| Polyoxometalates (POMs) constitute a class of compounds formed by early transition metals with unique photonic, electronic, magnetic properties and chemical reactivity that have promised dramatic applications including catalysis, medicine, and materials sciences. The electronic, redox and bonding properties, isomer stabilities, as well as optical properties those originate from the POMs, which have charge transfer have been receiving increasing attention. In the present thesis, quantum chemistry calculations have been carried out to investigate electronic properties and the second-order nonlinear optical (NLO) properties of a series of Keggin-type POM derivatives and Lindqvist-type POM derivatives. The present work has focus on the following four aspects:1. The NLO properties of porphyrin-metal-polyoxometalate (por-metal-POM) sandwich structures [(por)M(PW11O39)]5- (por=TPP, TPyP, TPPF20, M=Hf; por=TPP, M=Zr) and [(TPP)Hf(XW11O36)]6- (X=Si, Ge) are investigated by time-dependent density functional theory (TDDFT). The character of charge-transfer transition indicates that the porphyrin ligand acts as electron acceptor and lacunary Keggin-type POM acts as electron donor. Our results show that this kind of organic-inorganic hybrid compounds possesses remarkably large molecular second-order NLO polarizability, ~100×10-30 esu, and might be excellent second-order NLO materials. Furthermore, the NLO response can be tuned by the element substituents. The computedβ0 values increase as the auxiliary electron-accepting group on porphyrin ring (TPPF20 > TPyP > TPP) and a heavy central heteroatom (Ge > Si > P). The present investigation provides important insight into the NLO properties of this class of por-metal-POM sandwich compounds.2. The second-order NLO properties of Keggin-type organosilicone derivative [PW11O39(RSi2O)]3- are investigated by TDDFT. Our results indicated that the length of conjugated chain R has a crucial effect on the charge transfer (CT). The direction of CT alters with the end-cap-substituted electron donating or accepting moieties (NH2 or NO2), until the chain length reach a certain length, as the CT originates from the heteropolyanion to organosilicone moiety along the chain, further chain lengthening leaves this behavior invariant. The derivatives with long chain substitution and end-cap-substituted electron acceptor (NO2) display excellent second-order NLO response. The system 18 with the relevant long conjugated polyphenylethynyl chain and end-cap-substituted electron acceptor (NO2) has the largestβ0 value, 4538.1×10-30 esu. The present investigation provides important insight into the characteristic of CT and NLO properties of Keggin-type organosilicone derivatives.3. The present study focuses on probing the extent to which nonlinear optical effects as a guide to protonation sites ofα-Keggin heteropolyanions [SiW12O40]4- and [SiV3W9O40]7-. The second-harmonic generation (SHG) values of different protonated forms predict that the molecular nonlinear optical activity has a closely relationship with protonation site and provides a new potential experimental probe.4. DFT calculations were carried out to investigate the second-order NLO properties of a series of proposed porphyrin-polyoxometalate-based complexes related to [5-(3,5-dimethyl-4-hexamolybdate amino-phenyl-ethynyl)-15-(4-nitrophenyl -ethynyl)porphinato]zinc(II) which possess donor-πconjugated bridge-acceptor (D-π-A) configurations. Our calculations show that these species possess considerably large molecular second-order polarizability, ~2000×10-30 esu. Furthermore, it can be seen that {W6O18} exhibits stronger electron-donating ability than {Mo6O18}. And 2-D system with A-π-D-π-A structure might be a promising candidate for NLO materials based on the largeβ0 (4583.5×10-30 esu) and u (u =βzxx /βzzz) value.
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