Some notes on fluorescence intensity

The fluorescent emission spectra for 5 x 10(-6) M p-aminohippuric and p-aminobenzoic acids in mixtures of methyl alcohol and 1,2-propanediol have been determined. The results indicate that at almost invariant dielectric constant the quantum yield of fluorescence is a function of the viscosity of the solvent. The suggestion is made that a collisional quenching mechanism, which involves the rotational diffusion of solvent molecules, is significant in solutions of low viscosity and less so at high viscosities. A prediction, on the basis of the proposed mechanism, of augmentation of the emission spectra of p-aminohippuric acid, after binding to homologous antihapten antibody, is confirmed. A small red shift is also noted at higher viscosities in protein-free solutions or after binding to homologous antibody. It is suggested that, contrary to some interpretations in the literature, a red shift and/or an augmentation of quantum yield of fluorescence may, in specific instances, not be significant of a transfer of the fluorochrome to an environment of lower dielectric constant.