Main-chain polymers, nonlinear optics

D., Herman, W. N., and Ashley, P., Advances in main-chain syndioregic nonlinear optical polymers, in Polymers for Second-Order Nonlinear Optics (G. F. Lindsay and K. D. Singer, eds.), ACS Symp. Ser. 601, August 21-25, 1994, Washington, DC, American Chemical Society, Washington, DC 1995, Chapter 14, pp. 181-197. 

The increasing use of optical fibre in the telecommunications network will, ultimately, require all-optical signal processing to exploit the full bandwidth available. This has led to a search for materials with fast, large third order optical nonlinearities. Most of the current materials either respond in the nanosecond regime or the nonlinearity is too small (1-3). Organic materials are attractive because of their ultra-fast, broadband responses and low absorption. However the main problem in the materials studied to date, e.g. polydiacetylenes (4) and aromatic main chain polymers (5), has been the small nonlinear coefficients. 

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... 

R S. Nalwa. T. IMtanabe. A. Kakuta. and S. Miyata. (1993) Aromatic polyureas a new daas of chromophore main-chain polymers for second-order nonlinear optics. Nonlinear Optics. 8 137(1994)... 

Kurihara, K., Tomaru, S., Mori, Y., Hikita, M., and Kaino, T., Third-order optical nonlinearities of a processible main chain polymer with symmetrically substituted tris-azo dyes, Appl. Phys. 

However, the particular synthetic requirements in the preparation of conjugated polymers have thus far severely limited the number of similarly hierarchically structured examples. Pu et al. reported different types of conjugated polymers with fixed main-chain chirality containing binaphthyl units in their backbone which exhibited, for example, nonlinear optical activity or were used as enantioselective fluorescent sensors [42—46]. Some chirally substituted poly(thiophene)s were observed to form helical superstructures in solution [47-51], Okamoto and coworkers reported excess helicity in nonchiral, functional poly(phenyl acetylenejs upon supramolecular interactions with chiral additives, and they were able to induce a switch between unordered forms as well as helical forms with opposite helical senses [37, 52, 53]. 

Besides other intriguing properties, such as inherent planar chirality, metallocenes are of interest due to their considerable Lewis basicity. Their direct or conjugative attachment to a polymer chain should enhance the electron density along the main chain and therefore lower the HOMO-LUMO band gap. In addition, organometallic compounds and metallocene-based monomers and polymers represent interesting potential nonlinear optical materials, useful for frequency doubling, modulation, and switching, for three reasons ... 

This chapter concentrates on the design of efficient dipolar NLO chromophores and the different approaches for their incorporation in non-centrosymmetric materials, including guest-host polymer systems, chromophore-functionalized polymers (side-chain and main-chain), cross-linked chromophore-macromolecule matrices, dendrimers, and intrinsically acentric self-assembled chromophoric superlattices. The different architectures will be compared together with the requirements (e.g., large EO coefficient, low optical absorption, high stability, and processability) for their incorporation into practical EO devices. First, a brief introduction to nonlinear optics is presented. 

Typical functions of substituted poly acetylenes are based on their (i) high gas permeability and (ii) electronic and photonic properties. The former originates from the rigid main chain and bulky substituents. Though electrical insulators, substituted polyacetylenes are more or less conjugated polymers, and this feature has been utilized to develop their electronic and photonic functions such as photoconductivity, electrochromism, optical nonlinearity and ferromagnetism. 

One of the main properties of interest in the field of conjugated polymers is the study of their dynamic nonlinear optical (NLO) response [112, 113]. It is a major challenge to obtain reliable dynamics of interacting electron systems. While for short oligomers there exist reasonable approximations for computing these properties [114], for longer chains even within model Hamiltonian approximations, the dynamic NLO coefficients had proved elusive. Yet most interest lies in the longer chains since the dynamic NLO properties exhibit dominant finite-size effects. 

The polymers having delocalized r-electron in the main chain have been expected to possess extremely large third-order optical susceptibility.However, such an extended jr-electron conjugation generally rendered the polymers insoluble and infusible as well, which has seriously limited the fabrication of practical NLO devices. Recently, it was reported that the third-order nonlinear optical properties of poly(l,6-heptadiyne)s which were environmentally stable, soluble, and processable. The third-order optical nonlinearities of poly(l,6-heptadiyne)s bearing NLO active chomophores were evaluated for the first time. The third-order nonlinear susceptibility... 

Chapter V of this Handbook is entirely devoted to the potential applications of the side chain liquid crystal polymers, but it is interesting to mention here the main areas where FLCPs could play a role. As far as we know, the following applications can be considered for SmC LCPs nonlinear optics, pyroelectric detectors, and display devices. 

Random main-chain nonlinear optical polymers Amino-sulfone azobenzene chromophores for second harmonic generation, CAe/n. Mater. 5 1439 (1993). 

Ferrocene-containing polymers with long spacers have been prepared as new nonlinear optical (NLO) materials for second harmonic generation (SHG) applications (e. g. frequency doubling) [10], The use of ferrocene derivatives in this area is attractive as a result of their demonstrated large hyperpolarizability values combined with their thermal and photochemical stabilities [11], These factors make polyferrocenes desirable candidates for use as processable NLO materials. The polyurethane copolymer 4.4 was synthesized using a functional ferrocene monomer (Eq. 4.1) and has been well-characterized the molecular weight was estimated by GPC to be Mn=7600. The two possible orientations of the ferrocene NLO chromophore monomer unit, which correspond to opposite dipole orientations, were both present in the main chain. 

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