The mechanism of photochemical CO or CS bond cleavage in aryl (thio) ethers

Abstract

By photochemically induced 1H nuclear polarization ( 1H-CIDNP) spectroscopy, CO or CS bond cleavage is confirmed as the primary step in the photolysis of cyanomethyl-1-naphthyletther ( 1Aa), cyanomethyl-2-naphthylether ( 1Ba), 2-methylallyl-1-naphthylether ( 1Ab), 1,1-dimethylallyl-2-naphthylthioether ( 1Ce), 1,1-dimethylallyl-2-pyrenylether ( 1Dc) and 1,1-dimethylallyl-3-fluoranthenylether ( 1Ec). The polarization observed in the diamagnetic products formed via the primary aryloxy or naphthylthiyl and cyanomethyl-or methyl-substituted allyl radicals respectively is used to determine the spin multiplicity of the excited precursor states. Both the lowest excited singlet and triplet states are found to be photoreactive. These findings are supported by experiments using radical scavengers, triplet sensitizers or triplet quenchers. The kinetic constants of deactivation of the excited states are derived from the quantum yields, fluorescence decay times and laser flash spectroscopy of the transient triplets and radicals. Except for the rate constant of bond dissociation, these are very similar to those of related photostable methylaryl(thio)ethers. We conclude that bond cleavage is caused by a thermally activated crossing from bonding S 1 (ππ ∗ and T 1 (ππ ∗ states into dissociative πσ ∗ states in full support of an earlier theoretically study. The experimental conditions determine whether the singlet or triplet channel dominates. The spin densities of the radicals involved and the CO bond dissociation enthalpies are computed at the semiempirical AM1 level and are correlated with the experimentally observed product distributions and disappearance quantum yields.