Controlling the sense of molecular chirality with light: Toward a liquid crystal optical switch
Zhang, Mingbao
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Permalink
https://hdl.handle.net/2142/22717
Description
Title
Controlling the sense of molecular chirality with light: Toward a liquid crystal optical switch
Author(s)
Zhang, Mingbao
Issue Date
1995
Doctoral Committee Chair(s)
Schuster, Gary B.
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Organic
Language
eng
Abstract
This thesis deals with inventing new chiral compounds that are capable of inducing phase changes in liquid crystals upon irradiation. The concept is based on control of chirality with light, a light-induced switching of the stereochemistry of a chiral molecule. This can be done in one of the two ways: photoresolution of a racemate by circularly polarized light based on circular dichroism of chiral compounds and chirochromism, a new class of photochromism based on the photoepimerization of two diastereomers. The stereochemistry, in the former case, is controlled by the handedness of the circularly polarized light and in the latter case, controlled by the irradiating wavelength.
1,1'-Binaphthyl is chiral due to its restricted rotation about the inter-ring C-C bond. A series of 2,2'-bridged 1,1'-binaphthyl derivatives was investigated, with the aim of controlling the enantiomeric excess with light. It is found that the ee for these 1,1'-binaphthyl derivatives obtained by photoresolution is too small to induce a cholesteric phase, nevertheless, their photoracemization switches a cholesteric liquid crystal back to its nematic form which can be readily sensed optically. A series of chirochromic diastereomers of 1,1'-binaphthyl was also designed and synthesized. These 1,1'-binaphthyl derivatives contain a second non-switchable chiral chromophore which interacts with the photoswitchable binaphthyl chromophore resulting in a change in its absorption spectrum. The second chiral chromophore not only affects the photophysical properties of the binaphthyl chromophore, but also its photochemical properties. Both steady-state photolysis and time-resolved laser spectroscopy were employed to examine the photophysics and photochemistry of these potential chirochromic systems. It is found that when the second chiral chromophore is electron deficient, the system is chirochromic and could be developed into a liquid crystal phototrigger.
A series of novel 2,2'-binaphthyl derivatives was also prepared and evaluated as photoswitchable chiral chromophores for chirochromic systems. These binaphthyl derivatives generally have low activation barrier to racemization and do not show optical activity at room temperature, although with substitution at 3,3' and 4,4' positions, the activation barrier can be substantially raised. For example, with phenyl substitution at 3,3' and 4,4' positions, the racemization barrier is high enough to allow its optical resolution by chiral HPLC at room temperature. Its g-values are generally larger than that of 1,1'-binaphthyl derivatives, although its attempted photoresolution was unsuccessful.
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