Molecular orientation in single crystal thin films of N-(4-nitrophenyl)-(L)-prolinol
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Molecular orientation in single crystal thin films of N-(4-nitrophenyl)-(L)-prolinol Ligui Zhou and M. Thakur Auburn University, Auburn, Alabama 36849 (Received 19 February 1997; accepted 15 July 1997)
Thin single crystal films of N-(4-nitrophenyl)-(L)-prolinol (NPP) were prepared using the modified shear method. The surface orientation of the single crystal films was determined by x-ray diffraction and was found to be [101]. Polarized microscopy showed uniform birefringence and complete extinction when the thin film was rotated under crossed polarization, implying single crystal thin films with uniform surface were obtained. The molecular orientation in the NPP thin film was studied by polarized UV-visible and polarized micro-FTIR spectroscopy along with x-ray diffraction. The orientation of the NPP molecules was found to be almost parallel to the plane of the film. This parallel orientation is because of the polar (hydrogen bonding) interaction of the 2(OH) and the °° °O groups of the NPP molecule with the hydrophilic substrate surface. The results N° of the second harmonic generation (SHG) measurements are consistent with such a molecular orientation. These results show that the final molecular and crystallographic orientation in the film is determined by its initial molecule-substrate interaction.
I. INTRODUCTION
Currently, there is extensive interest in molecular crystals such as nitroanilines and nitropyridines with donor-acceptor substitution and molecular salt crystals for nonlinear optical applications.1–3 Extensive studies have been made in second harmonic generation (SHG) and electro-optic effects. The magnitude of nonlinear optical coefficient depends on the molecular structure and crystal structure. The material x 2 vanishes unless the chromophores are arranged in a noncentrosymmetric structure. Several approaches have been utilized as ways to bias molecules to pack in acentric structure in crystal growth, including metasubstitution, vanishing dipole moments, chiral substitution, and hydrogen bonds. Chirality, although guaranteeing a noncentrosymmetric structure, may not ensure a significant departure from centrosymmetry because the molecules favor the head-to-tail antiparallel arrangement due to the dipolar interaction forces. Hydrogen bonds are more energetic by one or two orders of magnitude than van der Waals and dipole-dipole potentials, and hydrogen bonds are opposed to dipolar potentials. The combination of chirality and hydrogen bonding can thus achieve more promising structures than one of them can by itself. NPP [N-(4-nitrophenyl)-(L)-prolinol] is an example of typical “molecular engineering” in which the chirality and hydrogen bonding are employed to achieve an optimum nonlinear effect.1 In this work, the NPP molecule was studied in detail for its crystal growth characteristics under specific molecule-substrate interaction. Thin single crystal films of NPP were grown using the modified J. Mater. Res., Vol. 13, No. 1, Jan 1998
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