Enhanced intersystem crossing via a high energy charge transfer state in a perylenediimide-perylenemonoimide dyad

The electronic relaxation processes of a photoexcited linear perylenediimide-perylenemonoimide (PDI-PMI) acceptor-donor dyad were studied. PDI-PMI serves as a model compound for donor-acceptor systems in photovoltaic devices and has been designed to have a high-energy PDI-center dot-PMII+center dot charge transfer (CT) state. Our study focuses on the minimal Gibbs free energy (Delta G(ET)) required to achieve quantitative CT and on establishing the role of charge recombination to a triplet state. We used time-resolved photoluminescence and picosecond photoinduced absorption (PIA) to investigate excited singlet (SI) and CT states and complemented these experiments with singlet oxygen ((1)Delta(g)) luminescence and PIA measurements on longer timescales to study the population of triplet excited states (T-1). In an apolar solvent like cyclohexene (CHX), photoinduced electron transfer does not occur, but in more polar solvents such as toluene (TOL) and chlorobenzene (CB), photoexcitation is followed by a fast electron transfer, populating the PDI-center dot-PMI-center dot CT state. We extract rate constants for electron transfer (ET; S-1 -> CT), back electron transfer (BET; S-1 <- CT), and charge recombination (CR) to lower-energy states (CT -> So and CT -> T-1). Temperature-dependent measurements yield the barriers for the transfer reactions. For ET and BET, these correspond to predictions from Marcus-Jortner theory and show that efficient, near quantitative electron transfer (k(ET)/k(BET) L 100) can be obtained when Delta G(ET) approximate to - 120 meV. With respect to triplet state formation, we find a relatively low triplet quantum yield (Phi(T) < 25%) in CHX but much higher values (Phi(T) = 30-98%) in TOL and CB. We identify the PDI-center dot-PMI-center dot state as a precursor to the T-1 state. Recombination to T1, rather than to the ground-state So, is required to rationalize the experimental barrier for CR. Finally, we discuss the relevance of these results for electron donor-acceptor films in photovoltaic devices.