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Singlet fission (SF) converts a singlet exciton into two triplet excitons in two or more electronically coupled organic chromophores, which may then be used to increase solar cell efficiency. Many known SF chromophores are unsuitable for device applications due to chemical instability or low triplet state energies. The results described here show that efficient SF occurs in derivatives of 9,10-bis(phenylethynyl)anthracene (BPEA), which is a highly robust and tunable chromophore. Fluoro and methoxy substituents at the 4- and 4′-positions of the BPEA phenyl groups control the intermolecular packing in the crystal structure, which alters the interchromophore electronic coupling, while also changing the SF energetics. The lowest excited singlet state (S1) energy of 4,4′-difluoro-BPEA is higher than that of BPEA so that the increased thermodynamic favorability of SF results in a (16 ± 2 ps)−1 SF rate and a 180% ± 16% triplet yield, which is about an order of magnitude faster than BPEA with a comparable triplet yield. By contrast, 4-fluoro-4′-methoxy-BPEA and 4,4′-dimethoxy-BPEA have slower SF rates, (90 ± 20 ps)−1 and (120 ± 10 ps)−1, and lower triplet yields, (110 ± 4)% and (168 ± 7)%, respectively, than 4,4′-difluoro-BPEA. These differences are attributed to changes in the crystal structure controlling interchromophore electronic coupling as well as SF energetics in these polycrystalline solids. [ABSTRACT FROM AUTHOR]

Abstract: The three solid-state indium/germanium selenides Cs4In8GeSe16, CsInSe2, and CsInGeSe4 have been synthesized at 1173K. The structure of Cs4In8GeSe16 is a three-dimensional framework whereas those of CsInSe2 and CsInGeSe4 comprise sheets separated by Cs cations. Both Cs4In8GeSe16 and CsInGeSe4 display In/Ge disorder. From optical absorption measurements these compounds have band gaps of 2.20 and 2.32eV, respectively. All three compounds are charge balanced. [Copyright &y& Elsevier]

Abstract: The compound Rb2Th7Se15 has been synthesized by the solid-state reaction at 1273K of Th, Rb2Se3, Se, and Ge, and its structure has been determined by single-crystal X-ray diffraction methods. Red crystals of Rb2Th7Se15 crystallize at 100(2)K with four formula units in a new structure type in the monoclinic space group −P21/c. The structure is three-dimensional and comprises Rb and Th atoms coordinated by Se atoms to form nine-, eight-, and seven-coordinate polyhedra. Infinite channels that contain Rb atoms are present. Crystals of Rb2Th7Se15 are red in color; an indirect band gap of 1.83eV was derived from single-crystal optical measurements. [Copyright &y& Elsevier]

The compounds ThOS, ThOSe, and ThOTe have been synthesized, and their structures have been determined by means of single-crystal X-ray diffraction methods. All three compounds adopt the PbFCl structure type in the tetragonal space group D4h7 - P4/nmm. More precise crystallographic data have been obtained for ThOS and ThOSe, which had previously only been known from X-ray powder diffraction data. ThOS, ThOSe, and ThOTe are yellow-, orange-, and black-colored, respectively. From single-crystal optical absorption measurements the band gaps are 2.22, 1.65, and 1.45 eV, respectively. Optical band gaps, ionic charges, and densities of states were calculated for the three compounds with the use of Density Functional methods. [ABSTRACT FROM AUTHOR]

Abstract: CsU2(PO4)3 was synthesized in highest yield by the reaction in a fused-silica tube of U, P, and Se in a CsCl flux at 1273K. It crystallizes with four formula units in space group P21/n of the monoclinic system in a new structure type. The structure of CsU2(PO4)3 is composed of U and Cs atoms coordinated by PO4 3− units in distorted octahedral arrangements. Each U atom corner shares with six PO4 3− units. Each Cs atom face shares with one, edge shares with two, and corner shares with three PO4 3− units. The structure shares some features with the sodium zirconium phosphate structure type. X-ray powder diffraction results demonstrate that the present CsU2(PO4)3 compound crystallizes in a structure different from the previously reported β′- and γ-CsU2(PO4)3 compounds. CsU2(PO4)3 is highly pleochroic, as demonstrated by single-crystal optical absorption measurements. [Copyright &y& Elsevier]

Abstract: The compound Ba2Cu2ThS5 has been synthesized by the reaction of BaS, S, Cu, and Th at 1173K. It crystallizes in the monoclinic space group with two formula units in a cell of dimensions a=13.5616(8)Å, b=4.1159(2)Å, c=9.3548(6)Å, β=115.89(1)° (100K). The structure comprises Ba2+ cations and layers. The two dimensional layers are formed by the connection of ThS6 octahedra and CuS4 tetrahedra along [001] with a sequence…. oct tet tet oct tet tet…. Optical measurements performed on Ba2Cu2ThS5 indicate an indirect band gap of 1.86eV. DFT calculations performed with the HSE06 functional yield band gaps of 1.7eV for Ba2Cu2ThS5 and 1.5eV for the isostructural compound Ba2Cu2US5. [Copyright &y& Elsevier]

The compounds Ba₂An(S₂)₂S₂ (An = U, Th) have been synthesized by reactions of the elements with BaS and S at 1273 and 1173 K, respectively. These isostructural compounds crystallize in a new structure type in the tetragonal space group D4h15-P4₂/nmc. The structure comprises Ba2+ cations and ²∞[An(S₂)₂(S)₂4-] layers. The An4+ cations in these layers are arranged linearly and are bridged by S2- anions. Coordination about the An center, which has symmetry 4m2, consists of two S₂2- ions and four S2- ions. Thus, the compounds are charge-balanced with An4+. No other alkali-metal actinide chalcogenides are known that contain chalcogen--chalcogen bonds. Optical measurements on Ba₂Th(S₂)₂S₂ indicate a direct band gap of 2.46(5) eV. Density functional theory calculations, performed with the HSE exchange-correlation potential, lead to band gaps of 2.2 and 1.8 eV for Ba₂Th(S₂)₂S₂ and Ba₂U(S₂)₂S₂, respectively, thus demonstrating the utility of applying this functional to 5f-electron systems. [ABSTRACT FROM AUTHOR]

Abstract: The interlanthanide compounds Er3SmS6, Er3SmSe6, and Er1.12Sm0.88Se3 have been synthesized from stoichiometric reactions of the elements in a KI salt flux at 1273, 1173, and 1123K, respectively. Er3SmS6 and Er3SmSe6, which are isostructural and ordered, crystallize in space group P21/m in the ScEr3S6 structure type whereas Er1.12Sm0.88Se3, in which the Er and Sm atoms are disordered, crystallizes in space group Pnma in the U2S3 structure type. Er3SmS6 is a paramagnet with a μ eff=11.25(1)μB/mol. From optical measurements a direct band gap of 2.0eV for light perpendicular to the (100) crystal face of Er3SmSe6 is derived whereas for isostructural Er3SmS6 an optical transition at 2.2–2.4eV and a broad absorption peak at lower energies are observed. [Copyright &y& Elsevier]

Three new compounds, Cs2Bi2ZnS5, Cs2Bi2CdS5, and Cs2Bi2MnS5, have been synthesized from the respective elements and a reactive flux Cs2S3 at 973 K. The compounds are isostructural and crystallize in a new structure type in space group Pnma of the orthorhombic system with four formula units in cells of dimensions at 153 K of a=15.763(3), b=4.0965(9), c=18.197(4) A˚, V=1175.0(4) A˚3 for Cs2Bi2ZnS5; a=15.817(2), b=4.1782(6), c=18.473(3) A˚, V=1220.8(3) A˚3 for Cs2Bi2CdS5; and a=15.830(2), b=4.1515(5), c=18.372(2) A˚, V=1207.4(2) A˚3 for Cs2Bi2MnS5. The structure is composed of two-dimensional ∞2[Bi2MS52−] (M=Zn, Cd, Mn) layers that stack perpendicular to the [100] axis and are separated by Cs+ cations. The layers consist of edge-sharing ∞1[Bi2S66−] and ∞1[MS34−] chains built from BiS6 octahedral and MS4 tetrahedral units. Two crystallographically unique Cs atoms are coordinated to S atoms in octahedral and monocapped trigonal prismatic environments. The structure of Cs2Bi2MS5, is related to that of Na2ZrCu2S4 and those of the AMM′Q3 materials (A=alkali metal, M=rare-earth or Group 4 element, M′= Group 11 or 12 element, Q=chalcogen). First-principles theoretical calculations indicate that Cs2Bi2ZnS5 and Cs2Bi2CdS5 are semiconductors with indirect band gaps of 1.85 and 1.75 eV, respectively. The experimental band gap for Cs2Bi2CdS5 is ≈1.7 eV, as derived from its optical absorption spectrum. [Copyright &y& Elsevier]