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2024
●Chiral Induction of a Tetrakis(porphyrin) in Various Chiral Solvent
Non-covalent interactions offer an alternative way for developing stimulus-responsive materials such as sensors, machines, and drug-delivery systems. We recently reported that a urethane-equipped tetrakis(porphyrin) forms one-handed helical supramolecular polymers in solution in response to chirality of chiral solvents. Conformational changes in helical sense were detected using circular dichroism (CD) spectroscopy, which showed that the tetrakis(porphyrin) can possibly be used as a sensor for determining the enantiomeric excess of a chiral analyte. Hence, we studied the scope and limitations of the chiral-induction behavior of tetrakis(porphyrin) to deepen the understanding of tetrakis(porphyrin)-based chiral sensing systems. Herein, we report the chiral-induction behavior of tetrakis(porphyrin) in various chiral solvents, which was found to be CD-active in many chiral solvents. Notably, the tetrakis(porphyrin) was CD active in a cryptochiral molecular solvent, which is exciting because the chiralities of acyclic saturated hydrocarbons are hard to sense. Consequently, this study highlights the potential advantages of supramolecular chiral sensors capable of targeting a wide range of analytes, including molecules that are absorption-silent in the UV/vis region, ones devoid of anchoring functional groups, and acyclic, saturated hydrocarbons.
Takehiro Hirao, Sei Kishino, Masaya Yoshida, Takeharu Haino, Chem. Eur. J., 2024, 30, e20240356.(Front Cover)
●Assessment of Edge Modification of Nanographene
Carboxy groups on the edges of nanographene (NG) enable functionalization for realizing NG-organic hybrid materials. Therefore, assessment of the edge-functionalization of the electronic structures of NGs is valuable for the rational design of functional carbon materials. In this study, the structures of model NGs comprising 174 carbon atoms with armchair edges and various functional groups at the edges were computed. To achieve the greatest possible similarity between the computed structure and the real one, the carbon framework was designed based on experimental observations. The functional groups can be accessed via suitable chemical reactions. The computations predicted that although the conversion of carboxyl groups with electron-withdrawing/donating groups influences the orbital energies, the HOMO-LUMO (H-L) gap is not significantly affected, except in a few cases. Among the evaluated examples, π-extension had the greatest influence on the H-L gap. Interestingly, for the Pd2+-coordinated NG, the participation of the low-lying LUMO localized on Pd2+ in the surface-to-metal transitions seemingly narrowed the H-L gap, and a surface-to-ligand transition was observed.
Ryo Sekiya and Takeharu Haino, ChemPysChem, 2024, 30, e202402922.(Front Cover)
●Latent Porosity of Planar Tris(phenylisoxazolyl)benzene
Interest in developing separation systems for chemical entities based on crystalline molecules has provided momentum for the fabrication of synthetic porous materials showing selectivity in molecular encapsulation, such as metal-organic frameworks, covalent organic frameworks, hydrogen-bonded organic frameworks, zeolites, and macrocyclic molecular crystals. Among these, macrocyclic molecular crystals have generated renewed interest for use in separation systems. Selective encapsulation relies on the sizes, shapes, and dimensions of the pores present in the macrocyclic cavities; thus, nonmacrocyclic molecular crystals with high selectivity for molecular encapsulation via porosity-without-pore behaviors have not been studied. Here, we report that planar tris(phenylisoxazolyl)benzene forms porous molecular crystals possessing latent pores exhibiting porosity-without-pore behavior. After exposing the crystals to complementary guest molecules, the latent pores encapsulate cis- and trans-decalin while maintaining the structural rigidity responsible for the high selectivity. The encapsulation via porosity without pores is a kinetic process with remarkable selectivity for cis-decalin over trans-decalin with a cis-/trans-ratio of 96:4, which is confirmed by single-crystal X-ray diffraction and powder X-ray diffraction analyses. Hirshfeld surface analysis and fingerprint plots show that the latent intermolecular pores are rigidified by intermolecular dipole‒dipole and π–π stacking interactions, which determines the remarkable selectivity of molecular recognition.
Yudai Ono, Takehiro Hirao, Naomi Kawata, and Takeharu Haino, Nat. Commun., 2024, 15, 8314.
DOI:10.1038/s41467-024-52526-9
●Conformation Regulation of Trisresorcinarene Directed by Cavity Solvation
This compound is a synthetic macrocycle comprising three pivaloyl-protected resorcinarene units connected by six pentylene chains. We conducted a conformational study using 1H-NMR, X-ray diffraction (XRD), and computational analyses. The macrocycle adopts two conformers, one open, the other closed. The ratio of the open to closed forms depended on the solvent used. Only the open form existed in [D8]toluene, both forms coexisted in [D6]benzene, and the closed form was the major conformer in [D1]chloroform. The benzene-solvated open form observed in the solid state suggests that cavity solvation by solvent molecules directs the open form. The open form was the major or only conformer in [D10]o- and [D10]m-xylene and [D12]mesitylene, whereas the closed form was the major conformer in [D6]acetone. The open and closed forms were equally populated in [D10]p-xylene, suggesting that the size, shape, and dimensions of the solvent molecules most likely influenced the conformation of the protected trisresocinarene.
Daisuke Shimoyama, Ryo Sekiya, Shoichiro Inoue, Naoyuki Hisano, Shin-ichi Tate, and Takeharu Haino, Chem. Eur. J. 2024, 30, e202402922.(Front Cover)
●Spectrally Selective Leakage of Light from Self-Assembled Supramolecular Nanofiber Waveguides Induced by Surface Plasmon Polaritons
We report surface plasmon polariton (SPP)-induced spectrally selective leakage of light waveguided in supramolecular nanofibers. The nanofibers are fabricated by self-assembly of tris(phenylisoxazolyl)benzene derivative molecules, and their diameter ranges from nanometers to hundreds of nanometers. Nanofibers with heights of more than 200 nm are shown to function as waveguides for fluorescence excited in one location by a focused 360 nm laser. The fluorescence can transfer the whole length of the nanofibers of tens of micrometers and is outcoupled from the nanofiber ends. The waveguiding phenomenon dramatically changes when the nanofibers are deposited on SPP-generating substrates. The substrates in the form of nanohole arrays are fabricated on a gold film with a pitch of 500 nm, a diameter of 250 nm, and a depth of 40 nm. On the SPP substrates, the nanofiber waveguides exhibit strong leakage of the guided light. The spectrum of the leaked light is consistent with the SPP resonance wavelength, and its polarization corresponds to the TE waveguided mode. We propose mechanisms of the observed phenomena that include either excitation of the SPPs via the waveguide evanescent field or direct enhancement of the leakage by the modified density of states near the plasmonic substrate.
Qiwen Tan, Nao Koishihara, Shun Omagari, Takehiro Hirao, Takeharu Haino, Martin Vacha, J. Phys. Chem. C 2024, 128, 10, 4295–4302.
●Selective encapsulation of carboxylic acid dimers within a size-regulable resorcinarene-based hemicarcerand
A cavity within a resorcinarene-based hemicarcerand was contracted and expanded through conformational changes induced by the complexation and decomplexation, allowing self-sorting of homo- and heterodimeric carboxylic acid pairs.
Kentaro Harada, Yudai Ono, Ryo Sekiya, and Takeharu Haino, Chem. Commun. 2024, 60, 6603(Front Cover)
●Kinetic Resolution of Secondary Alcohols Catalyzed at the Exterior of Chiral Coordinated Capsules
Confined spaces inside molecular hosts can function as reaction vessels. However, this concept significantly limits the scope of reactants. When the exterior of molecular hosts is used instead, we can ease the restriction because reactants are not necessary to be trapped inside molecular hosts, although studies along this line have not been reported. As a proof-of-concept of enantioselective reactions at the exterior of chiral molecular hosts, we utilized host‒guest complexes of enantiomerically enriched Cu-coordinated capsules and guests possessing a catalytic center to realize the kinetic resolution of secondary alcohols. Under suitable reaction conditions, a selectivity factor of 2.6 was realized, demonstrating that the reactions occur at the exterior of the capsules. A series of experiments indicated that the substituents on the 2,2’-bipyridyl arms and the alkyl chains on the lower rim contributed to the enantioselectivity of the reactions.
Kentaro Harada, Ryo Sekiya, Takeharu Haino, Chem. Eur. J., 2024, 30, e202304244.(Front Cover)
●Application of Exciton Coupling for Characterization of Nanographene Edge
The structural characterization of nonstoichiometric nanographenes (NG)-organic hybrid materials is usually difficult. The number of substituents on the edge and their arrangements are frequently questioned but are difficult to answer. Since the number of functional groups is closely related to the distance between the nearest neighbors (dISD), the extraction of dISD from spectroscopic data could provide important information on their structural characterization. We show that exciton coupling, which is a theoretical prediction of the absolute structures of discrete molecules, is a possible candidate to address this issue. The comparison of the calculated CD spectra of the chiral chromophores extracted from the model NG edge with the observed edge spectra indicated a dISD of ca. 8 Å; this corresponded to substitution on every other armchair edge. Furthermore, an up-up-down-down alternate orientation was found to be a possible edge structure. Although the procedure was limited to NGs carrying chiral substituents, our method could facilitate the detailed structural characterization of NG-organic hybrid materials.
Ryo Sekiya and Takeharu Haino, ChemPhysChem, 2024, 25, e202300740.(Front Cover)
●Synthesis and Cooperative Guest Binding of Tetrameric Porphyrin Macrocycle
A new family of macrocyclic host molecules that possess two bisporphyrin
clefts was synthesized. The tetrameric porphyrin macrocycle provides the
two guest binding sites in which two electron-deficient aromatic molecules
are encapsulated through donor–acceptor and π-π stacking interactions.
The tetrameric porphyrin macrocycle showed positive cooperativity, whereas
the nonmacrocyclic analog did not. The preorganization concept drives positive
cooperativity, which is directed by the entropy benefit to macrocyclization.
Kouta Tanabe, Naoyuki Hisano, and Takeharu Haino, ChemistrySelect, 2024, 9, e202305211.
●Intermediate Color Emission via Nanographenes with Organic Fluorophores
Photoluminescence (PL) color can be tuned by mixing fluorophores emitting the three primary colors in an appropriate ratio. When color tuning is achieved on a single substrate, we can simplify device structures. We demonstrated that nanographenes (NGs), which are graphene fragments with a size of tens of nanometers, could be utilized as carriers of fluorophores. The addition of red- and blue-light-emitting fluorophores on the edge successfully reproduced the purple light. The relative PL intensities of the fluorophores could be regulated by the excitation wavelength, enabling multicolor emission between blue and red light. Owing to the triphenylamine units of the fluorophores, the NGs showed PL enhancement due to aggregation. This characteristic was valuable for the fabrication of solid polymer materials. Specifically, the functionalized NGs can be dispersed into polyvinylidene difluoride. The resultant polymer films emitted red, blue, and purple color. Our study demonstrated the potential applicability of NGs for fluorophore carriers capable of reproducing intermediate colors of light.
Saki Arimura, Ikuya Matsumoto, Ryo Sekiya, and Takeharu Haino, Angew. Chem. Int. Ed., 2024, 63, e202315508.(Frontispiece)
●Supramolecular Polymerization Behavior of a Ditopic Self-folding Biscavitand
Reported herein is the supramolecular polymerization of a mixture of a feet-to-feet connected biscavitand and a homoditopic quinuclidinium guest that is regulated by cooperativity in the host–guest association. The diffusion ordered NMR spectroscopy (DOSY) was used to evaluate the supramolecular polymerization in toluene, CHCl3, and THF. Upon concentrating the solutions of the biscavitand with the quinuclidinium guest in CHCl3 and THF, the diffusion coefficient (D) values were meaningfully decreased, indicating that the host-guest complexation facilitated the supramolecular polymerization. In contrast, the slight change of the D value in toluene suggests that the supramolecular polymerization was suppressed, although the binding constant (K) between the cavitand and quinuclidinium guest was reported to be 105 L mol–1 in toluene. The viscosity measurements showed both the critical polymerization concentration (CPC) and entangled concentration (Ce) upon concentrating the CHCl3 solution of the mixture. Neither the CPC nor Ce was seen in the toluene solution of the mixture. Accordingly, the strong negative cooperativity in the 1:2 host-guest complexation of the biscavitand discouraged the supramolecular polymerization in toluene. These findings are valuable to deepen the understanding of the host–guest association-driven supramolecular polymerization behaviors regulated by a combination of cooperativity and K value in solution.
Haruna Fujimoto, Takehiro Hirao, and Takeharu Haino, Bull. Chem. Soc. Jpn, 2024, 97, uoad016.(Frontispiece)