Crystal engineering of second harmonic generation using N-Glycoside deravatives
by Kin-ning Hui
xv, 158, 111 leaves : ill. ; 30 cm
The aim of crystal engineering is to design new solids with desirable physical and chemical properties, utilizing the understanding of molecular packing in crystalline materials....[ Read more ]
The aim of crystal engineering is to design new solids with desirable physical and chemical properties, utilizing the understanding of molecular packing in crystalline materials.
In this project we have demonstrated the ability to engineer one such crystal tensor property, that of Second Harmonic Generation of laser light. Only materials which are noncentrosymmetric will be SHG active. Since, saccharides are chiral, all saccharide-derived compounds will crystallize in a noncentrosymmetric point group, and show SHG activity. By coupling nonlinear optical chromophore such as PNA to a wide range of saccharides, a series of compounds, N-glycosides, can be formed with widely varying SHG efficiencies.
By study of the powder X-ray diffraction patterns, each compound was found to have its own unique molecular packing arrangement. Details of a number of these have been revealed by single crystal X-ray structure analysis.
The crystal structures confirm that the compounds are β-N-aryglycosides and that the packing of these ambiphilic molecules is dominated by hydrogen bonding, such that each crystal contains hydrophilic and hydrophobic domains.
The hydrogen bond arrangement between saccharide moieties in the hydrophilic domain may affect the relative orientations of the hydrophobic chromophores to which they are attached, thus modifying their SHG efficiency. In D-Ara-PNA, a favourable packing arrangement is found such that the charge transfer axis of the chroniophores forms an angle of 50.7° with respect to the Crystallographic b-axis. This leads to a high SHG efficiency. Other crystals studied such as D-Gal-PNA have a less favourable arrangement with the molecular charge transfer axes nearly orthogonal to the monoclinic 21 axis.
Our studies show that for these families of saccharide adducts about 10% should crystallize in the more favourable monoclinic symmetry class and have acceptable chromophore orientations with respect to the crystallographic b-axis. Since 15-20 crystalline saccharide derivatives can be made for a given chromophore, this offers an attractive approach to optimizing its SHG response in the solicl-state.