The decay of triplet pyrazine and pyrazine-D4 in supersonic jets: Isotope effects

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Abstract
We have measured the intersystem crossing (ISC) rates of optically excited triplet pyrazine‐d4 in supersonic expansion. ISC rates ranging from 3.3×102 to 1.3×103 were found in the 1154 cm−1 range of excess vibrational energy above the T1 origin. These rates were substantially lower than the ISC rate of pyrazine. Our measurements were accompanied by model calculations of the ISC rates of pyrazine and pyrazine‐d4. According to this model, certain vibrational modes, which undergo large frequency reduction in the excited state, regulate the strong vibrational energy dependence of the T1→S0 ISC of pyrazine. The large frequency changes result from the interaction of the near lying 3nπ* and 3ππ* states (the ‘‘proximity effect’’). These calculations account quantitatively for the excess vibrational energy dependence of the ISC rates in pyrazine, as well as for the isotopic substitutional effect.