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

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 the 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,pi* 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 sigma,sigma* 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 cannot be excluded.