Intramolecular electronic relaxation and photoisomerization processes in the isolated azabenzene molecules pyridine, pyrazine and pyrimidine

Abstract

For the three azabenzenes pyridine, pyrazine and pyrimidine, quantum yields for fluorescence and intersystem crossing have been measured in the vapour phase as a function of the excitation energy, ranging from the zero-point level of S₁(n, π*) to the higher vibrational levels of S₂(π, π*). The picosecond fluorescence lifetimes have also been measured by exciting the molecules at various wavelengths. For all three azabenzenes a particularly fast non-radiative decay channel has been found. The appearance of this new decay channel has also been demonstrated by direct measurement of the picosecond lifetime. In the lower excitation-energy region of the S₁â†�S₀ transition the intersystem-crossing process is dominant, whereas above the excitation-energy threshold the non-radiative transition is governed by this decay channel. These new decay channels in azabenzenes may be related to photoisomerization processes, which are known to proceed by irradiation of u.v. light corresponding to the S₂â†�S₀ absorption transition. The discussion is developed in relation to “channel three” in the parent hydrocarbon, benzene.