Heterocyclic quinol-type fluorophores.: Part 1.: Synthesis of new benzofurano[3,2-b] naphthoquinol derivatives and their photophysical properties in solution and in the crystalline state

Isomeric pairs of novel benzofuranonaphthoquinol-type fluorophores (2a-2c and 3a-3c) have been synthesized and their absorption and fluorescence characteristics in solution and in the crystalline state have been studied. Big differences in the absorption and fluorescence spectra were observed: the quinols 2a-2c exhibit much stronger absorption and fluorescence intensities than the quinols 3a-3c in solution. However, the two isomeric quinols exhibit similar fluorescence intensities in the crystalline state. Semi-empirical molecular orbital calculations (AM1 and INDO/S) and X-ray diffraction analysis have been carried out to elucidate the differences in the photophysical properties of the two isomers (2 and 3) both in solution and in the crystalline state. On the basis of the results of calculations and the X-ray crystal structures, the relations between the observed photophysical properties and the chemical and crystal structures of the quinol-type fluorophores are discussed.