Luminescence spectra and lifetimes of tris-(2,2′-bipyridine)ruthenium(II) dichloride and potassium hexacyanochromate(III) in polymer films and hydrocarbon mulls at approximately 77 K: evaluation of alternative host media for studying unstable species in photochemical reaction pathways originating from non-volatile substrates

Abstract

Emission, polarized emission and luminescence lifetime measurements were carried out for [Ru(2,2′-bipyridine) 3]Cl 2 in poly(vinyl alcohol) films, Nujol mulls and poly(propylene oxide) mulls at approximately 77 K in order to evaluate the degree of molecular isolation afforded for substrates dispersed in these media compared with that in gas matrices at approximately 12 K. Structured emission and excitation spectra were obtained which showed band maxima shifts consistent with solvent effects. Some re-absorption effects were noted with poly(vinyl alcohol) films at higher concentrations, but no such re-absorption was encountered for Nujol and poly(propylene oxide) mulls. The excitation spectra and phosphorescence lifetimes of [Ru(2,2′-bipyridine) 3]Cl 2 in poly(vinyl alcohol) films showed a concentration dependence indicative of short-range quenching which was analogous to that found for C 6H 6 in rare gas matrices. The limiting phosphorescence lifetimes in poly(vinyl alcohol) films at 298 K (emission maximum at 610 nm), 77 K (emission maximum at 580 nm) and 77 K (emission maximum at 625 nm) were 1.14, 3.15 and 3.15 μs respectively. Single exponential decay curves for [Ru(2,2′-bipyridine) 3]Cl 2 were obtained beyond the limiting concentration (approximately 1 mg to 250 mg of poly(vinyl alcohol), suggesting that the photochemistry of substrates dispersed in poly(vinyl alcohol) films will remain unchanged but that quenching processes may reduce some quantum yields. The spectral resolution achieved with [Ru(2,2′-bipyridine) 3]Cl 2 in Nujol and poly(propylene oxide) mulls was remarkable given the simplicity of the sample preparation technique. The phosphorescence decay profiles corresponded to multiexponential decay profiles, as expected for the dispersion of microcrystalline substrates compared with the molecular dispersion achieved with poly(vinyl alcohol) films. Studies with K 3[Cr(CN) 6] in Nujol and poly(propylene oxide) mulls at approximately 77 K afforded well-resolved emission and excitation spectra with no concentration dependence. It is suggested that more use should be made of polymer films and various mulls, together with organic glasses, alkali halide discs and other media, to study unstable transients in photochemical reaction pathways at approximately 12–298 K, especially where the precursors are non-volatile and are not amenable to studies by the gas matrix isolation technique.