The Py scale of Dong and Winnick

The Py scale is based on the ratio between the intensity of components (0,0) Ii and (0,2) I3 of the fluorescence of monomeric pyrene (3) in various solvents. It was initially established from 95 solvents and spans values from 0.41 for the gas phase to 1.95 for DMSO. This scale is primarily used in biochemical studies, which usually involve fluorescent probes. However, it poses problems arising largely from the difficulty of obtaining precise values of the above-mentioned intensity ratio this has resulted in divergences among Py values determined by different laboratories. One further hindrance is that the mechanism via which low-polar solvents enhance the intensity of symmetry-forbidden vibronic transitions through a reduction in local symmetry is poorly understood. 

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