Switching of Adsorption Properties in a Zwitterionic Metal-Organic Framework Triggered by Photogenerated Radical Triplets

Article abstract of DOI:10.1021/acs.chemmater.6b03224

A new metal-organic framework (MOF) that features photoreactive zwitterionic pyridinium 4-carboxylate units has been designed. Upon UV light irradiation, these units form radical triplets permitted by intramolecular electron transfer between anionic carbo

Full text of DOI:10.1021/acs.chemmater.6b03224

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Article
Switching of Adsorption Properties in a Zwitterionic Metal-
Organic Framework Triggered by Photogenerated Radical Triplets
Wen An, Darpandeep Aulakh, Xuan Zhang, Wolfgang Verdegaal, Kim R. Dunbar, and Mario Wriedt
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03224 • Publication Date (Web): 17 Oct 2016
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Chemistry of Materials
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Switching of Adsorption Properties in a Zwitterionic Metal-
Organic Framework Triggered by Photogenerated Radical
Triplets
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Wen An, Darpandeep Aulakh, Xuan Zhang, Wolfgang Verdegaal, Kim R. Dunbar, and Mario
Wriedt*
Department of Chemistry & Biomolecular Science, Clarkson University, Potsdam, New York 13699, United States
Department of Chemistry, Texas A&M University, College Station, Texas 77845, United States
ABSTRACT: A new metal-organic framework (MOF) has been designed that features photo-reactive zwitterionic pyri-
dinium-4-carboxylate units. Upon UV-light irradiation, these units form radical triplets enabled by intramolecular elec-
tron transfer between anionic carboxylate and cationic pyridinium groups. This reversible light-responsive behavior cre-
ates on/off switchable charge gradients localized at the MOF's major adsorption sites and thus, significant control of the
gas sorption process. It is shown that this strategy offers new design routes to access stimuli-responsive materials.
cation within seconds, exhibiting stability up to several
■
INTRODUCTION
months. None of
these materials are reported to be porous however.
Stimuli-responsive properties are one of the most intri-
guing aspects of metal-organic frameworks (MOFs),1 as
the structure can undergo significant changes in response
In this vein, we recently reported a new design strategy
to access zwitterionic (ZW) MOFs from anionic viologen
derivatives.38 We have shown that an electrostatic field
gradient can be found on the zwitterionic molecular sur-
face due to their well-separated intramolecular charges
and therefore, their incorporation into MOFs can create
charged organic surfaces (COSs) within the pore envi-
ronment – a new means to polarize guest molecules.39, 40
This effect has the potential to increase host-guest inter-
actions and, hence, to enhance adsorption enthalpies (Fig
1A). More importantly, the fact that pyridinium-based
zwitterions can be reversibly photo-reduced to a radical
species may lead to the elimination of any charge gradi-
ents, thus representing an unprecedented mechanism to
design on/off switchable adsorption sites (Fig 1B). In the
current study, we hypothesize that intriguing opportuni-
ties for tunable adsorption properties are possible if this
behavior can be triggered in porous MOFs. In particular,
it is expected that this process does not lead to any major
structural changes, as only the electrostatic surface is
modified as opposed to conformational changes in azo-
benzene-/diarylethene-based materials. In particular, this
structurally inert behavior would be of interest for mem-
brane and thin film applications. Therefore, our strategy
is to introduce pyridinium moieties as light-switchable
functionalities into a porous MOF. As a proof of concept,
the effect of reversible radical formation of the ligand on
the gas adsorption properties of the MOF was investigat-
ed by CO2 adsorption experiments since its adsorption in
MOFs is typically significant at room temperature. This
approach of implementing stimuli-responsive adsorption
behavior based on the reversible photo-induced genera-
to physical or chemical stimuli such as temperature,2,
3
pressure,4, 5 light,6, 7 pH8, 9 or electricity.10-12 Among these
materials, optically responsive MOFs are receiving in-
creasing attention due to the controllable release of gas
molecules from their pores modulated by light irradia-
tion.13-16 Herein, the vast majority of light-responsive
MOFs17, 18 are azobenzene19-24 or diarylethene25, 26 contain-
ing frameworks that exhibit drastic changes in their gas
adsorption properties based on structural transformations
through the reversible and light-driven cis/trans isomer-
ization of azobenzene or ring opening/closing reactions
of diarylethene units.
At the same time, the formation of radicals in MOFs
have been studied extensively,27 including N-substituted-
pyridiniums and the better known N,N’-disubstituted-
4,4’-bipyridiniums (viologens). While bipyridinium cati-
ons are not themselves radicals, they are often readily
photo-reduced in a photochromic process to form deep
blue colored radical species. Subsequent exposure to the
atmosphere triggers a return to the diamagnetic state, a
process that can also be accelerated thermally. Recently,
it was demonstrated that such cations can be incorpo-
rated in cavities of anionic MOFs, leading to a new type of
composite material which, upon UV irradiation, can
maintain a radical character.28-30 In another approach,
carboxylate-functionalized pyridinium cations are used as
building-blocks in coordination polymers.31-37 In both cas-
es, the radical generation process was always described as
a photo-induced intermolecular electron transfer from
the donor carboxylate anion to the acceptor pyridinium
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tion of radicals to produce a switchable electrostatic pore solution of isonicotinic acid (0.555 g, 4.5 mmol) in DMF
1
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surface into a MOF is unprecedented, as none of the ex-
isting porous pyridinium-based ZW MOFs is reported to
exhibit any light-
(36 mL). After the mixture was stirred at 70 °C for 6
hours, the resulting precipitates were collected by filtra-
tion, washed with DMF and diethyl ether, and dried in
vacuum to give H3LBr3 as a light yellow powder (0.411 g,
yield 40.0%). Anal. Calcd for C27H24Br3N3O6 (726.21): C
44.66%, H 3.33%, N 5.79%; found: C 44.05%, H 3.69%, N
5.38%. 1H-NMR (400 MHz, D2O): δ = 8.95 (d, 2H); 8.34 (d,
2H); 7.59 (s, 1H); 5.88 (s, 2H) (Figure S2).
9
2. Synthesis of {[CdBr(L)]·(ClO4)·2DMF}n (1). Bulk
material was prepared by the following procedure: H3LBr3
(19.4 mg, 0.04 mmol) and Cd(ClO4)2·6H2O (62.4 mg, 0.2
mmol) were dissolved in 2 mL of DMF and the pH value
of the solution was adjusted to ~7 using 70 µL 1.0 mol L-1
aqueous NaOH solution. Ivory colored polycrystalline
precipitate was obtained after stirring the mixture in a
closed glass vial on a heating plate at 80 °C for 3 days. The
residue was removed by filtration, washed with water and
diethyl ether, and air dried. The purity was confirmed by
X-ray powder diffraction (Figure S3). Yield based on the
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Figure 1. (A) Schematic representation of a ZW MOF pore
showing well-separated charges on its surface. Their charge
gradients yield into a well-aligned intramolecular electrostat-
ic field which in turn leads to enhanced polarization effects
on guest molecules and thus, strong host-guest inter-actions;
(B) the ZW component can be reversibly transformed into a
radical species upon exposure to external stimuli, switching
off the COS and releasing adsorbed guest molecules; note:
the intramolecular electron transfer process is indicated by
the curved arrows in A; (C) structure of the tritopic ZW lig-
and.
ligand:
14.5
mg
(39.4%).
Anal.
Calcd
for
C27H25BrCdClN3O12 (811.267): C 39.97, H 3.11, N 5.18; found:
C 39.29, H 3.23, N 5.32%. Note that this elemental compo-
sition is calculated based on a dihydrate, as opposed to
the DMF solvate suggested by SC-XRD analysis. We as-
sume that the prolonged handling time during elemental
analysis promoted this solvent exchange. IR (KBr pellet,
cm-1): ꢀȬ = 3121 (w), 3057 (w), 1618 (s), 1563 (s), 1509 (w),
1455 (m), 1360 (s), 1255 (w), 1212 (w), 1181 (w), 1167(w), 1078
(s), 930 (w), 870 (m), 776 (s), 705 (m), 685 (m), 663 (w)
(Figure S4).
responsive adsorption behavior.41-47 Herein, we report the
design of a unique tritopic pyridinium-based ZW ligand
yielding a new ZW MOF, which exhibits an unprecedent-
ed light-responsive adsorption effect. We believe that our
findings constitute a potential route towards the devel-
opment of new CO2 scrubber materials with an ultra-low
energy requirement for their regeneration processes, as
opposed to the state-of-the-art, energy intensive tempera-
ture- or pressure-swing routes.
Single-crystals were prepared by the following proce-
dure: H3LBr3 (9.7 mg, 0.02 mmol) and Cd(ClO4)2·6H2O
(31.2 mg, 0.1 mmol) were dissolved in a mixture of 0.5 mL
water and 0.5 mL DMF. The pH was adjusted to 7 using
1.0 mol L-1 aqueous NaOH solution. The reaction mixture
was kept in a closed vial for three weeks at room tempera-
ture, forming colorless needle-shaped single crystals.
3. Powder X-Ray Diffraction. PXRD patterns were
collected using a Bruker D2 Phaser diffractometer
equipped with a Cu sealed tube (λ = 1.54178 Å). Powder
samples were dispersed on low-background discs for
analyses. Simulations of the PXRD data were performed
using single crystal data and the Powder Pattern module
of the Mercury CSD software package.48
■
EXPERIMENTAL SECTION
General Information. Commercially available rea-
gents were used as received without further purification.
The
zwitterionic
ligand
1,1′,1′′-(benzene-1,3,5-
triyl)tris(methylene)tris(4-carboxypyridinium) tribromide
(H3LBr3) was synthesized as described below.
4. Elemental analysis. Elemental analyses (C, H, and
N) were completed by Atlantic Microlab, Inc.
1. Synthesis of H3LBr3. Mesitylene (1.2 g, 10 mmol), N-
bromosuccinimide (4.43 g, 25 mmol) and benzoyl perox-
ide (0.06 g, 0.25 mmol) were dissolved in 20 mL benzene
and refluxed at 95 °C for 12 h. The reaction mixture was
filtered, and the filtrate was evaporated to yield a yellow
oil. Crystals were formed after 3 days, collected by filtra-
tion, washed with an ethanol/hexane mixture, and dried
under vacuum to give 1,3,5-tris(bromomethyl)benzene as
5. Spectroscopy. FT-IR data were recorded on a Ni-
1
colet iS10 from Thermo Scientific. H-NMR data were rec-
orded on Avance DMX-400 from Bruker.
6. Magnetic Measurements. Magnetic characteriza-
tion of the sample was carried out in a Quantum Design
MPMS XL SQUID magnetometer under an applied mag-
netic field of 1000 Oe in the temperature range of 2-300 K.
The activated sample was first measured in a sealed
quartz tube. Then the same sealed sample was subjected
to UV irradiation (365 nm, 100 W, lamp UVP B-100AP) for
48 h with frequent shaking.
1
a white powder (4.14 g, yield 57.1%). H-NMR (400 MHz,
CDCl3): δ = 4.45 (s, 6H), 7.35 (s, 3H) (Figure S1).
A solution of 1,3,5-tris(bromomethyl)benzene (0.543g,
1.5 mmol) in DMF (1.5 mL) was treated dropwise with a
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measurement. Oil-free vacuum pumps and oil-free pres-
7. Single-Crystal Structure Analysis. Data collec-
1
2
3
4
5
6
sure regulators were used for all measurements to prevent
contamination of the samples during the evacuation pro-
cess or of the feed gases during the isotherm measure-
ments.
tions were performed on single crystals coated with Para-
tone-N oil and mounted on Kapton loops. Single crystal
X-ray data of all compounds were collected on a Bruker
Kappa Apex II X-ray diffractometer outfitted with a Mo X-
ray source (sealed tube,
λ = 0.71073 Å) and an APEX II
The sequence of events for the characterization of the
stimuli-responsive adsorption effect of 1 were performed
using the same sample and are as follows: (1) the sample
was activated; (2) a CO2 isotherm was collected; (3) the
sealed sample was irradiated by UV light (365 nm, 100 W,
lamp UVP B-100AP) for 48 h with frequent shaking; (4) a
CO2 isotherm was collected; (5) the sample was exposed
to ambient air for 12 h; (6) the sample was re-activated
and (7) a final CO2 isotherm was collected.
CCD detector equipped with an Oxford Cryosystems
Desktop Cooler low temperature device. The APEX-II
software suite was used for data collection, cell refine-
ment, and reduction.49 Absorption corrections were ap-
plied using SADABS.50 Space group assignments were
determined by examination of systematic absences, E-
statistics, and successive refinement of the structures.
Structure solutions were performed using intrinsic phas-
ing methods implemented with ShelXT,51 and structure
refinements were performed by least-squares refinements
against |F|2 followed by difference Fourier synthesis using
ShelXL;52 both pieces of software are part of the ShelX53
program package. All non-hydrogen atoms were refined
with anisotropic displacement parameters. The C-H at-
oms were positioned with idealized geometry and were
refined with fixed isotropic displacement parameters
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Table 1. Selected crystal data and details on the struc-
ture determinations from single crystal data for 1.
Formula
MW [g·mol-1]
Crystal system
Space group
a [Å]
C33H35BrCdClN5O12
921.42
Orthorhombic
Pbcn
[Ueq(H) = -1.2·Ueq(C)] using a riding model with dC-H
=
28.9782(10)
17.9781(10)
12.4794(5)
90
0.95 Å (aromatic) and 0.99 Å (methylene). It should be
noted that one perchlorate and two DMF molecules could
be found in the electron density maps. However, a rea-
sonable structural model was only found for the perchlo-
rate in 1. Thus, the data were corrected using the
SQUEEZE option in PLATON.54 SQUEEZE estimated a
total count of 709.2 electrons per 1268.7 Å3 of solvent ac-
cessible volume, which is in good agreement with two
DMF molecules (Z = 8; DMF = 40 e-; 709.2 e-/8 = 88.7 e- ≈
2 DMF molecules) per asymmetric unit of 1. Details of the
structure determination are given in Table 1.
b [Å]
c [Å]
α
β
γ
[deg]
[deg]
[deg]
90
90
V [Å3]
6501.4(5)
170(2)
T [K]
Z
8
CCDC-1428227 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained
free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk.
Dcalc [g·cm-3]
1.883
µ [mm-1]
2.061
Min/max transmission
0.657/0.746
25.027
θ
max [deg]
8. Adsorption Analysis. Gas adsorption isotherms for
pressures in the range 1·10-5 to 1.1 bar were measured by a
volumetric method using a Micromeritics ASAP2020 sur-
face area and pore analyzer. A pre-weighed analysis tube
was charged with the sample, capped with a seal frit and
evacuated by heating at 100 °C under dynamic vacuum for
12 h. Note that this activation process removes all guest
molecules from the pores as evidenced by the TGA data
(SI, Section S5). The evacuated analysis tubes containing
the activated samples were then carefully transferred to
an electronic balance and weighed to determine the sam-
ple mass (121.0 mg). The tubes were then transferred to
the analysis port of the gas adsorption instrument. For all
isotherms, warm and cold free space correction meas-
urements were performed using ultra-high purity He gas
(UHP grade 5.0, 99.999% purity). All gases used are UHP
grade (99.999% purity). N2 and H2 isotherms at 77 K were
measured in liquid nitrogen, isotherms at 273 K were
measured using ice water and isotherms at 283 K and 295
K were measured using water baths. All temperatures and
fill levels were monitored periodically throughout the
Measured reflections
Unique reflections
27896
5749
1954
Reflections [F0 > 4
Parameter
Rint
σ(F0)]
388
0.1715
0.0686
0.1892
R1 [F0 > 4σ(F0)]
wR2 [all data]
GOF
0.804
ꢀρmax/ꢀρ
min [e·Å-3]
1.916/-1.008
9. Thermogravimetric Analysis. TGA data were
recorded using a TGA Q50 from TA Instruments. All
measurements were performed using platinum crucibles
in a dynamic nitrogen atmosphere (50 mL min-1) and a
heating rate of 3 °C min-1. The instrument was corrected
for buoyancy and current effects, and was calibrated using
standard reference materials.
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we initially investigated potential intermolecular reaction
10. Differential Scanning Calorimetry. DSC data
were recorded using a TGA Q20 from TA Instruments. All
measurements were performed using T zero aluminum
pans, a dynamic nitrogen atmosphere (50 mL min-1) and a
heating rate of 3 °C min-1. The instrument was calibrated
using standard reference materials.
1
2
3
4
5
6
7
8
pathways: In the present case, we identified the carbox-
ylate groups and the bromide anions as potential electron
donors. The perchlorate anions can be excluded as elec-
tron donor species due to their rather weak electron-
donating ability. Favorable conditions for the electron trans-
fer can be found if the electron donor is located perpendicu-
larly and in close proximity to the pyridinium N atom. Note
that inter-
11. Density Functional Theory Calculations. DFT
calculations were performed using Gaussian 09 software55,
namely the B3LYP exchange-correlation function56 in
combination with 6-31G* basis-set57 to evaluate the distri-
butions and energy levels of involved molecular orbitals.
Spin densities were computed at the same level. The or-
bital and spin density maps were plotted by Multiwfn
software version 3.3.8.58
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■
RESULTS AND DISCUSSION
1. Synthesis and Crystal Structure. The ZW ligand
1,1′,1′′-(benzene-1,3,5-triyl)tris(methylene) tris(4-
carboxypyridinium)tribromide (H3LBr3) was designed as
an exemplary stimuli-responsive ligand, as its tritopic
flexible nature is predicted to lead to interesting new
MOF topologies with favorable intra/intermolecular car-
boxylate-O to pyridinium-N orientations enabling acces-
sible electron transfer pathways for radical formation. Its
reaction with Cd(ClO4)2·6H2O in dimethylformamide at
80 °C leads to the formation of a new MOF material (SI,
Section S1). Single crystal X-ray diffraction analysis reveals
the composition {[CdBr(L)]·(ClO4)·2DMF}n (1). Powder X-
ray diffraction analysis confirmed phase-purity of the bulk
material (SI, Fig. S3). It crystallizes in the orthorhombic
space group Pbcn, with the asymmetric unit consisting of
one Cd(II) cation, one L ligand, one bromide and one per-
chlorate anion, and two non-coordinating DMF molecules
(SI, Fig. S5). All atoms are located on crystallographically
independent general positions. In the crystal structure,
each Cd(II) is hexa-coordinated with five carboxylate-O
atoms of three symmetry-related bridging L ligands and
one terminally bonded Br anion in a distorted trigonal
prismatic geometry (Fig. 2A). Enabled by the three meth-
ylene groups of L, their pyridinium rings are significantly
twisted from the central benzene moiety with dihedral
angles of 77.3, 74.2 and 86.0° (angles listed in order ring-
N11, -N21 and -N31). This arrangement facilitates the metal
centers to be bridged by two bidentate and one monoden-
tate carboxylate groups of L to form 2D layers (SI, Fig.
S6), which are further linked by strong interlayer hydro-
gen bonding interactions (SI, Table S1) to form a 3D su-
pramolecular framework with two pore types of similar
diameter (~4 Å, Fig. 2B). The interlayer voids are partially
occupied by the non-coordinating perchlorate anions,
serving as interlayer hydrogen bonding acceptors outside
the pores (SI, Fig. S7).
Figure. 2 Crystal structure of 1 with view of the metal coor-
dination environment (A) and 3D supramolecular coordina-
tion network showing two types of pores (B). Solvent acces-
sible surfaces are shown in orange and hydrogen atoms are
omitted for the sake of clarity. Color scheme: Cd, turquoise;
Br, brown; Cl, green; C, grey; N, blue and O, red.
molecular geometries and distances between donor and
acceptor units of 70-100° and 3.4-4.0 Å respectively are
reported to be suitable for generation radicals.31-37 A thor-
ough structural analysis of 1 reveals that such orientations
can be found for the carboxylate groups (SI, Table S2),
whereas unfavorable distances of >4.5 Å are found for the
bromide anion. However, it should be pointed out that
the aforementioned favorable donor-acceptor orienta-
tions are solely reported for viologen derivates and that to
the best of our knowledge an intermolecular electron
transfer has never been reported for isonicotinate deri-
vates. This is not surprising, as the direct linkage of the
carboxylate group to the pyridinium unit results in an
extended conjugation that delocalizes the carboxylate's
negative charge over the pyridinium ring. Thus, this
structural feature leads to the mitigation of both the elec-
tron-accepting ability of the pyridinium ring and the elec-
tron-donor ability of the carboxylate group,.33 Such struc-
tural attributes render the photoinduced electron transfer
unfavorable and thus, an intermolecular electron transfer
can be ruled out as a potential dominating radical form-
ing mechanism.
2. Mechanism of Radical Formation. A crucial step
towards the photo-induced radical formation in this MOF
is the occurrence of suitable electron transfer reactions
between the pyridinium acceptor units and organic elec-
tron donors. To determine the nature of such reactions,
Following this argument, density functional theory
(DFT) calculations have been carried out to explore po-
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Chemistry of Materials
ligand-only computation is sufficient in the current situa-
tential intramolecular charge transfer (CT) transitions in
1
2
3
4
5
6
7
8
L. The ground state geometry was adapted from the X-ray
data and further optimized (SI, Fig. S8). The computa-
tional calculations suggest that L exhibits a complete sep-
aration of the HOMOs and LUMOs. The majority of the
electron distribution of the HOMOs was located on the
carboxylate groups (Fig. 3A) and that of the LUMOs on
the pyridinium units (Fig. 3B). The complete localization
of HOMOs and LUMOs means the HOMO–LUMO tran-
sition becomes a typical photoinduced intramolecular CT
tion especially given the fact that the electronic excitation
of interest is ligand-centered and not ligand-to-metal in
nature.26, 61-63
3. Photo-Induced Radical-Driven Adsorption Ef-
fect. The above findings provide the basis for the hypoth-
esis that 1 should exhibit an optically induced radical-
driven adsorption effect. To test this hypothesis, the CO2
adsorption properties were characterized before and after
UV exposure. Note, that scattering from crystal defects
and absorption from competing chromophores renders
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photons unusually difficult to penetrate into the core of
the crystals.64 Thus, to enhance radical generation, the
bulk crystals were irradiated with UV-light (365 nm) for
an extended time of 48 h in air. In comparison to the pris-
tine activated sample, a significant decrease of 43.2 % in
CO2 uptake at 273 K/1 bar was recorded for the UV irradi-
ated sample (pristine: 18.3 cm3 g-1, after UV irradiation:
10.4 cm3 g-1). This change in uptake of 38.2% and 35.0% is
slightly less pronounced at 283 K and 295 K, respectively
(Figs. 4A and SI, Fig. S9). At the same time, it is worth
mentioning
Figure. 3 Spatial representations of (A) the HOMOs, energy -
5.85 eV; and (B) the LUMOs, energy -3.25 eV in the ZW form
of L. The orbital isosurfaces are given in isovalue of 0.03. Red
and blue represent positive and negative orbital phases, re-
spectively; (C) the spin density for the multiradical state of L,
which is distributed on all three ligand branches evenly, with
each branch having two unpaired electrons. For calculating
spin density, the isovalue was set to 0.05.
transition.59, 60 Accordingly, upon excitation the HOMO
donates one electron to the LUMO, resulting in the for-
mation of one diradical per branch of L with each set of
two unpaired electrons delocalized over the carboxylate
group and pyridinium unit (Fig. 1B). Spin density calcula-
tions confirm the delocalization in all three branches (Fig.
3C). From their nature it can be assumed that a radical
triplet is formed per branch with a total of six unpaired
electrons evenly distributed over the three branches, also
referred to as a radical septet. Interestingly, the formation
of a second radical triplet would result in the pairing of
radicals by Pauli Exclusion principles, thereby returning
to the zwitterion ground state. In principle, the third exci-
tation is observed, but is indistinguishable from the first,
allowing us to conclude that even excitations are silent in
this process with the observation of only the odd. Finally,
although these conclusions are sound, it should be noted
that the chemical environment of L in the framework of 1
may be a prerequisite for the quantum mechanical origin
of these CT transitions, and, at the very least, it is impos-
sible to decouple this effect. Nevertheless, even if one
posits that the ligand-metal interactions could indeed
perturb the linker's molecular orbitals, it is not expected
to change their relative patterns. As a result, this type of
Figure. 4 CO2 adsorption isotherms (A) and isosteric heats
of adsorption (B) of 1 showing a reversible alteration of the
adsorption behavior accompanied by a dramatic change in
the heats of adsorption. Note that in (A) the data points are
superimposed for the pristine (blue) and after UV/air expo-
sure (red) sample.
that the isotherms of the pristine sample can be fitted
using a dual-site Langmuir model, whereas the data after
UV can be roughly fitted only using a single-site model
(SI, Fig. S11, S12). This observation is in agreement with
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the relative adsorption behavior: two types of micro pores
the design of a photochromic diarylethene MOF.25 It is
1
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3
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7
8
are present in 1, which are each strongly adsorbed by CO2
in the pristine form, whereas after UV irradiation they
become more or less equivalent to the CO2 molecules as a
result of their weak adsorption. In contrast, the adsorp-
tion amount of N2 and H2 is negligible at 77 K and 295 K
(SI, Fig. S10, S14, S15). Although the pores are large
enough to host these guest molecules, their weak polar-
izability may explain this unusual adsorption-exclusion.
important to emphasize that these azobenzene- and dia-
rylethene-based materials undergo major conformational
changes upon photo-switching, whereas the current light-
driven reversible alteration of adsorption properties re-
ported herein is driven solely by a switchable electrostatic
pore surface – a new mechanistic approach that is un-
precedented in literature for any sorbent.
We also investigated the potential of 1 as a sorbent for
carbon capture. In general, for a sorbent competent in
carbon capture, high selectivity of CO2 over N2 and ener-
getically efficient sorbent regeneration are two prerequi-
sites. The latter was extensively shown above, as light is
abundant and a low-cost alternative towards tempera-
ture- or pressure-swing sorbent regeneration processes.
The selectivity was investigated using IAST simulations of
the mixed gas uptake (15% CO2, 85% N2), yielding re-
markable selectivity values of 47.3 at 273 K and 26.1 at 295
K, respectively (SI, Fig. S17, S18, and Table S3, S4). These
values fall in the mid-range of best performing MOFs,65-68
but significantly outperform well-known MOFs such as
HKUST-1 (~12), ZIF-8 (~4), MOF-5 (~3) and PCN-11 (~21).69
Another critical element in relation to the energy re-
quirement is the activation of the sorbent. Whereas most
promising MOF adsorbers require energy intense activa-
tion procedures such as solvent-exchange reactions,70 the
“as-synthesized” sample of 1 can be activated through a
simple thermal activation process. Most importantly, the
activated material can be kept in ambient laboratory air
for several weeks without showing any signs of hydration,
as indicated by TG experiments (SI, Fig. S20).
9
To further quantify the observed stimuli-responsive ad-
sorption effect, the heat of adsorption was calculated
from CO2 isotherms at three different temperatures (273,
283 and 295 K), exhibiting a significant decrease from 40.5
kJ mol-1 at zero coverage before UV irradiation to 27.3 kJ
mol-1 afterwards (Fig. 4B). Note that these Qst values were
derived from dual-site Langmuir fits and unfitted iso-
therms respectively (see SI, Fig. S13 for Qst data from sin-
gle-site fit). Most importantly, as a result of this study, it
is evident that the drastic change of the material's CO2
adsorption capacities is in accord with a drastic change in
Qst data.
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In addition, N2 isotherms of 1 collected at 77 K before
and after UV irradiation (Fig. S14) do not indicate any
change in the material's porosity. This observation
strengthens the conclusion that the aforementioned
stimuli-responsive adsorption effect is driven by changes
of the linker's electrostatic charges rather than by any
geometrical modifications of the material's pore space.
Subsequently, the reversibility of this stimuli-
responsive phenomenon was investigated, as the oxida-
tion of viologen radicals into their non-radical forms is
known to be facilitated by air exposure via a process of
oxidative quenching.28, 31-37 This experiment was initiated
with the exposure of the UV irradiated sample to ambient
air for 12 h followed by the characterization of CO2 ad-
sorption. Although the measurements were performed
sequentially using the same sample, the gas uptake at 295
K before UV irradiation and after returning from the radi-
cal back to the non-radical form were almost identical
(Fig. 4A, compare blue and red squares), demonstrating
the reversible alteration of CO2 capture upon photochem-
ical treatment. At the same time, this finding along with
any significant changes of PXRD patterns before and after
UV irradiation (SI, Fig. S16), exclude any significant struc-
tural collapse during and after UV irradiation.
4. Magnetic Characterization. To provide incontro-
vertible evidence that this light-responsive adsorption
effect is based on a radical generation mechanism with
delocalized spins, variable temperature magnetic data
were collected on 1 before and after 48 h of UV irradiation
over the range 2-300 K at 0.1 T (Fig. 5).
Roughly similar performance values can be found for
other light-responsive frameworks such as those reported
by Zhou et al. with a 53.3% decrease in CO2 uptake at 295
K upon UV irradiation of PCN-13 with pore walls lined by
photosensitive azobenzene units.64 An alternative ap-
proach was explored by Hill et al. by employing azoben-
zene units as building blocks into a MOF which resulted
in a 64% dynamic change of CO2 adsorption at 303 K.22
This approach was further adopted by Luo et al.20 and
later by Feng et al.19 to achieve a record decrease of 75%
and 21.4% at 298 K in CO2 capture/release under dynamic
conditions in respective azobenzene-based MOFs. Guo et
al. recorded a CO2desorption capacity of 75% at 298 K by
Figure. 5 Temperature dependence of the magnetic suscep-
tibility-temperature (χT) product of 1 (with sample holder)
before (black squares) and after UV irradiation (blue
squares). The difference is the contribution from the radicals
of the sample only (red squares, the solid line represents the
best fit to χT = χTIPT).
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Chemistry of Materials
The sample before UV irradiation showed diamagnetic
current energy-intense state-of-the-art temperature- or
pressure-swing sorbent regeneration processes.
1
signals with negative χT values due to both the sample
and sample holder. This is expected, as 1 is composed of
diamagnetic Cd2+ cations (d10) and diamagnetic ligands.
After UV irradiation, the sample exhibits paramagnetic
signals, which is direct evidence of the presence of radi-
cals. The difference in χT before and after UV irradiation
allows for the subtraction of the diamagnetic contribution
from the sample holder and diamagnetic atoms so one
can evaluate the contribution from the radicals of the
sample. It should be noted that the Curie Weiss Law is
not obeyed below 300 K, neither can a dimer (Bleaney-
Bowers) or alternating magnetic chain (Bonner-Fisher)
model be fit to the data. Instead, the linear trend in the χT
difference as the temperature changes can be described
using the temperature-independent paramagnetism (TIP)
term (χT = χTIPT ) with a χTIP of 8.7 × 10-3 emu mol-1, which
can be appropriately ascribed to delocalized radical spins.
This magnetic behavior is typical for a Fermi gas-like
state, also referred to as Pauli paramagnetism.71 Moreover,
the absence of any exchange coupling between nearest
neighbors is also consistent with the DFT calculations
suggesting the even distribution of delocalized spins
along all three isonicotinate branches of L.
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ASSOCIATED CONTENT
Supporting Information
Experimental details, synthetic procedure, structural details,
PXRD patterns, Langmuir fits, adsorption isotherms, TGA
data. This material is available free of charge via the Internet
at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*mwriedt@clarkson.edu
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
M.W. gratefully acknowledges Clarkson University for their
generous start-up funding and the Donors of the American
Chemical Society Petroleum Research Fund (56295-DNI10)
for support of this research. The magnetic measurements
were conducted in the Dept. of Chemistry SQUID Facility at
Texas A&M University with a magnetometer obtained by a
grant from the National Science Foundation (CHE-9974899).
The magnetic studies were performed by X.Z. in the Dunbar
laboratories with funds provided by the US Department of
Energy, Office of Science, Basic Energy Sciences, under
Award No. DE-SC0012582. K.R.D. also thanks the Robert A.
Welch Foundation (Grant A-1449) for summer salary for X.Z.
However, it should be noted that the power and dura-
tion of UV irradiation as well as the nature of the sample
in terms of crystallite size, may influence the magnetic
properties of 1. For example, preliminary studies indicate
a decrease in the paramagnetic signal when the material
was irradiated for only 12 h. This observation is not sur-
prising, as scattering from crystal defects and absorption
from competing chromophores renders photons unusual-
ly difficult to penetrate into the core of the crystals.
Therefore, an extensive irradiation for 48 h could result in
quantitative radical formation. Nevertheless, most im-
portantly, this study reveals a drastic change of the rela-
tive magnetic behavior before and after UV irradiation,
which is incontrovertible evidence of radical formation.
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■
CONCLUSIONS
The reversible alteration of CO2 capture upon light ir-
radiation or oxidative quenching has been observed in a
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