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 a nucleophilic center) has been drawn on in the design of two unprecedented synthetic routes towards these atypical xanthene dyes. The syntheses 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 they form low-lying quenching states, some dark and demonstrating a twisted intramolecular charge transfer (TICT) nature, depending on the medium (CHCl3 and water). Furthermore, two fluorophore compounds have been applied for imaging in paraformaldehyde-fixed A549 cells to gain insights into their permeation and localization.