Mixed-stack complexes which comprise columns of alternating donors and acceptors are organic conductors with typically poor electrical conductivity because they are either in a neutral or highly ionic state. This indicates that conductive carriers are insufficient or are mainly localized. In this study, mixed-stack complexes that uniquely exist at the neutral-ionic boundary were synthesized by combining donors (bis(3,4-ethylenedichalcogenothiophene)) and acceptors (fluorinated tetracyanoquinodimethanes) with similar energy levels and orbital symmetry between the highest occupied molecular orbital of the donor and the lowest unoccupied molecular orbital of the acceptor. Surprisingly, the orbitals were highly hybridized in the single-crystal complexes, enhancing the room-temperature conductivity (10-4-0.1 S cm-1) of mixed-stack complexes. Specifically, the maximum conductivity was the highest reported for single-crystal mixed-stack complexes under ambient pressures. The unique electronic structures at the neutral-ionic boundary exhibited structural perturbations between their electron-itinerant and localized states, causing abrupt temperature-dependent changes in their electrical, optical, dielectric, and magnetic properties.
An anomalously high valence state sometimes shows up in transition-metal oxide compounds. In such systems, holes tend to occupy mainly the ligand p orbitals, giving rise to interesting physical properties such as superconductivity in cuprates and rich magnetic phases in ferrates. However, no one has ever observed the distribution of ligand holes in real space. Here, a successful observation of the spatial distribution of valence electrons in cubic perovskite SrFeO3 by high-energy X-ray diffraction experiments and precise electron density analysis using a core differential Fourier synthesis method is reported. A real-space picture of ligand holes formed by the orbital hybridization of Fe 3d and O 2p is revealed. The anomalous valence state in Fe is attributed to the considerable contribution of the ligand hole, which is related to the metallic nature and the absence of Jahn-Teller distortions in this system.
Proton-coupled electron transfer (PCET) is a ubiquitous and fundamental process in biochemistry and electrochemistry performed by transition-metal complexes. Most synthetic efforts have been devoted to selecting the components, that is, metal ions and ligands, to control the proton-electron coupling. Here, we show the first example of controlling the proton-electron coupling using the cis-trans metal-ligand isomerization in a π-planar platinum complex, Pt(itsq)2 (itsq1-: o-iminothiosemiquinonate). Both the isomers, which were obtained separately, were characterized by single-crystal X-ray diffraction, and the cis-to-trans isomerization was achieved by immersing in organic solvents. Theoretical calculations predicted that the proton-electron coupling evaluated from the energetic stabilization of the lowest unoccupied molecular orbital by protonation varies greatly depending on the geometrical configuration compared to the metal substitution.
The synthesis of lanthanide metal-organic frameworks with terephthalate (Ln-BDC-MOFs) was investigated using a data-driven approach. Visually mapping the previously reported synthetic conditions suggested the existence of unexplored search spaces for novel Ln-BDC-MOFs. By focusing on the unexplored chemical reaction space, we successfully synthesized a series of new anionic Ln-BDC-MOFs, KGF-15, which demonstrated potential as luminescent sensors for Cu2+ ions. This synthetic exploration approach can significantly reduce the experimental effort required to discover new materials.
We observed the crystallization dynamics of halide perovskite crystals (CH3NH3PbI3) by in situ heating wide-angle X-ray scattering measurements. As a result, we revealed that crystal growth occurs during the conversion of complexes to perovskite crystals.
2,2',7,7'-Tetrakis(N,N-di-p-methoxyphenylamino)-9,9'-spirobifluorene (spiro-OMeTAD) is utilized as a p-type semiconductor layer in perovskite solar cells and solid-state dye-sensitized solar cells. Spiro-OMeTAD has been known to have a spiro center, leading to a random orientation. Although the molecular orientation of organic semiconductor materials influences the conductivity, which is directly related to semiconductor device characteristics, the molecular orientation of spiro-OMeTAD has not been fully discussed. In this study, we prepared spiro-OMeTAD layers on various substrates and investigated their orientation by grazing-incidence wide-angle X-ray scattering (GIWAXS) and near-edge X-ray absorption fine structure (NEXAFS). Additionally, we demonstrated that the molecular orientation of spiro-OMeTAD could be controlled by changing their surface energies by changing the substrate materials. Consequently, we could improve the electrical conductivity by improving its molecular orientation. The results of this study provide a guideline for the preparation of organic semiconductor material layers using the wet-coating method.
Metal halide perovskites are promising materials for light absorbers in solar cell applications. Use of the Br/I system enables us to control band gap energy and improves the efficiency of solar cells. Precise knowledge of lattice parameters and band gap energies as functions of compositions are crucially important for developing the devices using those materials. In this study, we have determined lattice parameters and band gap energies of CH3NH3Pb(I1-xBrx)3, one of the most intensively studied mix-halide perovskites, as functions of Br content x. We measured accurate Br contents and lattice parameters of CH3NH3Pb(I1-xBrx)3 (0 ≤ x ≤ 1) using single-crystalline samples by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) measurements, respectively. The CH3NH3Pb(I1-xBrx)3 crystal system is tetragonal for x ≤ 0.06 and cubic for x ≥ 0.08 at 300 K. Lattice parameters of CH3NH3Pb(I1-xBrx)3 strictly follow Vegard's law; i.e., they are linearly dependent on x. We give linear expressions of x of lattice parameters for the tetragonal and cubic phases of CH3NH3Pb(I1-xBrx)3 at 300 K. We have shown that these expressions can be used for determining the Br contents of CH3NH3Pb(I1-xBrx)3 polycrystalline thin-film samples based on XRD measurements and, in addition, demonstrated that XPS measurements on polycrystalline samples may be erroneous because of impure ingredients in the samples. Furthermore, we determined band gap energies of CH3NH3Pb(I1-xBrx)3 (0 ≤ x ≤ 1) at room temperature using absorption spectra of polycrystalline thin films taking account of excitonic effects.
