Intraligand photoreactions II: Photochemistry of Pentakis(methylisocyanide) (pyridine-4-aldehyde-4-nitrophenyl-hydrazone)ruthenium(II) tetrafluoroborate

Abstract

A parallel investigation of the photochemistry of [Ru(CNCH 3) 5(pyridine-4-aldehyde-4-nitrophenylhydrazone)](BF 4) 2 and of the organic ligand was carried out at various excitation wavelengths in different solvents. The intraligand syn—anti isomerization was the only photoreaction observed in the complex; it was found that the quantum yields decrease with the excitation wavelength and with solvent polarity and remain unaltered in the presence of triplet quenchers. The only effects produced by ruthenium coordination were a decrease in the quantum yields of isomerization and an increase in the fluorescence quantum yields. The syn—anti isomerization sensitized by 2′-acetonaphthone occurred with almost the same quantum yields in the free and the coordinated ligand. The thermal back reaction anti → syn was observed and the kinetic data were calculated A mechanism which involves rotation in the excited states via an adiabatic pathway was proposed for the direct photoisomerization of the free and bound ligand, taking into account the effects of the irradiation wavelengths, the solvents and the coordination on the quantum yields of isomerization and fluorescence.