Distinctive tunable photophysical properties of versatile environmentally-sensitive tribranched cyanopyridine fluorophores

Abstract

In the present work, twenty-four environmentally-sensitive cyanopyridine fluorophores bearing pyrene and/ or fluorene with different para-substituted-phenyl moieties that have been previously designed and synthesized by us are studied in depth for their photophysical properties. Initially, the optical performances of the compounds were investigated by employing UV–visible and fluorescence spectroscopic tools in various aprotic and protic solvents. All the compounds exhibited absorption bands between 310 and 452 nm, and emission bands between 454 and 633 nm. High sensitivity emission spectra with solvents of different polarities were recorded and studied. The fluorescence quantum yield (ϕf) increased in solvents of low polarity and decreased on increasing the polarity of solvents. On the other hand, in case of strong electron donating (–NMe2) and strong electron attracting (–CN) substitution, a pronounced increase in Stokes shifts (up to 252 nm, 14250 cm−1) were recorded. Lippert-Mataga and Reichardts correlations, applied for estimating the variation in dipole moments (Δμ), suggested that the emissive state of designed fluorescence 3-cyanopyridine derivatives is of strong ICT character. The aprotic and protic solvents gave a linear plot for the Stokes shifts in a Lippert–Mataga plot, which appeared as two distinct domains in ET(30) scales indicating the presence of hydrogen bondings. It was observed that for compounds 5b – 8b, with (-NMe2) group on the skeleton of phenyl ring, the Lippert-Mataga and Reichardt–Dimroth’s plots deviated from linearity signifying that 5b – 8b molecules were involved in specific interaction with protic solvents.