Shinya Moribe

Photoinduced intramolecular electron-transfer reactions in carbazole-fullerene and phenothiazine-fullerene linked compounds in benzene and benzonitrile as studied by fluorescence, transient absorption, time-resolved EPR, and magnetic field effects

Photoinduced intramolecular electron-transfer reactions in carbazole (Cz)-fullerene (C60) (Cz(8)C60) and phenothiazine (Ph)-C60 (Ph(n)C60 (n=8, 10, 12)) linked compounds have been investigated in benzene and benzonitrile by fluorescence, transient absorption, and time-resolved electron paramagnetic resonance measurements, and by magnetic field effects on the decay rate constants of the photogenerated biradicals. In benzonitrile, photoinduced intramolecular electron transfer from Cz to the singlet excited state of C60 (1C60*) occurred in Cz(8)C60, but not to the triplet excited state (3C60*), while the intramolecular electron-transfer to both1C60* and3C60* occurred in Ph(n)C60 (n=8, 10, 12). In benzene, on the other hand, no electron transfer to both1C60* and3C60* took place in all linked compounds. These results were interpreted in terms of the different Gibbs free energy changes in the two solvents.

Photoinduced intramolecular electron transfer reactions in fullerene—phenothiazine linked compounds: effects of magnetic field and spacer chain length

Spectroscopic and electrochemical properties of two fullerene(C60)-phenothiazine(PH) linked compounds with different spacer chain length have been compared in benzonitrile (polar solvent) and in benzene (non-polar solvent). Transient absorption and fluorescence spectra indicated that photoinduced intramolecular electron transfer occurred in benzonitrile, but not in benzene. The results are due to solvent effect on energy levels of the photogenerated biradical. The driving forces for the electron transfer were determined by measuring the redox potentials of the C60 and PH moieties. Thermodynamic parameters for the electron transfer processes were evaluated and compared. In benzonitrile, the lifetime of the photo-generated biradical was very long, in spite of being around the top region in Marcus theory. The decay rate of the biradicals was retarded in the presence of magnetic fields. The decay rate constant decreased quickly with increasing the magnetic field and became constant above about 0.2 T. The magnetic field effects verified that the triplet biradical was generated by the intramolecular electron transfer from PH to the triplet excited state of C60. The long lifetime is most probably ascribed to the spin multiplicities of the biradical.

Magnetic field effects and time-resolved EPR studies on photogenerated biradical from intramolecular electron transfer reactions in zinc-tetraphenylporphyrin-C60 linked compounds: contribution of relaxation mechanism due to spin–spin relaxation

Magnetic field effects (MFEs) and time-resolved EPR on photogenerated biradical from intramolecular electron-transfer reactions in zinc-tetraphenylporphyrin (ZnP)–C60 linked compounds (ZnP(4)C60 and ZnP(8)C60) with flexible four and eight methylene groups have been investigated in benzonitrile. At low temperature (288 K), the decay rate constant of the biradical decreased steeply in low magnetic fields (<0.1 T), and then increased gradually in middle magnetic fields (0.1 ≤ H ≤ 0.6 T) and finally became almost constant in high magnetic fields (0.6 ≤ H ≤ 1 T) in both ZnP(4)C60 and ZnP(8)C60. At high temperature (323 K), in contrast, the decay rate constant decreased slightly in low magnetic fields (<0.1 T), and then increased gradually in higher magnetic fields (0.1 ≤ H ≤ 1 T). Interestingly, the decay rate constants in higher magnetic fields (0.4 ≤ H ≤ 1 T) were larger than that in zero magnetic field. The reverse phenomena of the MFEs around 0.1 T and temperature dependence on the MFEs are explained by the contribution of spin–spin relaxation due to anisotropic Zeeman interaction to the relaxation mechanism and most likely ascribed to the properties of C60 anion radical due to the spherical π-system. The mechanism is also supported by the time-resolved EPR spectra.

Reverse phenomena of magnetic field effects and time-resolved EPR spectra in the photogenerated biradical from intramolecular electron-transfer in a phenothiazine-C_ linked compound with a semi-rigid spacer

Photoinduced electron-transfer reactions and magnetic field effects (MFEs) on the decay rates of the photogenerated biradical in a phenothiazine (Ph)–C60 linked compound with a biphenyl group (Ph(BP)C60) were examined in benzonitrile and benzene. Fluorescence and transient absorption spectra indicate that the intramolecular electron-transfer for Ph(BP)C60 from the Ph to the singlet or triplet excited state of C60 was suppressed by the biphenyl group. The decay rates of the photogenerated biradical decreased in the 0–0.2 T magnetic field range and increased in the 0.2–1 T magnetic field range. The reverse phenomena of the MFEs in Ph(BP)C60 were strongly enhanced with increasing temperature and similar to those in Ph(n)C60 (n = 6−12). The MFEs in Ph(BP)C60 can be governed by spin-lattice relaxation and/or spin-spin relaxation mechanisms as observed in Ph(n)C60 (n = 6−12). Time-resolved EPR spectra of Ph(BP)C60 showed absorption, emission, absorption and emission patterns, and are quite different from those in Ph(n)C60 (n = 4−12). The result indicates that the magnitude and distribution of the exchange interaction |2J| in Ph(BP)C60 are smaller than those in Ph(n)C60 (n = 4−12) and charge recombination occurs in the inverted region because the sign of the J is positive.

Effects of magnetic processing on electrochemical and photoelectrochemical properties of electrodes modified with nanoclusters of a phenothiazine-C60 linked compound

Effects of magnetic processing on morphological, electrochemical, and photoelectrochemical properties of electrodes modified with nanoclusters of a phenothiazine-C60 linked compound with four methylene group (Ph(4)C60) were examined in the absence and presence of magnetic processing with three different magnetic environments due to strong magnetic field. The AFM measurements indicated that the morphologies of nanostructures of Ph(4)C60 varied with magnetic enviroments as comparison with that in the absence of magnetic processing. At top position (5.6 T; − 940 T2/m) with hypogravity, large spherical nanoclusters (60~70 nm diameter) were observed as comparion with those (ca. 20 nm diameter) in the absence of magnetic processing. At middle positon (15 T; 0 T2/m) with normal gravity, the fiber-like nanostructure was observed. At bottom position (9.8 T; + 1070T2/m) with hypergravity, the rod-like nanostrucure was observed. The interesting results might be ascribed to the different solvent properties due to the different rates of evaporation of two solvents in the toluene-acetonitrile mixed solvent during drying process under various magnetic environments. First reduction peaks due to C60 moiety of Ph(4)C60 nanostrucures in the presence of magnetic processing at three different positions were negative-shifted as comparison with that in the absence of magnetic processing. Potential dependencies of the photocurrents of the electrodes modified with Ph(4)C60 nanostrucures in the presence of magnetic processing at three positions were also different from that in the absence of magnetic processing. The magnetic field effects in AFM, and electrochemical and photoelectrochemical measurements are most likely ascribed to the difference of the reduction potentials due to C60 clusters between the absence and presence of magnetic processing due to the morphological change of Ph(4)C60 nanostrucures.