Co-crystallization of a cationic Fe^II complex with a partially charged TCNQ^.δ− (7,7′,8,8′-tetracyanoquinodimethane) radical anion has afforded molecular materials that behave as narrow band-gap semiconductors, [Fe(tpma)(xbim)](X)(TCNQ)_1.5⋅DMF (X=ClO₄⁻ or BF₄⁻; tpma=tris(2-pyridylmethyl)amine, xbim=1,1′-(α,α′-o-xylyl)-2,2′-bisimidazole). Remarkably, these complexes also exhibit temperature-and light-driven spin crossover at the Fe^II center, and are thus the first structurally defined magnetically bistable semiconductors assembled with the TCNQ^.δ− radical anion. Transport measurements reveal the conductivity of 0.2 S cm⁻¹ at 300 K, with the low activation energy of 0.11 eV.

Co-crystallization of a cationic Fe^II complex with a partially charged TCNQ^.δ− (7,7′,8,8′-tetracyanoquinodimethane) radical anion has afforded molecular materials that behave as narrow band-gap semiconductors, [Fe(tpma)(xbim)](X)(TCNQ)_1.5⋅DMF (X=ClO₄⁻ or BF₄⁻; tpma=tris(2-pyridylmethyl)amine, xbim=1,1′-(α,α′-o-xylyl)-2,2′-bisimidazole). Remarkably, these complexes also exhibit temperature-and light-driven spin crossover at the Fe^II center, and are thus the first structurally defined magnetically bistable semiconductors assembled with the TCNQ^.δ− radical anion. Transport measurements reveal the conductivity of 0.2 S cm⁻¹ at 300 K, with the low activation energy of 0.11 eV.

Two novel tris-heteroleptic Ru–dipyrrinates were prepared and tested as sensitizers in the dye-sensitized solar cell (DSSC). Under AM 1.5 sunlight, DSSCs employing these dyes achieved power conversion efficiencies (PCEs) of 3.4 and 2.2 %, substantially exceeding the value achieved previously with a bis-heteroleptic dye (0.75 %). As shown by electrochemical measurements and DFT calculations, the improved PCEs stem from the synthetically tuned electronic structure, which affords more negative excited state redox potentials and favorable electron injection into the TiO₂ conduction band. Electron injection was quantified by nanosecond transient absorption spectroscopy, which revealed that the highest injection yield is achieved with the dye that acts as the strongest photoreductant.

Reactions between the complexes M₃(dpa)₄Cl₂ (M = Co, Ni; dpa = 2,2′-dipyridylamine), which contain linear trimetallic fragments, and (Bu₄N)₃[M′(CN)₆] (M′ = Fe, Co) result in the formation of CN-bridged coordination polymers. The analysis of the products obtained suggests that they have a two-dimensional structure, in which ditopic [M₃(dpa)₄]²⁺ linkers bridge 4-connected [M′(CN)₆]^3– nodes into an extended layer. The synthesis of {[Co₃(dpa)₄]_1.97[Fe(CN)₆]}Cl_0.8 (1) is accompanied by an electron transfer from the tricobalt to the hexacyanoferrate units, which results in the formation of [Co₃(dpa)₄]³⁺ and [Fe(CN)₆]^4– fragments. In {[Ni₃(dpa)₄]_1.74[Fe(CN)₆]}Cl_0.45 (3), a partial charge transfer between the trinickel and the hexacyanoferrate units leads to the temperature-dependent Fe^III/Fe^II mixed valence, and lower temperatures favor the thermodynamic Fe^III ground state. {[Co₃(dpa)₄]_2.06[Co(CN)₆]}Cl_1.1 (2) exhibits spin-glass behavior with a spin-freezing point of approximately 4.8 K, which is due to the magnetic superexchange between the paramagnetic [Co₃(dpa)₄]²⁺ (S = 1/2) units through the diamagnetic [Co(CN)₆]^3– linkers.

Three iron(II) complexes, [Fe(TPMA)(BIM)](ClO₄)₂⋅0.5H₂O (1), [Fe(TPMA)(XBIM)](ClO₄)₂ (2), and [Fe(TPMA)(XBBIM)](ClO₄)₂ ⋅0.75CH₃OH (3), were prepared by reactions of Fe^II perchlorate and the corresponding ligands (TPMA=tris(2-pyridylmethyl)amine, BIM=2,2′-biimidazole, XBIM=1,1′-(α,α′-o-xylyl)-2,2′-biimidazole, XBBIM=1,1′-(α,α′-o-xylyl)-2,2′-bibenzimidazole). The compounds were investigated by a combination of X-ray crystallography, magnetic and photomagnetic measurements, and Mössbauer and optical absorption spectroscopy. Complex 1 exhibits a gradual spin crossover (SCO) with T_1/2=190 K, whereas 2 exhibits an abrupt SCO with approximately 7 K thermal hysteresis (T_1/2=196 K on cooling and 203 K on heating). Complex 3 is in the high-spin state in the 2–300 K range. The difference in the magnetic behavior was traced to differences between the inter- and intramolecular interactions in 1 and 2. The crystal packing of 2 features a hierarchy of intermolecular interactions that result in increased cooperativity and abruptness of the spin transition. In 3, steric repulsion between H atoms of one of the pyridyl substituents of TPMA and one of the benzene rings of XBBIM results in a strong distortion of the Fe^II coordination environment, which stabilizes the high-spin state of the complex. Both 1 and 2 exhibit a photoinduced low-spin to high-spin transition (LIESST effect) at 5 K. The difference in the character of intermolecular interactions of 1 and 2 also manifests in the kinetics of the decay of the photoinduced high-spin state. For 1, the decay rate constant follows the single-exponential law, whereas for 2 it is a stretched exponential, reflecting the hierarchical nature of intermolecular contacts. The structural parameters of the photoinduced high-spin state at 50 K are similar to those determined for the high-spin state at 295 K. This study shows that N-alkylation of BIM has a negligible effect on the ligand field strength. Therefore, the combination of TPMA and BIM offers a promising ligand platform for the design of functionalized SCO complexes.

The front cover picture shows the clock tower, the “Campanile”, of Iowa State University where John Corbett did the ground-breaking research in polar intermetallics that forms the basis of his Viewpoint in this cluster issue. Superimposed on this background are structures and data to visualize the broad scope of topic. The complexity of structure is displayed by the ternary rare-earth cobalt gallides that contain interstitial atoms (top left, A. Mar et al.), a calcium-poor intermetallic phase of the Ca/Ni/Ge system (top right from the lab of T. Fässler), and a single crystal of a polymorph of thallium nickel gallide (bottom right, J. Chan et al.). The potentially general synthesis of colloidal nanoparticles – Au₃Li from the lab of R. E. Schaak – is outlined mid left. The groups of M. H. Whangbo and G. Miller devote their contributions to the theoretical aspects of bonding (depicted top centre, the plots showing Au–Au bonding and antibonding interactions in Dy₂Au₂In and mid right, the effects of ionic interactions on the structural properties of isoelectronic intermetallic compounds, respectively). Representative of the range of properties discussed is the magnetic susceptibility of Yb₅Ni₄Ge₁₀ (bottom left, M. G. Kanatzidis et al.). We thank the authors for the use of the graphics from their papers on the cover.

Ternary polar intermetallics Sm₁₁₇Co_55.6Sn₁₁₆, Tb₁₁₇Co₅₉Sn₁₁₁, and Dy₁₁₇Co₅₈Sn₁₁₁ were prepared by the arc-melting technique. They belong to the Tb₁₁₇Fe₅₂Ge₁₁₂ structure type and are characterized by a giant cubic unit cell (V > 25000 ų). The build-up of the structure can be understood by identifying three basic metal-centered polyhedra, a 14-vertex Co@Co₆Sn₈ at the edge centers and at the body center of the unit cell, a 14-vertex Sm@Co₈Sn₆ at the origin of the unit cell, and a 22-vertex Sm@Co₁₂Sn₁₀ at the center of each octant. The rest of the atoms in the crystal structure can be considered as forming concentric polyhedra around these basic units. Such an approach results in three types of large multishell polyhedra that share their outermost faces and provide complete space filling. The arrangement of these polyhedra follows the arrangement of atoms in the structure of Heusler alloy Cu₂MnAl. The description of this complex structure in terms of multishell clusters allows its separation into regions of different polarity, as each individual shell is composed of either Sm or Co/Sn atoms. Sm₁₁₇Co_55.6Sn₁₁₆ exhibits ferromagnetic ordering in the Sm sublattice at 86 K. Tb₁₁₇Co₅₉Sn₁₁₁ and Dy₁₁₇Co₅₈Sn₁₁₁ undergo consecutive paramagnet–ferromagnet and ferromagnet-canted–antiferromagnet phase transitions when the temperature is lowered. The transitions take place at 38 K and 20 K for Tb₁₁₇Co₅₉Sn₁₁₁ and at 30 K and 11 K for Dy₁₁₇Co₅₈Sn₁₁₁.