Synthesis and Characterization of Siloxane-terminated Liquid Crystals and Photochromic Fulgide Dopants for Liquid Crystal Photonics Applications

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Li, Li
liquid crystal , photochromic , siloxane , fulgide , photonics
The goal of this thesis is to design both liquid crystal hosts and photochromic dopants terminated with short siloxane oligomers in order to produce homogeneous liquid crystal mixtures containing a photochromic component for liquid crystal photoswitches. Some of the liquid crystals terminated with short siloxane oligomers were also found to form 'de Vries'-type SmA phases, which minimize the formation of chevrons and zigzag defects that severely degrade the quality of electro-optic devices. In the first part of the thesis, siloxane-terminated phenylpyrimidine liquid crystals were synthesized and characterized. They form SmC phases and, in some cases, SmA phases. Addition of a terminal halogen substituent on the alkoxy side-chain broadens the SmA temperature range. It was shown that combining a structural element that promotes the formation of a SmC phase with one that promotes the formation of a SmA phase in a mesogen with a 2-phenylpyrimidine core results in a maximum layer contraction of 1.6% for 2-(4-(11-(1,1,1,3,3,5,5-heptamethyltrisiloxanyl)undecyloxy)phenyl)-5-(1-chlorooctyloxy)pyrimidine (33a), which may be considered a ‘de Vries’ material. As an extension of the result obtained in the first project, siloxane-terminated biphenyl benzoate and phenyl benzoate liquid crystals were synthesized and their phase behavior studied. They form SmC phases and, in some cases, SmA and SmI phases. Addition of a terminal chloro substituent on alkoxy side-chain results in a broader SmA temperature range and, in some cases, also affects the nature of the mesophase formed. The compound 4-[6-(1,1,1,3,3,5,5-heptamethyltrisiloxanyl)hexanoyloxy]phenyl 4-(8-chlorooctyloxy)benzoate (40b) is characterized by a maximum layer shrinkage of only 1.7%, which may be considered to possess some ‘de Vries’ character. Finally, siloxane-terminated fulgide derivatives were designed as photochromic dopants for liquid crystal photoswitches. Compound 41 was designed and synthesized and proved to be an oil at room temperature. Solid fulgide precusors 42, 43a and 44b were then assessed as photochromic dopants in terms of fast switching and high quantum yield requirements of a photoswitch, and their photophysical properties were studied in solution. The isomer composition at the photostationary state suggests that the competition from E-Z isomerization is likely too high for compounds 42, 43a and 44b to be suitable as photochromic dopants for photoswitches.
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