Effect of halogenation on the photophysics of salicylideneimine-boron compound: An unusual behaviour with bromination

Abstract

Understanding and controlling intersystem crossing (ISC) is crucial to generate triplet states, which have diverse applications across various fields. A commonly employed strategy to enhance ISC involves the incorporation of heavy atoms within the molecule. However, the introduction of heavy atoms, such as halogens, not only influences the ISC process but also alters the overall photophysical properties of the molecule. In this study, we investigated the photophysics of an organoboron compound (SB) with a donor–acceptor–BF₂ (D–A–BF₂) framework, and explored how halogenation modulates its excited-state dynamics. While organoboron compounds are generally known for their strong luminescence, our parent SB compound exhibited remarkably low fluorescence quantum yield and lifetime. Halogenation with heavy atoms, such as bromine and iodine, was expected to further quench fluorescence, as observed in the iodo derivative of SB (5ISB). Surprisingly, the bromo derivative (5BrSB) shows the opposite effect, with enhanced fluorescence quantum yield and lifetime. Our study suggests that the low fluorescence in the parent SB compound is probably due to the presence of a conical intersection (CI) between the S₁ and S₀ states, which facilitates the rapid non-radiative decay from the S₁ state. The introduction of heavy atoms seems to hinder the structural relaxation process, limiting the molecule’s ability to achieve the geometry required to form the CI. Additionally, the heavy atom effect in 5BrSB is insufficient to substantially enhance spin–orbit coupling (SOC). As a result, an increased fluorescence observed in the bromo-substituted compound.