SHG-enhancement Study of Chained-Chromophores in their Polar Nematic Liquid Crystal Phase

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SHG property of poly(4-aminobenzoic acid), an aromatic polyamide with head-tail structure, was investigated by calculating the hyperpolarizability (β) for (H_2N-(C_6H_4CONH)_n-C_6H_4COOH) (AA oligoamides) and (H-(C_6H_4CONH)_n-C_6H_5)(BA oligoamides) at density functional theory (DFT) B3LYP/6-31G^*. The β values of AA and BA oligoamides increase with increasing number of amide bonds (n). Since the β values of AA oligoamides are always larger than those of BA oligoamides, the terminal effect on the β value for oligoamides is significant. The two-level model ascribes the origin of the large β of AA oligoamides mainly to the large transition moment between the ground state and the excited state. For both AA and BA oligoamides, the excitation energies are almost constant values with increasing the number of amide bonds so that the red-shift involved with polymerization may be small. The result indicates that poly(4-aminobenzoic acid) is a new candidate of SHG-active chromophore for improvi … More ng efficiency-transparency trade-off problem.Then N-methylation effect on aromatic amides were investigated by using the DFT calculations. Benzanilide (BA) adopts the trans conformation with lower energy of 4.60 kcal/mol than cis one. On the other hand, N-methylbenzanilide (MBA) adopts the cis conformation with lower energy of 3.23 kcal/mol than trans one. N-methylated para-linked aromatic diamide compounds were also studied by the DFT calculations and they also prefer the cis conformation. Since there are two stable conformations (cis-cis-syn and cis-cis-anti), N-methylated aromatic polyamides have enormous various conformations in addition to losing linearity of main chains. As a result, N-methylated aromatic polyamides is not liquid crystal but amorphous.Moreover, we studied various N-methylated aromatic diamide compounds by using the DFT calculations in order to check these structures are chiral or not. The relative energies for six stable conformers for N-methylated aromatic diamides were obtained. All of these compounds prefer the cis conformation, and most of them are chiral. Therefore, it is experimentally interesting to investigate whether the chilarity of these amides are induced by addition of a chiral dopant or not. We succeeded in observation of chirality induction on N-methy aromatic amides with chiral amines. This result indicates that N-methyl aromatic amides may be useful as a building block for constructing a new chiral material such as a chiral foldamer and liquid crystal. Less