Excited state proton transfer in 2,9-(di-2′-pyridyl)-4,7-di(t-butyl)carbazole

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.