An elastic metal-organic crystal with a densely catenated backbone.

What particular mechanical properties can be expected for materials composed of interlocked backbones has been a long-standing issue in materials science since the first reports on polycatenane and polyrotaxane in the 1970s1-3. Here we report a three-dimensional porous metal-organic crystal, which is exceptional in that its warps and wefts are connected only by catenation. This porous crystal is composed of a tetragonal lattice and dynamically changes its geometry upon guest molecule release, uptake and exchange, and also upon temperature variation even in a low temperature range. We indented4 the crystal along its a/b axes and obtained the Young's moduli of 1.77 ± 0.16 GPa in N,N-dimethylformamide and 1.63 ± 0.13 GPa in tetrahydrofuran, which are the lowest among those reported so far for porous metal-organic crystals5. To our surprise, hydrostatic compression showed that this elastic porous crystal was the most deformable along its c axis, where 5% contraction occurred without structural deterioration upon compression up to 0.88 GPa. The crystal structure obtained at 0.46 GPa showed that the catenated macrocycles move translationally upon contraction. We anticipate our mechanically interlocked molecule-based design to be a starting point for the development of porous materials with exotic mechanical properties. For example, squeezable porous crystals that may address an essential difficulty in realizing both high abilities of guest uptake and release are on the horizon.

NMR spectrum prediction for dynamic molecules by machine learning: A case study of trefoil knot molecule.

Nuclear magnetic resonance (NMR) spectroscopy is one of the indispensable techniques in chemistry because it enables us to obtain accurate information on the chemical, electronic, and dynamic properties of molecules. Computational simulation of the NMR spectra requires time-consuming density functional theory (DFT) calculations for an ensemble of molecular conformations. For large flexible molecules, it is considered too high-cost since it requires time-averaging of the instantaneous chemical shifts of each nuclear spin across the conformational space of molecules for NMR timescales. Here, we present a Gaussian process/deep kernel learning-based machine learning (ML) method for enabling us to predict, average in time, and analyze the instantaneous chemical shifts of conformations in the molecular dynamics trajectory. We demonstrate the use of the method by computing the averaged 1H and 13C chemical shifts of each nuclear spin of a trefoil knot molecule consisting of 24 para-connected benzene rings (240 atoms). By training ML model with the chemical shift data obtained from DFT calculations, we predicted chemical shifts for each conformation during dynamics. We were able to observe the merging of the time-averaged chemical shifts of each nuclear spin in a singlet 1H NMR peak and two 13C NMR peaks for the knot molecule, in agreement with experimental measurements. The unique feature of the presented method is the use of the learned low-dimensional deep kernel representation of local spin environments for comparing and analyzing the local chemical environment histories of spins during dynamics. It allowed us to identify two groups of protons in the knot molecule, which implies that the observed singlet 1H NMR peak could be composed of the contributions from protons with two distinct local chemical environments.

Chiral Calix[3]pyrrole Derivatives: Synthesis, Racemization Kinetics, and Ring Expansion to Calix[9]- and Calix[12]pyrrole Analogues.

Chiral pyrrolic macrocycles continue to attract interest. However, their molecular design remains challenging. Here, we report a calixpyrrole-based chiral macrocyclic system, calix[1]furan[1]pyrrole[1]thiophene (1), synthesized from an oligoketone. Macrocycle 1 adopts a partial cone conformation in the solid state, and undergoes racemization via ring inversion. Molecular dynamics simulations revealed that inversion of the thiophene is the rate determining step. Pyrrole N-methylation suppressed racemization and permitted chiral resolution. Enantioselective N-methylation also occurred in the presence of a chiral ammonium salt, although the stereoselectivity is modest. A unique feature of 1 is that it acts as a useful synthetic precursor to yield several calix[n]furan[n]pyrrole[n]thiophene products (n=2-4), including a calix[12]pyrrole analogue that to our knowledge constitutes the largest calix[n]pyrrole-like species to be structurally characterized.

Structural-transformation-induced Drastic Luminescence Changes in an Organic-Inorganic Hybrid [ReN(CN)4 ]2- Salt Triggered by Chemical Stimuli.

We synthesized a (1-propylpyridinium)2 [ReN(CN)4 ]-type organic-inorganic hybrid exhibiting water-vapor-induced drastic structural changes of the [ReN(CN)4 ]2- assemblies. Specifically, upon exposure to water vapor, dehydrated nitrido-bridged chains were converted to hydrated cyanido-bridged tetranuclear clusters via rearrangements of large molecular building units in the crystals. These switchable assembly forms display substantially different photo-physical properties, although in both cases the emission is caused by a metal-centered d-d transition. The nitrido-bridged chain exhibited a near-infrared (749 nm) emission, which blue-shifted as the temperature increased, while a visible (561 nm) emission and its red shift was demonstrated by the cyanido-bridged cluster.

Strain-Induced Ring Expansion Reactions of Calix[3]pyrrole-Related Macrocycles.

The recent discovery of calix[3]pyrrole, a porphyrinogen-like tripyrrolic macrocycle, has provided an unprecedented strain-induced ring expansion reaction into calix[6]pyrrole. Here, we synthesized calix[n]furan[3-n]pyrrole (n=1∼3) macrocycles to investigate the reaction scope and mechanism of the ring expansion. Single crystal X-ray analysis and theoretical calculations revealed that macrocyclic ring strain increases as the number of inner NH sites increases. While calix[1]furan[2]pyrrole exhibited almost quantitative conversion into calix[2]furan[4]pyrrole within 5 minutes, less-strained calix[2]furan[1]pyrrole and calix[3]furan were inert. However, N-methylation of calix[2]furan[1]pyrrole induced a ring-expansion reaction that enabled the isolation of a linear reaction intermediate. The mechanism analysis revealed that the ring expansion consists of regioselective ring cleavage and subsequent cyclodimerization. This reaction was further utilized for synthesis of calix[6]-type macrocycles.

Accumulated Lattice Strain as an Internal Trigger for Spontaneous Pathway Selection.

Multicomponent crystallization is universally important in various research fields including materials science as well as biology and geology, and presents new opportunities in crystal engineering. This process includes multiple kinetic and thermodynamic events that compete with each other, wherein "external triggers" often help the system select appropriate pathways for constructing desired structures. Here we report an unprecedented finding that a lattice strain accumulated with the growth of a crystal serves as an "internal trigger" for pathway selection in multicomponent crystallization. We discovered a "spontaneous" crystal transition, where the kinetically preferred layered crystal, initially formed by excluding the pillar component, carries a single dislocation at its geometrical center. This crystal "spontaneously" liberates a core region to relieve the accumulated lattice strain around the dislocation. Consequently, the liberated part becomes dynamic and enables the pillar ligand to invade the crystalline lattice, thereby transforming into a thermodynamically preferred pillared-layer crystal.

Triplet Carbene with Highly Enhanced Thermal Stability in the Nanospace of a Metal-Organic Framework.

Triplet carbenes (TCs) are of great interest due to their magnetic properties and reactivity, which descend from TCs' unique electronic state. However, the reactivity and stability of TCs are usually a trade-off, and it is difficult to achieve both at the same time. In this work, we were able to enhance the thermal stability of a TC species while maintaining its reactivity by confining them in the nanospace of a metal-organic framework (MOF). We synthesized a new MOF using a TC precursor; subsequently, TCs were generated by photostimulation. The TCs generated in the MOF nanospace were detectable up to 170 K, whereas their non-MOF-confined counterparts (bare ligand) could not be detected above 100 K. In addition, the reactivity of TC generated in MOF with O2 was drastically improved compared to that of bare ligand. Our approach is generally applicable to the stabilization of highly reactive species, whose reactivity needs to be preserved.

Calix[3]pyrrole: A Missing Link in Porphyrin-Related Chemistry.

A long-standing question in porphyrin chemistry is why pyrrole monomers selectively form tetrapyrrolic macrocycles, whereas the corresponding tripyrrolic macrocycles are never observed. Calix[3]pyrrole, a tripyrrolic porphyrinogen-like macrocycle bearing three sp3-carbon linkages, is a missing link molecule that might hold the key to this enigma; however, it has remained elusive. Here we report the synthesis and strain-induced transformations of calix[3]pyrrole and its furan analogue, calix[3]furan. These macrocycles are readily accessed from cyclic oligoketones. Crystallographic and theoretical analyses reveal that these three-subunit systems possess the largest strain energy among known calix[n]-type macrocycles. The ring-strain triggers transformation of calix[3]pyrrole into first calix[6]pyrrole and then calix[4]pyrrole under porphyrin cyclization conditions. The present results help explain the absence of naturally occurring three-pyrrole macrocycles and the fact that they are not observed as products or intermediate during classic porphyrin syntheses.

Double-Helix Supramolecular Nanofibers Assembled from Negatively Curved Nanographenes.

The layered structures of graphite and related nanographene molecules play key roles in their physical and electronic functions. However, the stacking modes of negatively curved nanographenes remain unclear, owing to the lack of suitable nanographene molecules. Herein, we report the synthesis and one-dimensional supramolecular self-assembly of negatively curved nanographenes without any assembly-assisting substituents. This curved nanographene self-assembles in various organic solvents and acts as an efficient gelator. The formation of nanofibers was confirmed by microscopic measurements, and an unprecedented double-helix assembly by continuous π-π stacking was uncovered by three-dimensional electron crystallography. This work not only reports the discovery of an all-sp2-carbon supramolecular π-organogelator with negative curvature but also demonstrates the power of three-dimensional electron crystallography for the structural determination of submicrometer-sized molecular alignment.

Trapping and Releasing of Oxygen in Liquid by Metal-Organic Framework with Light and Heat.

Nanoporous materials can adsorb small molecules into their nanospaces. However, the trapping of light gas molecules dissolved in solvents suffers from low concentration and poor adsorption affinity. Here, the reversible trapping and releasing of dissolved oxygen are shown through integrating photosensitization and chemical capturing abilities into a metal-organic framework (MOF), MOMF-1. 9,10-Di(4-pyridyl)anthracene (dpa) ligands in MOMF-1 generates singlet oxygen from triplet oxygen under photoirradiation without additional photosensitizers, and successively reacts with it to produce anthracene endoperoxide, forming MOMF-2, which is proved crystallographically. The reverse reaction also proceeds quantitatively by heating MOMF-2. Moreover, MOMF-1 exhibits excellent water resistance, and completely removes oxygen of ppm order concentrations in water. The new material shown in this report allows controlling of the amount of dissolved oxygen, which can be applicable in various fields relating to numerous oxidation phenomena.

Flexibility Control of Two-Dimensional Coordination Polymers by Crystal Morphology: Water Adsorption and Thermal Expansion.

Layer flexibility in two-dimensional coordination polymers (2D-CPs) contributes to several functional materials as it results in anisotropic structural response to external stimuli. Chemical modification is a common technique for modifying layer structures. This study demonstrates that crystal morphology of a cyanide-bridged 2D-CP of type [Mn(salen)]2 [ReN(CN)4 ] (1) consisting of flexible undulating layers significantly impacts the layer configuration and assembly. Nanoplates of 1 showed an in-plane contraction of layers with a longer interlayer distance compared to the micrometer-sized rod-type particles. These effects by crystal morphology on the structure of the 2D-CP impacted the structural flexibility, resulting in dual-functional changes: the enhancement of the sensitivity of structural transformation to water adsorption and modification of anisotropic thermal expansion of 1. Moreover, the nanoplates incorporated new adsorption sites within the layers, resulting in the uptake of an additional water molecule compared to the micrometer-sized rods.

A Temporarily Pore-Openable Porous Coordination Polymer for Guest Adsorption/Desorption.

Flexible porous coordination polymers (PCPs)/metal-organic frameworks are unique materials that have potential applications as components of highly efficient separation, sensor, and actuator systems. In general, the structures of flexible PCPs drastically change upon guest loading. In this investigation, we uncovered the rare one-dimensional PCP [Cu2(bza)4(2-apyr)] (1; bza = benzoate and 2-apyr = 2-aminopyrimidine), which exhibits a unique type of flexibility involving temporary pore opening. Single-crystal X-ray diffraction analysis revealed that desolvated 1 and ethyl acetate (AcOEt)-loaded (1·AcOEt) and CO2-loaded (1·CO2) 1 have isolated pores. In the case of 1, the pore structure prevents guest penetration. In addition, the isolated pore structures of 1·AcOEt and 1·CO2 block guest release. However, 1 participates in reversible adsorption/desorption of AcOEt and CO2 because pore opening occurs temporarily. The CO2 adsorption/desorption isotherms of 1 are type I and dissimilar to those observed in traditional flexible PCPs with adsorption/desorption hysteresis. The lesser conventional flexibility displayed by 1 could offer new insight into the design of flexible PCPs.

Photochemically Crushable and Regenerative Metal-Organic Framework.

A photochemically crushable and regenerative metal-organic framework (DTEMOF) was developed by complexation of photochromic ligand PyDTEopen and 5-nitroisophthalate (nip2-) with Cd2+ in DMF/MeOH. DTEMOF ([Cd(nip)(PyDTEopen)(H2O)(DMF)2]n) was obtained as colorless crystals. Its crystal structure revealed that DTEMOF adopts a tubular structure with interlocked coordination networks and can accommodate guest molecules in its one-dimensional pores. When DTEMOF suspended in DMF/MeOH was exposed to UV light, its crystalline network, though thermally stable up to 260 °C, was readily crushed to afford a homogeneous blue-colored solution, via ring-closing isomerization of the constituent PyDTEopen ligand into PyDTEclosed. Upon successive exposure of this solution to visible light, colorless MOF crystals identical to those of DTEMOF were regenerated. Light-responsive DTEMOF enabled highly efficient on-demand guest release.

Trans Influence across a Metal-Metal Bond of a Paddle-Wheel Unit on Interaction with Gases in a Metal-Organic Framework.

Metal-organic frameworks (MOFs) are known as promising adsorbent materials that can recognize gases specifically. In the frameworks, gases favor interacting with specific binding sites such as open metal sites (OMSs), which can consist of various metals and show characteristic adsorption properties. A recently reported framework possessing OMSs of rhodium paddle-wheel (Rh-PW) showed distinct adsorption properties between NO and CO. We investigated theoretically the reasons for stronger NO binding to the Rh-PW and different adsorption amounts between NO and CO using Rh-PW cluster models, as well as the frequently reported Cu-PW for comparison. We also analyzed the cases of CO2 and N2, which are often used to probe functions of MOFs. We observed an increase in binding energy of NO at the second adduction of NO. On the basis of energy decomposition analysis, we found that Rh-NO bond formation inducing a trans influence is important for the stronger binding than with CO. Furthermore, we proposed a reason for twice the adsorption amount of NO than CO. The results are consistent with experimental observations, giving us insight into design functions of MOFs by selecting metal species.

One-Step Synthesis of an Adaptive Nanographene MOF: Adsorbed Gas-Dependent Geometrical Diversity.

A layered metal-organic framework (MOF) comprising extra-large nanographene sheets, HBCMOF, was successfully synthesized using a dicarboxylic acid derivative of hexa-peri-hexabenzocoronene (HBCLH2), and its structure was characterized by single-crystal X-ray diffraction analysis. The crystal structure shows that 2D layers composed of a dinuclear Zn2+ complex unit and HBCL are located on top of each other through multiple weak interlayer bonds, affording HBCMOF, having three dimensionally connected nanopores with large nanographene surfaces. The HBC-based nanographene sheets are anchored to the MOF framework via two zinc carboxylate linkages and therefore have an axial rotational freedom. The sorption isotherms of gaseous molecules such as carbon dioxide and hydrocarbons (acetylene, propane, propylene, benzene, and cyclohexane) on HBCMOF all displayed a hysteretic profile with reversible structural changes, as observed by in situ powder X-ray diffraction studies.

Theoretical Insights into the Switching Off/On of 1 O2 Photosensitization in Chemicontrolled Photodynamic Therapy.

Density Functional Theory and time-dependent (TD) DFT calculations were carried out for recently reported 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-based photosensitizers (PSs) that could be activated by reactive oxygen species (ROS) to generate 1 O2 specifically in target tissues. To assess the applicability of the compounds as activatable PSs (a-PSs) in photodynamic therapy, absorption wavelengths; singlet-triplet energy gaps; and spin-orbit matrix elements for the radiationless transition, Sn →Tm , were investigated. A TD-DFT qualitative analysis indicated that only a Br-substituted BODIPY derivative with the chromanol ring of α-tocopherol linked by methylene functioned as an a-PS. The chromanol ring promotes photoinduced electron transfer to the BODIPY unit that reduces the probability of intersystem crossing and triplet-state population, and can turn off 1 O2 photosensitization. Therefore, 1 O2 photosensitization can be switched on only in target cells in which the chromanol ring is oxidized by ROS. The oxidation reaction pathways of the most promising derivative, by either 1 O2 or cumyloxyl radical as typical ROS, have been examined to reveal that oxidation by the cumyloxyl radical is more effective than that by 1 O2 .

Systematic Study of Photoluminescence Enhancement in Monolayer Molybdenum Disulfide by Acid Treatment.

Monolayer molybdenum disulfide (MoS2) is an atomically thin semiconducting material with a direct band gap. This physical property is attributable to atomically thin optical devices such as sensors, light-emitting devices, and photovoltaic cells. Recently, a near-unity photoluminescence (PL) quantum yield of a monolayer MoS2 was demonstrated via a treatment with a molecular acid, bis(trifluoromethane)sulfonimide (TFSI); however, the mechanism still remains a mystery. Here, we work on PL enhancement of monolayer MoS2 by treatment of Brønsted acids (TFSI and sulfuric acid (H2SO4)) to identify the importance of the protonated environment. In TFSI as an acid, different solvents-1,2-dichloroethane (DCE), acetonitrile, and water-were studied, as they show quite different acidity in solution. All of the solvents showed PL enhancement, and the highest was observed in DCE. This behavior in DCE would be due to the higher acidity than others have. Acids from different anions can also be studied in water as a common solvent. Both TFSI and H2SO4 showed similar PL enhancement (∼4-8 enhancement) at the same proton concentration, indicating that the proton is a key factor to enhance the PL intensity. Finally, we considered another cation, Li+ from Li2SO4, instead of H2SO4, in water. Although Li and H atoms showed similar binding energy on MoS2 from theoretical calculations, Li2SO4 treatment showed little PL enhancement; only coexisting H2SO4 reproduced the enhancement. This study demonstrated the importance of a protonated environment to increase the PL intensity of monolayer MoS2. The study will lead to a solution to achieve high optical quality and to implementation for atomically thin optical devices.

Photophysical Properties of S, Se and Te-Substituted Deoxyguanosines: Insight into Their Ability To Act as Chemotherapeutic Agents.

Guanine and guanosine derivatives have long been in use as anticancer drugs and recently have been proposed also as photosensitizers in photodynamic therapy. By means of density functional theory and its time-dependent formulation, the potential power as UVA chemotherapeutic agents has been investigated computing the photophysical properties (absorption spectra, excitation energies, and spin-orbit matrix elements) of sulfur, selenium, and tellurium-substituted deoxyguanosines. Different pathways for the population of the lowest triplet state have been considered. Results show that all the examined systems have the lowest triplet state lying above the energy required for the production of the highly cytotoxic excited molecular oxygen 1Δg and that the heavy atom effect ensures an efficient intersystem spin crossing.

Direct and cluster-assisted dehydrogenation of methane by Nb+ and Ta+: a theoretical investigation.

DFT calculations have been performed to examine both direct and cluster-assisted methane C-H bond activation by Nb+ and Ta+ cations. The commonly accepted dehydrogenation pathways, that are oxidative addition and reductive elimination, have been studied in detail for methane ligated clusters M(CH4)n+ (M = Nb, Ta and n = 1-4). For the second H atom transfer to the metal in the presence of additional CH4 molecules (n > 1) two alternative routes have been explored. Energy profiles for ground quintet and excited triplet and singlet spin states of both Nb+ and Ta+ cations have been calculated. Spin crossings occur for all the examined pathways. Clustering of methane ligands appears to favorably affect the process stabilizing all the intercepted minima and transition states and bringing all the calculated PESs below the reference energy of the separated reactants. The direct activation of methane (n = 1) can proceed efficiently only for Ta+, whereas dehydrogenation is endothermic for Nb+ by 9.0 kcal mol-1. When assisted by additional methane ligands, the dehydrogenation process becomes exothermic for both cations whatever the number of coordinated molecules.

Theoretical Exploration of Type I/Type II Dual Photoreactivity of Promising Ru(II) Dyads for PDT Approach.

Ru(II) dyads are a class of bioactive molecules of interest as anticancer agents obtained incorporating an organic chromophore in the light-absorbing metallic scaffold. A careful DFT and TDDFT investigation of the photophysical properties of a series of Ru(II)-polypiridyl dyads containing polythiophene chains of different lengths bound to a coordinating imidazo[4,5-f][1,10]phenanthroline ligand is herein reported. The modulation of the crucial chemical and physical properties of the photosensitizer with increasing number of thiophene units has been accurately described by investigating the UV-vis spectra and type I and type II photoreactions, also including spin-orbit coupling values (SOC). Results show that the low-lying 3IL states afforded as the number of thiophene ligands increases (n = 3, 4) are energetically high enough to ensure singlet oxygen production and can be also involved in electron transfer reaction, showing a dual type I/type II photeoreactivity.