Tuning Thermally Activated Delayed Fluorescence Emitter Photophysics through Solvation in the Solid State

Abstract

Solid-state solvation (SSS) is analogous to liquid-phase solvation but occurs within glassy matrices. Organic solutes with singlet charge transfer (1CT) excited states are especially susceptible to solvatochromism. Their 1CT states and photon emission energies decrease when surrounding molecules with sterically unhindered polar moieties reorient to stabilize them. Thermally activated delayed fluorescence (TADF) organic light-emitting diodes feature such solutes as emitters in the solid state, employing efficient reverse intersystem crossing to harvest the majority of electrogenerated triplets. Here we explore the potential of SSS to manipulate not only these emitters’ 1CT states but also, concurrently, their singlet–triplet energy gaps (ΔEST) that control TADF. By solvating the TADF emitter 2PXZ-OXD with progressively increasing concentrations of camphoric anhydride (CA) in a polystyrene film, we find that it is possible to finely tune the emitter’s photophysics. We observe a maximum increase in prompt lifetime and corresponding decrease in delayed lifetime of ∼60%. By contrast, the photoluminescence quantum yield peaks at an intermediate CA concentration, reflecting competition between increasing reverse intersystem crossing yield and decreasing singlet oscillator strength. Our findings demonstrate technologically relevant fine control of emitter photophysical properties, as varying the extent of SSS reveals the convolved evolution of different kinetic rates as a function of the 1CT state energy and ΔEST.