Xingguo Chen

A New Mixed Halide, Cs2HgI2Cl2: Molecular Engineering for a New Nonlinear Optical Material in the Infrared Region

A new mixed halide, Cs(2)HgI(2)Cl(2), which contains the highly polar tetrahedron of anion (HgI(2)Cl(2))(2-), has been designed and synthesized by reaction in solution. In its single crystal, the isolated (HgCl(2)I(2))(2-) groups are arranged to form chains. The chains are then further connected into a three-dimensional framework through the Cs atoms that occupy the empty spaces surrounded by halide atoms. All the polar (HgCl(2)I(2))(2-) groups align in such a way that gives a net polarization, leading it to show a phase matchable second harmonic generation (SHG) effect as strong as that of KH(2)PO(4) (KDP) based on the powder SHG measurement. It also displays excellent transparency in the range of 0.4-41 μm with relatively high thermal stability. A preliminary measurement indicates that its laser-induced damage threshold is about 83 MW/cm(2), about twice that of AgGaS(2). This study demonstrates that Cs(2)HgI(2)Cl(2) is a promising nonlinear optical material in the infrared region.

Rb2CdBr2I2: a new IR nonlinear optical material with a large laser damage threshold.

This paper describes the synthesis, crystal structure, and photophysical properties of a new compound Rb2CdBr2I2. It crystallizes in the noncentrosymmetric space group Ama2. In the crystal, all the distorted tetrahedron [CdBr2I2](2-) groups are arranged in a way such that all the Cd-I bonds are located in the same side of the Cd atoms resulting in a net polarization. Rb2CdBr2I2 showed a powder second harmonic generation (SHG) response 4 times that of KH2PO4 (KDP). The preliminary measurement indicated that it exhibits a large laser-induced damage threshold (LDT) of 190 MW/cm(2) which is 6 times that of AgGaS2. It also exhibits a wide transparent region (0.37-14 μm) with a relatively high (up to 490 °C) thermal stability. All these indicate that Rb2CdBr2I2 is a new promising candidate for NLO materials in the IR region.

New organic dyes containing tert-Butyl-capped N-Arylcarbazole moiety for Dye-sensitized solar cells

Two new organic dyes with tert-butyl-capped N-arylcarbazole as a donor, cyanoacrylic acid as an acceptor and a bithiophene unit as a π-linker (DH-11 and DH-12) have been synthesized and characterized for dye-sensitized solar cells (DSSCs). It is found that the introduction of tert-butyl-capped N-arylcarbazole as an electron donor can efficiently suppress the intermolecular aggregation and improve the photovoltaic performances. The DSSC devices based on the dyes show relatively high power conversion efficiency of 3.67 and 3.75% for DH-11 and DH-12, respectively, which reaches over 65% of the reference dye N719-based cell fabricated and measured under the same conditions. This infers that the tert-butyl-capped N-arylcarbazole unit is a promising electron-donor that can be employed to design metal-free sensitizers with a new structural skeleton.

Synthesis and photophysical properties of hyperbranched polyfluorenes containing 2,4,6-tris(thiophen-2-yl)-1,3,5-triazine as the core.

A series of new hyperbranched polymers containing a 2,4,6-tris(thiophen-2-yl)-1,3,5-triazine core unit and polyfluorene chain arms have been synthesized via Suzuki coupling, and characterized by NMR, IR and GPC. All the polymers exhibit good thermal stability with a high decomposition temperature. By changing the 2,4,6-tris(thiophen-2-yl)-1,3,5-triazine/fluorene ratio the UV-vis absorption and emission spectra can be partially tuned. It has been found that the polymers containing a low ratio of 2,4,6-tris(thiophen-2-yl)-1,3,5-triazine units (P1-P3) have an absorption maximum around 385 nm, localized in the polyfluorene chain, and a shoulder around 425 nm ascribable to a charge transfer state involving the fluorene and the 2,4,6-tris(thiophen-2-yl)-1,3,5-triazine core. Increasing the molar ratio of the 2,4,6-tris(thiophen-2-yl)-1,3,5-triazine unit enhances the charge transfer band which becomes dominant for P4. The LUMO level of these polymers is relatively low due to the electron affinity of the triazine group. The polymers show dual emission, with a structured band in the blue (410-440 nm), attributed to the polyfluorene, and a broad band in the red (470-500 nm) associated with the charge transfer state. All the polymers exhibit two-photon absorption activity in the range of 660 to 900 nm with the maximum two-photon absorption (TPA) cross-section red-shifted from the corresponding linear absorption. The values of the TPA cross-sections vary from 1000 to 5000 GM, following the 2,4,6-tris(thiophen-2-yl)-1,3,5-triazine/fluorene ratio.

Designing a thiophene-fused DPP unit to build an A–D–A molecule for solution-processed solar cells

A novel thiophene-fused diketopyrrolopyrrole unit (7H-pyrrolo[3,4-a]thieno[3,2-g]indolizine-7,10(9H)-dione, PTI) has been designed as an electron acceptor with the 4,4′-bis-(2-ethylhexyl)-dithieno[3,2-b:2′,3′-d]silole (DTS) unit as a donor to construct a new kind of A–D–A molecule (DTS–2TPTI) for solution-processed solar cells. DTS–2TPTI exhibits excellent thermal stability with the decomposition temperature over 400 °C and shows strong absorption from 550 to 750 nm with a high molar extinction coefficient. The optical band gap (Eg) estimated from the absorption edge of the thin film is about 1.44 eV. The highest occupied molecular orbital energy level of DTS–2TPTI determined from CV is about −4.99 eV. Through optimizing the photovoltaic performances of devices, the DTS–2TPTI/PC71BM-based solution processed bulk-heterojunction solar cell with 0.5% DIO as a solvent additive exhibits the best photovoltaic performance with a JSC of 11.28 mA cm−2, VOC of 0.64 V, FF of 59.5% and power conversion efficiency (PCE) of 4.28%, indicating that the PTI unit can act as an efficient acceptor moiety to construct D–A small molecules for OPVs.

A2BiI5O15 (A = K+ or Rb+): two new promising nonlinear optical materials containing [I3O9]3− bridging anionic groups

Two new alkali metal bismuth iodates, K2BiI5O15 and Rb2BiI5O15, have been synthesized by a hydrothermal method. They are isostructural with the same noncentrosymmetric (NCS) orthorhombic space group Abm2 and contain a unique [I3O9]3− bridging anionic group. Their powders showed phase-matchable second-harmonic generation (SHG) effects 3 times that of KH2PO4 (KDP) and the laser-induced damage threshold values of 84 and 72 MW cm−2, respectively. They also exhibit a wide transparent range (up to 12 μm) and good thermal stability (450 °C).

A promising nonlinear optical material in the Mid-IR region: new results on synthesis, crystal structure and properties of noncentrosymmetric β-HgBrCl

This paper reports, for the first time, the nonlinear optical property of β-HgBrCl, which was synthesized by a reaction of Hg2Cl2 and Br2 in EtOH. This reaction also gave α-HgBrCl. And the single crystal structures of both α- and β-HgBrCl are presented and compared. β-HgBrCl shows phase-matchable powder second harmonic generation (SHG) effect of about 2 times that of KH2PO4 (KDP). Its absorption edge in UV-vis spectrum locates at 366 nm, implying a wide band gap and therefore higher laser damage threshold. It also exhibits a relatively wide transparent window (from 0.4 to 20 μm) and relatively good thermal stability.

Symmetrical and unsymmetrical multibranched D–π–A molecules based on 1,3,5-triazine unit: synthesis and photophysical properties

A series of V-shaped (D–π–A–π–D) and star-shaped ((D–π–)3A) molecules based on the electron acceptor 1,3,5-triazine core (A) connected to different electron donor groups (D) by a carbon–carbon triple bond as a conjugation bridge (π) has been synthesized. The studied molecules can be separated into symmetrically and unsymmetrically substituted molecules depending on the combination of the electron donating branches connected to the triazine core. Their photophysical properties are characterized experimentally and the structure–properties relationship is analysed with the aid of theoretical calculations. The symmetrically branched 1,3,5-triazine-based molecules exhibit similar UV-vis absorption to the corresponding linear molecules, but an obvious blue-shift in the emission is observed with increasing dimensionality. The absorption of the unsymmetrically branched 1,3,5-triazine-based molecules is clearly localized on a specific branch, suggesting a weak interbranch conjugation in the ground state. Emission is mainly controlled by the branch with the lowest energy excited state, which corresponds to the one with the largest intramolecular charge transfer (ICT) effect. The two-photon absorption properties of selected molecules are studied. They exhibit strong two-photon absorption activities and a modest interbranch conjugation effect, enhancing the TPA cross-section beyond the additive effect of increasing branch number.

A study on K2SbF2Cl3 as a new mid-IR nonlinear optical material: new synthesis and excellent properties

To search for a new nonlinear optical (NLO) material to be used in the mid-IR region with excellent comprehensive performance including high laser damage threshold (LDT), K2SbF2Cl3 has been synthesized by the hydrothermal method, and its potential as such a new material is evaluated for the first time. Its powder shows phase-matchable second harmonic generation (SHG) response of approximately 4 times as strong as that of KH2PO4 (KDP). Its optical band gap is 4.01 eV, which is much wider than the band gaps of the currently commercialised IR NLO crystals. This means that K2SbF2Cl3 will exhibit a much higher LDT. Its powder also exhibits excellent transparency in the range of 0.31–14 μm and good thermal stability. The relationship between the composition, crystal structure and properties is discussed. These results indicate that K2SbF2Cl3 is a promising candidate for IR NLO materials.

A promising new nonlinear optical crystal with high laser damage threshold for application in the IR region: synthesis, crystal structure and properties of noncentrosymmetric CsHgBr3

Searching for new IR nonlinear optical (NLO) crystals with high laser damage threshold (LDT) has become one of the great challenges in this field. This paper reports the synthesis, single crystal structure and properties of noncentrosymmetric CsHgBr3, which is a new crystalline phase and shows a high LDT of about 226 MW cm−2, together with good comprehensive performance. Its crystal can be obtained by reaction of CsBr and HgCl2 in acetone. It crystallizes in the trigonal space group P32 (no. 145). Its powders show a phase-matchable second harmonic generation as strong as that of KH2PO4 (KDP). It also displays excellent transparency in the range of 0.42–31 μm and is thermally stable up to 250 °C. All these behaviors make it a promising new NLO crystal in the mid-IR region.