Photoinduced charge separation and charge recombination of oligothiophene-viologen dyads in polar solvent

Photoinduced intramolecular charge separation and charge recombination of oligothiophene-methyl viologen dyads (T-n-MV(2+.)2X(-), n = 4 or 8, X- = I- or PF6-), 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 CH3CN. Upon excitation of the T-8 moiety in T-8-MV(2+.)2X(-), the radical cation pair (Tg(.+)-MV(.-)2X(-)) was produced via the excited singlet state of T-8 (T-1(8)*), because the fluorescence lifetime of the T-1(8)* moiety in T-8-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, T-4-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 T-4 (T-3(4)*), because the fluorescence quenching of the T-1(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-n and MV moieties and the center-to-center distance between the positive charges (holes) on the T,, and MV moieties may be considered, in addition to the triplet spin character of T-n(.+)-MV(.+.)2X(-).