroscopy. In the case of C(60)-4TV-Fc, the CS state was indicated by the nanosecond transient absorption spectra. In C(60)-2TV-Fc, although the CS process was also confirmed by the fluorescence quenching in nonpolar and polar solvents, the lifetimes of the CS states were shorter than those of C(60)-4TV-Fc. It was revealed that the introduction of Fc donor moiety at the end of the longer nTV chain in the C(60)-nTV dyad systems effectively increases the CS efficiency and the lifetimes of CS states.

A fulleropyrrolidine end-capped platinum-acetylide triad: the mechanism of photoinduced charge transfer in organometallic photovoltaic cells

The fullerene end-capped platinum acetylide donor-acceptor triad Pt(2)ThC(60) was synthesized and characterized by using photophysical methods and photovoltaic device testing. The triad consists of the platinum acetylide oligomer Ph-[triple bond, length as m-dash]-Pt(PBu3)2-[triple bond, length as m-dash]-Th-[triple bond, length as m-dash]-Pt(PBu3)2-[triple bond, length as m-dash]-Ph (Ph=phenyl and Th=2,5-thienyl, stereochemistry at both Pt centers is trans) that contains fulleropyrrolidine moieties on each of the terminal phenylene units. Electrochemistry of the triad reveals relatively low potential oxidation and reduction waves corresponding, respectively, to oxidation of the platinum acetylide and reduction of the fulleropyrrolidine units. Photoluminescence spectroscopy shows that the singlet and triplet states of the platinum acetylide chromophore are strongly quenched in the triad assembly, both in solution at ambient temperature as well as in a low-temperature solvent glass. The excited state quenching arises due to intramolecular photoinduced electron transfer to produce a charge separated state based on charge transfer from the platinum acetylide (donor) to the fulleropyrrolidine (acceptor). Picosecond time resolved absorption spectroscopy confirms that the charge transfer state is produced within 1 ps of photoexcitation, and it decays by charge recombination within 400 ps. Organic photovoltaic devices fabricated using spin-coated films of Pt2ThC60 as the active material operate with modest efficiency, exhibiting a short circuit photocurrent of 0.51 mA cm(-2) and an open circuit voltage of 0.41 V under 100 mW cm(-2)/AM1.5 illumination. The results are discussed in terms of the relationship between the mechanism of photoinduced electron transfer in the triad and the comparatively efficient photovoltaic response exhibited by the material.

Photoinduced Electron Transfer in α-Cyclodextrin-Based Supramolecular Dyads: A Free-Energy-Dependence Study

Photoinduced electron transfer (PET) between alpha-cyclodextrin-appended pyrene (PYCD) and a few acceptor molecules was studied in aqueous solutions. The pyrene moiety in PYCD is located above the narrower rim of the alpha-CD and is fully exposed to water. The acceptors are monocyclic organic molecules and, upon dissolution in water in the presence of PYCD, a fraction of the donor-acceptor systems is present as supramolecular dyads and the remaining fraction as free molecules. Free-energy-dependence studies showed that electron transfer in the supramolecular dyads follows the Marcus equation. The donor-acceptor coupling and the reorganization energy were determined from fits of the data to the Marcus equation. The electronic coupling was found to be similar to those reported for hydrogen-bonded systems. It appears that the actual lambdaout values are somewhat lower than values calculated with the continuum model. The experimental design has also allowed, for the first time, a visual demonstration of the inverted region on the basis of the raw fluorescence lifetime data.

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).

Novel Reactions of [60]Fullerene with Amino Acid Esters and Carbon Disulfide

[reaction: see text] Novel reactions of C60 with amino acid ester hydrochlorides and CS2 in the presence of Et3N affording fullerene derivatives 2 and 3 containing biologically active amino acids, thioamide, and thiourea units have been investigated. The thioamide groups in compounds 2 are sensitive to moisture and can easily be hydrolyzed to amide groups.

Photoinduced Charge Separation and Charge Recombination in the [60]Fullerene−Diphenylbenzothiadiazole−Triphenylamine Triad: Role of Diphenylbenzothiadiazole as Bridge

Photoinduced electron-transfer processes of the newly synthesized [60]fullerene-diphenylbenzothiadiazole-triphenylamine (C60-PBTDP-TPA) triad in polar and nonpolar solvents have been studied by using time-resolved transient absorption and fluorescence measurements from picosecond to microsecond regions. By fluorescence lifetime measurements in picosecond time regions, excitation of the charge-transfer transition of the PBTDP-TPA moiety in C60-PBTDP-TPA induces energy transfer to the C60 moiety generating 1C60*-PBTDP-TPA, competitively with charge separation generating C60*--PBTDP-TPA*+. From 1C60*-PBTDP-TPA, which is generated directly and indirectly, charge separation occurs generating C60*--PBTDP-TPA*+ in polar solvents. The C60*--PBTDP-TPA*+ formed via the singlet excited states decayed within a few nanoseconds as revealed by the picosecond transient absorption spectra. In the nanosecond time region, C60*--PBTDP-TPA*+ is produced slowly, probably via 3C60*-PBTDP-TPA. Lifetimes of such slowly generated C60*--PBTDP-TPA*+ were longer than 1 micros, which are the longest values among the C60-bridge-TPA triad systems reported hitherto at room temperature. Roles of the PBTDP-TPA moiety with twisted intermolecular charge-transfer character playing as energy donor and electron donor in addition to the bridge have been disclosed.

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 Charge Separation and Charge Recombination in [60]Fullerene−Ethylcarbazole and [60]Fullerene−Triphenylamines in Polar Solvents

Molecules of C60 covalently connected with N-ethylcarbazole (EtCz) and triphenylamine (TPA) have been synthesized. Photoinduced electron transfer in C60-EtCz and C60-TPA has been studied in polar and nonpolar solvents using time-resolved transient absorption and fluorescence measurements. From the fluorescence lifetimes, the excited singlet state of the C60 moiety (1C60) of C60-TPA generates predominantly C60*--TPA*+, which decays quickly to the ground state within 6 ns even in polar solvents. In the case of C60-EtCz, on the other hand, about half of the 1C60 moiety generates short-lived C60*--EtCz*+, while the other half of the 1C60 moiety is transferred to the 3C60 moiety via intersystem crossing in dimethylformamide, in which the energy level of C60*--EtCz*+ is lower than that of 3C60. Thus, the charge separation takes place via 3C60 generating C60*--EtCz*+, having a lifetime as long as 300 ns, probably because of the triplet spin character of C60*--EtCz*+. A special property of the EtCz moiety to stabilize the hole in the charge-separated state was revealed.

Pyrazolino[60]fullerene-Oligophenylenevinylene Dumbbell-Shaped Arrays: Synthesis, Electrochemistry, Photophysics, and Self-Assembly on Surfaces

Symmetrically substituted oligophenylenevinylene (OPV) derivatives bearing terminal p-nitrophenylhydrazone groups have been prepared and used for the synthesis of dumbbell-shaped bis(pyrazolino[60]fullerene)-OPV systems. In these triad arrays, the OPV-type fluorescence is dramatically quenched as a consequence of ultrafast OPV-->C60 singlet energy transfer. In its turn the fullerene singlet state is quenched by pyrazoline-->C60 electron transfer, in line with the behavior of the corresponding reference fullerene molecule. The occurrence of electron transfer in the multicomponent arrays is evidenced by recovery of fullerene fluorescence at 77 K in CH2Cl2 and in toluene at 298 K. Under these conditions the OPV-->C60 energy transfer is unaffected. The rate of this process turns out to be higher for the OPV trimer than for the corresponding pentameric OPV arrays, in agreement with energy-transfer theory expectations. Scanning tunneling microscopy (STM) and scanning force microscopy (SFM) revealed that the bis(pyrazolino[60]fullerene)-OPV can self-assemble into ordered layered crystalline architectures on the basal plane of highly oriented pyrolitic graphite.

Photoinduced Intramolecular Electron-Transfer Processes in [60]Fullerene-(Spacer)-N,N-Bis(biphenylyl)aniline Dyad in Solutions

Intramolecular photoinduced charge-separation and charge-recombination processes in a covalently connected C60-(spacer)-N,N-bis(biphenylyl)aniline (C60-spacer-BBA) dyad, in which the center-to-center distance of the electron acceptor and electron donor is 15 A, have been studied by time-resolved fluorescence and transient absorption methods. The observed low fluorescence intensity and the short fluorescence lifetime of the C60 moiety of the dyad in PhCN and THF indicate that charge separation takes place via the excited singlet state of the C60 moiety at a quite fast rate and a high efficiency. The nanosecond transient absorption spectra in PhCN and THF showed the broad absorption bands at 880 and 1100 nm, which were attributed to C60(*-)-spacer-BBA(*+). The charge-separated state decays with a lifetime of 330-360 ns in PhCN and THF at room temperature. From temperature dependence of the charge-recombination rate constants, the reorganization energy was evaluated to be 0.77-0.87 eV, which indicates that the charge-recombination process is in the inverted region of the Marcus parabola. With lowering temperature, the contribution of charge separation via the excited triplet state of the C60 moiety increases due to an increase in solvation of C60(*-)-spacer-BBA(*+).