Nonlinear Optics of Chained-Chromophore Polymer

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Organic materials with high nonlinear optics (NLO) property tend to absorb visible light. This is a deficiency because they absorb second harmonic generated (SHG) light by themselves. Main aim of this project is to design novel liquid crystalline polymers (LCP) having high NLO property without such deficiency by using quantum chemical calculations and molecular dynamics (MD) calculations.(1) The internal rotation of the ester linkage for phenyl benzoate (PB) and p-hydroxybenzoic acid (HBA) dimer (HB) as models of aromatic polyesters was investigated by using the density functional theory. The rotation of S bond, which mainly determines the linearity of the molecule, leads to the trans- and cis-conformers of PB. Since the cis-conformer of PB is 7.69 kcal mol^<-1> higher than the trans-conformer, the cis-conformer has little population at standard condition. HB does not have the cis-conformer. These agree with the experimental understanding that poly(p-hydroxybenzoic acid) is the class o … More f stiff and extended polymer.(2) The aromatic copolyester of HBA and 6-hydroxy-2-naphtoic acid (HNA) shows strong SHG-activity in the nematic phase as well as the crystalline phase in spite of no absorption in the visible region. Quantum chemistry calculations show that the second-order hyperpolarizability of the HBA dimer is comparable to that of p-nitroaniline and it increases with the degree of polymerization. The large hyperpolarizability of the aromatic polyester is ascribed to the large electronic transition moment.(3) The most stable molecular packing systems of aromatic polyester have been investigated by MD simulations performed for decamer (ten repeating units) of HBA in the nematic phase. The MD simulations were started from eight possible packing structures. Among them, we found the polar ordering structure with the intermolecular ester carbonyl groups aligned in the same direction is energetically most favorable. This is consistent with our previous experimental observation for the SHG intensity measurement. Less