Reversible Single‐Crystal‐to‐Single‐Crystal Photochemical Formation and Thermal Cleavage of a Cyclobutane Ring

A [2+2] cycloaddition reaction has been observed in a number of solids. The cyclobutane ring in a photodimerized material can be cleaved into olefins by UV light and heat. The high thermal stability of the metal–organic salt K₂SDC (H₂SDC=4,4’‐stilbenedicarboxylic acid) has been successfully utilized to investigate the reversible cleavage of a cyclobutane ring. The two polymorphs of K₂SDC undergo reversible cyclobutane formation by UV light and cleavage by heat in cycles. Of these, one polymorph retains its single‐crystal nature during the reversible processes. Polymorphs are known to show different physical properties and chemical reactivities. This work reveals that the retention of single‐crystal nature is strongly associated with the packing of molecules, which is controlled by kinetics and thermodynamics. The photoemissive nature of the products makes this as a promising material for photoswitches and optical data storage devices.     

Crystal Structures and Emitting Properties of Trifluoromethylaminoquinoline Derivatives: Thermal Single‐Crystal‐to‐Single‐Crystal Transformation of Polymorphic Crystals That Emit Different Colors

2,4‐Trifluoromethylquinoline (TFMAQ) derivatives that have amine ( 1 ), methylamine ( 2 ), phenylamine ( 3 ), and dimethylamine ( 4 ) substituents at the 7‐position of the quinoline ring were prepared and crystallized. Six crystals including the crystal polymorphs of 2 (crystal GB and YG) and 3 (crystal B and G) were obtained and characterized by X‐ray crystallography. In solution, TFMAQ derivatives emitted relatively strong fluorescence (${\lambda {{{\rm f}\hfill \atop {\rm max}\hfill}}}$ =418–469 nm and Φ_f(s)=0.23–0.60) depending on the solvent polarity. From Lippert–Mataga plots, Δμ values in the range of 7.8–14 D were obtained. In the crystalline state, TFMAQ derivatives emitted at longer wavelengths (${\lambda {{{\rm f}\hfill \atop {\rm max}\hfill}}}$ =464–530 nm) with lower intensity (Φ_f(c)=0.01–0.28) than those in n‐hexane solution. The polymorphous crystals of 2 and 3 emitted different colors: 2 , ${\lambda {{{\rm f}\hfill \atop {\rm max}\hfill}}}$ =470 and 530 nm with Φ_f(c)=0.04 and approximately 0.01 for crystal GB and YG, respectively; and 3 , ${\lambda {{{\rm f}\hfill \atop {\rm max}\hfill}}}$ =464 and 506 nm with Φ_f(c)=0.28 and approximately 0.28 for crystal B and G, respectively. In both crystal polymorphs of 2 and 3 , crystals GB and G showed emission color changes by heating/melting/cooling cycles that were representative. By following the color changes in heating at the temperature below the melting point with X‐ray diffraction measurements and X‐ray crystallography, the single‐crystal‐to‐single‐crystal transformations from crystal GB to YG for 2 and from crystal B to G for 3 were revealed.     

Crystal‐to‐Crystal Synthesis of Helically Ordered Polymers of Trehalose by Topochemical Polymerization

The synthesis of crystalline helical polymers of trehalose via topochemical azide–alkyne cycloaddition (TAAC) of a trehalose‐based monomer is presented. An unsymmetrical trehalose derivative having azide and alkyne crystallizes in two different forms having almost similar packing. Upon heating, both the crystals undergo TAAC reaction to form crystalline polymers. Powder X‐ray diffraction (PXRD) studies revealed that the monomers in both the crystals polymerize in a crystal‐to‐crystal fashion; circular dichroism (CD) studies of the product crystals revealed that the formed polymer is helically ordered. This solvent‐free, catalyst‐free polymerization method that eliminates the tedious purification of the polymeric product exemplifies the advantage of topochemical polymerization reaction over traditional solution‐phase polymerization.     

Synthesis and Reversible Hydration of a Pseudoprotein,a Fully Organic Polymeric Desiccant by Multiple Single‐Crystal‐to‐Single‐Crystal Transformations

A diphenylalanine derivative, N3‐Phe‐Phe‐NHCH2CCH, was designed for topochemical azide–alkyne cycloaddition (TAAC) polymerization. This dipeptide adopted β‐sheet arrangement as designed, in its crystals, but the azide and alkyne were not fitly aligned for their topochemical reaction. However, the voids present around these groups allowed them to attain a reactive geometry upon heating and their consequent TAAC polymerization to a pseudoprotein in a single‐crystal‐to‐single‐crystal (SCSC) fashion. This motion led to the creation of channels in the product crystal and it absorbed water from the surroundings to fill these channels as H‐bonded water wire. The pseudoprotein undergo reversible hydration/dehydration in SCSC fashion many times under mild conditions: hydration at low relative humidity and dehydration at low temperature. Vapor sorption analyses suggest that this fully organic polymer might be useful as an energy‐efficient desiccant material for controlling indoor humidity.     

Formation of a Hydrogen‐Bonded Heptazine Framework by Self‐Assembly of Melem into a Hexagonal Channel Structure

Self‐assembly of melem C₆N₇(NH₂)₃ in hot aqueous solution leads to the formation of hydrogen‐bonded, hexagonal rosettes of melem units surrounding infinite channels with a diameter of 8.9 Å. The channels are filled with strongly disordered water molecules, which are bound to the melem network through hydrogen bonds. Single‐crystals of melem hydrate C₆N₇(NH₂)₃ ? xH₂O (x≈2.3) were obtained by hydrothermal treatment of melem at 200 °C and the crystal structure (R $\bar 3$ c, a=2879.0(4), c=664.01(13) pm, V=4766.4(13)×10⁶ pm³, Z=18) was elucidated by single‐crystal X‐ray diffraction. With respect to the structural similarity to the well‐known adduct between melamine and cyanuric acid, the composition of the obtained product was further analyzed by solid‐state NMR spectroscopy. Hydrolysis of melem to cyameluric acid during syntheses at elevated temperatures could thus be ruled out. DTA/TG studies revealed that, during heating of melem hydrate, water molecules can be removed from the channels of the structure to a large extent. The solvent‐free framework is stable up to 430 °C without transforming into the denser structure of anhydrous melem. Dehydrated melem hydrate was further characterized by solid‐state NMR spectroscopy, powder X‐ray diffraction, and sorption measurements to investigate structural changes induced by the removal of water from the channels. During dehydration, the hexagonal, layered arrangement of melem units is maintained whereas the formation of additional hydrogen bonds between melem entities requires the stacking mode of hexagonal layers to be altered. It is assumed that layers are shifted perpendicular to the direction of the channels, thereby making them inaccessible for guest molecules.     

Fabrication of a Metal Particle Array Based on a Self‐Assembled Template from a Two‐Armed Polymer

A two‐armed polymer with a crown ether core self‐assembles to produce macroporous films with pores perpendicularly reaching through the film down to the substrate. A possible assembling mechanism is discussed. The pore size can be conveniently adjusted by changing the solution concentration. These through‐hole macroporous films provide a template for fabricating an array of Cu nanoparticle aggregates.

     

Two‐Dimensional Nanoporous Networks Formed by Liquid‐to‐Solid Transfer of Hydrogen‐Bonded Macrocycles Built from DNA Bases

We present an approach that makes use of DNA base pairing to produce hydrogen‐bonded macrocycles whose supramolecular structure can be transferred from solution to a solid substrate. A hierarchical assembly process ultimately leads to two‐dimensional nanostructured porous networks that are able to host size‐complementary guests.     

Synthesis of Highly Active and Stable Spinel‐Type Oxygen Evolution Electrocatalysts by a Rapid Inorganic Self‐Templating Method

Composition‐adjustable spinel‐type metal oxides, Mn_xCo_3?xO_4?δ (x=0.8–1.4), were synthesized in ethanol solutions by a rapid inorganic self‐templating mechanism using KCl nanocrystals as the structure‐directing agent. The Mn_xCo_3?xO_4?δ materials showed ultrahigh oxygen evolution activity and strong durability in alkaline solutions, and are capable of delivering a current density of 10 mA cm^?2 at 1.58 V versus the reversible hydrogen electrode in 0.1 M KOH solution, which is superior in comparison to IrO₂ catalysts under identical experimental conditions, and comparable to the most active noble‐metal and transition‐metal oxygen evolution electrocatalysts reported so far. The high performance for catalytic oxygen evolution originates from both compositional and structural features of the synthesized materials. The moderate content of Mn doping into the spinel framework led to their improved electronic conductivity and strong oxidizing ability, and the well‐developed porosity, accompanied with the high affinity between OH^? reactants and catalyst surface, contributed to the smooth mass transport, thus endowing them with superior oxygen evolution activity.     

Crystal‐to‐Crystal Transformation: An HgII Metal–Organic Polymer Constructed with a Triazole Ligand

A new one‐dimensional double‐chain Hg^II coordination polymer containing the ligand 3,5‐bis(4‐pyridyl)‐4‐amino‐1,2,4‐triazole (bpatrz) and thiocyanate anions, namely, {[Hg₂(μ‐bpatrz)(μ‐SCN)₂(SCN)₂] · MeOH}_n ( 1 ), has been synthesized and structurally characterized by single‐crystal X‐ray diffraction. Compound 1 displays bright blue luminescence with emission maxima around 370 and 440 nm in solution and the solid state, respectively. Uptake and release of MeOH by 1 were investigated by powder X‐ray diffraction, thermogravimetric analysis, elemental analysis, and IR and NMR spectroscopy.     

Supramolecular Polymer Network‐Mediated Self‐Assembly of Semicrystalline Polymers with Excellent Crystalline Performance

A novel application of supramolecular interactions within semicrystalline polymers, capable of self‐assembling into supramolecular polymer networks via self‐complementary multiple hydrogen‐bonded complexes, is demonstrated for efficient construction of highly controlled self‐organizing hierarchical structures to offer a direct, efficient nucleation pathway resulting in superior crystallization performance. Herein, a novel functionalized poly(ε‐caprolactone) containing self‐complementary sextuple hydrogen‐bonded uracil‐diamidopyridine (U‐DPy) moieties is successfully developed and demonstrated excellent thermal and viscoelastic properties as well as high dynamic structural stability in the bulk state due to physical cross‐linking created by reversible sextuple hydrogen bonding between U‐DPy units. Due to the ability to vary the extent of the reversible network by tuning the U‐DPy content, this newly developed material can be readily adjusted to obtain the desired crystalline products with specific characteristics. Importantly, incorporating only 0.1% U‐DPy resulted in a polymer with a high crystallization rate constant, short crystallization half‐time, and much more rapid crystallization kinetics than pristine PCL, indicating a low content of U‐DPy moieties provides highly efficient nucleation sites that manipulate the nucleation and growth processes of polymer crystals to promote crystallization and chain alignment in bulk. This new system is suggested as a potential new route to substantially improve the performance of polymer crystallization.

     

Designed Synthesis of a 1D Polymer in Twist‐Stacked Topology via Single‐Crystal‐to‐Single‐Crystal Polymerization

To synthesize a fully organic 1D polymer in a novel twist‐stacked topology, we designed a peptide monomer HC≡CCH₂‐NH‐Ile‐Leu‐N₃, which crystallizes with its molecules H‐bonded along a six‐fold screw axis. These H‐bonded columns pack parallelly such that molecules arrange head‐to‐tail, forming linear non‐covalent chains in planes perpendicular to the screw axis. The chains arrange parallelly to form molecular layers which twist‐stack along the screw axis. Crystals of this monomer, on heating, undergo single‐crystal‐to‐single‐crystal (SCSC) topochemical azide–alkyne cycloaddition (TAAC) polymerization to yield an exclusively 1,4‐triazole‐linked polymer in a twist‐stacked layered topology. This topologically defined polymer shows better mechanical strength and thermal stability than its unordered form, as evidenced by nanoindentation studies and thermogravimetric analysis, respectively. This work illustrates the scope of topochemical polymerizations for synthesizing polymers in pre‐decided topologies.