Effect of tert-Butylation on the Photophysics of Thermally Activated Delayed Fluorescence Emitters

Thermally activated delayed fluorescence (TADF) emitters potentially can provide organic light-emitting diodes with 100% internal quantum efficiency by harvesting triplet excitons. Generally, TADF emitters are small molecules that are not applicable for solution processability. The addition of tert-butyl groups to the periphery of TADF emitters has proven to improve their solubility in various organic solvents, reduce aggregation-induced quenching, and enhance the photoluminescence quantum yield (PLQY). This article studies the photophysical influence of the tert-butyl group attached to an emitter with a carbazole acceptor and a triazine donor. The resulting t3CzTrz-F is a blue–green TADF emitter, in which the addition of a tert-butyl group increases the rate of reverse intersystem crossing (rISC), while simultaneously decreasing the nonradiative decay rate substantially. In addition, dilution of t3CzTrz-F in a host matrix in film results in an enhanced PLQY, which is associated with a decrease in the nonradiative decay constant, while there is no change in the rISC rate. Through a solid-state NMR study, the change in rISC and nonradiative rate upon tert-butylation by enlarged intermolecular spacing and reduced vibrational and rotational freedom is rationalized, resulting in improved photophysical performance.