Molecular Structure and Photochemistry of (E)- 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 derivatives, and their N,N-dimethylamide analog have been investigated. The E ester exists in the ground state as a mixture of anti and syn rotational isomers. The spectroscopic and photochemical behaviors of the individual anti and syn conformers were characterized with the assistance of comparisons with the behavior of the methylated indole derivatives. The major anti conformer of the E ester absorbs and emits at shorter wavelength than the minor syn conformer. The rate constant for singlet state isomerization of the anti conformer is substantially larger than that of the syn conformer, resulting in a shorter singlet lifetime and smaller fluorescence quantum yield for the anti conformer. The behavior of the E amide in both the ground and excited states is similar to that of the ester. The Z isomers of the ester and amide possess a relatively strong intramolecular hydrogen bond. Their singlet states are weakly fluorescent and photoisomerize inefficiently in nonpolar solvents. Thus photostationary states highly enriched in the Z isomers are obtained in nonpolar solvents. The red-shifted, structureless emission observed upon irradiating the Z amide in an EPA or methylcyclohexane glass at 77 K is attributed to an excited state tautomer formed via intramolecular hydrogen transfer.