Strategic electronic property control of self-assemblying pyrazine-acenes

• Main Author: Brownell, Lacie V
• Format: Dissertation
• Language: English
• Subjects: chemistry, organic
• ISBN: 9781303293641; 1303293641

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 E LUMO 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.