Isomeric Impact of o -Carborane-Decorated Diboron-Embedded Multiresonance TADF Compounds on Photophysical Properties

Isomeric engineering offers an effective approach to finely tune the photophysical properties of luminophores without altering their core molecular framework. Herein, we report two regioisomeric o-carboranyl multiresonance thermally activated delayed fluorescence (MR-TADF) compounds, 7CB-diBON (1) and 3CB-diBON (2), featuring methyl-o-carborane units at distinct sites (the 7- and 3-positions) on a diboron-based MR scaffold. Despite their structural similarity, the two compounds exhibit markedly different photophysical behaviors. In toluene, both compounds show deep-blue emission with high photoluminescence quantum yields (PLQYs) of 79-81% and narrow full widths at half-maximum of 14-15 nm. In the solid state, however, compound 2 outperforms compound 1, displaying a higher PLQY, greater resistance to PL quenching, and a faster reverse intersystem crossing. Electrochemical studies reveal elevated HOMO and LUMO energy levels for compound 2. Theoretical analyses confirm MR-core-localized frontier orbitals and significantly reduced low-frequency vibrational modes for both compounds, contributing to their narrowband emission. Moreover, compound 2 displays a smaller singlet-triplet energy gap and slightly enhanced spin-orbit coupling, which together contribute to more efficient exciton dynamics. These findings highlight the critical impact of o-carborane substitution position on excited-state properties and demonstrate isomeric control as an effective strategy for optimizing the performance of o-carboranyl MR-TADF emitters.