Photoinduced intramolecular charge separation and charge recombination of oligothiophene-methyl viologen dyads (Tn-MV 2+·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-MV 2+·2X-, the radical cation pair (T8 ·+-MV·+·2X-) was produced via the excited singlet state of T8 (1T8 *), because the fluorescence lifetime of the 1T 8* moiety in T8-MV 2+·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-MV 2+·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-.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry