Singlet states, and fluorescence

The medium in which a species is dissolved or on which it is adsorbed may exert considerable influence on the intensity and wavelength of the fluorescence. Polar materials such as alcohols or esters frequently increase the intensity of the fluorescence relative to non-polar hydrocarbon solvents. The solvent environment often prevents or inhibits intersystem crossing to a triplet state in favour of excitation to a singlet state and fluorescence, while in many cases the opposite is true. The dielectric constant of solvents has been shown to influence the fluorescence intensity and wavelength maxima of some compounds [33,34]. Fig.2.9 shows the effect of solvent dielectric constant on the fluorescence intensity of DNS-phenol, while Table 2.4 shows the corresponding effect on the fluorescence wavelength [34]. For DNS-phenol, solvents of low dielectric constants result in the most intense fluorescence and shift the wavelength maxima to lower values. 

Excited Singlet State and Fluorescence Spectral Properties... 

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