LAUSR

Thermally activated delayed fluorescence (TADF) compounds with rapid triplet upconversion in solutions frequently yield drastically lowered upconversion rates in solid hosts due to the conformational disorder, resulting in prolonged multiexponential TADF decay profiles. The dispersion of singlet–triplet gaps was shown to be the nature of this unwanted effect, minimized in compounds with more sterically confined molecular structure, though the observation of low solid-state disorder still is scarce. Therefore, here we present the unambiguous realization of rapid single-exponential solid-state TADF decay in the optimized acridine–pyrimidine TADF compound ACRPyr. The compound was designed to have a rigid molecular structure with negligible conformational disorder along with small singlet–triplet energy gaps, leading to rapid solid-state TADF with a lifetime of about 1.76 μs, a fluorescence quantum yield of about 0.67 and an exceptional rISC rate of 5.7 × 106 s−1. Furthermore, the ACRPyr based sky-blue OLED device showed a peak EQE of 14.3% with minor roll-off.