Colloids and Surfaces A: Physicochemical and Engineering Aspects 198 – 200 (2002) 275 – 280 Orientation of functional groups in polyelectrolyte multilayers studied by second-harmonic generation (SHG) M. Koetse a , A. Laschewsky a, *, A.M. Jonas b , T. Verbiest c a Department of Chemistry, Uniersite Catholique de Louain, Place Louis Pasteur 1, B-1348 Louain -la -Neue, Belgium b Department of Materials Science, Uniersite Catholique de Louain, Croix du Sud 1, B-1348 Louain -la -Neue, Belgium c Department of Chemistry, K.U. Leuen, Celestijnenlaan 200D, B-3001 Leuen, Belgium Received 26 August 2000; accepted 10 January 2001 Abstract The orientation of polymer bound, NLO-active dye disperse red in alternating polyelectrolyte multilayers was investigated by second harmonic generation. Film growth was followed by UV/Vis spectroscopy, and films were probed for internal order and roughness by small angle X-ray reflectivity. The orientation of the dyes was followed in-situ using Second Harmonic Generation (SHG). Some chromophore orientation was observed for most systems, which however, typically passes over a maximum with ongoing film growth. The intensity of the SHG could not be correlated with the molecular structure of the polymers employed. Surprisingly, heating/drying of certain films increased the SHG activity substantially. Attempts were made to increase and to stabilize the orientation of the chromophores by inclusion complexation with a charged cyclodextrin. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Ultrathin films; Polymer; Non-linear optics; Internal order www.elsevier.com/locate/colsurfa 1. Introduction The preparation of ultrathin films by alternat- ing adsorption of polyelectrolytes has made sub- stantial progress in recent years [1 – 3]. Typically, these films are amorphous and have no internal structure, as the polyelectrolyte chains are inter- digitated, though being more or less stratified. The lack of internal structure is not necessarily disadvantageous, but limits the use of such coat- ings for purposes in which functional fragments must be aligned, or even oriented, such as for molecular recognition or second-order non-linear optics. Surprisingly, Second Harmonic Generation (SHG) studies have evidenced the partial orienta- tion of chromophores in dye-functionalized multi- layer films in special cases [4–10]. The effects observed seem to result mostly from a surface induced orientation by the support in the first layers only [8–10], and not only to stagnate, but even to fade away with film growth [4–6]. The observed loss of SHG signal can be attributed to * Corresponding author. Tel.: +32-10-478-709; fax: +32- 10-472-836. E-mail address: laschewsky@cico.ucl.ac.be (A. Laschewsky). 0927-7757/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0927-7757(01)00940-2