Fluorescence enhancement of organic aggregates induced by bromine substituents: Heavy-atom effect and vibronic coupling

Organic fluorophores are essential for organic optoelectronic and bioelectronic applications. Heavier halogens have been found to enhance fluorescence of organic aggregates, breaking the traditional view that heavy-atom effect leads to fluorescence quenching. Herein, we proposed that the bromine atoms show a limited acceleration to intersystem crossing (ISC) process, but largely reduce the vibronic coupling of singlet excitons, by inspecting TPE fluorophores and brominated analogs, achieving fluorescence enhancement of bromo-organic aggregates. This is because the lowest singlet excited state S₁ have lower energy than triplet state T₂ and large gap ΔE_ST with T₁, and strong oscillator strength. Moreover, the excited-state dynamics further demonstrate that the bromine substituents hugely decelerate the internal conversion process, and make a slow ISC rates (∼10⁶ s⁻¹), in contrast to the fast radiative transition (∼10⁸ s⁻¹), which is responsible for the abnormal fluorescence enhancement in brominated organic aggregates.