Fabrication And Photoresponsive Properties Of Weak-interaction Soft Matter Based On Cholesterol-linked Azobenzene Derivatives

Since first proposed by Pierre-Gilles de Gennes, the concept of’soft matter’has come up for over two decades. Compared to the traditional’hard matter’such as metal, ceramic and semiconductor, soft matter is well known for its large response triggered by weak stimulus. In this dissertation, a series of cholesterol-linked azobenzene derivatives were synthesized and used to farbricate photoresponsive soft matter such as organogel and blue phase liquid crystal. The self-assembly structure and photoresponsive behaviours of the two systems were studied in detail. The contents of this dissertation are as follows:1. A series of new symmetric dicholesterol-linked azobenzene gelators with different spacer lengths have been synthesized. The compounds with spacers of zero, two or six methylene units are denoted as DCAZOO, DCAZO2and DCAZO6, respectively. A gelation test reveals that a subtle change in the length of the spacer can produce a dramatic change in the gelation behavior of the compounds. DCAZO2obtains the minimum gelation concentration among the three gelators. For cyclopentanone gel of DCAZO2, the reversible gel-sol transitions by irradiation with UV and visible light are investigated by UV-vis absorption and circular dichroism (CD) spectra, SEM, TEM and XRD analyses. Upon UV irradiation of the gel, trans-cis photoisomerization of the azobenzene groups occurs, the change in molecular polarity leads to the breaking of van der Waals interactions, resulting in the gel-sol transition. The gel can be recovered by the reverse cis-trans photoisomerization after the exposure to visible light. SEM, TEM and XRD studies reveal that the gelator molecules self-assemble into one-dimensional fibers with diameters of50-100nm in an anticlockwise direction, which further crossed-linked to form three-dimensional networks.2. By introducing methanol as a co-solvent into cyclopentanone, solvent effects on structure, photoresponse and speed of gelation of a dicholesterol-linked azobenzene organogel DCAZO2have been investigated. Optical microscopy (OM) and Scanning electron microscopy (SEM) images reveal that the gel fibers form macroscopic aggregates by adding methanol. Minimum gelation concentration (MGC) of the gels decreases with increasing methanol content, while the gel-to-sol transition temperature (Tgel) decreases firstly and increases afterwards. For photoisomerization (corresponding to gel-sol transition), the first stage rate constant kg1of the gels decreases with increasing methanol content. The photoisomerization process of the gel with5%methanol is different from the other five samples, whose first stage rate constant kg1is greater than its second stage rate constant kg. By increasing methanol content from0%to30%, gelation time (corresponding to sol-gel transition) of both heated gel and UV-visible light irradiated gel decreases from7hours to5minutes and6hours to0minute (before visible light irradiation is accomplished), respectively. Solubility parameters and Teas plots of the gelator and mixed solvents are systematically calculated to estimate the gelator-solvent interaction. It is found that gelation requires that the gelator-solvent interaction be neither too strong nor too weak, otherwise soultion or precipitate are formed.3. A photoresponsive azobenzene molecule DCAZO2with two cholesteryl groups linked to both sides of the azobenzene group is doped in a mixture of nematic liquid crystal E7and chiral dopant S811(61.9wt%E7,36.1wt%S811and2.0wt%DCAZO2). Cooled from isotropic phase to33.0℃, chiral nematic liquid crystal (N*LC) was formed in the sample and then the temperature was kept unchanged at33.0℃. UV light irradiation induces the trans-cis photoisomerization and thus an obvious phase transition. When the azobenzene groups isomerize to a cis saturated state, the UV light was turned off and the white light was turned on at the same time. The bent-shaped cis isomer then turns back to the planar trans isomer gradually. A blue-green platelet texture representing cubic blue phase (BP) was observed and the size of the platelets was increased along with the cis-trans isomerization. UV-vis absorption spectra indicates that the photoinduced BP exists when the isomerization degree is between79%to18%, and further cis-trans isomerization change BP back into N*LC. The large geometric structure of the cholesteryl groups and the large bent angle θ of the cis isomer are supposed to be responsible for the interesting result.4. A series of cholesterol-linked azobenzene derivatives with different spacer lengths have been synthesized and doped in a mixture of nematic liquid crystal E7and chiral dopant S811. Cooled from isotropic phase, blue phase (BP) was found in the samples doped with CAZO6. Increasing the doping ratio from1.3wt%to6.0wt%, the interactions between CAZO6and the host liquid crystal increase as well. However,when the doping concentration further increases to8.0wt%, the miscibility of CAZO6decreases and it crystallised and separated out from the host liquid crystal. UV light irradiation induces the trans-cis photoisomerization and thus an obvious phase transition. Textures of blue-green platelets with clear boundaries representing BP I were found in the samples with4.0wt%to6.0wt%CAZO6.