Construction of soft porous crystal with silole derivative: Strategy of framework design, multiple structural transformability and mechanofluorochromism

Article abstract of DOI:10.1039/c1jm12673c

Novel fluorescent organic soft porous crystals have been designed and prepared based on a multi-substituted silole bearing 1-phenyl-2,2-dicyanoethene moieties (molecule 8). 8 exhibited a series of emission colors, ranging from yellow to dark red with an over 70 nm shift of emission maximum. Molecule 8 also showed the ability to reversibly switch between different solid states, and a typical mechanofluorochromism was observed by cyclic operation of the grinding-heating-cooling processes. In addition, two single crystals (O and R) were successfully obtained in proper conditions, and the crystallographic data indicated that crystal O and R had reasonable hollow structures, inside which different solvent molecules were selectively encapsulated. More importantly, we have presented a proof-of-concept example of the strategy for the designation of organic soft porous crystals with a conjugated fluorophore and demonstrated the successful achievement of softness, porosity and crystallization ability. This design strategy is instructive to design and construct organic soft porous crystals with other conjugated building blocks and develop novel smart and stimuli-responsive photo/electronic materials.

Full text of DOI:10.1039/c1jm12673c

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PAPER
Construction of soft porous crystal with silole derivative: strategy of
framework design, multiple structural transformability and
mechanofluorochromism†‡
J. Mei,a J. Wang,a A. Qin,a H. Zhao,a W. Yuan,b Z. Zhao,b H. H. Y. Sung,b C. Deng,b S. Zhang,a
b
I. D. Williams, J. Z. Sun and B. Z. Tang
a
ab
*
*
Received 10th June 2011, Accepted 29th July 2011
DOI: 10.1039/c1jm12673c
Novel fluorescent organic soft porous crystals have been designed and prepared based on a multi-
substituted silole bearing 1-phenyl-2,2-dicyanoethene moieties (molecule 8). 8 exhibited a series of
emission colors, ranging from yellow to dark red with an over 70 nm shift of emission maximum.
Molecule 8 also showed the ability to reversibly switch between different solid states, and a typical
mechanofluorochromism was observed by cyclic operation of the grinding–heating–cooling processes.
In addition, two single crystals (O and R) were successfully obtained in proper conditions, and the
crystallographic data indicated that crystal O and R had reasonable hollow structures, inside which
different solvent molecules were selectively encapsulated. More importantly, we have presented
a proof-of-concept example of the strategy for the designation of organic soft porous crystals with
a conjugated fluorophore and demonstrated the successful achievement of softness, porosity and
crystallization ability. This design strategy is instructive to design and construct organic soft porous
crystals with other conjugated building blocks and develop novel smart and stimuli-responsive photo/
electronic materials.
exploited.5 With increasing scientific and technological interests
Introduction
in organic electronics and photonics, various organic crystals
based on conjugated molecules have come into being. All of these
crystalline solids, in a certain sense, may be soft and porous. But
for conventional conjugated organic solids, the softness and
porosity are very limited. In principle, the p-conjugation system
should be large enough to render expected photo/electronic
properties to the chromophores such as broad absorption band,
visible or near-infrared emission, and high charge mobility. As
a result, there exists strong p–p interaction between the chro-
mophores, which results in close packing of the p-conjugation
systems and leads to the formation of long range order. The
orderly p–p stacking endows the crystals with strong intermo-
lecular electronic communications and high charge mobility,
which are pursued by materials scientists working in the areas of
organic thin film transistors and photovoltaic cells.5–7 But the
close packing compresses the free space and blocks the pene-
trating of the pores inside the crystals. In addition, the rigidity of
the aromatic building blocks impairs the structural trans-
formability, thereby further constraining the softness of the
framework. Although a few original works demonstrated the
construction of SPCs with small organic molecules and gave
useful clues for molecular design, even a concept of ‘‘organic
zeolites’’ was proposed,1,8 common concepts aiming at the
construction of SPCs based on conjugated molecules are still
demanding. Without the inductive coordination between metal
Soft porous crystals (SPCs), a family of porous solids that
possess both highly ordered network and structural trans-
formability, have attracted intensive research interests for
a variety of purposes such as gas storage, separation, and cata-
lysts.1–4 Their photo/electronic aspects, however, have been less
aInstitute of Biomedical Macromolecules, MOE Key Laboratory of
Macromolecular Synthesis and Functionalization, Department of
Polymer Science and Engineering, Zhejiang University, Hangzhou,
310027, P. R. China. E-mail: sunjz@zju.edu.cn; Fax:
87953734; Tel: + 86-0571-87953734
+ 86-0571-
bDepartment of Chemistry, The Hong Kong University of Science &
Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China.
E-mail: tangbenz@ust.hk; Fax: + 852+852-2358-7375; Tel: + 852-2358-
1594
† Electronic
Characterization
1,1-dimethyl-2,5-bis(4-benzaldehyde)-3,4-diphenylsilole
supplementary
spectra
information
2-(4-bromophenyl)-1,3-dioxolane,
and
(ESI)
available:
of
8
(Fig. S1–S7), space filling view X-ray structures of crystal O and crystal
R (Fig. S8), representative frontier orbitals of molecule 8 in crystal O
and R (Fig. S9), fluorescence spectra of molecule 8 in crystal O and R
(Fig. S10), DSC curves of different solids of molecule 8 (Fig. S11)
supplementary data for crystal O and R (Table S1) and complete
citation of ref. 14 and ref. S1. CCDC reference numbers 829494 and
829495. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c1jm12673c
‡ Dedicated to Professor Daoben Zhu on the occasion of his 70th
birthday.
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cations and ligands as found in metal–organic frameworks
(MOFs) and multiple directional hydrogen-bonding as observed
in ‘‘organic zeolites’’,8b new constructive elements must be
discovered. Basically, two key points – one is the ingenious
balance between the rigidity of the conjugated blocks and the
softness of the expected buildings, and the other is the artful
combination of the open porosity and robust crystalline struc-
ture of the built frameworks – are the pivots of molecular design.
Consequently, it is of great significance, yet a great challenge, to
discover a rational strategy to construct SPCs with photo/elec-
tronic active conjugated organic molecules.
aggregates in different conditions, and the highly crystalline state
showed a blue-shifted emission feature and enhanced fluores-
cence efficiency.9e These examples displayed the softness and
regularity of the related organic frameworks. However, the
porosity is rather low and their practical usage cannot be counted
on. To prepare workable soft porous crystals, more spacious
voids need to be generated in the solid. It means the dimension of
the rotors should be extended. But the extension may impart
instability to the target frameworks due to the higher degree of
conformational freedom. Meanwhile, the introduction of more
rotatory aryls into the molecule will heavily interrupt p–p
interactions between adjacent building blocks and thus may lose
long-range order. Dendrimers can be viewed as extreme exam-
ples. Their porosity and softness usually take effect in proper
solutions, but they are too ‘‘soft’’ to sustain the necessary order
and robust frameworks.
Fluorescence molecules with aggregation-induced emission
(AIE) property provide some instructive clues to the construction
of conjugated SPCs. AIE active materials have received consid-
erable attention for their potential applications in organic
light-emitting devices and bio/chemosensor systems.9,10 1,1-
Dimethyl-2,3,4,5-tetraphenylsilole (Chart 1, molecule 1) and
tetraphenylethene (molecule 2) are shown as two representatives
and a series of their derivatives have been synthesized.10 Based on
the chemical structures and the available crystallography data,
we extracted some common features from these molecules. Every
molecule has a central p-conjugate (stator) and multiple
peripheral aryl groups (rotors); and each rotor links to the stator
via a single bond. The intramolecular rotations of the rotors
against the stator exhaust the energy of the excited states through
non-radiative channels, thus making the molecules non-emissive
in solution or other free states. In crystal or aggregated states, the
intramolecular rotations are restricted and the non-radiative
relaxation windows are closed, thereby evident fluorescence can
be observed. Due to the steric repulsion between relevant atoms
on the central stator and peripheral rotors (as illustrated by Ha
and Hb of the model compound 3 in Chart 1), these molecules can
never take planar shape, thus little p–p interaction is expected.
The existence of multiple rotatory aryls, non-planar shape and
weak p–p interaction imply that this kind of crystals may have
desirable structural transformability, loose molecular packing,
and interpenetrating channels. In other words, they are prom-
ising candidates for the construction of conjugated SPCs.
This anticipation was partially validated by previous investi-
gations. As shown in Chart 1, molecule 4, or 4-(biphenylyl)-
phenyldibenzofulvene was weakly luminescent in amorphous
phase but became highly emissive upon crystallization. This
polymorph-dependent fluorescent behavior allowed its emission
to be quite repeatedly switched between dark and bright states by
heating–cooling processes.9d Molecule 5, or 1,2-diphenyl-3,4-
bis(diphenylmethylene)-1-cyclobutene could also form different
Indeed, we have never obtained well-resolved crystals from 6
and 7, which have extended rotors attaching to the silole
cores,9f,10 although many conditions were tried to grow single
crystals. Thus, it is rational to introduce other factors into the
molecular design to make a balance between the softness and the
robustness of the organic framework. The dipole–dipole inter-
action is a permanent force, which is possible to hold the
aromatic building blocks together.11 Herein, we demonstrate our
strategy to construct SPCs with a sole conjugated building block,
in which the structural transformability is derived from the
conformational rotations of the multiple aromatic moieties and
the long-range order is maintained by permanent dipole–dipole
interaction of the polar functionalities, respectively. Considering
that nitrile is a strong electron-withdrawing group and can
impart high polarity to aromatic systems, we designed and
synthesized a novel silole derivative, 1,1-dimethyl-2,5-bis(40-
benzylidenemalononitrile)-3,4-diphenylsilole (molecule
8 in
Chart 1). The two malononitrile moieties attaching to the parent
silole (1) serve as the pivotal factors of both molecular polarizing
and rotor extending.
Results and discussion
Multiple solid states of molecule 8
As expected, molecule 8 does exhibit multistable solid states,
which is a direct indication of the softness. Solids of 8 in distinct
colors were obtained during the preparation process. The
product freshly gotten after purification but before vacuum
drying was yellow (Fig. 1a). When it was dissolved in THF and
dried in vacuum after rotary evaporation, crimson powder was
collected without exception (Fig. 1d). Also, powders in other
colors were acquired from different preparation conditions. A
spectrum of the colorful powders/crystals are displayed in Fig. 1.
The latest Cambridge Structural Database (CSD) tells that 5-
methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile (ROY)
is the most polymorphic substance. ROY has seven polymorphs
with solved structures.12 Searching in CSD also found zero
substances with six polymorphs, 2 with five, 11 with four, and 58
with three. Molecule 8 seems to have potential to be listed in the
compounds with multiple polymorphs. The growth of single
crystals and determination of their structures are under
investigation.
Chart 1 Molecular structure of relevant compounds.
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a unit cell of crystal O, four n-hexane molecules are encapsulated
inside the framework built from aromatic blocks of 8 (Table 1).
For crystal R, two acetone molecules are trapped in a unit cell
(Table 1). These results can account for the elementary analysis
data of the obtained solids. More importantly, these observa-
tions clearly validate that both crystals O and R are really
porous.
Fig. 1 Photographs of different solids of 8. (a) Yellow powder obtained
after purification without drying in vacuo. (b) Orange powder obtained by
vacuum drying the yellow crystal in (a). (c) Red powder obtained by
evaporating 8 from its DCM solution and being dried in vacuo. (d) Dark
red powder obtained by evaporating 8 from its THF solution and being
dried in vacuo. (e) Orange crystals grown from THF/hexane. (f) Red
crystals grown from acetone, DCM and THF at first and then grown
from methanol/DCM for a second time. Photos of (g), (h), (i), (j), (k) and
(l) are the images of (a), (b), (c), (d), (e) and (f) taken under UV light
(lex ¼ 365 nm), respectively.
Selective accommodation of solvent guests in the framework
Perspective on the X-ray crystal structure provides further
insight into the structural characteristics. In both crystals O and
R, the conjugated frameworks play the role of host to accom-
modate the solvent guests. For crystal O, the 10-phenyl-20,20-
dicyanoethene (PDCE) moieties at the 2,5 positions, the phenyl
rings at 3,4 positions, and the two methyl groups on the silole
core form one-dimensional, rhombus-shaped pores running at
the azimuth angle of ꢀ45ꢁ to the a axis (Fig. 2d and 2e). The
hexane molecules (highlighted in Fig. 2a, 2b, 2c and 2e, the space
filling view is presented in Fig. S8†) localize in the one-dimen-
sional pores with the long molecular axis aligning approximately
parallel to the axial direction of the pore. The robust aromatic
elements construct the pore wall, and the two methyl groups
project into the channel and obstruct the close queuing of the
hexane molecules. Intrinsically, hexane is a non-solvent to 8, thus
the encapsulation of hexane molecules inside the framework is
rationally ascribed to the admissible pores. When looking
through in the direction of axis a, it is found that the hexane
molecules are divided into two groups: one is inserted between
two PDCE segments on position 2 and the other on position 5 of
the correlated 8 molecules (pink skeleton, Fig. 2a); whereas the
others are between the two PDCEs on position 5 and 2 of the
correlating silole cores (orange skeleton, Fig. 2a). The two
The colorful solids may result, on one hand, from the expected
structural transformability of the conjugated organic framework,
and on the other hand, from the doping of certain unwanted
resultants. To affirm which one is the latent mechanism, special
attention had been paid to the purification of molecule 8.
Rigorous dryness and repeated silica gel column chromatog-
1
raphy were conducted, and the characterization results from H
NMR and mass spectroscopy confirmed that all the samples were
identical in chemical nature. But the elementary analysis results
were unsatisfactory, indicating the existence of alien species.
Alternately, we focused our efforts on the preparation of single
crystals. Slow diffusion of hexane vapour into THF solution of 8
gave orange crystals (Fig. 1e, crystal O). We also tried to grow
single crystals from other mixture solvents such as acetone/DCM
and acetone/THF, but the collected crystals were not good
enough for single crystal X-ray diffraction analysis. Unexpect-
edly, when we tried to grow crystals by redissolving the solid
derived from acetone/DCM in a methanol/DCM mixture for
a second time, red crystals (Fig. 1f, crystal R) were obtained. X-
Ray crystallographic analyses of crystals O and R have been
performed and the data are summarized in Table 1 and Table
S1.† To our surprise, both crystal O and R are not unmixed. In
Table 1 Crystallographic data and structure refinement for crystals O
and R
Cell parameters
Crystal O
Crystal R
Empirical formula
Color
Temperature
Wavelength
Crystal system
Space group
Unit cell dimensions
C44H40N4Si
Orange
133(2) K
1.54178 A
Monoclinic
C2/c
C41H32N4OSi
Red
173(2) K
1.54178 A
Monoclinic
P2/c
Fig. 2 X-Ray crystal structure for crystal O at 133 K. (a) View of a unit
cell. (b) View of the unit cell along the c axis, showing the shape of the
pores and the orientation of the hexanes. (c) A perspective view of the
lattice to display the two groups of hexane molecules in a herringbone-
like arrangement tilted to the a axis. (d) Perspective view of the frame-
work and the 1-dimensional channel approximately in the azimuth angle
of ꢀ45ꢁ to the a axis. The guest hexane molecules are removed for clarity.
(e) A perspective view in the azimuth angle of ꢀ45ꢁ to the a axis to show
the guest hexane molecules in the 1-dimensional channels. The host
framework atoms are represented as sticks and the guest molecules are
shown as stick-spheres. The hexane molecules are colored in orange and
pink to highlight their positions and relative orientations.
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
a ¼ 30.7917(4) A
a ¼ 13.268(3) A
ꢀ
b ¼ 10.23740(10) A
b ¼ 10.287(2) A
ꢀ
c ¼ 11.7082(2) A
c ¼ 13.771(4) A
a ¼ 90ꢁ
a ¼ 90ꢁ
b ¼ 90.4810(10)ꢁ
b ¼ 113.39(3)ꢁ
g ¼ 90ꢁ
g ¼ 90ꢁ
3
3
ꢀ
ꢀ
Volume
Z
Density (calculated)
3690.61(9) A
4
1725.2(7) A
2
1.175 Mg mꢀ3
1.203 Mg mꢀ3
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groups of hexanes stay in a similar environment (Fig. 2b and 2e)
but have tilted orientations (Fig. 2a and 2c).
For crystal R, the pore walls are also constructed by the
PDCEs and the phenyl rings. Perspective to the c axis, the pore is
in an hourglass shape (Fig. 3b and Fig. S8†). The guest molecules
are standing by the PDCEs (Fig. 3b and 3c), rather than inserting
between two adjacent PDCEs as observed in crystal O. Based on
X-ray analysis data, the six largest residual electrons in the cavity
were refined as an acetone molecule in two positions. In order to
have reasonable thermal U (eq)s, overlapping of oxygen and
carbon as two pairs, i.e. O (1S), C (1SA) and O (1SA), C (1S), are
adopted. The overall occupancy for this acetone molecule is 0.5
per asymmetric unit. It indicates that the acetone molecule can
flip-flap inside the pore, and the dynamical equilibrium config-
uration looks like a dimer with a head-to-tail arrangement
(Fig. 3e). It is noted that the acetone molecules can oscillate
inside the pores due to the large pore size and weak interaction
between the guest and host molecules, so that a strange feature
was presented by the crystallographic analysis program.
Similar to the classical SPCs, the variations of the pore volume
and shape are a result of the conformational changes of the
organic component. But in the present case, the conformational
changes are associated with the rotations of the PDCE segments
and the phenyls around the central silole core. In other words,
the rotatory units are rigid aryls on different positions of an
aromatic core, but not flexible units in the same molecular chain
as found in some peptide- or aliphatic segment-based MOFs.13 In
crystal O, as shown by Fig. 4a, the torsional angle of qO1 and qO2
(dihedral angle between the silole core and the PDCE segments at
the 2 and 5 positions) are both 41.34ꢁ; thus the relative dihedral
angle of the two PDCE segments is 82.68ꢁ, nearly perpendicular
Fig. 4 (a) and (b) ORTEP plots of single molecule 8 in crystals O and R,
respectively. The insets in (a) and (b) present the fluorescent images of
crystal O and R, respectively. (c) and (d), (e) and (f) Calculated electronic
distributions of the HOMO and LUMO levels for 8 in crystals O and R,
respectively. The bond lengths and angles are extracted from (a) and (b).14
A guest molecule (hexane or acetone) is also included for showing that the
guest molecule has no electronic interaction with the host. (g) and (h)
Perspective views showing the J-aggregation between the correlated
PDCE moieties; the distances between two correlated PDCE moieties are
also plotted. The host and guest atoms are represented as sticks and stick-
spheres for clarity.
to each other (Fig. 2a). The dihedral angles between the silole
core and phenyl rings on the 3 and 4 positions (qO3 and qO2) are
both ꢀ122.88ꢁ (Fig. 4a), indicating a large torsion between the
phenyls and the silole core. In crystal R, the corresponding
dihedral angles qR1 ¼ qR2 ¼ –6.42ꢁ, the dihedral angle of the two
PDCE segments is only ꢀ12.84ꢁ, indicating a nearly co-planar
configuration between the two PDCE segments and the silole
core. The dihedral angles of qR3 and qR4 are both ꢀ74.02ꢁ
(Fig. 4b). The data of the dihedral angles reveal that the rotors of
the phenyls and the PDCE segments in crystal O have a more
twisted conformation than those in crystal R, thereby the
porosity of crystal O is higher than that of crystal R. This result is
perfectly consistent with the facts that the density of crystal O
(1.175 Mg mꢀ3) is lower than that of crystal R (1.203 Mg mꢀ3),
and the pores in crystals O and R can accommodate bigger
(hexane) and smaller (acetone) molecules, respectively.
Fig. 3 X-Ray crystal structure for crystal R at 173 K. (a) View of a unit
cell. (b) View of the unit cell along the c axis, showing the hourglass shape
of the pores, and the location of the guest molecules. (c) A magnified view
of (b) to display the by-stander role of the guest molecules with respect to
the overlapped 10-phenyl-20,20-dicyanoethenyl segments. (d) Perspective
view of the framework and the 1-dimensional channel approximately in
the c axis. The guest molecules are removed for clarity. (e) Perspective
view of the framework and the 1-dimensional channel in the c axis. The
guest acetone molecules inhabit in the 1-dimensional channels high-
lighted in the inset. The host atoms are represented as sticks in all panels
and the guest atoms are shown as stick-spheres in panels (a)–(c); while the
guests also represented as red sticks for clarity.
According to the above analyses, a noticeable essence is that
the formation of the pores and the accommodation of the guest
molecules in the pores both correlate with the PDCE segment.
On one hand, the PDCE segments extend the dimension of the
rotors thereby offering expanded voids to accommodate the
guest molecules. And on the other hand, the PDCE segment has
a strong dipole due to the efficient electron-withdrawing dual-
cyano-functionalities. The dipole–dipole interaction helps to
engender a permanent force and to hold the irregularly shaped
molecules together. In Fig. 4g and 4h, the two intimately corre-
lated PDCE segments are parallel to each other and take a
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J-aggregation with oppositely aligned dipoles to stabilize the
whole system. These results indicate that our original intention to
build soft porous crystals from bulky conjugated molecules with
the aid of polar interaction is feasible.
color is also yellow-orange (YO-form, peak at 576.5 nm, Fig. 5a
and 5b). After being ground, the YO-form was converted to
a vivid cherry-colored solid showing a strong red fluorescence
upon irradiation of UV-light (R-form, peak at 600.5 nm, Fig. 5
and the inset). Inspired by this phenomenon, we tentatively
heated the obtained R-form at 150 ꢁC. The choice of this
annealing temperature was based on the differential scanning
calorimetry analyses data of the solid, which has a melting point
at around 228 ꢁC and a weak transition at around 122 ꢁC
(Fig. S11†). After thermal annealing, the red solid was turned
yellow-orange in no more than 30 s. The YO-solid recovered
from the R-form still emitted the intensive yellow-orange fluo-
rescence (peak at about 566 nm), approximately equal to the
original O-form.
Electronic structure
Different from the classical SPCs, not only are the volume and
shape of the pores tuned by the intramolecular rotations, but also
are the electronic structures and the electronic/photonic prop-
erties of the frameworks. The images in Fig. 1 sketchily
demonstrate this particularity. Specifically, upon excitation at
429 nm, crystal O emits fluorescence with a maximum at 566 nm
(Fig. S10†), and its fluorescence quantum yield is 1.2%. Under
the same conditions, crystal R displays an evidently red-shifted
emission feature with a peak at 633 nm and a fluorescence
quantum yield of 10.1% (Fig. S10†). These observations can be
rationally explained by the crystallographic data. In crystal O,
molecule 8 has a more twisted configuration than that in crystal
R. It indicates that 8 has a lower conjugation degree in crystal O,
and thus displays a shorter emission wavelength in comparison
with crystal R. Moreover, the distinct molecular packing char-
acteristics in crystals O and R may be responsible for the
different PL quantum yields as well. In crystal R, the molecular
packing is closer than that in crystal O. The aryl groups (rotors)
were restricted more efficiently in crystal R. Therefore, crystal R
displayed a stronger AIE property compared with crystal O.
Fig. 4c and 4d illustrate the calculated HOMO and LUMO levels
according to the real configuration of a single molecule 8 in
crystal O (see Fig. S9†).14
It is noted that both the O, YO and R forms were stable
enough to last for more than five months when they were kept at
ambient temperature. This is a typical phenomenon of mecha-
nochromic luminescence, which may find versatile applications
in fluorescence tuning, environment monitoring and chemical
sensing.15 To validate its reversibility, cycles of the grinding–
heating–cooling operation were adopted. The repeated cycles
shown in Fig. 5a demonstrated that the mechanochromic lumi-
nescence was readily reproducible.
To acquire further understanding of the mechanochromic
fluorescence, X-ray diffraction (XRD) measurements were
carried out for powder samples of both YO- and R-forms. The
XRD patterns of the single crystals of O and R are shown as
Fig. 6a and 6b for comparative analysis of the XRD patterns of
the YO- and R-forms. The diffraction peaks of crystal O can be
largely divided into three regions, i.e. 8ꢂ12ꢁ, 14ꢂ20ꢁ, and
22ꢂ30ꢁ. And the characteristic diffractions of R are composed of
an isolated peak (ꢂ11ꢁ) and two progressions (13ꢂ18ꢁ and
20ꢂ28ꢁ). The typical diffraction pattern of the pristine YO-form
solid is displayed as Fig. 6c. It exhibits some recognizable
diffraction peaks ascribable to a partially crystallized solid,
although the indexation cannot be achieved. After grinding,
most diffraction peaks disappear or become vague, while some
new ones at different diffraction angles emerge (Fig. 6d),
implying that the initial crystal lattice has been significantly
disrupted by mechanical force and the crystalline size of the new
polymorph is quite small. In addition, the diffraction pattern of
For a single molecule 8 in crystal R, the landscape of the
frontier electronic orbital is illustrated as Fig. 4e and 4f. The
theoretical calculation results demonstrate that the electrons tend
to localize in the central silole core for crystal O but dislocate
over the lateral PDCE moieties for crystal R. This means that in
crystal R 8 has a lower optical bandgap in comparison with
crystal O, which is in good agreement with the recorded fluo-
rescence spectra. Meanwhile, the calculation results also show
that the guest molecules (hexane and acetone) have negligible
electronic communication with the host conjugated blocks.
When evaluated from an intermolecular viewpoint, in crystal R,
the distance between the parallel-arranged two adjacent PDCE
ꢀ
segments in crystal O (d ¼ 7.294 A, Fig. 4g) is evidently longer
ꢀ
than that in crystal R (d ¼ 3.488 A, Fig. 4h), indicating a closer J-
aggregation state in crystal R. The smaller conformational
torsion, higher degree of electronic dislocation and stronger J-
aggregation all together bestow crystal R with red-shifted and
enhanced emission as compared with crystal O.
Mechanofluorochromism
The existence of multiple rotors in 8 provides it with the reality of
multistable crystalline states and fluorescent colors. In addition,
it also implies that the solid may have a potentiality to respond to
external stimuli such as mechanical force, pressure, humidity or
thermal treatment. Intentionally, we attempted to change the
solid-state emission of 8 by mechanical force. Firstly, the orange
powder displayed in Fig. 1b was dried over the vacuum oven and
subsequently furnished a yellow-orange solid, whose emission
Fig. 5 (a) Switching of the solid-state fluorescence of molecule 8 by
repeated grinding and heating (lex ¼ 429 nm). Insets show the fluores-
cence photographs of the R-form (after grinding), O-form (before
grinding) and heating at 150 ꢁC for 30 s. (b) The fluorescence spectra of
the YO-form and R-form upon excitation at 429 nm.
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electronic activities. The softness is rendered by the intra-
molecular rotations between the multiple rigid aromatic substi-
tutes and the conjugated core, which is basically different from
those observed for MOFs and organic frameworks. The porosity
is intrinsically generated by the non-coplanar configuration of
the extended rotatory aryl blocks, which shape hollow frame-
works to accommodate guest molecules. The long-range struc-
tural regularity is achieved by introducing strong polar groups
into the building blocks to hold the rugged-shaped molecules
together by the dipole–dipole interactions, but not by the cate-
nation between metal cations and organic ligands, the network of
multiple H-bonding, or the covalent bonding.16 More signifi-
cantly, the present SPC has demonstrated unique emission
behavior that is acutely sensitive to environmental stimuli such as
solvents, mechanical force and thermal treating. The up-take of
different guest species may induce interesting changes to the
electronic structures of the host frameworks, e.g. charge transfer
interactions17 and spin crossover.18 Herein, the change in elec-
tronic structure is associated with the conjugation between
PDCE segments and silole core. Conceiving the promising and
interesting applications of the ordered frameworks with elec-
tronic-optical activities and structural transformability in
chemical and biological sensing arrays,15,19 the present strategy to
build up SPCs has great significance and is helpful to construct
and develop novel organic porous materials with high order
based on various conjugated building blocks.
Fig. 6 (a) and (b) Calculated XRD patterns for crystals O and R
according to the single-crystallographic data. (c) XRD pattern of O-form
before grinding. (d) and (e) Typical XRD patterns of R-form obtained by
grinding O- or YO-form and YO-form after thermal annealing,
respectively.
Fig. 6d is mimetic with Fig. 6b, also composed of an isolated
peak and two progressions. The similar molecular packing mode
leads to the apparent red color of the ground crystalline form
similar to crystal R. The differences in diffraction angles are
associated with the presence or the absence of guest molecules in
the lattice, which can substantially modify the lattice parameters.
As the ground R-form crystal was treated by heating, the solid
was turned into YO again; and the XRD pattern (Fig. 6e) is
evidently different from that of the R-from, but is quite similar to
the pattern shown in curve 6a. In comparison with curve 6c
recorded before grinding, the diffraction peaks in curve 6e are
even sharper and clearer, implying the recovery of the primary
lattice and the healing of the interrupted molecular arrangement.
Hereby, mechanofluorochromism of 8 should be ascribed to
a reversible change of molecular packing or the transformation
of crystal forms induced by grinding and heating.15a,e–i
Experimental
Chemicals and materials
Toluene and THF were distilled from sodium benzophenone
ketyl under dry nitrogen immediately prior to use. All other
chemicals and regents were commercially available and used as
received without further purification. p-Toluenesulfonic acid and
magnesium sulfate were purchased from Sinopharm Chemical
Reagent Co., Ltd. 4-Bromobenzaldehyde was obtained from
Acros Organics Co. Ethylene glycol and malononitrile were
purchased from Alfa Aesar. Dichloro(N,N,N0,N0-tetramethyle-
nediamine)zinc (ZnCl2-TMEDA) was purchased from Aldrich
Chemical Co. Bis(triphenyl phosphine)palladium(II) dichloride
(Pd(II)) was purchased from ABCR GmbH & Co. KG. Di(phe-
nylethynyl)silanes were prepared by lithiation of phenylacetylene
followed by reaction with dichlorosilanes.20
Concluding remarks
In summary, we have shown that the solid formed by molecule 8
exhibits all the essentials of a typical SPC. Firstly, the solid is
soft. The softness is embodied intrinsically by the intramolecular
rotations of the aryl substitutes around the conjugated silole
core, and extrinsically by the multistable solid states. Moreover,
8 has the ability to reversibly switch between different states.
Secondly, 8 can form porous structures. The porosity is
demonstrated by the fact that different hollow cages can be built
up with the aromatic blocks and solvent molecules can be
selectively encapsulated inside the cages. And finally, it can form
crystals. Both single crystals O and R have been successfully
obtained, and other crystals such as form-YO and form-R can be
also obtained. The crystallinity endows the solid with highly
ordered networks.
General characterization
1H and 13C NMR spectra were measured on a Bruker AV 400
spectrometer in deuterated chloroform using tetramethylsilane
(TMS; d ¼ 0) as the internal reference. The elemental analysis
was carried out on a ThermoFinnigan Flash EA 1112. MALDI-
TOF mass spectra were recorded on a GCT premier CAB048
mass spectrometer. Absorption spectra were measured on
a Varian CARY 100 Bio UV-visible spectrophotometer. Fluo-
rescence spectra were recorded on a Perkin-Elmer LS 55 spec-
trofluorometer. The characterization data are summarized in the
Supporting Information and the related spectra are shown as
Fig. S1–S7 and S10.†
It is noticeable that the rationale for the construction of the
present SPC from the silole derivative is not parallel to those
reported for classic organic frameworks and MOFs. The building
blocks used here are pure conjugated ones possessing photo-
This journal is ª The Royal Society of Chemistry 2012
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Synthesis of 2-(4-bromophenyl)-1,3-dioxolane
To a 250 mL two necked round bottom flask, 4-bromo-
benzaldehyde (5.55 g, 30 mmol), ethylene glycol (4.14 g, 66
mmol) and p-toluenesulfonic acid were added. The mixture was
dissolved in newly prepared dried toluene and then refluxed with
a Dean–Stark apparatus at 140 ꢁC for 48 h. The solution was
cooled to room temperature and extracted by cold water/ethyl
acetate. The obtained organic layer was washed by brine and
then dried over magnesium sulfate. After filtration, the solvent
was evaporated under reduced pressure, and the residue was
purified by silicon-gel column chromatography using petroleum
ether/ethyl acetate ¼ 20 : 1 (v/v) as eluent, Rf 0.33. Colorless
1
liquid was obtained in a yield of 69.3% (4.76 g). H NMR (400
MHz, CDCl3): d (TMS, ppm) 7.55–7.47 (d, 2 H), 7.39–7.31 (d, 2
H), 5.78–5.73 (s, 1 H), 4.10–3.95 (m, 4 H). 13C NMR (80 MHz,
CDCl3): d (TMS, ppm) 137.3, 131.6, 128.4, 123.1, 102.9, 65.3.
Scheme 1 Synthetic route to molecule 8. Reagents and conditions: (a) p-
toluenesulfonic acid, anhydrous toluene, 140 ꢁC, 48 h, 69.3%; (b) step 1:
di(phenylethynyl)silanes, lithium naphthalenide (LiNp), anhydrous
THF, under N2 atmosphere, r.t., 3 h; step 2: ZnCl2-TMEDA, ꢀ10 ꢁC to
r.t., 1 h; step 3: Pd(II), 2-(4-bromophenyl)-1,3-dioxolane, 85 ꢁC, 12 h,
59.8%; (c) 1,1-dimethyl-2,5-bis(4-benzaldehyde)-3,4-diphenylsilole,
malononitrile, anhydrous ethanol, 88 ꢁC, 48 h, 38.1%.
Synthesis of 1,1-dimethyl-2,5-bis(4-benzaldehyde)-3,4-
diphenylsilole
The synthesis is followed the procedure described in literature.21
A solution of lithium naphthalenide (LiNp) was made by stirring
a mixture of naphthalene (2.10 g, 16 mmol) and granular lithium
(0.21 g, 30 mmol) in THF (20 mL) at room temperature for 12 h
under nitrogen atmosphere. The resultant dark green mixture
was transferred to a pre-deoxygenated two necked flask via
a cannula as soon as possible. A solution of di(phenylethynyl)
silanes (1.04 g, 4 mmol) in THF (10 mL) was added dropwise into
the solution of LiNp, and the resultant mixture was stirred forꢁ3 h
at room temperature. After the solution was cooled to ꢀ10 C,
ZnCl2-TMEDA (4.00 g, 16 mmol) and 40 mL of THF were
added. The fine suspension was stirred for 1 h and then 160 mg
Pd(II) and 2-(4-bromophenyl)-1,3-dioxolane (1.83 g, 8 mmol) in
THF (20 mL) were added quickly to this system. Afterwards the
mixture was refluxed at 85 ꢁC for 12 h. 1 M HCl aqueous solution
(50 mL) was added and then after stirring for 30 min, the mixture
was extracted with dichloromethane. The organic layer was
washed successively with brine, and dried over magnesium
sulfate. After filtration, the solvent was evaporated under
reduced pressure, and the residue was purified by silicon-gel
column chromatography using petroleum ether/ethyl acetate ¼
10 : 1 (v/v) as eluent, Rf 0.25. Light yellow green solid of 1,1-
3,4-diphenylsilole (0.12 g, 0.255 mmol) and malononitrile (0.07 g,
1 mmol) were dissolved in anhydrous ethanol and then refluxed
at 88 ꢁC for 48 h. After cooling to room temperature, the reddish
solution was dried under reduced pressure, and the residue was
purified by silicon-gel column chromatography using petroleum
ether/ethyl acetate ¼ 20 : 1 (v/v) as eluent, Rf 0.15. Orange solid
of 8 was obtained in 38.1% yield (0.055 g). 1H NMR (400 MHz,
CDCl3): d (TMS, ppm) 7.73–7.67 (d, 4 H), 7.63–7.59 (s, 2 H),
7.13–7.01 (m, 10 H), 6.80–6.74 (d, 4 H), 0.55–0.51(s, 6 H). 13C
NMR (80 MHz, CDCl3): d (TMS, ppm) 159.3, 157.2, 147.3,
142.4, 137.6, 131.1, 129.8, 128.7, 128.1, 127.6, 114.3, 113.2, 81.0,
ꢀ3.6. HRMS (MALDI-TOF), m/z Calad C38H26N4Si: 566.1927.
Found: 566.2630 (M+). Mp: 225 ꢁC and 228 ꢁC (DSC). UV
(THF): lmax (3max): 429 nm (10ꢀ5 mol Lꢀ1).
Preparations of crystals
The THF solution of molecule 8 was poured into a small glass
spawn bottle, and then this uncovered bottle was put into a wide-
mouth container which was filled with hexane. The container was
firmly sealed and stored at room temperature, and crystal O was
afforded in two weeks. We attempted to grow crystal R from the
mixture of acetone, DCM and THF at first, however, it is not
suitable for single crystal analysis, thus the condition was opti-
mized to the methanol and DCM mixture. From the above
solution, the crystal R was given in the conditions similar to
crystal O in about ten days. The grinding of the solid samples was
carried out in an agate mortar.
dimethyl-2,5-bis(4-benzaldehyde)-3,4-diphenylsilole
was
1
obtained in 59.8% yield (1.13 g). H NMR (400 MHz, CDCl3):
d (TMS, ppm) 9.89–9.88 (s, 2 H), 7.67–7.62 (d, 4 H), 7.09–7.05 (d,
4 H), 7.05–6.97 (m, 6 H), 6.80–6.76 (d, 4 H), 0.52–0.48 (s, 6 H).
13C NMR (80 MHz, CDCl3): d (TMS, ppm) 191.7, 155.9, 146.8,
142.3, 137.8, 133.8, 129.7, 129.6, 129.2, 127.8, 126.9, ꢀ4.2. Anal.
Calcd for C32H26O2Sꢁi: C, 81.66; H, 5.57. Found: C, 81.20; H,
5.88%. Mp: 207–208 C. UV (THF): lmax (3max): 377 nm (10ꢀ5
mol Lꢀ1).
Synthesis of (1,1-dimethyl-2,5-bis(4-benzylidenemalononitrile)-
3,4-diphenylsilole) (8)
Structural characterizations of 8
The melting points of the crystals were recorded on a SGW X-4
digital melting point apparatus at first and then verified by
differential scanning calorimetry again. Differential scanning
calorimetry (DSC) measurement was conducted on Perkin-
Elmer DSC 7 under N2 atmosphere at a heating/cooling rate of
The target silole derivative, i.e. molecule 8, was prepared from di
(phenylethynyl)silanes via key intermediate 1,1-dimethyl-2,5-
bis(4-benzaldehyde)-3,4-diphenylsilole according to the synthetic
route shown in Scheme 1. 1,1-Dimethyl-2,5-bis(4-benzaldehyde)-
4296 | J. Mater. Chem., 2012, 22, 4290–4298
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10 K minꢀ1. Single crystal X-ray diffraction intensity data were
collected at 133 K for crystal O and 173 K for crystal R on an
Xcalibur, Sapphire 3, Gemini ultra diffractometer with graphite
monochromated enhance ultra Cu-Ka X-ray source radiation.
Empirical absorption correction was done by using spherical
harmonics, implemented in SCALE 3 ABSPACK scaling algo-
rithm. CrysAlisPro, Oxford Diffraction Ltd., Version
1.171.33.55. The structure solution was measured by SHELXS-
97 (Sheldrick, 1990), and the structure refinement were con-
ducted using the SHELXL-97 (Sheldrick, 1997) suite of X-ray
programs. The analysis was carried out using Mercury Version
2.3. Powder X-ray diffraction (XRD) patterns were recorded on
X0pert PRO, PANalytical and Cu-Ka radiation was used at 40
kV and 40 mA.
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Theoretical calculations
The simulated XRD patterns of crystals were calculated from
their crystallographic data by Poudrix. All calculations on the
considered molecules were performed by using the Gaussian 09
program package.14 B3lYP/6-31g (d) was employed to do the
single point energy calculation based on the crystal structure.
The relative energies of the HOMO and LUMO levels were
obtained from the computed results.
Acknowledgements
This work was partially supported by the National Science
Foundation of China (21074113, 50873086, and 20974028), the
Ministry of Science and Technology of China (2009CB623605),
the Natural Science Foundation of Zhejiang Province
(Z4110056); the Research Grants Council of Hong Kong
(603509, 601608, HKUST2/CRF/10), the Innovation and Tech-
nology Commission (ITP/008/09NPandITS/168/09), the
University Grants Committee of Hong Kong (AoE/P-03/08). We
thank Prof. Z. G. Shuai (Tsinghua University, Beijing) for his
helps in theoretical calculation, and Prof. P. Lu and Y. G. Ma for
their help in solid-state quantum yield measurement. B.Z.T.
thanks the support from the Cao Guangbiao Foundation of
Zhejiang University.
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