Phosphorescence of Crystalline Pyrazine at 4.2°K

Abstract

The high‐resolution phosphorescencespectrum of pyrazine crystals has been recorded and analyzed. The fact that the crystal structure of pyrazine crystal is known and that the ground‐state molecular and lattice vibrations are assigned, made it possible to accurately analyze and assign the observed spectrum. The comparison of the observed spectrum obtained from pyrazine crystals with that obtained from pyrazine in cyclohexane matrix at 4.2°K leaves no doubt that the observed emission from the crystals results from the 3 B 3u ← 1 A g π*→n transition of the pyrazine molecule. The 0,0,0 band in emission is found to be 102 cm−1 lower in energy than the 0,0,0 band in absorption. This might suggest that the emission centers have slightly different environment and thus lower energy than those absorbing the radiation. This difference in environment also leads to the trapping of triplet excitons by pyrazine molecules located at physical defects. The different types of trapping mechanisms are briefly discussed in the light of the observed spectrum. There is a strong coupling between the electronic motion and the lattice modes of b 3g symmetry (the most prominent ones have frequencies of 57 and 82 cm−1). The intensity of the emission of the phononic and phonon‐vibronic bands is greater than that of the vibronic bands. This observation suggests that either the lattice parameters are different in the excited triplet state from those in the ground state or else there is a lattice‐induced emission resulting from the coupling of the different molecular electronic states of pyrazine with lattice vibrations. These two mechanisms of phonon enhancement of the emission are discussed in terms of the site symmetry of the emitting pyrazine molecules at the trapping sites. Spectral evidence is given showing the coupling between the intramolecular C–H in‐plane bending vibrations and the lattice modes. The b 3g lattice modes are found to have lower frequencies when coupled to the C–H in‐plane bending vibration than when it is coupled to the other vibrations. This observed phonon‐vibronic coupling as well as the phonon‐electronic coupling is consistent with the form of the force field proposed for the pyrazine crystal which consists of one term representing an intermolecular hydrogen bonding (C–H···:N) and two intermolecular hydrogen‐hydrogen repulsion terms. A change in the distribution of the hydrogen atoms (as produced by C–H vibrations) or the lone‐pair electron density (as produced in π*→n transition) can give rise to observed effects on lattice parameters and vice versa.