Semiempirical Theory of Excimer Luminescence. II. Comparison with Previous Theories and Consideration of the Transition Probability and the Stability of the Excimer Triplet State

The semiempirical theory of excimer luminescence in the Pariser–Parr–Pople scheme [Paper I, J. Chem. Phys. 48, 2589 (1968)] is shown to be formally equivalent to, but much simpler than, the theories based on the molecular exciton–charge resonance concept. On the basis of an assumption that excimers possess $D_{\text{2h}}$ equilibrium conformation, the question of the stability of the excimer triplet state and the thermal distortion of the excimer as the possible source of dipole‐allowed character of excimer fluorescence is examined. It is shown that excimer triplet states of many polycyclic hydrocarbons are very likely unstable with respect to dissociation into monomer triplet and monomer ground state, and that thermal excitation of torsional distortion can account for the observed radiative lifetime of excimer fluorescence in naphthalene.