High Reverse Intersystem Crossing Rate Diminishes the Impact of Conformational Disorder Phenomenon in Solid-State TADF

Abstract

Embedding donor–acceptor type thermally activated delayed fluorescence (TADF) molecules in a rigid surrounding lead to structural inhomogeneity, and deteriorating emission decay rates. Designing TADF structures with hampered rotational flexibility between donor and acceptor structural units is shown to lower the conformational disorder. However, in this work, it is shown that it is not always enough. In fact, the negative impact of conformational inhomogeneity may be reduced by lowering the singlet-triplet energy gap (ΔE_ST) and boosting the reverse intersystem crossing (rISC) rate while preserving the same donor-acceptor orientation. In such cases the lower ΔE_ST enables the early triplet upconversion even from the conformers with unfavorably low D-A twist angles, which is not observed in compounds with larger ΔE_ST. In this way, the temporal shifts of prompt and delayed fluorescence are evidently reduced. When the reverse intersystem crossing is inactive at low temperatures, nearly the same fluorescence peak shifts are observed, as expected for compounds with similar molecular geometry. In this way, low ΔE_ST and rapid rISC are shown to be of fundamental importance not only for TADF efficiency but also for the temporal dynamics in the solid-state.