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

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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 Otsubo
Photoinduced intramolecular charge separation and charge recombination of oligothiophene−methyl viologen dyads (Tn−MV2+·2X-, n = 4 or 8, X- = I- or PF6-), which were designed to generate positively charged radical ion pairs (Tn•+−MV•+·2X-), have been investigated by the time-resolved fluorescence and transient absorption spectra in CH3CN. Upon excitation of the T8 moiety in T8−MV2+·2X-, the radical cation pair (T8•+−MV•+·2X-) was produced via the excited singlet state of T8 (1T8*), because the fluorescence lifetime of the 1T8* moiety in T8−MV2+·2X- becomes short compared with pristine 1T8*. The lifetimes of T8•+−MV•+·2X- were evaluated to be 720−980 ns depending on the counteranion, I- or PF6-. On the other hand, T4−MV2+·2X- leads to lifetimes of T4•+−MV•+·2X- of 210−240 ns, although the charge separation seems to occur via the excited triplet state of T4 (3T4*), because the fluorescence quenching of the 1T4* moiety in T4−MV2+·2X- was not observed. As factors to achieve these relatively long lifetimes of Tn•+−MV•+·2X-, the charge recombination between of the positive charges (holes) on the Tn and MV moieties and the center-to-center distance between the positive charges (holes) on the Tn and MV moieties may be considered, in addition to the triplet spin character of Tn•+−MV•+·2X-.