Triplet state radiationless transitions in polycyclic hydrocarbons

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Abstract
The zero‐field EPR transitions and spin sublevel intersystem crossing rates for the lowest triplet states of tetracene, perdeuterotetracene, 1,2‐benzanthracene, and 1,2,3,4‐dibenzanthracene have been measured in n‐alkane solvents at 2°K by optical detection of magnetic resonance techniques. The rates of spin sublevel intersystem crossing are primarily nonradiative, and an analysis of these molecules, as well as the triplet state dynamics of several groups of polycyclic hydrocarbons, is presented with conclusions applicable to planar aromatic organic systems in general. Comparison of the triplet state intersystem crossing rates of the polycyclic hydrocarbons suggests that nonradiative transitions for the individual spin sublevels of the lowest triplet state may be qualitatively understood and predicted by a determination of the symmetry‐selected one‐center spin‐orbit coupling mechanism available to each triplet sublevel.