Photoinduced Substitution Reactions in Halothiophene Derivatives. Steady State and Laser Flash Photolytic Studies

Article abstract of DOI:10.1021/j100015a020

A photochemical and photophysical study by steady state and pulsed techniques of 5-bromo- and 5-iodo-2-thiophenecarbaldehyde and 2-acetyl-5-iodothiophene in benzene is presented.The quantum yields for the direct and sensitized formation of the phenylthiophenes following the photocleavage of the carbon-halogen bond were determined in the 293-343 K temperature range.The transient species (triplets and free radicals) originated by laser excitation were investigated by laser flash photolysis; their spectral and kinetic properties and their formation quantum yields were determined at room temperature.The halogen atoms were detected through their interactions with teh solvent to form benzene-X^. complexes that were identified by absorption spectra and decay lifetimes.The characteristics of the triplets were also determined in methylcyclohexane/3-methylpentane (9:1, v/v) at 77 K.Phosphorescence measurements at low temperature gave the spectral and kinetic characteristics of the lowest triplet states of halothiophenes.Through the effect of the solvent polarity on the triplet energy (E_T), evidence was obtained of the n,?^* nature of the lowest triplet.Results obtained by steady state and pulsed techniques gave strong indication that the sensitized photosubstitution takes place through an upper triplet state (likely ?,?^* in nature) localized on the carbon-halogen bond.A scheme showing the energies of the involved electronic states is presented.For the direct photoreaction the main route is probably the triplet again, although a singlet pathway cannopt be excluded.