On the formation, stabilization and luminescent decay of charged species in photo- or λ-ionized molecular solid solutions

Abstract

Transitory charged species produced at 77 K—either by photoionization of solutes in solid solutions or λ-irradiation of pure rigid solvents—have been studied at the stage of their neutralization by recording the optically stimulated luminescence. Three distinct aspects of separated ion pair formation and decay have been examined. I. The photoionization threshold energies I sol of a solute (TMPD or tryptophan) in various solid solvents have been determined from plots of stimulated luminescence (SL) intensity vs exciting light frequency. Systematic experiments have shown that (a) the more polar the solvent, the lower is I sol, (b) in the case of non-polar matrices, the more globular the solvent molecule, the lower is I sol, and (c) for these non-polar solvents, I sol is 0·–0·9 eV higher in solid solvents than in the corresponding liquids. Besides the formation of separated ion-pairs, the existence of compact ion-pairs seems to be evidence dby a specific spontaneous long-lived luminescence located around 430 nm, between the fluorescence and phosphorescence of the solute. II. The negatively charged species can be identified from the SL excitation spectra. Two types of physically trapped electrons have thus been differentiated in more or less relaxed 3MP and MCH glasses. If optically detrapped, the released electrons may then be captured by 3 methylpentyl- or methylcyclohexyl (R MCH .) radicals. The relative efficiences of R M CH formation vs bleaching wavelengths suggest that the electron attachment is of resonant type. I.N.D.O. calculations indicate that electron capture is not expected for an isolated R MCH . radical and that condensed phase attachment requires a distortion of R MCH . from π to σ structure. III. The dissociative or non-dissociative character of the neutralization process as well as the nature of electronically excited dissociation fragments can be deduced from the SL emission spectra. This has been considered in the case of λ-irradiated light and heavy polycrystalline ices. For both ices, the emission intensity is maximum around 380–390 nm. Since the detrapped electron has only about 1 eV initial kinetic energy, an electronic excitation of OH or OH − as well as temporary H 2O − formation may be disregarded. H 2O lower triplet formed upon H 3O + dissociative neutralization thus remains the most plausible emitting species.