Layer-by-Layer Deposited Organic/Inorganic Hybrid Multilayer Films Containing Noncentrosymmetrically Orientated Azobenzene Chromophores

Organic/inorganic hybrid multilayer films with noncentrosymmetrically orientated azobenzene chromophores were fabricated by the sequential deposition of ZrO₂ layers by a surface sol−gel process and subsequent layer-by-layer (LbL) adsorption of the nonlinear optical (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 the negatively charged ZrO₂ and the sulfonate groups of the 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 (I_2ω^1/2) increases with the increase of the azobenzene graft ratio in PAC-azoBNS as the number of deposition cycles of the ZrO₂/PAC-azoBNS/PDDA films remains the same, while the second-order susceptibility of the film decreases with the increase of the azobenzene graft ratio. Furthermore, 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 groups at one end and sulfonate groups at the other end were used. The present method was characterized by its simplicity and flexibility in film preparation, and it is anticipated to be a facile way to fabricate second-order nonlinear optical film materials.