Fluorescence quantum yield molecular glasses

In most investigations in solvents of medium or high viscosity, or in polymers above the glass transition temperature, the fluorescence quantum yields were in fact found to be a power function of the bulk viscosity, with values of the exponent x less than 1 (e.g. for p-N,N-dimethylaminobenzylidenemalononitrile, x = 0.69 in glycerol and 0.43 in dimethylphthalate). This means that the effective viscosity probed by a molecular rotor appears to be less than the bulk viscosity >/ because of free volume effects. 

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