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. The E isomer exists in the ground state as a mixture of two rotational isomers, whereas the Z isomer exists predominantly as a single conformer due to the presence of an intramolecular hydrogen bond. The structure and energy of the configurational and conformational isomers have been investigated by means of NMR spectroscopy and molecular mechanics. In the case of the E isomer, the less polar conformer is favored in nonpolar solvents and the more polar conformer in polar solvents. The more polar conformer has a low barrier for singlet state isomerization and thus has a short singlet lifetime and high isomerization quantum yield. 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 similar to that of the long-lived rotamer. The hydrogen-bonded conformer of the Z isomer does not undergo photoisomerization due to either the occurrence of rapid nonradiative decay or a hydrogen bond dependent barrier for twisting. As a consequence, irradiation of the E isomer results in one-way E --> Z photoisomerization in both polar and nonpolar solvents. A minor conformer of the Z isomer may be responsible for the observation of weak fluorescence and inefficient photoisomerization in polar solvents. The N-methyl derivative of the Z isomer undergoes photoisomerization.