Effect of Chirality on Ultrafast Triplet Exciton Formation in Electron Donor-Acceptor Cocrystals

Triplet excitons formed in organic donor-acceptor cocrystals are desirable for solar energy, optoelectronic, and quantum information applications, and recent work on chirality-induced spin selectivity (CISS) provides an indication that chirality may influence triplet exciton formation in potentially useful ways. Here, we investigate the effect of a chiral electron acceptor on the charge transfer dynamics and triplet exciton formation in a series of donor-acceptor cocrystals, each having a coronene donor paired with one of three acceptors: achiral N,N-bis(3'-pentyl)perylene-(3,4:9,10)-bis(dicarboximide) and two enantiomeric chiral PDIs, N,N'-bis((R)- or (S)-3'-sec-butyl)perylene-(3,4:9,10)-bis(dicarboximide). X-ray diffraction of the three cocrystals confirms that they have nearly identical structures. Transient absorption microscopy shows that photoexcitation of the cocrystals results in charge-transfer (CT) state formation in <0.3 ps followed by triplet exciton formation in ∼100 ps, with yields of 38 ± 2, 51 ± 5, and 50 ± 6% for the achiral, (R)-, and (S)-cocrystals, respectively. Time-resolved electron paramagnetic resonance spectroscopy confirms that the triplet excitons form by spin-orbit CT intersystem crossing. The increase in triplet yield of the cocrystals with the chiral acceptor relative to that of the achiral acceptor is attributed to CISS increasing the triplet character of the CT state.