The Photophysics of Naphthalimide‐Phenoselenazine Electron Donor‐Acceptor Dyads: Revisiting the Heavy‐Atom Effect in Thermally Activated Delayed Fluorescence

Abstract

We prepared thermally activated delayed fluorescence (TADF) emitter dyads, NI‐PTZ, NI‐PTZ‐2Br and NI‐PSeZ, with naphthalimide (NI) as electron acceptor and 10H‐phenothiazine (PTZ) or 10H‐phenoselenazine (PSeZ) as electron donor to study the heavy‐atom effect on the intersystem crossing (ISC) and reverse ISC (rISC) in the TADF emitters. The delayed fluorescence lifetimes of the dyads containing heavy atoms ( =5.9 μs for NI‐PSeZ and =16.5 μs for NI‐PTZ‐2Br, respectively) are longer than the heavy atom‐free counterpart NI‐PTZ ( =2.0 μs). Nanosecond transient absorption (ns‐TA) spectral study and the time‐resolved electron paramagnetic resonance (TREPR) spectra show the presence of both 3LE and 3CS states. These findings represent solid experimental evidences for the spin‐vibronic coupling mechanism of TADF. Moreover, the ns‐TA spectra show that the heavy atoms don't have a significant effect since the lifetime of the triplet transient species (1.3 μs for NI‐PTZ) is not shortened in their presence (4.5 μs for NI‐PSeZ and 5.3 μs for NI‐PTZ‐2Br). These results show that the previously claimed heavy‐atom effect on rISC and TADF is not a universal principle. The femtosecond transient absorption (fs‐TA) spectra of the compounds indicate the occurrence of fast charge separation within 1–2 ps, and the charge recombination is slow (>4 ns)