Triplet MLCT photosensitization of the ring-closing reaction of diarylethenes by design and synthesis of a photochromic rhenium(I) complex of a diarylethene-containing 1,10-phenanthroline ligand

Synthesis of the diarylethene-containing ligand L1 based on Suzuki cross-coupling reaction between thienyl boronic acid and the dibromophenanthroline ligand is reported. On coordination to the rhenium(I) tricarbonyl complex system, the photochromism of L1 could be photosensitized and consequently extended from intraligand excitation at lambda< or =340 nm in the free ligand to metal-to-ligand charge-transfer (MLCT) excitation at lambda< or =480 nm in the complex. The photochromic reactions were studied by (1)H NMR, UV/Vis, and steady-state emission spectroscopy. Photosensitization was further probed by ultrafast transient absorption and time-resolved emission spectroscopy. The results provided direct evidence that the formation of the closed form by the MLCT-sensitized photochromic process was derived from the (3)MLCT excited state. This supports the photosensitization mechanism, which involves an intramolecular energy-transfer process from the (3)MLCT to the (3)IL(L1) state that initiated the ring-closure reaction. The photophysical and electrochemical properties of the complex were also investigated.     

Solid-state photochromism of chromenes: enhanced photocoloration and observation of unstable colored species at low temperatures

Solid-state photochromism of benzopyrans and naphthopyrans (chromenes) was investigated in the temperature range between 300 and 80 K. Variable-temperature diffuse reflectance spectroscopy of microcrystalline powders showed that the extent of photocoloration was greatly enhanced at low temperatures. All the chromenes examined exhibited solid-state photochromism at low temperatures, even when they showed little or no photocoloration at room temperature. The solid-state photochromic properties of the chromenes were quite similar to those reported for analogous photochromic compounds of spiropyrans and spirooxazines, which indicates that these classes of compounds are generally photochromic even in the solid state. Photobleaching reactions of the colored merocyanine forms proceeded at low temperatures through the formation of a colorless intermediate, instead of directly resuming the original closed form. In addition to two stable planar merocyanine forms, which are usually observed in the photochromic reactions in solution, photoreactions at low temperatures allowed us to observe unstable colored species, which were tentatively assigned as nonplanar cisoid forms, and were stabilized in the solid state at low temperatures.     

Solid‐State Thermolysis of a fac‐Rhenium(I) Carbonyl Complex with a Redox Non‐Innocent Pincer Ligand

The development of rhenium(I) chemistry has been restricted by the limited structural and electronic variability of the common pseudo‐octahedral products fac‐[ReX(CO)₃L₂] (L₂=α‐diimine). We address this constraint by first preparing the bidentate bis(imino)pyridine complexes [(2,6‐{2,6‐Me₂C₆H₃N?CPh}₂C₅H₃N)Re(CO)₃X] (X=Cl 2 , Br 3 ), which were characterized by spectroscopic and X‐ray crystallographic means, and then converting these species into tridentate pincer ligand compounds, [(2,6‐{2,6‐Me₂C₆H₃N?CPh}₂C₅H₃N)Re(CO)₂X] (X=Cl 4 , Br 5 ). This transformation was performed in the solid‐state by controlled heating of 2 or 3 above 200 °C in a tube furnace under a flow of nitrogen gas, giving excellent yields (≥95 %). Compounds 4 and 5 define a new coordination environment for rhenium(I) carbonyl chemistry where the metal center is supported by a planar, tridentate pincer‐coordinated bis(imino)pyridine ligand. The basic photophysical features of these compounds show significant elaboration in both number and intensity of the d–π* transitions observed in the UV/Vis spec tra relative to the bidentate starting materials, and these spectra were analyzed using time‐dependent DFT computations. The redox nature of the bis(imino)pyridine ligand in compounds 2 and 4 was examined by electrochemical analysis, which showed two ligand reduction events and demonstrated that the ligand reduction shifts to a more positive potential when going from bidentate 2 to tridentate 4 (+160 mV for the first reduction step and +90 mV for the second). These observations indicate an increase in electrostatic stabilization of the reduced ligand in the tridentate conformation. Elaboration on this synthetic methodology documented its generality through the preparation of the pseudo‐octahedral rhenium(I) triflate complex [(2,6‐{2,6‐Me₂C₆H₃N?CPh}₂C₅H₃N)Re(CO)₂OTf] ( 7 , 93 % yield).     

Reactions of Ruthenium Vinylidene and Acetylide Complexes Containing Trichloromethyl Groups: Preparation of a Cyclobutenonyl Complex by Solid‐State Photolysis

Solid‐state route to a cyclobutenone : Ruthenium perchlorocyclobutenonyl complex 2 is obtained by solid‐state photoisomerization of ruthenium trichloroacetyl acetylide complex 1 . The four‐membered ring is sufficiently robust that transfer of the intact ligand could be readily achieved in a reaction of 2 with an enyne. Cyclobutenedionyl complex 3 was obtained by hydrolysis of 2 in H₂O/THF.

     

Superstructure-dependent optical and electrical properties of an unusual face-to-face, pi-stacked, one-dimensional assembly of dehydrobenzo[12]annulene in the crystalline state

To develop a novel pi-conjugated molecule-based supramolecular assembly, we designed and synthesized trisdehydrotribenzo[12]annulene ([12]DBA) derivative 2 with three carboxyl groups at the periphery. Recrystallization of 2 from DMSO gave a crystal of the solvate 23 DMSO. Crystallographic analysis revealed, to our surprise, that a face-to-face pi-stacked one-dimensional (1D) assembly of 2 was achieved and that the DMSO molecule played a significant role as a "structure-dominant element" in the crystal. This is the first example of [12]DBA to stack completely orthogonal to the columnar axis. To reveal its superstructure-dependent optical and electrical properties, 2 and its parent molecule 1, which crystallizes in a herringbone fashion, were subjected to fluorescence spectroscopic analysis and charge-carrier mobility measurements in crystalline states. The 1D stacked structure of 2 provides a red-shifted, broadened, weakened fluorescence profile (lambda(max) = 545 nm, phi(F) = 0.01), compared to 1 (lambda(max) = 491 nm, phi(F) = 0.12), due to strong interactions between the p orbitals of the stacked molecules. The charge-carrier mobility of the single crystal of 23 DMSO, as well as 1, was determined by flash photolysis time-resolved microwave conductivity (FP-TRMC) measurements. The single crystal of 23 DMSO revealed significantly-anisotropic charge mobility (sigma(mu) = 1.5x10(-1) cm(2) V(-1) s(-1)) along the columnar axis (crystallographic c axis). This value is 12 times larger than that along the orthogonal axis (the a axis).     

Visual observation of contact-induced intercrystalline migration of aromatic species adsorbed in zeolites by fluorescence microscopy.

Intercrystalline migration and a migration-assisted chemical reaction of adsorbed aromatic species between zeolite particles in physical contact were visualized by fluorescence microscopy coupled with a particle manipulation technique. The luminescence color characteristics of particular zeolite particles originating from the specific photochemistry of adsorbed species was exploited to follow the migration of the molecules. Two examples are shown that are relevant to the visualization of the time-dependent migration process: A one guest-two sets of zeolite crystals system: chrysene (Chry)-loaded zeolite Na+ -X (the sodium form of zeolite X) crystals were placed in contact with unloaded Tl+ -X (thallium-exchanged X) crystals and allowed to stand at room temperature. Initially, the blue fluorescence of Chry was detected only from the Na+ -X particles, but later, the development of green phosphorescence emission was discernible from the Tl+ -X which suggests that Chry migrated from the Na+ -X to the Tl+ -X crystals. A two guest-species systems: Electron-donating Chry-loaded Na+ -X crystals were placed in contact with electron-accepting 1,2,4,5-tetracyanobenzene (TCNB)-loaded Na+ -X or Na+ -Y crystals. With time, the former system (Chry/Na+ -X and TCNB/Na+ -X) gave rise to the emission of Chry-TCNB charge-transfer complexes resulting mainly from the migration of Chry while the latter system (Chry/Na+ -X and TCNB/Na+ -Y) afforded the same emission resulting largely from the migration of TCNB. The present investigation reveals that there is a certain direction for guest migration depending on the zeolite host and the nature of host-guest or guest-guest interaction.     

Cooperative photochemical reaction mechanism of femtosecond laser-induced photocoloration in spirooxazine microcrystals.

Microcrystalline powders of spirooxazine and spiropyran compounds do not show photocoloration under steady-state illumination, whereas they undergo photochromism on intense femtosecond laser-pulse excitation. We investigated the characteristic mechanism of the crystalline photochromism by studying the photocoloration of spironaphthooxazine (SNO) and its chloro-substituted derivative (Cl-SNO) with our femtosecond diffuse-reflectance spectroscopic system. In particular, femtosecond double-pulse excitation using 390+780-nm pulses and 390+390-nm pulses, with a variable time interval between the two pulses, was applied to reveal an intermediate species involved in the photocoloration. Although 780-nm excitation of an intermediate produced by 390-nm excitation did not lead to isomerization, the 390+390-nm excitation resulted in photocoloration. The yield for SNO decreased on increasing the interval from 40 ps to 5 ns, while that for Cl-SNO was constant. The photocoloration mechanism in the crystalline phase is considered from the viewpoint of the time-dependent density of short-lived transient species, and it is concluded that cooperative interactions of excited states and nonplanar open forms play an important role in femtosecond laser-induced photochromism in these crystals.     

Formation of 1 D and 3 D coordination polymers in the solid state induced by mechanochemical and annealing treatments: bis(3-cyano-pentane-2,4-dionato) metal complexes

Bis(3-cyano-pentane-2,4-dionato) (CNacac) metal complex, [M(CNacac)(2)], which acts as both a metal-ion-like and a ligand-like building unit, forms supramolecular structures by self-assembly. Co-grinding of the metal acetates of Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) with CNacacH formed a CNacac complex in all cases: mononuclear complex was formed in the cases of Mn(II), Cu(II) and Zn(II), whereas polymeric ones were formed in the cases of Fe(II), Co(II) and Ni(II). Subsequent annealing converted the mononuclear complexes of Mn(II), Cu(II) and Zn(II) to their corresponding polymers as a result of dehydration of the mononuclear complexes. The resultant Mn(II), Fe(II), Co(II), Ni(II) and Zn(II) polymeric complexes had a common 3 D structure with high thermal stability. In the case of Cu(II), a 1 D polymer was obtained. The Mn(II), Cu(II) and Zn(II) polymeric complexes returned to their original mononuclear complexes on exposure to water vapour but they reverted to the polymeric complexes by re-annealing. Co-grinding of metal chlorides with CNacacH and annealing of the mononuclear CNacac complexes prepared from solution reactions were also examined for comparison. [Mn(CNacac)(2)(H(2)O)(2)], [M(CNacac)(2)(H(2)O)] (M=Cu(II) and Zn(II)) and [M(CNacac)(2)](infinity) (M=Mn(II), Fe(II) and Zn(II)) are new compounds, which clearly indicated the power of the combined mechanochemical/annealing method for the synthesis of varied metal coordination complexes.     

Multidimensional solid‐state NMR studies of the structure and dynamics of pectic polysaccharides in uniformly 13C‐labeled Arabidopsis primary cell walls

Plant cell wall (CW) polysaccharides are responsible for the mechanical strength and growth of plant cells; however, the high‐resolution structure and dynamics of the CW polysaccharides are still poorly understood because of the insoluble nature of these molecules. Here, we use 2D and 3D magic‐angle‐spinning (MAS) solid‐state NMR (SSNMR) to investigate the structural role of pectins in the plant CW. Intact and partially depectinated primary CWs of Arabidopsis thaliana were uniformly labeled with ¹³C and their NMR spectra were compared. Recent ¹³C resonance assignment of the major polysaccharides in Arabidopsis thaliana CWs allowed us to determine the effects of depectination on the intermolecular packing and dynamics of the remaining wall polysaccharides. 2D and 3D correlation spectra show the suppression of pectin signals, confirming partial pectin removal by chelating agents and sodium carbonate. Importantly, higher cross peaks are observed in 2D and 3D ¹³C spectra of the depectinated CW, suggesting higher rigidity and denser packing of the remaining wall polysaccharides compared with the intact CW. ¹³C spin–lattice relaxation times and ¹H rotating‐frame spin–lattice relaxation times indicate that the polysaccharides are more rigid on both the nanosecond and microsecond timescales in the depectinated CW. Taken together, these results indicate that pectic polysaccharides are highly dynamic and endow the polysaccharide network of the primary CW with mobility and flexibility, which may be important for pectin functions. This study demonstrates the capability of multidimensional SSNMR to determine the intermolecular interactions and dynamic structures of complex plant materials under near‐native conditions. Copyright © 2012 John Wiley & Sons, Ltd.