PHOSPHORESCENCE ENHANCEMENT DUE TO EXCITON SPLITTING IN HYDROGEN BONDED 7-AZAINDOLE DIMER.

splits as a result of dimerization. This is a manifestation of the exchange of excitation energy between the two molecules forming the dimer due to exciton interaction. Since the $S_{2}\leftarrow S_{0}$ transition in 7-azaindole is long axis polarized, the high energy exciton state is predicted to be allowed, and the low energy component is predicted to be forbidden in a first order approximation. The coupling of the two exciton states via intramolecular vibrational modes qualitatively accounts for the appearance of the forbidden lower exciton state. Lowest triplet exciton states remain essentially degenerate since spin exchange and dipole-dipole interactions are both small. After excitation to the upper allowed singlet exciton state, rapid internal conversion (I.C.) occurs to the lower forbidden singlet exciton state. The lifetime of this state is expected to be in the millisecond range, due to its small transition probability. Consequently the radiationless intersystem crossing (I.S.) process $(k_{I.S.} \approx 10^{7} \sec^{-1})$ is expected to be the important next step. Highly efficient triplet excitation of the dimer is therefore expected, leading to a great increase in phosphorescence. Fluorescence quenching and dramatic phosphorescence enhancement of 7-azaindole due to dimer formation have been observed.