Rie Suizu

Molecular, crystal, and thin-film structures of octathio[8]circulene: release of antiaromatic molecular distortion and lamellar structure of self-assembling thin films.

Carbon–sulfur compounds have been studied extensively as key materials in organic/molecular electronics. A variety of polyand oligothiophenes have been synthesized by organic synthesis and electrochemistry to elucidate their electronic properties. TTF (=1,4,5,8-tetrathiafulvalene) is a very important molecular skeleton as a donor for organic metals and superconductors; [TTF]ACHTUNGTRENNUNG[TCNQ (7,7,8,8-tetracyanoquinodimethane)] was the first organic metal to be developed into organic superconductors in the salts of TMTSF (= tetramethyltetraselenafulvalene), BEDT-TTF (bis(ethylenedithio)tetrathiafulvalene). In these materials, intermolecular S···S contacts play a crucial role in bringing about multidimensional electronic structures that are advantageous to electrical conduction. Cyclic oligothiophenes and thiocirculenes possess promising molecular structures from this perspective; the sulfur atoms are exposed to the outside of the molecular rings. Intermolecular S···S contacts are naturally expected in their crystals. In addition, their molecular structures are of considerable interest because they involve the radialene framework that has been a theoretical and experimental matter of concern with regard to exocyclic double bonds, aromaticity, and p-conjugation. Recently, Nenajdenko et al. synthesized octathio[8]circulene (2) from the parent compound, tetrathiophene (1), and 2 named “sulflower”. They obtained 2 as a red powder and estimated molecular and crystal structures from powder X-ray diffraction data, concluding the presence of a closed packing structure in the solid state with short intermolecular S···S contacts. Although the synthesis of 2 has attracted much attention, there is no single-crystal X-ray analysis; the molecular structure of 2 is open to debating on bond lengths, antiaromaticity. In our previous work, we carried out crystal growths and thin-film fabrications of a planar macrocyclic molecule, tetrakis(thiadiazole)porphyrazine, with strong self-assembling capabilities due to electrostatic S···N contacts and p–p interactions. The investigations revealed a highly ordered molecular arrangement in the thin films, which is nearly the same as that in the bulk crystals. In the present work, we prepared the single crystals and thin films of the sulflower 2. The molecular, crystal, and thin-film structures of 2 are described in detail, on the basis of the results of X-ray singlecrystal analysis, in comparison with those of 1. Single crystals of the parent compound 1 were grown by slow evaporation of the Et2O solution at room temperature. While the X-ray structure of 1 has been reported previously, we obtained a new polymorph belonging to the monoclinic P21/c space group. [17] In this new polymorph, there are two crystallographically independent molecules, whereas their molecular structures are nearly the same. Figure 1 shows top and side views of one of the two molecules. The bond lengths of 1 are shown in Figure S1 (Supporting Information) and Table 1, where the endo bond is the one shared

The key role of vibrational entropy in the phase transitions of dithiazolyl-based bistable magnetic materials

The neutral radical 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) is a prototype of molecule-based bistable materials. TTTA crystals undergo a first-order phase transition between their low-temperature diamagnetic and high-temperature paramagnetic phases, with a large hysteresis loop that encompasses room temperature. Here, based on ab initio molecular dynamics simulations and new X-ray measurements, we uncover that the regular stacking motif of the high-temperature polymorph is the result of a fast intra-stack pair-exchange dynamics, whereby TTTA radicals continually exchange the adjacent TTTA neighbour (upper or lower) with which they form an eclipsed dimer. Such unique dynamics, observed in the paramagnetic phase within the whole hysteresis loop, is the origin of a significant vibrational entropic gain in the low-temperature to high-temperature transition and thereby it plays a key role in driving the phase transition. This finding provides a new key concept that needs to be explored for the rational design of novel molecule-based bistable magnetic materials.

Monovalent and Mixed-Valent Potassium Salts of (1,2,5)Thiadiazolo(3,4-f )(1,10)phenanthroline 1,1-Dioxide: A Radical Anion for Multidimensional Network Structures

A novel phenanthlorine derivative, [1,2,5]thiadiazolo[3,4-f][1,10]phenanthroline 1,1-dioxide (tdapO(2)), was prepared to act as a radical-anion building block for coordination polymers. The crystal structures and magnetic properties of the monovalent and mixed-valent radical-anion salts K·tdapO(2) and K·(tdapO(2))(2) were elucidated and confirm the possibility of tdapO(2) to act as a bridging ligand and its capability to exhibit magnetic ordering at 15 K.

Multidimensional Network Structures and Versatile Magnetic Properties of Intermolecular Compounds of a Radical–Anion Ligand, [1,2,5]Thiadiazolo[3,4-f][1,10]phenanthroline 1,1-Dioxide

The crystal structures and magnetic properties of seven kinds of [1,2,5]thiadiazolo[3,4-f][1,10]phenanthroline 1,1-dioxide (tdapO2) radical-anion salts, namely, K·tdapO2, K·tdapO2·0.5MeCN, K·(tdapO2)2, Rb·(tdapO2)2, Cs7·(tdapO2)6·ClO4, (NH4)2·tdapO2·I, and Hppda·tdapO2·MeCN, were investigated. Single-crystal X-ray analyses of these radical-anion salts revealed formation of π-stacking columns and the presence of intercolumnar coordination bonding or hydrogen bonding. The intermolecular magnetic coupling constants in these salts range from strong antiferromagnetic (J/kB = -310 K) to ferromagnetic (J/kB = 24 K). Ab initio calculations performed on the nearest-neighbor radical pairs in the π-stacking columns suggested that the magnetic interactions are strongly governed by the overlap between the two anionic radical species and well explain the observed ferromagnetic and antiferromagnetic interactions. In addition, calculations of a hypothetical oxygen-less tdap analogue suggested that the presence of oxygen in tdapO2 significantly reduces the hopping integral and enhances the probability of ferromagnetic interaction.

Discovery of the K4 Structure Formed by a Triangular π Radical Anion.

The K4 structure was theoretically predicted for trivalent chemical species, such as sp(2) carbon. However, since attempts to synthesize the K4 carbon have not succeeded, this allotrope has been regarded as a crystal form that might not exist in nature. In the present work, we carried out electrochemical crystallization of the radical anion salts of a triangular molecule, naphthalene diimide (NDI)-Δ, using various electrolytes. X-ray crystal analysis of the obtained crystals revealed the K4 structure, which was formed by the unique intermolecular π overlap directed toward three directions from the triangular-shape NDI-Δ radical anions. Electron paramagnetic resonance and static magnetic measurements confirmed the radical anion state of NDI-Δ and indicated an antiferromagnetic intermolecular interaction with the Weiss constant of θ = -10 K. The band structure calculation suggested characteristic features of the present material, such as a metallic ground state, Dirac cones, and flat bands.

Fe(II) spincrossover complex of [1,2,5]thiadiazolo[3,4-f][1,10]phenanthroline

We prepared an iron(II) spin crossover complex with a new ligand, [1,2,5]thiadiazolo[3,4-f][1,10]phenanthroline, which is a 1,10-phenanthroline analogue annulated with a thiadiazole moiety. X-Ray crystal analysis indicated a 3D network structure formed by short S⋯S contacts, and magnetic measurements revealed a broad transition above room temperature.

Spatially inhomogeneous, stepwise phase transitions in a thiazyl diradical: a structural mismatch induced by lattice transformation

A heterocyclic thiazyl diradical, bis(1,2,3,5-dithiadiazolyl)-4,4′-biphenylene (1), was synthesized, and its structural and thermodynamic properties were characterized. This compound exhibits unique spatially inhomogeneous, first-order phase transitions at 306 and 359 K, in a stepwise fashion. The unit cell in the high-temperature phase above 359 K consists of four regular π-stacking columns with the same stacking manners having a uniform interplanar distance d. These four columns are connected by intermolecular S⋯S and S⋯N contacts between the thiazyl radical moieties, but the molecular planes are shifted by a distance of d/4 due to a steric hindrance between the bulky biphenylene moieties. Below 359 K, three of the four stacking columns exhibit structural transitions toward zigzag chain structures, formed by an alternating dimerization between the radical moieties, while one column maintains the uniform π stacking. In the low-temperature phase below 306 K, the last column finally exhibits the radical dimerization toward a zigzag chain structure. These unique phase transitions in 1 can be understood in terms of the stepwise transitions from a “dimer liquid” to a “dimer solid” through a “dimer soliton phase”.

Self-assembled honeycomb lattice in the monolayer of cyclic thiazyl diradical BDTDA (= 4,4'-bis(1,2,3,5-dithiadiazolyl)) on Cu(111) with a zero-bias tunneling spectra anomaly.

Scanning tunneling microscopy (STM) observation reveals that a cyclic thiazyl diradical, BDTDA (= 4,4′-bis(1,2,3,5-dithiadiazolyl)), forms a well-ordered monolayer honeycomb lattice consisting of paramagnetic corners with unpaired electrons on a clean Cu(111) surface. This BDTDA lattice is commensurate with the triangular lattice of Cu(111), with the former being 3 × 3 larger than the latter. The formation of the BDTDA monolayer structure, which is significantly different from its bulk form, is attributed to an interaction with the metal surface as well as the intermolecular assembling forces. STM spectroscopy measurements on the BDTDA molecules indicate the presence of a characteristic zero-bias anomaly centered at the Fermi energy. The origin of this zero-bias anomaly is discussed in terms of the Dirac cones inherent to the honeycomb structure.