Unprecedented Coumarin-Pyronin Hybrid Dyes: Synthesis, Fluorescence Properties and Theoretical Calculations

A novel class of rosamine dyes bearing a 7-substituted 4-hydroxycoumarin unit as meso-heteroaryl ring is presented. The latent C-nucleophilic character of 4-hydroxycoumarin derivatives (i.e., their C-3 position as nucleophilic center) has been drawn on in the designing of two unprecedented synthetic routes towards these atypical xanthene dyes. They are based on an effective formal Knoevenagel condensation with either pyronin derivatives or a mixed bis-aryl ether bearing both an aldehyde and a masked phenylogous amine, possibly applicable to a wide range of latent cyclic C-nucleophiles. We also report experimental and theoretical photophysical investigations of these unique coumarin-pyronin hybrid structures and particularly their form low-lying quenching states, some of dark twisted intramolecular charge transfer (TICT) nature, depending on the medium (CHCl₃ and water). Furthermore, two fluorophore compounds 9 and 11 have been applied for imaging in paraformaldehyde-fixed A549 cells to gain insights into their permeation and localization.