Phenyl ring dynamics in a tetraphenylethylene-bridged metal-organic framework: Implications for the mechanism of aggregation-induced emission

Article abstract of DOI:10.1021/ja306042w

Molecules that exhibit emission in the solid state, especially those known as aggregation-induced emission (AIE) chromophores, have found applications in areas as varied as light-emitting diodes and biological sensors. Despite numerous studies, the mechanism of fluorescence quenching in AIE chromophores is still not completely understood. To this end, much interest has focused on understanding the low-frequency vibrational dynamics of prototypical systems, such as tetraphenylethylene (TPE), in the hope that such studies would provide more general principles toward the design of new sensors and electronic materials. We hereby show that a perdeuterated TPE-based metal-organic framework (MOF) serves as an excellent platform for studying the low-energy vibrational modes of AIE-type chromophores. In particular, we use solid-state ²H and ¹³C NMR experiments to investigate the phenyl ring dynamics of TPE cores that are coordinatively trapped inside a MOF and find a phenyl ring flipping energy barrier of 43(6) kJ/mol. DFT calculations are then used to deconvolute the electronic and steric contributions to this flipping barrier. Finally, we couple the NMR and DFT studies with variable-temperature X-ray diffraction experiments to propose that both the ethylenic C' - 'C bond twist and the torsion of the phenyl rings are important for quenching emission in TPE, but that the former may gate the latter. To conclude, we use these findings to propose a set of design criteria for the development of tunable turn-on porous sensors constructed from AIE-type molecules, particularly as applied to the design of new multifunctional MOFs.

Full text of DOI:10.1021/ja306042w

Article
pubs.acs.org/JACS
Phenyl Ring Dynamics in a Tetraphenylethylene-Bridged Metal−
Organic Framework: Implications for the Mechanism of Aggregation-
Induced Emission
Natalia B. Shustova,† Ta-Chung Ong,†,‡ Anthony F. Cozzolino,† Vladimir K. Michaelis,†,‡
Robert G. Griffin,*,†,‡ and Mircea Dinca*,†
̆
†Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
‡Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
S
* Supporting Information
ABSTRACT: Molecules that exhibit emission in the solid
state, especially those known as aggregation-induced emission
(AIE) chromophores, have found applications in areas as
varied as light-emitting diodes and biological sensors. Despite
numerous studies, the mechanism of fluorescence quenching
in AIE chromophores is still not completely understood. To
this end, much interest has focused on understanding the low-
frequency vibrational dynamics of prototypical systems, such as tetraphenylethylene (TPE), in the hope that such studies would
provide more general principles toward the design of new sensors and electronic materials. We hereby show that a perdeuterated
TPE-based metal−organic framework (MOF) serves as an excellent platform for studying the low-energy vibrational modes of
AIE-type chromophores. In particular, we use solid-state 2H and 13C NMR experiments to investigate the phenyl ring dynamics
of TPE cores that are coordinatively trapped inside a MOF and find a phenyl ring flipping energy barrier of 43(6) kJ/mol. DFT
calculations are then used to deconvolute the electronic and steric contributions to this flipping barrier. Finally, we couple the
NMR and DFT studies with variable-temperature X-ray diffraction experiments to propose that both the ethylenic CC bond
twist and the torsion of the phenyl rings are important for quenching emission in TPE, but that the former may gate the latter.
To conclude, we use these findings to propose a set of design criteria for the development of tunable turn-on porous sensors
constructed from AIE-type molecules, particularly as applied to the design of new multifunctional MOFs.
and luminescent polymers16,17 have sparked a rapid expansion
of the field in the past decade. Despite these advancements, the
exact mechanism of AIE continues to be a subject of interest for
INTRODUCTION
■
The relaxation of singlet excited states in light-absorbing
molecules occurs either by emission of a photon, giving rise to
theoreticians and experimentalists alike; deciphering it
unequivocally would clearly be beneficial for the ab initio
development of new classes of AIE molecules.18,19 Generally,
AIE arises because rotor-containing molecules exhibit low-
frequency vibrational modes in the gas phase or in dilute
solutions. These modes are responsible for very fast non-
radiative decay of the singlet excited state but are eliminated in
the solid state due to intermolecular steric interactions. For
instance, tetraphenylethylene (TPE), one of the most accessible
and simplest AIE-type chromophores, exhibits low-frequency
phenyl torsion modes and CC twist modes (Figure 1) that
are deactivated in the solid state by close intermolecular
arene···H and Ph···Ph interactions.5,19,20 Understanding the
relative contribution and effect of these vibrational modes and
conformational changes is one of the keys to making more
efficient and more sensitive fluorescence turn-on sensors from
rotor-containing chromophores.
fluorescence, or nonadiabatically, through nonradiative decay
pathways.1 In most cases, chromophores that show high
fluorescence quantum yields in dilute solutions become
nonfluorescent in colloids and in the solid state, where
intermolecular interactions, such as π-stacking, often cause
self-quenching.2 This effect, sometimes referred to as
aggregation-caused quenching, poses significant difficulties for
the development of solid-state fluorescence devices, such as
organic light-emitting diodes and luminescence-based sen-
sors.2−4 A diametrically opposed effect is operative, however, in
a select class of chromophores that exhibit weak or almost no
fluorescence in dilute solutions but show high-fluorescence
quantum yields in colloidal aggregates and in the solid state.5,6
This opposite effect, known as aggregation-induced emission
(AIE), is observed in molecules that contain groups executing
fast discrete diffusion, such as two- or three-fold hops by phenyl
or trimethylsilyl rotors, respectively. These moieties are bonded
to relatively inflexible backbones, such as ethylenic CC
bonds, or rigid rings, such as silole.7−9 The discovery of the AIE
effect and its wide potential for applicability in biological and
environmental sensors,10−14 solid-state lighting devices,4,6,15
Received: June 20, 2012
Published: August 13, 2012
© 2012 American Chemical Society
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Ph···Ph interactions, as is typical for molecular constructs of
rotor-containing chromophores. Accordingly, we propose that
both the CC bond twist and the torsion of the phenyl rings
are important for quenching emission in TPE but that the
former may gate the latter. We use these findings to propose a
set of design criteria for the development of tunable turn-on
porous sensors constructed from AIE-type molecules.
RESULTS
■
Figure 1. The planes used to define the twist in the ethylene core
(left) and a portion of the X-ray crystal structure of Zn2(TCPE) that is
representative of both 1H and 1 (right). Orange, red, blue, and gray
spheres represent Zn, O, N, and C atoms, respectively. H/D atoms
were removed for clarity.
Synthesis and Temperature-Dependent Structural
Studies. Synthesis of a deuterated TPE-based MOF started
from deutero-tetra(4-carboxy)phenylethylene, H4TCPE-d16,
which was accessed from perdeuterated benzene in four
steps, shown in Scheme 1. Treatment of C6D6 with oxalyl
chloride in carbon disulfide produced benzophenone-d10, which
was subsequently homocoupled under McMurry condensation
conditions23 to yield TPE-d20. Bromination of TPE-d20 with
neat Br2 followed by copper-catalyzed halide-for-cyanide
exchange and basic hydrolysis of the nitrile groups gave the
desired tetracarboxylate ligand, H4TCPE-d16 in 31% overall
yield. Heating a solution of H4TCPE-d16 and Zn(NO3)2·6H2O
in a mixture of ethanol and N,N-diethylformamide (DEF) at 75
°C for 3 days produced yellow block-shaped crystals of
Zn2(TCPE-d16)(DEF)2·2DEF (1a).
An X-ray diffraction study of a single crystal of 1a revealed a
structure in which Zn2(O2C−)4 paddlewheel units are bridged
by TCPE4−-d16 ligands in infinite two-dimensional (2D) sheets
whose connectivity is identical to that found in 1H.21 The
sheets adopt a staggered conformation to give similar but not
identical lattice parameters to those of 1H, as shown in Table
S1. Despite the slight shift in the stacking arrangement of the
2D sheets in 1a relative to 1H, the two related structures
exhibit almost identical fluorescence spectra and thermal
behavior, evidenced in the TGA traces shown in Figure S1.
As in 1H, thermal treatment of 1a produces several significant
structural transformations. Since these are crucial for the
interpretation of the NMR data, we undertook variable-
temperature X-ray diffraction studies of both TPE-d20 and 1.
Thus, the X-ray crystal structure of TPE-d20 was determined at
93, 298, and 373 K. TPE-d20 maintains the monoclinic P21
space group at all three temperatures, with no significant
changes in lattice parameters, molecular packing, or Ph···Ph
ring intermolecular distance. As shown in Table S2, the shortest
interchromophore contacts (Ph···Ph ring contacts) are
3.583(3)−3.635(5) Å, while the twist angle of the CC
bond (Figure 1) is 8.84−10.16°. Over the entire temperature
range, the shortest intermolecular TPE contacts change by no
more than 0.052(6) Å, and the change in the CC twist angle
is less than 1.32°. Single crystal X-ray structures of 1 were also
determined at 100 and 373 K. As shown in Figure 2, 1 adopts a
monoclinic structure at 100 K (1a) but undergoes a symmetry-
increasing transformation to an orthorhombic phase while
To this end, we sought to understand the mechanism that
induces fluorescence in a TPE-based metal−organic framework
(MOF) reported recently by us.21 Although the formation of
close intermolecular contacts had previously been presumed
necessary for turning on emission in rotor-containing
chromophores,5 we showed that coordination of phenyl groups
to metal atoms within MOFs also turns on the fluorescence of
the TPE cores. One such material, Zn2(TCPE)(solvent)2 (1H,
TCPE = tetrakis(4-carboxyphenyl)ethylene), exhibits arene···H
and Ph···Ph interactions on neighboring TPE cores that are 1.5
Å longer than in molecular TPE aggregates (Figure 1).21
Although these distances are sufficiently large to allow
unimpeded rotation/flipping of the phenyl rings,22 1H is
fluorescent. We surmised that because the carboxylate groups in
H4TCPE are installed in the para position, phenyl ring flipping
and/or libration in 1H is not completely eliminated, and that
understanding the mechanism of fluorescence turn-on in 1H
would therefore aid in the design of efficient emitters and more
sensitive, guest-induced turn-on fluorescence sensors. Our
interest in studying the dynamics of phenyl ring motion in
TPE-based MOFs was therefore motivated by the possibility of
providing general principles toward the formation of high-
surface area turn-on fluorescent sensors from AIE-type
chromophores. In doing so, we were also hoping to shed
more light on the mechanism of aggregation-induced emission
and thereby provide guidance for the development of new
chromophores in this rapidly expanding area.
With these goals in mind, we synthesized a deuterated TPE-
based MOF that is structurally analogous to 1H and employed
2H NMR spectroscopy and 13C cross-polarized magic angle
spinning solid-state (CP MAS) NMR spectroscopy to
determine the activation barrier for phenyl ring flipping in
this material. In conjunction with temperature-dependent
single-crystal and powder X-ray diffraction analysis, and density
functional theoretical calculations, these results reveal that
fluorescence is turned-on in TPE-based MOFs by drawing of
the TPE core rather than the presence of close intermolecular
Scheme 1. Synthesis of H4TCPE-d16
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extension of the shortest Ph···Ph contacts from 4.744(9) to
5.10(1) Å and a reduction of the ethylene twist angle from
5.35° to 3.83°. The lack of guest DEF molecules in 1b,
formulated as Zn2(TCPE-d16)(DEF)2, is in agreement with the
thermogravimetric analysis (TGA) and elemental analysis data
(vide infra).
Continued heating at 200 °C caused a complete loss of Zn-
coordinated DEF molecules. PXRD analysis revealed that this is
also accompanied by a drastic structural rearrangement to a
desolvated form of 1, Zn2(TCPE-d16) (1c). Because single
crystals of 1b do not survive their transformation into 1c, we
sought to match the observed PXRD pattern of 1c with a
structural model. This was accomplished by implementing an
original computational routine in Matlab, which simulates
PXRD patterns of possible phases by changing the interlayer
distance and relative displacement of 2D layers. In this case, the
structure of 1b was used as an initial model, and we considered
the possibility that 1c is related to the former by simple
translations of the 2D sheets in the ab plane and/or by changes
in the intersheet separation. Modulation of these parameters
using our routine provided a structural model for 1c that
exhibited a good match with the observed pattern (Figure S2).
Although the relatively poor crystallinity of 1c prevented a full
Rietveld refinement even from synchrotron-collected data, our
computational routine revealed that 1c is a new orthorhombic
phase with parameters of 12.66, 8.40, and 21.62 Å.
Figure 2. Temperature-dependent X-ray diffraction studies of 1. Left
column: PXRD patterns of (a) 1a calculated from the X-ray crystal
structure determined at 100 K, (b) 1a collected at room temperature,
(c) 1b calculated from X-ray structure at 373 K, (d) the sample used in
the 2H NMR study, and (e) 1c. Right column: X-ray crystal structures
of 1a collected at 100 K, 1b collected at 373 K, and the simulated
structure of 1c based on the PXRD data. Golden, red, blue, and gray
spheres represent Zn, O, N, and C atoms, respectively. Guest DEF
molecules are shown in pink. H/D atoms have been removed for
clarity.
The one notable difference between 1b and 1c is the much
reduced interlayer distance, which decreases from 8.7 Å in the
former to 4.2 Å in the latter (Figure 2). The contraction of the
interlayer distance brings the Zn2(O2C−)4 paddlewheel units in
neighboring 2D sheets in close proximity and prompts the
formation of covalent linkages between Zn atoms in one sheet
and carboxylate oxygen atoms in adjacent sheets. The absence
of all DEF molecules from 1c was confirmed by TGA, which
showed a mass loss of 36.2% below 200 °C, in agreement with
the 35.4% expected for the elimination of four DEF molecules
from 1a (Figure S1).
heating to 373 K, which we designate as 1b. Importantly,
powder X-ray diffraction (PXRD) analysis revealed that the 2D
sheets do not change their relative positioning upon trans-
formation from 1a to 1b. Determination of the unit cell
parameters of 1a at room temperature confirmed only very
small deviations from the orthorhombic cell determined at 373
K for 1b. Apart from the small deviation in overall symmetry,
important structural differences between the structures of 1a
and 1b include the lack of guest DEF molecules in the latter, an
2
1H, 13C, and H NMR Spectroscopic Studies. Variable-
1
temperature H NMR spectra of TPE and H4TCPE were
recorded in CD2Cl2 and CD3OD, respectively, between 183
2
Figure 3. Experimental and simulated quadrupolar spin−echo solid-state H NMR spectra of 1a during heating and transformation into 1b (left)
and of 1b during cooling (right).
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and 293 K. The phenyl ring protons appear as a pair of doublets
with chemical shifts of 7.14 and 7.81 ppm (3JHH = 8 Hz) for
H4TCPE and two multiplets with chemical shifts at 7.03 and
7.10 ppm for TPE itself. As shown in Figure S3, cooling to 183
1
K broadens the H resonances in both TPE and H4TCPE, but
the two different proton signals do not coalesce, suggesting that
the phenyl rings in both molecules are in the fast exchange
regime in solution even at 183 K. To confirm the expected slow
exchange regime of phenyl rings in TPE-d20 in the solid state,
2H NMR spectra of crystalline samples of this molecule were
recorded between 298 and 423 K. As shown in Figure S4, the
2H NMR spectra of solid TPE-d20 in this temperature range
exhibit two peaks separated by Q = 128 kHz. This yields a Pake
pattern characteristic of C−2H vectors in the slow exchange
regime (τ∼ 10−3−10−4 s).24 2H quadrupolar echo spectroscopy
was also used to investigate the phenyl ring dynamics in 1a and
1c. As shown in Figure S5, 1c showed almost identical Pake
patterns up to 423 K, the highest temperature achievable with
our NMR probe. In contrast, freshly synthesized 1a showed
Pake patterns only between room temperature and approx-
imately 323 K. Heating 1a above 323 K caused the line shape to
evolve into a pattern, wherein a second set of symmetric peaks
with a smaller splitting of Q/4 = 32 kHz emerged along with a
third wider splitting being −5Q/−4 = 160 kHz. As shown in
Figure 3, the intensity of this central set gradually increased at
the expense of the original outer signal up to 423 K. An
isotropic signal also became apparent above 323 K, likely
indicative of the increased mobility of the guest DEF molecules.
Indeed, this isotropic signal disappeared after prolonged
heating at 423 K, indicating the loss of the guest molecules
and conversion to 1b. Upon cooling of 1b, the reverse
evolution of the quadrupolar signal was observed; the two-fold
flip pattern at 423 K gradually evolved into a typical slow-
exchange Pake pattern at 323 K.
Figure 4. Arrhenius plot of the two-fold phenyl exchange rate in 1b
during cooling.
Figure 5. 13C CP-MAS NMR spectra of fully desolvated 1H (top) and
1c (bottom).
To simulate the spectra, we assumed a model consisting of a
single population of phenyl rings undergoing discrete two-fold
flips. The model was used for simulations of the 2H
quadrupolar line shapes for five temperatures during the
cooling cycle of 1b between 421 and 321 K.25,26 The
simulations yielded flipping rates of 1.2 × 106, 2.0 × 105, 3.2
× 104, 1.8 × 104, and 1.0 × 104 Hz at 421, 369, 345, 321, and
300 K, respectively. To obtain an activation energy and pre-
exponential factor for phenyl ring flipping in 1b, the natural
logarithm of the rates was plotted against the inverse of the
respective temperatures to give an Arrhenius plot. A line fit to
this graph, shown in Figure 4, gave activation energy and pre-
exponential factor values of 43(6) kJ/mol and 2.2 × 1011 Hz,
respectively. Although 2H NMR revealed a wealth of
information about the phenyl ring dynamics in 1b, it was not
suitable to interrogate the same in 1c, where the phenyl ring
motion remains in the slow regime (<104 Hz) regardless of the
temperature (vide supra). Because 13C CP MAS NMR
spectroscopy can be used to probe motions down to
frequencies of ∼102 Hz,27 13C CP MAS NMR spectra were
acquired for 1c and its protonated relative (fully desolvated
1H) at room temperature. As shown in Figure 5, both
deuterated and protonated versions of the MOFs exhibit
isotropic peaks at 135, 137, and 153 ppm for the phenyl ring
carbon atoms and 147 and 181 ppm for the ethylene and
carboxylate carbon atoms, respectively.
Figure 6. DFT-calculated structures of truncated formate-capped
models of 1b with a fixed orientation of one phenyl ring at 125° (left)
and 5° (middle) and of TPE with a fixed orientation of the phenyl ring
at 0° (right). The scheme illustrates the distortion in the TPE core that
occurs to minimize the steric repulsion, namely in-plane bends of the
CAr−CC angles and the CC twist. The models are depicted
without hydrogen atoms for clarity. Yellow, red, and gray spheres
represent Zn, O, and C atoms, respectively. The carbon atoms that
define the dihedral angles used to model the PESs are shown in purple.
surface (PES) of TCPE4¯ bound by four Zn2(O2C−)4
paddlewheels. The metal coordination sphere was completed
with three bridging formate ligands and two terminal water
ligands (Figure 6).
The PES was constructed by varying one CAr−CAr−CC
dihedral angle from 0 to 180° and is depicted in Figure 7. The
Zn and oxygen atom coordinates were fixed in order to mimic
the rigidity imposed by the framework. Notably, a very similar
PES could be obtained using H4TCPE with the oxygen atom
coordinates fixed to those found in the 1 (Figure S7) with
significant savings in computational resources. The lowest
Theoretical Studies. Density functional theory (DFT) was
employed to calculate the activation barrier for ring flipping in
1b. The barrier was estimated by modeling the potential energy
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significant structural deviation from the minimum energy
structure involving the CAr−CC−CAr and CAr−CAr−CC
dihedral angles as well as the CAr−C(ethylene)−CAr and CAr−C
C bond angles (Figure 6), whereas the constraints imposed by
the rigid framework in 1b prevent the ethylene core from
undergoing similar distortions (Tables S3−S6). The structural
distortions in TPE correspond to the lowest energy vibrational
modes (Table 1) that occur well below kT (206 cm−1 at 298
K).
Table 1. DFT-Calculated Low-Energy Vibrational Modes for
TPE
energy (cm−1
)
vibrational mode
Figure 7. PES for the flipping of one phenyl ring in a truncated model
6
ν1
ν2
ν3
ν4
ν5
ν6
ν7
ν8
ν9
C−CC−C torsion
CAr−CAr−CC torsion
CAr−CAr−CC torsion
aryl rocking
○
of 1b ( ) and sum of the electron density at the C−C single bond
29
39
54
58
65
69
72
78
●
critical points ( grey ). The electron density axis has been reversed
and scaled for clarity. Lines have been added as a visual guide.
CAr−CAr−CC torsion
energy structure from the PES, which was deemed closest to
the absolute minimum energy conformation, was used as a
starting point for a geometry optimization to find the absolute
minimum. Because of the size of the system under
investigation, a transition state was not modeled. Under these
parameters, the activation energy for a ring flip in 1b was
estimated at 49 kJ/mol.
The DFT-estimated activation barrier for phenyl ring flipping
in TPE in the gas phase is 24 kJ/mol. This value was
determined by first modeling the PES by varying the CAr−CAr−
CC dihedral angle from 0 to 180° and 180 to 0° with no
additional constraints (Figure 8). To correct for false maxima
aryl rocking
CAr−CAr−CC torsion
CAr−CAr−CC torsion
CAr−CAr−CC torsion
In order to deconvolute the steric from the electronic effects
in the barriers in the PESs for the truncated model of 1b and
for TPE, PESs for the ring flipping of the phenyl ring in styrene
and benzoic acid were constructed under the assumption that
the PESs for these two systems provide a rotational barrier that
is free of steric effects. In both cases, the minimum in energy
occurs when the CAr−CAr−CC dihedral angle is 0°, which
corresponds to the geometry that maximizes the conjugation
between the phenyl ring and the pendant group (Figure S6).
The associated calculated activation energies for phenyl ring
flipping in styrene and benzoic acid are 18 and 27 kJ/mol,
respectively.
DISCUSSION
■
The structure of 1a consists of a 2D framework composed of
paddlewheel Zn2(O2C−)4 secondary building units that are
bridged by TCPE4¯-d16 ligands (Figures 1 and 2). The structure
contains both bound DEF molecules, which occupy the axial
sites on Zn atoms in the Zn2 paddlewheels, and guest DEF
molecules, which occupy the pores. The latter likely prevents
2
fast flipping of the TPE phenyl rings, and H NMR spectra of
1a accordingly reveal Pake patterns characteristic of slow
exchange (<104 Hz). The Pake patterns persist up to 373 K, but
heating 1a above this temperature starts liberating the guest
DEF molecules, thereby activating the phenyl ring flips. Indeed,
2H NMR spectra at 373, 396, and 423 K reveal an isotropic
signal that can be attributed to solvent motion and dynamic
quadrupole patterns that can be fit to discrete 180° phenyl ring
flips with respective frequencies shown in Figure 3. Because
both guest solvent loss and activation of phenyl ring dynamics
take place during heating of 1a, the two processes are
convoluted and prevent an Arrhenius analysis. Instead, the
sample was kept at 423 K for 24 h to eliminate all of the guest
solvent molecules and complete conversion of 1a into 1b, as
identified by the disappearance of the isotropic signal attributed
Figure 8. PES for the flipping of one phenyl ring in a model of TPE.
The solid line with black circles ( ) indicates the lowest energy
surface constructed from the forward (solid gray line and open circles,
●
○
) and the reverse (hashed gray line) direction ring-flip PESs.
that could arise due to the high number of degrees of freedom,
a minimum energy PES was constructed by convoluting PESs
calculated in the forward and reverse directions of phenyl ring
rotation.
As before, the lowest energy structure on the convoluted PES
was used as a starting point for a geometry optimization and
was confirmed by a frequency calculation that provided no
negative values. Notably, we calculated a barrier of 49 kJ/mol
for the truncated model of 1b in the vicinity of 0°, a value that
is approximately 25 kJ/mol higher in energy than that
calculated for TPE at the same angle. The structure of TPE
at the maxima reveals an ethylene core that has undergone
2
to mobile DEF. H NMR data were again collected for 1b on
cooling back to room temperature, with data points at 421, 369,
345, 321, and 300 K. Because no guest solvent molecules are
present in 1b, the data could be plotted in Arrhenius fashion, as
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shown in Figure 4. The experimentally determined activation
energy for the phenyl ring flip in 1b, 43(6) kJ/mol, is larger
than that expected for free TPE by approximately 20 kJ/mol.
This suggests that, indeed, the torsion of the phenyl ring in 1b
is impeded relative to solution-phase TPE and is likely the
cause of fluorescence turn-on in the TPE-based MOF. The pre-
exponential factor, which can be interpreted as the barrier-less
flipping rate of the pure phenylene bridge,28,29 is 2.2 × 1011 Hz
and is somewhat smaller than those of phenylene bridges in
related porous materials, such as MOF-5,30,31 and periodically
ordered mesoporous organosilica.22 It is conceivable, however,
that intramolecular steric effects converge to decrease the pre-
exponential factor in TPE derivatives relative to phenylene
itself.
One essential aspect of the NMR data interpretation relates
to the stability and identity of the sample during the heating
cycle. As for 1H, heating of 1a above 150 °C causes loss of both
bound and unbound DEF molecules and is accompanied by
significant structural changes and formation of a new phase, 1c.
In 1c, fused 2D sheets bring phenyl rings on adjacent TPE
cores in close proximity, giving rise to short Ph···Ph contacts of
∼5 Å (measured between the centroids of the phenyl rings), in
line with those observed in molecular crystals of TPE
derivatives and solid TPE itself.32
considering ring flipping in styrene and benzoic acid, as well as
vinylbenzoic acid (Figures S6 and S8). These molecules have
similar electronic structures to the benzoate units in the
truncated model of 1b, but their phenyl groups lack vicinal
phenyl rings that could sterically hinder rotation. The barrier to
phenyl ring flipping in these can therefore be assumed to be
completely electronic in origin. The electronic component of
the PES for ring flipping in TPE could therefore be
reconstructed from the PES of styrene, even though the energy
contributions were not necessarily expected to be additive. This
implied that the barrier for ring flipping in gas-phase TPE is
almost completely electronic in origin, and the steric
interactions expected to occur at a CAr−CAr−CC dihedral
angle of 0° are avoided due to a number of small geometrical
distortions that correspond to low-energy vibrational modes
(Table 1). Rationalizing the shape of the PES of 1b is more
complicated because it cannot be reconstructed by simply
summing the contributions from the PESs of styrene and
benzoic acid. Because in 1b itself the ethylene core is
perpendicular to the carboxylate groups, the effect of the
electronic contribution to the overall barrier for ring flipping is
expected to be rather insignificant (Figure S8). To attest this,
an atoms-in-molecules analysis of the C−C single bonds at
select points on the PES was performed (Table S7).33
Expectedly, just like TPE, 1c exhibits Pake patterns at both
low and high temperature, reinforcing the observation that
close-packed TPE cores prohibit torsional motion of their
phenyl(ene) components. Importantly, however, if 1b is heated
below 150 °C (i.e., the temperature range of our NMR
experiments), its structure and the large Ph···Ph separation
conducive to fast phenyl ring flipping are maintained. This
important fact was verified by both single crystal and powder X-
ray analysis. Thus, single crystal X-ray diffraction of 1b at 100
°C showed that no significant structural changes occur relative
to 1a. Although single crystals of 1b do not survive heating at
150 °C, powder X-ray analysis of the sample used for the NMR
experiments showed a pattern that matched that of 1b, with
only small peaks corresponding to the completely desolvated
phase, 1c (see Figure 2 and below). Because phenyl ring
motion in 1c is in the slow-exchange regime in this temperature
range, its presence does not affect the dynamic line shapes used
for the Arrhenius plot for 1b and is a minor contributor only to
the Pake singularities with large quadrupolar splitting. In
addition, 13C CP MAS NMR spectra of 1c and of fully
desolvated 1H illustrate that both of these compounds exhibit
similar resolution and line shape, which is consistent with a
rigid lattice with motion that is slower than what is detectable
with this technique (<102 Hz) (Figure 5). This finding agrees
with the 2H NMR results, which show that the ring motions are
in the slow exchange regime.
To understand the origin of the activation barrier in 1b,
especially in comparison to TPE itself, the ring flipping process
in both 1b and TPE was probed by DFT calculations. The
calculated values of the activation barriers for ring flipping in a
truncated model of 1b and gas-phase TPE are 49 and 24 kJ/
mol, respectively. Clearly, despite the axial symmetry of
phenylene rings in H4TCPE, which should allow fast flipping
in a sterically unhindered environment such as the pores of 1b,
phenyl ring flipping in 1b is much more sluggish than in TPE
itself. To understand the origin of the increased barrier in 1b
and the differences between the PESs of 1b and TPE, a more
detailed look at the steric and electronic contributions to these
was performed. The electronic contribution was probed by
Since the density at the critical point is indicative of the bond
order, the sum of the electron densities at each of the C−C
single bond critical points should be indicative of the amount of
electron delocalization throughout the molecule and, by
extension, the stability of the conformation at each point.
Figure 7 illustrates how the sum of the densities at the C−C
critical points mirrors the shape of PES. A key point is that the
lowest total density, which should correspond to the least stable
conformation, is found at a local maximum. This indicates that
the global maximum found at 5° (49 kJ/mol) is not entirely
electronic in origin and must have a considerable steric
contribution. Investigation of the geometry at the maximum in
the PES of 1 (Figure 6) shows that the ortho-hydrogen atom
on one phenyl ring is directed into the π-cloud of the vicinal
cis-phenyl ring. Unlike in gas-phase TPE, where low-energy
geometric distortions to the TPE core allow the steric
maximum to be avoided (Figure 6), the TPE core in 1b is
drawn tight, thereby forcing the phenyl rings to remain in close
proximity during the ring flipping process.
This computational analysis highlights the following points:
(1) Low-energy vibrational modes in the TPE core minimize
inter-ring steric interactions and allow ring flipping to occur
with a low barrier (25 kJ/mol); and (2) The drawing of the
TPE core by the framework forces these steric interactions to
occur, leading to a significantly higher barrier for ring flipping
(49 kJ/mol) that is in good agreement with the experimentally
derived barrier.
The relative importance of the CC bond twist and phenyl
ring torsion in quenching the fluorescence in molecular TPE
derivatives has been addressed before, and it was concluded
that the latter has a higher contribution to the nonradiative
decay of the excited state.34 Our results are in line with this
observation and allow us to establish a connection between the
two: diminution of the CC twist angle by drawing of the
TPE core in 1b causes a larger steric barrier for phenyl torsion/
flipping, suggesting that a relatively large CC twist angle or a
flexible core is required for a low-barrier phenyl ring torsion.
The activation barrier for ring flipping in 1b is comparable to
the activation energies for phenylene-linked porous materi-
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Table 2. Activation Energies and Discrete Flipping Models for Known, Structurally-Rigid Porous Materials
a
BDC = 1,4-benzenedicarboxylate; BDC-NH2 = 2-amino-1,4-benzenedicarboxylate; BDC-Br = 2-bromo-1,4-benzenedicarboxylate; Pz = pyrazine;
b
STP = 2-sulfoterephthalate; NDC = 1,4-naphthalenedicarboxylate; and dabco =1,4-diazabicyclooctane. The torsion angle refers to the dihedral
angle defined by the two para carboxylate groups. Pre-exponential factor in Arrhenius equation.
c
als.22,29,30,36 For instance, activation energies for 1,4-aromatic
dicarboxylate-based MOFs range from 21−53 kJ/mol (Table
2). Although some of these are higher than the activation
energy for ring flipping in 1b, the differences can be entirely
attributed to conjugation-stabilized conformations in which the
carboxylate groups and the phenylene ring are coplanar. We
confirmed computationally that the PES constructed for ring-
flipping in terephthalic acid gives an activation energy of 50 kJ/
mol when both carboxylate groups are held coplanar, in good
agreement with experimentally observed activation energies for
MOFs constructed from this ligand. In the absence of
conjugating groups, exemplified by the pyrazine-bridged
structures, a much lower activation energy is found. In these,
because pyrazine is primarily a σ-donating ligand, there is little
energetic cost for ring flipping, which must only overcome a
weak π-type interaction with the d10 metal ion.
Importantly, because the origin of the activation barrier in 1b
is enforced partly by coordination in a rigid lattice and is
therefore not inherently borne in the ligand, strategies can be
envisioned for reducing the activation barrier for ring-flipping.
These strategies include: (1) Designing MOFs where AIE-type
chromophores are well separated spatially. This is necessary to
avoid aggregation in the empty material and to ensure porosity
for analyte adsorption; (2) maintaining the flexibility in the
TPE core to ensure that low-energy vibrational modes are not
eliminated in the empty material. This could be implemented,
for instance, by leaving two dangling/unsubstituted phenyl
rings which should maintain dynamics in the fast flipping
regime; and (3) minimizing ligand conjugation to reduce the
contribution of an electronic component to the ring-flipping
barrier. This could be achieved, for instance, by enforcing a
perpendicular orientation between the ethylene core and the
metal-binding functional groups, as in 1, by using acetylene
spacers to ‘insulate’ the phenyl ring from orientation-inducing
conjugation, or by using nonconjugating ligating groups.
The criteria outlined above would allow the design of true
turn-on MOF sensors. In such sensors, AIE-type chromophores
with low-barrier ring flipping would completely quench the
fluorescence in the empty porous materials. Fluorescence
would then only be turned-on in the presence of analyte guests
that can hinder the rotation of the phenylene ring, thereby
eliminating the low-energy nonradiative excited-state quench-
ing pathways. MOFs are ideal candidates for incorporating such
strategies because they lend themselves to modular synthetic
design.43,48−56 We envision that these strategies are not limited
to TPE-based ligands and should be more broadly applicable to
the construction of switchable luminescent MOFs from a wide
variety of AIE-type ligands.
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between the two pulses. Phenyl ring motional dynamics were
EXPERIMENTAL SECTION
■
2
determined by simulations of the experimental H powder lineshapes
Materials. Zn(NO3)2·6H2O (98%, Strem Chemicals), Br2
(≥99.5%, Sigma-Aldrich), CuCN (99%, Strem Chemicals), Zn (dust,
98.6%, Mallinckrodt), oxalyl chloride (98%, Alfa Aesar), TiCl4 (≥99%,
Sigma-Aldrich), MgSO4 (98%, VWR), AlCl3 (≥99%, Sigma-Aldrich),
N,N′-dimethylethylenediamine (99%, Sigma-Aldrich), dichlorome-
thane (HPLC grade, Honeywell), methanol (99.9%, VWR), DEF
(>95%, TCI America), ethanol (ACS grade, Mallinckrodt), ethylene
glycol (AR grade, Mallinckrodt), ethyl acetate (VWR), tetrahydrofuran
(ACS grade, Mallinckrodt), toluene (Sigma-Aldrich, ACS), C6D6
(Cambridge Isotopes), CDCl3 (Cambridge Isotopes), CD3OD
(Cambridge Isotopes), and DMSO-d6 (Cambridge Isotopes) were
used as received.
Tetraphenylethylene-d20 (C26D20, TPE-d20). The synthetic
sequence for the preparation of this material is shown in Scheme 1.
Benzophenone-d10 was synthesized from benzene-d6 based on a
known procedure57 and was then heated (5.47 g, 0.03 mmol) to reflux
in the presence of TiCl4 (8.60 g, 0.05 mmol) and Zn dust (5.90 g, 0.09
mol) under McMurry conditions23 to give 4.60 g (0.01 mol) of
using TURBOPOWDER.62
13C MAS NMR Spectroscopy. Experiments were performed at
16.4 T (697.8 MHz, 1H) using a home-built spectrometer (courtesy of
Dr. Dave Ruben) and a 3.2 mm Chemagnetics triple-channel magic-
angle spinning probe. Samples were ground using a mortar and pestle
and packed in 3.2 mm ZrO2 rotors (∼28 μL sample volume). Spectra
were acquired at spinning frequencies of 10 kHz, with 512−4096
coadded transients and recycle delays between 3 and 120 s, using
either a Bloch decay or cross-polarization63 (υrf of 83 kHz for H and
1
13C, τCP = 2.0 ms) and two pulse phase modulation (TPPM) proton
decoupling64 for naturally abundant 13C deuterated and protonated
samples. 13C experiments were referenced to adamantane at 38.48
ppm relative to TMS.65
Computational Details. Calculations were performed using the
ORCA 2.8 quantum chemistry program package from the develop-
ment team at the University of Bonn.66 In all cases the LDA and GGA
functionals employed were those of Perdew and Wang (PW-LDA,
PW91).67 Calculations were performed using the TZV basis set for
hydrogen, the TZV(p) basis set for main group atoms, and TZV(2pf)
for zinc.68 Spin-restricted Kohn−Sham determinants have been chosen
to describe the closed-shell wave functions, employing the RI
approximation and the tight SCF convergence criteria provided by
ORCA. Numerical frequency calculations were performed on the
optimized structures when size would permit. The atoms-in-molecules
analysis was performed using Xaim.69
2
perdeutero-tetraphenylethylene (87% yield). H NMR (CHCl3): δ =
7.05 (br) ppm; 13C NMR (CDCl3, 500 MHz): δ = 126.20 (t), 127.24
(t), 131.00 (t), 140.93 (s), 143.68 (s) ppm. IR (neat, cm−1): 2281 (s),
2269 (s), 1617 (w), 1563 (m), 1385 (w), 1322 (s), 1279 (w), 1202
(w), 959 (w), 878 (w), 855 (s), 841 (m), 822 (vs), 788 (w), 763 (w).
Elemental analysis calculated for C26D20: C, 88.57; H(D), 6.07. Found:
C, 88.67; H(D), 5.87.
Tetrakis(4-cyanophenyl)ethylene-d16 (C30D16N4, H4TCNPE-
d16). H4TCNPE-d16 was prepared from TPE-d20 following a recently
published synthetic route for the protonated analogue.21 13C NMR
(CD2Cl2, 500 MHz) δ = 111.78 (s), 118.56 (s), 131.56 (t), 132.14 (t),
141.75 (s), 145.82 (s) ppm. IR (neat, cm−1): 2294 (w), 2225 (vs),
1573 (s), 1414 (w), 1321 (m), 1291 (w), 1109 (m), 869 (w), 827 (m),
759 (w), 743 (w), 718 (w), 677 (w). Elemental analysis for calculated
for C30D16N4: C, 80.36; H(D), 3.72; N, 12.49. Found: C, 80.10; H(D)
3.75; N, 12.30.
Tetrakis(4-carboxyphenyl)ethylene-d16 (C30H4D16O8,
H4TCPE-d16). H4TCPE-d16 was synthesized by hydrolysis of the
corresponding nitrile following the published procedure for the
protonated analogue.21 2H NMR (CH3OH, 500 MHz): δ = 7.19 (br),
7.86 (br) ppm; 13C NMR (CDCl3, 500 MHz): δ = 128.8 (m), 129.29
(s), 130.49 (m), 141.10 (s), 146.32 (s), 166.96 (s) ppm. IR (neat,
cm−1): 2972 (w, b), 2225 (w), 1687 (vs), 1578 (s), 1542 (w), 1439
(m), 1376 (w), 1327 (w), 1259 (b, s), 1206 (s), 1078 (w), 871 (w),
841 (w), 816 (w), 786 (w), 746 (w), 691 (w). Elemental analysis
calculated for C30H4D16O8·H2O: C, 66.4; H(D), 4.21. Found: C,
66.09; H(D), 3.93.
Other Physical Measurements. TGA was performed on a TA
Instruments Q500 thermogravimetric analyzer at a heating rate of 0.5
°C/min under a nitrogen gas flow of 90 mL/min. Infrared spectra
were obtained on a PerkinElmer Spectrum 400 FT-IR/FT-FIR
spectrometer equipped with a Pike Technologies GladiATR
attenuated total reflectance accessory. Solution NMR spectra were
collected on a Varian 300 or a Varian Inova-500 NMR spectrometer.
2H spectra were referenced to the natural abundance 2H peak in
1
protonated solvents; 13C and H spectra were referenced to natural
1
abundance 13C peaks and residual H peaks of deuterated solvents,
respectively. PXRD patterns for 1a and 1b were recorded on a Bruker
Advance D8 diffractometer using nickel-filtered Cu−Kα radiation (λ =
1.5418 Å), with accelerating voltage and current of 40 kV and 40 mA,
respectively. A PXRD pattern for 1c was collected at station 11-B at
the Argonne National Laboratory using synchrotron radiation (λ =
0.413073 Å). Samples for PXRD were prepared by placing a thin layer
of the appropriate material on a silicon (510) crystal plate for 1a and
1b and by sealing 1c in a Kapton capillary.
Synthesis of Zn2(TCPE-d16)(DEF)2·2DEF (1a). This compound
was synthesized in an identical procedure as 1H. IR (neat, cm−1): 2979
(w), 2939 (w), 2878 (w), 2272 (w), 1634 (vs), 1578 (m), 1559 (m),
1442 (s), 1382 (vs), 1309 (w), 1265 (w), 1215 (w), 1106 (w), 881
(w), 832 (w), 820 (w), 703 (w), 677 (w). Elemental analysis
calculated for 1a·H2O: C, 55.92; H(D), 5.91; N, 5.22. Found C 55.74,
H 5.73, N 5.10.
ASSOCIATED CONTENT
* Supporting Information
X-ray structure refinement tables and details, NMR spectra,
TGA traces, DFT-calculated molecular conformations and
PESs, and emission spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
S
X-ray Crystal Structure Determination. Diffraction-quality
single crystals of 1a, 1b, and TPE-d20 were mounted using mineral
oil and epoxy on Kapton loops. Diffraction data (φ- and ω-scans) at
100, 298, and 373 K were collected on a Bruker-AXS X8 Kappa Duo
diffractometer coupled to a Smart APEX II CCD detector with MoKα
radiation (λ = 0.71073 Å) from an IμS-micro source. Absorption and
polarization corrections were applied using SADABS.58 The structure
was solved by direct methods using SHELXS and refined against F2 on
all data by full-matrix least-squares with SHELXL-97.59 All non-
hydrogen atoms were refined anisotropically and were included in the
model at geometrically calculated positions. The crystallographic data
for TPE-d20 and 1 are shown in Table S1.
AUTHOR INFORMATION
Corresponding Author
mdinca@mit.edu, rgg@mit.edu
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported as part of the Center for Excitonics,
an Energy Frontier Research Center funded by the U.S.
Department of Energy, Office of Science, Office of Basic Energy
Sciences under award number DE-SC0001088 (MIT). We
would like to thank the 11 B-sector team at the Advanced
Photon Source, Argonne National Laboratory, which was
supported by the U.S. Department of Energy, Office of Science,
2H NMR Spectroscopy. Experiments were conducted on a home-
built spectrometer (courtesy of Dr. Dave Ruben) operating at 61 MHz
for 2H using a single-channel transmission line probe with 3.2 mm coil.
Spectra were obtained using a quadrupolar echo sequence60 with an 8-
step phase cycling61 using a π/2 pulse of 2.0 μs and a delay of 30 μs
15068
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Stimson, E. R.; Meinwald, Y. C.; Freed, J. H.; Scheraga, H. A. J. Am.
Chem. Soc. 1981, 103, 7707−7710.
Office of Basic Energy Sciences, under contract no. DE-AC02-
06CH11357. Grants from the NSF also provided instrument
support to the DCIF and single crystal X-ray diffraction facility
at MIT (CHE-9808061, DBI-9729592, CHE-0946721). R.G.G.
is supported through the National Institute of Health (NIH
grants EB001960 and EB002026). V.K.M. thanks the Natural
Sciences and Engineering Research Council of Canada for a
postdoctoral fellowship. We thank Tarun Narayan for writing
the Matlab routine for modeling the structure of 1c and
Minyuan Li for assistance with the production of the TOC
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