Heterocyclic quinol-type fluorophores. Synthesis of novel imidazoanthraquinol derivatives and their photophysical properties in benzene and in the crystalline state

An isomeric pair of novel imidazoanthraquinol fluorophores 2 and 3 exhibiting two tautomeric forms (A and B) that are possible for the imidazole ring have been developed. In order to control the tautomeric form to improve the fluorescence properties, the N-butylated imidazole derivatives 6 and 7 have further been synthesized. The fluorescence intensity of the quinols was in the order 3 ≪ 6 < 7 < 2 in benzene, which is quite different from the order 3 ≪ 2 ≪ 6 < 7 in the crystalline state. To investigate the effect of N-alkylation of imidazole ring on the photophysical properties of the quinol fluorophores in solution and in the crystalline state, we have performed semi-empirical molecular orbital calculations (AM1 and INDO/S) and X-ray crystallographic analysis. On the basis of the results of calculations and the X-ray crystal structures, the N-alkylation effects on the chemical structure of the quinol fluorophores and on their solid-state photophysical properties are discussed.