UV-induced transformations in matrix-isolated 6-methoxyindole

The UV-induced transformations of 6-methoxyindole isolated in both xenon and argon matrixes are discussed. Experimental evidence about the role played by the N-H dissociation channel in the photochemistry of the compound is provided, demonstrating easy formation of the 6-methoxy-indolyl radical plus H atom as primary photoproducts. The two radicals were found to recombine after annealing of the matrixes to a higher temperature, to yield a new prototropic tautomer of the original compound. Other two photoinduced processes were also observed: (1) isomerization between the two conformers of 6-methoxyindole, which was found to be selective in relation to the excitation wavelength and also sensitive to the matrix media; (2) formation of 5-methyl- and 7-methyl-6-indolones, which takes place via O-CH3 dissociation and initial formation of 6-indoxyl and methyl radicals that promptly recombine, within the primarily occupied matrix cage, to produce the two observed indolones. The different observed photoprocesses are mechanistically interpreted with support from quantum chemical calculations. The wavelength selectivity for the isomerization/decomposition pathways was achieved by using narrowband (spectral width = 0.2 cm(-1)) UV-light. Detection and characterization of the different photoproducts was accomplished by using infrared probing, complemented by theoretical calculations. (C) 2016 Elsevier B.V. All rights reserved.