Red Light-Emitting Thermally-Activated Delayed Fluorescence of Naphthalimide-Phenoxazine Electron Donor-Acceptor Dyad: Time-Resolved Optical and Magnetic Spectroscopic Studies

We prepared an orthogonal compact electron-donor (phenoxazine, PXZ)-acceptor (naphthalimide, NI) dyad (NI-PXZ), to study the photophysics of the thermally-activated delayed fluorescence (TADF), which has a luminescence lifetime of 16.4 ns (99.2 %)/17.0 mu s (0.80 %). A weak charge transfer (CT) absorption band was observed for the dyad, indicating non-negligible electronic coupling between the donor and acceptor at the ground state. Femtosecond transient absorption spectroscopy shows a fast charge separation (CS) (ca. 2.02 similar to 2.72 ps), the majority of the singlet CS state is short-lived, especially in polar solvents (tau(CR) = 10.3 ps in acetonitrile, vs. 1.83 ns in toluene, 7.81 ns in n-hexane). Nanosecond transient absorption spectroscopy detects a long-lived transient species in n-hexane, which is with a mixed triplet local excited state ((LE)-L-3) and charge separated state ((CS)-C-3), the lifetime is 15.4 mu s. In polar solvents, such as tetrahydrofuran and acetonitrile, a neat (CS)-C-3 state was observed, whose lifetimes are 226 ns and 142 ns, respectively. Time-resolved electron paramagnetic resonance (TREPR) spectra indicate the existence of strongly spin exchanged (LE)-L-3/(CT)-C-3 states, with the effective zero field splitting (ZFS) |D| and |E| parameters of 1484 MHz and 109 MHz, respectively, much smaller than that of the native (NI)-N-3 state (2475 and 135 MHz). It is rare but solid experimental evidence that a closely-lying (LE)-L-3 state is crucial for occurrence of TADF and this (LE)-L-3 state is an essential intermediate state to facilitate reverse intersystem crossing in TADF systems.