Reversible Transition between Discrete and 1D Infinite Architectures: A Temperature-Responsive Cu(I) Complex with a Flexible Disilane-Bridged Bis(pyridine) Ligand.

A thermoresponsive structural change based on a disilane-bridged bis(pyridine) ligand and CuI is reported. Single-crystal X-ray analysis revealed that there are two polymorphs in the Cu(I) complex: octanuclear copper(I) complex at 20 °C and 1D staircase copper(I) polymer complex at -173 °C. The formation of these polymorphs is due to the flexibility of the ligand. Cu-I bond formation is observed upon cooling the sample from -10 °C to -170 °C. The temperature-induced phase transition progression was clarified by DSC, VT-PXRD, and VT-photoluminescence measurements and indicated a reversible temperature-controlled crystal-to-crystal phase transition. Observation on a VT-stage using a high-speed camera showed crystal cracking during single-crystal to single-crystal transitions between these polymorphic forms.

Thermally responsive morphological changes of layered coordination polymers induced by disordering/ordering of flexible alkyl chains.

The search for a method to control structural transformations of layered coordination polymers is highly desirable to modulate their properties and functions. Herein, we report the construction of a novel coordination polymer named ZnC16 with Zn(II) ions coordinated to isophthalate ligands bearing an n-hexadecyloxy chain (C162-). Its structure consists of a layer-by-layer structure of a rigid two-dimensional coordination network and an assembly of alkyl chains as a thermally responsive moiety. Single crystals of ZnC16 exhibit a thermal crystal-to-crystal phase transition behaviour dominated by disordering/ordering of alkyl chains, which induces the expansion and shrinkage of the distance within the rigid 2D coordination networks. Microscopic observation revealed that the thermal phase transition of ZnC16 induces a significant change in their crystal morphology: a reversible macroscopic elongation/shrinkage of crystal dimensions driven by the displacement of interlayer distances and an irreversible delamination and polycrystal slippage driven by constraints generated from this phase transition. Our result provides a new direction to modulate the dynamic behaviour and related properties and functions of layered coordination polymers where the thermally responsive character of flexible alkyl chains plays an important role in tuning interlayer interactions.

Luminescence color change of [3,4-difluoro-2,6-bis(5-methyl-2-pyridyl)phenyl-κ3N,C1,N']cyanidoplatinum(II) by aggregation.

We have investigated the color and luminescence color changes of novel Pt(L)CN (L = 4,6-difluoro-1,3-di(2-(4-methyl)pyridyl)benzene) in solution and crystalline states that resulted from aggregation-induced emission (AIE). In the solution state, the AIE results from excimer and trimer formation in the excited states at high concentrations. We determined the emission lifetimes of the excimer and trimer to be τE = 1.72 µs and τT = 0.43 µs, respectively, and the emission quantum yields to be ϕE = 20% and ϕT = 12%, respectively, which are slightly smaller yet comparable to τM = 8.85 µs and ϕM = 67% of the monomeric species. In the crystalline state, the purple color of Pt(L)CN with no solvent of crystallization changes to red upon exposure to chloroform vapor, and the invisible emission turns to bright red emission. This phenomenon can be applied to inexpensive devices for the fast chloroform detection. The exposure of purple crystals to dichloromethane vapor causes a further redshift of the invisible emission and blue coloration, which suggests the capability of the discrimination of chloroform from dichloromethane.

Reversible and Stepwise Single-Crystal-to-Single-Crystal Transformation of a Platinum(II) Complex with Vapochromic Luminescence.

The vapochromic single-crystal-to-single-crystal (SCSC) transformation of a highly luminescent PtII complex bearing an N-heterocyclic carbene [Pt(CN)2 (tBu-impy)] (tBu-impyH+ =1-tert-butyl-3-(2-pyridyl)-1H-imidazolium) is reported. The trihydrate form of the complex, which exhibits blue 3 MMLCT emission owing to weak Pt⋅⋅⋅Pt interactions, changed its luminescence color from blue to yellowish-green upon the desorption of water molecules while keeping the high emission quantum yield of more than 0.45. Variable-temperature and continuous in-situ tracking of single-crystal X-ray diffraction measurements revealed that the SCSC transformation proceeds reversibly by the release and reabsorption of water molecules, thereby changing the stacked structure slightly. As a result, the dynamics of vapor-induced SCSC transformation were elucidated: that the anhydrous form returned to the original trihydrate form in a two-step process under a water vapor atmosphere. In addition, the PtII complex exhibited a similar SCSC response accompanied by a luminescence color change in the presence of methanol vapor, while being inactive toward ethanol vapor.

Monitoring photo-induced transformations in crystals of 2,6-difluorocinnamic acid under ambient conditions.

Several conditions need to be fulfilled for a photochemical reaction to proceed in crystals. Some of these conditions, for example, geometrical conditions, depend on the particular type of photochemical reaction, but the rest are common for all reactions. The mutual directionality of two neighbouring molecules determines the kind of product obtained. The influence of temperature on the probability of a photochemical reaction occurring varies for different types of photochemical reaction and different compounds. High pressure imposed on crystals also has a big influence on the free space and the reaction cavity. The wavelength of the applied UV light is another factor which can initiate a reaction and sometimes determine the structure of a product. It is possible, to a certain degree, to control the packing of molecules in stacks by using fluoro substituents on benzene rings. The crystal and molecular structure of 2,6-difluorocinnamic acid [systematic name: 3-(2,6-difluorophenyl)prop-2-enoic acid], C9H6F2O2, (I), was determined and analysed in terms of a photochemical [2 + 2] dimerization. The molecules are arranged in stacks along the a axis and the values of the intermolecular geometrical parameters indicate that they may undergo this photochemical reaction. The reaction was carried out in situ and the changes of the unit-cell parameters during crystal irradiation by a UV beam were monitored. The values of the unit-cell parameters change in a different manner, viz. cell length a after an initial increase starts to decrease, b after a decrease starts to increase, c increases and the unit-cell volume V after a certain increase starts to decrease. The structure of a partially reacted crystal, i.e. containing both the reactant and the product, namely 2,6-difluorocinnamic acid-3,4-bis(2,6-difluorophenyl)cyclobutane-1,2-dicarboxylic acid (0.858/0.071), 0.858C9H6F2O2·0.071C18H12F4O4, obtained in situ, is also presented. The powder of compound (I) was irradiated with UV light and afterwards crystallized [as 3,4-bis(2,6-difluorophenyl)cyclobutane-1,2-dicarboxylic acid toluene hemisolvate, C18H12F4O4·0.5C7H8] in a space group different from that of the crystal containing the in-situ dimer.