MOLECULAR-STRUCTURE AND PHOTOCHEMISTRY OF (E)-2-(2-(2-PYRIDYL)ETHENYL)INDOLE AND (Z)-2-(2-(2-PYRIDYL)ETHENYL)INDOLE - A CASE OF HYDROGEN-BOND DEPENDENT ONE-WAY PHOTOISOMERIZATION

The molecular structure, electronic spectra, and photoisomerization of (E)- and (Z)-2-(2-(2-pyridyl)ethenyl)indole have been investigated. T he E isomer exists in the ground state as a mixture of two rotational isomers, whereas the Z isomer exists predominantly as a single conform er due to the presence of an intramolecular hydrogen bond. The structu re and energy of the configurational and conformational isomers have b een investigated by means of NMR spectroscopy and molecular mechanics. In the case of the E isomer, the less polar conformer is favored in n onpolar solvents and the more polar conformer in polar solvents. The m ore polar conformer has a low barrier for singlet state isomerization and thus has a short singlet lifetime and high isomerization quantum y ield. The less polar conformer has a longer singlet lifetime and lower quantum yield for photoisomerization. The N-methyl derivative of the E isomer exists as a single rotamer with photochemical behavior simila r to that of the long-lived rotamer. The hydrogen-bonded conformer of the Z isomer does not undergo photoisomerization due to either the occ urrence of rapid nonradiative decay or a hydrogen bond dependent barri er for twisting. As a consequence, irradiation of the E isomer results in one-way E --> Z photoisomerization in both polar and nonpolar solv ents. A minor conformer of the Z isomer may be responsible for the obs ervation of weak fluorescence and inefficient photoisomerization in po lar solvents. The N-methyl derivative of the Z isomer undergoes photoi somerization.