Illuminating Weak Bonds: Non-Classical C–H···Br Hydrogen Bonding and Bright Emission in a BODIPY Fluorophore

Abstract

In this study, we report and thoroughly analyze an instance of non-classical intermolecular C–H···Br hydrogen bonding in a newly synthesized BODIPY-based fluorophore. The compound was fully characterized by high-resolution mass spectrometry (HRMS), NMR spectroscopy (¹H, ¹³C, ¹¹B), and single-crystal X-ray diffraction, which revealed a C–H···Br interaction with a donor–acceptor distance of 2.79 Å. To gain deeper insight into the nature of this interaction, we employed Bader’s quantum theory of atoms in molecules (QTAIM), which confirmed the presence of a bond critical point and topological parameters indicative of a weak but significant non-covalent interaction. Additionally, Hirshfeld surface analysis and 2D fingerprint plots were used to explore the full spectrum of intermolecular contacts. While other interactions such as C–H···π contacts were also identified and discussed, the C–H···Br interaction emerged as the dominant force in shaping the crystal packing. This study presents a rare example of bromine-involved hydrogen bonding and offers a comprehensive analysis of the supramolecular architecture in organoboron fluorophores. To rigorously interpret the origin of the optical behavior and state ordering, we complemented TD-DFT with advanced multireference calculations. The results rationalize the small Stokes through a rigid excited-state potential energy surface and modest geometric relaxation, and they predict large oscillator strengths consistent with the observed high fluorescence quantum yield. For the brominated scaffold, spin–orbit-coupling-inclusive estimates indicate only minor enhancement of intersystem crossing, aligning with the retention of bright fluorescence.