Chemistry and Chemical Biology ETDs

Publication Date

1-15-1970

Abstract

To elucidate the paths and rates of energy degradation in transition-metal complexes following photoexcitation, a comprehensive experimental study was carried out on a total of fourteen complexes of RU(II), Os(II), IR(III), and Rh(III). Photoluminescence lifetimes, spectra, wavelength dependences of the quantum yields, and absolute quantum yields were determined. The complexes all contained organic ligands—pyridine, 2,2’bypyridine, 2,2’2”-terpyridine, and 1,10-phenanthroline.

The ruthenium, osmium, and iridium complexes all exhibited charge transfer (CT) phosphorescences with quantum yields of 1-60% and lifetimes of 0.6-11 musec in an alcoholic glass at 77 K. The rhodium complexes gave d-d* phosphorescences with yields in the range 3-20% and lifetimes of 10-500 musec.

Intrinsic lifetimes, radiative rate constants, and quenching rate constants were evaluated. In the series of dihalo- substituted rhodium complexes, a large (factor of ~40) “heavy atom” shortening of the intrinsic lifetimes occurred when the halogen was changed from chlorine to iodine.

Intersystem crossing yields were shown to be ~1 in the transdibromotetra(pyridine)rhodium(III) bromide, tris(2,2’-bibypridine)ruthenium(II) chloride, and the osmium complexes. Also, in this ruthenium complex, the relaxation efficiency of the pi-pi* ligand localized states to the emitting charge transfer triplet is shown to proceed with unit efficiency. Inferences from these data regarding the behavior of other transition metal complexes with unfilled d-shells are discussed. A general rule is propose: In the absence of upper excited state photochemistry, luminescent transition metal complexes with unfilled d shells will exhibit wavelength independent quantum yields and will emit only from the lowest excited state in the molecule or from those states which can achieve a significant Bolzmann population relative to this state.

The CT and d-d* luminescences of these complexes have previously been assigned to spin-forbidden processes. Based on the existing data, extensive arguments are given supporting this assignment. In addition, successful semiempirical spin-orbit coupling models were developed for predicting intrinsic lifetimes:

In the case of the CT emitters a heavy atom perturbed “intensity borrowing” mechanism between the emitting triplet and the lowest energy singlet charge transfer state was employed. Using two semi-empirical spin-orbit coupling matrix elements (one for osmium and one for ruthenium( and absorption and emission data, the intrinsic lifetimes of nine compounds with intrinsic lives ranging from ~10-50 musec are predicted to within 40% of the measured values.

For the halogenated d-d* emitters an average spin-orbit fields concept involving only the metal and the six nearest coordinating atoms was used. This model predicts to within a factor of two the observed forty-fold change in intrinsic lifetimes observed in a halogenated series.

Observed quenching rate constants are discussed qualitatively with the regard to the current theory of radiationless transitions and the available data on aromatics and rare earth compounds. The effect of deuterated solvents on the lifetimes of several complexes is described.

The Strickler-Berg formula for calculating intrinsic lifetimes from absorption and emission data is shown to work very poorly for transition metal complexes exhibiting CT or d-d* phosphorescence.

Details of the construction and calibration of the versatile visible-infrared sensitive (400-1100 nm) spectrofluorimeter used for carrying out the luminescence measurements are given. Experimental techniques are discussed at length.

Because of the importance of absolute quantum yield measurements to the present study, a comprehensive, critical review of the literature on quantum yields covering the period 1920 to mid-1969 comprises about one-third of the dissertation.

Language

English

Document Type

Dissertation

Degree Name

Chemistry

Level of Degree

Doctoral

Department Name

Department of Chemistry and Chemical Biology

First Committee Member (Chair)

Glenn Arthur Crosby

Second Committee Member

Roy Dudley Caton Jr.

Third Committee Member

Nicholas Ernest Vanderborgh

Fourth Committee Member

Raymond N. Castle

Fifth Committee Member

Illegible

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