Rigidity and Polarity Effects on the Electronic Properties of Two Deep Blue Delayed Fluorescence Emitters

Molecules that undergo reverse intersystem-crossing (RISC), which enables thermally activated delayed fluorescence, represent an important advance in the development of organic-based light-emitting diodes (OLEDs). The current study focuses on two blue-emitting RISC molecules employing carbazole as the donor and benzothiazole or benzoxazole derivative as the acceptor (BTZ/BOX-carbazole (CBZ)). Although the emission maxima of these compounds are deep blue (∼410 nm) in hydrocarbon solvents, their spectra broaden, red shift, and decrease in intensity with even a modest increase in solvent polarity because of their strong charge-transfer (CT) character. These effects are qualitatively predicted from time-dependent density functional theory calculations using the state-specific polarizable continuum model, though the emission spectral shifts are significantly overestimated. The desired blue emission peak of both compounds (∼425 nm) is recovered by rigidifying the environment, either in low-temperature glasses or in room-temperature polymer films, independent of local polarity. The polarity-induced emission red shift is therefore due to the solvent orientational polarizability. The effects of an applied electric field on the spectra (Stark effect) are used to quantify the CT character of the absorbing and emitting states. Significantly less field-induced emission quenching is observed in BOX-CBZ versus that in BTZ-CBZ. Minimizing this effect is important for the performance in the large (1–10 MV) fields present within OLED devices.