Amino-containing donor-acceptor biaryls with lactone bridging units as three-state halochromic pi-expanded coumarins

Abstract

This report describes the synthesis and study of two new amino-containing donor-acceptor lactone-bridged biaryls as pi-expanded, or benzannulated, coumarins based on the 6H-benzo[c]chromen-6-one (benzo[c]coumarin) structural unit. These fluorescent and planar compounds exhibit strong intramolecular charge transfer (ICT) through the conjugated biaryl unit via a diethylamino donor on one arene and either a nitro or cyano acceptor unit on the opposing arene. The ICT of these systems can be reversibly altered via two different stimuli: 1) addition of a strong base (hydroxide) results in nucleophilic cleavage of the lactone bridge which dramatically weakens ICT by disrupting conjugation between the individual arenes and 2) addition of a strong acid protonates the diethylamino donor unit, disrupting its ability to donate, thereby causing ICT to be attenuated. The presence of strong acid or base results in dramatic changes in optical absorption and emission properties of these systems which can be readily observed both spectroscopically and visually. Ultimately compounds containing the benzo[c]coumarin structural unit with amine donors could act as three-state halochromic molecular switches and dyes.