Long-lived photogenerated states of alpha-oligothiophene-acridinium dyads have triplet character.

Photoinduced processes, leading to charge-transfer states with extended lifetimes, are of key importance for solar-energy-conversion applications. Utilizing external heavy-atom effect allowed us to photogenerate long-lived transients of electron donor-acceptor dyads. For an electron acceptor and a principal chromophore of the dyads, we selected N-methylacridinium, and for electron donors thiophene, bithiophene, and terthiophene were selected. While the photoinduced charge transfer, mediated by the investigated dyads, occurred in the picosecond time domain, the lifetime of the transients extended to the microsecond time domain. We ascribed the relatively long lifetimes to the triplet character of the observed transients. An increase in the size of the donor lowered the energy of the charge-transfer states of the dyads. When the energy level of the acridinium triplet lies below the energy level of the charge-transfer state, the locally excited triplet accounted for the long-lived transient. For the conjugates with charge-transfer states lying below all other excited states, the long-lived transients were, indeed, the charge-transfer species.