Intensity, microenvironment polarity effect

Symmetry-lowering effects of the solvent are referred to as the Ham effect (Ham, 1953 Platt, 1962). Thus, in rigid or fluid solutions, symmetry-forbidden vibrational components of the Bju ( Lb) absorption band of benzene appear with increasing intensity as the polarizability and the polarity of the solvent increase. The fact that for pyrene and 2-methylpyrene the direction of the Lb transition moment is inclined on the average 40° and 20° away from the y axis, respectively, has also been ascribed to a symmetrylowering perturbation by the environment, related to the Ham effect (Lang-kilde et al., 1983). The associated intensification of otherwise weak vibronic peaks in the Lb band can be used for an investigation of microenvironments such as micelles. 

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