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Osmium (II) complexes containing organic coordinating ligands 2.2'-bipyridine, 1.10-phenanthroline, 2.2' .2"­terpyridine were studied spectroscopically. Syntheses were carried out by published methods, and each complex was checked by chemical analysis and thin layer chromatography. The complexes were shown to be d6 strong field systems by magnetic susceptibility measurements performed at room temperature in air.

Room temperature absorption spectra in the visible and near-ultraviolet were measured in solvents of different polarities (H2O, MeOH, CH3CN, DMF, CHCl3) and solvent shifts of the band maxima of 0.3 to 3 kK were found. Low temperature (approximately 82-90°K) absorption spectra were taken in rigid glass (EtOH-MeOH 4:1; v/v) solutions in a cryostat especially built for the Cary 14 spectrophotometer. Photoluminescence spectra of the complexes in the same rigid glasses at 77°K were recorded in the visible and near-infrared regions.

Luminescence lifetimes were measured in rigid glasses at 77°K for most of the complexes. The measured lifetimes varied from 4.03 microseconds for [Os(terpy)2]I2 to 0.58 microseconds for [Os(py)2(bipy)2 ]I2. The same trends were found for intrinsic lifetimes calculated from quantum yields and measured lifetimes as those calculated from the overlap of absorption and emission spectral bands. These results were interpreted as indicating that the highest energy emission peak and the first low energy absorption peak represent transitions between the same electronic levels.

Changes in the energy of absorption bands with temperature, pressure, and solvents of different polarities were correlated with a changing-internuclear-distance model.

An equation (E = Q x 10Dq where Q is an experimental parameter for a given series of complexes, 10Dq the calculated average ligand field, and E the energy of emission) was shown to predict the energy of d-d emission for a homologous series of complexes to within 0.3 kK, but was shown to be unsuccessful for two homologous series of molecules displaying electron transfer luminescence.

Criteria for the identification of d-d and electron transfer transitions in both absorption and emission were summarized and evaluated against the experimental data from the complexes studied. The more important criteria identifying electron transfer transitions were found to be the following:

(a) The absorption bands have a molar extinction coefficient greater than 1,000,

(b) There is no apparent correlation of observed emission energies with those predicted by ligand field theory for d-d transitions.

(c) Plots of the transition energies of the absorption bands against solvent polarity (Kosower Z value) are linear.

(d) The peaks in electron transfer luminescence spectra are sharp and the energy separation between peaks is similar (approximately 1.3 kK) when nitrogen-coordinated heterocyclic ligands are present.

(f) The energies of electron transfer absorption and emission bands show the same changes when ligands are varied from complex to complex.

(g) Solvent and temperature dependence studies show that the energies of electron transfer absorption bands of complexes shift differently than d-d transitions.

A model to explicate the differences and similarities among spectra of analogous series of homologous osmium (II) and ruthenium(II) complexes was presented. The model assumes that ligand orbitals in osmium complexes are approximately 1 kK lower in energy than in analogous ruthenium complexes. Splitting, considered to be due to spin-orbit coupling and the ligand field, is 2 kK greater in an osmium complex than in an analogous ruthenium complex. The overall result is that the emitting level of an osmium complex is approximately 3 kK lower in energy than that in an analogous ruthenium complex as observed experimentally. Spin-orbit coupling was discussed and was considered to be important.



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Department of Chemistry and Chemical Biology

First Committee Member (Chair)

Glenn Arthur Crosby

Second Committee Member

Roy Dudley Caton Jr.

Third Committee Member

Raymond N. Castle

Fourth Committee Member

Lee Duane Hansen

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