Synthesis of Benzocoumarins and Characterization of Their Photophysical Properties

Abstract

Benzocoumarins (benzochromenones) are pi-extended coumarins and constitute a promising family of photonic materials. Disclosed here are syntheses and characterization of the photophysical properties of a series of benzo[g]coumarins and benzo[f]coumarins that contain both an electron donor and an electron acceptor which are electronically conjugated through the benzocoumarin core. The maximum absorption and emission wavelengths of benzo[g]coumarins are at longer wavelengths than the corresponding benzo[f]coumarins, as the former family has favourable intramolecular charge transfer from the donor to the acceptor moiety due to linear conjugation. The emission behaviour of the benzocoumarins is dependent on solvent due to their dipolar nature. The fluorescence of benzo[f]coumarins that have a dimethylamino donor at the C9 or C7 position is much weaker in polar media such as dimethyl sulfoxide (DMSO) and water compared with that of the corresponding benzo[g]coumarins. Comparison of a benzo[g]coumarin and a benzo[h]coumarin that have the same donor and acceptor groups shows that the Stokes shift of the former is larger than the latter. The benzo[g]coumarin series is highly promising for bioimaging applications because they are known to be two-photon excitable around 900 nm, an optimum biological optical window. The synthetic routes established here provide a basis to obtain functional benzocoumarin derivatives.