Effects of methyl substitution on electronic relaxation processes of pyrimidine vapor

Abstract

The pressure and excitation-energy dependence of the fluorescence quantum yields of pyrimidine, 5- and 4-methylpyrimidines and 4,6-dimethylpyrimidine in the vapor phase has been investigated in the pressure range 10 −3–10 2 torr. The results indicate that not only pyrimidine but also its methyl derivatives give both fast and slow fluorescence emissions at low pressure, in conformity with the intermediate case of electronic relaxation. The total fluorescence quantum yields for the 0-0 transition, extrapolated to zero pressure, are 0. 17, 0.0079, 0.0057 and 0.0010 for pyrimidine, 5- and 4-methylpyrimidines and 4,6-dimethylpyrimidine, respectively. The methyl substitution significantly reduces the yield of the slow fluorescence component in essentially the same manner as the increase of excitation energy in the unsubstituted pyrimidine. The decrease of the slow-fluorescence yield resulting from the methyl substitution or the excitation-energy increase is interpreted as due to the increase of the number of triplet vibronic It is shown that there is a fairly good correlation between the number of the effectively coupled triplet levels and the vibrational state density in t