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Date of Award

1996

Document Type

Thesis - Pacific Access Restricted

Degree Name

Master of Science (M.S.)

Department

Physics

First Advisor

Carl E. Wulfman

First Committee Member

Alex Granik

Second Committee Member

Larry O. Spreer

Third Committee Member

Bruce W. Shore

Fourth Committee Member

Richard L. Perry

Abstract

Quantum mechanical calculations were used to examine the polarization of excited singlet states of twisted conjugated systems. This study began as an investigation of systems which are symmetric about a central double bond. Symmetry requires that when twisting about such a bold, there must be a crossing of two low-lying states. At the one election level of theory, this leads to a degeneracy between the ground state and first excited state at 90°of twist. Slight perturbations to the twisted system will mix the two degenerate states and lead to a highly polar condition. This was confirmed on the one-electron level with calculations on several symmetrically twisted conjugated systems.

Further calculations indicated that there were crossings between two low-lying states for twisting about certain non-central double bonds as well. It was hypothesized from the results of simple calculations, that the pairing theorem of alternant hydrocarbons provides a framework that explains where and why these non-symmetry determined crossings occur. In general, the implications of the pairing theorem are that twisting a neutral even-alternant hydrocarbon about any double bond should lead to a crossing of two low-lying states.

Upon further i:tivestigation of the connection between the pairing theorem and twisted excited states, the theorem's predictions were confirmed by MNDO calculations. Furthermore, it was shown that small perturbations applied to these systems when the two states become nearly degenerate led to a dramatic polarization across the twisted double bond.

It was found that there are also highly polarizable twisted excited states whose existence is not predicted by the pairing theorem.

Pages

123

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