Phosphonate-Stabilized Titanium-Oxo Clusters with Ferrocene Photosensitizer: Structures, Photophysical and Photoelectrochemical Properties, and DFT/TDDFT Calculations.

The metal-to-core charge transfer (MCCT) transition in sensitized titanium-oxo clusters is an important process for photoinduced electron injection in photovoltaic conversion. This process resembles most closely the Type II photoinjection in dye-sensitized solar cells. Herein we report the synthesis and photophysical and photoelectrochemical (PEC) properties of the phosphonate-stabilized titanium-oxo clusters containing the ferrocenecarboxylate ligands. These ferrocene-containing clusters exhibit intense visible absorption extended up to 600 nm along with low optical band gaps of ∼2.2 eV. The low-energy transitions of these clusters were systematically investigated by UV-vis spectroscopy and DFT/TDDFT calculations. The combined experimental and computational studies suggest that the ferrocenecarboxylate-substituted titanium-oxo clusters form a donor-acceptor (D-A) system. The low-energy transition of these clusters primarily involves the MCCT from the iron center to TiO cluster core. The TiO core structure and phosphonate ligands both have great influence on the PEC properties of the clusters. This work provides valuable examples for the sensitized titanium-oxo clusters in which electron injection takes place via MCCT transition.

Syntheses and characterization of three new sulfides with large band gaps: acentric Ba4Ga4SnS12, centric Ba12Sn4S23 and Ba7Sn3S13.

The desirable development of infrared nonlinear optical (IR NLO) materials is to design new compounds which exhibit wide band gaps and strong second harmonic generation (SHG) responses. Herein, we report three new sulfides, Ba4Ga4SnS12 (1), Ba12Sn4S23 (2) and Ba7Sn3S13 (3), with wide band gaps of 2.90, 2.98 and 3.0 eV, respectively, which have been successfully synthesized for the first time. Significantly, compound 1 exhibited a large SHG coefficient (34 × KDP), illustrating a good balance between the band gap and the SHG response. Single crystal X-ray diffraction determined that compound 1 crystallizes in the non-centrosymmetric space group P4[combining macron]21c and it was characterized as an interesting kite-shaped linkage motif of ∞[Ga4SnS12]. Compounds 2 and 3 crystallize in the space groups of P21c and Pnma, respectively. In addition, compounds 2 and 3 were characterized as zero-dimensional (0D) structures comprising isolated SnS4 tetrahedra with Ba2+ cations and S2- anions located between them. However, compound 2 contains extra disulfide S22- anions in its isolated structure. Moreover, the theoretical calculations demonstrated that SHG responses for compound 1 could be ascribed to the transitions from S-3p and Ga-4p states to Ba-5d, Ga-4p and Sn-5p states. By analysing the relationship between the structures and properties for Pb4Ga4GeS12-type compounds, it was concluded that site disorder could be an effective way to improve optical properties.

A 4-dimethylaminobenzoate-functionalized Ti6-oxo cluster with a narrow band gap and enhanced photoelectrochemical activity: a combined experimental and computational study.

Organic donor-π-bridge-acceptor (D-π-A) dyes with arylamines as an electron donor have been widely used as photosensitizers for dye-sensitized solar cells (DSSCs). However, titanium-oxo clusters (TOCs) functionalized with this kind of D-π-A structured dye-molecule have rarely been explored. In the present study, the 4-dimethylaminobenzoate-functionalized titanium-oxo cluster [Ti6(µ3-O)6(OiPr)6(DMABA)6]·2C6H5CH3 (DMABA = 4-dimethylaminobenzoate) was synthesized and structurally characterized by single-crystal X-ray diffraction. For comparison, two other Ti6-oxo clusters, namely [Ti6(µ3-O)6(OiPr)6(AD)6] (AD = 1-adamantanecarboxylate) and [Ti6(µ3-O)2(µ2-O)(µ2-OiPr)4(OiPr)10(DMM)2] (DMM = dimethylmalonate), were also studied. The DMABA-functionalized cluster exhibits a remarkably reduced band gap of ∼2.5 eV and much enhanced photocurrent response in comparison with the other two clusters. The electronic structures and electronic transitions of the clusters were studied by DFT and TDDFT calculations. The computational results suggest that the low-energy transitions of the DMABA-functionalized cluster have a substantial charge-transfer character arising from the DMABA → {Ti6} cluster core ligand-to-core charge transfer (LCCT), along with the DMABA-based intra-ligand charge transfer (ILCT). These low-energy charge transfer transitions provide efficient electron injection pathways for photon-to-electron conversion.

A ferrocenecarboxylate-functionalized titanium-oxo-cluster: the ferrocene wheel as a sensitizer for photocurrent response.

Sensitized titanium-oxo clusters (TOCs) have attracted growing interest. However, reports on TOCs incorporated with a metal complex as photosensitizers are still very rare. In the present work, the organometallic complex ferrocene was used as a sensitizer for a titanium-oxo cluster. A ferrocenecarboxylate-substituted titanium-oxo cluster [Ti6(µ3-O)6(OiPr)6(O2CFc)6] (Fc = ferrocenyl) was synthesized and structurally characterized, in which the ferrocene wheel performs as a sensitizer for photocurrent response. For comparison, naphthalene-sensitized titanium-oxo clusters [Ti6(µ3-O)6(OiPr)6(NA)6] (NA = 1-naphthoate) and [Ti6(µ3-O)6(OiPr)6(NAA)6] (NAA = 1-naphthylacetate) with the same {Ti6} core structure were also synthesized. The structures, optical behaviors, electronic states and photoelectrochemical properties of these sensitized {Ti6} clusters were investigated. It is demonstrated that the introduction of ferrocene groups into the titanium-oxo cluster significantly reduces the band gap and enhances the photocurrent response in comparison with the naphthalene-sensitized clusters. The substantially reduced band gap of the ferrocene-sensitized cluster was attributed to the introduction of Fe(ii) d-d transitions and the possible contribution from the Fc → {Ti6} charge transfer. For the naphthalene-sensitized clusters, the better electronic coupling between the dye and the {Ti6} core in the 1-naphthoate (NA) substituted cluster results in higher photoelectrochemical activity.

Tailored synthesis of nonlinear optical quaternary chalcohalides: Ba4Ge3S9Cl2, Ba4Si3Se9Cl2 and Ba4Ge3Se9Cl2.

Three novel zero-dimensional quaternary chalcohalides, Ba4Ge3S9Cl2, Ba4Si3Se9Cl2 and Ba4Ge3Se9Cl2, which crystallize in the polar noncentrosymmetric space group P63 (no. 173), have been rationally synthesized by a tailored approach on the basis of unique [M3Q9]6- (M = Ge, Q = S; M = Ge/Si, Q = Se) units with Ba2+ cations and Cl- anions occupying the interspaces. The [M3Q9]6- units which consist of three Q-corner-sharing [MQ4]4- tetrahedra, are arranged along the 63 screw axis. Remarkably, Ba4Ge3S9Cl2 exhibits a strong powder second harmonic generation (SHG) response that is 2.4 times that of benchmark AgGaS2 at a laser radiation of 2.05 µm in the same particle size range of 46-74 µm. Furthermore, theoretical studies based on the density functional theory helped to gain insight into the origin of the SHG.

Pb5Ga6ZnS15: a noncentrosymmetric framework with chains of T2-supertetrahedra.

A novel noncentrosysmmetric sulfide, Pb5Ga6ZnS15 (1), was synthesized for the first time. Its crystal structure revealed the presence of [GaS2](-)∞ (chain 1, chains of [GaS4](5-) tetrahedra) and [Ga4S9](6-)∞ (chain 2, chains of T2-supertetrahedra) connected by isolated [ZnS4](6-) tetrahedra. Structure correlation with the network constructed solely using chain 1 (Pb4Ga4GeQ12-type) is discussed. Pb5Ga6ZnS15 was calculated to have a suitable static briefringence (Δn) of 0.1137, and a large nonlinear optical susceptibility d31 of 58.32 pm V(-1) at a wavelength of 2.05 µm (0.6 eV), much higher than that of the benchmark AgGaS2 (18.14 pm V(-1)).