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Differentiating Π Interactions by Constructing Concave/Convex Surfaces Using a Bucky Bowl Molecule, Corannulene in Liquid Chrom

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Differentiating Π Interactions by Constructing Concave/Convex Surfaces Using a Bucky Bowl Molecule, Corannulene in Liquid Chrom Article Cite This: Anal. Chem. 2019, 91, 2439−2446 pubs.acs.org/ac Differentiating π Interactions by Constructing Concave/Convex Surfaces Using a Bucky Bowl Molecule, Corannulene in Liquid Chromatography † † † ‡ § Eisuke Kanao, Takuya Kubo,*, Toyohiro Naito, Takatoshi Matsumoto, Tomoharu Sano, ∥ † Mingdi Yan, and Koji Otsuka † Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ‡ Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan § Center for Environmental Measurement and Analysis, National Institute for Environmental Studies, Onogawa 16-2, Tsukuba, Ibaraki 305-8506, Japan ∥ Department of Chemistry, University of Massachusetts Lowell, One University Avenue, Lowell, Massachusetts 01854, United States *S Supporting Information ABSTRACT: Convex−concave π conjugated surfaces in hemispherical bucky bowl such as corannulene (Crn) have shown increasing utility in constructing self-assembled new functional materials owing to its unique π electrons and strong dipole. Here, we investigate these specificmolecular recognitions on Crn by developing new silica-monolithic capillary columns modified with Crn and evaluating their performance in the separation of different aromatic com- pounds by liquid chromatography (LC). We synthesized two Crn derivatives and conjugated them onto the surface of a silica monolith. The first Crn derivative was edge functionalized, which can undergo free inversion of a convex−concave surface. The second Crn derivative was synthesized by modifying the spoke of Crn, which suppresses the convex−concave inversion. Results of LC suggest that each surface showed different shape recognition based on π interaction. Furthermore, the concave surface of Crn showed strong CH−π interaction with a planar molecule, coronene, demonstrated by the shifts of the 1H NMR signals of both Crn and coronene resulting from the multiple interactions between Crn and π electrons in coronene. These results clearly demonstrated the presence of CH−π interactions at multiple points, and the role of shape recognition. he π interaction is a type of noncovalent interaction with Lawton in 1966,9 has attracted much interest because of its T aromatic compounds and plays an important role in the many unique properties including hemispherical structure,10 11 12,13 Downloaded via UNIV OF MASSACHUSETTS LOWELL on May 6, 2019 at 13:40:48 (UTC). molecular recognition processes in biological systems and − large dipole moment, high electron acceptability, and See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. 1 5 organic functional materials. For example, Nakagawa et al. bowl to bowl inversion.14,15 The bucky bowls have been widely revealed an oncogenic promoter recognition mechanism used in the synthesis of new functional materials.16,17 For −π caused by the CH interaction between kinase C and example, Sygula et al. designed a clip as a fullerene host, in 6 π indole-V derivatives. Recently, many studies suggest that the which two bucky bowl units were connected through a rigid interaction is profoundly involved in photo and electronic benzo cyclooctatetraene linker.18 Mack et al. synthesized behaviors of organic functional materials. Okamoto et al. extended bucky bowl π-systems having potential applications developed organic transistors based on a laminating π−π as blue emitters in organic light emitting diodes.19 interaction, which exhibited 10 times higher electron mobility 7 Generally, it is challenging to study π interactions especially than conventional transistors. Wu et al. developed an organic π thin film capable of regulating visible singlet and near-infrared in the presence of other molecular interactions because triplet emissive properties by CH−π interaction-assisted self- interactions are much weaker than most molecular inter- assembly.8 As can be observed by these and many other actions, such as hydrophobic interaction, hydrogen bonding, reports, a deep understanding and the ability to control π and electrostatic bonding. Computational approaches to study interactions will greatly facilitate the development of new functional materials. Received: November 13, 2018 The first bowl-shaped π-conjugated molecule, corannulene, Accepted: December 24, 2018 known as a “bucky bowl” and first synthesized by Barth and Published: December 24, 2018 © 2018 American Chemical Society 2439 DOI: 10.1021/acs.analchem.8b05260 Anal. Chem. 2019, 91, 2439−2446 Analytical Chemistry Article Scheme 1. Preparation of the Crn-PFPA Column: (a) Synthesis of Crn-PFPA-CO2CH3 and Crn-PFPA; (b) Surface Modification of a Silica Monolith by Crn-PFPA molecular interactions using quantum mechanical models have azide) columns. The Crn-ester column was prepared from a seen much progress in recent years due to the significant Crn derivative that was edge functionalized with a −CH OH − 2 improvement in both algorithms and computing power.20 23 group, which was then conjugated to a carboxy-functionalized Also, nuclear magnetic resonance (NMR) spectroscopy has silica monolith. It was anticipated that both surfaces of the Crn been successfully applied to study strong molecular inter- structure could interact with solutes37 in the Crn-ester column. − actions such as H bonding.24 27 In this study, we suggest a The Crn-PFPA column was prepared from a Crn derivative further straightforward experimental method to measure the that was functionalized with PFPA to form an azridine on a 38 strength of π interactions and estimate the interaction spoke of Crn. In this case, the aziridine formation converts 2 mechanism, and then the computational study and spectro- two sp hybridized carbon atoms of corannulene into 3 scopic approaches will be more reliable. pyramidal sp centers, and it breaks the possibility of inversion − High performance liquid chromatography (HPLC) is a of convex concave surface. Using these two new columns, we π powerful separation technique that is able to distinguish the evaluated the strength of interactions between Crn and partition coefficients of solutes between the mobile and severalPAHsbynormalphaseliquidchromatography stationary phases and can sensitively reflect the strength of (NPLC), in which hydrophobic interaction was completely π 33 molecular interactions.28,29 In our previous studies, we reduced and thus interaction could be examined. To further understand the π−π interactions between Crn structure successfully immobilized C60-fullerene (C60) and C70-fullerene (C70) onto a silica-monolithic capillary and evaluated the on the stationary phase and Crn as a solute, computational − 1 characteristics of π interactions of fullerenes.30 34 In these simulations were carried out. In addition, H NMR spectros- copy was employed to examine the interaction between Crn studies, we succeeded in separating several polycyclic aromatic fi hydrocarbons (PAHs) by the effective π−π interactions. We and coronene in detail. This represents the rst report that evaluates the shape-based specific interactions on π-conjugated also showed that fullerenes exhibited specific π−π interaction hemispherical surface and multiple CH−π interactions in with corannulene resulting from the hemispherical recognition bucky bowls. and induced dipole of fullerenes. In this study, we developed new Crn-coated silica monoliths for the precise understanding of π interactions on the curved π- ■ EXPERIMENTAL SECTION conjugated surface using LC. Crn is known to have convex and Synthesis and Instruments. We synthesized Crn concave surfaces,35,36 which is expected to lead to different derivatives and prepared Crn-modified monolithic capillaries molecular recognition at each surface. Toward this end, we as shown in Scheme 1, and Supporting Information, Scheme developed two kinds of Crn-functionalized silica monoliths, S1. The detailed experimental procedures and results are namely Crn-ester column and Crn-PFPA (perfluorophenyl summarized in the Supporting Information. Our capillary 2440 DOI: 10.1021/acs.analchem.8b05260 Anal. Chem. 2019, 91, 2439−2446 Analytical Chemistry Article liquid chromatographic system consists of a DiNa S (Kya aromatic region (Supporting Information, Figure S7). Technologies Co., Tokyo, Japan) as the pump, CE-2070 Computation of the orbital energies of PFPA and Crn (Jasco, Tokyo, Japan) as the UV detector, Cheminert (Valco supports the reaction between the LUMO of the azide in Instruments Co., Huston, TX) as the sample injector, and PFPA and the HOMO (Supporting Information, Figures S10 Chemco capillary column conditioner model 380-b (Chemco and S11). Results of the stabilization energy computed from Co. Osaka, Japan) as the column oven. The HPLC system is a the Hartree−Fock (HF) method (Supporting Information, Prominence series (Shimadzu Co., Kyoto, Japan). FT-IR, Table S1) for structures A and C indicated that the most stable NMR, elemental analysis, and direct analysis in real-tyime mass structure is structure A, which is consistent with our spectroscopy (DARTMS) were carried out on a Nicolet iS5 experimental data. fi ATR (Thermo Fisher Scienti c Inc., Waltham, MA, USA), These Crn derivatives were then conjugated to a CO2H- or fi JNM-ECA500 spectrometer (JEOL, Tokyo, Japan), Flash NH2-functionalized silica monolith by esteri cation or EA1112 (Thermo Fisher Scientific), and DART (JEOL), amidation to give the Crn-ester column and Crn-PFPA respectively. column, respectively (Scheme 1, and Supporting Information, Surface Modification of
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