Modulating Radiative Efficiency in ortho-Carboranyl Luminophores through Electron-Donating Substituents: Insights from Intramolecular-Charge-Transfer-Based Emissions

Abstract

In this study, we introduce six ortho-carboranyl compounds derived from 9,9-dimethyl-9H-fluorene 1CH‒6CP, each possessing substituents with varying electron-donating capabilities. Their geometric features were comprehensively characterised through single X-ray crystallography. In rigid states (solutions at 77 K, films, aggregates, and crystals), these compounds exhibited intense intramolecular charge-transfer (ICT)-based emissions in the bluish-green spectrum. Notably, the emission quantum efficiency and radiative constant in the crystalline state were proportional to their electronic features, particularly the molecular dipole moment. Furthermore, theoretical modeling of the ground (S₀) and first excited (S₁) state-optimised structures indicated a gradual enhancement in the orbital occupation at each lowest unoccupied molecular orbital (LUMO) level, correlating with an increase in dipole moment. These findings strongly suggest that the ICT-based radiative efficiency in π-conjugated o-carborane is significantly influenced by the electron-donating properties of the substituents attached to the o-carborane.