Quantum Mechanical Investigation Of Electronic Properties In Lindqvist-type Polyoxometalate Derivatives: Inorganic-Organic Hybrid Functional Materials

The design and synthesis of novel compounds hybridizing polyoxometalates (POMs) with a variety of organic ligands through supramolecular arrays have aroused particular interest and accelerated the development of this new kind of materials. We have quantum chemically designed these hybrid compounds, which are generally called anion-cation salts and host-guest solids, and which can possess active electrical and magnetic properties as well as desorption-readsorption properties. POMs can accept and release a certain number of electrons without decomposition, thus serving as multielectron relays. This attribute has made these inorganic-organic hybrids very attractive in electrode modification and electrocatalytic research. These POM-based inorganic-organic materials have immense potential in the field of electronics, photonics, and information technology. We have found new and novel insights into the properties of inorganic-organic hybrid functional materials.In this thesis, the quantum chemical approach has been carried out to investigate electronic properties, dipole polarizabilities, change of dipole moments (Δμ), density of states, and second-order nonlinear optical (NLO) properties of POM-based inorganic-organic hybrid functional materials by employing time dependent density functional theory (TDDFT).1- The first research report is a valuable addition in the field of advanced functional materials where the direction of charge transfer had been altered by changing the length of organic ligand in which hexamolybdates-to-organoimido charge transfer was a vital determinant to increase the NLO response of proposed functional material. The electron accepting property of POM cluster has been changed as it acts as a donor and organoimido ligand acts as an acceptor (D-bridge-A) via charge-transport property ofÏ€-conjugated bridge which has established the identity of POM as a donor as well in a rigorous way and has opened new horizons in the field of functional material chemistry. Here the quantum design of 1-D (D-D-A-A), 2-D, and 3-D nonlinear optical materials have also been suggested based on inorganic-organic hybrids with remarkably large NLO responses.2- In second research report the tunable NLO behavior of aryldiazenido hexamolybdates has been demonstrated and it may provide a new means for experimentalists to design high-performance functional materials based on inorganic-organic hybrid composites. The inclusion of organic ligand in inorganic cluster (POM) has changed the nature and direction of charge transfer which might be a surprising addition in the field of functional materials. The nonlinear optical properties of aryldiazenido hexamolybdates were studied by DFT analysis. An electron donor in the aryldiazenido ligand enhanced the first hyperpolarizability, and the NLO properties could be improved strikingly by increasing the conjugation path of the ligand. Thus, the NLO behavior of aryldiazenido hexamolybdates can be tuned for the design of high-performance NLO materials.3- In third research report the inclusion of the terpyridine moiety (organic pendant) in substituted hexamolybdates (POM) is shown to be important because they may act as efficient molecular precursors for application in supramolecular chemistry. The polyoxomolybdate cluster connected to pendant terpyridine units via a bridging group to generate donor-acceptor complexes suitable for applications as non linear optical materials. The study varied the substituents pendant to the terpyridine ligand and the results provide synthetic targets for the development of new functional materials with improved responses through the incorporation of various substituents. The orbital analysis shows that the degree of charge transfer (CT) between POM and terpyridine segment was increased in 2D and organometallic/POM hybrid systems. This investigation provides important and thought provoking insight into the robustly large NLO properties of terpyridine substituted hexamolybdates.4- In this work, NLO response of organoimido-substituted hexamolybdates has been tuned from 218.61×10^-30 esu to 490.10×10^-30 esu. The dipole polarizabilities and second-order nonlinear optical (NLO) properties of organoimido derivatives of hexamolybdates have been investigated by using time-dependent density functional response theory (TDDFT). The electron with drawing ability of F (fluorine) has played an important role to tune second-order NLO response in this class of organic-inorganic hybrid compounds particularly system 6 [Mo6O18(NC16H8F2(CF3)2I)]2- with the static second-order polarizability (βvec) computed to be 490.10×10^-30 esu. Thus, our studied systems have feasibility to be excellent tunable second-order nonlinear optical materials. The analysis of the major contributions to theβvec value suggests that the CT from POM to organic ligand (D–A) along the z-axis has been enhanced with addition of F atoms at the end phenyl ring which directs head (POM) to tail (fluorinated ring) charge transfer. The computedβvec values have been tuned by incorporation of different halogen atoms at the end phenyl ring of organoimido segment. Furthermore, substitution of two trifluoromethyl (-CF₃) groups sidewise along with iodine (I) at the terminus of end phenyl ring in organoimido ligand has striking influence to tune the optical nonlinearity as CT from POM to organoimido ligand was significantly increased. The systematic small changes in molecular composition by substitution of different halogen groups cause ripple effect which leads to tuning the NLO response; the so-called"ripple effect"catches this point nicely. The NLO response of systems (1-6) has been increased by increasing the strength of terminal group in the following order: CH₃ (1) < F (2) < Cl (3) < Br (4) < I (5) < CF3 (6). So, we can use the principle of Ripple Effect to magnify the actions of electron acceptors and their effects on NLO responses of studied systems. Thus present investigation provides thought provoking insight into the tunable NLO properties of organoimido substituted hexamolybdates. 5- A dramatic increase in second-order NLO response of terpyridine-substituted hexamolybdates has been observed from 886.55×10^-30 esu (system 1) to 4622.92×10^-30 esu (system 7). The dipole polarizabilities and second-order nonlinear optical (NLO) properties of terpyridine derivatives of hexamolybdates have been investigated by using time-dependent density functional response theory (TDDFT). The Quantum mechanical design suggests that [Mo₆O₁₈(N₄C₂₅H₁₄(CF₃)₂ (CN)₂)]^2- (system 7) is the best choice among all studied systems to improve nonlinearity. The electron withdrawing ability of electron acceptor groups (F, Cl, Br, I, CF3 and CN) at the end of terpyridine ligand directs the CT from POM-cluster to terpyridine segment along the z-axis which leads to an efficient second-order NLO molecular designing of our studied systems. These small changes in molecular composition by substitution may have disproportionally huge effects on the NLO properties, the so-called"butterfly effect"catches this point nicely.6- In this work, DFT calculations were carried out on vanadosilicate [Si8V14O50]12- and other related anions, the effect of substitution of Ge^IV, P^V, As^V with Si^IV on the electronic and redox properties was investigated. The substitution of Si^IV with Ge^IV, P^V, and As^V may open up possibilities for the experimentalists to synthesize other mimicked vandosilicates, vanadogermenates, vanadophosphates and vanadoarsenates as well as their heteropolyanions respectively.