Side-chain, nonlinear optical

Liquid crystal polymers are also used in electrooptic displays. Side-chain polymers are quite suitable for this purpose, but usually involve much larger elastic and viscous constants, which slow the response of the device (33). The chiral smectic C phase is perhaps best suited for a polymer field effect device. The abiHty to attach dichroic or fluorescent dyes as a proportion of the side groups opens the door to appHcations not easily achieved with low molecular weight Hquid crystals. Polymers with smectic phases have also been used to create laser writable devices (30). The laser can address areas a few micrometers wide, changing a clear state to a strong scattering state or vice versa. Future uses of Hquid crystal polymers may include data storage devices. Polymers with nonlinear optical properties may also become important for device appHcations. 

Optically active polymers are potentially very useful in areas such as asymmetric catalysis, nonlinear optics, polarized photo and electroluminescence, and enantioselective separation and sensing.26 Transition metal coupling polymerization has also been applied to the synthesis of these polymers.27 For example, from the Ni(II)-catalyzed polymerization, a regioregular head-to-tail polymer 32 was obtained (Scheme 9.17).28 This polymer is optically active because of the optically active chiral side chains. 

Our initial work involved the synthesis of side chains which have the molecular characteristics required for a nonlinear optical response. Compounds 1-3 were prepared by the use of Horner-Emmons-Wadsworth Wittig methodology (22). Compound 4 was commercially available (Aldrich) as the dye, Disperse Red 1. As outlined in Scheme I, in the first step in the synthesis of 1-3,... 

Another significant helical amplification in optically active copolymers with preferential screw-sense helicity is known as the majority rule phenomenon [ 17,18]. In this case, the screw sense of a helical main chain bearing nonracemic chiral side groups is controlled by the ee only and a population of preferential screw-sense helicity and optical activity were nonlinearly amplified by ee of chiral side groups. Since Pino et al. first reported this phenomenon in poly-a-olefins made of vinyl co-monomers bearing nonracemic chiral moieties [21], this majority rule has already been established in stiff polyisocyanates bearing a nonracemic chiral side chain [17,18]. 

Van der Vorst CPJM, Picken SJ (1996) Electric field poling of nonlinear optical side chain polymers. In Shibaev VP (ed) Polym Electroopt photoopt Act Media. Springer, Berlin Heidelberg New York, p 173... 

Figure 2. Schematic structure of side chain liquid crystal polymers with dichroic dyes (imparting nonlinear optical properties) and mesogenic side groups.

Van der Vorst CPJM and Picken SB, "Electric Field Poling of Nonlinear Optical Side Chain Polymers", in Shibaev VP (Ed.) "Polymers as Electro-optical and Photo-optical Active Media", Springer Verlag, Berlin, 1996, Chap. 5 "Electric Field Poling of Acceptor-Donor Molecules", J Opt Soc Am B 7 (1990) 320. 

Wendorff, J. H., and Eich, M. Nonlinear optical phenomena in liquid crystalline side chain polymers. Mol. Cryst. Liq. Cryst. 169, 133 (1989). 

This synthetic approach offers many opportunities, both with molecular weight control and side-chain chemistry, for tailoring molecular structure to prepare different mesophases and optimize physical properties for nonlinear optical applications. 

Part of the rationale for investigating liquid crystal polyphosphazenes is for nonlinear optical (NLO) applications. In general, for NLO activity, polymers must either contain noncentrosymmetric side chains (eg., side-chain liquid crystal polymers) or highly delocalized (conjugated) backbones (16). Alternatively, polymers can be doped with low molar mass compounds to obtain NLO activity. 

In the polymerization of the (—)-monomers with various ee s, enantiomer selection was observed though the selectivity was lower compared with that of the polymerization of IDPDMA.83-87 In this experiment, a nonlinear relation was observed between the ee of the monomer in the feed and the optical activity of the obtained polymer (Figure 6). This indicates that the optical activity of the polymer is not based only on the side chain chirality. Furthermore, the chirality of a one-handed helical part induced by a successive sequence of the (—)-monomeric units (monomeric units derived from a (—)-monomer) can overcome the opposite chiral induction by the sporadic (+)-monomeric units. In other words, once a one-handed helical radical comes under the influence of the (—)-monomeric units, an entering (+)-monomer becomes a part of the one-handed helix whose direction may be unfavorable to the chiral nature of the (+)-monomer. 

Side-chain liquid-crystalline polymers with controlled molecular weights have been obtained by the polymerization of FM-25 with 1-22 (X = Br)/CuBr/ L-3 in the bulk at 100 °C, to examine the thermotropic transition as a function of the MWD.324 Second-order nonlinear optical materials with branched structure were prepared by the copper-catalyzed radical polymerization of FM-26 and FM-27 using hyperbranched poly[4-(chloromethyl)styrene] as a multifunctional initiator.325... 

As discussed earlier, while the scale of the fillers is substantially different, nanocomposite materials concepts and technology are very similar to those of conventional composite materials. This is clearly demonstrated in the case of new thermosets for nonlinear optical (NLO) applications, " " where nanocomposite of liquid crystalline thermosets, IPNs, and simple filled thermosets are evaluated. Tripathy et al. discussed four different ways to prepare nonlinear optical polymers. (1) The polymer matrix is doped with NLO moieties in a guest/host system (2) In side-chain polymer systems, NLO polymers with active moieties are covalently bonded as pendant groups (3) In the main chain polymer, the chromo-phores are incorporated as parts of the main polymer backbone to enhance the temporal stability of the NLO properties and (4) Stability of the optical noninearity in sol-gel-based thermosets is related to... 

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