Keitaro Nakatani

Multiferroics by Rational Design: Implementing Ferroelectricity in Molecule‐Based Magnets

Multiferroics (MF) are materials that exhibit simultaneouslyseveral ferroic order parameters. Among the multiferroicmaterials, those combining antiferro- or ferroelectricity (FE)and antiferro-, ferri-, or ferromagnetism (FM) within thesame material are highly desirable: the coexistence of thepolar and magnetic orders paves the way towards four-levelmemories while their interactions through the magnetoelec-tric effect makes it possible to control the magnetization byelectric fields and hence to develop electronically tuneablemagnetic devices, which are an essential feature for spin-tronics.

Spirooxazine- and spiropyran-doped hybrid organic-inorganic matrices with very fast photochromic responses

Both spirooxazine and spiropyran dyes have been embedded into two different hybrid matrices, which were formed from hydrolysis and cocondensation between diethoxydimethylsilane and zirconium propoxide and between methyldiethoxysilane (DH) and triethoxysilane (TH) respectively. The nature and the kinetics of the photochromic response depend strongly on the hydrophobic/hydrophilic balance (HHB) of the hybrid material. The HHB controls the competition between direct and reverse photochromism. The photochromic behaviour of the strongly hydrophobic spirooxazine-doped DH/TH coatings is direct, highly efficient (ΔA>1), reversible and extremely fast (thermal bleaching time constant, k=0.2 s -1 ). The photochromic kinetics of this hybrid material are, to the best of our knowledge, much faster than those reported for spirooxazine in any other solid matrix.

Azo-azulene derivatives as second-order nonlinear optical chromophores

The molecular and solid state nonlinear optical (NLO) properties of several (phenylazo)-azulenes are investigated. In particular, (4-nitrophenylazo)-azulene (2b) exhibits a quadratic hyperpolarizability (beta(vec)) of 80 x 10(-30) cm5esu recorded at 1.907 microm by the electric field-induced second-harmonic (EFISH) technique. This molecular material, which crystallizes in the monoclinic noncentrosymmetric space group Pc, exhibits an efficiency 420 times that of urea in second-harmonic generation. The origin of the optical nonlinearity in azo-azulene is discussed in relation with crystal structures and semiempirical calculations within the INDO/SOS formalism, and compared with that of the well known disperse red one (DR1) organic dye.

Separation of Photoactive Conformers Based on Hindered Diarylethenes: Efficient Modulation in Photocyclization Quantum Yields

Endowing both solvent independency and excellent thermal bistability, the benzobis(thiadiazole)-bridged diarylethene system provides an efficient approach to realize extremely high photocyclization quantum yields (Φo-c , up to 90.6 %) by both separating completely pure anti-parallel conformer and suppressing intramolecular charge transfer (ICT).

Second-Order Nonlinear Optical Properties of Polyoxometalate Salts of a Chiral Stilbazolium Derivative

The synthesis of nonlinear optical (NLO) active salts with stilbazolium derivatives and polyoxometalate (POM) counterions has been investigated. With known nonchiral stilbazolium derivatives, such as MOMS(+), compounds with centrosymmetric structures have been isolated, like for instance the centrosymmetric salt (MOMS)(4)[Mo(8)O(26)] (1), synthesized under hydrothermal conditions. A new chiral derivative of the known DAMS(+) molecules, named here CHIDAMS(+), has therefore been synthesized in order to force the crystallization of the hybrid ionic salts in noncentrosymmetric space groups. The CHIDAMS(+) cation has been crystallized under two polymorphic PF(6)(-) salts, (CHIDAMS)PF(6) (2a and 2b), and its reactivity with various POMs has been investigated. The ionic salt (CHIDAMS)(2)[Mo(5)O(13)(OEt)(4)(NO){Na(H(2)O)(0.5)(DMF)(0.5)}] (4) crystallizes in the noncentrosymmetric P2(1) group, but the push-pull axis of the CHIDAMS(+) cations adopts a quasi-antiparallel alignment. The ionic salt (CHIDAMS)(3)[PW(12)O(40)].2DMF (5) associating three CHIDAMS(+) cations and a PW(12)O(40)(3-) Keggin anion crystallizes also in the P2(1) space group, but the disposition of the cations in the solid state is far more favorable. Diffuse reflectance experiments have evidenced a charge transfer between the organic and inorganic components in 5, and Kurtz-Perry experiments show that this salt exhibits a second harmonic generation efficiency more than 10 times higher than those of the PF(6)(-) salts 2a and 2b, the hybrid salt 4, and all of the other NLO active POM molecular materials reported in the literature.

Unprecedented Stability of a Photochromic Bisthienylethene Based on Benzobisthiadiazole as an Ethene Bridge

Photochromic systems are of increasing interest as a result of their applications in ophthalmic lenses and as smart molecular materials that act as photoresponsive self-assembling systems, molecular switches, logic gates, and information storage media. In particular, bisthienylethenes (BTEs) have become one of the most promising families because of their thermal irreversibility and outstanding fatigue resistance since the 1,3,5-hexatriene section can adopt an appropriate conformation to undergo a conrotatory 6p-electron photocyclization. To date, the rational design of symmetric or asymmetric BTEs has mainly been carried out by modifying the thiophene rings. In contrast, the central ethene bridges necessary for the versatility of the BTE architectures reported so far have been mostly limited to a cyclopentene unit or its strong electron-withdrawing analogues, such as perfluorocyclopentene, maleic anhydride, or maleic imide. As alternatives to this, several research groups have investigated the use of six-membered rings containing a C=C bond, such as 1,10-phenanthroline and cyclohexene obtained by a [4+2] cycloaddition of butadiene with a dienophile, as the central ethene bridge of BTEs. Recently, we incorporated highly polar and electron-withdrawing chromophores of 2,1,3benzothiadiazole and naphthalimide units as a novel family of six-membered rings that act as ethene bridges and result in good photochromic performance with moderate fatigue resistance in solution or in organogel systems. Moreover, their fluorescence can be modulated by solvatoand photochromism, eventually leading to a combined NOR and INHIBIT logic operation system. However, the two building blocks have a degree of aromaticity, which can facilitate the undesirable thermal back reaction in the dark as a result of the large loss of aromatic stabilization energy upon photocyclization from the open to the closed form. Accordingly, a lower aromaticity of the central ethene moiety in BTEs is expected to lead to a higher thermal stability of the closed form. Since the ethene bridge within a six-membered ring has its own advantages, such as a higher quantum yield in its closed-ring form and longer absorption wavelength, we envisioned that the use of a non-aromatic sixmembered ring with a C=C bond as the central ethene bridge might widely extend the diversity of the thermally irreversible photochromic systems. With this in mind, we report herein an original BTE photochromic system (BTTE, Figure 1) based on the benzo[1,2-c :3,4-c’]bis[1,2,5]thiadiazole (abbreviated as benzobisthiadiazole) chromophore as a novel six-membered ring containing a central bridging ethene unit, which is expected to have a low aromaticity, and yield a thermally stable closed form (c-BTTE). As depicted in Figure 1A, BTTE was easily prepared by conventional Suzuki cross-coupling with the corresponding boronic acid. The key intermediate of 4,5-dibromobenzo[1,2c :3,4-c’]bis[1,2,5]thiadiazole (BBT) was conveniently synthesized from benzothiadiazole according to the reported procedure. The Suzuki coupling of BBT and 2,5-dimethylthien-3-yl boronic acid in the presence of a [Pd(PPh3)4] catalyst in a mixture of aqueous Na2CO3 (2m) and 1,4-dioxane under reflux gave the target molecule BTTE, which was fully characterized by H NMR and C NMR spectroscopy as well as HRMS (see the Supporting Information). Interestingly, the H NMR spectrum of BTTE shows two well-resolved sets of signals for the methyl protons which correspond to the parallel (d = 2.09 and 2.42 ppm) and antiparallel (d = 1.96 and 2.46 ppm) conformations commonly found in BTE systems (Figure 1D). In general, the two conformations, parallel (photochromic inactive) and antiparallel (photochromic active) conformations, undergo very fast single-bond rotation in most BTEs, thereby resulting in only one set of time-averaged signals in the H NMR spectrum. Only in the cases of diarylethenes bearing substituents can the rotation of the aryl groups be slowed down sufficiently to give two sets of signals. As found by integration of the H NMR spectrum, the ratio between the parallel and antiparallel conformations of BTTE in CDCl3 is 55:45. A solution of BTTE in THF is colorless, with an intense absorption band at 288 nm and a moderate one at 361 nm (Table 1). As expected, irradiation of the solution of BTTE with light at a wavelength of 365 nm results in the solution [*] Prof. Dr. W. Zhu, Y. Yang, Dr. Q. Zhang, Prof. Dr. Y. Xie, Prof. Dr. H. Tian Shanghai Key Laboratory of Functional Materials Chemistry Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology Shanghai 200237 (China) E-mail: whzhu@ecust.edu.cn

Synthesis, characterization and nonlinear optical (NLO) properties of a push–pull bisboronate chromophore with a potential electric field induced NLO switch

A new bisboronate with five six-membered ring heterobicycles was prepared by reaction of [2-(2-hydroxy-4-diethylaminobenzylidene)amino]-5-nitroaminophenol and 1,4-phenyldiboronic acid, and its quadratic nonlinear optical (NLO) response was compared to that of a related monoboronate species. A computational investigation conducted within the framework of the DFT theory, at the B3PW91/6-31G* level, indicates that, while the diboronate derivative exhibits a centrosymmetric conformation in the ground state, and hence a vanishing quadratic hyperpolarizability (β), the application of an external electric field provides a gradual intramolecular rotation of the two “push–pull” sub-units which reaches 128° for a field intensity of 10−3 a.u. and leads to a β value equal to 49.2 × 10−30 cm5 esu−1. This result suggests a possibility for a molecular NLO switch induced by an electric field in such systems. Experimentally, the NLO response measured by the electric field induced second harmonic (EFISH) technique indicates an NLO response (expressed as the μ × β product) 1.95 times larger for the bisboronate than for the monoboronate analogue, thus suggesting that, although the ground state conformation is centrosymmetric, the “push–pull” sub-units are in quasi-free rotation at room temperature.

Dithiazolylethene-based molecular switches for nonlinear optical properties and fluorescence: synthesis, crystal structure and ligating properties

Two photochromic dithiazolylethene-based derivatives (1a and 2a) incorporating a push–pull structural motif D–π–A are synthesized and structurally characterized. The photochemical reactions, which reversibly modify the connection between the electron donor and acceptor, result in simultaneous and effective photo-modulations of their NLO and fluorescent properties. The photo-switchable NLO behaviors are investigated theoretically by performing INDO calculations. The presence of chelating sites in 2a allows the synthesis of various metal complexes: Cu(hfac)2·2a, Mn(hfac)2·2a and Ag(2a)2CF3SO3. Their crystal structures are determined.

Synthesis and photochromism of two new 1,2-bis(thiazolyl)perfluorocyclopentenes with chelating sites

Two new dithiazolylethenes, 1a {1,2-bis[5′-methyl-2′-(2′′-pyridyl)thiazolyl]perfluorocyclopentene} and 2a {1,2-bis[4′-methyl-2′-(2′′-pyridyl)thiazolyl]perfluorocyclopentene}, have been synthesized. Their photochromic behavior has been fully investigated in solution as well as in the crystalline phase. They display both fatigue-resistant and thermally irreversible photochromic reactions, with more than 80% of the closed-ring forms (1b and 2b, respectively) in the photo-stationary state. Upon UV irradiation, a colorless solution of 1a turns purple while the yellow color of a 2a solution becomes more intense. Such marked differences in their electronic absorption spectra are reproduced by DFT calculations. Finally, although structurally closely related, only 1a shows photochromism in the crystalline phase whereas 2a displays reversible photo-modulation of fluorescence in solution.

Photochromic Magnetic Materials

Incorporation of organic photochromes into magnetic systems not only provides new ways to photo-control magnetic behavior but may also lead to interesting photochromic behavior. Cationic spiropyrans exhibit this type of behavior depending on whether the structure is open or closed (see Figure). These structures can also be incorporated into rigid layered MnPS3 materials where reversible photo-switching of magnetic coercivity is observed.