Radiationless transitions of pyridine, pyrazine and pyrimidine studied by intermolecular energy transfer to biacetyl

Abstract

The intramolecular nonradiative processes of azabenzenes were studied by the use of intermolecular triplet—triplet energy transfers from azabenzenes to biacetyl in the gas phase. The triplet (T 1) lifetimes of the donor molecules were determined by the observation of the rise time of sensitized phosphorescence from biacetyl to be 60μs, more than 0.5 ms and less than 10 μs for pyrazine, pyrimidine and pyridine, respectively. Comparison of the excitation spectra of the sensitized phosphorescence of biacetyl in an azabenzene—biacetyl mixture and the absorption spectra of azabenzenes showed the occurrence of a new channel of nonradiative transition in the 1000 cm −1 excess energy region of S 1 (1 nπ*) in both pyridine- h 5 and - d 5, whereas in pyrazine and pyrimidine gradual decreases of the energy transfer efficiency were found in the 3000 cm −1 excess energy region of S 1 at high pressure. The excitation spectrum of sensitized phosphorescence of biacetyl is very sensitive to pressure in pyrazine-biacetyl, moderately sensitive in pyrimidine—biacetyl and insensitive in pyridine-biacetyl in the pressure range between 10 torr and 0.1 torr. The difference was ascribed to the nature of the intramolecular nonradiative transition and the lifetime of the triplet state of a donor molecule.