Simultaneously Strong Charge-Transfer Absorption and Efficient Triplet Formation in Organic Donor-Acceptor Molecules

Molecular triplet excited states play a crucial role in applications such as photocatalysis and photodynamic therapy. Spin-orbit charge-transfer intersystem crossing (SOCT-ISC) in pure organic donor-acceptor molecules offers an effective means to convert singlet charge-transfer states into triplet products without relying on heavy atoms. To date, however, quantitative insights into SOCT-ISC systems remain limited, and it is unclear how various recombination processes compete with the triplet formation channel. Here, we conduct a systematic study of excited-state dynamics in a PBI-4Cz pentad, comprising four carbazole donor units situated at the ortho-positions of a perylene bisimide acceptor, in different environments and under different temperatures. We find that the PBI triplet formation yield in 2-methyltetrahydrofuran solvent increases from 10% to 60% as the temperature decreases from 300 to 80 K, whereas it is temperature-independent (45%) in a poly(methyl methacrylate) matrix. Global analysis of transient absorption kinetics shows that the temperature dependence in solvent arises from more substantial suppression of the singlet recombination channel than the triplet formation channel due to gradually frozen molecular configuration with decreasing temperature. Importantly, we establish that an intermediate donor-acceptor dihedral angle (i.e., neither coplanar nor orthogonal) can simultaneously allow for efficient charge-transfer absorption, ultrafast charge separation, and efficient triplet formation, which allows us to demonstrate red-light-driven photon upconversion using the PBI-4Cz pentad as a sensitizer. These unprecedented insights for SOCT-ISC provide guidelines for the design and control of optical properties and triplet formation yields in pure organic systems.