Preparation And Functionalization Of Layer-by-Layer Assembled Multilayer Films Containing Azobenzene Compounds

Since layer-by-layer (LbL) assembly technique was introduced by G. Decher for the construction of layered ultrathin films based on electrostatic interaction in 1991, it has attracted more and more attention of scientists. A few commercial products based on LbL assembled multilayer films appeared in recent years, for example, Contact Lens, Yasa-Sheet, Metal Rubber. The appearance of these products are exciting to scientists because the ultimate goal of research work is to produce useful products for the improvement of people's daily life. In this dissertation we mainly focus on fabrication of functionalized multilayer films by LbL assembly technique.In chapter 1, we introduced a few methods to prepare multilayer film, including Langmuir-Blodgett film technique, self-assembly technique based on chemical adsorption, alternated deposition process and sequential surface sol-gel process. Driving forces for the preparation of multilayer films include electrostatic interaction, hydrogen bonding, coordination bonding, charge transfer, host-guest interaction, molecular recognition, and so on. In general, the driving force for the construction of LbL assembled multilayer films is based on the synergetic interactions of several kinds of interaction, but not only one kind of interaction. We also introduced functionalities of LbL assembled multilayer films, including luminescent film, conductive film, antireflection, antifogging and self-clean film, organic second order nonlinear optical film, separation and permselective film, antibacterial film, and so on. In chapter 2, we synthesized azobenzene-containing polymer PAC-azoBNS with different graft ratio and three small azo molecules MH-azoBNS, CH-azoBNS and THA-azoB. The azobenzene-containing polymer PAC-azoBNS has 3 graft ratios, 29%, 44% and 62% as confirmed by 1HNMR spectroscopy. All azobenzene compounds were characterized by 1HNMR, IR and UV-vis spectroscopies.In chapter 3, we prepared organic/inorganic hybrid multilayer films with noncentrosymmetrically oriental azobenzene chromophores. These kinds of films have potential applications of second order nonlinear optical film materials. In the first section of chapter 3, organic/inorganic hybrid multilayer films with noncentrosymmetrically orientated azobenzene chromophores were fabricated by sequential deposition of ZrO2 layer by surface sol-gel process and subsequent layer-by-layer (LbL) adsorption of NLO-active azobenzene-containing polyanion PAC-azoBNS and poly(diallyldimethylammonium chloride) (PDDA). Noncentrosymmetric orientation of the NLO-active azobenzene chromophores was achieved because of the strong repulsion between negatively charged ZrO2 and sulfonate groups of azobenzene chromophore in PAC-azoBNS. Regular deposition of ZrO2/PAC-azoBNS/PDDA multilayer films was verified by UV-vis absorption spectroscopy and quartz crystal microbalance measurements. Both UV-vis absorption spectroscopy and transmission second harmonic generation (SHG) measurements confirmed the noncentrosymmetric orientation of the azobenzene chromophores in the as-prepared ZrO2/PAC-azoBNS/PDDA multilayer films. The square root of the SHG signal (I2ω1/2) increases with the increase of the azobenzene graft ratio in PAC-azoBNS as the deposition cycle of the ZrO2/PAC-azoBNS/PDDA films is the same. While the second order susceptibility of the film decreases with the increase of the azobenzene graft ratio. The present method was characterized by its simplicity and flexibility in film preparation, and is anticipated to be a facile way to fabricate the second-order nonlinear optical film materials. In the second section of chapter 3, the present method was successfully extended to realize the noncentrosymmetric orientation of azobenzene chromophores in multilayer films when small organic azobenzene compounds with carboxylic acid and/or hydroxyl group at one end and sulfonate group at the other end were used. In the third section of this chapter, we simplified the process to fabricate multilayer films with noncentrosymmetrically oriented azobenzene chromophores by the successful alternative deposition of ZrO2 layer and THA-azoB layer. Our initial results confirmed the noncentrosymmetric orientation of azobenzene chromophores in ZrO2/THA-azoB films. Although further confirmation by second order SHG measurement is still required.In chapter 4, we reported a way to fabricate robust ion-permselective multilayer films. The azobenzene-containing polyanion PAC-azoBNS was alternately assembled with the polycation diazoresin (DAR) to construct photo-cross-linkable multilayer films of PAC-azoBNS/DAR that contain photolabile groups of azobenzene. Upon mild UV irradiation, the interaction between PAC-azoBNS/DAR multilayers was converted from electrostatic interaction to covalent bonds. Because of the free carboxylic acid groups presented in the films, the photo-cross-linked PAC-azoBNS/DAR films with intense UV irradiation, azobenzene groups decompose to produce imine groups, and a photo-cross-linked robust film containing free carboxylic acid and imine groups was fabricated. The resultant film allows the permeation of negatively charged species and meanwhile shows a pH-switchable permselectivity for positively charged species. Because of the covalently cross-linking structure, the photolyzed cross-linked PAC-azoBNS/DAR film shows high reversible switching behavior and has high stability in solution with high ionic strength.