Blue-shifted hydrogen bond—concept

The concept of polarity covers all types of solute-solvent interactions (including hydrogen bonding). Therefore, polarity cannot be characterized by a single parameter. Erroneous interpretation may arise from misunderstandings of basic phenomena. For example, a polarity-dependent probe does not unequivocally indicate a hydrophobic environment whenever a blue-shift of the fluorescence spectrum is observed. It should be emphasized again that solvent (or microenvironment) relaxation should be completed during the lifetime of the excited state for a correct interpretation of the shift in the fluorescence spectrum in terms of polarity. 

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