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

Article abstract of DOI:10.1021/acs.analchem.8b05260

Convex-concave p 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 p electrons and strong dipole. Here, we investigate these specific molecular recognitions on Crn by developing new silica-monolithic capillary columns modified with Crn and evaluating their performance in the separation of different aromatic compounds 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 p interaction. Furthermore, the concave surface of Crn showed strong CH-p interaction with a planar molecule, coronene, demonstrated by the shifts of the¹H NMR signals of both Crn and coronene resulting from the multiple interactions between Crn and p electrons in coronene. These results clearly demonstrated the presence of CH-p interactions at multiple points, and the role of shape recognition.

Full text of DOI:10.1021/acs.analchem.8b05260

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Article
Differentiating # 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
Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b05260 • Publication Date (Web): 24 Dec 2018
Downloaded from http://pubs.acs.org on December 27, 2018
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Differentiating π interactions by constructing
concave/convex surfaces using a bucky bowl molecule,
corannulene in liquid chromatography
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†
*,†
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‡
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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 Ave., Lowell, MA
01854, USA
Corresponding author
Takuya Kubo
Tel: +81-75-383-2448
Fax: +81-75-383-2450
E-mail: kubo@anchem.mc.kyoto-u.ac.jp
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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 specific molecular recognitions on
Crn by developing new silica-monolithic capillary columns modified with Crn, and evaluating their
performance in the separation of different aromatic compounds by liquid chromatography (LC). We
synthesized two Crn derivatives and conjugated them onto the surface of a silica monolith. The first
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Crn derivative was edge functionalized, which can undergo free inversion of 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
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interaction with a planar molecule, coronene, demonstrated by the shifts of the H-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.
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The π interaction is a type of non-covalent interaction with aromatic compounds, and plays an
important role in the molecular recognition processes in biological systems and organic functional
materials.1–5 For example, Nakagawa et al. revealed an oncogenic promoter recognition mechanism
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caused by the CH-π interaction between kinase C and indole-V derivatives. Recently, many studies
suggest that the π interaction is profoundly involved in photo and electronic behaviors of organic
functional materials. Okamoto et al. developed organic transistors based on a laminating π-π
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interaction, which exhibited ten times higher electron mobility than conventional transistors. Wu et
al. developed an organic thin film capable of regulating visible singlet and near infrared triplet
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emissive properties by CH-π interaction-assisted self-assembly. As can be observed by these and
many other reports, a deep understanding and the ability to control π interactions will greatly facilitate
the development of new functional materials.
The first bowl-shaped π-conjugated molecule, corannulene, known as a “bucky bowl” and first
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synthesized by Barth and Lawton in 1966, has attracted much interest because of its many unique
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properties including hemispherical structure, large dipole moment, high electron acceptability,
and bowl to bowl inversion.14,15 The bucky bowls have been widely used in the synthesis of new
functional materials.16,17 For example, Sygula et al. designed a clip as a fullerene host, in which two
bucky bowl units were connected through a rigid benzo cyclooctatetraene linker.18 Mack et al.
synthesized extended bucky bowl π-systems having potential applications as blue emitters in Organic
Light Emitting Diode.19
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Generally, it is challenging to study π interactions especially in the presence of other molecular
interactions because π interactions are much weaker than most molecular interactions, such as
hydrophobic interaction, hydrogen bonding, and electrostatic bonding. Computational approaches to
study molecular interactions using quantum mechanical models have seen much progress in recent
years due to the significant improvement in both algorithms and computing power.20–23 Also, nuclear
magnetic resonance (NMR) spectroscopy has been successfully applied to study strong molecular
interactions such as H bonding.24–27 In this study, we suggest a further straightforward experimental
method to measure the strength of π interactions and estimate the interaction mechanism, and then
the computational study and spectroscopic approaches will be more reliable.
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High performance liquid chromatography (HPLC) is a powerful separation technique that is able
to distinguish the partition coefficients of solutes between the mobile and stationary phases and can
sensitively reflect the strength of molecular interactions. 28,29 In our previous studies, we successfully
immobilized C -fullerene (C60) and C -fullerene (C70) onto a silica-monolithic capillary and
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evaluated the characteristics of π interactions of fullerenes.30–34 In these studies, we succeeded in
separating several polycyclic aromatic hydrocarbons (PAHs) by the effective π–π interactions. We
also showed that fullerenes exhibited specific π–π interaction with corannulene resulting from the
hemispherical recognition 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-conjugated surface using LC. Crn is known to have convex and concave
surfaces,35,36 which is expected to lead to different molecular recognition at each surface. Towards
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this end, we developed two kinds of Crn-functionalized silica monoliths, namely, Crn-ester column
and Crn-PFPA (perfluorophenyl azide) column. The Crn-ester column was prepared from a Crn
derivative that was edge functionalized with a -CH OH group, which was then conjugated to a
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carboxy-functionalized silica monolith. It was anticipated that both surfaces of the Crn structure could
interact with solutes 37 in the Crn-ester column. The Crn-PFPA column was prepared from a Crn
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derivative that was functionalized with PFPA to form an azridine on a spoke of Crn. In this case,
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the aziridine formation converts two sp hybridized carbon atoms of corannulene into pyramidal sp
centers, it breaks the possibility of inversion of convex–concave surface. Using these two new
columns, we evaluated the strength of π interactions between Crn and several PAHs by normal phase
liquid chromatography (NPLC), in which hydrophobic interaction was completely reduced and thus
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π interaction could be examined . To further understand the π-π interactions between Crn structure
on the stationary phase and Crn as a solute, computational simulations were carried out. In addition,
1H-NMR spectroscopy was employed to examine the interaction between Crn and coronene in details.
This represents the first report that evaluates the shape-based specific interactions on π-conjugated
hemispherical surface and multiple CH-π interactions in bucky bowls.
EXPERIMENTAL SECTION
Synthesis and instruments
We synthesized Crn derivatives and prepared Crn-modified monolithic capillaries as shown in
Schemes 1 and Scheme S1. The detailed experimental procedures and results are summarized in
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Supporting Information. Our capillary liquid chromatographic system consists of a DiNa S (KYA
Technologies Co., Tokyo, Japan) as the pump, CE-2070 (JASCO, Tokyo, Japan) as the UV detector,
CHEMINERT (Valco Instruments Co., Huston, TX) as the sample injector, and Chemco capillary
column conditioner Model 380-b (Chemco Co. Osaka, Japan) as the column oven. The HPLC system
is a Prominence series (Shimadzu Co., Kyoto, Japan). FT-IR, NMR, elemental analysis, and direct
analysis in real time mass spectroscopy (DARTMS) were carried out on a Nicolet iS5 ATR (Thermo
Fisher Scientific Inc., Waltham, MA, USA), JNM-ECA500 spectrometer (JEOL, Tokyo, Japan),
Flash EA1112 (Thermo Fisher Scientific), and DART (JEOL), respectively.
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Surface modification of the silica monolith with Crn-PFPA-NHS
The silica-monolithic capillary was treated with 1.0 M aqueous HCl at 40 C for 3 h, and washed
with water and methanol. It was then filled with APTMS in methanol (10%, v/v), remained at room
temperature for 24 h and then washed with methanol to give NH -modified silica monolith. A solution
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of Crn-PFPA-NHS in toluene (1.0 mg mL ) was charged into the NH -modified column. After
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remaining at room temperature for 24 h, the column was washed with toluene and methanol to give
the Crn-PFPA column (see Scheme 2-(b)). A control was prepared by charging the NH -modified
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silica monolith with PFPA-NHS in toluene (1.0 mg mL ) at room temperature for 24 h, and washing
with toluene and methanol to give the PFPA column. Additionally, C60 column and C70 column
were prepared following the protocols in our previous reports. The detailed preparation procedures
are described in Scheme S1.
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Scheme 1. Preparation of the Crn-PFPA column (a) synthesis of Crn-PFPA-CO CH and Crn-
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PFPA; (b) surface modification of a silica monolith by Crn-PFPA.
RESULTS AND DISCUSSION
Preparation of Crn-modified silica monoliths
Two types of Crn derivatives were used to construct the columns: one was edge functionalized
with hydroxymethyl (Crn-CH OH) and the other was functionalized PFPA-NHS (Crn-PFPA-NHS)
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Scheme 1). Crn-CH OH was synthesized by first reacting Crn with an excess amount of
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dichloromethyl methyl ether in the presence of TiCl at room temperature for 24 hours to give the
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aldehyde derivative Crn-CHO, which was then reduced with NaBH to give Crn-CH OH in an overall
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51% yield .
PFPA-derivatized Crn, Crn-PFPA-NHS, was synthesized by heating Crn with PFPA-NHS in
chlorobenzene at 108 C for 5 days. PFPA undergoes cycloaddition with double bonds to form
aziridine structures. Perfluorination of phenyl azide lowers the LUMO of the azide, facilitating its
reaction with dipolarophiles and electrophiles 39. PFPAs have also demonstrated good reactivity
towards carbon materials such as fullerenes30,40, carbon nanotubes 41 and graphene 42–44 which are
otherwise quite inert chemically. There are four types of double bonds in Crn, and therefore four
different products are possible from the reaction with PFPA (Figure. S9). To facilitate structure
determination of the product, a PFPA methyl ester, PFPA-CO CH , was used as a model compound
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for its simpler structure than PFPA-NHS. The aliphatic region of the H NMR spectrum of the product
only showed the methyl peak from PFPA-CO CH , thus ruling out the arizidine products from the
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addition to the flank and rim of Crn (B and D, Figure. S9) because these products include aliphatic
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CH moiety. The H-NMR spectrum of the product showed the peak patterns in the aromatic region
(Figure S7). Computation of the orbital energies of PFPA and Crn supports the reaction between the
LUMO of the azide in PFPA and the HOMO (Figures S10, S11). Results of the stabilization energy
computed from the Hartree–Fock (HF) method (Table S1) for structures A and C indicated that the
most stable structure is structure A, which is consistent with our experimental data.
These Crn derivatives were then conjugated to CO H- or NH -functionalized silica monolith by
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esterification or amidation to give the Crn-ester column and Crn-PFPA column, respectively (Scheme
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1 and S1). Columns that were modified with only the linkers, Ester column and PFPA column, were
also prepared and used as controls. Figure. 1 (a) shows the surface structure of the stationary phase
in Crn-ester column, Crn-PFPA column, ester column and PFPA column.
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Figure. 1 (b) plots the retention factor k (k = (t –t / t ; t , retention time of a solute; t , elution
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time of non-retained solute (acetone)) of different alkylbenzenes (ABs) and PAH in these columns
vs. the water/octanol partition coefficient (Log PO/W), which indicates the hydrophobicity of the
molecule. When the columns were modified with Crn (i.e., Crn-ester and Crn-PFPA columns), they
showed stronger retention of ABs than other columns. Log k increased linearly with Log PO/W
.
Additionally, Crn-ester column and Crn-PFPA column showed stronger retention for hydrophobic
PAHs, while this was not observed on the control columns (Ester and PFPA columns) that were not
modified with Crn. The results confirmed the existence of the hydrophobic interaction and π
interaction derived from the Crn structure. Furthermore, these results clearly demonstrated the
successful immobilization of Crn on Crn-ester and Crn-PFPA columns.
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Figure. 1. (a) Surface structure of the stationary phase in Crn-ester column, Crn-PFPA column
and the controls, Ester column and PFPA column. (b) Log k vs. log Po/w in each column. Condition:
Crn-ester column (32.0 cm  100 μm i.d.), Crn-PFPA column (32.0 cm  100 μm i.d.), Ester column
–1
(
32.0 cm  100 μm i.d.), PFPA column (32.0 cm  100 μm i.d.); flow rate, 2.0 μL min ; mobile
phase, water / methanol = 1 / 9; detection, UV 254 nm; temperature 40 ºC.
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Retention selectivity for hemispherical structure
In our previous reports, C60 and C70 showed higher recognition for Crn than typical planar PAH
structures owing to the specific π-π interaction between the spherical π-conjugated surfaces 31,32. We
expect that Crn-modified silica monoliths should show similarly spherical recognition owing to the
hemispherical structure of Crn. To test the hypothesis, we included benzo[a]pyrene, which has the
same number of π electrons as Crn but has a planar structure rather than the hemispherical structure
in Crn, and evaluated the elution behavior of Crn and benzo[a]pyrene using Crn modified silica
monoliths. For a comparison, C60 and C70 columns, and commercially available LC columns that
are known for their effective π interaction (PYE and πNAP (Nacalai Tesque, Kyoto, Japan)) were
tested under the same conditions.
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Chromatograms of the mixed sample of Crn and benzo[a]pyrene on each column are shown in
Figure. 2. On the commercially LC columns PYE and πNAP, there were no differences in the strength
of π interactions between Crn and benzo[a]pyrene, likely due to the small-planar π-conjugated
structure that are used to modify the columns (Figure. 2 (a)). On the other hand, fullerene-modified
silica monoliths showed stronger retention for Crn than that of benzo[a]pyrene, which can be
attributed to spherical recognition (Figure. 2 (b)). Surprisingly, Crn-modified silica monoliths showed
lower retentions for Crn than benzo[a]pyrene (Figure. 2 (c)). This result implies a significantly lower
interaction between Crn structures themselves than their interaction with fullerene. In our earlier
study, we confirmed that the effective retention of Crn on C70 column was caused by the dipole of
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Crn inducing dipoles in C70. Hence, we hypothesize that the repulsion between the hemispherical
structures of Crn is caused by the dipole moment.
A computational study was carried out in order to understand the charge state of each carbon atom
on Crn and Crn derivatives, The detailed calculation conditions are described in Supporting
Information. Figure. 3 shows the charge on each carbon atom and the charge distribution in Crn and
Crn derivatives. The carbon atoms in Crn exhibited alternating negative and positive charges (Figure.
S12 (b) and (c)). Crn derivatives exhibited similar trend, with the exception of those carbon atoms
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that were functionalized (C3, and to a lesser extent C2 and C4 in Crn-CH OH; C1 and C2, and to a
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lesser extent C3 and C4 in Crn-PFPA, Figure. S12 (b)). As such, if Crn in the mobile phase overlaps
with the Crn structure on the stationary phase, electrostatic repulsions would occur between their
dipole moments. In this case, Crn-modified silica monoliths would not be able to interact with Crn in
the mobile phase due to the electrostatic repulsion resulting from the same arrangements of atomic
charges in the Crn structures. Even in Crn-ester column, in which both concave and convex surfaces
of the Crn structure could interact with Crn, the repulsion effect was observed. Similar result was
reported by Wang et al. that neighboring Crn structures were not completely stacked in the crystal
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structure, which is consistent with our observation .
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Figure. 2. Chromatograms of the mixed sample of Crn and benzo[a]pyrene on (a) πNAP (Nacalai
Tesque, 50 mm × 2.0 mm i.d.), 5PYE (Nacalai Tesque, 150 mm × 4.6 mm i.d.), (b) C60 column (32.0
cm  100 μm i.d.), C70 column (32.0 cm  100 μm i.d.), (c) Crn-ester column (32.0 cm  100 μm
–
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i.d.), Crn-PFPA column (32.0 cm  100 μm i.d.). Condition: flow rate, (a) 2.0 mL min , (b), (c) 2.0
–1
µLmin ; mobile phase, chloroform / n-hexane = 3 / 7; detection, UV 254 nm; temperature 40 ºC.
Based on the above results, we hypothesize that intramolecular dipoles in aromatic compounds
could decrease the strength of π-π interactions with other polar aromatic compounds resulting from
charge repulsion. To test this, we evaluated separation behaviors between the non-polar naphthalene
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and the polar azulene , both of which are of planar structures and have the identical number of π
electrons. As shown in Figure S13, We found that Crn-modified silica monoliths showed weaker
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retention and lower selectivity to azulene than the fullerene-modified columns. On the other hand,
Crn-modified silica monoliths exhibited stronger retentions than fullerene-modified columns for
naphthalene due to intramolecular dipoles in Crn structures and induced dipole in naphthalene. Thus,
we suggest that polar aromatic compounds showed lower affinity to other polar aromatic compounds
than nonpolar compounds regardless of plane or curved structures.
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Retention behaviors of PAHs with different π electron numbers
In the last section, we clarified the interaction of Crn with polar aromatic compounds and
elucidated the retention behavior based on the nature of charge interactions. In this section, we
evaluate the retention selectivity of non-polar and planar PAHs in Crn-modified silica monoliths. The
typical chromatograms obtained by normal phase mode of PAHs, including phenanthrene, pyrene,
chrysene, benzo[a]pyrene, and coronene are shown in Figure. 3 (a) and (b). Both Crn-ester and Crn-
PFPA columns strongly retained PAHs. The retention increased with the number of π electrons, and
showed high separation resolution owing to the strong π-π interaction. To demonstrate the retention
selectivity in the Crn-modified silica monoliths, the retention factor for each PAH is plotted against
the number of π electrons. As shown in Figure. 3 (c), PAHs were retained in Crn columns slightly
better than other columns, which can be attributed to the dipole of Crn. Interestingly, in both Crn-
ester and Crn-PFPA columns, coronene was significantly retained compared to other planar PAHs,
whereas a linear relation was observed for all the PAHs in the C60, C70 (Scheme S2), PYE and πNAP
columns. Especially, the retention of coronene was much higher in Crn-PFPA column than in Crn-
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ester column. In Crn-PFPA, since PFPA derivatization occurs on the spoke of Crn (Scheme 1), only
the concave surface of Crn structure in Crn-PFPA column could interact with coronene because of
the large steric hindrance of nitrogen atom on the concave surface. In Crn-ester column, however,
because the derivatization occurs at the edge of the structure, both concave and convex surfaces of
the Crn structure could interact with coronene.
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Figure. 3. Retention behaviors of nonpolar and planar PAHs in Crn-modified silica monoliths.
Chromatograms of the mixed sample of PAHs on (a) Crn-ester column, (b) Crn-PFPA column. (c)
Plots of retention factor on each column vs. the number of π electrons in PAH. Condition: column,
C60 column (32.0 cm  100 μm i.d.), C70 column (32.0 cm  100 μm i.d.), Crn-ester column (32.0
cm  100 μm i.d.), Crn-PFPA column (32.0 cm  100 μm i.d.), πNAP (Nacalai Tesque, 50 mm × 2.0
mm i.d.), 5PYE (Nacalai Tesque, 150 mm × 4.6 mm i.d.); mobile phase, n-hexane; detection, UV 300
nm; temperature 40 ºC.
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To elucidate the additional intramolecular interactions between coronene and Crn structures, the
H NMR spectra of Crn-CH OH or Crn-PFPA-CH OCH in the presence of coronene were recorded
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and compared to those of coronene or the Crn derivatives. No significant peak shifts were observed
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in either coronene or Crn-CH OH when the two molecules were mixed (Figure. S14 (a)). However,
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when coronene was mixed with Crn-PFPA-CH OCH , the aromatic peaks in Crn-PFPA-CH OCH
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shifted upfield and the width of several peaks were also broadened, while no shift was observed for
the methyl protons.
In general, π interactions can be interfered by polar molecules such as halogenated compounds,
thus the interaction between Crn and coronene may not be fairly evaluated in chloroform. In fact, the
retentions due to π interactions were dramatically decreased by adding chloroform to the mobile phase
in LC 34. Therefore, to better evaluate the chemical shifts in NMR, we added n-hexane-d14 to
chloroform-d because π interactions appear more strongly in n-hexane, due to its lower dielectric
constant, than in chloroform. As expected, significant shifts of the aromatic protons in both coronene
and Crn-PFPA-CH OCH were observed in n-hexane-d /chloroform-d. As shown in Figure. 4, the
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aromatic peak in coronene shifted downfield after mixing with Crn-PFPA-CH OCH , whereas all
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Crn aromatic protons in Crn-PFPA-OCH shifted upfield. In addition, these peaks were bifurcated
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and broadened. These drastic changes suggest the presence of extremely strong intermolecular
interactions. Thus, the protons on the Crn structure were placed in a more electron rich environment
(more shielded), whereas the protons on coronene were placed in a more electron poor environment
(less shielded). In other words, as the Crn structure approached the π conjugated surface of coronene,
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it attracted the π electrons in coronene making its aromatic protons more shielded .
This is likely mediated by the strong electron-withdrawing PFPA on Crn-PFPA-CH OCH , which
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pulls the electron density away from Crn, and subsequently coronene. Briefly, strong CH-π
interaction at multiple points working between the hydrogen atom of Crn and the aromatic ring of
coronene was confirmed. Matsuno et al. recently reported multipoint and strong π interaction between
(P)-(12,8)-[4]cyclo-2,8-chrysenylene, which is a cylindrical molecules made of linked chrysen
structure in a hoop-like structure, and Crn. Our results are also strongly supported from this report.48
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Figure. 4. H NMR spectral comparisons of coronene, Crn-PFPA-CH OCH or their mixture in
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hexane-d /chloroform-d = 1/1.
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Figure. 5 shows a schematic diagram illustrating the π interaction of coronene with Crn in Crn-
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ester or Crn-PFAP column. On the Crn-ester column, Crn was edge-functionalized and the linker is
away from the Crn surface. As such, both concave and the convex surfaces of Crn can interact with
coronene. On the Crn-PFPA column, because the modification by PFPA occurs on the spoke of Crn,
the strong steric hindrance puts the PFPA group on the convex surface of the Crn structure. This
leaves mainly the concave surface of Crn to interact with coronene. The fact that the Crn-PFPA
column had significantly higher retention for coronene than the Crn-ester column demonstrate that
the CH-π interaction on this concave surface is much stronger than the interaction from both surfaces
in Crn-ester.
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Figure. 5. Schematic diagram of π interaction between coronene and Crn in Crn-ester or Crn-
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Shape recognition
We show in last section that both convex and concave surfaces of the Crn structure contributed
to the molecular recognition in the Crn-ester column, while only the concave surface of the Crn
structure contributed in the Crn-PFPA column. To investigate whether the shape recognition plays a
role, we evaluated the retention selectivity of naphthacene and triphenylene, which are planar
aromatic compounds that have the same number of π electrons and the only difference is the molecular
shape. The chromatograms obtained by normal phase mode of naphthacene and triphenylene are
shown in Figure. 6. Interestingly, the elution order was reversed; triphenylene eluted faster than
naphthacene on the Crn-PFPA column, while the opposed result was obtained on the Crn-ester
column.
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To investigate the difference in shape recognition, we consider the difference in the polarizability
of these PAHs. In the case of nonpolar molecules in π interaction, the strength of π interaction is
considered to be due to induced dipole–dipole interaction or induced dipole–induced dipole
interaction 49,50. The potential energy of induced dipole–induced dipole interaction is given as
follows:
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휇 훼
퐺induced dipole ― dipole = ―
1 (4휋휀 휀 ) 푟
(1)
0 푟
The potential energy of induced dipole–dipole interaction is given as follows:
A훼1훼2
2 6
퐺induced dipole–induced dipole = ―
(4휋휀 휀 ) 푟
(2)
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where, A is a constant depending on the ionization energy, µ is the dipole moment of polar
molecules, α are polarizabilities of the molecules, ε is the permittivity of vacuum, ε is the
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permittivity of the solvent, and r is the distance between the molecules, respectively 51,52. In this case,
the strength of π interaction increases as the polarizability of the solute increases.
Then, we consider that the difference in shape recognition was caused by the polarizability of these
PAHs. The polarizability of PAHs was summarized in Table S2, and the polarizability is plotted
against the number of π electrons (Figure. S15). As shown in Figure. S15, the polarizability of each
solute increases roughly with the number of π electrons. This trend is consistent with increasing in
retention and the stronger π interaction as the number of π electrons increases. For those PAHs with
identical number of π electrons, small differences in the polarizability were observed; e.g.
naphthacene, benzo[a]anthracene, chrysene, and triphenylene. As further examination regarding the
shape recognition (Figure S16), the convex surface of Crn contributes to retention in the Crn-ester
column in contrast to the Crn-PFPA column that the concave surface contributed to the retention. In
summary, the concave surface of Crn in Crn-PFPA column dominated the interactions with the
solutes.
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Figure. 6. Chromatograms of the mixed sample of naphthacene and triphenylene on (a) Crn-ester
column, (b) Crn-PFPA column. Condition: column, Crn-ester column (32.0 cm  100 μm i.d.), Crn-
PFPA column (32.0 cm  100 μm i.d.); mobile phase, n-hexane; detection, UV 280 nm; temperature
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CONCLUSIONS
In this report, we revealed the molecular recognition of Crn by evaluating the retention of Crn as well
as a number of aromatic compounds on Crn-modified silica monoliths in LC. We synthesized two
kinds of Crn derivatives, Crn-ester by introducing the functional group on the edge of Crn, and Crn-
PFPA by modifying the spoke structure of corannulene, and successfully prepared the Crn-modified
columns Crn-ester and Crn-PFPA. Both columns showed low retention for Crn, despite its
hemispherical structure. Computer simulation of Crn and Crn derivatives suggested electrostatic
repulsion resulting from the same arrangements of charge and charge distributions in the Crn
structures. On the other hand, both columns exhibited strong interactions with the planar molecule,
coronene, especially Crn-PFPA which showed significantly strong interactions. The evaluation of
1H-NMR shifts suggested that the specific retention caused by CH-π interaction at multiple points
between the hydrogen atoms of the concave surface of Crn structure and the planar π conjugated
surface of coronene. Furthermore, we demonstrated that the molecular recognition in Crn-ester was
due to π–π interaction on the convex surface of Crn. We believe that this report greatly advanced our
understanding on the π interactions, which should aid the development of novel functional materials.
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ASSOCIATED CONTENT
Supporting Information
The detailed experimental procedures, synthesis of 4-azido-2,3,5,6-tetrafluorophenyl succinate
(PFPA-NHS), preparation of a silica-monolithic capillary, computational methods, structure of PAHs,
synthesis of Crn-CH OH, synthesis of Crn-PFPA-CO CH , calculations for the reaction between
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PFPA and Crn, and preparation of C60/C70 columns. This material is available free of charge via the
Internet at http://pubs.acs.org.
ACKNOWLEDGMENTS
This research was partly supported by the Grant-in Aid for Scientific Research (No. 25620111 and
15K13756) from the Japan Society for the Promotion of Science, an Environment Research and
Technology Development Fund (5-1552) from the Ministry of the Environment, Japan. M.Y. thanks
the financial support from the National Science Foundation (CHE-1112436, CHE-1808671).
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TOC
Semi-click Reaction
Riech Reaction
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