Date of Award

Spring 1999

Document Type

Dissertation - Restricted

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Kincaid, James R.

Second Advisor

Yi, Chae S.

Third Advisor

McKinney, Michael A.

Abstract

One of the most important goals of current chemical research is the development of a molecular system which is capable of capturing sunlight and converting the energy into useful chemical fuels. The zeolite-entrapped polypyridine complexes of divalent ruthenium hold promise as efficient photocatalysts for net charge separation and such efficiencies are further enhanced by organized incorporation of donor and acceptor components. This research work deals with the construction and spectroscopic investigation of zeolite-entrapped organized molecular assemblies which may be useful in the development of solar energy conversion schemes.

A bis-terpyridine complex of ruthenium (II) (Ru(tpy)2 2+ ) has been prepared in zeolite Y supercages and characterized by electronic absorption, electronic emission, and resonance Raman spectroscopy. In free solution this complex is practically non-luminescent, having a very short excited state lifetime (250 ps) at room temperature. However, entrapment within the zeolite supercage results in dramatic increases in emission intensity and excited state lifetime (140 ns) at room temperature. The observed temperature dependence of the excited-state lifetime has been modeled by a kinetic equation with two thermal terms corresponding to the so-called fourth 3 MLCT state and ligand field state (LF), respectively. It is shown that the increased lifetime of the entrapped complex results from zeolite-induced destabilization of the LF state, a conclusion which is in agreement with results obtained for a number of other zeolite-entrapped ruthenium(II) polypyridine complexes.

The heteroleptic ruthenium(II) complex Ru(bpy)2 (dpp) 2+ (where bpy = 2,2' -bipyridine and dpp = 2,3-bis(2-pyridyl)pyrazine) has been prepared in the supercages of Y-zeolite and characterized by diffuse reflectance, electronic absorption, electronic emission and resonance Raman (RR) spectroscopy, as well as lifetime measurements. The spectral results confirm the identity of the entrapped complex. The excited state lifetime measurement shows no dramatic changes upon entrapment within the zeolite supercages. Temperature dependent lifetime measurements indicate that the excited state decays via two thermally accessible upper states.

An organized molecular assembly composed of two ruthenium polypyridine complexes, Ru(bpy)2 (bpz)2+ and Ru(bpy)2 (H 2 O)22+ (where bpy = 2, 2' -bipyridine and bpz = 2, 2' -bipyrazine) has been prepared in the adjacent supercages of Y-zeolite. This material has been characterized by diffuse reflectance, electronic absorption, electronic emission and resonance Raman (RR) spectroscopy, as well as lifetime measurements. Photoinduced electron transfer from Z-Ru(bpy) 2 (bpz)2+ to methyl viologen followed by reductive quenching of the oxidized sensitizer by the adjacent cage donor complex is evidenced by diffuse reflectance spectroscopy.

Resonance Raman (RR) and Time-resolved Resonance Raman (TR3 ) spectra are reported for the complexes of divalent ruthenium with the ligands 4, 4' -bipyrimidine (bpm) and 2,2' -bipyridine (bpy) ie., Ru(bpm)32+ and Ru(bpy)2(bpm) 2+ . Ground state RR studies of the latter, employing excitation lines at 413.1, 457.9 and 514.5 nm, permit assignment of an electronic absorption band maximizing at 428.5 nm to a Ru(II)[arrow right]bpy MLCT transition, while the maximum observed at 516.0 run is assigned to the Ru(II)[arrow right]bpm charge transfer band. The TR 3 studies, employing the third harmonic (354.7 nm) of a 10 nsec pulsed Nd-YAG laser, provide definitive evidence for selective population of the bpm-localized excited state for the heteroleptic complex, Ru(bpy)2 (bpm)2+ .

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