Heavy-Atom Effect on the Thermally Activated Delayed Fluorescence Property of Naphthalimide-Phenoselenazine Electron Donor-Acceptor Dyads: Electron Transfer and Intersystem Crossing

Abstract

We prepared two electron donor-acceptor dyads based on naphthalimide (NI) as electron acceptor and 10H-phenoxazine (PXZ) or 10H-phenoselenazine (PSeZ) as electron donor. Both NI-PXZ and NI-PSeZ show the thermally activated delayed fluorescence (TADF) property. The purpose of this study is to unveil the heavy atom effect on the TADF property, which was believed to enhance the reverse intersystem crossing (rISC), thus more dark 3 CS and 3 LE state will be transformed to the emissive 1 CT state, so that the TADF property can be enhanced. Steady state UV-Vis absorption and fluorescence study show negligible interaction between the donor and acceptor at ground state (S0), yet charge transfer (CT) emission was observed, indicating interaction between the radical anion and radical cation of the CS state. We didn't observe shortened delayed fluorescence lifetime for the NI-PSeZ (τPF = 14.9 ns, τDF = 91.1 μs) as compared to the analogues of NI-PXZ (τPF = 24.3 ns, τDF = 57.8 μs) and the previously reported NI-PTZ (τPF = 11.9 ns, τDF = 82.1 μs), therefore the heavy atom effect for the rISC is not significant. Moreover, femtosecond transient absorption spectroscopy only captures the 1 CS state as the final species in nonpolar solvent (CHX). Nanosecond transient absorption spectra show the low-lying 3 NI state in non-polar solvent (τT = 41.3 μs), admixture of 3 NI state and 3 CS state (τT = 23.5 μs) in solvent with intermediate polarity, and only 3 CS state (τT = 0.8 μs) in polar solvent was observed. Together with femtosecond/nanosecond spectra findings, these observations support the SOCT-ISC mechanism. The results further show the rISC is not enhanced by the heavy atom effect, for NI-PSeZ, it is krISC = 7.0 × 10 4 s -1 , in comparison, it is krISC = 1.3 × 10 5 s -1 for NI-PXZ, and krISC = 1.4 × 10 7 s -1 for NI-PTZ. Time-resolved electron paramagnetic resonance (TREPR) spectral study show that the localized triplet state ( 3 NI) is the last triplet state for the dyads in frozen solution at 80 K, and based on the selective population of the three sublevels of the T1 state, spin orbit charge transfer ISC may contribute to the formation of the triplet states.