Structural features and photophysical behavior of carborane-appended BODIPY dyes

A series of carborane-appended BODIPY derivatives (compounds 4 - 8 ) were synthesized and investigated to understand the influence of carborane substitution on molecular conformation, electronic structure, and fluorescence behavior. The compounds were prepared using established synthetic routes, including Sonogashira and Suzuki cross-coupling, followed by dipyrromethane-based BODIPY formation and carborane incorporation. Density functional theory (DFT) calculations at the B3LYP/6-31G(d,p) level with solvent effects modeled via CPCM revealed that ortho- and meta -carborane substitutions (compounds 5 and 6 ) led to nearly planar geometries and partial delocalization of the LUMO onto adjacent phenyl rings, yet showed lower fluorescence quantum yields due to efficient photoinduced electron transfer (PET). In contrast, control compound 4 and compound 8 exhibited larger dihedral angles that disrupted π-conjugation but retained higher quantum yields, attributed to reduced PET. UV-Vis absorption and fluorescence spectra showed minimal shifts across all derivatives, with similar HOMO-LUMO gaps (2.24-2.92 eV) and emission in the 663–667 nm range. The findings underscore the delicate interplay between molecular conformation and excited-state processes in determining fluorescence efficiency. Notably, the integration of boron-rich carborane clusters with optically active BODIPY scaffolds presents a promising strategy for dual-functional cancer theranostics, including boron neutron capture therapy (BNCT) and photodynamic therapy (PDT), by combining boron delivery with fluorescent imaging capabilities.