Chemistry and Chemical Biology ETDs

Publication Date

5-29-1970

Abstract

Two experimental spectroscopic investigations spanning the temperature range 3000 K to 20 K were carried out on molecular complexes. The nature of charge-transfer phenomena in transition-metal chelates was studied extensively, and the thermal quenching of the luminescence of terbium (III) chelates was briefly investigated. The experimental methods employed were absorption and luminescence spectroscopy including photoflash excitation for studying transient luminescence decays.

Measurements were performed on three osmium and three ruthenium complexes, all containing bidentate or tridentate heterocyclic aromatics as ligands. The compounds were studied in rigid solvent glasses, in poly(methyl methacrylate) (PMM), in an isomorphic host of tris(2,2'-bipyridine)zinc(II) sulfate, and in the pure solid form. All the osmium and ruthenium complexes were available; no preparations were performed. The complexes all exhibited charge-transfer absorption and strong visible luminescences which were studied between 77 K and 2 K. Intrinsic decay times at low temperatures which exceeded those calculated from 77°K data for all the transition-metal complexes coupled with the extreme temperature dependences of the luminescence spectra of the osmium species firmly established the existence of more than one luminescing state contributing to the charge-transfer luminescence in these molecules, The intrinsic lifetime of tris(2,2•·-bipyridine)ruthenhun(II) sulfate, for example, increased from 13,9 microseconds at 77°K to an extrapolated value of 1.1 milliseconds at o°K,

The growing in of very sharp bands in the 4. 2°K luminescence spectra of tris(2,:2'-bipyridine)osmium(II) sulfate and bis(2,2'2"-terpyridine)osmium(II) iodide more than 50 cm to the high energy side of the normal 77°K emission spectra strongly indicated that spi1~ lattice relaxation was retarded significantly at temperatures below 20°K in the osmium complexes.

The 4,2°K luminescence lifetimes of the osmium complexes did not change when subjected to an external magnetic field of 14,000 gauss. Tris(2,2'-bipyridine)ruthenium(II) sulfate doped in the isomorphic zinc host and tris(4,4'-diphenyl-2,2'-bipyridine)ruthenium(II) chloride in PMM (both D symmetry), however, showed decreases ( ~ 10% ) of their luminescence lifetimes when subjected to the same field. The ili-dicyanobis(2,2'bipyidine)ruthenium(II) chloride in PMM showed a corresponding decrease of about 15%, This additional field dependence of the luminescence lifetime was attributed to the lower symmetry (c2 ) of this system.

A model was developed to describe charge-transfer phenomena ind6 complexes, It relies entirely on group theoretic considerations and estimates of the appropriate interaction energies from experimental data. The model is based on the assumption that charge-transfer a1bsorption in ad metal chelate leaves the metal ion core in a a5 configuration. The excited states are derived, to first-order , from the states of a d5 system, The interaction of the electron promoted to the ligand orbital is considered in second-order, The resultant three lowest electronic excited states (A1, E, and A2 i n n3: ordered in increasing energies) are close in energy in the ruthenium complexes (A1 to E, 6 E=lO cm -1 ; A1 to A2, 6 E=-1) 50 cm and can account for the observed temperature dependence of the charge-transfer luminescence.

Thermally activated luminescence quenching in the two terbium(III) complexes (the tetrakis(benzoylacetonato) and the tris (acetato) l ,10-phenanthroline) was studied between 77°K and J00°K, Arrhenius plots of both the luminescence lifetimes and intensities yielded activation energies comparable to the energy gaps between the 5n4 level of Tb)+ ion and the lowest triplet state of the ligand. These results strongly indicate that thermally activated quenching in these molecules proceeds via the lowest ligand triplet state and that upper state temperature dependent quenching is negligible.

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

Lee Duane Hansen

Third Committee Member

Guido Herman Daub

Fourth Committee Member

Milton Kahn

Comments

Air Force Office of Scientific Research and the University of New Mexico financial assistance

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