A guest-induced reversible switching of a self-assembled H-bonded supramolecular framework

Article abstract of DOI:10.1039/c1cc13124a

A new class of organic crystalline 2,2′-biphenol-based H-bonded material displaying 1D-channels encapsulating solvent molecules is described. A reversible guest-induced crystal-to-crystal conversion between the solvated H-bonded phase and a compact H-bond

Full text of DOI:10.1039/c1cc13124a

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COMMUNICATION
A guest-induced reversible switching of a self-assembled H-bonded
supramolecular frameworkw
Pierre Mobian,* Clarisse Huguenard and Marc Henry*
Received 27th May 2011, Accepted 13th July 2011
DOI: 10.1039/c1cc13124a
A new class of organic crystalline 2,20-biphenol-based H-bonded
material displaying 1D-channels encapsulating solvent molecules
is described. A reversible guest-induced crystal-to-crystal conversion
between the solvated H-bonded phase and a compact H-bonded
non-solvated phase was observed. The energy competition
between intramolecular H-bonds formation and solvation of
organic pores has been characterized using PACHA calculations.
(Phase I) (Fig. 1d) with 1D channels along the a-axis. A close
observation of the structure indicated that each http unit
interacts with two neighbours through intermolecular OHꢀ ꢀ ꢀO
hydrogen bonds with an Oꢀ ꢀ ꢀO distance of about 3 A to form
a 1D columnar assembly shown in Fig. 1b. It is also worth
noting that in the solid state the two central crystal structure
phenyl rings of the http unit adopts anticlinal conformation
(measured torsion angle: 1161) that renders this molecule
chiral. Consequently, in the crystal, these 1D columnar assem-
blies could also be regarded as an infinite self-association of
http heterochiral dimers (Fig. 1a). Each column is then sur-
rounded by four analogues through CH–p interactions
(Fig. 1c). As shown in the ESIw, the other OH moiety is not
H-bonded but rather engaged in a O–Hꢀ ꢀ ꢀpi bonding with an
interaction energy very similar to the intermolecular H-bond.
The repetition of this latest supramolecular motif creates
distorted hexagonal 1D channels with an inner diameter
ranging from 4.4 A to 5.3 A (see ESIw). The solvent-accessible
free volume of these channels is estimated to be 388 A3
representing 14% of the total crystal volume.8
Clathrates are a fascinating class of supramolecular crystalline
solids that elicit major interest for fundamental purposes and
practical issues.1 A particular class of clathrates that attracts
considerable attention are dynamic systems where reversible
guest-induced crystal-to-crystal transformation occurs.2 Many
efforts have been made to characterize dynamic porous
coordination polymers showing dynamic behaviours.3 However,
focusing on crystalline materials formed by the self-assembly
of organic building blocks, few examples of dynamic clathrates
have been reported.4 For these particular systems, removing
the guest molecules trapped in the porous frameworks usually
led to the irreversible collapse of the structures into dense
forms.5,6 A remarkable exception concerns organic hydrogen-
bonded networks which often exhibit flexibility.7
The presence of guest molecules trapped inside the channels
was evidenced by thermogravimetric analysis (TGA), which
showed a rapid weight loss (13%) upon heating from 75 1C to
95 1C. XRD analysis was not able to resolve the molecular
disorder, but 1H and 13C solid-state NMR analyses enabled us
to identify the solvents trapped in the as-synthesized solid.
Herein, we report the dynamic behavior of a new class of
organic crystalline materials based on the readily available
2,20-biphenol derivative, i.e. 20,20 0-dihydroxy-3,5,30 00,500 0-tetra-
methyl-1,10:30, 30 0:10 0,10 0 0-tetraphenyl (http).
Crystals suitable for X-ray analysis (space group Pna21)
were obtained by allowing n-pentane to diffuse into a dichlor-
omethane solution of http. Structure analysis revealed that
http solid-state organization led to a honeycomb-like structure
´
Laboratoire de Chimie Moleculaire de l’Etat Solide, UMR 7140,
Universite´ de Strasbourg, 67070 Strasbourg, France.
E-mail: mobian@unistra.fr, henry@unistra.fr
w Electronic supplementary information (ESI) available: Synthetic
pathway and characterization of http, X-ray diffraction data, NMR
experimental, TGA, DSC thermograms and PACHA calculations.
CCDC 817017, 817018. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1cc13124a
Fig. 1 X-Ray crystal structure of http (Phase I). (a) Capped sticks
representation of a http heterochiral dimer, the blue dotted line represents
the hydrogen bond existing between two http units. (b) View along the
b-axis of a 1D columnar assembly. (c) View along the a-axis of the
pentameric assembly of 1D columns. (d) Spacefill model of the resulting
porous network (in all cases hydrogens are omitted for clarity).
c
9630 Chem. Commun., 2011, 47, 9630–9632
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Reproducible NMR signals due to CH2Cl2 species were indeed
observed on 1H magic angle spinning (MAS) spectrum (4.9 ppm)
as well as 13C cross-polarization (CP) MAS (54.2 ppm)
spectrum. Signals from n-pentane were also observed at
(Fig. 2b). XRPD and 13C CPMAS systematic investigations
allowed us to establish some other features of the system
described below. Phase II may also be obtained as the direct
product of n-pentane diffusion into a http CH2Cl2 solution if
the temperature of the medium is not controlled. However, for
a constant temperature (4 1C), Phase I is most frequently
obtained. Similarly, using n-hexane or cyclohexane instead of
n-pentane also led to solvated phases analogous to Phase I
including hydrocarbon molecules. It was also observed that
Phase II crystallized from a n-hexane/CH2Cl2/http system. The
structure of Phase II was in fact resolved from this latter
system that provided crystals suitable for X-ray analysis
(P21/n space group). In this case, the three-dimensional
arrangement of the http units was shown to be compact.
Fig. 3 presents the supramolecular organization of the http
molecules in the crystal. First, two http molecules interact via
an intermolecular OHꢀ ꢀ ꢀO hydrogen bond (Oꢀ ꢀ ꢀO = 3.1 A).
Interestingly enough, such a dimer is built from two different
http conformers. The first http conformer presents a synclinal
arrangement of the two central phenyl rings, whereas the
second http conformer displays an anticlinal conformation.
The measured torsion angles between the two central phenol
rings were, respectively, found to be 521 due to an intramolecular
hydrogen bond (http1) and 1161 (http2). The repetition of this
dimeric motif (Fig. 3b) along the b-axis leads to an infinite 1D
columnar assembly that is self-associated through van der Waals
interactions to form the compact network (Fig. 3c).
1
0.75/1.2 ppm on H MAS and at 14.3/22.2/34.2 ppm on 13C
1
CPMAS spectra (Fig. 2a). See ESIw for H spectra.
Using a 10 kHz MAS speed, the overlap of the numerous 1H
signals of http gives rise to a broad pattern but signals of
CH2Cl2 and n-pentane appear well separated and thin enough
(150 Hz linewidth) to allow an estimation of the trapped
solvents molar proportion. The as-synthesized product includes
about 70% of n-pentane and 30% of CH2Cl2 molecules.
Taking into account the TGA-weight loss, a host : guest ratio
of 1 : 0.8 is inferred for Phase I. After twenty days storage in
air, 80% of CH2Cl2 was spontaneously lost whereas less than
10% of n-pentane had left the solid, illustrating the predominant
role of this hydrocarbon in Phase I stability. DSC traces
displayed two intense endotherms, the first one at 90 1C and
a second one at 155 1C which were attributed, respectively, to
removal of solvent molecules and to the melting temperature
of the product (mp: 156 1C) (see ESIw).
The characterization of the apohost phase (Phase II) thermally
synthesized from Phase I was then performed. Despite the
a priori amorphous aspect, X-ray powder diffraction (XRPD)
(see Fig. S5, ESIw) and 13C CPMAS solid-state NMR analysis
showed the apohost material to be crystalline. The X-ray
diffraction pattern and the 13C CPMAS spectrum are strongly
different from those obtained for the starting porous material
To investigate the reversibility of the formation of the
compact structure i.e. Phase II-to-Phase I conversion, a
sample of Phase II prepared by heating Phase I at 120 1C
was exposed to an atmosphere saturated with dichloro-
methane and pentane vapours at room temperature. TGA
analysis of the resulting solid indicated a weight loss of 12%.
Furthermore, it was evident from infrared spectroscopy,
XRPD and 13C CPMAS solid-state NMR analysis that the
starting Phase II was converted to Phase I. This taking-up of
solvent molecules affected only very partially the infrared
spectrum of Phase II. The hydrogen-bonded O–H stretch
region (u = 3489–3473 cmꢁ1) became slightly sharper which
might indicate a perturbation of the columnar hydrogen net-
works, and a new band at u = 2953 cmꢁ1 was observed that
Fig. 2 13C CPMAS NMR spectra of http powder (a) as-synthesized
by diffusion of n-pentane into CH2Cl2 solution (Phase I), (b) after one
night at 100 1C (Phase II), and (c) after further exposure to CH2Cl2/
n-pentane vapors for 36 h. Top: http methyl and trapped solvents
signals range; bottom: http C, CH and CO signals range, see the text
for details. Note: in (a) and (c) cases, CH3 sites of http are identified at
20.2, 22.2 and 23.1 ppm (1 : 2 : 1) and one of the resonance coming
from n-pentane overlap with the central signal.
Fig. 3 X-Ray crystal structure of http (Phase II). (a) Capped sticks
representation of a http dimer formed by two different conformers
interacting through an intermolecular hydrogen bond, the blue dotted
line represents the hydrogen bond existing between two http units. (b)
View along the b-axis of a 1D columnar assembly. (c) View along the
a-axis of the resulting compact network (in all cases hydrogens are
omitted for clarity).
c
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Chem. Commun., 2011, 47, 9630–9632 9631

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may correspond to absorption arising from asymmetrical C–H
stretching of methyl groups in pentane (see ESIw). Fig. 2 shows
a series of 13C CPMAS solid-state NMR spectra correspond-
ing to Phase I, Phase II and the re-solvated Phase I. The
symmetry changes caused by solvent removal and uptake are
clearly demonstrated as two (150.0 and 150.5 ppm) and four
(148.4, 150.3, 151.1 and 152.0 ppm) inequivalent CO sites are
observed for Phase I and Phase II, respectively, in agreement
with single-crystal XRD. Chemical shifts for CO sites are
slightly different (150.2 and 150.5 ppm) in the re-solvated
phase compared to the starting material and some other slight
chemical shift differences are observed, but the re-solvated
phase exhibits essentially the same spectrum as Phase I. These
shifts are certainly induced by a different pore occupation due
to a higher CH2Cl2 proportion in the re-solvated phase than in
the starting material as evidenced in the top part of Fig. 2 and
pentane molecules and channels half-filled with dichloromethane
was computed and found to be strongly stabilizing (ꢁ40 kJ molꢁ1).
Finally, the interaction energy involving dichloromethane
molecules and channels half-filled with pentane was also computed
and found to be less stabilizing (ꢁ14 kJ molꢁ1). It follows
from this energy analysis that the only way to be competitive
with the strong intramolecular H-bond (ꢁ50 kJ molꢁ1) in
Phase II is to fill the channels first with dichloromethane and
then encapsulate pentane molecules (ꢁ40 ꢁ 32 = ꢁ72 kJ molꢁ1
)
while filling first with pentane and then encapsulate dichloro-
methane is much less favorable (ꢁ14 ꢁ 32 = ꢁ46 kJ molꢁ1).
These oligophenylene structures being light emitters,12 solid-
state fluorescent measurements were also performed both on the
solvated and compact state of the material. It was thus found
that upon excitation at 340 nm, both phases exhibited luminescence
centered at lmax = 400 nm.
In conclusion, we were able to characterize a dynamic behavior
of an organic crystalline solid. This dynamic phenomenon corres-
ponds to the reversible guest-induced crystal-to-crystal conversion
of a solvated to a compact phase. From a quantitative energy
analysis it was found that the presence of both dichloromethane
and pentane molecules within the channels was necessary for the
occurrence of the reversible Phase II to Phase I transition.
We are indebted to Dr Kyritsakas for X-ray structure resolu-
tion. We thank Prof. Mir Wais Hosseini for fruitful discussions.
1
in H MAS spectra (see ESIw).
In order to better characterize the role of solvent molecules
in the formation of Phase I and Phase II, we have used the
PACHA (Partial Atomic Charges Analysis) approach that
allows us to compute reliable interaction energies from the sole
knowledge of crystalline structures (see ESIw for computational
details).9,10 Concerning Phase I, the intermolecular H-bond energy
responsible for H-bonded chains of http molecules was found to
be ꢁ22 kJ molꢁ1, a value that may be compared to the energy
associated by the stacking of these chains into a 3D structure
of about ꢁ1 kJ molꢁ1. The 1D nature of Phase I is thus clearly
established. Concerning Phase II, molecular interactions
between http1 molecules in the absence of http2 were found
to be slightly destabilizing (+1 kJ molꢁ1) whereas molecular
interactions between http2 molecules in the absence of http1
molecules were found to be slightly stabilizing (ꢁ3 kJ molꢁ1).
Interweaving these two sub-networks was neatly stabilizing
with an interaction energy of ꢁ12 kJ molꢁ1. The inter-
molecular H-bond energy for H-bonded chains of http mole-
cules was found to be ꢁ26 kJ molꢁ1, whereas the energy
associated with the intramolecular H-bonds was about twice
This work was done at Universite de Strasbourg with public
´
funds allocated by CNRS and the French government.
Notes and references
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this value at ꢁ50 kJ molꢁ1
.
In order to get a good understanding of the role played
by solvent molecules, atomic coordinates were computed for
dichloromethane and pentane molecules encapsulated within the
network channel after minimization of steric energy using
Gordin–Kim potentials.11 Assuming a 1 : 1 host : guest ratio
(1 : 0.8 measured) and channels entirely filled with dichloro-
methane, 1D chains of dichloromethane molecules (halogen bond
energy of ꢁ18 kJ molꢁ1) were obtained with an encapsulating
energy of ꢁ26 kJ molꢁ1. By contrast, for channels entirely filled
with pentane, interactions between pentane molecules or with
channels walls were found to be both rather weak (ꢁ0.3 kJ molꢁ1
and ꢁ1 kJ molꢁ1 respectively). In both cases there is clearly not
enough energy to overcome the strong intramolecular H-bond
observed in Phase II. Consequently, a network with channels
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ꢁ32 kJ molꢁ1 while the average interaction energy with channels
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c
9632 Chem. Commun., 2011, 47, 9630–9632
This journal is The Royal Society of Chemistry 2011