Enhanced Intersystem Crossing via a High Energy Charge Transfer State in a Perylenediimide−Perylenemonoimide Dyad

Abstract

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^−•-PMI^+• charge transfer (CT) state. Our study focuses on the minimal Gibbs free energy (ΔG_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 (S₁) and CT states and complemented these experiments with singlet oxygen (¹Δ_g) luminescence and PIA measurements on longer timescales to study the population of triplet excited states (T₁). 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^−•-PMI^+• CT state. We extract rate constants for electron transfer (ET; S₁→CT), back electron transfer (BET; S₁←CT), and charge recombination (CR) to lower-energy states (CT→S₀ and CT→T₁). 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 ≥ 100) can be obtained when ΔG_ET ≈ −120 meV. With respect to triplet state formation, we find a relatively low triplet quantum yield (Φ_T < 25%) in CHX but much higher values (Φ_T = 30−98%) in TOL and CB. We identify the PDI^−•-PMI^+• state as a precursor to the T₁ state. Recombination to T₁, rather than to the ground-state S₀, 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.