Fusion of pyrene and phenanthrene through 5H-imidazo[1,2-a]azepine scaffolds: structural tuning for fluorescence labeling and bacterial imaging

Abstract

This study investigates polycyclic aromatic fluorophores featuring pyrene and phenanthrene fused through either an imidazo[1,2-a]azepinone or an imidazo[1,2-a]azepinol central ring. These fluorophores were synthesized via a one-pot pyrene-4,5-dione condensation reaction and a subsequent metal hydride reduction. The molecular structures of two representative fluorophores were unequivocally determined by single-crystal X-ray diffraction (SCXRD) analysis, while their electronic absorption and emission properties were comprehensively characterized using UV-vis absorption and fluorescence spectroscopy. Excitation–emission matrix (EEM) fluorescence spectroscopy was employed to gain further insight into their emission behavior as a function of excitation energy. A combination of spectroscopic and density functional theory (DFT) studies revealed that the fluorescence behavior of azepinone-centered fluorophores is governed by a planarization-induced intramolecular charge transfer (PLICT) mechanism, resulting in pronounced solvatofluorochromism but relatively low fluorescence quantum yields. In contrast, azepinol-based fluorophores exhibited significantly higher fluorescence efficiency, albeit with much weaker solvatofluorochromic effects. To assess their potential for bioimaging applications, we investigated the interactions of these fluorophores with bovine serum albumin (BSA) as a model protein using fluorescence titration. All fluorophores bound to BSA, quenching its tryptophan fluorescence at 354 nm while emitting at their characteristic wavelengths. Detailed binding parameters were derived by fitting the data to the Stern–Volmer equation and a 1 : 1 binding isotherm. Molecular docking and molecular dynamics (MD) simulations further elucidated the fluorophore–BSA interactions at the atomic level. Finally, the fluorophores were incubated with two types of bacterial cells to evaluate their fluorescence imaging performance. The results demonstrated promising utility in optical sensing and imaging of bacteria.