Role-allocated combination of two types of hydrogen bonds towards constructing a breathing diamondoid porous organic salt

Article abstract of DOI:10.1002/chem.201202959

A diamondoid porous organic salt (d-POS) composed of 8-hydroxyquinoline-5- sulfonic acid (HQS) and triphenylmethylamine (TPMA) shows reversible structure contraction and expansion ( breathing ) in response to guest desorption and adsorption. This flexible structure is designed hierarchically by utilizing two different types of hydrogen bonds. X-ray crystallographic analysis reveals that the two types of hydrogen bonds are formed separately to play respective roles for constructing the d-POS. The strong charge-assisted hydrogen bond between the sulfonate anion of HQS and the ammonium cation of TPMA serves as a static node to provide a supramolecular cluster for a building block. In contrast, the complementary neutral hydrogen bond between the hydroxyl and quinolyl groups of HQS acts as a dynamic linker to connect the clusters. Consequently, these two types of hydrogen bonds yield the d-POS with one-dimensional channels through the formation of diamondoid networks. We clarify that the d-POS undergoes dynamic structure transformation that originates in the cleavage and reformation of the complementary neutral hydrogen bond during guest desorption and adsorption. From the comparative studies, it is also demonstrated that applying the complementary neutral hydrogen bond in the d-POS provides significant advantages in terms of the responsivity of the structure over applying other weak noncovalent interactions for the connection of the clusters. Furthermore, the resultant d-POS also modulates fluorescent profiles dynamically responsive to guest adsorption and desorption. Breathing architecture: A hierarchical design by using an organic salt combining charge-assisted hydrogen bonds and complementary neutral hydrogen bonds provides a flexible diamondoid porous organic salt (d-POS; see figure). The structure and fluorescence of the resultant d-POS are reversibly changed in response to guest desorption and adsorption, originating in the role-allocated association of the hydrogen bonds. Copyright

Full text of DOI:10.1002/chem.201202959

DOI: 10.1002/chem.201202959
Role-Allocated Combination of Two Types of Hydrogen Bonds towards
Constructing a Breathing Diamondoid Porous Organic Salt
Atsushi Yamamoto,[a] Tetsuya Hasegawa,[a] Tomoya Hamada,[a] Tomofumi Hirukawa,[a]
Ichiro Hisaki,[a] Mikiji Miyata,[a] and Norimitsu Tohnai*[a, b]
Abstract: A diamondoid porous organ-
ic salt (d-POS) composed of 8-hydroxy-
quinoline-5-sulfonic acid (HQS) and
triphenylmethylamine (TPMA) shows
reversible structure contraction and ex-
pansion (“breathing”) in response to
guest desorption and adsorption. This
flexible structure is designed hierarchi-
cally by utilizing two different types of
hydrogen bonds. X-ray crystallographic
analysis reveals that the two types of
hydrogen bonds are formed separately
to play respective roles for constructing
the d-POS. The strong charge-assisted
hydrogen bond between the sulfonate
anion of HQS and the ammonium
cation of TPMA serves as a static node
to provide a supramolecular cluster for
a building block. In contrast, the com-
plementary neutral hydrogen bond be-
tween the hydroxyl and quinolyl
groups of HQS acts as a dynamic
linker to connect the clusters. Conse-
quently, these two types of hydrogen
bonds yield the d-POS with one-dimen-
sional channels through the formation
of diamondoid networks. We clarify
that the d-POS undergoes dynamic
structure transformation that originates
in the cleavage and reformation of the
complementary neutral hydrogen bond
during guest desorption and adsorp-
tion. From the comparative studies, it
is also demonstrated that applying the
complementary neutral hydrogen bond
in the d-POS provides significant ad-
vantages in terms of the responsivity of
the structure over applying other weak
noncovalent interactions for the con-
nection of the clusters. Furthermore,
the resultant d-POS also modulates flu-
orescent profiles dynamically respon-
sive to guest adsorption and desorp-
tion.
Keywords: cluster compounds
host–guest systems hydrogen
bonds porous structures
supramolecular chemistry
·
·
·
·
Introduction
such as p–p interactions or hydrogen bonds have also been
of interest in terms of free of toxicity associated with metal
content and good workability, allowing easy solvent-based
fabrication and reassembly.[7] These materials also have the
potential to gain structural flexibility, owing to the weak
connection of their components. This structural flexibility
leads to unique properties such as dynamic and guest-re-
sponsive adsorption behaviors.[8] These behaviors are hardly
achieved by conventional robust porous structures. Further-
more, the structural response of these materials to external
stimuli enables cooperative switching of the interlaced prop-
erties by combining structural flexibility with other physical
properties such as optoelectronic properties.[4b,9] The cooper-
ative switching of the properties is expected to facilitate the
development of porous materials for molecular sensors and
memory devices. In connection with this, several reports
have emerged recently for constructing new flexible organic
porous structures. Tang and co-workers designed flexible
porous structures based on multisubstituted silole deriva-
tives.[10] Meanwhile, Rissanen and Raatikainen utilized halo-
gen bonds combined with hydrogen bonds to yield porous
structures with solvent-induced adaptation of the nanosized
channels.[11] Nevertheless, flexible organic porous structures
often irreversibly end up close-packing and non-porous
structures by guest desorption to maximize interactions be-
tween the components. Therefore, it is still a challenging
task to construct organic porous materials whose structures
Microporous materials employing organic molecules have
attracted much attention not only in terms of gas storage,[1]
separation,[2] and catalysis[3] but also as guest-responsive
multifunctional materials.[4] In this context, the preparation
of metal-organic frameworks (MOFs)[5] and covalent-organ-
ic framework (COFs)[6] have been greatly advanced. In
these materials, the open frameworks are robustly stabilized
by strong bonds such as coordination or covalent bonds to
maintain their permanent porosity. On the other hand, or-
ganic porous materials using weak noncovalent interactions
[a] A. Yamamoto, T. Hasegawa, T. Hamada, T. Hirukawa, Dr. I. Hisaki,
Prof. Dr. M. Miyata, Dr. N. Tohnai
Department of Material and Life Science
Graduate School of Engineering
Osaka University, 2-1 Yamadaoka, Suita
Osaka 565-0871 (Japan)
Fax : (+81)6-6879-7404
E-mail: tohnai@mls.eng.osaka-u.ac.jp
[b] Dr. N. Tohnai
Japan Science and Technology Agency (JST)
PRESTO, 4-1-8 Honcho Kawaguchi
Saitama 332-0012 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202959.
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FULL PAPER
reversibly contract and expand in response to guest desorp-
tion and adsorption.
Regarding the construction of porous structures, we focus
on organic salts consisting of ammonium sulfonates. The us-
ability of the organic salts for constructing porous structures
has been demonstrated by numerous reports.[12] For exam-
ple, Ward et al. yielded distinguished grid-like structures
with one-dimensional open channels based on guanidine
and disulfonic acids.[12a–c] On the other hand, we recently re-
ported the preparation of porous organic salts (POSs) by
various sulfonic acid derivatives and amines.[12d,e] Among
these POS systems, organic salts composed of sulfonic acid
derivatives having polyaromatic groups and triphenylme-
thylamine (TPMA) provide specific POSs, diamondoid
porous organic salts (d-POSs).[13] In the d-POSs, sulfonic
acids and TPMA construct bulky tetrahedral [4+4] supra-
molecular clusters by charge-assisted hydrogen bonds. Sub-
sequently, the clusters are arranged by p–p interactions be-
tween polyaromatic groups to produce porous structures
through the formation of diamondoid networks correspond-
ing to sulfonic acid and guest species. The d-POS system, in
which the cluster serves as a subunit, is a powerful strategy
for providing a variety of porous architectures in structural
and functional characters through diversity of both the clus-
ter itself and its arrangement. Most importantly, the clusters,
and therefore the resultant architectures, are readily tunable
with the use of suitable sulfonic acids as needed.
On the basis of the fruitful d-POS system, in this paper
we demonstrate a d-POS that shows reversible expansion
and contraction, similar to “breathing”. Figure 1a shows the
hierarchical design of the structure. We conceived that the
hierarchical utilization of different types of hydrogen bonds
should be a suitable approach to yield the desired flexible d-
POS. Hydrogen bonds are powerful tools for controlling
molecular arrangements in the solid state.[14] Among the dif-
ferent types of hydrogen bonds, charge-assisted hydrogen
bonds produce exceptionally strong interactions.[14d,e] Thus,
charge-assisted hydrogen bonds can serve as static nodes be-
tween components to produce stable building blocks for re-
taining void space. The sterically bulky supramolecular clus-
ter constructed by charge-assisted hydrogen bonds is a good
candidate for the building block.[15] In contrast, neutral hy-
drogen bonds have the appropriate strength and flexibility
to provide reversible association and dissociation in re-
sponse to the surrounding environment.[14b] In particular,
complementary hydrogen bonds also produce efficient re-
formability owing to their directionality and precise recogni-
tion.[14a,f,g] Therefore, the complementary neutral hydrogen
bonds are expected to act as dynamic linkers in response to
guest desorption and adsorption. In this respect, we pre-
pared an organic salt 2a composed of 8-hydroxyquinoline-5-
sulfonic acid (HQS) and TPMA (Scheme 1). In addition, we
prepared two different organic salts: organic salt 2b com-
bined naphthalene-1-sulfonic acid (1-NS) with TPMA and
organic salt 2c combined pyrene-1-sulfonic acid (1-PyS)
Figure 1. a) Schematic representation of the hierarchical design of
“breathing” diamondoid porous organic salt (d-POS). b) Hierarchical as-
sembly of HQS and TPMA by using two types of hydrogen bonds. Sulfo-
nate groups and ammonium groups construct a cubic-like charge-assisted
hydrogen-bond network to form a supramolecular cluster. The comple-
mentary hydrogen bonds are formed between hydroxyl and quinolyl
groups to connect the clusters.
of aromatic guest molecules was obtained from organic salt
2a. In the structure, two types of hydrogen bonds play a
role for hierarchically assembling the molecules. That is,
charge-assisted hydrogen bonds construct a supramolecular
cluster and complementary neutral hydrogen bonds connect
the clusters. Interestingly, the structure of the d-POS shrinks
but maintains porosity upon guest desorption. Subsequently,
the shrunken structure reversibly expands to the original
structure by guest adsorption. It is revealed that these be-
haviors are accompanied by dynamic alteration of the com-
plementary hydrogen bonds. Furthermore, comparative
studies demonstrate that the structure reversibility is greatly
attributed to the reformability of the complementary neutral
hydrogen bonds.
Results and Discussion
Construction of d-POS by hierarchically applying two types
of hydrogen bonds: HQS, which we selected for constructing
a “breathing” d-POS, not only has a sulfonic acid group but
with TPMA for comparative studies. As expected,
a
“breathing” d-POS in response to desorption and adsorption
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N. Tohnai et al.
bonds (Figure 2d). The distances of the complementary hy-
drogen bonds are 2.85 (N1-O8) and 2.72 ꢁ (N2-O4). These
complementary hydrogen bonds connect the clusters to each
other, providing diamondoid networks (Figure 2e). Owing
to large void space in the networks, independent networks
eventually interpenetrate and stabilize their structures by
CH-p and p–p interactions between aromatic groups of the
clusters to give a 4-fold host framework having a one-di-
mensional (1D) open void-space (Figure 2b and 2e). Ac-
cording to calculations using PLATON/VOID,[17] the void
space holds 30% of the volume of the whole crystal struc-
ture with the shortest width of 5.1 ꢁ and the longest width
of 10.4 ꢁ. In the void space, three anisole molecules are in-
cluded as guests per one cluster (Figure 2b).
The crystallographic study of d-POS-2a·Anis clarifies that
two types of hydrogen bonds, charge-assisted hydrogen
bonds and neutral complementary hydrogen bonds, play dif-
ferent roles for constructing the porous structure. The
strong charge-assisted hydrogen bonds work as rigid nodes
to assemble HQS and TPMA into robust clusters. In con-
trast, the complementary neutral hydrogen bonds act as
linkers to accumulate the clusters. The neutral hydrogen
bonds, which are flexible compared with the charge-assisted
hydrogen bonds, are expected to show a dynamic response
to external stimuli and environmental change. In particular
it should be noted that these different types of hydrogen
bonds are not mixed and are completely segregated to fulfill
each role. This segregated formation is explained by the
hardness and softness of the intermolecular interaction prin-
ciple of the hydrogen bonds.[18] This principle indicates that
hard donors such as sulfonate groups prefer to form hydro-
gen bonds between hard acceptors such as amino groups. In
contrast, soft donors such as hydroxyl groups prefer to form
hydrogen bonds between soft acceptors such as quinolyl
groups. Furthermore, complementarity can strictly define
hydrogen bond behavior owing to their precise recognition.
These results indicate that well-designed combinations of
different strength of hydrogen bonds should be a useful ap-
proach for controlling the assembly of organic molecules
into porous structures.
Scheme 1. Chemical structures of sulfonic acid derivatives and TPMA for
the organic salts and guest molecules.
also hydroxyl and quinolyl groups available for the forma-
tion of hydrogen bonds. We expected that the sulfonic acid
groups of HQS and the amino groups of TPMA form a
cubic-like charge-assisted hydrogen-bond network to pro-
vide a supramolecular cluster, whereas the hydroxyl and
quinolyl groups of HQS build complementary neutral hy-
drogen bonds with each other to accumulate the clusters, as
shown in Figure 1b. Recrystallization of the organic salt 2a
comprising HQS and TPMA gave bipyramidal crystals from
a mixture of methanol and anisole at 408C. Single-crystal X-
ray analysis revealed that HQS and TPMA give a diamond-
oid porous organic salt (d-POS-2a) including anisole (d-
POS-2a·Anis, Figure 2a and 2b).[16] In d-POS-2a·Anis, the
two types of hydrogen bonds contribute to the hierarchical
construction of the structure (Figure 2c–2e). First, four
HQS and four TPMA form a cubic-like charge-assisted hy-
drogen-bond network between the sulfonate anions and the
ammonium cations to assemble into a [4+4] supramolecular
cluster (Figure 2c). The distances of the charge-assisted hy-
drogen bonds in the cubic hydrogen-bond network range
from 2.73 (N3-O1) to 2.81 ꢁ (N4-O3). The cluster has a
spherical shape with a diameter of approximately 17 ꢁ, in
spite of the cubical shape of the core (Figure 2e). Between
the clusters, the quinolyl groups of HQS do not form any p–
p stacked arrangements. Instead, as we expected, the nitro-
gen atoms in the quinolyl groups and the hydroxyl groups
face each other to form complementary neutral hydrogen
Reversible structure transformation responsive to guest de-
sorption and adsorption: The segregated formation of the
two types of hydrogen bonds, especially the complementary
neutral hydrogen bonds, is expected to undergo dynamic
transformation upon guest desorption and adsorption. Ther-
mal gravimetric analysis (TGA) of d-POS-2a·Anis displays
a 14.2% weight loss around 70–1008C before decomposition
of the components at over 2008C (Figure S1a, the Support-
ing Information). The weight loss corresponds to the
amount of the incorporated guest molecules observed in the
crystal structure. Figure 3a and 3b show the powder X-ray
diffraction (PXRD) patterns before and after guest desorp-
tion, respectively. These PXRD patterns indicate the dy-
namical change of the host framework from the d-POS-2a
into a transformed structure d-POS-2a’, by guest desorption.
Before guest desorption, only one sharp peak is observed at
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Figure 2. Structure of a diamondoid porous organic salt comprising an organic salt 2a including anisole (d-POS-2a·Anis). a) View along the c axis. Guest
molecules are omitted for clarity. Hydrogen atoms are also omitted for clarity except for the space-filling model of a supramolecular cluster. b) Visualiza-
tion of the void space. Guest molecules in the sky-blue-colored void space are represented by green balls and sticks. Hydrogen atoms of the guest mole-
cules are omitted for clarity. Representation of the two types of hydrogen bonds: c) the cubic-like charge-assisted hydrogen bonds and d) the comple-
mentary neutral hydrogen bonds. e) Hierarchical interpretation for constructing the host framework. In the diamondoid network and the 4-fold host
framework, the triphenylmethyl groups are omitted for clarity. The independent diamondoid networks are indicated by magenta, green, orange, and
blue.
6.08 (d=14.7 ꢁ) in the low 2q region (Figure 3a). This peak
is derived from the (110) plane corresponding to a distance
between the 1D void space of d-POS-2a. After guest de-
sorption by heat treatment at 908C, however, the PXRD
pattern of d-POS-2a’ did not show any peaks at 6.08. In-
stead, two new peaks appear at 6.5 and 6.78 (d=13.6 and
13.2 ꢁ, respectively, Figure 3b). In contrast, a peak at 12.98
(d=6.9 ꢁ) is still observed after guest desorption. This peak
originates from the (0 0 4) plane corresponding to half the
distance between the clusters along the 1D channels. The
two split peaks suggests anisotropic shrinkage of the struc-
ture with an approximately 10% reduction of the void diam-
eter, whereas one preserved peak indicates the clusters
themselves and their alignment along the channels are main-
tained even after guest removal. To consider the structure of
d-POS-2a’, the unit cell parameters are also determined
from the PXRD data (Table S2, the Supporting Informa-
tion). The space group is dramatically changed from higher
Figure 3. PXRD patterns of the crystal composed of HQS and TPMA:
symmetrical P42/n to lower symmetry of P-1. The unit cell
a) Before guest desorption (d-POS-2a·Anis); b) After guest desorption
parameters also undergo drastic changes that precluded
exact crystal structure determination. However, the shrunk-
(d-POS-2a’) by heat treatment at 908C for 1 h; c) After exposure to ani-
sole vapor for 3 h at 258C.
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N. Tohnai et al.
en structure d-POS-2a’ is also supposed by FT-IR measure-
ments (Figure S2, the Supporting Information). Before guest
adsorption data based on the Horvath-Kawazoe (HK)
method[19] confirms that the average diameter of the pore is
6.8 ꢁ, which also suggests shrinkage of the structure by ap-
proximately 10% from d-POS-2a. These results suggest that
an externally accessible void space is still present even after
the shrinkage, which is in contrast to many other flexible or-
ganic structures. The isotherm also shows significant hystere-
sis between adsorption and desorption, although the total
adsorption amount is not so large compared with those of
other porous organic materials.[7a,b,e,f] The hysteretic behavior
is generally observed in porous materials with flexible small
pore and/or trapping sites.[2g,8d–f] CO2 molecules not only
have the smallest kinetic diameter among the three adsor-
bates, but also have a potential to interact by using their
quadrupole moments. Therefore, the contracted small pore
covered with aromatic groups in d-POS-2a’ may cause coop-
erative diffusion and strong interactions between CO2 mole-
cules and the void surface to provide the hysteretic behav-
ior. The hysteretic behavior is favorable for storage, because
a large hysteresis allows gas loading at higher pressures and
store at relatively lower and safer pressures. It should be
also considered to be the effect of gas adsorption on struc-
ture breathing. So far, there are only a few reports of gas-in-
duced transformation and expansion of the organic struc-
tures such as tris-o-phenylenedioxycyclotriphosphazene
(TPP).[8e,f] A non-porous form of TPP undergoes a 23% ex-
pansion of the structure by pressurization with CO2 to trans-
form a nanoporous form with accompanying large uptake of
gas. Nevertheless, d-POS-2a’ shows no sharp increase of
uptake during the gas adsorption under the experimental
conditions. Theoretical full-loading capacities calculated
from the accessible void volume for CO2 molecules indicate
that d-POS-2a and d-POS-2a’ have the potential to absorb
15.4 and 11.2 CO2 molecules per one cluster, respectively.
Therefore, adsorption under these conditions is not enough
to fill up the void space. Furthermore, d-POS-2a’ expands
to its structure by vapor adsorption of organic solvents as
discussed in detail below. From these results, it is expected
to undergo further adsorption and possibly gas-induced
structure expansion by pressurized gas loading.
À
desorption, the band of O H stretching forming comple-
mentary hydrogen bonds appears at 3303 cmÀ1 (Figure S2a,
the Supporting Information). However, this band shifts to
3368 cmÀ1 in higher wavenumber region after guest desorp-
tion (Figure S2b, the Supporting Information). This spectral
change indicates that hydroxyl groups vibrate more freely
than those before guest desorption owing to cleavage of the
complementary hydrogen bonds. These results strongly sug-
gest that guest desorption causes shrinkage of the structure
and rearrangement of the clusters concomitant with the
cleavage of the complementary hydrogen bonds (Figure 4a).
To estimate the porosity of d-POS-2a’, gas adsorption be-
haviors were investigated by using N2, O2 and CO2 gases
(Figure S3, the Supporting Information). The adsorption iso-
therms of N2 and O2 show no adsorption except for surface
adsorption at 77 K. However, d-POS-2a’ selectively adsorbs
CO2 gas at 194 K. The maximum adsorption amount is
19.5 cm3 gÀ1 (STP) at P/P0 =0.98, corresponding to 1.7 CO2
molecules per one cluster. Analysis of the low-pressure CO2
In contrast, adsorption of organic molecules at ordinary
conditions induces structure expansion as well with similar
results as the TPP system. When d-POS-2a’ was exposed to
anisole vapor at 258C for 3 h, the PXRD pattern of d-POS-
2a’ was dynamically changed (Figure 3c). After the expo-
sure, the two peaks at 6.5 and 6.78 disappear and one sharp
peak recovers at 6.08 in the PXRD pattern. This PXRD pat-
tern is in good agreement with the PXRD pattern of d-
POS-2a·Anis. In addition, the TGA data exhibits a weight
loss of 13.2% up to 2008C, which is also almost equal to
that of d-POS-2a·Anis (Figure S1c, the Supporting Informa-
tion). These results suggest that d-POS-2a’ swells its struc-
ture to cause reformation of the structure d-POS-2a by
guest adsorption. This reversible transformation is also con-
firmed by the FT-IR measurements (Figure S2c, the Sup-
Figure 4. Schematic representation of the “breathing” behavior of d-
POS-2a. a) Guest desorption induces shrinkage of the structure and
cleavage of the complementary hydrogen bonds to transform the expand-
ed structure d-POS-2a into the shrunken structure d-POS-2a’. b) Guest
adsorption induces the expansion of the structure and reformation of the
complementary hydrogen bonds, resulting in transformation from d-POS-
2a’ into d-POS-2a.
porting Information). After guest adsorption, the band of
À1
À
O H stretching appears at 3306 cm , indicating reformation
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of the complementary neutral hydrogen bonds concomitant
with structure swelling (Figure 4b).
companying structure swelling is potentially attractive in
terms of not only molecular separation but also molecular
recognizable properties such as chemical sensing.
Surprisingly, the structure d-POS-2a’ is selective in the
size of molecules that it adsorbs to swell the structure
(Table S3, the Supporting Information); it adsorbs less-
bulky molecules such as benzene and toluene with the con-
comitant structural swelling at 258C for 3 h, however, it
does not adsorb comparatively bulkier molecules, as repre-
sented by 1,3,5-trimethylbenzene. It is particularly noted
that the structure shows unusual adsorption behaviors for
xylene isomers. In general, rigid porous structures preferen-
tially adsorb p-xylene, which has the smallest kinetic diame-
ter of 6.7 ꢁ. However, the flexible structure d-POS-2a’ pre-
fers to adsorb o-xylene, which has a kinetic diameter of
7.4 ꢁ, rather than m- or p-xylene. These unique behaviors
should reflect different interactions between the host frame-
work and xylene isomers, which are also observed in flexible
MOFs.[20] The adjacent substitution position of methyl
groups in o-xylene should provide effective p–p and CH–p
interactions between o-xylene and the channel wall covered
with aromatic rings to facilitate the accommodation for ex-
panding the structure. These results indicate that d-POS-2a’
is capable of acting like affinity molecular sieves and col-
umns. The selective adsorption of organic molecules with ac-
Effect of the complementary hydrogen bonds on reversible
structure transformation: To clarify the effect of the comple-
mentary neutral hydrogen bonds on reversible transforma-
tion of the structure, a comparative study was performed
with an organic salt 2b composed of naphthalene-1-sulfonic
acid (1-NS) and TPMA (Scheme 1). 1-NS has a molecular
configuration similar to that of HQS, except that there is no
functional group to form the complementary hydrogen
bonds. The organic salt 2b yielded block crystals by recrys-
tallization from ethanol mixed with anisole. According to
single-crystal X-ray crystallographic analysis, 1-NS and
TPMA provide a porous organic salt (d-POS-2b) including
anisole, similar to d-POS-2a of HQS and TPMA (d-POS-
2b·Anis, Figure 5a and 5b).[16] 1NS and TPMA construct a
barrel-like [4+4] supramolecular cluster (major and minor
axis are 17 and 16 ꢁ, respectively) through charge-assisted
hydrogen bonds between sulfonate and ammonium groups
(Figure 5c and 5d). The clusters are accumulated through a
hierarchical process similar to that for d-POS-2a to con-
struct a 3-fold host framework with 1D open void-space in-
Figure 5. Structure of a porous organic salt comprising an organic salt 2b including anisole (d-POS-2b·Anis). a) View along the c axis. Guest molecules
are omitted for clarity. Hydrogen atoms are also omitted for clarity except for the space-filling model of a supramolecular cluster. b) Visualization of 1D
void-space. Guest molecules in the light-gray void-space are represented by gray balls and sticks. Hydrogen atoms of the guest molecules are omitted for
clarity. c) Hierarchical interpretation for constructing the host framework. In the diamondoid network and the 3-fold host framework, triphenylmethyl
groups are omitted for clarity. The independent diamondoid networks are indicated by deep gray, light gray, and gray. d) Cubic-like hydrogen-bond net-
work in the cluster constructed by 1-NS and TPMA. e) Close up view of the naphthyl groups between the clusters.
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N. Tohnai et al.
corporating disordered anisole molecules. The naphthyl
groups face to their edges between the clusters, leading to
minimal interactions through CH–p contacts (Figure 5e).
The void space has 28% of the volume of the whole crystal
structure with the longest width of 10.8 ꢁ and the shortest
width of 5.4 ꢁ (Figure 5b).
TGA data of d-POS-2b·Anis displays a guest release of
12.7% weight loss corresponding to three anisole molecules
per one cluster before decomposition of the components
over 2008C (Figure S4, the Supporting Information). PXRD
measurements revealed that d-POS-2b also transforms as
well as d-POS-2a by guest desorption. Figure 6a and 6b
show the PXRD patterns before and after guest desorption,
respectively. Before guest desorption, one sharp peak de-
rived from the (110) plane is observed at 5.78 (d=15.5 ꢁ).
However, this peak disappears after guest desorption where-
as other strong peaks appear at 6.78 (d=13.2 ꢁ) and 7.58
(d=11.8 ꢁ). These changes suggest that d-POS-2b causes
greater anisotropic shrinkage (approximately 15%) than d-
POS-2a to yield d-POS-2b’. In addition to this, d-POS-2b’
could not swell to revert back to d-POS-2b even after expo-
sure to anisole at 258C for 3 h (Figure 6c). Figure 6d illus-
trates the structural transformation of d-POS-2b. These re-
sults demonstrate that suitable interactions between supra-
molecular clusters are required for the reversible structure
transformation.
To estimate the advantage of the complementary neutral
hydrogen bonds with other interactions, an additional com-
parative study was performed with an organic salt 2c com-
posed of pyrene-1-sulfonic acid (1-PyS) and TPMA
(Scheme 1). 1-PyS has a larger polyaromatic pyrenyl group
compared with the quinolyl and naphthyl groups of HQS
and 1-NS. The pyrenyl groups should cause more intensive
hydrophobic effects than the other groups to stabilize
porous structures. Indeed, we previously demonstrated that
an organic salt comprising 1-PyS and TPMA yields d-POS-
2c, including 1,3,5-trimethylbenzene within 2D open void-
spaces (d-POS-2c·Mes, Figure 7a and 7b).[16] The structure
is hierarchically constructed by tetrahedral clusters of 1-PyS
and TPMA through formation of a diamondoid network as
shown in Figure 7c. First, 1-PyS and TPMA assemble into a
tetrahedral [4+4] supramolecular cluster through charge-as-
sisted hydrogen bonds (Figure 7c and 7d). The longest dis-
tance between neighboring sides for the cluster is approxi-
mately 25 ꢁ. Subsequently, the tetrahedral clusters are accu-
mulated by p–p interaction between face-to-face pyrenyl
groups at a distance of 3.40 ꢁ to form a diamondoid net-
work (Figure 7c and 7e). The cluster and diamondoid net-
work of 1-PyS have larger steric hindrance than those of
HQS. Consequently, only two independent diamondoid net-
works interpenetrate each other to yield a 2-fold host frame-
work possessing 2D open void-space (Figure 7b and 7c).
The 2D void-space consists of a main 1D channel, with the
longest width of 7.1 ꢁ and the shortest width of 5.7 ꢁ and a
small channel perpendicular to the main channel (Fig-
ure 7b). The volume of the void space is 29% (calculated
by PLATON/VOID). In the void space, two trimethylben-
zene molecules are included per cluster.
TGA data of d-POS-2c·Mes shows a gradual weight loss
of 11.1% up to the decomposition of the components over
2008C (Figure S5, the Supporting Information). The weight
loss corresponds to desorption of two trimethylbenzene mol-
ecules per one cluster. PXRD measurements indicate that
guest desorption allows for structural transformation from
d-POS-2c to d-POS-2c’, although adequate p–p and CH–p
interactions work between the clusters to construct the
porous structure. As shown in Figure 8a, one sharp peak de-
rived from the (200) plane of channel distance is observed
at 6.68 (d=13.4 ꢁ) in the PXRD pattern before guest de-
sorption. After guest desorption, on the other hand, three
peaks are observed at 6.5 (d=13.6 ꢁ), 6.9 (d=12.8 ꢁ), and
7.28 (d=12.3 ꢁ) in the low 2q region, suggesting transfor-
mation of the structure (Figure 8b). Considering the peak at
6.58, which is almost equal to the position to the sharp peak
Figure 6. PXRD patterns of the crystal composed of 1-NS and TPMA:
a) Before guest desorption (d-POS-2b·Anis); b) After guest desorption
(d-POS-2b’) by heat treatment at 1408C for 1 h; and c) After exposure
to anisole vapor at 258C for 3 h. d) Schematic representation of the dy-
namic structure transformation. d-POS-2b performs irreversible shrink-
age of the structure owing to lack of the complementary hydrogen bonds
between the clusters.
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Breathing Diamondoid Porous Organic Salt
FULL PAPER
Figure 7. Structure of a diamondoid porous organic salt comprising an organic salt 2c including 1,3,5-trimethylbenzene (d-POS-2c·Mes). a) View along
the c axis. Guest molecules are omitted for clarity. Hydrogen atoms are also omitted for clarity except for the space-filling model of a supramolecular
cluster. b) Visualization of 2D void-space. Guest molecules in the light-gray-colored void-space are represented by gray balls and sticks. Hydrogen atoms
of the guest molecules are omitted for clarity. c) Hierarchical interpretation for constructing the host framework. In the diamondoid network and the 2-
fold host framework, triphenylmethyl groups are omitted for clarity. The independent diamondoid networks are indicated by light gray and gray, respec-
tively. d) The charge-assisted hydrogen bonds in the cluster constructed by 1-PyS and TPMA. e) The p-stacked arrangement of the pyrenyl groups be-
tween the clusters.
observed before guest desorption, the relative position be-
tween the clusters should be still maintained even after
guest desorption. However, this transformed structure d-
POS-2c’ could not recover to the original structure d-POS-
2c by exposure to trimethylbenzene and any other aromatic
solvents at 258C for 3 h (Figure 8c). Figure 8d shows the
schematic representation of the structure transformation of
d-POS-2c. The irreversible structure transformation arises
from following two possible reasons. One reason is that hy-
drophobic effects between the clusters do not have enough
strength to maintain and recover the void space during
guest desorption and adsorption. Another reason is that hy-
drophobic effects stabilize not only the original porous
structure (d-POS-2c) but the transformed structure (d-POS-
2c’) because of their non-directionality and non-selective ef-
fects. In contrast, complementary hydrogen bonds have
more strength, directionality, and bond specificity than hy-
drophobic effects. Therefore, only the structure d-POS-2a
applying the complementary neutral hydrogen bonds shows
the reversible structure transformation accompanied with
guest desorption and adsorption. The two comparative stud-
ies of d-POS-2b and d-POS-2c strongly emphasize that the
complementary neutral hydrogen bonds are particularly
suitable for the construction of flexible porous structures
providing reversible structure stretching in response to guest
desorption and adsorption.
Dynamic fluorescence modulation of the d-POS constructed
by HQS and TPMA: The structure d-POS-2a also changes
its fluorescence reversibly concomitant with guest desorp-
tion and adsorption. As shown in Figure 9a, d-POS-2a·Anis
exhibits broad fluorescence from 450 to 700 nm. However,
the relative fluorescent intensity in longer wavelength
region decreases upon guest desorption (Figure 9b). Subse-
quently, the relative intensity is recovered by adsorption of
anisole molecules (Figure 9c). These fluorescence modula-
tions relate to interaction changes between HQS and the
guest molecules. HQS, which consists of a hydroxyquinolyl
group and an electron-withdrawing sulfonate group, has the
potential to serve as an electron-acceptor molecule. In con-
trast, anisole is an electron-donor molecule. Thus, before
guest desorption, weak charge-transfer (CT) interactions
work between HQS and anisole molecules to emit longer
wavelength fluorescence. Meanwhile, the CT interactions do
not work after guest desorption, resulting in only shorter
wavelength fluorescence. The CT interactions affect the flu-
orescence behaviors more prominently in the case of ad-
sorption of N,N-dimethylaniline. Exposure of d-POS-2a’ to
N,N-dimethylaniline vapor at 258C gives d-POS-2a·Dimea-
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N. Tohnai et al.
Figure 9. Fluorescent spectra and photographs of the crystals composed
of HQS and TPMA. a) d-POS-2a·Anis; b) d-POS-2a’; c) d-POS-2a’ after
adsorption of anisole. d) d-POS-2a·Dimeani. Excitation wavelength for
the emission spectra and photographs of fluorescent crystals is 365 nm.
uted to the structural flexibility of d-POS-2a. Such guest-re-
sponsive fluorescent modulation is an attractive property in
terms of chemical detection by using porous structures.
Conclusion
We have successfully prepared a new d-POS from 8-hydrox-
yquinoline-5-sulfonic acid (HQS) and triphenylmethylamine
(TPMA), which exhibits reversible structural “breathing” in
response to guest desorption and adsorption. This d-POS is
constructed by the hierarchical utilization of two different
types of hydrogen bonds. In the crystal structure, strong
charge-assisted hydrogen bonds serve as a static node to
construct supramolecular clusters of HQS and TPMA as
stable building blocks, in conjunction with complementary
neutral hydrogen bonds that work as dynamic linkers to
connect the clusters, leading to the porous structure through
the formation of a diamondoid network. The dynamic link-
ing of the complementary neutral hydrogen bonds perform
reversible bond-cleavage and reformation in response to
guest desorption and adsorption. These behaviors produce
dynamic structural transformations in response to desorp-
tion and size-selective adsorption of guest molecules con-
comitant with fluorescence modulation. To the best of our
knowledge, porous structures hierarchically incorporating
different types of hydrogen bonds have not been demon-
strated. The hierarchical design applying different types of
hydrogen bonds should enable easy modification of structur-
al flexibility by tailoring of the dynamic-linking part. Thus,
we believe that these results may lead to new useful ap-
proaches for constructing flexible and restorable organic
porous structures.
Figure 8. PXRD patterns of the crystal composed of 1-PyS and TPMA:
a) Before guest (1,3,5-trimethylbenzene) desorption (d-POS-2c·Mes);
b) After guest desorption (d-POS-2c’) by heat treatment at 1458C for
1 h; and c) After adsorption of 1,3,5-trimethylbenene at 258C for 3 h.
d) Schematic representation of the dynamic structure transformation.
Non-directional hydrophobic effects stabilize not only the original struc-
ture d-POS-2c but also the transformed structure d-POS-2c’ to provide
the irreversible structure transformation.
ni. The PXRD pattern of d-POS-2a·Dimeani is in good
agreement with that of d-POS-2a·Anis (Figure S6, the Sup-
porting Information). TGA of d-POS-2a·Dimeani shows
weight loss a 16.1% up to 2008C, which corresponds to
three N,N-dimethylaniline molecules per cluster (Figure S7,
the Supporting Information). By the adsorption of N,N-di-
methylaniline, the relative fluorescent intensity in the longer
wavelength region increases more than those of d-POS-
2a·Anis and d-POS-2a’ (Figure 9d). N,N-dimethylaniline is
one of the most electron-rich species and is often used as a
donor in charge- and electron-transfer complexes.[21] There-
fore, this remarkable enhancement of the fluorescent inten-
sity is attributed to strong charge-transfer interactions be-
tween the host framework and the guest molecules. These
reversible desorption and adsorption of guest molecules
concomitant with fluorescent modulations should be attrib-
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Breathing Diamondoid Porous Organic Salt
FULL PAPER
and Grant-in-Aids for Scientific Research on Innovative Areas “Coordi-
nation Programming” (area 2107, No. 24108723) from the Ministry of Ed-
ucation, Culture, Sports, Science and Technology (MEXT), Japan. The
synchrotron radiation experiments were performed at BL38B1 in the
Spring-8 with the approval of the Japan Synchrotron Radiation Research
Institute (JASRI) (Proposal No. 2007B1988, 2007B2005, 2007B2039,
2008A1422, and 2012A1580).
Experimental Section
General methods: All chemicals and solvents were commercially availa-
ble and used without any purification. The potassium salt of pyrene-1-sul-
fonate was synthesized according to reference to the published proce-
dure.[22] The salt was converted to the acid form, 1-PyS, by passing its
aqueous solution through an ion-exchange column filled with an Amber-
lite IR 120B NA resin (ORGANO). The organic salts 2a–c comprising
sulfonic acid derivatives and triphenylmethylamine (TPMA) were pre-
pared by mixing of sulfonic acid derivatives and TPMA in a 1:1 molar
ratio in methanol. The solution was evaporated under reduced pressure
to obtain crude salts of 2a-c.
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Preparation of single crystals: The crude salt 2a was dissolved in ethanol
and then anisole was added to the solution. Slow evaporation of the sol-
vent at 408C gave single crystals of d-POS-2a. The crude salt 2b was dis-
solved in ethanol and then anisole was added to the solution. Slow evap-
oration of the solvent at room temperature gave single crystals of d-
POS-2b. Crystals of d-POS-2c were also obtained by slow evaporation
of the solvent after dissolution of the crude salt 2c to the ethanol mixed
with 1,3,5-trimethylbenzene at room temperature. Single crystals were
picked from the solution by filtration and used for the measurements.
Crystallographic analysis of single crystals: X-ray diffraction data were
collected on a Rigaku R-AXIS RAPID diffractometer with a 2D area
detector by using graphite-monochromatized CuKa radiation (l=
1.54178 ꢁ). Direct methods (SIR-2004 and SHELXS-97) were used for
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SHELXL-97. All non-hydrogen atoms were refined with anisotropic dis-
placement parameters, and they were placed in idealized positions and
refined as rigid atoms with the relative isotropic displacement parame-
ters. The data were also refined by using the SQUEEZE routine function
in PLATON to solve the structures without the influence of the disor-
dered guest molecules.
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Powder X-ray diffraction analysis: Powder X-ray diffraction (PXRD)
patterns were measured by using Rigaku Ultima IV by using graphite-
monochromatized CuKa radiation (l=1.54178 ꢁ) at room temperature.
The cell parameters of the crystalline powder of d-POS-2a’ were deter-
mined by N-TREOR, an indexing program equipped in PDXL software
from Rigaku Corporation.
Fluorescence spectroscopy measurements: Fluorescence spectra were
measured on a FP-6500 spectrofluorometer (Jasco). The samples for the
measurements in the solid-state were encapsulated in a quartz cell (30ꢂ
30ꢂ0.3 mm) conditions. The excitation wavelength was 365 nm.
FT-IR measurements: FT-IR spectra of the crystalline powders composed
of HQS and TPMA were recorded by ATR (attenuated total reflectance)
technique using a JASCO FT/IR-4200 spectrometer.
Thermoanalysis measurements: Thermogravimetric (TG) analysis was
performed on Rigaku Thermoplus TG8120 with N2 gas flow
(100 mLminÀ1) from 30 to 2008C at a heating rate of 58C minÀ1
.
Molecular graphics and calculations: The inclusion space volumes filled
by the solvent molecules in the crystal structures were calculated from
the atomic coordinates by using the PLATON program. The atomic radii
were adopted as follows: H 1.20, C 1.70, O1.60, N 1.65, and S 1.80. The
probe radius was fixed at 0.7.
Gas adsorption measurements: Gas adsorption measurements were per-
formed on BELSORP-max from BEL, Japan. The adsorption isotherms
for N2 and O2 were corrected at 77 K. The adsorption isotherm for CO2
was corrected at 194 K. Before all measurement, the sample was dried
under reduced pressure at 363 K for 2 h.
Acknowledgements
This work was financially supported by the Research fellowships of the
Japan Society for the Promotion of Science (JSPS) for Young Scientists
Chem. Eur. J. 2013, 19, 3006 – 3016
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N. Tohnai et al.
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Received: August 20, 2012
Revised: November 21, 2012
Published online: January 10, 2013
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