## Photoinduced Charge Separation and Charge Recombination of Oligothiophene−Viologen Dyads in Polar Solvent

journal contribution

posted on 02.12.2004, 00:00 by Yasuyuki Araki, Hongxia Luo, Takumi Nakamura, Mamoru Fujitsuka, Osamu Ito, Hiroki Kanato, Yoshio Aso, Tetsuo OtsuboPhotoinduced intramolecular charge separation and charge recombination of oligothiophene−methyl viologen
dyads (T

*−MV*_{n}^{2+}·2X^{-},*n*= 4 or 8, X^{-}= I^{-}or PF_{6}^{-}), which were designed to generate positively charged radical ion pairs (T_{n}^{•+}−MV^{•+}·2X^{-}), have been investigated by the time-resolved fluorescence and transient absorption spectra in CH_{3}CN. Upon excitation of the T_{8}moiety in T_{8}−MV^{2+}·2X^{-}, the radical cation pair (T_{8}^{•+}−MV^{•+}·2X^{-}) was produced via the excited singlet state of T_{8}(^{1}T_{8}*), because the fluorescence lifetime of the^{1}T_{8}* moiety in T_{8}−MV^{2+}·2X^{-}becomes short compared with pristine^{1}T_{8}*. The lifetimes of T_{8}^{•+}−MV^{•+}·2X^{-}were evaluated to be 720−980 ns depending on the counteranion, I^{-}or PF_{6}^{-}. On the other hand, T_{4}−MV^{2+}·2X^{-}leads to lifetimes of T_{4}^{•+}−MV^{•+}·2X^{-}of 210−240 ns, although the charge separation seems to occur via the excited triplet state of T_{4}(^{3}T_{4}*), because the fluorescence quenching of the^{1}T_{4}* moiety in T_{4}−MV^{2+}·2X^{-}was not observed. As factors to achieve these relatively long lifetimes of T_{n}^{•+}−MV^{•+}·2X^{-}, the charge recombination between of the positive charges (holes) on the T*and MV moieties and the center-to-center distance between the positive charges (holes) on the T*_{n}_{n}and MV moieties may be considered, in addition to the triplet spin character of T_{n}^{•+}−MV^{•+}·2X^{-}.