DOI: 10.1002/adma.200702582 Nonlinear Optical Properties of Schiff-Base-Containing Conductive Polymer Films Electro-deposited in Microgravity** By Agostino Pietrangelo, Bryan C. Sih, Britta N. Boden, Zhenwei Wang, Qifeng Li, Keng C. Chou, * Mark J. MacLachlan, * and Michael O. Wolf* Materials with large and rapid nonlinear optical (NLO) res- ponses are required for future optical and photonic technol- ogies that will be used for data transmission and processing. [1] Molecular and polymeric materials are promising candidates as they offer the possibility of tuneable NLO properties coupled with solution processability. It has been found that the hyperpolarizability of materials may be enhanced by extending electron delocalization (conjugation) and by incorporating transition metals with polarizable d-electrons. Moreover, the molecular order imparted by processing significantly affects the NLO properties of the materials. Conjugated polymers have excellent NLO properties [2,3] owing to their fast electronic responses and large NLO coefficients, where the observed hyperpolarizabilities have been linked to the degree of bond alternation in these systems. [4,5] For example, polythiophene (PT) exhibits large x (3) values as a consequence of an extended p-conjugated backbone. [6–8] Because the conjugation length in conducting polymers is dependent on the orientation of the polymeric backbone, small improvements in alignment have been shown to lead to large improvements in NLO response. It has been shown that an enhancement in x (3) is observed when the order parameter of PT thin films is increased by stretching or flow alignment. [9,10] In addition to the inherent hyperpolarizabilities of the individual chromophores, an optimized NLO response is also associated with the relative orientation and structural proper- ties of the overall chromophore assembly. For example in the case of polymer composites, [11] phthalocyanines [12–14] and poly- diacetylenes, [15] significant improvement of their NLO proper- ties is observed when processing is performed in a microgravity environment. The magnitude of the NLO response is directly related to the degree of order in the film. Schiff base metal complexes have been identified as promising NLO materials. [16,17] The NLO response in these compounds is enhanced in the excited state owing to significant metal-to-ligand charge transfer character. [16] The metal also lends environmental and thermal stability to the complexes. As a result, these molecules have large second- and third-order NLO responses and are stable to over 300 8C. Moreover, the metals may introduce electronic or magnetic characteristics that permit additional functionality, such as chemical sensing. Schiff base complexes containing metals have been previously incorporated into both conjugated and non-conjugated poly- mers. [18,19] These hybrid conjugated polymer films are of interest as they combine the properties of both components leading to new and potentially useful functions. [20] Complexes containing copper(II), nickel(II), and cobalt(II) with pendant thiophene units have been electropolymerized to generate conductive polymer films; [21–26] however, the NLO properties of these films have not been reported. In addition to modifying composition, it is possible to enhance the NLO properties of materials by processing and alignment strategies. Microgravity offers an environment that is free of gravity-driven convective currents. Polymers grown by free-radical polymerization under microgravity display different morphologies, permeabilities, and NLO properties than those grown on Earth. [15] Electropolymerization, an effective method to prepare thin films of conjugated polymers, is virtually unexplored in microgravity, even though studies indicate that electrochemical processes are significantly influenced by the effect of gravity. [27,28] Electrochemically generated density gradients form owing to the charge differences between the reactant, product, and the associated counterions, altering the structure of the fluid near the electrode surface. The density difference that emerges between the bulk solution and the diffusion layer creates a buoyancy force that induces convective flow in the solution. This disturbance is expected to affect the quality of electrodeposited material at the electrode surface. During electropolymerization, soluble monomers are oxi- datively coupled to form insoluble polymer films. Although the method is convenient and rapid, [29] films grown in this fashion may exhibit unpredictable degrees of intermolecular order. [30] COMMUNICATION [*] Prof. K. C. Chou, Prof. M. J. MacLachlan, Prof. M. O. Wolf, A. Pietrangelo, B. C. Sih, B. N. Boden, Dr. Z. Wang, Dr. Q. Li Department of Chemistry University of British Columbia 2036 Main Mall, Vancouver, BC V6T 1Z1 (Canada) E-mail: kcchou@chem.ubc.ca; mmaclach@chem.ubc.ca; mwolf@chem.ubc.ca [**] Support by the Natural Sciences and Engineering Research Council (NSERC) of Canada and the Canadian Space Agency is acknowledged. Supporting Information is available from Wiley InterScience or from the authors. Crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-653536 and CCDC-653547. Copies of the data can be obtained free of charge from www.ccdc.cam.ac.uk/ conts/retrieving.html or from the authors. 2280 ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2008, 20, 2280–2284