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Theoretical Studies Of Several Nanomolecule And Small Molecule Systems On Nonlinear Optical Properties

Posted on:2012-05-19Degree:DoctorType:Dissertation
Country:ChinaCandidate:Z J ZhouFull Text:PDF
GTID:1101330335452921Subject:Physical chemistry
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In today's high-technology world, photonic materials are very important in many fields, such as optical process of information, optical computer, optical communication, etc. The research of the nonlinear optical materials is an important field of its development and is one of the international frontier topics at the present. The searching and designing of high-performance NLO materials is an important base in this field. In this thesis, the structures and nonlinear optical properties of the modified graphene nano-molecules and some important small molecules are investigated by the quantum chemical methods-ab initio and density functional theory (DFT). The main contributions are as followings:1. Graphene nanoribbon (GNR) as an excellent conjugated bridge was used in a donor-conjugated bridge-acceptor (D-B-A) framework to design high performance second-order nonlinear optical materials. Owing to the unique diradical planar conjugated bridge of GNR, (NH2)-GNR-(NO2) exhibits exceptionally large static first hyperpolarizability (β0) up to 2.5×106 a.u. (22000×10-30 esu) for H2N-(7,3)ZGNR-NO2 (ZGNR=zigzagedged GNR), which is about 15 times larger than the recorded value ofβ0 (1470×10-30 esu) for the D-A polyene reported by Blanchard-Desce et al. Interestingly, we have found that the size effect of GNR plays a key role in increasingβ0 for the H2N-GNR-NO2 system, in which the width effect of GNR perpendicular to the D-A direction is superior to the length effect along the D-A direction. It enriches the design and research of conventional organic NLO materials.2. Based on the donor-GNR conjugated bridge-acceptor (D-B-A) framework, we have designed smaller scale and high-performance nonlinear optical (NLO) materials.1) In the NH2-GNR-NO2 system, different shaped conjugated bridges of GNR with the same number of carbon atom are designed. We find that the more the fused bezene ring, the larger theβ0 value. Besides, the shape effect of GNR plays a very important role in enhancingβ0 for NH2-GNR-NO2 systems, such as the NH2- triangle-GNR-NO2 system displays a very largeβ0 value up to 1.7×105 au at the CAM-B3LYP/6-31G*.2) Substituting a donor (-NH2) on the edge of GNR forms one donor-acceptor mode. Meanwhile, Li doping into GNR forms another donor-acceptor mode. The twofold donor-acceptor mode greatly enhances NLO properties but not enlarge the size of systems.3. On the basis of ab initio method, we propose a physical method of external electric field (Eext) to drive the proton transfer from acid (HCl) to base (NH3/H2O). Our results show that when Eext along the proton-transfer direction achieves or exceeds the critical electric field (Ec), the proton transfer occurs. At the same time, theβ0 value of H3N-HCl and H2O-HCl is greatly increased. It is shown that the charge transfer has a key role in enhancing NLO property. This work inspired us to apply the external electric field to design NLO materials for our future work.4. The effects of substitution and cooperativity on spectrum and nonlinear optical properties in H3C…ClF complex has been studied with quantum chemical calculations at the UMP2(Full)/aug-cc-pVTZ level. The electron-withdrawing group (F atom) in the methyl (H3C-) makes the blue shift and the first hyperpolarizability (β0) decrease, whereas the electron-donating group (methyl group) in the methyl (H3C-) causes them increase. The cooperativity between different two types of halogen bonds in H3C…ClF…ClF complex enhances the strength of single-electron halogen bond and hyperpolarizability. The results of AIM and NBO show that the charge transfer plays an important role in enhancing the interaction, spectrum, and nonlinear optical properties.
Keywords/Search Tags:Graphene nanoribbon, charge transfer, nonlinear optics, electric field, density fuctional theory, ab initio
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