Non-Linear Optical Property-Structure relationship of N-(4-nitrophenyl)-N-[(1S)-1-phenylethyl]thiourea

Bao Chau Tran, Tram Anh Pham, Donald Responte, Bao Chau Tran, Tram Anh Pham, Donald Responte, Werner Kaminsky Chemistry Department, University of WashingtonWerner Kaminsky Chemistry Department, University of Washington

OutlookOutlook

We are currently synthesizing structurally related chiral crystals and measuring properties like the optical rotation. These studies will allow use to correlate the molecular structures with optical properties. Subsequently, we will be able to rationally design compounds with desired optical features in the desire to finding and optimizing potential photonic materials.

ResultsResults

It is evident that the orientation of the molecules and the anisotropy of the optical rotation and the electro-optic effect are coupled. For instance, the direction perpendicular to the plane defined by the nitro-phenyl groups contains the least induced dipoles and corresponds with the smallest refractive index as well as pseudo symmetries of the optical properties. Morphology calculations employing the Bravais-Friedel, Donnay-Harker model fitted the observed sizes of crystal face indicating mainly non-directional molecular forces.

Refractive Index of Alpha-MBARefractive Index of Alpha-MBA

As light passes from one transparent medium to another, it changes speed and the direction bends as a result of refraction. How much this happens depends on the refractive index of the mediums and the angle of the light ray to the surface separating the 2 mediums. The index of refraction is defined as the ratio of the speed of light in a vacuum to the speed of light in a particular medium

Using a micron scale microscope and the three height method which is derived from the Snell’s Law of Refraction, we were able to determine the refractive indices.

We measured the refractive indices and birefringence of the crystal face perpendicular to the c-axis of the crystal. The values and directions are approximately 1.61(2) along the a-axis and 1.49(2) perpendicular to that in the projection of the crystal face. Turning the crystal around we found that these two are to be extended by a third, bigger value.

The calculated refractive indices with similar polarisations as the observed ones are

1.504, 1.573, 1.767

h3

h2

h1

d

objective

L2 L1

Focus insidethe crystal

crystal top

crystal bottom

CalculationsCalculations

There are five elements in this structure: H, C, O, S, and N. Each reacts to light in a different way represented by a number, the electronic polarizability volume. The larger the electronic polarizability volume the stronger the atoms interaction.

The calculations which involve dipole-dipole interaction in crystals allow predicting values for different optical properties. we calculated the amount of optical rotation of linear polarized light and the amount to which the refractive indices change when a static electric field is applied to the crystal (electro-optic effect)

Below: The structure, representation surface of the calculated electro-optic effect and crystal morphology in different directions.

IntroductionIntroduction

Synthesis of-MBA: (N-(4-nitrophenyl)-N-[1(S)-1-phenylethyl]thiourea

We synthesized this compound by reacting 4-nitro-phenyl-isothyocyanate with enantiopure alpha-methyl-benzyl-amine and recrystallized it in ethanol solution. When R2 is an enantiopure chiral ligand the product is also chiral

In -MBA (space group P1) is no symmetry restriction to any of the optical properties due to the absence of any other but simple translational symmetry in the crystals.

Methods

Crystal Growth Crystals were grown by slow evaporation.

X-ray diffraction Data obtained from x-ray diffraction is used to determine molecular structures, index each crystal, and generate a model of its morphology.

Research PlanWe estimate non-linear optical properties using a theory which calculates the cumulative effect of dipole-dipole interactions between all atoms in the crystal lattice. This model needs the atomic structure and electronic polarizabilities as input parameters. In order to estimate the atomic polarizabilities we need to measure the refractive indices to which those calculated are compared. We employed the three height method derived from Snell’s law of refraction.

ReferencesSymmetrically substituted thiourea derivatives. Ramnathan, Sivakumar et al. Acta Crystallographica, Section C (1995), C51(11), 2446-50.

E. Humeres, C Zucco, M. Nunes, N. A. Debacher, R. J. Nunes: Hydrolysis of N-aryl thioncarbamate esters. Modified marcus equation for reactions with assymmetric intrinsic barriers. J Phys. Org. Chem. 2002 15 570-575