Emission-Tunable Soft Porous Organic Crystal Based on Squaraine for Single-Crystal Analysis of Guest-Induced Gate-Opening Transformation

Article abstract of DOI:10.1021/jacs.0c12153

Soft porous crystals (SPCs) with both crystallinity and flexibility have evolved as emerging materials for lots of applications. However, the development of purely organic SPCs (SPOCs) with advanced functionalities significantly lags behind. Herein, we report the construction of an emission-tunable SPOC with a rationally designed squaraine derivative (named as SPOC-SQ). SPOC-SQ is featured with a squaraine core and four peripheries with electron donor-π-acceptor (D-π-A) characteristics, which facilitates the formation of porous crystal framework stabilized by π-πinteractions and H bonds and at the same time provides structural flexibility through phenyl rotations. This SPOC can be easily obtained from its dichloromethane (DCM) solution and exhibits reversible stimuli-responsive single-crystal-to-single-crystal (SCSC) structural transformation, accompanied by bright and tunable emission. In addition, this activated SPOC (SPOC-SQ-a) selectively recognizes and absorbs acetylene (C2H2) over other gases without destroying the single crystallinity, enabling the single-crystal XRD analysis of the structural transformation. Close inspection of single-crystal XRD results of SPOC-SQ-C2H2 facilitates the understanding of the host-guest interactions. More interestingly, upon interacting with C2H2, a one-dimensional (1D) channel is formed in the crystal to adopt C2H2, which proves the SCSC process and provides molecular-level insights into the gate-opening process. Furthermore, C2H2 adsorption dynamics can be monitored in real time by tracking the fluorescence wavelength changes of SPOC-SQ framework. Thus, the unique gate-opening sorption attribute of SPOC-SQ-a crystals toward C2H2 enables its potential applications for gas separation.

Full text of DOI:10.1021/jacs.0c12153

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Article
Emission-Tunable Soft Porous Organic Crystal Based on Squaraine
for Single-Crystal Analysis of Guest-Induced Gate-Opening
Transformation
Lin Li, Huili Ma, Jingyan Zhang, Engui Zhao, Jian Hao, Huiming Huang, Hui Li, Pengfei Li,
Xinggui Gu,* and Ben Zhong Tang
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ABSTRACT: Soft porous crystals (SPCs) with both crystallinity
and flexibility have evolved as emerging materials for lots of
applications. However, the development of purely organic SPCs
(SPOCs) with advanced functionalities significantly lags behind.
Herein, we report the construction of an emission-tunable SPOC
with a rationally designed squaraine derivative (named as SPOC-
SQ). SPOC-SQ is featured with a squaraine core and four
peripheries with electron donor−π−acceptor (D−π−A) character-
istics, which facilitates the formation of porous crystal framework
stabilized by π−π interactions and H bonds and at the same time
provides structural flexibility through phenyl rotations. This SPOC
can be easily obtained from its dichloromethane (DCM) solution
and exhibits reversible stimuli-responsive single-crystal-to-single-
crystal (SCSC) structural transformation, accompanied by bright and tunable emission. In addition, this activated SPOC (SPOC-
SQ-a) selectively recognizes and absorbs acetylene (C2H2) over other gases without destroying the single crystallinity, enabling the
single-crystal XRD analysis of the structural transformation. Close inspection of single-crystal XRD results of SPOC-SQ-C2H2
facilitates the understanding of the host−guest interactions. More interestingly, upon interacting with C2H2, a one-dimensional (1D)
channel is formed in the crystal to adopt C2H2, which proves the SCSC process and provides molecular-level insights into the gate-
opening process. Furthermore, C2H2 adsorption dynamics can be monitored in real time by tracking the fluorescence wavelength
changes of SPOC-SQ framework. Thus, the unique gate-opening sorption attribute of SPOC-SQ-a crystals toward C2H2 enables its
potential applications for gas separation.
frameworks (HOFs),15−18 or easily collapsed architectures to
INTRODUCTION
■
polycrystalline during guest-induced lattice rearrangement.19,20
Soft porous crystals (SPCs), which can undergo structural
transformations in response to external stimuli, such as light,
After extensive endeavors, several SPOCs, such as porous
organic cages (POCs)-based SPCs and flexible HOFs with
flexible framework, have been constructed in recent years,21−26
exemplifying the feasibility constructing SPOCs. Despite the
successes in constructing SPOC, it remains challenging to
unveils the host−guest interplays and transformation mecha-
nism,27−31 as large guest-loaded SPOC single crystals suitable
for single-crystal X-ray diffraction (SCXRD) analysis are rather
difficult to obtain, especially for gas guests.32−35 The difficulties
lie in the lack of well-designed organic functional molecules
temperature, pressure, and guest molecules, have attracted
considerable interest in the fields of both science and
technology.1−5 Currently, most SPCs are constructed from
flexible metal−organic frameworks (FMOFs) and employed
for extensive applications, such as gas adsorption, molecular
recognition, sensing, and catalysis.6−10 The framework
flexibility of these SPCs is mostly derived from the conforma-
tional changes of organic ligands, which enables their gate-
opening transformation in response to external stimuli.1,11−13
Because in most MOFs, strong coordination interactions are
involved, it could thus be expected that SPCs constructed from
purely organic molecules (SPOCs) may undergo more flexible
conformation changes and are more adaptive to ambient
environment changes, which may result in diverse function-
alities and applications.14 However, most reported purely
organic porous crystals are featured with either permanent
porosity, such as well-known hydrogen-bonded organic
Received: November 23, 2020
Published: March 4, 2021
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and the extreme challenges in controlling the single
crystallinity without severe fracturing during stimuli-responsive
structural transformations.19,36,37
electron donor−π−acceptor (D−π−A) characteristic. The
D−π−A structure of 4-chloro-aminophenyl linkers could not
only facilitate the strong π−π interactions between adjacent
SPOC-SQ molecules, which is essential for stabilizing the
porous crystal framework together with H bonds, but also
endow the framework with structural flexibility through phenyl
rotations. Single crystals of SPOC-SQ can be facilely obtained
from its dichloromethane (DCM) solution and exhibit
reversible SCSC structural transformation, accompanied by
highly efficient and tunable emissions. Activated SPOC-SQ
crystals can be prepared through driving out the DCM
molecules in vacuum. These activated SPOC-SQ crystals could
be cooperatively gate-opened by C2H2 molecules, undergoing
C2H2-induced reversible SCSC gate-opening/-closing behav-
iors without destroying the quality of the crystal. More
intriguingly, C2H2 molecules could be accommodated inside
the single crystal of SPOC-SQ for studying C2H2 locations and
specific C2H2-framework interactions through SCXRD, directly
promoting molecular-level insights into the C2H2-induced
gate-opening mechanism. Furthermore, SPOC-SQ crystals
experience luminescence changes upon interacting with
C2H2, which facilitates in situ monitoring of the unique gate-
opening adsorption dynamics. The unique gate-opening
sorption attribute of SPOC-SQ-a crystals toward C2H2 enables
its potential applications in gas separation.
Squaric acid architected by a highly symmetric four-
membered ring has been recently employed as an organic
linker to construct porous frameworks.38 The intrinsic oxygen
atoms in the skeleton of squaric acid can act as natural
functional sites for framework construction and selective
molecular recognition through hydrogen bonds (H bonds)
or Lewis acid/base interactions with special guest species,
without the need for postsynthetic modification.39 In addition,
a variety of squaraines (SQs) dyes derived from squaric acid
exhibit favorable photophysical properties with intense
absorption and emission in the wide range from visible to
near infrared, which are sensitive to the surrounding
environment.40,41 Recently, a new type of SQs have been
reported to exhibit unique solid-state tunable luminescence
properties, which can be modulated through control of
molecular excited-state vibration and electron behavior.42
Some of these SQs could achieve temperature-/solvent
vapor-triggered SCSC reversible structure transformation
associating with tunable luminescence behavior,42,43 implying
their structural flexibility and structure-related luminescence.
In this regard, SQs are expected to be promising candidates for
construction of functional SPOCs with diverse photophysical
properties through sensible molecular design.
RESULTS AND DISCUSSION
Herein, we report a novel SPOC prepared from a
luminescent SQ derivative (SPOC-SQ, Figure 1a). SPOC-SQ
is featured with a squaraine core and four peripheries with
■
SPOC-SQ was synthesized facilely with a high yield of around
90% (Scheme S1) and characterized by NMR spectroscopy
(Figures S1 and S2) and high-resolution mass spectrometry
(HRMS) (Figure S3), from which satisfactory results
corresponding to its molecular structure were obtained. Within
the structure of SPOC-SQ, the electron-withdrawing chloride
(Cl) was linked to the electron-donating nitrogen-derived SQ
skeleton through a phenyl ring, thus generating four symmetric
4-chloro-aminophenyl D−π−A linkers. The electrostatic
potential (ESP) of SPOC-SQ was shown in Figure 1a, and it
was found that the ESP changed from positive to negative
along the D−π−A structure, which would facilitate the strong
intermolecular D−A interactions between two antiparallel 4-
chloro-aminophenyl units in adjacent SPOC-SQ molecules to
form π−π interactions during crystallization. On the other
hand, the negative ESP of the intrinsic oxygen in SPOC-SQ
was favorable for the formation of H bond, which was critical
for framework construction and molecular recognition. Be-
sides, the single-bond linkage between the squaric derived core
and the 4-chloro-aminophenyl units can rotate in response to
external stimuli, which would bring about the flexibility of
molecular conformation. Thus, it was expected that both
crystallinity and flexibility could be realized in the SPOC-SQ
crystals for construction of SPOCs.
By slow evaporation of the DCM solution, rhombus-shaped
SPOC-SQ single crystals (SPOC-SQ-DCM) were obtained,
which were subjected to SCXRD analysis. SPOC-SQ-DCM
Figure 1. (a) Chemical structure and electrostatic potential (ESP)
diagram of SPOC-SQ. (b) 2D supramolecular network formed by
SPOC-SQ molecules and (c) the intermolecular interactions (the
strong π−π interactions (red dotted lines) and weak H bonds (blue
dotted lines) between two antiparallel 4-chloro-aminophenyl D−π−A
linkers). (d) Packing diagram and 1D pore channel of SPOC-SQ-
DCM, with the solvent-accessible void space visualized by yellow/
gray (inner/outer) curved planes generated with a probe of 1.4 Å. The
whole process is determined by SCXRD. Color code: gray, C; blue,
N; red, O; green, Cl; white, H.
̅
crystallized in triclinic system and P1 space group (Table S1)
with DCM molecules in the crystal lattice. For this SPOC-SQ-
DCM crystal, each SPOC-SQ molecule was connected with
four neighboring molecules to form a two-dimensional (2D)
supramolecular network (Figure 1b) through the strong π−π
interactions (3.360 and 3.320 Å) and weak intermolecular H
bonds (O···H−Ar: 2.496 and 2.534 Å) between two
antiparallel D−π−A-structured 4-chloro-aminophenyl linkers
as designed (Figure 1c). These interactions were further
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Figure 2. (a) Thermogravimetric analysis (TGA), (b) variable-temperature powder X-ray diffraction (PXRD), and (c) differential scanning
calorimetry (DSC) for SPOC-SQ-DCM crystals upon heating. (d) Single-crystal-to-single-crystal (SCSC) reversible structure transformation
between SPOC-SQ-DCM and SPOC-SQ-a crystals by activation and DCM fuming, with the changes of torsion angles for the peripheral phenyl
rings. The whole process is determined by SCXRD. Color code: gray, C; blue, N; red, O; green, Cl; white, H.
confirmed by density functional theory (DFT) calculation
(Figure S4). All these results revealed the synergistic effect of
π−π interactions and H bonds on stabilizing the 2D
supramolecular network. The 2D supramolecular network
stacked in a layer-by-layer fashion to form a three-dimensional
(3D) porous crystal framework (Figures 1d and S5). Within
this framework, one-dimensional (1D) irregular pore channels
with large cavities and narrow windows along the a axis (Figure
1d and Figure S6) can be discerned, where DCM molecules
were trapped in the cavity between two layers through H
bonds (Cl···H−Ar: 2.943, 2.836, 2.820, and 2.863 Å; O···H−
C: 2.538 Å) and C−H···π interactions (2.846 Å) (Figure S7).
The SPOC-SQ-DCM crystals could be activated by heating
under vacuum to obtain DCM-free SPOC-SQ-a crystals. In the
heating process, DCM molecules were released from crystal
lattices, as was confirmed by thermogravimetric analysis
(TGA) of SPOC-SQ-DCM crystals with a gradual weight
loss of about 10% from room temperature to 80 °C (Figure
2a). The activated SPOC-SQ-a crystals exhibited excellent
thermal stability with negligible weight loss even when the
temperature was raised up to about 330 °C (Figure S8).
During the release of DCM molecules, the interactions
between DCM molecules and porous crystal framework
changed and crystal structure varied, which resulted in
detectable signal changes in powder X-ray diffraction
(PXRD). As shown in Figures 2b and Figure S9, the PXRD
patterns varied when the temperature was raised from 40 to 80
about 732 and 1263 cm−1 were disappeared and decreased,
respectively, which could be assigned to DCM (Figure S11).
Meanwhile, large endothermic peak at about 73 °C was found
in differential scanning calorimetry (DSC) curve (Figure 2c),
which could be assigned to the DCM removal process.
Collectively, these results demonstrated the successful
activation of SPOC-SQ-DCM crystals by removing DCM
molecules from the porous crystal framework, and revealed the
crystal structure transformation during the activation pro-
cesses.
We also tried to grow single crystals in other common
solvents with suitable solvating power toward SPOC-SQ
(Table S2) and porous crystals were obtained in chloroform
(CHCl3), acetonitrile, toluene, and dioxane, with solvent
molecules trapped inside the porous channels (Table S3,
Figures S12−S15). While nonporous crystals were formed in
ethyl acetate and acetone with the solvent molecules residing
inside the cages (Figures S16 and S17). By comparison, the
preparation of porous crystals in DCM is more high-efficiency
due to the relatively large dissolved amount (4.46 mg/mL,
Table S2). However, these crystals lost their crystallinity upon
removal of solvent molecules by vacuum. This may be ascribed
to the large sizes of these solvent molecules as compared with
DCM, whose release destroyed the pores or channels.
Due to the “soft” nature of these crystals obtained in DCM,
the single crystals structures were pertained during the
activation process and remained suitable for SCXRD analysis,
which enabled the direct identification of the structural
transformation during activation by SCXRD (Table S1).
1
°C. The removal of DCM was confirmed by H NMR. The
chemical shift at 5.30 ppm corresponding to DCM disappeared
completely after activation (Figure S10), which could be
recovered upon fuming the crystals with DCM. DCM removal
was further characterized with IR spectrometry. IR peaks at
̅
SPOC-SQ-a retained the same triclinic system and P1 space
group as SPOC-SQ-DCM crystals, but the whole volume of
SPOC-SQ-a shrunk about 8.26% and the unit-cell parameters
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changed largely. In sharp contrast to SPOC-SQ-DCM crystals,
SPOC-SQ molecules in SPOC-SQ-a crystals packed more
closely and exhibited a more compact nonporous framework,
featuring porous-to-nonporous SCSC transformation process
during activation (Figures 2d and Figure S18). In SPOC-SQ-
DCM, all SPOC-SQ molecules were in the same conformation,
which was changed to four different conformations in SPOC-
SQ-a with the corresponding torsion angles of chlorophenyl
units varying from 42.72 and 43.05° in SPOC-SQ-DCM to
27.15, 59.46, 8.29, 82.20, 39.70, 37.54, 40.97, and 33.76° in
SPOC-SQ-a crystals (Figure 2d). During the activation
process, SPOC-SQ molecules adaptively adjusted their
molecular conformation, accompanying with the destruction
and reconstruction of π−π interactions to eventually induce
the crystal lattice rearrangement from SPOC-SQ-DCM to
SPOC-SQ-a crystals. The nonporous SPOC-SQ-a crystals
could be facilely transformed to the porous SPOC-SQ-DCM
crystals by fuming the crystals with DCM vapor (Figure 2d and
Figure S19, Table S1). These crystallographic results clearly
demonstrated the reversible SCSC structural transformation
from SPOC-SQ-DCM to SPOC-SQ-a crystals, which was
facilitated by the π−π interactions and weak H bonds as well as
flexible phenyl rotations.
DCM, which enabled the direct monitoring of the structure
changes with fluorescence microscopy (Figure S21 and Movie
S1). When DCM was dropped onto SPOC-SQ-a crystal, some
solvent droplets formed and their sizes diminished quickly,
showing an interesting “inhaling” behavior. The emission color
of the crystals was simultaneously switched to greenish blue,
suggesting the structural transformation behind. To under-
stand these emission behaviors, DFT calculation was carefully
carried out. In gas phase, the transition from low-lying excited
1
state S1 to ground state was (n+σ, π*) transition, which was
electric dipole-forbidden dark state (Figure 3b and Figure S22,
Table S5). According to the Kasha rule,42 it resulted in weak or
even no emission in dilute solution. While in both SPOC-SQ-
DCM and SPOC-SQ-a crystals, the electronic transition
1
natures of the S1 states were bright (π, π*) with oscillator
strengths (f) of 0.8435 and 0.7974, respectively. Thus, the
different electronic transitions of S1 in crystals and solutions
can explain their varied fluorescence efficiencies.42 On the
other hand, the different aggregation microenvironments inside
crystals would lead to varied emission behaviors for SPOC-SQ-
DCM and SPOC-SQ-a crystals. The free volume ratio in
SPOC-SQ-DCM and SPOC-SQ-a crystals were 6.51 and
11.84%, respectively (Table S6 and Figure S23), suggesting a
more confined microenvironment in SPOC-SQ-DCM crystals.
The more confined microenvironment in SPOC-SQ-DCM
crystals will restrict the intramolecular motions of SPOC-SQ to
a larger extent and favor radiative decay with higher emission
efficiency. Moreover, the excitation energy of SPOC-SQ in
SPOC-SQ-a crystals (2.83 eV) was lower than that in SPOC-
SQ-DCM crystals (2.90 eV) (Figure 3b), which explained the
red-shifted luminescence of SPOC-SQ-a crystals as compared
with SPOC-SQ-DCM crystals. These results clearly demon-
strated the stimuli-responsive SCSC structure transformation
and unusual solid-emission-tunable property of the as-
constructed SPOC-SQ crystals.
The SPOC-SQ-DCM crystals emitted strong greenish blue
light peaked at 515 nm with an impressive efficiency of 0.96
(Figure 3a and Table S4), while the dilute DCM solution of
The intrinsic oxygen atoms in SPOC-SQ could also serve as
functional sites to form specific interactions with guest
molecules, making it promising candidate for selective gas
molecular recognition. Surprisingly, the SPOC-SQ-a crystals
could respond to and adsorb C2H2 above a critical pressure of
0.44 bar at 298 K (Figure S24). Below this threshold pressure,
almost no C2H2 was adsorbed, presenting unique gate-opening
behavior. The uptake capacity of C2H2 was determined to be
greater than 35.0 cm3/g under 1 bar (Figure 4a). During the
desorption process, a large hysteretic loop appeared, indicating
that it was difficult to release the adsorbed C2H2 in SPOC-SQ-
a crystals. Thus, it allowed for characterization of the C2H2
adsorption by the solid-state 13C NMR spectrum, in which an
obvious chemical shift of acetylenic carbon at 73.3 ppm was
observed for C2H2-adsorbed SPOC-SQ-a crystals (Figure
S25). Furthermore, C2H2 adsorption/desorption cycling
could be repeated at least five times without any significant
loss of uptake capacity under almost unchanged gate-opening
pressure at 298 K (Figure S26), apparently proving the
reversibility of C2H2-induced gate-opening sorption.
Figure 3. (a) PL spectra of SPOC-SQ crystals after vacuum activation
and then DCM fuming: SPOC-SQ-DCM, SPOC-SQ-a, and SPOC-
SQ-a upon DCM fuming. Insets show fluorescent photos were taken
under fluorescence microscopy. (b) TD-DFT theoretical calculation
of vertical excitation energies, nature of the excited states, and
oscillator strengths (f) for SPOC-SQ under different states (gas
phase, SPOC-SQ-DCM, and SPOC-SQ-a crystals).
When the temperature was lowered to 285 and 273 K, the
C2H2 sorption isotherms of SPOC-SQ-a crystals also exhibited
similar gate sorption behavior and adsorption−desorption
hysteresis with a slightly enhanced uptake of 37 g/cm3 at 1 bar
(Figure 4a). However, the threshold of gate-opening pressure
was reduced remarkably to 0.23 and 0.11 bar, respectively,
suggesting the easier C2H2-induced gate-opening sorption at
lower temperature. The relatively low gate opening enthalpy of
SPOC-SQ fluoresced weakly (Figure S20), presenting the
fascinating crystallization-induced emission enhancement
(CIEE) phenomenon.44 After removal of DCM, such emission
of the crystals was red-shifted to greenish yellow with an
emission peak at 525 nm and a relatively low emission
efficiency of 0.25 (Figure 3a and Table S4). The emission
wavelength and efficiency of the crystals could be switched
reversibly between these two states by heating and fuming with
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3.358 Å) and H bonds (O···H−Ar: 2.514 and 2.611 Å) (Figure
S28a). Four molecular conformations with distinguished
phenyl torsion angles in SPOC-SQ-a crystals were interestingly
transferred to one (Figure S29), evidently proving the vital
roles of phenyl rotations in C2H2-induced structural trans-
formation. Different from SPOC-SQ-DCM crystals, multiple H
bonds (Cl···H−Ar: 2.905 and 2.935 Å) were observed between
two adjacent 2D supramolecular layers (Figure S28b), which
stabilized the framework for crystallographic study. Further-
more, the C2H2-induced porous SPOC-SQ-C2H2 crystals
could be converted back to nonporous SPOC-SQ-a crystals
upon desorption of C2H2 molecules, which was in consistent
with the reversible sorption and PXRD results. All these results
proved that the C2H2-induced gate-opening proceeded
through reversible SCSC structure transformation.
With SCXRD results of SPOC-SQ-C2H2, the host−guest
interactions could be further understood. Although the
location and configuration of the C2H2 molecules within the
pores are disordered, two possible modes (SPOC-SQ-C2H2-I
and -II) of C2H2 molecules resided in SPOC-SQ framework
were observed (Figure 5a), where C2H2 molecules interacted
Figure 4. (a) Sorption isotherms of SPOC-SQ-a for C2H2 at 273, 285,
and 298 K. Filled and open symbols represent adsorption and
desorption, respectively. Inset shows the plot of the gate opening
pressure (ln PGO) vs temperature (1/T) for sorption isotherms of
C2H2. (b) PXRD patterns of SPOC-SQ-a crystals upon adsorption
and desorption of C2H2. (c) C2H2-induced gate-opening process with
reversible SCSC structure transformation determined by SCXRD.
The packing diagram and 1D pore channel of SPOC-SQ-C2H2 were
depicted with the gas-accessible void space visualized by yellow/gray
(inner/outer) curved planes generated with a probe of 1.4 Å. Color
code: gray, C; blue, N; red, O; green, Cl; white, H.
33.0 kJ/mol was also revealed in Figure 4a.23 Therefore, the
unique C2H2 sorption behavior of SPOC-SQ-a could be
uncovered: sufficient C2H2 coverage on the SPOC-SQ-a
crystals could cooperatively drive the gate-opening of non-
porous crystal structure, and then C2H2 would be effectively
trapped by particular C2H2-framework interactions.45 Mean-
while, these specific host−guest interactions would be stronger
at lower temperature, and fewer C2H2 molecules were required
to provide sufficient force to trigger the framework rearrange-
ment for gate opening.
To reveal the structural changes, we used PXRD to study the
SPOC-SQ-a crystals during C2H2 sorption. The PXRD
patterns obviously changed after adsorption of C2H2, and
then recovered upon desorption (Figures 4b and Figure S26c),
indicating reversible structural transformation. Fortunately, the
C2H2-adsorbed single crystal, named SPOC-SQ-C2H2, was
obtained after exposure of SPOC-SQ-a single crystal to C2H2
atmosphere for about 48 h at 1 bar and 0 °C, which prompted
us to elucidate C2H2-induced gate-opening structural tran-
sitions by single-crystal analysis. The as-prepared SPOC-SQ-
C2H2 single crystal was analyzed by SCXRD at 110 K, and
Figure 5. (a) Two modes (SPOC-SQ-C2H2-I and -II) of C2H2
molecules residing in the SPOC-SQ framework with locations and
host−guest interactions of the C2H2 molecule in a SPOC-SQ-C2H2
single crystal. (b) Packing diagram and 1D pore channel of SPOC-
SQ-C2H2 viewed along the lateral view, with the gas-accessible void
space visualized by yellow/gray (inner/outer) curved planes
generated with a probe of 1.4 Å. The C2H2 gas molecules in voids
are omitted for clarity. (c) Potential energy profile of a C2H2 molecule
passing from outside into interior region of the SPOC-SQ-C2H2 along
the central axis of the 1D channel; inset shows the profile of the 1D
channel based on the total electron density derived from the DFT
calculation. Color code: gray, C; blue, N; red, O; green, Cl; white, H.
̅
crystallized in triclinic P1 with a larger (4.94%) unit cell (a =
8.7325 Å, b = 8.8710 Å, c = 9.6585 Å, α = 94.715°, β =
115.018°, γ = 90.156°, V = 675.13 Å3, Z = 1) compared with
SPOC-SQ-a (Table S1). The framework in SPOC-SQ-C2H2
was fully opened and underwent nonporous-to-porous phase
transformation during C2H2 sorption (Figure 4c). Similar to
SPOC-SQ-DCM, the 1D porous channel was formed orderly
(Figure S27), stabilized by strong π−π interactions (3.321 and
differently with SPOC-SQ framework through specific host−
guest interactions. In SPOC-SQ-C2H2-I and SPOC-SQ-C2H2-
II modes, the acidic H in C2H2 was deduced to form H bonds
with the electron-rich oxygen sites in SPOC-SQ (O···H−C
CH: 2.477 and 2.727 Å) and interacted with aromatic rings
through C−H···π interactions (2.752 Å), respectively. Mean-
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while, the π orbitals of C2H2 were also considered to
participate in the formation of the moderate-to-strong π−π
interactions (3.239 and 3.248 Å in SPOC-SQ-C2H2-I mode)
with the π electrons of phenyl rings, and the C−H···π
interactions (2.683 and 2.690 Å in SPOC-SQ-C2H2-II mode)
with the aromatic H. The above analysis supported the
important role of host−guest interactions on dynamic
structural rearrangement, providing the molecular level under-
standing for C2H2-induced gate opening.
In addition to the host−guest interactions, the pore shape of
the framework contributed to gate opening by trapping C2H2
molecules inside the channel. As shown in Figure 5b and
Figure S30, a periodically arranged large cavity (d = 11.039 Å)
and narrow neck (d = 4.936 Å) were observed at the lateral
view of the 1D channel, which formed a cavity structure to lock
C2H2 molecules inside the crystal. This observation was
consistent with DFT calculations. As shown in Figure 5c, the
adsorption of C2H2 was an exothermic process (ΔE = −0.38
eV) with mostly negative potential energy when a C2H2
molecule passed from outside into interior region of the
SPOC-SQ-C2H2 along the central axis of the pore. However,
when the C2H2 molecule passed through the narrow neck from
the large cavity, the adsorption energy increased drastically
with an energy barrier of 0.47 eV, thus preventing the release
of C2H2 molecule from SPOC-SQ-C2H2 crystals. Collectively,
both the specific C2H2-framework interactions and channel
structure played vital roles in the stable accommodation of
C2H2 molecules in the framework, contributing to the large
hysteresis in experimental adsorption−desorption isotherm
and the acquisition of C2H2-loaded SPOC-SQ-C2H2 crystals.
As C2H2 adsorption was an exothermic process, low temper-
ature would be more energetically favorable for the C2H2
adsorption process by strengthening the H bonds between
C2H2 and SPOC-SQ,46 thus leading to the easier gate-opening
behavior at low temperature as mentioned above .
Figure 6. (a) PL spectrum of SPOC-SQ-C2H2 crystals. Inset shows
the TD-DFT theoretical calculation of vertical excitation energies,
nature of the excited states and oscillator strengths (f) for SPOC-SQ
in SPOC-SQ-C2H2 crystals. (b) Measurement of PL spectra for
SPOC-SQ-a during the adsorption of acetylene. (c) Plot of the
wavelength blue-shifted change (Δλem) against C2H2 adsorption time.
from the synergistic effect of the host structure change and
specific host−guest interaction.
Because such fluorescence blue shift was correlated with the
C2H2 sorption process, it enabled monitoring of the adsorption
dynamics of C2H2 in situ. Thus, the fluorescence changes of
SPOC-SQ-a crystals upon C2H2 adsorption were carefully
recorded at 1 bar and 298 K. As suggested in the plot of
wavelength change (Δλem) against adsorption time (Figure
6c), the whole adsorption process could be divided to three
steps. In step I, the blue shift of fluorescence was relatively
slow because of the presence of the gate-opening process at the
beginning. This process occurred in a short period of time,
implying the fast gate-opening behavior. In step II, the
fluorescence wavelength changed rapidly with a hypsochromic
shift. In step III, the fluorescence blue shift reached a steady
platform, revealing the saturated adsorption of C2H2 for
SPOC-SQ-a crystals. In this way, the dynamic C2H2 adsorption
processes could be monitored in real time by tracking the
fluorescence changes, which was advantageous as compared to
the generally used approaches.23
During the C2H2-induced gate-opening process, the
fluorescence of SPOC-SQ-a crystals changed accordingly,
which motivated us to further investigate the solid-emission-
tunable behaviors of SPOC-SQ-a crystals upon C2H2
adsorption. As presented in Figure 6a, the as-obtained
SPOC-SQ-C2H2 crystals emitted at 510 nm with a quantum
efficiency of 0.67, higher than that of SPOC-SQ-a crystals.
DFT calculations revealed that the increased emission
efficiency was mainly due to the decrease of the free volume
from 11.84% to 5.19% upon C2H2 adsorption, which resulted
in more confinement on SPOC-SQ molecules, and largely
inhibited nonradiative relaxation by the restriction of the
intramolecular motions (Table S6 and Figure S23). On the
other hand, the fluorescence of SPOC-SQ-C2H2 crystals was
blue-shifted (Figure 6b), which may be related to the crystal
structure change and the specific host−guest interactions.
According to the DFT simulations (Table S7), the excitation
energy of SPOC-SQ molecule increased from 2.8301 to 2.8666
eV as the crystal structure changed from nonporous SPOC-
SQ-a to porous SPOC-SQ-C2H2 without C2H2 inside. The
excitation energy was increased to 2.8809 eV by considering
the specific H-bonding interactions between C2H2 and SPOC-
The unique gate-opening sorption attribute of SPOC-SQ-a
crystals toward C2H2 enables its potential applications for gas
separation. Single-component adsorption−desorption experi-
ments of varied gases, such as C2H2, C2H6, C2H4, CH4, CO2,
N2, and H2, were conducted on SPOC-SQ-a crystals at 298 K
to test their feasibility for gas separation. As shown in Figure
S24, SPOC-SQ-a crystals exhibit selective absorption of C2H2,
whereas they almost did not adsorb C2H6, C2H4, CH4, CO2,
N2, and H2. The high selectivity of SPOC-SQ-a crystals to
C2H2 could be mainly ascribed to the host−guest interactions
and pore morphology as mentioned above. The unique
selective sorption of C2H2 enables SPOC-SQ-a crystals to be
employed in separation of C2H2 from C2H4, which is very
important in industrial processes.15,48,49 To investigate the
potential for such separation, Henry selectivity, which
1
SQ in the crystal framework. The (π, π*)-featured excitation
energy could be further increased by shortening the distance
between the H atom of C2H2 and oxygen atom of SPOC-SQ
(H···O) (Figure S31).42,47 Collectively, these theoretical
calculations reveal that the blue shift in PL spectra arises
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Article
determines the separation capability, was calculated. The
Henry selectivity value was estimated to be 22.556 at 298 K,
and further increased to be 26.126 at 273 K (Figures S32 and
S33, Table S8), exhibiting the remarkable selectivity. The high
selectivity originates from the selective gate-opening effect for
C2H2 and the strong and specific C2H2-framework interactions.
Thus, SPOC-SQ-a featuring high C2H2 uptake capacity and
high C2H2/C2H4 adsorption selectivity could serve as a
superior SPOC material to separate C2H2 from C2H2/C2H4
mixtures. As shown in Figure 7, kinetically controlled real-time,
recognize C2H2 molecules and undergo gate-opening sorption
with an enthalpy of 33.0 kJ/mol. During C2H2 sorption, the
crystal framework experienced reversible C2H2-induced gate-
opening transformation in the manner of SCSC. Fortunately,
C2H2-loaded SPOC-SQ single crystal (SPOC-SQ-C2H2) was
obtained, which enabled us to understand C2H2 locations and
specific C2H2-framework interactions, and provided crystallo-
graphic understanding of C2H2-induced gate-opening mecha-
nism. Furthermore, by tracking the fluorescence changes of
SPOC-SQ crystals during the adsorption of C2H2, we were
able to monitor C2H2 adsorption dynamics in real time. In
addition, such unique gate-opening sorption behavior of
SPOC-SQ-a crystals toward selective adsorption of C2H2
facilitates the potential applications for gas separation. This
work is expected to deepen the fundamental understanding of
host−guest interactions in pure organic material and promote
the development of more functionality-tunable SPOC
materials for scientific research and industrial applications.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c12153.
■
sı
Materials and instrumentation, synthetic procedures,
characterization, preparation of crystals, experimental
details, single-crystal data and analysis, details of
theoretical calculations, PXRD, sorption properties,
and so on (PDF)
Figure 7. Experimental column breakthrough curves for C2H2/C2H4
separation (50/50 mixture, v/v) on SPOC-SQ-a (3.1138 g) at 273 K
and 1 bar. Breakthrough experiments were conducted in a column
(inside diameter, 10 mm) at a flow rate of 5 mL/min.
Accession Codes
CCDC 2034377−2034380 and 2057643−2057648 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.
fixed-bed column breakthrough experiments were conducted
by using binary gas mixture of C2H2/C2H4 (50/50, v/v) at 273
K, and the results demonstrated good separation performance
of SPOC-SQ-a crystals toward C2H2 for the binary C2H2/
C2H4 mixture. Compared with MOF-featured SPCs, this
SPOC-SQ-a possesses the advantages of reusability, low cost,
and easy preparation.
AUTHOR INFORMATION
Corresponding Author
■
Xinggui Gu − Beijing Advanced Innovation Center for Soft
Matter Science and Engineering, State Key Laboratory of
Chemical Resource Engineering, College of Materials Science
and Engineering, Analysis and Test Center, Beijing University
of Chemical Technology, Beijing 100029, China;
orcid.org/0000-0002-1678-2823; Email: guxinggui@
mail.buct.edu.cn
CONCLUSIONS
■
In summary, this work presented an emission-tunable SPOC
based on a well-designed SQ derivative, SPOC-SQ, which
contained conformation-rotatable 4-chloro-aminophenyl
D−π−A linkers and intrinsic oxygen sites to achieve both
the porous crystal framework by π−π and H-binding
interactions and the structural deformation through phenyl
rotations. This SPOC could be facilely prepared by slowly
evaporating its DCM solution (SPOC-SQ-DCM). Porous
SPOC-SQ-DCM crystals with DCM inclusions could be
transformed to nonporous SPOC-SQ-a crystals upon activa-
tion. The crystal framework could undergo reversible SCSC
structure transformations between SPOC-SQ-DCM and
SPOC-SQ-a crystals through heating and fuming with DCM,
which demonstrated its excellent crystallinity and flexibility.
Meanwhile, SPOC-SQ crystals exhibited highly efficient
emission, originating from crystallization-induced reverse
from dark to bright excited states. Such crystal-state emission
varied during the above-mentioned SCSC structural trans-
formations. The intrinsic oxygen atoms of SPOC-SQ
molecules could serve as functional sites to form H bonds
with acidic H, enabling SPOC-SQ-a crystals to selectively
Authors
Lin Li − Beijing Advanced Innovation Center for Soft Matter
Science and Engineering, State Key Laboratory of Chemical
Resource Engineering, College of Materials Science and
Engineering, Analysis and Test Center, Beijing University of
Chemical Technology, Beijing 100029, China
Huili Ma − Key Laboratory of Flexible Electronics (KLOFE)
& Institute of Advanced Materials (IAM), Nanjing Tech
University (NanjingTech), Nanjing 211816, China;
orcid.org/0000-0003-0332-2999
Jingyan Zhang − Beijing Advanced Innovation Center for Soft
Matter Science and Engineering, State Key Laboratory of
Chemical Resource Engineering, College of Materials Science
and Engineering, Analysis and Test Center, Beijing University
of Chemical Technology, Beijing 100029, China
3862
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Article
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Nanoporous Soft Crystal. Science 2019, 363, 387−391.
Engui Zhao − School of Science, Harbin Institute of
Technology Shenzhen, HIT Campus of University Town of
Shenzhen, Shenzhen 518055, China; orcid.org/0000-
0003-4953-0750
̌
(7) Taylor, M. K.; Runcevski, T.; Oktawiec, J.; Bachman, J. E.;
Siegelman, R. L.; Jiang, H.; Mason, J. A.; Tarver, J. D.; Long, J. R.
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Jian Hao − Beijing Advanced Innovation Center for Soft
Matter Science and Engineering, State Key Laboratory of
Chemical Resource Engineering, College of Materials Science
and Engineering, Analysis and Test Center, Beijing University
of Chemical Technology, Beijing 100029, China
Huiming Huang − Beijing Advanced Innovation Center for
Soft Matter Science and Engineering, State Key Laboratory of
Chemical Resource Engineering, College of Materials Science
and Engineering, Analysis and Test Center, Beijing University
of Chemical Technology, Beijing 100029, China
Hui Li − Beijing Advanced Innovation Center for Soft Matter
Science and Engineering, State Key Laboratory of Chemical
Resource Engineering, College of Materials Science and
Engineering, Analysis and Test Center, Beijing University of
Chemical Technology, Beijing 100029, China
Pengfei Li − Advanced Research Institute of Multidisciplinary
Science, Beijing Institute of Technology, Beijing 100081,
China
Ben Zhong Tang − Department of Chemistry, Hong Kong
Branch of Chinese National Engineering Research Center for
Tissue Restoration and Reconstruction, and Institute for
Advanced Study, The Hong Kong University of Science and
Technology, Kowloon, Hong Kong, China; orcid.org/
0000-0002-0293-964X
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c12153
Author Contributions
All authors have given approval to the final version of the
manuscript.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partially supported by the National Natural
Science Foundation of China Grant (21702016), Beijing
National Laboratory for Molecular Sciences (BNLMS201813),
the Research Grant Council of Hong Kong (16305518,
C6009-17G and A-HKUST605/16), and the Innovation and
Technology Commission (ITC−CNERC14SC01). B.Z.T,
acknowledges the financial support from the National Science
Foundation of China (21788102).
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