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 (Φet) for the generation of the radical anion of C60 (C60•-) via 3C60* were quite low even in polar solvents; nevertheless, the free-energy changes (ΔGet) of electron transfer from Fc to 3C60* are sufficiently negative. In benzonitrile (BN), the Φet 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 (Φet = 0.46 for decamethylferrocene in BN) and an increase in solvent polarity (Φet = 0.58 in BN:DMF (1:2) for decamethylferrocene) were effective

Photooxidation of Olefins Sensitized by Bisazafullerene (C₅₉N)₂ and Hydroazafullerene C₅₉HN: Product Analysis, Emission of Singlet Oxygen, and Transient Absorption Spectroscopy

The photooxidation reactions of olefins sensitized by the excited triplet states of bisazafullerene (C59N)2 and hydroazafullerene C59HN have been studied. Oxidation yields were compared with those of pristine C60. The singlet oxygen yields are also determined directly from the emission intensities, which are in good agreement with the oxidation yields. The triplet states of (C59N)2 and C59HN have been identified by the time-resolved spectroscopic method by observing the triplet−triplet absorption spectra, which decay in the presence of oxygen. It has been proven that (C59N)2 and C59HN have the ability to sensitize the reactions via singlet oxygen in about half of the efficiency of that of pristine C60. For both azafullerenes, the triplet lifetimes are shorter than that of pristine C60, which may be related to the nitrogen atom embedded in the C60 moiety

A Dramatic Elongation of the Lifetime of Charge-Separated State by Complexation with Yttrium Triflate in Ferrocene−Anthraquinone Linked Dyad

Seven million times elongation of the lifetime of charge-separated state is attained in the presence of yttrium triflate [Y(OTf)3] in the photoinduced electron-transfer reaction of a ferrocene−anthraquinone dyad (Fc−AQ) with a rigid amide spacer in benzonitrile at 298 K as compared with the lifetime in its absence. Such remarkable elongation of the CS lifetime in the presence of Y(OTf)3 results from the strong binding of Y(OTf)3 with the AQ•- moiety of Fc+−AQ•-

Onium Butyltriphenylborates as Donor−Acceptor Initiators for Sensitized Photopolymerizations of Vinyl Monomer

Donor−acceptor initiators, onium butyltriphenylborates (DPIB, DMPSB) containing both an onium cation (electron-accepting radical generator) and a borate anion (electron-donating radical generator) within the same compound were able to be sensitized by two aromatic ketones (2-chlorothioxanthone and Michler's ketone) to initiate polymerization of tetrahydrofurfuryl acrylate monomer. The photopolymerization efficiency of the onium butyltriphenylborates was higher than that of the onium salts (DPI, DMPS) or the borate compound (TBAB). The photoinitiation processes between the photosensitizers and the initiators have been investigated by laser flash photolysis. It has been clarified that two kinds of radical species are generated by a photoinduced electron-transfer mechanism

Photoinduced Microsecond-Charge-Separation in Retinyl-C₆₀ Dyad

Intramolecular photoinduced charge separation and recombination processes in a retinyl-C60 dyad molecule (Ret-C60) have been investigated in various solvents by time-resolved absorption and fluorescence techniques. Upon laser excitation of the C60-moiety in nonpolar toluene, the intersystem crossing proceeded from the excited singlet state of the C60-moiety (Ret-1C60*) to the excited triplet state (Ret-3C60*), followed by energy transfer yielding the excited triplet state of the retinyl-moiety (3Ret*-C60) without charge separation. On the other hand, in polar solvents such as N,N-dimethylformamide and benzonitrile, the charge separation occurred from Ret-1C60* at rate on the order of 1010 s-1. The quantum yield was close to unity in these polar solvents. Most parts of the ion pair (Ret•+-C60•-) changed to Ret-3C60* and 3Ret*-C60 by the charge recombination which took place at rate on the order of 109 s-1. However, some parts of the charge-separated state were kept in microsecond time-region:  The lifetimes of Ret•+-C60•- were 16 μs and 19 μs in DMF and benzonitrile, respectively, which were as long as those of Ret-3C60* and 3Ret*-C60, suggesting an equilibrium between the charge-separated state and the excited triplet states

Onium Butyltriphenylborates as Donor−Acceptor Initiators for Sensitized Photopolymerizations of Vinyl Monomer

Donor−acceptor initiators, onium butyltriphenylborates (DPIB, DMPSB) containing both an onium cation (electron-accepting radical generator) and a borate anion (electron-donating radical generator) within the same compound were able to be sensitized by two aromatic ketones (2-chlorothioxanthone and Michler's ketone) to initiate polymerization of tetrahydrofurfuryl acrylate monomer. The photopolymerization efficiency of the onium butyltriphenylborates was higher than that of the onium salts (DPI, DMPS) or the borate compound (TBAB). The photoinitiation processes between the photosensitizers and the initiators have been investigated by laser flash photolysis. It has been clarified that two kinds of radical species are generated by a photoinduced electron-transfer mechanism

Supramolecular Pattern of Fullerene on 2D Bimolecular “Chessboard” Consisting of Bottom-up Assembly of Porphyrin and Phthalocyanine Molecules

Two-component adlayers consisting of zinc(II) phthalocyanine (ZnPc) and a metalloporphyrin, such as zinc(II) octaethylporphyrin (ZnOEP) or zinc(II) tetraphenylporphyrin (ZnTPP), were prepared by immersing either an Au(111) or Au(100) substrate in a benzene solution containing those molecules. The bimolecular adlayers thus prepared were investigated in 0.1 M HClO4 by cyclic voltammetry (CV) and electrochemical scanning tunneling microscopy (EC-STM). A supramolecularly organized “chessboard” structure was formed for the ZnPc and ZnOEP bimolecular array on Au(111), while characteristic nanohexagons were found in the ZnTPP and ZnOEP bimolecular adlayer. EC-STM revealed that the surface mobility and the molecular re-organization of ZnPc and ZnOEP on Au(111) were tunable by manipulating the electrode potential, whereas the ZnTPP and ZnOEP bimolecular array was independent of the electrode potential. A “bottom-up” hybrid assembly of fullerene molecules was formed successfully on an alternate array of bimolecular ZnPc and ZnOEP molecules. The bimolecular “chessboard” served as a template to form the supramolecular assembly of C60 by selective trapping in the open spaces. A supramolecular organization of ZnPc and ZnOEP was also found on the reconstructed Au(100)−(hex) surface. A highly ordered, compositionally disordered but alternate array of ZnPc and ZnOEP was formed on the reconstructed Au(100)−(hex) surface, indicating that the bimolecular adlayer structure is dependent on the atomic arrangement of underlying Au in the formation of supramolecular nanostructures composed of those molecules. On the bimolecular array consisting of ZnPc and ZnOEP on the Au(100)−(hex), no highly ordered supramolecular assembly of C60 was found, suggesting that the supramolecular assembly of C60 molecules is strongly dependent upon the bimolecular packing arrangement of ZnPc and ZnOEP

Zinc Porphyrins Covalently Bound to the Side Walls of Single-Walled Carbon Nanotubes via Flexible Bonds: Photoinduced Electron Transfer in Polar Solvent

Single-walled carbon nanotubes (SWCNTs) tethering zinc porphyrins (ZnP) with flexible aliphatic bonds, ZnP−(spacer)−SWCNT, were prepared with applying the Prato reaction. Under illumination by visible light (>500 nm), ZnP−(spacer)−SWCNT effectively catalyzed the reduction of the benzylviologen dication (BV2+) by 1-benzyl-1,4-dihydronicotinamide in dimethylformamide (DMF). The H2 evolution was also confirmed when the ZnP−(spacer)−SWCNT aqueous solution was illuminated by visible light in the presence of Pt particles. Furthermore, the photocurrent and photovoltage were observed to respond to the on−off cycles of the visible light irradiation in a solar cell consisting of a transparent glass electrode modified with ZnP−(spacer)−SWCNT. The time-resolved fluorescence and absorption spectral measurements revealed the photoinduced electron-transfer mechanism

Fluorescence Up-Conversion Study of Excitation Energy Transport Dynamics in Oligothiophene−Fullerene Linked Dyads

Photoinduced excitation energy transport dynamics in oligothiophene−fullerene linked dyads, nT-C60 (n = 4, 8, and 12), have been investigated by femtosecond fluorescence up-conversion. In 8T-C60 and 12T-C60, each time profile of the fluorescence due to the 1nT* moiety consists of two components. The sub-picosecond component and a few picosecond components were experimentally evaluated depending on the lengths of oligothiophenes (n =8 and 12) and on the analyzing wavelength of the fluorescence. However, the time trace of the fluorescence due to 14T*-C60 decayed with a single short component in ∼300 fs due to direct excited energy transfer (EET) from the 14T* moiety to the C60 moiety. On the basis of the kinetic models considering the short and long locally π-conjugative thiophene segments in 8T-C60 and 12T-C60, the rate parameters of the elemental processes were evaluated. Sub-picosecond time constants of nT-C60 were found to be EET from the thiophene segment vicinal to the C60 moiety and intrachain energy transfer. Slower picosecond dynamics mainly corresponds to EET from the thiophene segments apart from the C60 moiety

Prolonged Charge-Separated States of Starburst Tetra(diphenylaminofluoreno)[60]fullerene Adducts upon Photoexcitation

Construction of starburst C60(>DPAF-C9)4 pentads was coupled with the use of highly fluorescent diphenylaminofluorene-C9 (DPAF-C9) addends as donor components in conjunction with the fullerene acceptor during single-photon excitation processes. High quantum yields (ΦCS) of charge-separation processes in the range 0.83−0.90 for C60(>DPAF-C9)n (n = 1, 2, or 4) were obtained in the formation of C60•-(>DPAF•+-C9)(>DPAF-C9)n-1 transient states. The lifetime of the radical ion-pairs (τRIP) was found to be 900 ns for starburst C60(>DPAF-C9)4 (3) samples, which is 6-fold longer than that of the linear analogue C60(>DPAF-C9) (1), with a ca. 2 times increase of the charge-separation rate (kCS) compared to that of 1. These data implied the important role of sterically hindered DPAF-C9 pendants arranged in a starburst-like environment that encapsulates the central C60 core on extending the τRIP. We interpreted the phenomena by the occurrence of intramolecular migration or exchange of electron or positive charge among multiple DPAF-C9 pendants of C60•-(>DPAF•+-C9)(>DPAF-C9)n-1, which gives an increased rate in charge generation and delayed charge recombination