Nonlinear Optical Side Chain Monomers

Multifunctional monomers have been synthesized whose polymers are exhibiting both photoconductivity and nonhnear optical (NLO) properties. Second-order optical nonlinearity requires the total system does not possess a center of symmetry. Therefore, the monomers have a rather complicated structure. The functionalities can be introduced by standard reactions in organic chemistry. 

Examples for this type of monomers are methacrylic acid 6-[3-(2-cyano-2-(4-nitrophenyl)-vinyl)-carbazol-9-yl]hexyl ester, methacrylic acid 6-[3-[2-(4-nitrophenyl)-vinyl]-carbazol-9-yl]hexyl ester, and methacrylic acid 6-[3-(diphenyl-hydrazonomethyl)-carbazol-9-yl]hexyl ester. 

Investigation of the photoconductivity showed that some of the polymers are photoconductive without any addition of sensitizer and charge transporting agent. 

2-Methyl-acrylic acid 6-[3-(diphenyl-hydrazonomethyl)-carbazol-9-yl]hexyl ester 

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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]. 

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. 

Cooperative effects of the monomer units along the polymer backbone may result in a nonlinear relation between the specific optical rotation and the enantiomeric excess (ee) of chiral units present in the polymer [91]. For stiff helical isocyanates, these cooperative effects have led to observations referred to as majority-rules. The role of cooperative effects in chiral rr-PATs has also been examined using a series of PTs containing various ratios of chiral and achiral side chains. Data show that the cooperative interaction between the side chains affects the optical activity in a nonlinear fashion, while the majority-rules principle is applicable to chiral rr-PATs, the magnitude is less pronounced than with helical main-chain isocyanates [91]. 

In these isotactic polymers, the optical purity of the monomer affected the optical activity via the relationship to the excess helical sense of the polymer (Figure 1). ° In the case of isotactic poly((S)-4-methyl-l-hexene) (2) and poly((i )-3,7-dimethyl-l-octene) (3), an increase in the optical purity of the monomers resulted in an increase in the optical activity of the polymers in a nonlinear fashion the optical activity of the polymers leveled off when the optical purity of the monomer reached -80%. By contrast, in the case of isotactic poly((S)-5-methyl-l-heptene) (4), the relation was linear. These findings imply that the side-chain chiral centers of 4, which are separated from the main chain by three covalent bonds, may be too far from the main chain to affect the helical conformation. 

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