Rotational modulation of fluorescence decay and quantum beats in expansion-cooled azabenzenes: Small molecule behavior of rotationally cold s-triazine in the lower vibronic levels of the à 1E″ state

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Abstract
The measurements of the fluorescence decay and quantum beats as a function of photoselected rovibronic levels in the 1E″(nπ*) state of jet‐cooled s‐triazine indicate that the intramolecular dynamics of the molecules excited to low rotational states of the 610 level, and still lower rotational states of the 620 level, fall into the small molecule limit. For the 620 (E″) level, changes from the small molecule decay behavior to the intermediate case decay behavior can be induced by rotational excitation of the molecule. Comparison of the fluorescence decay characteristics of s‐triazine with those of pyrimidine and pyrazine indicates that s‐triazine has much stronger singlet–triplet coupling and much smaller triplet level density as compared to the other azabenzenes. The strong singlet–triplet coupling in s‐triazine can be attributed to the efficient spin‐orbit coupling between the 1E″(nπ*) and 3A′(ππ*) states, while the small density of triplet levels is consistent with its small singlet–triplet energy gap, smaller vibrational degrees of freedom and higher symmetry of the molecule, relative to those of the other azabenzenes. The theory of intermediate case radiationless decay provides satisfactory qualitative accounts of all the experimental results.