Award Date

5-1-2013

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Chemistry

First Committee Member

Dong-Chan Lee

Second Committee Member

Kathleen A. Robins

Third Committee Member

David W. Hatchett

Fourth Committee Member

Monika Neda

Number of Pages

93

Abstract

Control of electronic properties in organic semiconductor materials is essential for electro-optical applications such as field-effect transistors, light-emitting diodes, and photovoltaic devices. This work is focused on two systems that highlight different approaches for the manipulation of electronic properties: (I) the development of electron-deficient (n-type) materials by selective lowering of ELUMO and (II) low energy gap materials by controlling both ELUMO and EHOMO

To specifically lower ELUMO, a pyrazine-acene π-platform was extended using electron-withdrawing moieties. These include: pyridine, pyrazine, and benzothiadiazole (system I). From the base pyrazine-acene, the most significant change in ELUMO of 0.83 eV was observed with benzothiadiazole π-extender, while pyridine and pyrazine lowered ELUMO by 0.15 eV and 0.42 eV, respectively. EHOMO was relatively unaffected by these π-extenders.

The mechanism for ELUMO control in this system was illustrated by theoretical evaluation. We found that the LUMO orbital localized on the more electron deficient part of the pyrazine-acene including the π-extenders, while the HOMO orbital localized on the more electron rich portion, away from the π-extenders. This enabled us to achieve specific control of ELUMO depending on the type of π-extenders.

To achieve low energy gap (Egap) materials, structures containing electron rich (donor) thiophene attached to an electron deficient (acceptor) pyrazine-acene π-core were synthesized as system II. The effects of planarity, type of solubilizing side group (i.e. alkyl vs. alkoxy), and type of pyrazine-acene π-core on Egap have been evaluated both experimentally and theoretically. Extension of the π-core with a thiadiazole moiety further enhanced the electron deficiency of the acceptor part of the molecule resulting in a decrease of ELUMO from -3.32 eV to -3.90 eV. An impressive Egap compression of 1.21 eV was achieved with this donor-acceptor configuration, which is rare for a small molecule.

Through systematic theoretical investigation, it was found that the dihedral angle between the pyrazine-acene acceptor and the thiophene donor affects EHOMO rather than ELUMO. This was also supported experimentally with planar and nonplanar systems.

Most of the compounds of system I and II assembled into one-dimensional (1D) fibers with a width of 10 nm to 2µm via organogelation, phase-transfer assembly, or simple-drop casting. This demonstrates the utility of the presented molecular design in generating beneficial self-assemble fibers.

Keywords

rganic semiconductors – Electric properties; Pyrazines; Pyridine; Thiophenes

Disciplines

Chemistry | Organic Chemistry | Semiconductor and Optical Materials

Language

English


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