Photophysics and Photoreactivity of Cross-conjugated Enediynyl Aggregates: Applications to Multi-parametric Sensing of Microheterogeneity and Reversible Fluorescence Switching

Abstract

Fluorescent aggregates find wide-spread applications in modern optoelectronics. Here, we study the photophysics of organic fluorophores composed of benzene, pyrene, naphthalene, and anthracene moieties connected via a Y-shaped enediyne π-spacer in water-acetonitrile solvent mixtures and solid forms. The fluorophores show a predominant aggregate emission band, which is ca. 4000 cm−1 red-shifted compared to their locally excited (LE) emission energies in water-acetonitrile. The aggregate emission is however blue-shifted compared to an intramolecular charge transfer (ICT) emission band. The interplay of the LE, ICT, and aggregate emissive states depends on peripheral aromatic units, electronic substituents and microenvironments. The LE to aggregate emission spectral shift is exploited to sensitively probe micro-heterogeneity in bile salts, which are complemented by fluorescence intensity and fluorescence anisotropy changes. The solution phase aggregate emissions closely resemble the solid state aggregate emission bands, which are attributed to excimer states. The aggregate fluorescence states in the solids are altered by external stimuli and reversibly switched multiple of cycles. The aggregates undergo photoreactions in mixed-aqueous solvents, but they remain unreactive in solid powder forms.