NLO-active

An important goal of this study is to characterize the noncentrosymmetric arrangement of the azobenzene branches in dendrimers and the effect of these dendrimers on macroscopic second-order susceptibihty. The NLO activity of the dendrimers can be characterized by their molecular hyperpolarizabihty. The hyper-Rayleigh scattering (HRS) method is a reliable way of measuring the mol-... 

NLO active molecules can be embedded in or chemically anchored to a sol-gel-matrix without changing the optical absorption spectrum. Disperse Red 1, a very efficient molecule for NLO applications, was embedded in a sol-gel-matrix, synthesized by hydrolysis and condensation of 3-glycidoxypropyltrimethoxysilane in the presence of N-methylimidazole. The dye-doped gel was applied to glass substrates and thermally cured to form a layer of perfect optical transparency. Currently, poling experiments and NLO measurements with these layers are being performed. 

The non-linear optical (NLO) active aromatic chalcone l-(2-furyl)-3-(4-benzamidophenyl)-2-propene-l-one (FAPPO), shown in Fig.l, was found to give a rather big SHG activity of 1-2 order of magnitude higher than crystalline urea [6]. Due to their conjugated system all discussed NLO active molecules are approximately fiat. 

The design of molecules, supermolecules and materials presenting NLO activity involves molecular and supramolecular engineering. At the molecular level, a high polarizability, leading to large quadratic ft and cubic y hyperpolarizability coeffi-... 

It is clear that combining the design of NLO active molecules with the manipulation of selective intermolecular interactions may produce novel NLO materials that take into account supramolecular features. 

We decided to explore the possibility of performing electric field poling of a NLO-active compound at room temperature and solidifying the material by in-situ photopolymerization. In essence, instead of cooling the sample below Tg, we proposed to raise the Tg of the material above the poling temperature. This paper reports on systems in which the active NLO species is present either as a dopant in a photocrosslinkable host matrix or as the matrix itself. We also describe the preparation and use of several new donor-acceptor stilbene compounds similar to 1. 

Some representative examples of NLO-active guest compounds are shown in Figure 1. Some of these compounds, or their synthetic precursors, were... 

Figure 1. Representative NLO-active guest compounds and maximum solubility (wt %) in host 7.

To increase the density of NLO-active chromophores in the crosslinked matrix, it was desirable to completely omit the inactive binder 7 and make the NLO dyes themselves photocrosslinkable. Therefore, we synthesized and studied chromophores substituted with two acrylate or methacrylate groups (27). 

Molecular recognition directed self-assembling of organized phases has been described recently in the formation 1) of mesophases by association of complementary molecular component, as in 13 (23) 2) of supramolecular liquid crystalline polymers of type 14 (24) and 3) of ordered solid state structures, such as that represented by 15 (25). In all these cases, the incorporation of NLO active groups may be expected to produce materials whose SHG properties would depend on molecular recognition induced self-organization. 

The results presented above illustrate how combining the design of NLO active molecules with the manipulation of selective intermolecular interactions may produce novel NLO materials. Bringing together two basic features of supramolecular chemistry -molecular recognition and self-organization- with the optical properties of the components, opens ways towards the design of supramolecular photonic devices. 

Problems to address in the future include 1) Increasing the density of NLO active units in the phase 2) Orientation of functional arrays with larger P and 3) Developing materials with better processibility. 

Some debate is evident about the role of the phosphazene polymer backbone in the generation of third-order NLO activity.255 256... 

Electrostatic self-assembly was combined with supramolecular chemistry to obtain inclusion complexes of a polymeric nonlinear optical (NLO) active dye and modified [3-cyclodextrin with induced chromophore orientation [37], The polyanion is a N,N-diallyl-aniline and sodium-2-acrylamido-2-methylpropanesulfonate copolymer functionalized with pendant azo group. The modified /i-cyclodextrin oligo-cation was obtained by treatment of hcptakis(6-dco y-6-iodo-/i-cyclodcxtrin) with excess pyridine. A linear polyamine, chitosan, was also combined with the polyanion, for comparison. Films were deposited on glass slides by dipping them alternatively in aqueous solutions of the cation and the polyanion. UV-visible spectra indicate dye aggregation and suggest the formation of an inclusion complex of the dye with the cyclodextrin, thus isolating the chromophores. 

---