Keitaro Nakatani

Donor–acceptor Stenhouse adducts (DASA) are negative photochromic compounds exhibiting a multi-step photoisomerization mechanism.

Photochromic compounds are promising for a variety of applications, including molecular solar thermal (MOST) energy storage. The energy release step and cyclability are critical issues to be addressed for the development of this technology. We report herein the synthesis and characterization of two diarylethene molecules featuring one (1) or two (2) pyridine groups as protonatable moieties. Upon UV irradiation, both molecules undergo a cyclization reaction from the open form (OF) to the closed form (CF). Both CF are stable for a few days in acetonitrile, and the addition of acid leads to a 600 (1) or 1500-fold (2) acceleration of the ring-opening reaction, even in catalytic amounts. A kinetic model is proposed to simulate the reaction, elucidating the contribution of each step to the kinetics and evidencing the importance of the kinetic control over the protonation thermodynamic equilibrium. Data fitting leads to the rates of elementary steps and turnover numbers (TON). Following a complete reaction cycle, neutralization of the acid by an equivalent amount of base allowed further cycles. This study represents a significant advancement in the cyclability and the control of the on-demand triggering of the energy-releasing ring-opening reaction of diarylethenes for future MOST applications.

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 (Φ_o‐c, up to 90.6 %) by both separating completely pure anti‐parallel conformer and suppressing intramolecular charge transfer (ICT).

Vapor deposition method was used to obtain nanocrystals of N‐(3,5‐di‐tert‐butylsalicylidene)‐4‐ aminopyridine (1) and N‐(3,5‐di‐tert‐butylsalicylidene)‐4‐iodoaniline (2), and nanoscale thin layers (65 nm thick) of cis‐1,2‐dicyano‐1,2‐bis(2,4,5‐trimethyl‐3‐thienyl)ethene (3). Compound 3 was also deposited on a substrate previously covered by gold nanoparticles (Ø = 100 nm). Nanoscale crystals of 1 and 2 showed similarities with bulk crystals: they exhibit photochromic properties and they have similar shapes according to Atomic force microscopy (AFM) investigations. The thin layers of 3, with and without gold nanoparticles, showed light‐induced absorption change in the visible. Copyright © 2007 John Wiley & Sons, Ltd.

Heat capacities of the metal-assembled ferromagnet MnCu(obbz)·H₂O (obbz = oxamidobis(benzoato)) have been measured in the temperature region between 0.1 and 301 K by adiabatic calorimetry. A broad phase transition due to the three-dimensional ferromagnetic ordering was observed around 17 K. Above the transition temperature, a heat capacity tail arising from the short-range order characteristic of a low-dimensional magnet was found. The enthalpy and entropy gained by the phase transition are 227 J mol^-1 and 12.7 J K^-1 mol^-1, respectively. This entropy acquisition is very close to the value Rln 5 ( = 13.4 J K^-1 mol^-1) expected for the spin manifold of the spin quantum number S = 2 ground state, where R stands for the gas constant. The thermal anomaly above the transition temperature can be accounted for in terms of the S = 2 two-dimensional ferromagnetic Heisenberg model rather than the one- or three-dimensional models. This suggests that this complex can be regarded as a two-dimensional ferrimagnet, in which strong antiferromagnetic interaction exists between a pair of Mn(II) (S = 5/2) and Cu(II) (S = 1/2) but the magnetic coupling between the pairs is very weak. Magnetic measurements for the complex revealed a bend of the hysteresis loop above the transition temperature and a magnetic relaxation below the transition temperature. These results lead to the conclusion that the broadness of the phase transition may be attributed to superparamagnetism caused by the fact that the present complex is prepared only in a fine powder.