Radiationless Paths in the Diazines

Abstract

Fluorescence was observed from pyrazine, pyrimidine, and pyridazine and their substituted derivatives in both liquid and rigid solutions. The fluorescence was assigned as emission from the allowed singlet (n, π*) excited state. Fluorescence and phosphorescence quantum yields of pyrazine, pyrimidine, and pyridazine, their deuterated analogs, and their substituted derivatives were measured by comparison of their emission with a standard substance of known quantum yield. Intersystem crossing yields were measured by Stephenson and Hammond by a chemical quenching method. The results of these measurements are presented in Table I. The rates of the radiationless processes, internal conversion, and intersystem crossing were calculated from the quantum-yield data. In all cases, internal conversion was found to have the fastest rate and was the process responsible for the weak fluorescence intensity in the diazines. From the deuteration and other substituent effects on the quantum yields, it was concluded that internal conversion proceeds from the allowed singlet excited (n, π*) state to a lower forbidden singlet excited (n, π*) state. Intersystem crossing between the lowest triplet state and the ground state of pyrazine and pyrimidine was found to be noncompetitive with the radiative rates. In the ortho-diazine, a different situation was observed to exist. In the latter case, deactivation of the triplet state was attributed to an intersystem crossing process to a low-lying excited singlet state. Intersystem crossing rates were shown to be largely dependent on factors other than the first-order electronic perturbation matrix element.