Li-Kai Yan

Redox-Switchable Second-Order Nonlinear Optical Responses of Push−Pull Monotetrathiafulvalene-Metalloporphyrins

The redox-active tetrathiafulvalene (TTF) is a good electron donor, and porphyrin is highly delocalized in cyclic pi-conjugated systems. The direct combination of the two interesting building units into the same molecule provides an intriguing molecular system for designing nonlinear optical (NLO) molecular materials. In the present paper, the second-order NLO properties of a series of monoTTF-porphyrins and metalloporphyrins have been calculated by density functional theory (DFT) combined with the finite field (FF) method. Our calculations show that these compounds possess considerably large static first hyperpolarizabilities, approximately 400 x 10(-30) esu. Since the TTF unit is able to exist in three different stable redox states (TTF, TTF(*+), and TTF(2+)), the redox switching of the NLO response of the zinc(II) derivative of monoTTF-metalloporphyrin has been studied, and a substantial enhancement in static first hyperpolarizability has been obtained in its oxidized species according to our DFT-FF calculations. The beta values of one- and two-electron-oxidized species are 3.6 and 8.7 times as large as that of the neutral compound, especially for two-electron-oxidized species, with a value of 3384 x 10(-30) esu. This value is about 3 times that for a push-pull metalloporphyrin, which has an exceptionally large hyperpolarizability among reported organic NLO chromophores. Meanwhile, to give a more intuitive description of band assignments of the electron spectrum and trends in NLO behavior of these compounds, the time-dependent (TD)DFT method has been adopted to calculate the electron spectrum. The TDDFT calculations well-reproduce the soret band and Q-type bands of the monoTTF-porphyrin, and these absorption bands can be assigned to the pi --> pi* transition of the porphyrin core. On the other hand, the oxidized process significantly affects the geometrical structures of the TTF unit and porphyrin ring, and the two-electron-oxidized species has a planar TTF unit and a high conjugative porphyrin ring. This effect reduces the excited energy, changes the CT feature, and thus enhances its static first hyperpolarizability.

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.

Three novel 3D (3,8)-connected metal–organic frameworks constructed from flexible-rigid mixed ligands

Three novel 3D (3,8)-connected networks, namely [Cd2(L1)2(1,3-bdc)] 1, [Cd2(L2)2(1,4-bdc)]·3H2O 2 and [Cd3(L3)2(1,3-bdc)2] 3, where L1 = 3,5-bis(pyridin-4-ylmethoxy)benzoic acid, L2 = 3,4-bis(pyridin-4-ylmethoxy)benzoic acid, L3 = 3,5-bis(pyridin-3-ylmethoxy)benzoic acid, bdc = benzenedicarboxylate, have been synthesized hydrothermally and characterized. Complex 1 exhibits a twofold interpenetrating network constructed from Cd2-based secondary building units (SBUs) [Cd2(CO2)2O2N4] with (42·6)2(44·622·82) topology. Complex 2 is constructed from Cd2-based SBUs [Cd2(CO2)4N4] with (43)2(46·618·84) topology. The structure of 3 is based on linear Cd3-based SBUs [Cd3(CO2)6N2] with a Schlafli symbol (42·5)2(44·56·610·75·82·9). In addition, X-ray powder diffraction (XRPD) and luminescent properties of 1–3 were also investigated in detail.

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

The static first hyperpolarizabilities and origin of nonlinear optical (NLO) properties of [(2-methylnaphthyl)imido]hexamolybdates derivatives have been investigated by density functional theory (DFT). The [(2-methylnaphthyl)imido]hexamolybdate has considerable large first hyperpolarizability, 6.780 x 10(-30) esu, and it is larger than that of [(2,6-dimethylphenyl)arylimido]hexamolybdate due to the double aromatic rings in the naphthylimido ligand. The naphthylimido ligand acts as an electron-donor and the polyanion acts as an electron-acceptor. The substituent position on the naphthylimido is a key factor to determine the first hyperpolarizability of (naphthylimido)hexamolybdate derivatives. The derivative, which the iodine atom locates on the para nitrogen on the naphthylimido ligand, has the largest betao(o) value among the iodine-substituted derivatives. It suggests that the iodine atom is quasi linear with nitrogen and Mo, which is bonded to thenitrogen atom, could generate a large static electronic field and give the large contribution to NLO response.The introducing of electron-donors significantly enhances the first hyperpolarizabilities of (naphthylimido)hexamolybdates comparing with the electron-acceptors as the electron-donating ability is significantly enhanced when the electron-donor is attached to the naphthylimido segment. The present investigation provides important insight into NLO properties of (arylimido)molybdate derivatives.

Reversible redox-switchable second-order optical nonlinearity in polyoxometalate: a quantum chemical study of [PW11O39(ReN)]n- (n = 3-7).

In this paper, the relationship between the reversible redox properties and the second-order nonlinear optical (NLO) responses for the title series of complexes has been systematically investigated by using the time-dependent density functional theory (TDDFT) method combined with the sum-over-states (SOS) formalism. The results reveal that the successive reduction processes of five PW11ReN redox states should be PW11ReVII (1) --> PW11ReVI (2) --> PW11ReV (3) --> PW11ReV1e ( 4) --> PW 11ReV2e (5). Furthermore, their electrochemical properties have been reproduced successfully. It is noteworthy that the second-order NLO behaviors can be switched by reversible redox for the present studied complexes. Full oxidation constitutes a convenient way to switch off the second-order polarizability (system 1). The incorporation of extra electrons causes significant enhancement in the second-order NLO activity, especially for the third reduced state (system 4), whose static second-order polarizability (betavec) is about 144 times larger than that of fully oxidized 1. The characteristic of the charge-transfer transition corresponding to the dominant contributions to the betavec values indicates that metal-centered redox processes influence the intramolecular donor or acceptor character. Therefore, these kinds of complexes with the facile and reversible redox states could become excellent switchable NLO materials.

Prediction of second-order optical nonlinearity of porphyrin–metal–polyoxometalate sandwich compounds

The second-order nonlinear optical (NLO) properties of porphyrin-metal-polyoxometalate (por-metal-POM) sandwich structures [(por)M(PW(11)O(39))](5-) (por = TPP, TPyP, TPPF(20), M = Hf; por = TPP, M = Zr) and [(TPP)Hf(XW(11)O(36))](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 an electron acceptor and the lacunary Keggin-type POM acts as an electron donor. Our results show that this kind of organic-inorganic hybrid compound possesses remarkably large molecular second-order NLO polarizability, approximately 100 x 10(-30) esu, and might be an excellent second-order NLO material. Furthermore, the NLO response can be tuned by the element substituents. The computed beta(0) values increase with the auxiliary electron-accepting group on the porphyrin ring (TPPF(20) > 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 compound.

N-Carbon coated P-W2C composite as efficient electrocatalyst for hydrogen evolution reactions over the whole pH range

Development of a highly efficient and stable hydrogen evolution reaction (HER) electrocatalyst based on non-precious-metal remains a great challenge. Herein, we report P-modified tungsten carbide encapsulated in N-doped carbon (P-W2C@NC) as an efficient and stable HER electrocatalyst over the whole pH range (0–14), prepared by annealing polyoxometalate (H3PW12O40·xH2O) and dicyandiamide at a high temperature. To achieve a current density of 10 mA cm−2, P-W2C@NC required an overpotential of 89 mV in acid, 63 mV in alkaline solution and 185 mV in neutral solution. This is the first case of a tungsten-carbide-based HER electrocatalyst operating at all pH values with high activity and stability, and might provide some new routes for the exploration of tungsten-based Pt-like electrocatalysts for the HER.

Quantum Chemical Studies on High-Valent Metal Nitrido Derivatives of Keggin-Type Polyoxometalates ([PW11O39{MVIN}]4− (M = Ru, Os, Re)): MVI−N Bonding and Electronic Structures

High-valent M(VI)N (M = Ru, Os) species are important reagents in nitrogen transfer reactions; the unique withdrawing properties of polyoxometalate (POMs) ligands would possibly modify the reactivity of the M(VI)N functional group. In the present paper, density functional theory (DFT) and natural bond orbital (NBO) analysis have been employed to calculate electronic structures, M(VI)-N bonding, and redox properties of high-valent metal nitrido derivatives of Keggin-type POMs, [PW(11)O(39) {M(VI)N}](4-) (M = Ru, Os, Re). Our calculations show that [PW(11)O(39){RuN}](4-) possesses stronger antibonding interaction between metal and nitrogen atoms compared with anions [PW(11)O(39){OsN}](4-) and [PW(11)O(39){ReN}](4-). A large increase in the Ru-N bond length of anion [PW(11)O(39){RuN}](4-) in the excited states has been found; the effective order and composition of the molecular orbital in anion [PW(11)O(39){RuN}](4-) is a key factor in determination of the increase of the Ru-N bond length in the excited states. The substitution effects of central tetrahedron heteroatoms (XO(4), X = Al, Si, P, As) in anions [XW(11)O(39){RuN}](4-) affect the relative energy of the LUMO; the relevant orbital energy increases in the order Al(III) < Si(IV) < P(V) approximately As(V). The RuN unit is the reduced center. NBO analysis of the extent of the bonding interaction between the ruthenium and the nitrogen centers in [PW(11)O(39){Ru(VI)N}](4-) shows that the Ru-N bond possesses a covalent feature and displays triple-, double-, and single-bond character when moving along the change of spin state ((1)1 --> (3)1 --> (5)1).

Inorganic-organic hybrid compounds based on the co-existence of different isomers or forms of polymolybdate†

Three novel compounds co-existing isomers or forms of polymolybdate, namely, {[Ni(γ-Mo8O26)(H2L1)2(H2O)2](γ-Mo8O26)(H2L1)}·2H2O (1), {[Co(γ-Mo8O26)(H2L1)2(H2O)2](β-Mo8O26)[Co(H2O)6]}·6H2O (2) and [CuI6L24(β-Mo8O26)(Mo6O19)] (3), where L1 = 1,1′-bis(pyridin-3-ylmethyl)-2,2′-biimidazole, L2 = 3-((1H-1,2,4-triazol-1-yl)methyl)pyridine, have been successfully synthesized. Crystal structure analysis reveals that 1 is a 1D + 1D supramolecular structure containing both γ-[Mo8O26N2] and γ-[Mo8O28] units; 2 has γ-[Mo8O26]4− as well as β-[Mo8O26]4− anions and 3 is a (3,4,6)-connected 2D network containing both β-[Mo8O26]4− and [Mo6O19]2− anions. According to quantum chemical calculations, the transformation from β-[Mo8O26]4− to [Mo6O19]2− is spontaneous in aqueous solution. Furthermore, we have investigated the optical band gap of compounds 1 and 3, with 3 showing a lower band gap (1.69 eV). Additionally, the photoluminescent property of 3 is also studied, which can be assigned to metal-to-ligand charge transfer (MLCT).

Theoretical investigation on electronic structure and second-order nonlinear optical properties of novel hexamolybdate-organoimido-(car)borane hybrid.

We report a theoretical study based on density functional theory (DFT) on the geometric and electronic structure, linear optical and second-order nonlinear optical properties of a series of new inorganic-organic hybrid hexamolybdate-organoimido-(car)boranes. By the incorporation of borane/carborane at the end of the phenyl ring of the organoimido segment, the studied systems show excellent nonlinear optical (NLO) response than the organoimido-substituted hexamolybdate. The computed static first hyperpolarizability β(vec) value of [Mo(6)O(18)(NC(8)H(8))(B(12)H(11))](4-) (II) is largest, -167.2 × 10(-30) esu, and a higher β(vec) value of [Mo(6)O(18)(NC(8)H(8))(C(2)B(10)H(11))](2-) (III-2p) is 58.6 × 10(-30) esu. Moreover, the time-dependent (TD)DFT calculation illustrates that the maximum absorption, which is helpful for the large NLO responses, is mainly assigned to the charge transfer (CT) from (car)borane and organoimido segment to the hexamolybdate cluster. The density of density (DOS) calculations further illustrate the excitation from valence orbitals of boron atoms to that of Mo and O atoms in hexamolybdate can be responsible for larger NLO responses. The linear and nonlinear optical properties of species III both vary with the position of the vertex on the carborane. Furthermore, the order of the β(vec) values is consistent with the bathochromic shift of the maximum absorption for our studied systems, and the studied systems show a wider transparency range extending into the entire visible and infrared (IR) region.