Chromophoric Self-Assembled Nonlinear Optical Multilayer Materials. Synthesis, Properties, and Structural Interconversions of Assemblies with Rodlike Alkynyl Chromophores

The communication describes an approach to the synthesis of thin-film second-order nonlinear optical (NLO) materials in which self-assembled chromophore-containing multilayer structures are built up on clean glass or quartz surfaces in the repeating reaction sequence p-Cl₃Si(CH₂)₂C₆H₄CH₂Cl (coupling layer), 4-[N,N-bis(3-hydroxypropyl)amino]phenylethynyl-4′-pyridine (chromophore layer), Cl₃SiOSiCl₂OSiCl₃ (capping layer). The multilayer structures have been characterized by optical and X-ray photoelectron spectroscopy, advancing contact angle measurements, and polarized second harmonic generation (SHG) properties. It is found that the initially formed coupling-chromophore monolayer exhibits a rather weak SHG response which increases substantially upon capping with the Si reagent. The proposed structural reorganization of the chromophore dipoles (β_zzz direction) to an average orientation more coincident with the surface normal is supported by optical spectroscopy. The square root of the SH intensity scales linearly with the number of chromophore layers up to nine layers. For a single coupling-chromophore layer, [formula omitted] esu at ℏω = 1.17 eV.