RADIATIONLESS PROCESSES IN PHOTOCHEMISTRY OF STILBAZOLES AND 1,2-BISPYRIDYLETHYLENES

Reactions of excited states of several stilbazoles and 1,2-bispyridylethylenes have been studied. Quantum efficiencies of fluorescence, intersystem crossing, and internal conversion to the ground state have been determined. Rate constants for these processes were calculated by a comparison of the quantum yields, using the rate constants for fluorescence calculated from absorption spectra. Rate constants for intersystem crossing for several of the azastilbenes (assuming a triplet mechanism for the direct isomerization) are calculated to be quite high even though quantum efficiencies are low. Internal conversion from the excited singlet to the ground state not involving geometric change is shown to be an important process for 2-stilbazole and all of the bispyridylethylenes in contrast to the behavior of excited singlets of stilbene. In this respect the behavior of the azastilbenes resembles that of pyridine, the diazines and other N-heteroaromatics in which radiationless deactivation of the singlet s ate is the major process. Rates of the various deactivation processes depend on nitrogen location. Possible mechanisms for the enhanced internal conversion processes are discussed. © 1969, American Chemical Society. All rights reserved.