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State- and water repellency-controllable molecularglass of pillar[5]arenes with ﬂuoroalkyl groups byguest vapors†
Katsuto Onishi, a Shunsuke Ohtani, a Kenichi Kato, a Shixin Fa, a
Yoko Sakata, bc Shigehisa Akine, bc Moe Ogasawara,c Hitoshi Asakawa,
Shusaku Nagano, e Yoshinori Takashima, fgh Motohiro Mizuno cd
and Tomoki Ogoshi *ab

bcd

Molecular glasses are low-molecular-weight organic compounds that are stable in the amorphous state atroom temperature. Herein, we report a state- and water repellency-controllable molecular glass by nalkane 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
Received 9th February 2022
Accepted 3rd March 2022

guest vapors, respectively. Furthermore, the n-alkane guest vapor-induced reversible changes in thewater contact angle were also observed: water contact angles increased and then reverted back to the

DOI: 10.1039/d2sc00828a

original state by the uptake and release of the n-alkane guest vapors, respectively, along with thechanges in the chemical structure and roughness on the surface of the molecular glass. The water

rsc.li/chemical-science

repellency of the molecular glass could be controlled by tuning the uptake ratio of the n-alkane guest vapor.

Introduction
Low molecular weight organic compounds are generally easy-tocrystalize compared with high-molecular-weight organic
compounds. Design of low-molecular-weight organic glasses,aDepartment of Synthetic Chemistry and Biological Chemistry, Graduate School ofEngineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan. E-mail:ogoshi@sbchem.kyoto-u.ac.jpb

WPI Nano Life Science Institute (WPI-NanoLSI), Kanazawa University, Kakumamachi, Kanazawa, Ishikawa, 920-1192, Japanc

Graduate School of Natural Science and Technology, Kanazawa University, Kakumamachi, Kanazawa, 920-1192, Japand

Nanomaterials Research Institute, Kanazawa University, Kakuma-machi, Kanazawa,
920-1192, Japane

Department of Chemistry, College of Science, Rikkyo University, 3-34-1 NishiIkebukuro, Toshima, Tokyo, 171-8501, Japanf

Department of Macromolecular Science, Graduate School of Science and Project
Research Center for Fundamental Sciences, Graduate School of Science, Osaka
University, Toyonaka, Osaka 560-0043, Japang
Institute for Advanced Co-Creation Studies, Osaka University, Suita, Osaka 565-0871,
JapanhInnovative Catalysis Science Division, Institute for Open and Transdisciplinary
Research Initiatives (OTRI), Osaka University, Suita, Osaka 565-0871, Japan† Electronic supplementary information (ESI) available: Experimental procedures,
NMR, UV-vis spectra, single-crystal X-ray structural analysis, PXRD, gas
adsorption, DSC, TGA, contact angle and movie. CCDC 2121263–2121267 and2144343. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/d2sc00828a

4082 | Chem. Sci., 2022, 13, 4082–4087

which form stable amorphous states even at room temperature,
is an interesting research area as they are superior to highmolecular-weight organic glasses in terms of transparency,
homogeneity and lm-forming properties.1,2 Owing to theseproperties, organic glasses have been used as organic lightemitting diodes and resist materials.3–8 One of the molecular
designs to create organic molecular glasses is introduction of
non-planar structures. For example, molecules with twisted
small p-units such as triphenylamine and tetraphenylmethane
derivatives can be organic molecular glasses, as these skeletons
are non-planar structures with diverse molecular conformations, which prevent the ordered periodic crystalline assemblies.5,9 Introducing bulky groups and/or intermolecularinteractions is another design strategy to obtain molecularglasses.10 Furthermore, design of stimuli-responsive functional
molecular glasses, which can easily exchange amorphous–
crystalline states by external stimuli and exhibit new functions
resulting from the stimuli-responsive transitions, is the nextchallenge. Until now, only photo-responsive molecular glasswith an azobenzene group showing an amorphous–crystallinetransition has been reported.11
Pillar[n]arenes, which were reported by our group in 2008,12
are highly symmetrical pillar-shaped macrocyclic host molecules. Aer their discovery, pillar[n]arenes became key players
in supramolecular chemistry due to their superior functionalities, host–guest properties and unique pillar-shaped structures.13–18 In most cases, pillar[n]arenes form crystalline solids© 2022 The Author(s). Published by the Royal Society of ChemistryView Article Online

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at room temperature due to their highly symmetrical pillarshaped structures and crystalline solids form host–guest
complexes by exposure to n-alkane guest vapors.19–26 Moreover,
an important feature of pillar[5]arenes is their high functionalities;27 they have 10 reaction sites, meaning that various
functional groups can be installed. The functional groups on
both the rims signicantly aﬀect the physical properties of pillar[5]arenes.28,29 In this study, we discovered that a pillar[5]arene
with C2F5 uoroalkyl groups on both rims could be a molecularglass. As in crystalline pillar[n]arenes, the transparent amorphous molecular glass took up n-alkane guest vapors by exposure to the vapors. Interestingly, the uptake caused an
appearance change from transparent to turbid lms due to theformation of the crystalline structure. The turbid lm reverted
back to the transparent lm again by removing the guest
through heating. The amorphous to crystalline and crystalline
to amorphous state transitions were completely reversible by
the uptake and release of the guest vapors, respectively. The
guest vapor-responsive transitions were not observed in pillar[5]arenes with short and long uoroalkyl groups, thus the transitions can be achieved by installing the C2F5 uoroalkyl groups.
Furthermore, based on the guest vapor-responsive transitions,switching and tuning of the water repellency could bedemonstrated.

Results and discussion
Syntheses and phase changes of pillar[5]arenes with
uoroalkyl groups
We synthesized pillar[5]arene derivatives with various lengthsof uoroalkyl groups and a compound with no uorine atoms(F5, F3, F13 and C5 in Fig. 1a). Pillar[6]arene with C2F5 groups([6]F5) and a Monomer Unit with no macrocyclic structure werealso synthesized.30 All new compounds were characterized by

Chemical Science1

H, 13C and 19F NMR spectroscopy and mass spectrometry(details in the ESI†).
F5 exhibits three states as shown in Fig. 1b. Initially, whitesolid F5 was obtained at 25  C aer evaporation of dichloromethane (DCM) containing F5. The solid contained no solvents,
which was conrmed by 1H NMR (Fig. S16†). Powder X-ray
diﬀraction (PXRD) measurement (Fig. 1c) of the solid showed
many sharp peaks, which suggested that the solid F5 existed in
the crystalline state (denoted as c-F5). From diﬀerential scanning calorimetry (DSC), the melting point of c-F5 was 116  C(Fig. S17†), thus c-F5 became a viscous liquid (denoted as l-F5,
Fig. 1b) when heated to 160  C. Unexpectedly, by cooling, l-F5
did not go back to the pristine white crystalline solid c-F5, but
a new state, transparent lm. In PXRD measurement (Fig. 1d),the transparent lm did not show sharp peaks, indicating that it
was an amorphous solid, i.e., an organic molecular glass(denoted as a-F5). The phase transition was completely reversible. a-F5 changed to l-F5 on heating at 160  C and then reverted
back to a-F5 aer cooling to 25  C. a-F5 could also change to cF5 by dissolving a-F5 in DCM, and then completely removing
the solvent under reduced pressure at 60  C. In contrast to F5,these three states were not observed in F3 with short CF3groups, C5 with no uorine atoms, F13 with long C6F13 groups
and the Monomer Unit with no macrocyclic structure by heating
and then cooling treatments (details in Fig. S18 and S19†).31 To
investigate the eﬀect of the C2F5 groups, we also prepared pillar[6]arene with C2F5 groups ([6]F5). Like F5, [6]F5 becamea molecular glass (details in Fig. S20†). Therefore, the installation of the C2F5 groups into pillar[n]arene enabled the reversible
changes between crystalline and amorphous phases, which arerare in macrocyclic compounds.31,32 To understand the eﬀect of
the C2F5 groups on formation of molecular glasses, DFT
calculations were performed (Fig. S30†). From molecular electrostatic potential maps, F5 had little diﬀerences in the electron
density distribution between both the rims and the cavity. On
the other hand, electron-poor rims and electron-rich cavity wereobserved in typical pillar[5]arenes with methoxy groups. These
results suggested that F5 was unlikely to form the orderedstructure through intermolecular C–H/p and p–p interactions
as observed in typical crystalline pillar[5]arenes. Thus, even in
the highly symmetrical pillar-shaped structure of pillar[n]arenes, the organic molecular glasses can be produced by installing
the C2F5 uoroalkyl groups.

Guest vapor-responsive state transitions(a) Chemical structures of F5, [6]F5, F3, C5, F13 and the
Monomer Unit. (b) Photographs of c-F5, l-F5 and a-F5. PXRD patternsof (c) c-F5 and (d) a-F5.

Fig. 1© 2022 The Author(s). Published by the Royal Society of Chemistry

It is known that crystalline pillar[5]arenes take up n-alkane
guest vapors in their cavities.15,18 Therefore, we exposed a-F5 ton-hexane vapors. a-F5 took up n-hexane vapors in 1 : 1 host : guest ratio, which was checked by 1H NMR (Fig. S21†). Interestingly, a-F5 changed from a transparent lm to a turbid lmby exposure to n-hexane vapors (Fig. 2a). The appearancechange occurred within only several minutes; thus, we could
visually follow the events (Movie S1†). More interestingly, thechange was completely reversible: c-(F5IH) returned toa transparent lm a-F5 by heating at 160  C and then cooling to
25  C, and the resulting transparent lm changed to a turbid
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Chemical Science(a) Photographs, (b) transmittance changes at 500 nm and (c)PXRD patterns of F5 by exposing a-F5 to n-hexane vapors for 17 h andheating c-(F5IH) at 160  C within 1 minute. (d) Single-crystal structures of F5IH; all labeled distances of intermolecular C–H/F
hydrogen bonds are given in the order of angstrom. F, O, C and H
atoms are represented by yellow, red, black and light blue capped
sticks, respectively.
Fig. 2

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state even aer the exposure to vapors, suggesting that aggregation of the long C6F13 groups in F13 inhibited the statetransition. These results concluded that the installation of the
C2F5 groups enabled F5 to form amorphous state a-F5 and thevapor-induced amorphous to crystalline transitions. The C2F5
groups also allowed amorphous [6]F5 to be in the crystalline
state by exposure to cyclohexane vapors (Fig. S42†), indicatingthat the C2F5 groups are good substituents to produce the guest
vapor-responsive pillar[n]arene molecular glasses.
To reveal the roles of the C2F5 groups in c-(HIF5), we obtained single crystals of F5 from chloroform under n-hexane
vapors (F5IH). From single-crystal X-ray structural analysis(Fig. 2d), F5 formed a 1 : 1 host–guest complex with n-hexane,
corresponding to the aforementioned 1H NMR study of c(F5IH) (Fig. S21†). The complex formed a highly symmetricalpillar-shaped structure, resulting in the formation of onedimensional channel assemblies. Fluoroalkyl layer formation
was induced by intermolecular C–H/F hydrogen bonds. The
PXRD pattern simulated from a single crystal of F5IH wasclose to that of c-(F5IH) (Fig. S28†), indicating that theassembled structure of c-(F5IH) was similar to that of a single
crystal of F5IH.
To obtain the structural information of a-F5, we performed
solid-state 13C NMR (Fig. S29†). Obvious peak shis of the
carbon signals from the uoroalkyl groups of F5 were observedin the samples before and aer exposure to n-hexane vapors,
indicating that the structure of the uoroalkyl groups mainly
changed along with the transition from a-F5 to c-(F5IH).
We investigated the uptake of other organic vapors by a-F5
from 1H NMR measurements (Fig. S31–S35†). a-F5 took up
organic guest vapors such as n-pentane, methanol, ethanol,
toluene and 1,4-dicyanobutane. The uptake of these organic
guest vapors triggered amorphous to crystalline transitions(Fig. S36†) and it only took several minutes for the state transitions to begin as in the case of n-hexane. Solubilization of a-F5
was observed by exposing a-F5 to DCM, 1,2-dichloroethane andchloroform vapors, which are good solvents for F5.

Guest vapor-responsive water contact angles
lm by exposure to n-hexane vapors again (Fig. 2a). We monitored the transitions by UV-vis transmittance spectroscopy,
showing the transmittance change between around 98% (a-F5)
and 10% (c-(F5IH)) (Fig. 2b and Table S1†). The reversible
appearance changes were caused by the transitions between
amorphous and crystalline states. From PXRD measurements(Fig. 2c), the reversible transitions between amorphous andcrystalline states by uptake and release of n-hexane vapors could
be conrmed. Contrary to F5, the appearance change induced
by exposure to n-hexane guest vapors was not observed in any of
the reference compounds, i.e., F3, C5, F13 and the Monomer
Unit (Fig. S22†).
F3, C5 and the Monomer Unit were crystalline states both
before and aer exposure to n-hexane vapors, indicating that
amorphous state a-F5 was successfully obtained by the installation of more uorine atoms on the side chains of pillar[5]arenes. On the other hand, F13 remained in the amorphous4084 | Chem. Sci., 2022, 13, 4082–4087

Because the n-hexane guest vapor uptake triggered the formation of the uoroalkyl layers, we measured the water contact
angles. Fig. 3a shows the change of the water contact angle
along with the state transition of F5 by the complexation. By then-hexane vapor uptake, the contact angle value increased from98  3 (a-F5) to 112  1 (c-(F5IH)).
We also measured the contact angles of other complexes
composed of F5 with n-pentane, methanol, ethanol, toluene and1,4-dicyanobutane (Table S2†). Apart from the complex of F5
with n-pentane (c-(F5IP)), none of the complexes showed asmuch increase in the contact angles as c-(F5IH). Surprisingly,
n-pentane vapors dramatically increased the contact angle value
to 119  2 (Fig. 3a), which is higher than that of c-(F5IH) (112
 1 ) and unmodied polytetrauoroethylene (114 ).33
In general, the increase in water repellency can be ascribed
to two main factors: one is the increased hydrophobicity of thesurface chemical structure and the other is the increased© 2022 The Author(s). Published by the Royal Society of ChemistryView Article Online

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Chemical Science
vapors. In contrast to these results, reference compounds F3,
C5, F13 and the Monomer Unit showed a small change in watercontact angles even aer exposure to n-hexane or n-pentane
vapors and the contact angles were lower than those of c(F5IH) and c-(F5IP) (Table S4†). This indicated that the
number of uorine atoms on the rims of pillar[5]arenes and theformation of the uoroalkyl layers by the guest vapor uptake areimportant factors for the increase in the contact angles (detailed
discussion in Fig. S57†).
Water repellency control by uptake of n-pentane guest vapors

Fig. 3 (a) Water contact angle changes of F5 by exposing a-F5
(middle) to n-hexane (left) or n-pentane (right) vapors. (b) AFM imagesof a-F5 (middle), F5IH (left) and F5IP (right). Range of grey scales:
20 nm. Size of AFM images: 400  400 nm2. (c) Single-crystal structures of F5IH (left) and F5IP (right); F, O and C atoms are represented by yellow, red and black capped sticks, respectively. Hydrogenatoms and n-hexane guest molecules are omitted for clarity. The
assembled structure of a-F5 (middle) was proposed from the results ofsolid-state 13C NMR and DFT calculations.

surface roughness.34–38 To elucidate these factors, we performed
single-crystal X-ray structural analysis and atomic force
microscopy (AFM) analysis to investigate the chemical structure
and roughness on the surface, respectively.
It is again noted that the uoroalkyl layers were formed in
the c-(F5IH) structure. A similar uoroalkyl layer formationinduced by C–H/F interactions was also observed in singlecrystal X-ray structural analysis (Fig. 3c and S37†) of F5IP,
which were obtained in a mixture of chloroform and n-pentane.
AFM analysis (Fig. 3b) of c-(F5IH) showed at terraces with
1 nm thickness attributed to a layer structure on the crystalline
surfaces perpendicular to the glass substrate. The root meansquare (RMS) value of a-F5 (0.917 nm) was similar to that of c(F5IH) (0.859 nm), indicating that surface roughness did notchange clearly even aer the assembled structure change
induced by n-hexane vapors. Contrary to this, in the case of c(F5IP), there were many crystallites on the surface and theRMS value of c-(F5IP) was 5.017 nm, suggesting a rougher
surface compared with those of a-F5 and c-(F5IH). These
results indicated that diﬀerent reasons contributed to theseincreases in the water contact angles: in the case of n-hexane
guest vapors, the main reason is the formation of uoroalkyl
layers. In the case of n-pentane, not only the formation of uoroalkyl layers but also the increase in surface roughness
resulted in the increase. Therefore, a greater increase wasobserved in the contact angle of c-(F5IP) than that of c-(F5IH)(Fig. 3). Oil repellency of a-F5, c-(F5IH) and c-(F5IP) also
supported this explanation (detailed discussion in Table S3†).
Overall, changes in macroscopic physical properties such as
water and oil repellency were achieved along with the amorphous–crystalline transitions by uptake and release of the guest© 2022 The Author(s). Published by the Royal Society of Chemistry

As in the case of reversible state transitions by n-hexane vapors,
crystalline c-(F5IP) returned to amorphous a-F5 by the releaseof n-pentane guest vapors on heating at 160  C. Exposure to npentane vapors again reverted a-F5 back to c-(F5IP) by uptakeof n-pentane vapors. The n-pentane guest vapor-triggered
reversible amorphous–crystalline and transmittance changes
were conrmed by PXRD measurements (Fig. S38†) and UV-vis
transmittance spectroscopy (Fig. S39†), respectively. Furthermore, the contact angles were reversibly changed between
approximately 98 and 120 by uptake and release of n-pentane
vapors (Fig. S40†).39 These results indicated that the contact
angles of F5 reversibly changed based on the state transitions by
the uptake and release of n-pentane vapors.
By tuning the exposure time to that of n-pentane vapors,
gradual n-pentane uptake in a-F5 was observed, which wasmonitored by 1H NMR measurements (Table S5 and Fig. S44†).
There was a plateau aer 1 h exposure to n-pentane vapors(Fig. 4), indicating that n-pentane uptake by a-F5 had reacheda saturated state only by 1 h exposure. Thus, we investigated therelationship between the uptake ratio of n-pentane to F5 and thewater repellency (Fig. 4 and Table S6†). Contact angle
measurements showed that a longer exposure time of 1 h
allowed the contact angles to increase gradually from 98 toaround 120 , which corresponded to the contact angle of thesaturated state c-(F5IP). Therefore, contact angles of a-F5 weretuned by controlling the uptake ratio of n-pentane. From UV-vis

Fig. 4 Time-dependent changes in uptake ratios of n-pentane to a-F5

(blue triangles) and water contact angles (red circles). The uptake ratios
were calculated by 1H NMR measurements. The error bars represent
one standard error from ﬁve independent measurements.

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Chemical Sciencetransmittance spectroscopy, it can be observed that transmittance decreased gradually as the exposure time was longer(Fig. S45 and Table S7†), suggesting that a-F5 also showed
exposure time-dependent appearance change from transparent
to turbid states.
To understand these mechanisms, we investigated timedependent changes in the formation of the crystalline structure by PXRD measurements and AFM analyses (Fig. S46 andS47†). From PXRD measurements, gradual appearances of
sharp diﬀraction peaks were observed along with increasing
ratio of n-pentane vapors, suggesting that amorphous to crystalline transitions of F5 gradually occurred by n-pentane vapor
uptake. From AFM analyses, it can be observed that crystalline
structures with clear edges were gradually formed by exposure
to n-pentane vapors, which was consisted with the PXRD
results.

Conclusions
We observed that a pillar[5]arene with C2F5 uoroalkyl groups
could be an organic molecular glass as a transparent lm bycooling the isotropic molten liquid of pillar[5]arene. Quite
distinct from typical molecular glasses, the pillar[5]arene
molecular glass showed guest vapor-responsive amorphous–
crystalline transitions because it has a cavity to capture theguest vapors based on the host–guest system. By exposing thetransparent lm to the guest vapors, the transparent lmchanged to a turbid lm by complexation with the guest
vapors. In addition, release of the guest vapors by heating
triggered the crystalline to amorphous transition. Taken
together, completely reversible amorphous–crystalline statetransitions occurred by the uptake and release of the guest
vapors. There are a few examples of amorphous macrocyclic
compounds,31,32,40–42 but investigation of these amorphousmacrocyclic compounds in terms of their macroscopic property changes was not reported. Vapochromism is a color/uorescence change which occurs by exposing crystals tovapors.43–45 In this study, n-alkane vapors can be directly
monitored by the transparency changes of the lm. Alkane
vapor-responsive materials are very rare46 because it is diﬃcult
for the C–H and C–C groups in alkanes to form stable interactions with adsorption materials. The transparent-turbidchanges come from amorphous–crystalline state transitions
by uptake and release of the guest vapors. The amorphous–
crystalline transitions triggered a macroscopic propertychange such as water repellency. Uptake and release of theguest vapors cause changes in the chemical structure androughness on the surface, enabling control of the contact
angle. Another macroscopic property, adhesiveness, is also
highly related to the chemical structure and roughness on thesurface, thus both these changes demonstrated in this study
along with amorphous–crystalline transitions will be applied
to switchable adhesive materials. Coating materials for selective vapor uptake would also be another idea for the application of the pillar[5]arene molecular glass because pillar[5]arenes can capture n-alkane guest vapors and molecularglasses can be integrated in polymer and glass coatings.

4086 | Chem. Sci., 2022, 13, 4082–4087

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Data availability
All relevant data are presented in the main text and ESI.†

Author contributions
T. O. conceptualized and supervised the work. K. O., S. O., K. K.,
S. F., Y. S., S. A., M. O., H. A., S. N., Y. T. and M. M. designed andperformed experiments. The rst dra of the manuscript was
written by K. O. and T. O. All authors contributed to manuscript
review.

Conﬂicts of interest
There are no conicts to declare.

Acknowledgements
This work was supported by the Grant-in-Aid for Scientic Research on Innovative Areas: p-System Figuration (JP15H00990
and JP17H05148, T.O.), So Crystals (JP18H04510 and
JP20H04670, T.O., JP18H04511 and JP 20H04667, Y.S.,
JP20H04666, M.M.), and K.A. (JP19H00909, T.O.) from MEXT
Japan, JST CREST (JPMJCR18R3, T.O.), and the World PremierInternational Research Center Initiative (WPI), MEXT, Japan.
This work was partially supported by Kanazawa University
JIKOCHOKOKU project (H.A.).

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Chemical Science
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