Characterization of the Reversible Intersystem Crossing Dynamics of Organic Photocatalysts using Transient Absorption Spectroscopy and Time-Resolved Fluorescence Spectroscopy

Thermally activated delayed fluorescence (TADF) emitters are molecules of interest as homogeneous organic photocatalysts (OPCs) for photoredox chemistry. Here, three classes of OPC candidates are studied in dichloromethane (DCM) or N,N-dimethyl formamide (DMF) solutions, using transient absorption spectroscopy and time-resolved fluorescence spectroscopy. These OPCs are benzophenones with either carbazole (2Cz-BP, 2tCz-BP) or phenoxazine / phenothiazine (2PXZ-BP and 2PTZ-BP) appended groups, and the dicyanobenzene derivative 4DP-IPN. Dual lifetimes of the S1 state populations are observed, consistent with reverse intersystem crossing (RISC) and TADF emission. Example fluorescence lifetimes in DCM are (5.18 ± 0.01) ns and (6.22 ± 1.27) μs for 2Cz-BP, (1.38 ± 0.01) ns and (0.32 ± 0.01) μs for 2PXZ-BP, and (2.97 ± 0.01) ns and (62.0 ± 5.8) μs for 4DP-IPN. From ground state bleach recoveries and time-correlated single photon counting measurements, triplet quantum yields in DCM are estimated to be 0.62 ± 0.16, 0.04 ± 0.01, and 0.83 ± 0.02 for 2Cz-BP, 2PXZ-BP and 4DP-IPN respectively. 4DP-IPN displays similar photophysical behaviour to the previously studied OPC 4Cz-IPN. Independent of the choice of solvent, 4DP-IPN, 2Cz-BP and 2tCz-BP are shown to be TADF emitters, whereas emission by 2PXZ-BP and 2PTZ-BP depends on the molecular environment, with TADF emission enhanced in aggregates compared to monomers. Behaviour of this type is representative of aggregation-induced emission luminogens (AIEgens)