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

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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>τ</mml:mi> <mml:mrow><mml:mi>D</mml:mi> <mml:mi>F</mml:mi></mml:mrow> </mml:msub> <mml:annotation>${{\tau }_{{\rm D}{\rm F}}}$</mml:annotation> </mml:semantics> </mml:math> =5.9 μs for NI-PSeZ and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>τ</mml:mi> <mml:mrow><mml:mi>D</mml:mi> <mml:mi>F</mml:mi></mml:mrow> </mml:msub> <mml:annotation>${{\tau }_{{\rm D}{\rm F}}}$</mml:annotation> </mml:semantics> </mml:math> =16.5 μs for NI-PTZ-2Br, respectively) are longer than the heavy atom-free counterpart NI-PTZ ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>τ</mml:mi> <mml:mrow><mml:mi>D</mml:mi> <mml:mi>F</mml:mi></mml:mrow> </mml:msub> <mml:annotation>${{\tau }_{{\rm D}{\rm F}}}$</mml:annotation> </mml:semantics> </mml:math> =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).