Characterization
of the Reversible Intersystem Crossing
Dynamics of Organic Photocatalysts Using Transient Absorption Spectroscopy
and Time-Resolved Fluorescence Spectroscopy
posted on 2023-12-14, 19:35authored byWilliam Whitaker, Igor V. Sazanovich, Yonghwan Kwon, Woojin Jeon, Min Sang Kwon, Andrew J. Orr-Ewing
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-dimethylformamide (DMF) solutions, using transient absorption
spectroscopy and time-resolved fluorescence spectroscopy. These OPCs
are benzophenones with either carbazole (2Cz-BP and 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 behavior 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. Behavior
of this type is representative of aggregation-induced emission luminogens
(AIEgens).