Beyond the Static Approximation: Assessing the Impact of Conformational and Kinetic Broadening on the Description of TADF Emitters

Abstract

Abstract Thermally activated delayed fluorescence (TADF) is a promising route toward high-efficiency, metal-free organic light-emitting diodes (OLEDs). However, the characterization of TADF kinetics in solid-state thin films is often complicated by pronounced multiexponential photoluminescence decays that prevent standard biexponential modeling. In this work, we introduce the “Gamma-Fit” method, a streamlined analytical framework based on the gamma distribution that accounts for the continuous distribution of decay rates inherent in disordered molecular ensembles. By treating the decay as a result of conformational and kinetic heterogeneity, we accurately extract kinetic parameters for the benchmark emitters 4CzIPN and 5CzBN, as well as a series of novel diphenylamine (DPA)-based systems. Our results reveal that accounting for the local environment in thin films remains an important part in determining OLED efficiency. Our experimental findings are complemented by a computational semiclassical Marcus approach. We evaluate the reliability of this conventional single-conformation rate calculation method and find that its deviations for flexible emitters do not arise from the Marcus approximation itself, but from neglecting the conformational ensemble and spin–vibronic coupling to higher-lying triplet states. A nuclear ensemble treatment that restores both effects recovers an order of magnitude of the RISC rate for the most flexible emitter 3DPA2FBN while accurately reproducing the rate of the rigid benchmark emitter 4CzIPN.