Versatile Hydrogen-Bonded Assemblies of Twisted Tetra[3,4]thienylene Tetracarboxylic Acid with Selective Solvent Sorption

Article abstract of DOI:10.1021/acs.cgd.9b00664

We demonstrated versatile hydrogen-bonded assemblies of a twisted πsystem with newly synthesized tetra[3,4]thienylene tetracarboxylic acid ThTCA. Due to its twisted saddle-shaped central πaromatic unit with an anisotropic arrangement of carboxyl units, a variety of dimensional solvate molecular crystals could be obtained, in contrast to typical aromatic carboxylic acid derivatives with low-dimensional assemblies. The versatile molecular assemblies could be converged to ThTCA_desol by desolvation with a simple annealing technique. ThTCA_desol showed selective solvent sorption behavior, which was governed by the solvent association energy and saturated vapor pressure. Especially, ThTCA_desol showed the selective sorption of EtOH, among primary alcohols.

Full text of DOI:10.1021/acs.cgd.9b00664

Article
pubs.acs.org/crystal
Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX
Versatile Hydrogen-Bonded Assemblies of Twisted
Tetra[3,4]thienylene Tetracarboxylic Acid with Selective Solvent
Sorption
Takashi Takeda,*,†,‡ Masataka Ozawa,‡ and Tomoyuki Akutagawa†,‡
†Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Miyagi 980-8577, Japan
‡Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Sendai, Miyagi 980-8579, Japan
S
* Supporting Information
ABSTRACT: We demonstrated versatile hydrogen-bonded
assemblies of a twisted π system with newly synthesized
tetra[3,4]thienylene tetracarboxylic acid ThTCA. Due to its
twisted saddle-shaped central π aromatic unit with an
anisotropic arrangement of carboxyl units, a variety of
dimensional solvate molecular crystals could be obtained, in
contrast to typical aromatic carboxylic acid derivatives with low-
dimensional assemblies. The versatile molecular assemblies
could be converged to ThTCAdesol by desolvation with a simple
annealing technique. ThTCAdesol showed selective solvent
sorption behavior, which was governed by the solvent
association energy and saturated vapor pressure. Especially, ThTCAdesol showed the selective sorption of EtOH, among
primary alcohols.
structures based on COT derivatives would be an interesting
research topic; they could form a variety of supramolecular
assemblies depending on intermolecular hydrogen bonding
between the host and guest.
INTRODUCTION
■
Hydrogen-bonding interaction plays a significant role in
biochemistry and supramolecular chemistry.1 Due to its strong
bonding energy and high directivity, it serves to determine the
higher-order structures of proteins to develop their function2
and enables the design of unique artificial supramolecular
structures.3 Hydrogen-bonding interaction also contributes to
the molecular assembly of organic molecules. The molecular
assembly of hydrogen-bonded organic molecules has been
investigated. Especially, benzene carboxylic acid derivatives have
been used for this purpose because of their rigid molecular
frameworks and easy derivatization. Depending on the direction
of hydrogen bonding, they form well-defined versatile molecular
assemblies, including rod,4 ring,5 and lamellar6 structures. If we
consider their planar molecular structures and the coplanar
connection of a carboxylic acid dimer to a central planar π unit,
they preferentially form low-dimensional molecular assemblies,
and it is difficult to use them to construct three-dimensional
supramolecular architectures (Figure 1). The connection of
twisted π units via a carboxylic acid dimer could lead to unique
spatial molecular assemblies with unique functions.7,8
While π extended COT derivatives have been synthesized, the
introduction of hydrogen-bonding carboxylic acid units to the
parent COT is difficult. Tetra[3,4]thienylene12,13 is a COT
derivative that consists of four thiophene units fused in the COT
π core. Thanks to its high molecular symmetricity and the
reactivity of the α position of the thiophene subunit, symmetric
functionalization is possible, which would enable us to connect
tub-shaped π units to construct spatially unique molecular
assemblies (Figure 1). Herein, we report hydrogen-bonded
crystal structures of tetra[3,4]thienylene tetracarboxylic acid
(ThTCA, Figure 1). Recrystallization of ThTCA afforded
versatile crystal structures, which were governed by intermo-
lecular hydrogen-bonding interactions between the hosts and
those between the host and guest solvents. The solvents could be
removed by simple annealing to give identical desolvated
crystals, which exhibited reversible and selective sorption of
EtOH.
While most cyclic π conjugated compounds form a planar
molecular geometry due to their aromatic character, there are
several exceptions. For example, cyclooctatetraene (COT)
forms a tub-shaped structure due to the nonaromatic nature of
its 8π electronic system.9 The fundamental nature of COT,
including its aromaticity, structure, and flipping, has been
investigated.10,11 If we consider the three-dimensional structure
and flexibility of the COT core, hydrogen-bonded network
EXPERIMENTAL SECTION
■
General Methods. Commercially available reagents and solvents
were used as received. Tetra[3,4]thienylene 1 was prepared from 3,4-
Received: May 22, 2019
Revised: June 25, 2019
Published: June 25, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.cgd.9b00664
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Article
Figure 1. (a) Hydrogen-bonding assemblies of benzene carboxylic acids and ThTCA. (b) Structural features of ThTCA.
dibromothiophene as reported in the literature.14 1H (400 MHz) NMR
spectra were recorded on a Bruker Avance III 400 NMR spectrometer.
Chemical shifts (δ) are expressed in ppm relative to tetramethylsilane
(1H 0.00 ppm) or residual nondeuterated solvent (DMSO-d6; 13C 39.5
ppm) as an internal standard. Mass spectra were recorded on a JMS-700
spectrometer at the NMR and MS Laboratory, Graduate School of
Agriculture, Tohoku University. Elemental analyses were performed on
a Microcoder JM10 at the Elementary Analysis Laboratory, Institute of
Multidisciplinary Research for Advanced Materials, Tohoku University.
IR spectra were measured on a Thermo Scientific NICOLET 6700 FT-
IR spectrometer. Thermogravimetric analysis was performed on a
Rigaku Thermoplus EVO2 at a scan rate of 10 K min−1. Variable-
temperature X-ray diffraction (VT-XRD) was conducted on a Rigaku
RAPID-II diffractometer with a rotating anode fitted with a multilayer
confocal optic using Cu−Kα (λ = 1.54187 Å) radiation.
measured at 298 K using an automatic volumetric adsorption apparatus
(BELSORP-Max II, MicrotracBEL Corp.). Before the measurements,
the crystals were maintained at 473 K under high vacuum conditions for
3 h to remove adsorbed molecules.
RESULTS AND DISCUSSION
■
The preparation of ThTCA is summarized in Scheme 1. Since
selective 4-fold lithiation of tetra[3,4]thienylene was reported by
Wang et al.,15 we used this method to prepare ThTCA. Reaction
with n-BuLi afforded the tetraanion, which was treated with dry
ice to give the corresponding tetracarboxylic acid ThTCA.
Surprisingly, this nucleophilic reaction did not proceed with 1-
bromodecane; only starting tetra[3,4]thienylene was recovered.
This is probably due to the severe steric congestion around the
bay region. The photophysical properties of ThTCA were quite
similar to those of tetra[3,4]thienylene (Figure S1 in the
Supporting Information).
Preparation of ThTCA. To a solution of tetra[3,4]thienylene14
(101 mg, 307 μmol) in dry THF (10 mL) was slowly added n-BuLi in
hexane (1.60 M, 1.15 mL, 1.82 mmol) at −78 °C. After being stirred for
1 h at this temperature, the mixture was warmed to 60 °C. After being
stirred at this temperature for 2 h, the mixture was cooled to −78 °C,
and then, dry ice (excess) was added in one portion. The mixture was
gradually warmed to room temperature and stirred overnight. 1.0 M
aqueous HCl solution (3 mL) was added, and the resulting precipitate
was collected by filtration and washed with THF to give ThTCA (106
mg, 68%) as a yellow solid.
Fortunately, we could obtain a series of single crystals of
ThTCA by recrystallization from several solvents with hydro-
gen-bonding ability. Figure 2 shows the molecular structure and
hydrogen-bonded structures of ThTCA observed in the
obtained solvated crystals. All of the obtained crystals contained
solvate molecules, which formed hydrogen bonds between
ThTCA and contributed to a close-packing arrangement. All of
the terminal ThTCA carboxyl groups formed hydrogen bonds
with carboxyl groups of neighboring molecules or guest solvent
molecules. In contrast to typical planar aromatic carboxylic acid
derivatives, such as benzoic acid, terephthalic acid, and trimesic
acid, ThTCA did not form an unsolvated crystal, which could be
accounted for by the highly twisted molecular structure of
ThTCA. ThTCA formed a highly twisted saddle-shaped
structure, as demonstrated by the large dihedral angle of the
central COT ring (46.4−51.3°). The dihedral angles showed
good consistency with the theoretically optimized structure
(49.3°). Such a twisted molecular structure made it impossible
to form energetically stable nonsolvated single crystals with both
Mp > 300 °C; 1H NMR (400 MHz, DMSO-d6) δ 12.8 (br s, 4H),
7.76 (s, 4H); 13C NMR (CDCl3) δ 162.5, 138.4, 137.6, 132.3, 129.2; IR
(KBr) 3440, 3092, 1674, 1416, 1251, 784 cm−1; HR-MS(FAB) calcd
for C20H9O8S4 [(M + H)+] 504.9180, found 504.9178; Anal. Calcd for
C20H8O8S4 + 5H2O: C, 40.40; H, 3.05; N, 0.00. Found: C, 40.49; H,
3.20; N, 0.00.
Single-Crystal Structure Analyses. Solvate single crystals of
ThTCA suitable for X-ray structural analyses were obtained by
recrystallization from solvents, as summarized in Table 1. Structural
refinements were carried out using the full-matrix least-squares method
on F2. Calculations were performed using the Crystal Structure and
SHELEX software packages. Parameters were refined using anisotropic
temperature factors except for the hydrogen atoms.
Adsorption−Desorption Isotherm Experiment. The adsorp-
tion isotherms of desolvated crystals of ThTCA for solvents were
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DOI: 10.1021/acs.cgd.9b00664
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Article
Table 1. Crystal Parameters of the Solvated Crystals of ThTCA
solvent
H2O
methanol
ethanol
2-propanol
formula
C20H8O8S4·5H2O
colorless, platelet
594.59
−173
tetragonal
C20H8O8S4·nCH4O
colorless, platelet
504.52 + n*44.01
−173
triclinic
̅
P1(#2)
9.4768(2)
11.9080(2)
15.0858(3)
86.6431(13)
79.1888(14)
76.4635(13)
1625.57(6)
2
1.031
29.669
18487
5832
0.0755
0.0934
0.2061
1.059
C20H8O8S4·4C2H6O
colorless, prism
688.79
C20H8O8S4·4C3H8O
colorless, platelet
744.90
−173
monoclinic
color, habit
formula weight
temperature, °C
crystal system
space group
a, Å
b, Å
c, Å
α, deg
β, deg
−173
monoclinic
P21/n(#14)
18.1324(3)
8.82435(16)
41.1816(8)
I41/a(#88)
9.56536(17)
C2/c(#15)
14.2289(11)
11.5757(10)
23.0807(19)
52.8198(10)
100.9136(11)
105.921(3)
γ, deg
V, Å3
4832.81(15)
8
1.634
42.458
27629
2227
0.0396
0.0427
0.1187
1.086
6470.1(2)
8
1.414
32.237
68614
11835
0.0629
0.0845
0.1838
1.021
3655.8(5)
4
1.353
28.926
20504
3333
0.0872
0.1222
0.2787
1.141
Z
D
calc, g·cm−1
μ, cm−1
reflections measured
independent reflection
R1(>2σ)
R1(all)
wR2(all)
GOF
CCDC No.
solvent
1917324
1917327
1917332
AcOEt
1917333
1,4-dioxane
THF
formula
color, habit
formula weight
temperature, °C
crystal system
space group
a, Å
C20H8O8S4·4C4H8O
colorless, block
792.94
−173
monoclinic
C20H8O8S4·2C4H8O2
colorless, block
680.73
C20H8O8S4·3C4H8O2
colorless, needle
768.84
−148
−173
monoclinic
C2/c(#15)
12.7907(4)
12.8168(3)
19.3205(6)
monoclinic
P21/c(#14)
14.9684(3)
15.1218(3)
16.0335(3)
C2/c(#15)
12.1620(10)
13.3051(12)
23.2758(17)
b, Å
c, Å
α, deg
β, deg
105.805(5)
90.147(2)
110.0435(7)
γ, deg
V, Å3
3624.0(5)
4
1.453
29.577
19847
3314
3167.31
4
1.427
32.92
17623
2854
0.0748
0.0994
0.2222
1.101
3409.38
4
1.498
31.727
39316
6218
0.0561
0.0719
0.1676
1.190
Z
D
calc, g·cm−1
μ, cm−1
reflections measured
independent reflection
R1(>2σ)
R1(all)
wR2(all)
0.071
0.0903
0.2179
1.105
GOF
CCDC No.
solvent
1917357
DMSO
1917363
2-picoline
1917367
quinoline
formula
C20H8O8S4·4C2H6SO
colorless, plate
817.03
−173
triclinic
C20H8O8S4·4C6H7N·nH2O
white, platelet
877.03 + 18.01n
−173
C20H8O8S4·5C9H7N
colorless, needle
1150.32
−173
triclinic
color, habit
formula weight
temperature, °C
crystal system
space group
a, Å
b, Å
c, Å
α, deg
β, deg
triclinic
P1
̅
(#2)
P1
̅
(#2)
̅
P1(#2)
9.42829(17)
13.1189(2)
15.8463(3)
112.6640(7)
90.4716(9)
95.8238(9)
10.8743(2)
15.6692(3)
16.4644(3)
62.7349(10)
73.9536(11)
76.1621(7)
13.0729(2)
14.6614(3)
17.4177(3)
64.4728(9)
66.3112(10)
67.4175(11)
γ, deg
C
DOI: 10.1021/acs.cgd.9b00664
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Crystal Growth & Design
Table 1. continued
solvent
Article
DMSO
1796.93(6)
2-picoline
2375.66(8)
quinoline
2667.33(9)
V, Å3
Z
D
2
2
2
calc, g·cm−1
1.510
51.130
20498
6434
0.044
0.0514
0.1438
1.065
1917368
1.226
22.717
21501
8534
0.0478
0.0665
0.1305
1.124
1917374
1.432
21.782
30550
9569
0.0659
0.0927
0.1802
1.101
1917377
μ, cm−1
reflections measured
independent reflection
R1(>2σ)
R1(all)
wR2(all)
GOF
CCDC No.
significant effect of molecular shape on the molecular assemblies
of ThTCA in the crystal. While all three solvent molecules are
protic with both hydrogen-donating and hydrogen-accepting
ability, the molecular assemblies with ThTCA were completely
different from each other. All four carboxyl groups of ThTCA
formed hydrogen bonds with H2O molecules in ThTCA·
(H2O)5 (Figure 4). The involved H2O molecules connected
ThTCA molecules by two-way hydrogen bonding (Owater···
HOCO and Owater···OCOH, Figure 4b). As a result, a
zigzag molecular arrangement of ThTCA and water molecules
with alternating layered assemblies was formed with a layer
distance of 13.20 Å (Figure 4c). The torsion angle of ThTCA in
the crystal (46.3−46.5°) is slightly different from those in other
solvated crystals (47.1−51.3°) as well as that of the optimized
structure (49.3°), which could be accounted for by dense
hydrogen bonding with solvent H2O molecules.
In contrast to ThTCA·(H2O)5, intermolecular hydrogen
bonding between ThTCA and between ThTCA and EtOH
coexisted in the crystal of ThTCA·(EtOH)4 (Figure 5). One of
four carboxyl groups of ThTCA formed intermolecular
hydrogen bonds between carboxyl groups to form the
fundamental ThTCA dimer structure. The other three carboxyl
groups of ThTCA formed hydrogen bonds with EtOH
molecules. The EtOH molecules connected to the dimer via
Scheme 1. Preparation of ThTCA
complete intermolecular hydrogen bonding between terminal
carboxylic acid units and closest-packing arrangement.
To achieve a close-packing arrangement and the full
hydrogen-bonding interaction of carboxyl groups of ThTCA,
the solvate crystals gave versatile assemblies governed by shapes
and sizes of solvent molecules, as summarized in Figure 3. The
molecular assemblies of ThTCA in the crystals include two-
dimensional and three-dimensional hydrogen bonding between
ThTCA carboxyl groups to form a channel structure, hydrogen
bonding between ThTCA via solvate molecules to form an
alternating layer structure, and hydrogen bonding between four
carboxyl groups with solvate molecules to form an alternating
staggered structure, a one-dimensional layer, and zigzag
structures (Figure 3).
Figures 4−6 show crystal structures of ThTCA·(H2O)5,
ThTCA·(EtOH)4, and ThTCA·(2-PrOH)4 to discuss the
Figure 2. Molecular structure and hydrogen bonding of ThTCA in the crystal. (a) ORTEP drawing of ThTCA; (b) hydrogen bonding of the carboxyl
group of ThTCA; (c) plot of the relative energy of ThTCA and the structural isomer X with and without terminal carboxyl groups (1 and Y) against the
bend angle θ.
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Figure 3. Versatile molecular assemblies of highly twisted ThTCA crystals governed by solvated molecules. Blue lines indicate intermolecular
interaction between ThTCA molecules. (a, b) Continuous three-dimensional intermolecular carboxyl hydrogen bonding to form a channel structure
[ThTCA·(MeOH)n, ThTCA·(AcOEt)2]. (c) ThTCA dimer formation to form a channel structure [ThTCA·(EtOH)4]. (d) Linear layer structure
[ThTCA·(H2O)5] and (e) staggered layer structure [ThTCA·(2-PrOH)4]. (f) One-dimensional layer [ThTCA·(DMSO)4] (top) and zigzag
structure [ThTCA·(quinoline)5] (bottom).
Figure 4. Crystal structure of ThTCA·(H2O)5. (a) Photograph of single crystal mounted on litholoops. (b) Two-way hydrogen bonding of H2O
molecule with ThTCA to form a significant dimer structure. Packing arrangement viewed from the (c) ac plane and (d) bc plane. The interlayer
distance was 13.20 Å with a molecular length of 9.94 Å along the c axis.
additional EtOH molecules or directly. As a result, two different
spaces to accommodate EtOH molecules were formed. Half of
the EtOH molecules were located in the channel formed by two
ThTCA dimers (site A, Figure 5b,c). The two ThTCA dimers
formed a rectangular space to accommodate four EtOH
molecules. The ThTCA dimers layered along the b axis, which
resulted in a one-dimensional channel of 7.9 × 6.3 Å2. Other
EtOH molecules were located between the channel structures to
form a narrow two-dimensional layer structure (site B, Figure
5b,d). While a similar molecular assembly was obtained with
MeOH as a solvent molecule, the obtained crystal was quite
unstable and efflorescent under ambient conditions and solvent
MeOH molecules could not be identified in the crystal structure
analysis due to severe disorder (Figure S2). These results clearly
indicate that the size of the guest molecule significantly affected
the stability and assembly of the crystal of ThTCA.
(EtOH)4 with ThTCA·(2-PrOH)4. This channel structure was
drastically changed by replacement of EtOH with 2-PrOH,
which has only one additional carbon atom. Recrystallization of
ThTCA from 2-PrOH afforded ThTCA·(2-PrOH)4. All of the
carboxyl groups of ThTCA formed hydrogen bonds with 2-
PrOH (Figure 6a), in contrast to the results for ThTCA·
(EtOH)4. As a result, the ThTCA framework in the crystal did
not form a channel structure. Instead, staggered assemblies were
constructed in the (1 1 0) plane with a one-dimensional channel
of 2-PrOH (Figure 6b,c). 2-PrOH molecules formed two-
dimensional hydrogen bonds between two ThTCA molecules to
stabilize this molecular assembly.
As discussed above, the molecular assemblies of ThTCA were
drastically influenced by the molecular size and structure. The
detailed molecular assemblies of ThTCA with other solvent
molecules, including AcOEt, THF, dioxane, DMSO, and
quinoline, are discussed in the Supporting Information, which
led to a similar conclusion. It is interesting to compare the
The significant size effect on the molecular assembly of
ThTCA could also be confirmed by comparison of ThTCA·
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DOI: 10.1021/acs.cgd.9b00664
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Article
Figure 5. Crystal structure of ThTCA·(EtOH)4. (a) Hydrogen bonding between two ThTCA molecules to form a fundamental dimer structure. (b)
Packing arrangement viewed from the ac plane. Light blue and violet molecules indicate EtOH molecules in site A and site B, respectively. Hydrogen
bonding arrangement of EtOH in (c) site A and (d) site B. One of eight independent EtOH molecules had disordered structure.
Figure 6. Crystal structure of ThTCA·(2-PrOH)4. (a) Hydrogen bonding of ThTCA with 2-PrOH. (b) Alternating staggered assemblies of ThTCAs
and solvated molecules. (c) Two-dimensional hydrogen bonding between ThTCA and 2-PrOH.
Figure 7. (a) TG analyses of host−guest molecular crystals of ThTCA·(H2O)5, ThTCA·(EtOH)4, and ThTCA·(2-PrOH)4. Quantitative desorption
of the solvents was observed during heating. (b) Powder XRD pattern of ThTCAdesol prepared from ThTCA·(H2O)5, ThTCA·(EtOH)4, and ThTCA·
(2-PrOH)4. Similar diffraction patterns were obtained. The TG curves and powder XRD patterns after annealing for other solvate host−guest crystals
can be found in the Supporting Information.
versatile crystal structures of ThTCA to those of tetra[2,3]-
thienylene tetracarboxylic acid (X, Figure 2c), which is a
structural isomer of ThTCA that forms a uniform hydrogen-
bonded diamond structure with 6-fold interpenetration
assembly to form hydrogen-bonded organic frameworks with a
variety of solvents.16 The torsion angle of the central COT ring
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DOI: 10.1021/acs.cgd.9b00664
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Article
of X (32.1−34.4°) is smaller than that in the optimized structure
(40.6°). This difference could be understood in terms of the
different substitution fashion of terminal carboxyl units. Figure
2c shows the theoretically calculated relative energies of ThTCA
and X and those without terminal carboxyl units (1 and Y)
against the bend angle θ. As demonstrated for 1 and Y without
terminal carboxyl groups, the tetrathienylene π unit is
energetically flexible; only a few kcal/mol are necessary to
change the COT bend angle 10° from the optimized structure.
While this flexible structural feature is maintained in X with
terminal carboxyl units, the flexibility of the central tetra[3,4]-
thienylene π unit significantly decreases with the introduction of
carboxyl groups (ThTCA), which is due to the steric repulsion
of carboxyl groups. Due to the less symmetric arrangement of
terminal carboxyl groups and the higher energy required to
modulate the molecular structure of the central twisted π unit,
ThTCA forms versatile solvated molecular assemblies with
hydrogen-bonding interactions between guest solvent mole-
cules.
Table 2. Adsorption of the Solvents to ThTCAdesol
single
single
a
solvent
adsorption
crystal
solvent
adsorption
crystal
H2O
MeOH
EtOH
1-PrOH
2-PrOH
1-BuOH
t-BuOH
benzene
×
×
○
×
○
×
×
×
○
○
○
×
○
×
hexane
THF
×
○
○
○
○
×
×
×
e
○
○
○
b
c
dioxane
DMSO
pyridine
quinoline
CH3CN
AcOEt
d
e
e
f
△
○
×
○
×
×
×
a
b
Availability of single crystal by recrystallization. 2 equiv of solvent
molecules were adsorbed overnight, followed by an additional 2 equiv
c
of solvent molecules. 2.5 equiv of solvent molecules were adsorbed.
d
e
2 equiv of solvent molecules were adsorbed. The same amount of
solvent molecules was adsorbed as in the corresponding solvate single
f
crystals. Poor crystallinity.
hand, solvents from which single crystals of ThTCA did not
form were not adsorbed to ThTCAdesol crystal.
Encouraged by the versatile solvated assemblies of ThTCA
and the flexible twisted ThTCA π unit, we performed desorption
and readsorption17,18 of solvent molecules with ThTCA solvate
crystals. The thermogravimetry (TG) curve of ThTCA revealed
the quantitative desorption of solvate molecules before
decomposition (>340 °C) from all of the obtained solvate
crystals (Figure 7a and Figure S8). For example, weight losses of
ca. 15 and 25% were observed at 120 °C for ThTCA·(H2O)5
and ThTCA·(EtOH)4, respectively, which correspond to the
quantitative desorption of solvate molecules. The plateaus were
maintained up to 330 °C, demonstrating the high thermal
stability of the ThTCA molecule even after desorption. There
was no distinct color change during desorption of the solvent
from solvate ThTCA crystals. Unfortunately, we could not
obtain nonsolvated single crystals to discuss the molecular
assembly. Thus, we performed powder XRD measurements for
the desolvated crystals. The desolvated crystals were prepared by
annealing solvated single crystals at an appropriate temperature
(determined on the basis of the TG curve) under ambient
atmosphere. Figure 7b shows the powder XRD pattern of
desolvated crystal prepared from a series of solvated crystals.
While completely different assemblies of ThTCA were observed
in solvated crystals, the diffraction pattern of the crystals
converged to the same diffraction pattern by desolvation, which
indicates the formation of the same molecular assemblies of
ThTCA from different solvate crystals by solvent desorption.
This could be accounted for by the high bonding energy of
hydrogen bonds between carboxyl groups and the relatively easy
reorganization of the molecule due to the flexible twisted
ThTCA π core.
The possible readsorption of solvents of the desolvated crystal
ThTCA (ThTCAdesol) was systematically screened with a
variety of solvents, including solvents that afford solvated
ThTCA crystals and other solvents such as longer primary
alcohols, including 1-PrOH and 1-BuOH, tert-BuOH, and the
aprotic solvents benzene and hexane. The ThTCAdesol crystals
were exposed to vapor of the solvents in separate closet-vials.
After the samples were exposed to the vapor overnight, solvent
readsorption was confirmed by TG measurements, as
summarized in Table 2. The solvents that gave solvate single
crystals such as EtOH, 2-PrOH, THF, dioxane, and DMSO were
readsorbed to ThTCAdesol crystal, albeit there were a few
exceptions, including AcOEt, MeOH, and H2O. On the other
To reveal the detailed mechanism of the selective read-
sorption of the solvents to ThTCAdesol, we compared the
powder XRD patterns during desorption and readsorption.
Figure 8a shows powder XRD patterns of crystal ThTCA·
(EtOH)4, ThTCAdesol, and the readsorbed sample of ThTCA·
(EtOH)4. The thermal annealing of ThTCA·(EtOH)4 crystals
at 423 K revealed a shift and disappearance of Bragg diffraction
peaks, which was consistent with a change in the packing
structure from ThTCA·(EtOH)4 to ThTCAdesol while main-
taining high crystallinity. After readsorption of the EtOH
molecule, the PXRD pattern was completely consistent with that
of as-grown ThTCA·(EtOH)4, indicating a reversible and cyclic
structural transformation between desolvated ThTCA crystals
and solvated ThTCA·(EtOH)4 upon the adsorption−desorp-
tion of EtOH. A similar change in the PXRD pattern was
observed with ThTCA·(2-PrOH)4 during desorption and
readsorption (Figure S10). Figure 8b shows the adsorption−
desorption isotherms of desolvated ThTCA crystals for primary
alcohols. Gate-opening EtOH adsorption behavior was observed
at P/P0 ∼ 0.7, and the amount of saturated adsorption was
similar to 2.0 mol mol−1. This amount was inconsistent with that
determined by TG measurement, which is due to fast adsorption
to one of two sites (site A or B, Figure 5), followed by slow
adsorption to the other site (see the Supporting Information for
details). Once an EtOH molecule was adsorbed, the adsorption
state was maintained until P/P0 ∼ 0.1 in the desorption process.
This gate-opening sorption mechanism with huge hysteresis for
adsorption−desorption is characteristic of adsorption accom-
panied by a structural change. The adsorption isotherm clearly
demonstrated the selective adsorption of EtOH among the
primary alcohols.
The selective solvent sorption behavior could be understood
in terms of the stabilization energy upon solvation and the
saturated vapor pressure. Table S1 shows the calculated solvent
association energies of ThTCAdesol crystals (ΔE, per ThTCA
molecule), which were estimated by the energy differences
between the host−guest complex ThTCA·(solvent)n (EHG) and
the sum of the isolated host ThTCA (EH) and solvent (EG) {ΔE
= −[EHG − (EH + EG)]}. The association energies ΔE for EtOH
(23.9 kcal mol−1) and 2-PrOH (23.2 kcal mol−1) were more
than 2 times larger than those which were not adsorbed to
ThTCAdesol (ΔE for AcOEt: 8.12 kcal mol−1). Sorption with
solvents in which single crystals could not be prepared did not
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Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
Figure 8. EtOH sorption behavior of ThTCAdesol. (a) Change in powder XRD patterns of ThTCAdesol during readsorption of EtOH and annealing.
The red dotted line indicates the simulated diffraction pattern obtained from a single-crystal structure of ThTCA·(EtOH)4. (b) Adsorption−
desorption isotherms of ThTCAdesol for the primary alcohols of EtOH, PrOH, and BuOH.
Accession Codes
occur due to the impossibility of the formation of a molecular
assembly with close-packing arrangement and maximum
hydrogen bonding with ThTCA. A large stabilization energy
of solvation was necessary to induce a significant change in the
assembly of ThTCAdesol. While a huge stabilization energy could
be obtained with H2O and quinoline, ThTCAdesol did not
readsorb these solvents, which was due to the quite low
saturated vapor pressure at room temperature.
CCDC 1917324, 1917327, 1917332−1917333, 1917357,
1917363, 1917367−1917368, 1917374, and 1917377 contain
the supplementary crystallographic data for this paper. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.uk,
or by contacting The Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223
336033.
CONCLUSION
■
AUTHOR INFORMATION
We demonstrated versatile molecular assemblies of tetra[3,4]-
thienylene tetracarboxylic acid ThTCA. Due to its twisted
saddle-shaped central π aromatic unit with the anisotropic
arrangement of carboxyl units, a variety of dimensional solvate
molecular crystals could be obtained. The versatile molecular
assemblies could be converged to ThTCAdesol by desolvation
with simple annealing. ThTCAdesol showed selective solvent
sorption behavior, which was governed by the solvent
association energy and the saturated vapor pressure. Especially,
ThTCAdesol showed selective sorption of EtOH among primary
alcohols. Molecular assemblies of a flexible twisted π system
could lead to the development of unique functionality. Studies
from this perspective are underway in our laboratory.
■
Corresponding Author
*E-mail: takashi@tohoku.ac.jp.
ORCID
Takashi Takeda: 0000-0002-5254-2819
Tomoyuki Akutagawa: 0000-0003-3040-1078
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partly supported by JSPS KAKENHI Grant
Number JP17K05769 and the research program “Network Joint
Research Center for Materials and Devices”.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.cgd.9b00664.
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