Theoretical Study on the Considerable Second-Order Nonlinear Optical Properties of Naphthylimido-Substituted Hexamolybdates

Theoretical exploration on switchable NLO response induced by redox properties of polyoxometalates [XNbW11O40]n-/(n+1)- (X = Al, Si, P, S, Ga, Ge, As, Se)

Calculations based on density functional theory (DFT) have been performed to investigate the relationship between redox properties and second-order nonlinear optical (NLO) responses of Keggin-type anions [XNbW₁₁O₄₀]^n- (X = Al, Si, P, S, Ga, Ge, As, Se) and their corresponding heteropoly blues. The frontier molecular orbitals, NLO responses and electronic transition properties of fully oxidized anions and heteropoly blues were discussed. It is noticeable that the second-order NLO behaviors can be switched by reduced electrons for these Keggin-type anions. The calculated β₀ value of heteropoly blues [XNbW₁₁O₄₀]^(n+1)- (X = P, As, S) are 21-23 times as large as those of fully oxidized anions [XNbW₁₁O₄₀]^n- (X = P, As, S) according to CAM-B3LYP/6-31G* (LANL2DZ basis sets for metal atom) calculations in CH₃CN. The aim of this work is to combine the attractive redox properties and NLO response of polyoxometalates to obtain the switchable NLO materials.

Theoretical exploration to the cation effect on the second-order nonlinear optical properties of Strandberg-type polyoxometalates

The combination of cations with octahedral coordinated d0 transition metal ions has been proved to be an effective way for designing new polar materials. So we investigate the second-order nonlinear optical (NLO) properties of Strandberg-type polyoxometalates (POMs) with alkali metal cations M 6 Mo 5 X 2 O 23 ( M = K +, Rb +, Cs +; X = P , As ) and M 4 M o5 X 2 O 21 ( M = K +, Rb +, Cs +; X = S , Se , Te ) by density functional theory (DFT) method. The calculated results show that this kind of Strandberg-type POMs possesses remarkably large molecular second-order NLO polarizability, especially for the Cs 6 Mo 5 P 2 O 23 (system Ic), which has a computed β0 value of 12526 a.u. and might be an excellent second-order NLO material. Moreover, the cations have important impact on the second-order NLO polarizabilities. Therefore, a careful choice of appropriate cations may allow the control of the second-order NLO response on these Strandberg-type POMs, which may provide a new route to design efficient NLO materials.

Exploration of charge transfer and absorption spectra of porphyrin–polyoxometalate hybrids to search for high performance sensitizers

Lindqvist-, Keggin-, and Anderson-type polyoxometalate organic–inorganic hybrids were designed to investigate their charge transfer character and screen them as high performance p-type sensitizers.

Theoretical study on the rectifying performance of organoimido derivatives of hexamolybdates

We design a new type of molecular diode, based on the organoimido derivatives of hexamolybdates, by exploring the rectifying performances using density functional theory combined with the non-equilibrium Green's function. Asymmetric current-voltage characteristics were obtained for the models with an unexpected large rectification ratio. The rectifying behavior can be understood by the asymmetrical shift of the transmission peak observed under different polarities. It is interesting to find that the preferred electron-transport direction in our studied system is different from that of the organic D-bridge-A system. The results show that the studied organic-inorganic hybrid systems have an intrinsically robust rectifying ratio, which should be taken into consideration in the design of the molecular diodes.

The self-assembly mechanism of the Lindqvist anion [W6O19]2- in aqueous solution: a density functional theory study

The formation mechanism is always a fundamental and confused issue for polyoxometalate chemistry. Two formation mechanisms (M1 and M2) of the Lindqvist anion [W(6)O(19)](2-) have been adopted to investigate it's self-assembly reaction pathways at a density functional theory (DFT) level. The potential energy surfaces reveal that both the mechanisms are thermodynamically favorable and overall barrierless at room temperature, but M2 is slightly dominant to M1. The formation of the pentanuclear species [W(5)O(16)](2-) and [W(5)O(15)(OH)](-) are recognized as the rate-determining steps in the whole assembly polymerization processes. These two steps involve the highest energy barriers with 30.48 kcal mol(-1) and 28.90 kcal mol(-1), respectively, for M1 and M2. [W(4)O(13)](2-) and [W(4)O(12)(OH)](-) are proved to be the most stable building blocks. In addition, DFT results reveal that the formation of [W(3)O(10)](2-) experiences a lower barrier along the chain channel.

Structure, properties and reactivity of polyoxometalates: a theoretical perspective

In the thematic review dedicated to polyoxometalate (POM) chemistry published in Chemical Reviews in 1998, no contribution was devoted to theory. This is not surprising because computational modelling of molecular metal-oxide clusters was in its infancy at that time. Nowadays, the situation has completely changed and modern computational methods have been successfully applied to study the structure, electronic properties, spectroscopy and reactivity of POM clusters. Indeed, the progress achieved during the past decade has been spectacular and herein we critically review the most important papers to provide the reader with an almost complete perspective of the field.

Theoretical investigation of the mechanism of primary amines reacting with hexamolybdate: an insight into the organoimido functionalization and related reactions of polyoxometalates

The functionalization of polyoxometalates (POMs), especially with an amino group to yield organonitrogenous derivatives of POMs, is an efficient approach to the enrichment of their structures and the diversification of their properties for various applications. The mechanism for the formation of organonitrogenous-derivatized hexamolybdates was explored by investigating the monofunctionalization of the [Mo(6)O(19)](2-) ion with methylamine using the density functional theory (DFT) method. The calculations show that the direct imidoylization of hexamolybdate with methylamine is both kinetically and thermodynamically unfavorable. However, this imidoylization was found to take place readily in the presence of dimethylcarbodiimide (DMC), for which the free-energy barrier was calculated to be +32.5 kcal mol(-1) in acetonitrile. Moreover, various factors controlling the efficiency of the imidoylization were examined. The calculations show that [W(5)MoO(19)](2-) has a relatively lower reactivity than [Mo(6)O(19)](2-), and that the imidoylization of [W(6)O(19)](2-) is an unfavorable process. With respect to the effect of carbodiimides, it is found that the catalytic activity is directly proportional to the electron-withdrawing effects of the substituents. As to the reactivity of R-NH(2) , the computation results indicate that the free-energy barriers of the substitution reactions are linearly correlated with the basicity constants (pK(b)) of the amino groups. It is noteworthy that the introduction of the proton dramatically decreases the free-energy barrier of the imidoylization of [Mo(6)O(19)](2-) catalyzed by DMC to 24.3 kcal mol(-1) in acetonitrile.

Theoretical investigation of second-order nonlinear optical response — Hexamolybdate as a superior donor over metal carbonyl complexes in the D–π–A model

Density functional theory (DFT) calculations were carried out to investigate the nonlinear optical (NLO) response for the donor-conjugated bridge-accepter (D–π–A) model of p-nitroaniline (PNA) – hexamolybdate derivatives and PNA–metal–carbonyl complexes. The bond length alternation (BLA) values decrease with lengthening of the π-conjugated bridge, especially for PNA–hexamolybdate derivatives, which dramatically enhances the NLO response. In addition, the introduction of Mo≡N in PNA–hexamolybdate derivatives is expected to provide a better electron transition channel, consequently generating lower BLA values and an outstanding NLO response compared with PNA–metal–carbonyl complexes. It is shown that the hexamolybdate acts as an electron donor when incorporating metal–carbonyl complexes into one molecule. All these behaviors reflect the superiority of hexamolybdate as a donor moiety in the D–π–A model for the design of potential NLO materials.

Tuning Second-Order Non-linear (NLO) Optical Response of Organoimido-Substituted Hexamolybdates through Halogens: Quantum Design of Novel Organic-Inorganic Hybrid NLO Materials

The second-order non-linear optical (NLO) response of organoimido-substituted hexamolybdates has been tuned from 218.61 × 10–30 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 the time-dependent density functional response theory (TDDFT). The electron withdrawing ability of F (fluorine) has played an important role in tuning the 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 the feasibility to be excellent tuneable second-order NLO materials. The analysis of the major contributions to the βvec value suggests that the charge transfer (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 computed βvec 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 sideways along with iodine (I) at the terminus of end phenyl ring in the organoimido ligand has a striking influence on tuning the optical non-linearity, as CT from POM to the organoimido ligand was significantly increased. These systematic small changes in molecular composition by substitution of different halogen groups leads to a tuning the NLO response; the so-called ‘ripple effect’ catches this point nicely. Thus, the present investigation provides thought provoking insight into the tuneable NLO properties of organoimido-substituted hexamolybdates.

Prediction of remarkably large second-order nonlinear optical properties of organoimido-substituted hexamolybdates

The dipole polarizabilities, dipole moments, density of states, and second-order nonlinear optical (NLO) properties of organoimido derivatives of hexamolybdates have been investigated by using time-dependent density functional response theory. This class of organic-inorganic hybrid compounds possesses remarkably large and eye-catching molecular second-order NLO response, especially [Mo(6)O(17)(NC(16)H(12)NO(2))(FeNC(10)H(9))](2-) (7) and [Mo(6)O(17)(NC(16)H(12)NO(2))(NC(6)H(2)(NH(2))(3))](2-) (6) with static second-order polarizability (beta(vec)) computed to be 15766.27 x 10(-30) esu and 6299.59 x 10(-30) esu, respectively. Thus, these systems have the possibility to be excellent second-order nonlinear optical materials. Analysis of the major contributions to the beta(vec) value suggests that the charge transfer (CT) from polyanion to organic segment (D-A) along the z-axis plays the key role in NLO response; the polyanion acts as a donor (D) whereas organoimido acts as an acceptor (A) in all the studied systems. The computed beta(vec) values increase by incorporation of an electron acceptor (-NO(2)) at the end of the phenyl ring of the organoimido segment. Furthermore, substitution of amino (-NH(2)) or ferrocenyl (-FeC(10)H(9)) at the outer side of polyanion and an electron acceptor (-NO(2)) at the end of the phenyl ring in organoimido segment simultaneously is more important to enhance the optical nonlinearity. Orbital analysis shows that the degree of CT between the polyanion and organoimido segments was increased when ferrocenyl donor was introduced. The present investigation provides important insight into the remarkably large NLO properties of organoimido-substituted hexamolybdates.