Luminescence and photochemistry of some tervalent chromium complexes.
Creators: name: Laver, James Leonard
Publisher: name: University Of Tasmania
Year: 2023

description: A number of investigations have been carried out into different aspects of the luminescence and photochemistry of tervalent chromium complexes over a temperature range from near liquid helium to room temperature. A literature review has been made of previous studies in this field. From the review it is evident that although the luminescence properties of many individual complexes have been studied, no systematic examination of the excited state properties of series of complex salts with different coordinating ligands or different anions has been made. In the studies reported here, the relationship between luminescence and photochemical properties and chemical constitution has been examined from a number of points of view. Firstly, the temperature dependence of the emission intensity of a series of microcrystalline tervalent chromium complexes with oxygen--; nitrogen and carbon-coordinated ligands has been investigated. The correlation observed between the degree of temperature dependence. and the nature of the coordinating ligand has been explained in terms of' differing positions of the electronic energy levels of the complexes leading to variation in the rate of non-radiative deactivation of the 2E energy . level through repopulation of the 4T2 level. Changes in the, temperature dependence of the luminescence lifetimes of the complexes with different ligands also have been examined and have been found to parallel those of the emission intensities. This result has been shown to support the explanation proposed above and to indicate that when the 4T2 level is thermally accessible from the 2E level, the thermal repopulation process 2E-4T2 is of major importance in the non-radiative relaxation of the 2 E level and thus in determining the emission characteristics of the complexes at room temperature. Secondly, the result of changes both outside and within the coordination sphere on the electronic absorption and emission spectra of trisethylenediamine complexes of chromium has been examined. The variation in the low temperature emission spectra of the [Cren 3]3+ species with different anions has been studied and the effect of disubstitution to give- the lower symmetry cis- or trans-bisethylene- diamine complexes has been investigated through examination of the reflectance and low temperature emission spectra of these salts. The emission spectra of the complexes at 7K and the temperature dependence of the intensity distribution of the emission bands are reported for the first time. The theory required for the assignment of the spectra the cis-complexes has. been developed and the assignments of other authors for the spectra of the trans-complexes have been critically discussed. Finally, the effect of variation of the anion on the photochemistry of the [Cren3]3+ species has been investigated. A systematic study of the photochemistry of the series in the solid state has been made in order to resolve current conflicting theories in the literature regarding the decomposition process. The effect of temperature on the photo-- decomposition of the complexes and the likely reaction products have been discussed and a reaction mechanism involving a thermally unstable reaction intermediate has been proposed.