Isoelectronic Push-Pull Fluorescent Difluoroborates: Halogen Bonding and Photophysical Properties

The structural and photophysical properties in the halogen bonding environment were thoroughly studied for a newly synthesized series of fluorescent dyes and their model derivatives. The analysis revealed that the ground-state interactions among both series are likewise. The fluorescent dyes have push-pull topology, and there is a low-lying charge-transfer (CT) excited state in their electronic structure. In order to study the effect of intermolecular interactions on the photophysical parameters of the CT excited state, a palette of solvents was used (C₆F₆, C₆F₅Cl, C₆F₅Br, and C₆F₅I). Our studies revealed that the weak halogen bonding between the perfluorohaloarene solvent and the heterocyclic core of the dyes enhances the CT in their excited states. The results also demonstrated that the position of the heterocyclic nitrogen atom in the acceptor core simultaneously controls the directionality of the intermolecular interaction and influences both the emission wavelength and the fluorescence quantum yield. Experimental data were further supported by the results of quantum-chemical calculations. Overall, the study establishes a direct link between the topology of a moiety prone to specific intermolecular interactions and the photophysical properties of fluorescent probes.