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

Abstract

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.

Absorption and emission of light in red emissive carbon nanodots

“…The structural rigidity of the molecule prevents energy loss through non radiative rotational and vibrational relaxation processes, which occur in less rigid or freely rotating functional groups. [48][49][50] The nitrogen-based heterocycles or amine functionalized compounds undergo extensive intersystem crossing, excited state predissociation and internal conversion, which reduce the uorescence intensity. 51,52 Upon aggregation, such processes are greatly hindered and increase the uorescence intensity.…”

Blue TADF Emitters Based on Indenocarbazole Derivatives with High Photoluminescence and Electroluminescence Efficiencies

“…showing clear broadening and red-shifted emission with the increasing solvent polarity. 37 PL emissions and the shift in different solvents both imply that InCz23DMeTz has the strongest ICT state and the lowest singlet energy. Energy gap between S 1 and T 1 ; g) Calculated S 1 and T 1 energy gaps by TDDFT.…”

Donor (HO-) and Acceptor (Nitro-) Substituted BODIPY as a Solvent Turn-on Fluorescence Dye

“…Exploring solvent effects on excited-state dynamics enables one to understand the fluorescence properties. [25][26][27] Report by Hu's group indicates that the nitrophenyl BODIPY remained quenched due to photoinduced electron transfer process from BODIPY to the acceptors. [28] Contrary to their result in which little or no attention was given to solvent polarity and the environment of the BODIPY-nitrophenyl acceptor conjugate.…”