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Synthesis and Characterization of Novel Discotic Liquid Crystal Porphyrins for Organic Photovoltaics

Zhou, Xiaoli
http://rave.ohiolink.edu/etdc/view?acc_num=kent1231463813

Abstract Details

2009, PHD, Kent State University, College of Arts and Sciences / Chemical Physics.
The ability to control molecular self-assembly by non-covalent interactions is a major driving force in the bottom-up nanofabrication of molecular devices. Large π-conjugated discotic liquid crystals (LCs) capable of being uniformly aligned represent a striking example of self-assembly mainly driven by π-π interactions. Such discotic LCs are viewed as a new generation of organic semiconductors due to the ability to carry charges more efficiently than conventional conjugated polymers, and provide tremendous opportunities as well as challenges in fundamental science that could open the door for applications such as photovoltaics, light-emitting diodes, and field effect transistors. The alignment control of discotic LCs is a crucial hinge on which device performance hangs. This dissertation focuses on the improvement of the homeotropic nature of porphyrin through modification of the porphyrin molecule itself for organic photovotaics. Firstly, nonfluorinated target porphyrins having a hexagonal liquid crystal phase were synthesized and characterized. These materials can be spontaneously homeotropically aligned by controlling the cooling rate and the thickness of thin films. The preliminary photovoltaic result using the synthesized nonfluorinated porphyrin as an electron donor is promising. Secondly, porphyrins with partial chain perfluorination were synthesized which show an exceptionally enhanced tendency to self-assemble into uniformly homeotropic alignment. The interest behind the design of new discotic liquid crystals with partially perfluorinated chains mainly results from perfluoroalkyl group having more rigidity, lower van der Waals forces and larger steric bulk than the corresponding hydrogenated analogs. These differences greatly reduce intercolumnar interactions and enhance the stability of columnar phase of the perfluorinated molecules. Strong evidence through polarized optical microscopy together with synchrotron X-ray diffraction confirmed the ordered nanostructure. To the best of our knowledge, these are the first fluorinated LC porphyrins capable of being homeotropically and homogenously aligned to be reported as well as the first partially perfluoroalkylated LC porphyrins to be synthesized. Interestingly, the intermediate acid of the fluorinated porphyrin exhibits a very interesting sequence of cubic and hexagonal columnar phases, whereas its methyl ester and its nonfluorinated analogue do not show a liquid crystalline phase. To the best of our knowledge, this is the first compound with the general structure of 3,4,5-tris(partially perfluorinated alkoxy)benzoic acid that exhibits a cubic phase. In order to investigate the effect of metalation on the alignment, phase behavior and charge mobility, zinc, copper and magnesium metal complexes were synthesized and characterized. The thermal behavior of all the metalloporphyrins was studied by polarized optical microscopy and differential scanning calorimetry. Unique optical properties were obtained for both the copper and magnesium complexes. In addition to improving homeotropic alignment, the effect of chirality in the columnar liquid crystal system was studied for potential application in nonlinear optics. Porphyrins with a chiral moiety were synthesized and characterized. Two of them seem to exhibit liquid crystal phases and have optical properties that are similar to those of the previous synthesized porphyrins. Further characterization and verification of the phase behavior for the chiral porphyrins is needed to ascertain their relevance in device application.
Quan Li (Advisor)
Engineering; Organic Chemistry
Photovoltaic; organic synthesis; characterization; discotic liquid crystal; porphyrin

Recommended Citations

Citations

  • Zhou, X. (2009). Synthesis and Characterization of Novel Discotic Liquid Crystal Porphyrins for Organic Photovoltaics [Doctoral dissertation, Kent State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=kent1231463813

    APA Style (7th edition)

  • Zhou, Xiaoli. Synthesis and Characterization of Novel Discotic Liquid Crystal Porphyrins for Organic Photovoltaics. 2009. Kent State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=kent1231463813.

    MLA Style (8th edition)

  • Zhou, Xiaoli. "Synthesis and Characterization of Novel Discotic Liquid Crystal Porphyrins for Organic Photovoltaics." Doctoral dissertation, Kent State University, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=kent1231463813

    Chicago Manual of Style (17th edition)

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This open access ETD is published by Kent State University and OhioLINK.