Yoshiaki Shuku

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.

Synthesis, optical properties and charge transport characteristics of a series of novel thiophene-fused phenazine derivatives

A series of tetrathienophenazine derivatives (t-TTP, l-TTP, m-TTP and their alkyl-substituted derivatives) have been synthesized via a simple condensation reaction between diketones and diamines. The redox potentials, UV-vis absorption spectra and fluorescence spectra of these derivatives are significantly affected by the positions of the sulfur atoms and the alkyl groups. The observed electronic properties were well reproduced by theoretical calculations based on density functional theory. X-ray analyses of these derivatives reveal extensive π–π interactions and short S⋯S contacts. The alkyl-substituted derivatives afford highly crystalline thin films by vapor deposition, and show reasonable field effect transistor performance. The conductivity of doped single crystals of these materials was also investigated, and showed an increase by several orders of magnitude upon I2 vapor doping. The π–π stacked structures of these crystals and conductive properties indicate that these thiophene-fused phenazines are useful materials for application in organic electronics.

Planar Ni(II), Cu(II) and Co(II) tetraaza[14]annulenes: structural, electronic and magnetic properties and application to field effect transistors

A series of planar Ni(II), Cu(II) and Co(II) tetraaza[14]annulenes has been prepared and studied optically, electrochemically and magnetically. Thin films of each of these complexes have been prepared by vacuum deposition to evaluate the field-effect transistor (FET) performance as well as the morphology and crystallinity of the film formed. The electrochemistry and UV/Vis absorption studies indicate the materials are redox active and highly coloured, with molar extinction coefficients as large as 80 000 M−1 cm−1 in the visible region. The paramagnetic complexes display weak antiferromagnetic interactions, fit to the Bonner–Fisher chain model. Each of the materials formed polycrystalline films when vacuum deposited and showed field-effect transistor behaviour, with charge carrier mobilities in the range of 10−5 to 10−9 cm2 V−1 s−1. SEM imaging of the substrates indicates that the central metal ion, and its sublimation temperature, has a crucial role in defining the morphology of the resulting film.

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”.

Controlling the crystallinity and crystalline orientation of “shuttlecock” naphthalocyanine films for near-infrared optoelectronic applications

The thin film properties of tin(II) 2,3-naphthalocyanine (SnNPc) were interrogated and various strategies for controlling the crystallinity and crystalline orientation within the films were assessed. SnNPc is shown to crystallize in the space group P21/c (Z = 4), where the molecular arrangement consists of alternating layers of concave and convex overlap, induced by the out-of-plane Sn atoms, resulting in a 3D slipped-π-stack network structure analogous to that reported for Phase I of titanyl phthalocyanine. The thin films were studied by X-ray diffraction, atomic force microscopy and absorption spectroscopy and are highly sensitive not just to the conditions during growth, but also to substrate pre- and post-deposition treatment. While the films grown at room temperature were largely amorphous, the crystallinity was enhanced with substrate temperature, with the molecules orienting in a standing molecular geometry. A thin layer of 3,4:9,10-perlenetetracarboxylic dianhydride induces a lying molecular geometry of the same polymorph as that of the single crystal, while different polymorphs are accessible through solvent vapor annealing of amorphous films. Transient photocurrent measurements showed a dramatic improvement in photodetector device bandwidth for the lying molecular geometry, which was attributed to enhanced photoconductivity along the π-stacking axis, while solvent vapor annealing could be used to tune the photosensitivity across the near-infrared region.

Transition Metal Complexes and Radical Anion Salts of 1,10-Phenanthroline Derivatives Annulated with a 1,2,5-Tiadiazole and 1,2,5-Tiadiazole 1,1-Dioxide Moiety: Multidimensional Crystal Structures and Various Magnetic Properties

Advances in the molecular variety and the elucidation of the physical properties of 1,10-phenanthroline annulated with 1,2,5-thiadiazole and 1,2,5-thiadiazole 1,1-dioxide moieties have been achieved, and are described herein. A 1,2,5-thiadiazole compound, [1,2,5]thiadiazolo[3,4-f][1,10]phenanthroline (tdap), was used as a ligand to create multidimensional network structures based on S•••S and S•••N intermolecular interactions. A 1,2,5-thiadiazole 1,1-dioxide compound, [1,2,5] thiadiazolo[3,4-f][1,10]phenanthroline, 1,1-dioxide (tdapO2), was designed to create a stable radical anion, as well as good network structures. Single crystal X-ray structure analyses revealed that transition metal complexes of tdap, and radical anion salts of tdapO2 formed multidimensional network structures, as expected. Two kinds of tdap iron complexes, namely [Fe(tdap)2(NCS)2] and [Fe(tdap)2(NCS)2]•MeCN exhibited spin crossover transitions, and their transition temperatures showed a difference of 150 K, despite their similar molecular structures. Magnetic measurements for the tdapO2 radical anion salts revealed that the magnetic coupling constants between neighboring radical species vary from strongly antiferromagnetic (J = −320 K) to ferromagnetic (J = 24 K), reflecting the differences in their π overlap motifs.