First principles and x-ray studies of the charge density and optical properties of octupolar molecules

Nonlinear optical materials have been extensively used in many electro-optic devices. For such applications, which rely mainly on the capability of frequency conversion of light (second harmonic generation, SHG), the materials must have large molecular hyperpolarizability and crystallize into a noncentrosymmetric structure to exhibit efficient bulk nonliner optical activity. The crystal engineering of new and more efficient materials with such properties relies on basic investigation of the physical mechanisms determining the optical response at both the molecular and intermolecular level.

Non-linear optical (NLO) materials have been based on either inorganic salts (such as KDP) or dipolar molecules with high polarizability. However, the SHG power of dipolar molecules is dependent on the polarization of the incident light, and moreover, these molecules have a tendency to couple their dipole moments in centrosymmetric supermolecular crystallline arrangements. Therefore, it is advantageous to design new materials based on octupolar molecules, that lack a ground state dipole moment. The polarizability of the electronic charge density distribution is the key parameter of the optical response. Due to the ever increasing computing power and recent theoretical developments in density functional theory (DFT) ab-initio calculations of the optical properties are now feasible.

The aim of this project is to combine state of the art ab-initio DFT calculations of the non-linear optical properties of organic octupolar molecules with accurate x-ray diffraction determinations of their charge density.

The ab-initio calculations will be performed at the Centro de Física Computacional (CFC-FCTUC) and the X-ray diffraction measurements will be performed at the Centro de Estudos de Materiais por Difracção de Raios-X (CEMDRX-FCTUC). This project will benefit from an interdisciplinary synergy between these two research centers.

Building on our previous work on diphenylguanidine and L-arginine compounds, we propose to investigate the electronic and optical properties of pi-conjugated systems such as di- and tri-aryl substituted guanidine molecules. Single-crystals of the samples to be studied will be grown in house and examined with X-rays to provide accurate charge densities that are a stringent test on the quality of the calculations.

The high-performance computing facilities of the Centro de Física Computacional will be used to calculate the optical response of both isolated molecules and different crystalline structures of these molecules. Emphasis will be put on Second Harmonic Generation (SHG), and on the calculation of static and dynamic hyperpolarizability tensors using Time-Dependent DFT.

Status: Concluded

Starting date:

End date:

Financing: 90.000 Euros

Financing entity: Fundação para a Ciência e Tecnologia

Project ID: POCI/FIS/58309/2004

Person*month: 0

Group person*month: 0