The isoquinolinone derivative 2-methyl-1,4-diphenylbenzo[g] isoquinolin-3(2H)-one (MDP-BIQ) shows dual fluorescence emission with band positions and intensities that depend sensitively on the solvent. We show that this behavior arises from the fact that MDP-BIQ has two valence tautomers, one aromatic and one conjugated but non-aromatic, each of which are separately fluorescent. The aromatic tautomer, which has significant zwitterionic character, is stabilized by trace amounts of hydrogen-bond donors or Lewis acids. The relatively high fluorescence quantum yield of the aromatic tautomer (0.127 versus 2.4 x 10(-3) for the non-aromatic tautomer) makes this and similar molecules outstanding candidates for use in sensors and other optoelectronic applications. (C) 2009 Elsevier B. V. All rights reserved.