A computational analysis on the first hyperpolarizability (£]) of 5-6-5 heterocyclic rigid-rod benzobisthiazole-based chromophores and their derivatives has been systematically investigated in this study by the software Gaussian03, in an effort to provide a molecular design criterion for better opto-electonic properties. The specific objectives of this study have been focused on (1) understanding of 5-6-5 heterocyclic rigid-rod chromophores and other similar compounds in nonlinear optical property, (2) control on the nonlinear optical properties through the manipulation of the conjugated composition of the chromophores, (3) understanding the effect of the donor and acceptor of chromophore on the nonlinear optical property. The results show that £] value of a chromophore with its 5-6-5 heterocyclic rigid-rod structure near the acceptor is 30 % higher than the one with the same structure near the donor; this reflects a unique property of 5-6-5 heterocyclic rigid-rod structure quite different from other chromophores. In conjugated chromophores, those having double bonds or thiophenes, which have soft structures or heteroatoms, would offer higher £] values. For example, a 5-6-5 heterocyclic rigid-rod chromophore with 15 double bonds has a £] value up to 1.44 x 10-26esu. In the study of multi-donors and acceptors, from 1 donor- 1 acceptor to 2 donors- 2 acceptors, the difference in £] could be up to 116%; however, this difference reduces to only 5% if the chromophere structure becomes unstable. From the band-resolved analysis, it is found that £] values of chromophores are affected by the electronic donor via its HOMO part, the electronic acceptor via its LUMO part, while the conjugated bridge via its molecular orbital exclusive of HOMO and LUMO, depending on the type and degree of conjugation, i.e., HOMO-2 and LUMO+1 for double bonds and HOMO-2, HOMO-1, and LUMO+1 for phenyl moieties. For the 5-6-5 heterocyclic rigid-rod structure, £] value is mainly affected by those molecular orbital below HOMO, i.e., HOMO-2 for those containing double bonds. Further detailed elucidation on the relation between molecular orbital and molecular structure of chromophores can be ambiguously depicted through analyses of electronic distribution on the molecular orbital.

The first hyperpolarizability £], which is an important factor in the second-order nonlinear optical effect, of the 5-6-5 heterocyclic rigid-rod chromophores-based derivatives has been systematically investigated in this work, applying the first principle based software CASTEP and the semi-empirical software MOPAC There are four factors chosen for this computational analysis, including the effects of (i) various rigid-rod molecules, (ii) auxiliary ability of various five-member rings, (iii) donor ability, and (iv) conjugation length. Results indicate that the benzobisimidazole (BBI)-based derivatives have larger £] values than those of benzobisthiazole (BBT) and benzobisoxazole (BBO)-based derivatives. The auxiliary donor ability of the pyrrole is larger than that of the thiophene and furan. In general, the £] value of various derivatives studied is parallel to the original donor strength and the trend in the donor is -N(CH3)2 > -NH2 > -OH > -CH3. In addition, the greater the conjugation length in the derivatives is, the higher the £] value. Further, from the analyses on the band-resolved £] values and the orbital density figures, it is found that the major contribution to the enhanced £] comes primarily from an efficient charge transfer from the donor side via the intermediate heterocyclic bridge to the acceptor side. Finally, a comparison between CASTEP and MOPAC reveals that CASTEP is more appropriate and accurate in predicting the optical properties of the heterocyclic-ring-based derivatives.

The linear absorption spectrum and resonance Raman spectrum of a series of charge-transfer molecules in solution are measured. The results are fitted to theoretical expressions. The dispersion of the first hyper-polarizability is investigated by substituting the parameters, which are obtained from the fit. The results are compared with Wang's formula modified from the simple Oudar and Chemla two-level model. The resonance Raman spectra and Raman profiles show that only one excited state exists in two of the charge-transfer molecules and two excited states are present in three other molecules investigated in this expeariment. It is concluded that Wang's modified formula is only valid to account for the dispersion of the first hyperpolarizability when there is only one excited vibronic state.

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Abstract The first hyperpolarizability(£]) of five charge-transfer molecules are determined using the hyper-Rayleigh scattering (HRS) technique at two excitation wavelengths : 1064nm and 1907nm. The 1064nm excitation wavelength is derived from a Nd: YAG pulsed laser, and the 1907nm excitation wavelength is obtained by shifting the 1064nm laser light by stimulated Raman scattering of pressurized H2 gas. Four of the five samples contains thiophene and thiazole are synthetized by Prof. Shu Ching-Fong at the National Chiao Tung University (NCTU) and the other sample is synthetized by Prof. Hong, Jin-Long at the National Sun Yat-sen University (NSYSU). The measured £] values are used to calculate the intrinsic molecular hyperpolarizabilities using the two-level model previously developed by Oudar and Chemla. The results are related to molecular structure.

The technique of hyper-Rayleigh scattering is employed to measure the first molecular hyperpolarizabilities£]of nonlinear optical chromophores provided by Industrial Technology Research Institute. £]0. In the thesis, the effect of the molecular structure and the solvent on the hyperpolarizabilities will be discussed further.

Submitted by Luciana Ferreira (lucgeral@gmail.com) on 2015-10-20T12:28:15Z No. of bitstreams: 2 Dissertação - Leonardo Martins de Abreu - 2011.pdf: 1808129 bytes, checksum: 57f554d3a354cf1b50f5c6811cf545b5 (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Approved for entry into archive by Luciana Ferreira (lucgeral@gmail.com) on 2015-10-20T12:32:59Z (GMT) No. of bitstreams: 2 Dissertação - Leonardo Martins de Abreu - 2011.pdf: 1808129 bytes, checksum: 57f554d3a354cf1b50f5c6811cf545b5 (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Made available in DSpace on 2015-10-20T12:32:59Z (GMT). No. of bitstreams: 2 Dissertação - Leonardo Martins de Abreu - 2011.pdf: 1808129 bytes, checksum: 57f554d3a354cf1b50f5c6811cf545b5 (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Previous issue date: 2015-08-02 / Using Hartree-Fock (HF) and the second-order Møller-Plesset perturbation theory (MP2) methods with the 6-31G(d) basis set, we have investigated the chain length dependence of the dipole moment, linear polarizability, first and second hyperpolarizabilities of fluorinated polyacetylene chains (PFHA). Such properties were calculated using converged geometries obtained at the HF/6-31G(d) e MP2/6-31G(d) levels theory. The converged geometric parameters were obtained from equilibrium geometry of the oligomer with 40 carbon atoms. Our results show that the dipole moment and the first hyperpolarizability grow initially and exhibit a saturation pattern with the size of chain. In opposition, the linear polarizability and second hyperpolarizability present an increase with increasing chain length. It is found that the effects of electron correlation have an important role on the equilibrium geometry and on the electric properties. For the first hyperpolarizability, in particular, the asymptotic value including the electron correlation effects is 17 times larger than the uncorrelated one. These calculations were performed using the GAUSSIAN03, GAUSSIAN09 and Finito.f programs. / Utilizando os métodos ab initio de Hartree-Fock (HF) e de Møller-Plesset em segunda ordem (MP2) com o conjunto de funções Base 6-31G(d), investigamos o comportamento do momento de dipolo, da polarizabilidade linear, da primeira e da segunda hiperpolarizabilidades para cadeias de poliacetileno fluoretadas. Tais propriedades foram calculadas usando geometrias convergidas nos níveis HF/6-31G(d) e MP2/6-31G(d), construídas a partir dos parâmetros geométricos convergidos da geometria de equilíbrio do oligômero com 40 átomos de carbono. Os resultados obtidos mostram que o momento de dipolo e a primeira hiperpolarizabilidade deste sistema crescem inicialmente e apresentam um regime de saturação com o crescimento da cadeia poliênica. Diferentemente, a polarizabilidade linear e a segunda hiperpolarizabilidade apresentam um comportamento crescente com o incremento de subunidades no sistema. É importante destacar que o efeito de correlação eletrônica tem um impacto importante sobre a geometria de equilíbrio e sobre as propriedades elétricas das cadeias de PFHA. Para a primeira hiperpolarizabilidade, em particular, o valor assintótico obtido incluindo o efeito de correlação eletrônica é, aproximadamente, 17 vezes o valor obtido sem estes efeitos. Todos os cálculos foram realizados utilizando os programas GAUSSIAN03, GAUSSIAN09 e Finito.f.

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The work presented in this thesis has broadly established a few findings about the structure¬hyperpolarizability relation in molecular compounds: First, by measuring βHRS of an organic keto-enol tautomer, benzoylacetanilide in a binary solvent, I have shown that the first hyperpolarizability can be manipulated favourably by changing the composition of the solvent or by altering the pH of the solution. BA which exists in the pure keto form in water and as a keto-enol tautomer in ethanol, co-exists in equilibrium with the keto and enol forms at pH 11 in aqueous solution. The β value of the anion form is 709 x 10¬30 esu , whereas that of the enol is 232 x 10-30 esu and of the keto is 88 x 10-30 esu. There is an enhancement of β by ~ 8 times for the anion and ~3 times for the enol compared to the keto form. This opens up the possibility of finding large nonlinearities in organic molecules by simply ionizing it. Second, in organometallic complexes of divalent Ru, the first hyperpolarizability could be manipulated by altering the valence state of the metal center by oxidation or reduction or by introducing highly polarisable halogen atoms as substitutions in ligands attached to the metal center. The enhancement of first hyperpolarizability was observed in mononuclear [RuII(acac)2(CH3CN)2] complex by 1.7 times when the metal center was oxidized from RuII to RuIII. As it is already known that the complexes like [(acac)2Ru-bptz-Ru(acac)2] produce stable mixed valent compound, the enhancement of β by ~1.6 times is appearing because of that species only. Exploring Large Nonlinearity in Tautomers In this thesis I have taken a linear ketone for studying the effect of structure on β via the enol and anion formation mediated by solvent and pH of the medium. In the present study the proton transfer in BA took place in the ground state of the ketone and the enol or anion are produced in the ground states. The proton transfer reaction (tautomerism) can also happen in the excited state as well in some molecules where there is a substantial barrier to the proton transfer reaction in the ground state. In such systems, once the ketone is excited using ultraviolet light the barrier to proton transfer in the medium is overcome and a proton transfer in the excited state takes place and the enol is produced. Since such a system will be at higher energy, it will be interesting to do a two-laser experiment where the excited state hyperpolarizability is measured in a time resolved manner and the β value is determined in the excited state. Building Molecular Nonlinearity in Step-by-Step Electron Transfer In this thesis, I have dealt with a binuclear complex of Ru(II) which in one-step electrochemical oxidation produced a mixed valence compound which had substantially higher β value compared to the unoxidized complex. In this way it is possible to build a multicentered complex and see if sequential one-electron transfer and subsequent oxidation/reduction of the metal centers produce a mixed-valent metal compound with large molecular nonlinearity. The indication from the present study is that such a scheme should double the β value in each one-electron transfer step. Also the linker group/moiety between the successive metal centers will play an important role in dictating the efficiency of electron transfer. If the metal d-electrons in a multinuclear complex are linked through a π-conjugation, one would expect manifold enhancement of β. Such metal arrays can also be designed in 2 or 3 dimensions. The dimensionality of the multinuclear metal complexes can easily be changed by supramolecular design and synthesis strategy. Such metal networks may or may not generate large β molecules since electronic polarization in such systems may not be superimposable in a coherent fashion and may not add in a positive sense. All these remain to be tested and explored in the future.