Vertically π-expanded coumarin – synthesis via the Scholl reaction and photophysical properties

Abstract

A short and efficient access to a unique type of π-expanded coumarin is achieved. The strategic placement of naphthalene at the 4-position of coumarin allowed us to fuse these two moieties via aromatic dehydrogenation under Scholl conditions. The intriguing optical properties of this π-expanded coumarin are discussed on the basis of quantum chemical calculations. The fluorescence quantum yield (∼20%) is significantly higher than that obtained for the classical 7-hydroxycoumarin. The ratio of emission versus radiationless deactivation is governed by the following factors: decrease in the oscillator strength of the SS transition (vs. perylene), low yield of intersystem crossing and strong internal conversion originating from the activity of the number of vibronic states.