Chun-Guang Liu

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.

Second-Order Nonlinear Optical Properties of Trisubstituted Keggin and Wells−Dawson Polyoxometalates: Density Functional Theory Investigation of the Inorganic Donor-Conjugated Bridge−Acceptor Structure

The donor-conjugated bridge-acceptor (D-A) model, as a simple molecular scheme, has been successfully used in the development of second-order organic compound, organometallic compound, and metal complex nonlinear optical (NLO) materials. However, for the totally inorganic molecules, the use of this model is still prohibitive. In the present paper, time-dependent density functional theory (TDDFT) was used to investigate the second-order NLO properties of vanadium- and molybdenum-trisubstituted Keggin and Wells-Dawson polyoxometalates (POMs). The results show that these POM clusters possess D-A structures. The oxygen atoms in the cap region and metal (vanadium and molybdenum) atoms in another cap region in these POM clusters can be viewed as the electron donor and acceptor, respectively. The vanadium ion derivatives possess larger second-order NLO responses and dipole moment than molybdenum ions derivatives; thus, the three vanadium atoms in the cap region act as a strong acceptor related to the three molybdenum atoms in cap region in our D-A scheme. The vanadomolybdate with Wells-Dawson structure displays the good second-order NLO response because of the relevant long conjugated bridge and strong acceptor. This D-A model may be an effective approach for optimizing the first hyperpolarizabilities of inorganic POM clusters.

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.

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).

Theoretical studies on phosphoraniminato derivatives of Keggin-type polyoxometalates [PW 11 O 39 {M V NPPh 3 }] 3− (M = Fe, Ru): Electronic structures and bonding features

Transition metal phosphoraniminato derivatives of Keggin-type polyoxometalates (POMs) are important intermediates in N-transfer reactions. Density functional theory (DFT) has been employed to calculate the electronic structures, bonding features and redox properties of the iron and ruthenium phosphoraniminato derivatives of Keggin-type POMs, [PW11O39{MVNPPh3}]3− (M = Fe, Ru). Our DFT calculations show that both anions have the same qualitative M-N single bond features. However, the calculations predict that the FeN system possesses a lower energy and more accessible metal-nitrogen antibonding orbital than the RuN system. This results in a greater weakening of the Fe-N bond in the reduction process, and thus enhances its N-transfer reactivity.

DFT STUDIES ON ELECTRONIC STRUCTURES AND THIRD-ORDER NONLINEAR OPTICAL PROPERTIES OF A SERIES OF Pt–Pt BOND-CONTAINING METAL COMPLEXES

In this paper, electronic structures and third-order nonlinear optical (NLO) properties of a series of Pt–Pt bond-containing metal complexes have been calculated by using the density functional theory (DFT) combining with the sum-over-states (SOS) method. In order to check the reliability of this method, the electron correlations and basis sets have been compared. Our calculated results show that introduction of electron donor, thiophene ring and lengthing of organic conjugated ligand can enhance third-order NLO responses. The electronic structure analysis shows that the [Pt–Pt(H2P(CH2)PH2)2] fragment displays strong electron-withdrawing character in these systems. Meanwhile, the third-order NLO response of Pt–Pt bond also has been estimated by using TDDFT–SOS method. An enhancement of third-order NLO response has been observed because of the introduction of the Pt–Pt bond. This is mainly due to the intense and low-lying metal-to-ligand charge transfer (MLCT) and intraligand (IL) CT transitions.