Wataru Fujita

Pyrrole-Fused Azacoronene Family: The Influence of Replacement with Dialkoxybenzenes on the Optical and Electronic Properties in Neutral and Oxidized States

A novel pyrrole-fused azacoronene family was synthesized via oxidative cyclodehydrogenation of the corresponding hexaarylbenzenes as the key step, and the crystal structures of tetraazacoronene 3b and triazacoronene 4a were elucidated. The photophysical properties for neutral compounds 1-4 were investigated using steady-state UV-vis absorption/emission spectroscopy and time-resolved spectroscopy (emission spectra and lifetime measurements) at both room temperature and 77 K. The observation of both fluorescence and phosphorescence allowed us to estimate the small S1-T1 energy gap (ΔES-T) to be 0.35 eV (1a), 0.26 eV (2a), and 0.36 eV (4a). Similar to the case of previously reported hexapyrrolohexaazacoronene 1 (HPHAC), electrochemical oxidation revealed up to four reversible oxidation processes for all of the new compounds. The charge and spin delocalization properties of the series of azacoronene π-systems were examined using UV-vis-NIR absorption, ESR, and NMR spectroscopies for the chemically generated radical cations and dications. Combined with the theoretical calculations, the experimental results clearly demonstrated that the replacement of pyrrole rings with dialkoxybenzene plays a critical role in the electronic communication, where resonance structures significantly contribute to the thermodynamic stability of the cationic charges/spins and determine the spin multiplicities. For HPHAC 1 and pentaazacoronene 2, the overall aromaticity predicted for closed-shell dications 1(2+) and 2(2+) was primarily based on the theoretical calculations, and the open-shell singlet biradical or triplet character was anticipated for tetraazacoronene 3(2+) and triazacoronene 4(2+) with the aid of theoretical calculations. These polycyclic aromatic hydrocarbons (PAHs) represent the first series of nitrogen-containing PAHs that can be multiply oxidized.

Complex phase transitions in stable thiazyl radicals: spin-gap, antiferromagnetic ordering and double melting

We examined the magnetic and thermal properties of a stable thiazyl radical 1,3,2-benzodithiazolyl, BDTA. While the diamagnetic room-temperature phase had been known for BDTA, we found a new phase transition to a paramagnetic solid phase at 346 K and a superheating process that resulted in the double melting (melt recrystallization–melt process) above 360 K. The supercooling of the paramagnetic high-temperature phase also occurred, leading to an antiferromagnetic ordering at 11 K with a λ-type anomaly in the temperature dependence of the heat capacity. Based on these data, the complex phase diagram of BDTA has been determined.

Organic ferromagnetism of Tc = 6.7 K driven by evaporation of crystal solvent

Abstract The crystals of the organic cation radical salt, BBDTA (benzo[1,2-d:4,5-d′]bis[1,3,2]dithiazole)·GaCl4·CH3CN, was found to consist of a ladder-type structure of BBDTA, coordinated by the crystal solvent, CH3CN. While this material exhibited diamagnetic properties, removal of the crystal solvent resulted in a drastic change from diamagnetic to paramagnetic, probably caused by a packing modification of the BBDTA molecules. The desolvated form made bulk, ferromagnetic ordering at 6.7 K. This finding has superseded the highest Curie temperature of organic ferromagnets based on ferromagnetic interactions,

1H-NMR study of the 2D spin-gap system m-MPYNN·BF4

Abstract The organic radical salt m-MPYNN·BF4 is characterized as a spin-1 kagome antiferromagnet and has been suggested to have a singlet ground state with a finite spin gap. 1 H -NMR measurements for this salt have been carried out in the temperature range down to 0.05 K and under the magnetic fields up to 70 kOe . It is found that nuclear spin–lattice relaxation rate T1−1 increases with decreasing temperature below about 1 K under magnetic fields below about 8 kOe . Further, a gapped magnetic state is found at higher fields. Our results are interesting from the viewpoint of the crossover between gapless and gapped regimes, which has been studied intensively for quasi-1D spin-gap systems.

Controllable magnetic properties of layered copper hydroxides, Cu2(OH)3X (X = carboxylates)

Abstract Physical and chemical properties of organic/inorganic hybrid nanocomposites are attractive, because they are potential materials yielding new functionality, such as a cooperative phenomenon, multi-functional property and so on. Layered copper hydroxides, Cu 2 (OH) 3 X (X=carboxylate anions), are two dimensional magnetic materials whose structures consist of alternating stacking of the magnetic copper hydroxides layer and the carboxylate layer. In this paper we report the preparations, structures and magnetic properties of various copper hydroxides. In the series of Cu 2 (OH) 3 ( n -C m H 2 m +1 COO), the carboxylate anions form an interdigitated monolayer in the m =0 and 1 materials, while those do a bilayer in the m =6–11 materials. The intermediate materials ( m =2–5) exhibit both mono- and bi-layer structures, which are governed by the condition at their preparations. The temperature dependence of the paramagnetic susceptibilities of the m =0 and 1 materials suggests an intralayer ferromagnetic interaction, while that of the m =6–11 materials does an antiferromagnetic interaction. Further, the longer alkyl chain materials show a divergence of the ac magnetic susceptibility at 22 K, indicating a ferromagnetic order. It is found that the materials show a drastic change in magnetism, depending on the molecular orientation of the carboxylate anions. The Cu K-edge EXAFS spectroscopy at 20 K indicates a characteristic difference in local structure between the shorter and the longer alkyl chain materials, they are large enough to explain the drastic magnetic change. In this report, we also show controllable magnetic properties of Cu 2 (OH) 3 (8-(( p -(phenylazo)phenyl)oxy)octanoate), derived from cooperation between the inorganic layer carrying magnetism and the organic layer playing a role of sensor. A reversible structural transformation occurs as a result of soaking in hot methanol and acetonitrile. The X-ray diffraction patterns indicate that the organic layer in the material exhibits an interdigitated monolayer structure in hot methanol, while it does a membrane-like bilayer structure in acetonitrile. The monolayer phase is paramagnetic down to 3 K, while the bilayer phase shows a ferromagnetic order below 10.8 K. The reversible packing change of the organic guest molecules results in the drastic modification in the magnetic properties of the inorganic layer. The material in the bilayer phase exhibits a trans -to- cis isomerization reaction of the azobenzene moiety by UV light illumination, although there is little change of the magnetic properties after the photoreaction. The photoreaction is irreversible under irradiation of VIS light.

Spontaneous magnetization below 44 K in (benzo[1,2-d:4,5-d′]bis[1,3,2]dithiazole) ·FeCl4 driven by evaporation of crystal solvent

Abstract An organic/inorganic hybrid system, (benzo[1,2-d:4,5-d ′ ]bis[1,3,2]dithiazole (=BBDTA))·FeCl 4 ·CH 3 COCH 3 , is shown to exhibit a drastic change in magnetism after evaporation of the volatile crystal solvent, acetone. The solvated material, with a crystal structure consisting of ladder-like BBDTA and a honeycomb network of Fe(III)Cl 4 , exhibits antiferromagnetic ordering at 6.3 K. Magnetic measurements and Mossbauer spectra suggest that the desolvated material includes a ferrimagnetic phase with T N =44.0 K .

Structural Study of a Dimerization Process in an Organic Radical Magnet, BBDTA⋅InBr4

The time required for the formation and dissociation of bonds in chemical processes, either in gaseous or in condensed phases, is usually very short. Therefore, it is difficult to access structural information for short-lived species, such as intermediates, or for molecular distortion in transition states. The spin-Peierls (SP) phase transition is a paramagnetic–diamagnetic transition observed in organic radical crystals at a characteristic temperature, TSP, concomitant with a progressive dimerization between neighboring magnetic species below TSP. [4] The SP transition may be regarded as chemical bond formation in organic radical crystals. Thus, Xray crystal structure analyses of a dimerization process in the SP system may provide structural information on a chemical bond-formation process. Herein, we describe the magnetic properties and detailed crystal structure analysis of an organic radical magnet, BBDTA·InBr4 (1, BBDTA = benzo[1,2-d :4,5d’]bis(1,3,2-dithiazole)). We present the SP-like transition of this material at the higher temperature of 250 K and the temperature dependence of structural details on its dimerization process.

Crystal structures and magnetic properties of (m- or p-MPYNN)2 NiII(tdas)2

Abstract Novel NiII(tdas)2 complexes, (m- or p-MPYNN)2Ni(tdas)2 (MPYNN=N-methylpyridinium α-nitronyl nitroxide and tdas=1,2,5-thiadiazole-3,4-dithiolate), were prepared. The Ni(tdas)2 anions are located on the lattice points and at the lattice center in the crystal of (m-MPYNN)2Ni(tdas)2. The m-MPYNN cations form a centrosymmetric dimer, and the anion is sandwiched by a pair of the dimers, forming alternate stacks along the a axis. In the crystal of the (p-MPYNN)2Ni(tdas)2 complex, the Ni(tdas)2 anions are located on the lattice points and at the center of the bc plane. The anion is surrounded by four p-MPYNN cations. The p-MPYNN cations also form a centrosymmetric dimer. Each cation has a short contact with a different Ni(tdas)2 anion. The magnetic susceptibilities of both complexes decrease with decreasing temperature, indicating antiferromagnetic interaction within the radicals. The behavior of (m- and p-MPYNN)2Ni(tdas)2 was understood with the singlet–triplet model and the Curie–Weiss law.

Canted antiferromagnetism and spin glasslike behavior in a family of two-dimensional organic/inorganic nanocomposites

We report magnetic properties of a new class of hybrid organic/inorganic layered materials, Cu2(OH)3(CmH2m+1COO), m=7, 9, and 11. The frequency dependence of the linear ac susceptibility and the irreversibility in the field-cooled/zero-field-cooled magnetization show that the geometrical frustration of the triangular lattice is strong enough to cause glassiness. The strong peak in the second harmonic of the nonlinear ac susceptibility indicates that frustration is not sufficient to prevent the system from developing a spontaneous moment. We propose that the interplay of Heisenberg antiferromagnetic exchange and Dzyaloshinskii–Moriya interaction leads to the unusual coexistence of glassiness and canted antiferromagnetism.

High-pressure effects on a manganese hexacyanomanganate ferrimagnet with TN=29 K

II w III .x The effects of high pressure, up to 1 GPa, on a Mn-based Prussian blue ferrimagnet, Mn Mn CN P 12H OP 36 2 2 . 1.7 CH OH with T s 29 K, were studied. The transition temperature gradually increases with an increase in pressure and 3N becomes ; 40 K at 1 GPa. The pressure dependence of T can be semi-quantitatively interpreted in terms of the shrinkage N