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

Halogenation with heavy atoms like bromine and iodine is expected to enhance the inter-system crossing and thus quench the fluorescence intensity and lifetime. Though iodo-derivative of salicylideneimine-boron compound (SB) shows such behaviors, the bromo-derivative exhibits unexpectedly enhanced fluorescence quantum yield and lifetime. Through the ultrafast spectroscopic measurements , we propose that an incorporation of halogen in SB restrict the structural relaxation, and prevents the molecule to adopt a geometry needed for rapid excited state relaxation through a conical intersection • Organo-boron compound with a D–A–BF 2 framework is synthesized, and its excited-state relaxation dynamics is studied. • Despite most organo-boron compounds being good emitters, the synthesized compound is nearly non-fluorescent. • Iodo-derivative shows further reduction in fluorescence, while bromo-derivative unexpectedly exhibits an enhancement. • Halogen incorporation restricts relaxation, preventing the geometry needed for rapid excited-state decay. 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–BF 2 (D–A–BF 2 ) 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 S 1 and S 0 states, which facilitates the rapid non-radiative decay from the S 1 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.