The lowest singlet and triplet excited states of pyrazine

Abstract

The phosphoresence, fluorescence and absorption spectra of pyrazine- h 4 and - d 4 have been obtained at 4°K in a benzene matrix. For comparison, those of the isotopically mixed crystal pyrazine- h 4 in d 4 were also taken. All these spectra show extremely sharp and well-resolved lines and reveal detailed vibronic structure. The coincidence of origins of absorption and emission shows that the lowest singlet state is an allowed transition, properly designated as 1B 3u ← 1A 1g . The forbidden component 1 B 2 g , predicted by both “exciton” and MO theories to be below the allowed component, must lie higher. Its exact location still remains uncertain. The phosphorescence spectrum, when compared with the singlet-triplet excitation spectrum, indicates that the lowest triplet state is also symmetry allowed, showing a strong 0-0 band and the coincidence of origins of absorption and emission. In accordance with previous work, the triplet state is designated as 3 B 3 u . The vibronic structure of the phosphorescence spectrum is very complicated. The first work on the analysis of this spectrum concluded that a simple progression of ν 6 a exists. Under the high resolution attainable in the present experiments, the supposed ν 6 a progression proves to have a composite structure, starting from the second member of the progression. Not only is the ν 9 a hydrogen-bending mode present as shown by the appearance of the CD bending mode in the d 4 spectrum, but a band assigned as 2 ν 6 b was also identified. The latter's anomalous intensity in the phosphorescence spectrum is interpreted as due to the Fermi resonance with 2 ν 6 a and ν 9 a bands. To help resolve the present controversy over the origin of the phosphoresence spectrum observed in crystalline pyrazine, detailed vibrational analyses of the emission spectra were made. The fluorescence spectrum has essentially the same vibronic structure as the phosphorescence spectrum.