Size effects of oligothiophene on the dynamics of electron transfer in π-conjugated oligothiophene-perylene bisimide dyads

The photophysical properties of a series of pi-conjugated oligothiophene-perylene bisimide dyads nT-PBI (n = 1, 2, and 3) in solutions were investigated. A sharp fluorescence quenching of the perylene bisimide moiety in these dyads nT-PBI (n = 1, 2, and 3) was observed upon photoexcitation of the perylene bisimide moiety, indicating a fast and efficient electron-transfer process. The properties of photoinduced electron transfer of these dyads nT-PBI (n = 1, 2, and 3) in solutions were investigated by time-resolved fluorescence spectra and femtosecond transient absorption spectra in the visible region. It was found that as the number of thiophene repeat units, i.e., the size, increases, the rate constants of charge separation and charge recombination increase about 3 orders and over 1 order of magnitude, respectively, because of the combined effects of energetics and electronic coupling. These results demonstrate that a change in size of donor could change and modulate the dynamics of electron transfer.