Identification of dual conformers of Coumarin 153 under jet-cooled conditions

Coumarin 153, examined under supersonic jet conditions, is found to exist in two conformations. These have the same rotational coherence signals and fluorescence decay times, and they show similar spectral shifts for two different attachment sites in the presence of a single water molecule. However, their vibronic spectra in the low-frequency region are significantly different. Semi-empirical calculations (AM1) suggest structure assignments corresponding to syn and anti conformations of the saturated rings. Coumarin 337, which has a similar ring structure, also reveals two conformers having similar properties to those of Coumarin 153.