Third-order NLO polymers

Electronic structural requirements for third-order NLO polymers are different from those for second-order polymers [3,4]. The third-order NLO properties are very sensitive to the length of the ir-electron conjugation. An interesting third-order NLO polymer is polydiacetylene, which can be prepared by solid-state polymerization (Fig. 49.11). The backbone of polydiacetylene is derived from a diacetylene skeleton, RiC = CC = CR2, therefore, depending on the nature of substituents a variety of derivatives can... 

TABLE 49.8. values of a number of third-order NLO polymers. 

Third-Order NLO Measurements of Acetylide Polymer Complexes"... 

A New Molecular Crystal DEANST. Another example of a low molecular weight compound with a large third-order NLO property is a styrene derivative. By selecting an appropriate -conjugation length and chemical structures of donor and acceptor, several new organic molecular crystals such as stilbene and azo benzene derivatives which exhibit %(3) larger than 10 12 esu can be developed. The %(3) of 4-(N,N-diethylamino)-4 -nitrostilbene (DEANS), an intramolecular CT compound, has been reported in a solution, in a polymer matrix and in a crystalline state(27.29.30). 

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

Although several authors have reported on symmetrical and unsymmetrical homoleptic dithiolene complexes for third-order NLO applications (368, 436-440), the most complete and significant studies have been carried out by Underhill and co-workers (397, 431, 433, 434, 441-448). Two of these systems are shown in Scheme 25. One of the conclusions (434) is that an optimized material should absorb 800 nm (433) in order to have the highest ratio x(3 /a. For obtaining the material in a useful form, it is also proposed to embed the dithiolene complexes as guests in polymers such as poly(methylmethacrylate) (PMMA) (434, 449, 450). Along this line, the most efficient material in this series is the bis[l-butyl-2-phenylethene-l,2-dithiolato(2 )-S,S ]nickel(II) compound (29) shown in Scheme 25 (451). 

Conjugated polymers satisfy these requirements and have thus emerged as the most widely studied materials for their susceptability. Some of the examples of conjugated polymers, that have been studied for their third order NLO properties, are polydiacetylenes, poly-p-phenylenevinylenes and polythiophenes. However, CVD has only been used in the case of poly-p-phenylenevinylenes (PPV) [section 3.4], although values have not been reported. An excellent review of third order nonlinear optical properties of PPV in general, can be found in literature. Recently, McElvain et al. ° reported the values of CVD polyazomethines to be... 

Important criteria for conjugated polymers with large third order NLO response are, therefore, the following ... 

Polyimides with third-order NLO properties have been prepared by Lu et al. [56] by polycondensation pyromellitic dianhydride with either benzoguanamine or 3,3 -diaminobenzophenone under microwave irradiation conditions (Scheme 14.27). The polymers obtained under microwave conditions were characterized by large third-order nonlinearities and time response. 

Lu et al. have obtained poly(amic acid) side-chain polymers by polycondensation of benzoguanamine and pyromellitic dianhydride under microwave irradiation conditions [65-67]. The reactions were performed in a household microwave oven in which 100 mL DMF solution of 33 mmol benzoguanamine and an equimolar amount of pyromellitic dianhydride were stirred and irradiated for 1 h at 60 °C (Scheme 14.31). The resulting poly(amic acid) was precipitated from the solution and then modified to obtain side-chain polymers with fluorescent and third-order NLO properties. 

INCORPORATION OF LADDER POLYMER SUBUNITS IN FORMAL COPOLYMERS FOR THIRD ORDER NLO APPLICATIONS... 

Chart 3.3 Polymers exhibiting third-order NLO activities. R, Rn, and R2 denote aliphatic or aromatic groups. 

Coordination polymers have also been investigated for third-order NLO properties. " Thus, [Co(NCS)2(bpms)2] (bpms = l,2-bis(4-pyridylmethyl)disulfenyl) affords a 1-D double zigzag chain coordination polymer (1-D double chain Scheme 5c) and shows significant third-order NLO properties.Mn-4,4 -bpy polymers also show third-order NLO properties. Recently, the 1-D quantum wire [ Ni(l,2-chxn)2Br 2Br] j (1-D linear chain Scheme 5a) demonstrated gigantic third-order NLO activity. ... 

Among the many types of processible NLO polymers, polyurethane with symmetrical substituted tris-azo dye with fluorinated alkyl units (PSTF for short) was selected as a waveguide material. PSTF is a novel third-order NLO main-chain polyurethane with tris-azo dye incorporated into the main chain with a fluorinated alkyl backbone [40]. This polymer is similar to the SBAC polymer family whereby a symmetrically substituted 7t-conjugated molecule was incorporated in a main-chain polymer. Azo dye is an effective NLO dye investigated by many researchers and the tris-azo dye is very special for NLO applications. Fluorinated alkyl units are expected to be effective for reducing the polymer... 

Another important class of photonic materials is polysilanes (Fig. 49.15) [217-222]. In polysilanes the delocalization of (T orbitals of the Si atom along the polymer backbone plays a significant role in their NLO properties. Their physical and chemical properties can be tailored by the choice of an appropriate substituent, which also determines the conformation of the polymer and its solubility. What is of more interest is their transparency in their NLO properties, values of a series of polysilanes are given in Table 49.9. Tables 49.10-49.13 contain y and values of some other third-order materials, namely metallophthalocyanine, bis-metallophthalocyanine, metallonaphthalocyanines, fuller-enes, and j8-carotenes. Although these molecules are not considered as polymers, they are macromolecules that have drawn a considerable amount of attention as third-order NLO materials. For example, the highly stable icosa-... 

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