Crystallographic and computational studies of organic non-linear optical materials

Abstract

This thesis presents work on the determination of both linear and non-linear optical properties of organic molecular crystals from high-resolution X-ray diffraction data. The eventual goal of this work is to obtain accurate and reliable estimates of the non-linear optical properties for these materials of proven technological importance and to further our understanding of the factors affecting the relationship between molecular structure and macroscopic properties in order to aide our quest in designing new and better non-linear optical materials. The basic theory of crystallography is discussed in Chapter 1, with a particular emphasis on obtaining accurate charge densities from high-resolution X-ray diffraction data. Chapter 2 summarises the theory behind modern quantum chemistry calculations for single molecules and periodic materials. Also introduced is the relatively new method for obtaining ‘experimental' wave functions by constraining the wavefunction with structure factors obtained from X-ray diffraction experiments. This technique, devised by D. Jayatilaka, is the basis for much of the work carried out in this thesis. In Chapter 3, the definitions of the dipole (hyper) polarisabilities and related bulk susceptibilities are given along with a scheme for the calculation of approximate dipole polarisabilities attributed to Sylvain and Csizmadia. Chapter 4 discusses the equations required for the calculation of dipole рolararisabilities and hyperpolarisabilities derived from coupled-perturbed Hatree-Fock theory (СРHF). In addition, a scheme was presented for the calculation of refractive indices proposed by Rohleder and Munn. Routines for the calculation of these quantitles were implemented in the Tonto quantum chemistry package. This has allowed us for the first time to determine CPHF polarisabilities and hyperpolarisabilities from constrained wavefunction calculations. Constrained wavefunction calculations were performed on three compounds, benzene, urea and 2-methyl-4-nitroaniline. CPHF polarisabilities and related refractive indices were calculated and compared with the Sylvain-Csizmadia values from the previous study by A.E. Whitten at the University of New England. The CPHF polarisabilities and refractive indices were comparable to experimental values and those obtained from the Sylvain-Csizmadia approach, but unfortunately no significant improvement was observed using the more rigorous CPHF approach. Similar constrained wavefunction calculations were performed on three well known organic non-linear optical materials, 4-(N, N -dimethylamino)-3-acetamidomtrobenzene (DAN), 2-(N-L-prolinol)-5-nitropyridine (PNP) and (S)-2-(α-methylbenzylamino)- 5-nitropyridine (MBANP), selected from the literature due to their importance in the field of non-linear optics. Enhancements in the calculated dipole polarisability, hyperpolarisability and refractive indices were observed after wavefunction fitting, which is attributed to the effects of the crystal field and intermolecular interactions included by way of the X-ray diffraction data. A comparison between the CPHF(Electric Field Induced Second Harmonic generation) experiments, showed that the former were underestimated by on average 16.7 X 10(^-51)Cm(^3)V(^-2). A comprehensive comparison of various properties determined from wavefunction fitting and multipole refinements of the same X-ray diffraction data, was reported in Chapter 6, in order to further our understanding of the effect of wavefunction fitting and the nature of the 'experimental' wavefunctions. Notable differences were observed between properties obtained from the multipole model and experimental wavefunction, with large differences observed for the electron densities of the atomic core regions. Finally, Chapter 7 presents a charge-density study on the non-linear optical prototype material N,N-dimethyl-4-nitroaniline (NNDPNA). The multipole model obtained suggests a dipole moment enhancement of some 24 Debyes over that of the isolated molecule. Unreasonable estimates for the electrostatic properties such as this, are thought to be the result of the limitations of using X-ray diffraction data alone to obtain accurate charge densities.