Twisted Acceptors in the Design of Deep-Blue TADF Emitters: Crucial Role of the Excited-State Relaxation on the Photophysics of Methyl Substituted s-Triphenyltriazine Derivatives
Among the most popular approaches recently applied to design of blue thermally activated delayed fluorescence (TADF) emitters, the one utilizing alkyl substitution distorting acceptor or donor fragments seems to have very high potential and wide applicability to organic materials. Here, we take a first detailed systematic look on how such a distortion affects singlet/triplet states, (reverse) intersystem crossing rates, (non)radiative deactivations, and intermolecular interactions in donor-acceptor TADF emitters on the example of six new emitters with unplanar s-triaryltriazine acceptor. Our results evidence that despite frequently used approach, emissive features of such twisted molecules cannot be explained using their crystal or ground-state geometries due to excited-state structural relaxation and planarization. Likewise, determination of ΔEST from fluorescence/phosphorescence spectra is not reliable. Moreover, the discussed approach enables deep-blue TADF only in highly viscous medium inhibiting excited-state planarization. Consequently, we designed an efficient (deep)-blue TADF emitter with highest rISC rate among the investigated compounds in condensed phase. Its highly twisted structure provides high excited-states’ energies, small gap between them, inhibits nonradiative processes. This enables high PLQY of 77% and 60% of deep-blue TADF with CIE coordinates (0.16, 0.16) and (0.15, 0.06) in PMMA films and crystals, respectively; and 17.6% EQEmax in a blue OLED device.
- This article is part of the themed collection: Journal of Materials Chemistry C HOT Papers