MOLECULAR-STRUCTURE AND PHOTOCHEMISTRY OF (E)-ETHYL 3-(2-INDOLYL)PROPENOATE AND (Z)-ETHYL 3-(2-INDOLYL)PROPENOATE - GROUND-STATE CONFORMATIONAL CONTROL OF PHOTOCHEMICAL BEHAVIOR AND ONE-WAY E-]Z PHOTOISOMERIZATION

The molecular structure, electronic spectra, and photoisomerization of (E)- and (Z)-ethyl 3-(2-indolyl)-propenoate, two methylated indole de rivatives, and their N,N-dimethylamide analog have been investigated. The E ester exists in the ground state as a mixture of anti and syn ro tational isomers. The spectroscopic and photochemical behaviors of the individual anti and syn conformers were characterized with the assist ance of comparisons with the behavior of the methylated indole derivat ives. The major anti conformer of the E ester absorbs and emits at sho rter wavelength than the minor syn conformer. The rate constant for si nglet slate isomerization of the anti conformer is substantially large r than that of the syn conformer, resulting in a shorter singlet lifet ime and smaller fluorescence quantum yield for the anti conformer. The behavior of the E amide in both the ground and excited slates is simi lar to that of the ester. The Z isomers of the ester and amide possess a relatively strong intramolecular hydrogen bond. Their singlet state s are weakly fluorescent and photoisomerize inefficiently in nonpolar solvents. Thus photostationary states highly enriched in the Z isomers are obtained in nonpolar solvents. The reel-shifted, structureless em ission observed upon irradiating the Z amide in an EPA or methylcycloh exane glass at 77 K is attributed to an excited state tautomer formed via intramolecular hydrogen transfer.