Self-Inclusion Complexation of Electron-Accepting Guest into Electron-Donating Cyclic Host by Photoexcitation.

Self-inclusion complexes consisting of host-guest conjugates are one of the unique supramolecular structures because they form in-state and out-state depending on the external stimuli. Despite many reports of the stimuli-responsive self-inclusion complex formation, study of the structural relaxation from out-state to in-state by photoexcitation has been unexplored. Herein, we report that an electron-donating host and an electron-accepting guest conjugate exhibits the structural relaxation from out-state to in-state by photoexcitation. Formation of the in-state in the excited state resulted in exciplex emission along with the locally excited emission from the out-state. Moreover, this structural relaxation by photoexcitation was suppressed not only by temperature, but also by the presence of guest molecules, resulting in changes in the ratio of the dual emission intensities.

Silapillar[n]arenes: Their Enhanced Electronic Conjugation and Conformational Versatility.

During recent decades, methylene-bridged macrocyclic arenes have been widely used in supramolecular chemistry. However, their π-conjugations are very weak, as the methylene bridges disrupt the electronic communication between π orbitals of the aromatic units. Herein, we successfully synthesized a series of silapillar[n]arenes (n = 4, 6, and 8) using silylene bridging. These showed enhanced electronic conjugation compared with the parent pillar[n]arenes because of σ*-π* conjugation between σ* (Si-C) orbitals and π* orbitals of the benzenes. Owing to the longer Si-C bond compared with the C-C bond, silylene-bridging provides additional structural flexibility into the pillar[n]arene scaffolds; a strained silapillar[4]arene was formed, which is unavailable in the parent pillar[n]arenes because of the steric requirements. Furthermore, silapillar[n]arenes displayed interesting size-dependent structural and optical properties.

Induced chirality at the surface: fixation of a dynamic M/P invertible helical Co3 complex on SiO2.

Dynamic M/P invertible helicity was successfully induced at a SiO2 surface immobilized with a dynamic helical trinuclear cobalt complex, [LCo3(NHMe2)6](OTf)3, using chiral ((R) or (S))-1-phenylethylamine. Solid-state CD spectra and theoretical calculations suggested that the fixation of the M/P helical complex on the surface via coordination interactions was the key factor of the induced chirality at the surface.

Friedel-Crafts Acylation for Accessing Multi-Bridge-Functionalized Large Pillar[n]arenes.

Pillar[n]arenes can be constructed using a Friedel-Crafts alkylation process. However, due to the reversible nature of the alkylation, mixture of large pillar[n]arenes (n≥7) are obtained as minor products, and thus laborious purification are necessary to isolate the larger pillar[n]arenes. Moreover, inert methylene bridges are introduced during the alkylation process, and the multi-functionalization of the bridges has never been investigated. Herein, an irreversible Friedel-Crafts acylation is used to prepare pillar[n]arenes. Due to the irreversible nature of the acylation, the reaction of precursors bearing carboxylic acids and electron-rich arene rings results in a size-exclusive formation of pillar[n]arenes, in which the ring-size is determined by the precursor length. Because of this size-selective formation, laborious separation of undesired macrocycles is not necessary. Moreover, the bridges of pillar[n]arenes are selectively installed with reactive carbonyl groups using the acylation method, whose positions are determined by the precursor used. The carbonyl bridges can be easily converted into versatile functional groups, leading to various laterally modified pillar[n]arenes, which cannot be accessed by the alkylation strategy.

Acceleration and deceleration of chirality inversion speeds in a dynamic helical metallocryptand by alkali metal ion binding.

We report that the chirality inversion kinetics of a trinickel(II) cryptand can be controlled by guest recognition in the cryptand cavity. When the guest was absent, the nickel(II) cryptand underwent a dynamic interconversion between the P and M forms in solution, preferring the M form, with a half-life of t1/2 = 4.99 min. The P/M equilibrium is reversed to P-favored by binding with an alkali metal ion in the cryptand cavity. The timescale of this M→P inversion kinetics was both notably accelerated and decelerated by the guest binding (t1/2 = 0.182 min for K+ complex; 186 min for Cs+ complex); thus, the equilibration rate constants differed by up to 1000-fold depending on the guest metal ions. This acceleration/deceleration can be explained in terms of the virtual binding constants at the transition state of the P/M chirality inversion; K+ binding more stabilizes the transition state rather than the P and M forms to result in the acceleration.

Stapling strategy for slowing helicity interconversion of α-helical peptides and isolating chiral auxiliary-free one-handed forms.

In nature, α-helical peptides adopt right-handed conformations that are dictated by L-amino acids. Isolating one-handed α-helical peptides composed of only achiral components remains a significant challenge. Here, this goal is achieved by optical resolution of the corresponding racemic (quasi-)static α-helical peptide with double stapling, which effectively freezes the interconversion between the right-handed (P)- and left-handed (M)-α-helices. An as-obtained doubly stapled analogue having an unprotected L-valine residue at the C-terminus transforms from a kinetically trapped (M)-α-helix to a thermodynamically stable (P)-α-helix upon heating. In contrast, the corresponding singly stapled α-helical peptide undergoes an acid/base-triggered and solvent-induced reversible inversion of its preferred helicity within minutes. The interconversion rates of the singly and doubly stapled α-helical peptide foldamers are approximately 106 and 1012 times slower, respectively, than that of a non-stapled dynamic helical peptide. Therefore, the enantiopure doubly-stapled (quasi-)static α-helical peptide would retain its optical activity for several years at 25 °C.

Dynamic-to-Static Planar Chirality Conversion in Pillar[5]arenes Regulated by Guest Solvents or Amplified by Crystallization.

Controlling dynamic chirality and memorizing the controlled chirality are important. Chirality memory has mainly been achieved using noncovalent interactions. However, in many cases, the memorized chirality arising from noncovalent interactions is erased by changing the conditions such as the solvent and temperature. In this study, the dynamic planar chirality of pillar[5]arenes was successfully converted into static planar chirality by introducing bulky groups through covalent bonds. Before introducing the bulky groups, pillar[5]arene with stereogenic carbon atoms at both rims existed as a pair of diastereomers, and thus showed planar chiral inversion that was dependent on the chain length of the guest solvent. The pS and pR forms, regulated by guest solvents, were both diastereomerically memorized by introducing bulky groups. Furthermore, the diastereomeric excess was amplified by crystallization of the pillar[5]arene. The subsequent introduction of bulky groups yielded pillar[5]arene with an excellent diastereomeric excess (95 % de).

Adaptive Planar Chirality of Pillar[5]arenes Invertible by n-Alkane Lengths.

Chirality of host molecules can be induced and/or inverted by the guest molecules. However, the adapting chirality of hosts to the length of n-alkanes remains a great challenge because n-alkanes are neutral, achiral, and linear molecules, resulting in a weak interaction with most compounds. Herein, we report a system with chirality adapted to n-alkane lengths, using a pillar[5]arene-based macrocyclic host, S-Br, which contains five stereogenic carbons and five terminal bromine atoms on each rim. The electron-rich cavity of S-Br could include n-alkanes and the planar-chiral isomers sensitively inverted in response to the lengths of the complexed n-alkanes. The inclusion of a short n-alkane such as n-pentane made S-Br more inclined to be in the pS-form, whereas the inclusion of long n-alkanes such as n-heptane made the pR-form more favorable. The difference in the stability of the isomers was supported by the crystal structures and the theoretical calculations. Furthermore, temperature drives the adaptive chirality of S-Br with n-alkanes. An n-alkane with middle length, n-hexane, showed the dominance of the pR-form of S-Br at a higher temperature, whereas the pS-form was shown at a lower temperature.

Speed Tuning of the Formation/Dissociation of a Metallorotaxane.

Switching between the formation/dissociation of rotaxanes is important to control the function of various types of rotaxane-based materials. We have developed a convenient and simple strategy, the so-called "accelerator addition", to make a static rotaxane dynamic without apparently affecting the chemical structure. As an interlocked molecule that enables tuning of the formation/dissociation speed, a metallorotaxane was quantitatively generated by the complexation of a triptycene-based dumbbell-shaped mononuclear complex, [PdL2 ]2+ (L=2,3-diaminotriptycene), with 27C9. As a result of the inertness of the Pd2+ -based coordination structure, the metallorotaxane was slowly formed (the static state). This rotaxane formation was accelerated 27 times simply by adding Br- as an accelerator (the dynamic state). A similar drastic acceleration was also demonstrated during the dissociation process when Cs+ was added to the metallorotaxane to form the free axle [PdL2 ]2+ and the 27C9-Cs+ complex.

Synthesis of Isomeric Tb3+ -Phthalocyanine Double-Decker Complexes Depending on the Difference in the Direction of Coordination Plane and Their Magnetic Properties.

Invited for the cover of this issue are Kentaro Tanaka at Nagoya University and co-workers. The image depicts three isomers of a terbium(III) phthalocyanine double-decker complex made from C4h symmetrically substituted phthalocyanines and their magnetic properties. Read the full text of the article at 10.1002/chem.202203272.

Synthesis of Isomeric Tb3+ -Phthalocyanine Double-Decker Complexes Depending on the Difference in the Direction of Coordination Plane and Their Magnetic Properties.

A C4h symmetrically substituted phthalocyanine, 1,8,15,22-tertrakis(2,4-dimethylpent-3-oxy)phthalocyanine (H2 TdMPPc), was used to synthesize Tb3+ -phthalocyanine double-decker complexes ([Tb(TdMPPc)2 ]s). Because H2 TdMPPc has C4h symmetry, S,S, R,R, and meso isomers of [Tb(TdMPPc)2 ] were obtained depending on the difference in the direction of the coordination plane of two C4h -type phthalocyanines with respect to a central Tb3+ ion. We investigated the physical properties of these [Tb(TdMPPc)2 ] isomers, including their single-ion magnetic properties, and found that the spin-reversal energy barrier (Ueff ) of the meso isomer was apparently higher than that of the enantiomers. Detailed crystal structural analyses indicated that the meso isomer has a more symmetrical structure than do the enantiomers, thereby suggesting that the higher Ueff of the meso isomer originated from the more highly symmetrical structure.

Synthesis of amphiphilic chiral salen complexes and their conformational manipulation at the air-water interface.

A series of amphiphilic salen complexes, [L1a,bM] and [L2a,bM], were designed and synthesized. These complexes consist of two or four hydrophilic triethylene glycol (TEG) chains and a hydrophobic π-extended metallosalen core based on naphthalene or phenanthrene. The obtained amphiphilic complexes, [L1bM] (M = Ni, Cu, Zn), formed a monolayer at the air-water interface, while the monocationic [L1bCo(MeNH2)2](OTf) did not form a well-defined monolayer. The number of hydrophilic TEG chains also had an influence on the monolayerformation behavior; the tetra-TEG derivatives, [L1bNi] and [L2bNi], showed a pressure rise at a less compressed region than the bis-TEG derivatives, [L1aNi] and [L2aNi]. In addition, the investigation of their compressibility and compression modulus suggested that the tetra-TEG derivatives, [L1bNi] and [L2bNi], are more flexible than the corresponding bis-TEG analogues, [L1aNi] and [L2aNi], and that the phenanthrene derivatives [L1a,bNi] were more rigid than the corresponding naphthalene analogues, [L2a,bNi]. The Langmuir-Blodgett (LB) films of one of the complexes, [L1bNi], showed CD spectra slightly different from that in solution, which may originate from the unique anisotropic environment of the air-water interface. Thus, we demonstrated the possibility of controlling the chiroptical properties of metal complexes by mechanical compression.

A Twisted Chiral Cavitand with 5-Fold Symmetry and Its Length-Selective Binding Properties.

Controlling bottom-up syntheses from chiral seeds to construct architectures with specific chiralities is currently challenging. Herein, a twisted chiral cavitand with 5-fold symmetry was constructed by bottom-up synthesis using corannulene as the chiral seed and pillar[5]arene as the chiral wall. After docking between the seed and the wall, their dynamic chiralities (M and P) are fixed. Moreover, the formed hedges also exhibit M and P chirality. Through dynamic covalent bonding, the thermodynamically stable product is obtained selectively as a pair of enantiomers (MMM and PPP), where all three subcomponents, i.e., the corannulene, hedges, and pillar[5]arene, are tilted in the same direction. Furthermore, the twisted cavitand exhibits length-selective binding to alkylene dibromides, with three maximum binding constants being unexpectedly observed.

Real-time chirality transfer monitoring from statistically random to discrete homochiral nanotubes.

Real time monitoring of chirality transfer processes is necessary to better understand their kinetic properties. Herein, we monitor an ideal chirality transfer process from a statistically random distribution to a diastereomerically pure assembly in real time. The chirality transfer is based on discrete trimeric tubular assemblies of planar chiral pillar[5]arenes, achieving the construction of diastereomerically pure trimers of pillar[5]arenes through synergistic effect of ion pairing between a racemic rim-differentiated pillar[5]arene pentaacid bearing five benzoic acids on one rim and five alkyl chains on the other, and an optically resolved pillar[5]arene decaamine bearing ten amines. When the decaamine is mixed with the pentaacid, the decaamine is sandwiched by two pentaacids through ten ion pairs, initially producing a statistically random mixture of a homochiral trimer and two heterochiral trimers. The heterochiral trimers gradually dissociate and reassemble into the homochiral trimers after unit flipping of the pentaacid, leading to chirality transfer from the decaamine and producing diastereomerically pure trimers.

π-Stacked Ion Pairs: Tightly Associated Charged Porphyrins in Ordered Arrangement Enabling Radical-Pair Formation.

π-Electronic ion pairs are of interest for fabricating electronic materials that use intermolecular interactions based on electrostatic and dispersion forces, defined as iπ-iπ interactions, to provide dimension-controlled assemblies. Porphyrin ions, whose charge is delocalized in the core units, are suitable for ordered arrangement and assemblies by ion pairing. Herein, charged porphyrins were found to form solid-state assemblies and solution-state stacked ion pairs according to the peripheral electron-donating groups (EDGs) and electron-withdrawing groups (EWGs). The concentration-dependent 1H NMR signal shifts of a porphyrin ion pair, comprising a meso-EWG cation and a meso-EWG anion, provided a hetero-dimerization constant of 2.8 × 105 M-1 in CD2Cl2 at 20 °C. In the ion pair of a meso-EWG cation and a meso-EDG anion, the electron transfer in the steady and excited states according to solvent polarity and photoexcitation, respectively, produced the radical pairs. The electron spin resonance analysis in frozen toluene revealed the formation of a heterodiradical in a closely stacked structure by the antiferromagnetic dipolar interaction and temperature-dependent spin transfer behavior.

Rotaxane formation by an allosteric pseudomacrocyclic anion receptor utilising kinetically labile copper(I) coordination properties.

Rotaxanes, which are composed of ring and axle components, are important interlocked molecules with wide applications such as molecular machines and switchable catalysts. The construction of interlocked structures targeting anions is an important issue, as evidenced by the fact that anionic groups are usually abundant in many biomacromolecules. We now report an allosteric pseudomacrocyclic anion receptor as a ring that spontaneously generates a rotaxane in an auto-clipping way, which does not require the successive ring forming reaction like usual clipping, in the presence of an axle with an anionic station. We designed a linear ligand 1 bearing three anion recognition moieties, i.e., one thiourea group at the centre and two urea groups near the 2,2'-bipyridine ends of 1. The complexation of 1 with Cu+ proceeded in an intramolecular manner that quantitatively led to a macrocyclic structure [1·Cu]+. Compared to 1, the anion binding ability of [1·Cu]+ was significantly larger (positive allosteric effect) due to the macrocyclization arrangement of the three anion recognition moieties in a cyclic fashion and electrostatic interaction. In addition, the kinetically labile but thermodynamically stable coordination properties of the pseudomacrocyclic ring unit promoted the spontaneous rotaxane formation with a phosphate axle at room temperature.

Noncooperative guest binding by metal-free [2 + 2] Schiff-base macrocycles.

Salphen-based [n + n] macrocycles have been widely explored for their unique chemical and topological properties following metal ion coordination. Despite having vastly different reactivity than their coordinated counterparts, fewer studies have focused on metal-free salphen macrocycles. We investigated the binding of [2 + 2] Schiff-base macrocycle host 3, which contains a central 18-crown-6-like cavity and two N2O2 moieties. This macrocycle strongly binds to spherical cationic guests (K11 ≈ 103-104 M-1, DCM/MeCN). The most robust binding was shown for K+ and Na+, followed by Li+ and Rb+. More sterically demanding cationic guests like dibenzylammonium (DBA+) showed almost no binding. The binding pocket in 3 is slightly smaller than 18-crown-6, resulting in binding outside the cavity, which provides a scaffold appropriate for 2 : 1 complexes, where two host molecules sandwich the guest. All host-guest complexes follow a 2 : 1 noncooperative binding model, where each successive binding event is less likely than the previous, unlike coordinated versions of 3, where most binding is 1 : 1.

State- and water repellency-controllable molecular glass of pillar[5]arenes with fluoroalkyl groups by guest vapors.

Molecular glasses are low-molecular-weight organic compounds that are stable in the amorphous state at room temperature. Herein, we report a state- and water repellency-controllable molecular glass by n-alkane guest vapors. We observed that a macrocyclic host compound pillar[5]arene with the C2F5 fluoroalkyl groups changes from the crystalline to the amorphous state (molecular glass) by heating above its melting point and then cooling to room temperature. The pillar[5]arene molecular glass shows reversible transitions between amorphous and crystalline states by uptake and release of the n-alkane guest vapors, respectively. Furthermore, the n-alkane guest vapor-induced reversible changes in the water contact angle were also observed: water contact angles increased and then reverted back to the original state by the uptake and release of the n-alkane guest vapors, respectively, along with the changes in the chemical structure and roughness on the surface of the molecular glass. The water repellency of the molecular glass could be controlled by tuning the uptake ratio of the n-alkane guest vapor.

Transient chirality inversion during racemization of a helical cobalt(III) complex.

SignificanceWe first observed a transient chirality inversion on a simple unimolecular platform during the racemization of a chiral helical complex [LCo3A6]3+, i.e., the helicity changed from P-rich (right-handed) to M-rich (left-handed), which then racemized to a P/M equimolar mixture in spite of the absence of a reagent that could induce the M helix. This transient chirality inversion was observed only in the forward reaction, whereas the reverse reaction showed a simple monotonic change with an induction time. Consequently, the M helicity appeared only in the forward reaction. These forward and reverse reactions constitute a hysteretic cycle. Compounds showing such unique time responses would be useful for developing time-programmable switchable materials that can control the physical/chemical properties in a time-dependent manner.