2,9-(Di-2'-pyridyl)-4,7-di(t-butyl)carbazole exhibits dual fluorescence in nonpolar and polar, protic and aprotic, solvents. At 293 K, both emissions are very weak and short-lived, whereas lowering the temperature to 90 K increases the quantum yield of the primary florescence by nearly two orders of magnitude; the lower energy emission also becomes stronger. These results are interpreted as a manifestation of intramolecular excited state tautomerization along the pre-existing hydrogen bond. Strong dependence of the radiationless deactivation of the phototautomer on temperature, viscosity, and deuterium substitution was observed. (c) 2006 Elsevier B.V. All rights reserved.