New Synthetic Approaches to Self-Assembled Chromophoric Multilayers as Second-Order Nonlinear Optical Materials
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ABSTRACT This contribution describes the synthesis and properties of NLO-active self-assembled chromophoric multilayers. The stilbazolium self-assembled multilayers were prepared by new topotactic approaches based on siloxane self-assembly technology. X-ray photoelectron spectroscopy (XPS), advancing aqueous contact angle (0a) measurements, transmission optical spectroscopy, polarized second harmonic generation (SHG), and specular X-ray rellectivity (XRR) show that the resulting self-assembled chromophoric superlattices have very high structural regularity and very large second-order nonlinear optical susceptibility.
INTRODUCTION There is much current interest in molecule-based nonlinear optical (NLO) materials because they offer attractions such as large nonresonant response, ultrafast response times, low dielectric constants/losses, intrinsic tailorability. Although considerable progress has been made in the fabrication of organic NLO materials in recent years, particularly in the areas of the poled glassy polymersl2, 31 and Langmuir-Blodgett films[ 41, significant synthetic challenges have impeded the realization of optimum materials.l 1, 21 These challenges include maximizing chromophore hyperpolarizability and number density as well as maximizing the degree and thermal/temporal stability of induced microstructural acentricity. An attractive and intuitively superior alternative to the above approaches [241 is a covalent self-assembly[ 51 process in which acentric chromophoric superlattices[ 6l are built up in a layer-by-layer fashion.l 6 ,71 In the past, the efficiency of this approach has been limited by the slow rate (days) at which the large hyperpolarizability stilbazolium chromophore layers[61 could be assembled and the resulting, less than optimum superlattice structural regularity. We report here a new solvent-free, topotactic approach, effected in either a solid state or chemical vapor deposition mode, which in combination with other building block modifications, allows the expedient assembly of robust chromophoric architectures with high structural regularity and large nonlinear optical response.
EXPERIMENTAL PROCEDURE All the operations were done using standard Schlenck techniques. Pentane and heptane were distilled from Na/K alloy immediately before use. X-ray reflectivity measurements were performed at Beam Line X23B of the National Synchrotron Light Source using the 95 Mat. Res. Soc. Symp. Proc. Vol. 392 0 1995 Materials Research Society
instrumentation and analysis procedures described in reference 8. Polarized second harmonic generation measurements were carried out as described in reference 6b. X-ray photoelectron spectra were recorded on a VG Scientific, Ltd, Escalab MK II spectrometer using Mg Kj irradiation.191 (1) Self-assembly of (3-bromopropyl)trichlorosilane. Freshly cleaned glass or silicon single crystal substrates were immersed in a 1:100 (v/v) solution-of (3-bromopropyl)trichlorosilane in heptane for 20 min and washed with copious amount of pentane and sonicated in acetone for 1 m
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