Oscillator strength, substituent effect

In general, for benzyl and other arylmethyl radicals there is a large spectral red shift between the observed absorption maxima of the ground-state radicals and the emission maxima for the excited states. The origin of this shift lies in the low oscillator strength for the D0 —> Z), transition which renders it nearly unobservable for most of the radicals studied. Thus, the observed absorption is due to the Dq — Dn transition. Excitation into this band partially leads to relaxation to the >, state followed by emission. The effect of substituents on the emission has been studied extensively and has shown that substitution usually leads to a spectral red-shift. 

There are also effects observed from the symmetry being reduced. The degenerate benzene states split in all chromophores, with the gap being smallest for toluene. The oscillator strength of the lowest dark state increases when the symmetry of the benzene molecule is lost due to substituents. The effect is strongest in 4-hydroxytoluene. Quite interestingly the lowest dark state in benzene becomes the absorbing/fluorescent state in the chromophores present in the amino acids. 

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