Dissertation/Thesis Abstract

From oligomers to polymers: Theoretical studies of the geometrical, electronic, and vibrational properties of conjugated systems
by Yang, Shujiang, Ph.D., Georgetown University, 2007, 184; 3339910
Abstract (Summary)

Theoretical methods were first screened and validated with available experimental data on conventional conjugated polymers and/or their oligomers, and then they were used to predict and design various novel intrinsic low band gap conducting polymers. We proposed a series of novel types of ladder polymers with conventional conjugated polymers, polyacetylene, polydiacetylene, and polytriacetylene serving as sidepieces, and with conjugated acetylenic blocks (-C≡C-)m serving as crosspieces. The band gap of the so-constructed acetylenic coupled ladder polymers can be reduced to as low as 0.3-0.4 eV without doping, significantly lower than the parent sidepiece polymers. In addition to the popular aromatic→quinonoid transition concept, we also proposed the aromatic-quinonoid matching concept to minimize the band gap through aromatic-quinonoid copolymers.

The recent discovery of very long carbon chain encapsulated within carbon nanotubes were investigated with first principles methods. The combined system is predicted to be metallic with a high density of states at the Fermi level. The bond length alternation of the sp-hybridized carbon chain inside the carbon nanotube can be suppressed through charge transfer and orbital hybridization with the nanotube. The geometry, energetics, and vibrational properties of the isolated finite linear carbon clusters and the infinite carbon chain (polyyne) were also studied with state of the art theoretical methods.

To accurately predict vibrational spectroscopy of extensively conjugated systems has been challenging to computational chemists due to the strong electron-phonon coupling in these systems. Currently available theoretical models failed for this problem, especially for the extremely conjugated system, polyyne. A novel linear/exponential hybrid quantum mechanical force field scaling scheme was applied successfully, yielding highly accurate frequency prediction for various isotopes of polyacetylene in addition to polyyne. The new method can also be extended to other extensively conjugated systems.

Indexing (document details)
Advisor: Kertesz, Miklos
School: Georgetown University
School Location: United States -- District of Columbia
Source: DAI-B 69/12, Dissertation Abstracts International
Subjects: Physical chemistry, Polymer chemistry
Keywords: Conjugated polymers, Low-bandgap, Oligomers, Polymers, Vibrational
Publication Number: 3339910
ISBN: 978-0-549-93958-0
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