Study on fluorescence properties in solutions and the solid states of three N,O-chelated difluoroboron compounds and their application in latent fingerprint imaging and ink-free writing†

To further promote the development of multifunctional organoboron fluorescent dyes, three different methyl-substituted pyridine-based difluoroboron compounds (1-BF₂, 2-BF₂, and 3-BF₂) were designed and successfully synthesized to explore their photoluminescence properties in solutions and in the solid-state. Results suggested that all three compounds had solvent-dependent behaviors in seven organic solvents, and the solid powder was luminescent under a UV lamp. Furthermore, compounds 1-BF₂ and 3-BF₂ exhibited aggregation-induced emission activities in DMSO/water mixtures, but compound 2-BF₂ displayed an aggregation-caused quenching phenomenon. Importantly, the solid-state powder of compound 1-BF₂ exhibited irreversible mechanofluorochromism due to its packing mode change between the crystalline phase and the amorphous state, according to the XRD diffraction results, which endowed it with ink-free writing characteristics. The abovementioned photophysical properties were explained by time-dependent density functional theory calculations. It was also found that compounds 1-BF₂ and 3-BF₂ could be used for latent fingerprint imaging and could provide preliminary evidence to match personal identity.