Photoactive and physical properties of an azobenzene-containing coordination framework

Article abstract of DOI:10.1071/CH17215

A new three-dimensional coordination framework, [Zn4(tbazip)3(bpe)2(OH)2]·bpe·{solvent} (where bpe≤1,2-di(4-pyridyl)ethene) containing the novel photoactive ligand tbazip (tbazip≤5-((4-tert-butyl)phenylazo)isophthalic acid) has been synthesised and crystallographically characterised. The photoactivity of discrete tbazip was investigated and compared with its photoactivity while incorporated within the framework. The effect of isomerisation of the incorporated azobenzene on the chemical and physical properties of the framework were investigated using UV-vis and Raman spectroscopies. The framework is porous only to hydrogen gas at 77K, but displayed an appreciable uptake for CO2 at 195K.

Full text of DOI:10.1071/CH17215

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem.
Full Paper
https://doi.org/10.1071/CH17215
Photoactive and Physical Properties of an Azobenzene-
Containing Coordination Framework*
A
A
B
B
James S. Caddy, Thomas B. Faust, Ian M. Walton, Jordan M. Cox,
B
A
A
Jason B. Benedict, Marcello B. Solomon, Peter D. Southon,
Cameron J. Kepert, and Deanna M. D’Alessandro
A
,
A C
A
School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia.
Department of Chemistry, University at Buffalo, The State University of New York,
Buffalo, NY 14260-3000, USA.
B
C
Corresponding author. Email: deanna.dalessandro@sydney.edu.au
A new three-dimensional coordination framework, [Zn (tbazip) (bpe) (OH) ]ꢀbpeꢀ{solvent} (where bpe ¼ 1,2-di(4-pyridyl)
4
3
2
2
ethene) containing the novel photoactive ligand tbazip (tbazip ¼ 5-((4-tert-butyl)phenylazo)isophthalic acid) has been
synthesised and crystallographically characterised. The photoactivity of discrete tbazip was investigated and compared with
its photoactivity while incorporated within the framework. The effect of isomerisation of the incorporated azobenzene on the
chemical and physical properties of the framework were investigated using UV-vis and Raman spectroscopies. The
framework is porous only to hydrogen gas at 77 K, but displayed an appreciable uptake for CO at 195 K.
2
Manuscript received: 20 April 2017.
Manuscript accepted: 17 June 2017.
Published online: 8 August 2017.
Introduction
Functional inorganic materials that may be altered by the
[20]
diarylethenes, spiropyrans, and azobenzenes.
[
MOFs incor-
[24]
18,21–23]
porating both diarylethenes
and spiropyrans
have
[
application of an external stimulus, such as pressure, tem-
1]
previously been reported. Azobenzenes in particular have
gained significant attention and are very well studied because
of their ability to undergo facile and rapid trans-cis isomerisa-
tion in high quantum yields around their central azo moiety upon
[
2]
[3]
perature, or light, have been rapidly developing over the
past decade. Light stimulus is of particular interest because it is
highly accessible, tunable, and may be used as a way to modify
the structure of a functional inorganic material to induce a
[
25]
exposure to UV light at 365 nm.
There lies great scope to
[
4]
response (referred to as photoresponsivity). A particular class
of materials that has been at the centre of these studies is metal-
organic frameworks (MOFs). The prospect of incorporating
photoactivity into MOF systems is garnering significant atten-
tion at present, owing to the prospect of exploiting the light
functionalise the azobenzene core to immobilise it within a
MOF; once incorporated, the change in geometry between the
trans and cis isomers alters the distance of the two para carbons
of the azobenzene from 9 to 5.5 A, respectively,
great potential to affect the macroscopic properties of a MOF.
[
26]
˚
which has
[
5]
activity to perform useful functions such as ‘cargo delivery’,
The physical change is accompanied by an induced dipole
[27]
[6]
tunable molecular separations, controlled CO adsorption/
2
moment change from 0 to 3 Da, respectively,
[28]
and a change
An important property of
[7–9]
[10]
desorption,
[11]
and catalysis (both heterogeneous
tocatalysis ). A particular interest in controlled CO adsorp-
and pho-
in melting point of azobenzene.
these molecules is the reversibility of the light-induced changes,
2
[29–31]
tion/desorption stems from the need to mitigate the excess CO2
released into the atmosphere, which has been identified as a
enabling the material to be used over multiple cycles. The
reverse reactions (from the cis to the trans form) require visible
[
12–16]
[32,33]
global challenge.
This makes it particularly relevant to
light irradiation or heating the sample.
explore the interaction of different gases with a framework
material.
The reversible isomerisation properties of the azo functional
group have received significant attention for inducing photoac-
tivity in MOFs. Yanai et al. incorporated discrete azobenzene
molecules into [Zn (terephthalate) (triethylenediamine)], where
One strategy to integrate photoactivity into a MOF is to
exploit its tunability and to incorporate an organic linker that can
2
2
[7,8,17,18]
change conformation upon light irradiation.
Photo-
cis-trans isomerism altered the unit cell of the MOF from
[34]
chromes are substances that undergo a chemical change upon
exposure to electromagnetic radiation, forming products with
tetragonal to orthorhombic. Hermann et al. performed similar
studies on MOF-5, MIL-68(M) (M¼ Ga, In), and MIL-53(Al),
where it was demonstrated that photoactive guests can only
induce structural changes in MOFs if the pore size of the MOF
[
19]
significantly different absorption or emission spectra. There
exist many examples of photochromes in the literature, such as
*
Associate Professor Deanna D’Alessandro is the recipient of the 2017 Australian Academy of Science Le F e´ vre Prize and the 2017 RACI Alan Sargeson
Lectureship (Inorganic Division).
Journal compilation � CSIRO 2017
www.publish.csiro.au/journals/ajc

B
J. S. Caddy et al.
allows for it, and if there is no dense packing of the azoben-
[35]
zene.
functionalised azobenzene ligands. In the latter case, the ability
to target porous structures with an optimal pore geometry is
advantageous.
More recently, efforts have focussed on using visible
light for the capture and release of CO . Hill et al. have
2
incorporatedmethylred (an azobenzene derivative) into the pores
of Mg-MOF-74 and MIL-53(Al), which assisted in the adsorption
Experimental
[9]
of CO after exposure to visible light.
Materials
2
A strategy to control the density of azobenzene sites in MOFs
has involved their installation as pendant groups onto ligands that
form the backbone of the framework scaffold. This approach was
first employed by Stock et al., who post-synthetically installed an
All chemicals were obtained from Aldrich, Merck, Alfa Aesar,
and Cambridge Isotope Laboratories (for deuterated solvents)
and were used without further purification unless otherwise
1
13
1
stated. H and C{ H} NMR spectra were recorded at 298 K on
1
a Bruker AVANCE300, operating at 300 MHz for H and
0
[36]
azo functionality within a 4,4 -bipyridine (bpy) scaffold. The
same group subsequently reported the incorporation of an azo-
13
5 MHz for C{ H}. Chemical shifts were standardised against
1
7
published deuterated solvent shifts.
[
37]
based ligand into MIL-101-NH (Cr),
2
whereby reversible
[46]
light-induced cis-trans isomerisation of the azo bond occurred,
although the reversibility was relatively low. A family of
frameworks incorporating the azo-appended bpy scaffold have
Synthesis
5-(4-(tert-Butyl)phenylazo)isophthalic Acid (tbazip)
[
38]
since been synthesised.
Subsequent work has investigated
The method was adapted from that of Priewisch and R u¨ ck-
[
the effects of the switching of the azo group. In particular, Zhou
et al. synthesised a MOF-5-type framework with appended
47]
Braun.
1,3-Dimethyl-5-aminoisophthalate (2.1 g, 10 mmol)
[
39]
was dissolved in dichloromethane (80 mL). To this solution was
added Oxone (12 g, 20 mmol) dissolved in water (250 mL). The
two-phase mixture was stirred vigorously for 6 h under N . The
azobenzenes which protruded into the pores.
Light depen-
dent gas adsorption studies were undertaken, showing that the
amount adsorbed was reliant on whether the cis or trans isomer
was dominant in the material. The authors suggested that this
difference was due to the pendant azo group blocking and
unblocking access to the metal centre, respectively. MOFs
incorporating azobenzenes have also been found to facilitate
the trapping and release of dye molecules from the pores through
a gating effect, where the movement of the azobenzene
2
organic layer was removed and the aqueous layer was washed
with dichloromethane (3 ꢁ 50 mL). The combined organic
layers were washed with HCl (1 M, 200 mL), saturated aqueous
NaHCO solution (200 mL), H O (200 mL) and brine (200 mL),
3
2
and dried over MgSO . The solvent was removed by rotary
4
evaporation, giving an impure yellow product. The unpurified
1,3-dimethyl-5-nitrosoisophthalate (1.5 g) was dissolved in gla-
cial acetic acid with 4-tert-butylaniline (1.0 mL, 0.94 g,
6.3 mmol). The reaction was stirred at room temperature for
24 h. Ethyl acetate (EtOAc) (250 mL) was added, and the
[17]
increases the mobility of the dye. A similar paddling motion
has been observed for the mechanism of CO uptake and release
2
in azo-decorated isoreticular MOFs, where light induced iso-
7]
merisation affects the adsorption of CO2.
[
organic phase washed with copious water and sat. NaHCO3
A further strategy for the development of photoresponsive
MOFs is the direct incorporation of azobenzenes as pillaring
ligands. In this case, the structural rigidity of a MOF may
preclude the photoinduced change, or may initiate the degrada-
solution then dried over MgSO . The solvent was removed by
4
rotary evaporation, giving an orange solid that was purified by
silica column chromatography (cyclohexane 90 : 10 EtOAc) and
washed with MeOH. The product, 5-(4-(tert-butyl)phenylazo)
isophthalate (0.84 g, 2.4 mmol), was dissolved in a mixture of
THF (70 mL), MeOH (10 mL), and aqueous NaOH solution
(20 %, 10 mL). The reaction was refluxed at 508C overnight, the
organic solvent removed by rotary evaporation and the aqueous
phase neutralised with HCl (1 M). The orange product was
[
40]
tion of the framework itself.
0
The direct incorporation of
,4 -azobenzenedicarboxylic acid (abdc) into a UiO-67-type
4
framework was reported by Schaate et al., although no cis-trans
isomerisation could be observed in the MOF. Lyndon et al.
investigated the framework [Zn(abdc)(bpe)], previously
reported by Chen et al. Here, a photoresponse was observed
in the framework for the first time, where the azobenzene
[
41]
[8]
[
42]
1
collected by vacuum filtration and dried. Yield: 0.79 g, 27 %. H
3
NMR (300 MHz, [D6]DMSO) d ¼ 0.59 (s, 9H), 6.82 (d, J
underwentcis-trans isomerisation in thepillars of the MOF, which
led to measurable variations in the uptake and release of CO .
H-
3
4
¼
9 Hz, 2H), 7.12 (d, J ¼ 9 Hz, 2H), 7.87 (d, J ¼ 1.5 Hz,
H
H-H
H-H
2
4
13
1
1
1H), 7.94 (t, J ¼ 1.5 Hz, 1H); C{ H} NMR (75 MHz, [D6]
More recently, an azo-based MOF, Co (dpia) (azdc) {dpia¼ N ,
H-H
3
2
3
3
0
43]
DMSO) d ¼ 22.1, 26.5, 114.6, 117.9, 118.9, 123.8, 124.5, 142.2,
N -dipyridin-4-ylisophthalamide, azdc ¼ 4,4 -diazene-1,2-diyldi-
ꢂ1
[
144.8, 147,6, 158.8. FTIR n(C=O) 1714, 1698 cm , n(j-N)
benzoate} was reported by Gong et al.
who observed a
ꢂ1
ꢂ1
1
281 cm , n(N=N) 1448, 1411 cm . ESI-MS: (ESIþ, MeOH)
breathing of the MOF upon irradiation with UV light, and CO2
capture and release performance of 45 % and 75 % under static
and dynamic conditions, respectively. In addition to the findings of
photoresponsive function, research has suggested that MOFs
incorporating azobenzene ligands display an increased affinity
þ
m/z calculated for C H N O [MþH] : 327.13, found: 327.09.
18 18
2 4
Anal. Calc. for C H N O : C 66.25, H 5.56, N 8.58 %. Found: C
18 18
2 4
66.42, H 5.43, N 8.33 %.
[44]
[Zn (tbazip) (bpe) (OH) ] ꢀ bpe ꢀ 2H O
for CO2, and that azobenzene exhibits a degree of N phobicity
2
4
3
2
2
2
[45]
that enhances its selectivity for CO over N .
5-(4-(tert-Butyl)phenylazo)isophthalic
acid
(33 mg,
0.10 mmol), 1,2-di(4-pyridyl)ethene (18 mg, 0.10 mmol), Zn
2
2
Herein, we detail the synthesis and characterisation of a new
coordination framework incorporating the novel azo-containing
ligand 5-((4-tert-butyl)phenylazo)isophthalic acid. The photo-
activity of both the ligand and the framework were investigated
to gain insight into the modulation of the properties upon light
activation. This work demonstrates some of the interesting
effects of azo-incorporation into frameworks, and aids in the
future development of novel multifunctional materials featuring
(AcO) ꢀ2H O (22 mg, 0.10 mmol), KOH (5.5 mg, 0.10 mmol),
2
2
EtOH (6 mL), and water (6 mL) were combined and sealed in a
Teflon insert inside a Parr pressure reaction vessel. The vessel
was heated at 1208C for 24 h and then cooled to 258C over 24 h,
giving a mixture of orange prismatic crystals and yellow
powder, which was separated by density separation (CHCl3
7 : 1 EtOH) to give the crystalline product. Yield: 23 mg, 38 %

Photoactive Frameworks
C
based on tbazip. Anal. Calc. for [Zn (tbazip) (bpe) (OH) ] ꢀ
Microscope equipped with an Argon ion laser exciting at 325 nm
at ꢁ 40 resolution. All data were collected on single crystals and
all measurements were composed of 10 static scans of 1 s
duration.
4
3
2
2
bpe ꢀ 2H O: C 58.39, H 4.57, N 9.08 %. Found: C 58.17, H 4.19,
2
N 10.07 %.
Single Crystal X-Ray Diffraction (SCXRD)
Gas Adsorption
X-Ray diffraction data for [Zn (tbazip) (bpe) (OH) ]ꢀbpeꢀ
4
3
2
2
Adsorption isotherms over the 0–1 bar pressure range were
recorded in glass analysis tubes using an ASAP2020 instrument
from Micromeritics Instruments Inc. Each sample (40–90 mg)
was degassed at either 373 or 403 K overnight before analysis.
{solvent} were collected using a Bruker SMART APEX2 CCD
diffractometer installed at a rotating anode source (MoKa
˚
radiation, l ¼ 0.71073 A), and equipped with an Oxford Cryo-
systems (Cryostream 700) nitrogen gas-flow apparatus. The
data were collected at 90 K by the rotation method with 0.58
frame-width (v scan) and 45 s exposure time per frame. Three
sets of data (360 frames in each set) were collected, nominally
covering the complete reciprocal space. Additional details
regarding the data collection and refinement may be found in the
Supplementary Material.
N isotherms were measured at 77 K using a liquid nitrogen bath
2
and the Brunauer-Emmett-Teller (BET) surface area calculated.
H isotherms were measured at 77 K in a liquid nitrogen bath.
2
N , CO , O , and H isotherms were also collected at 195 K
2
2
2
2
using a dry ice and acetone bath. CO isotherms were also col-
2
lected at 298 K using a circulating water bath.
Crystal data for [Zn (tbazip) (bpe) (OH) ]ꢀbpeꢀ{solvent},
4
3
2
2
Thermogravimetric Analysis (TGA)
C H N O Zn (M ¼ 1815.14), CCDC 1538659: triclinic,
9
0
80 12 14
4
˚
space group P-1 (#2), a ¼ 10.9673(7) A, b ¼ 16.0053(11)
Measurements were performed on a TA Instruments Hi-Res
TGA 2950. Each sample (2–3 mg) was loaded onto a tared
platinum sample pan and its mass recorded. The sample equil-
ibrated for 10 min at 25–308C and then heated to 6008C at
˚
˚
A, c ¼ 26.0812(18) A, a ¼ 91.5674(18)8, b ¼ 94.1464(17)8,
3
˚
g ¼ 92.2275(17)8, V ¼ 4560.6(5) A , Z ¼ 2, T ¼ 90.0 K,
ꢂ1
3
m(MoKa) ¼ 1.106 mm , D ¼ 1.322 g/mm , 75190 reflections
calc
ꢂ1
ꢂ1
.
measured (3.726 # 2y # 69.974), 36791 unique (R ¼ 0.0398,
int
28C min under a constant N flow rate of ,0.1 L min
2
Rsigma ¼ 0.0750) which were used in all refinements. The final R
1
was 0.0560 (I .2s(I)) and wR was 0.1485 (all data).
Results and Discussion
2
Ligand Synthesis
Powder X-Ray Diffraction (PXRD)
The ligand tbazip was synthesised employing a similar method
[
27]
All data were collected on a PANalytical X’Pert PRO MPD
diffractometer equipped with a PIXcel detector and CuKa
to that reported by Park et al.
Amino isophthalate was first
oxidised to a nitroso compound using Oxone. The azo bond was
formed by an acid catalysed Mills reaction between the iso-
phthalate and tert-butyl aniline. The resultant esterified azo
group was then deprotected to form the dicarboxylic acid. This
procedure provides an effective pathway to generate a family of
azo-functionalised isophthalic acids, as the same nitroso iso-
phthalate may be reacted with a large number of substituted
anilines.
˚
radiation (l ¼ 1.5418 A). Capillary PXRD measurements were
conducted on samples under vacuum or a CO atmosphere. The
2
samples were added to glass quartz capillaries and degassed
overnight at either 373 or 403 K, then flame sealed with an
oxygen micro-torch. Le Bail refinements were performed using
[
48]
GSAS.
UV-vis-Near-Infrared Spectroscopy
All measurements were undertaken using a CARY 5000 UV-vis-
near IR spectrophotometer. Solid state spectra were obtained
using a diffuse reflectance Praying Mantis attachment and were
recorded on a BaSO substrate against BaSO as a background
Trans-cis Isomerisation of Discrete tbazip
Solid state UV-vis diffuse reflectance spectroscopy showed that
tbazip exhibits the characteristic bands for a trans-azo com-
pound (Fig. S1, Supplementary Material). Specifically, it fea-
4
4
standard. A Kubelka–Munk analysis was performed on the dif-
2
ꢂ1
tures an intense band centred at 347 nm (28850 cm ), assigned
as a p-p* transition, and a relatively less intense band at 456 nm
fuse reflectance data. The KM factor, F(R) ¼ (1ꢂR) /2R, where
R is the diffuse reflectance of the sample as compared with the
BaSO background was used to derive the absorption spectrum of
ꢂ1
(
21990 cm ), which is assigned as the n-p* transition. Addi-
4
tional peaks are observed below 285 nm and are attributed to
higher energy p-p* transitions. The spectrum for the ligand is
similar to that reported for azobenzene itself, which features an
the compound. Excitation was achieved in the dark using a 20W
Nelson Compact Fluorescent ultraviolet light bulb located 5 cm
from the material. Solution state spectra were measured in EtOH.
Excitation was achieved in the dark using a 4 W UV light Model
ENF-240C/FE produced by Spectroline using the 365 nm
wavelength setting located 5 cm from the sample.
ꢂ1
n-p* transition at 457 nm (21900 cm ) and a p-p* transition
centred at 325 nm (30750 cm ).
ꢂ1 [49]
UV light dependent studies were undertaken on tbazip to
establish its photoactivity. Photoirradiation of tbazip with UV
light led to a marked change in band intensities corresponding to
trans-cis isomerisation. Upon excitation with a 365 nm light
source, tbazip was partially excited from its ground trans to
excited cis state, as shown by the increase in intensity of the
n-p* band, and its blue shift from 441 to 435 nm (22660 to
Solid-State Fourier Transform Infrared Spectroscopy (FTIR)
FTIR spectra were collected using a Bruker Tensor 27 infrared
spectrometer. Samples were measured in a dry KBr matrix
against a KBr background for 64 scans over the range
ꢂ1
4
00–4000 cm
ꢂ1
.
22980 cm ) (Fig. 1a). The reaction reached its photostationary
state after 60 min. The decay of the excited state of tbazip was
also monitored by UV-vis spectroscopy (Fig. 1b). The cis isomer
was relatively stable in the dark, returning to its initial state after
120 min. When exposed to ambient light, however, cis-trans
Resonance Raman Spectroscopy
Data were collected at the Mark Wainright Analytical Centre at
the University of New South Wales using an inVia Raman

D
J. S. Caddy et al.
(a) 5
(b)
4
4
3
2
1
0
3
2
1
0
p-p*
p-p*
20000
22000
24000
26000
20000
22000
24000
26000
n-p*
n-p*
20000
22000
24000
26000
28000
20000
22000
24000
26000
28000
Wavenumber [cmϪ1
]
Wavenumber [cmϪ1
]
Fig. 1. (a) Variation of the UV-vis spectrum of tbazip upon irradiation with light of wavelength 365 nm, showing trans-cis isomerisation. The blue curve
shows the initial spectrum, and the following curves show the progression, with the red curve being the spectrum after 60 min of UV irradiation. (b) Decay of the
absorption spectrum of tbazip showing cis-trans isomerism; the green curve shows the spectrum after 120 min of UV irradiation, while the purple curve shows
the spectrum 120 min after the UV light source was turned off. Each grey curve shows the decay of the isomerism over 20 min intervals.
(a)
(b)
Fig. 3. (a) Polyhedral representation of the metal SBU in [Zn
4 3
(tbazip)
(
bpe) (OH)
2
2
]ꢀbpeꢀ{solvent}. (b) Diagram showing the ‘extended paddle-
wheel’ structure; the zinc atoms and bridging carboxylate groups comprising
the ends of the paddlewheel are illustrated using a ball and stick
representation.
800
1000
1200
1400
1600
1800
2000
Wavenumber [cmϪ1
]
consists of four zinc centres, three deprotonated tbazip ligands,
-
Fig. 2. Raman spectra of tbazip initially (black) and after irradiation of UV
three bpe molecules, and two m -OH ions (Fig. S3, Supple-
3
light for 60 s (red).
mentary Material). Two of the zinc centres are six-coordinate and
adopt octahedral configurations, while the remaining two zinc
atoms are four-coordinate and exhibit a tetrahedral geometry. The
coordination sphere of the tetrahedral zinc centres consists of a
nitrogen atom from a pyridyl group and three oxygen atoms, two
from two carboxylate groupsand one from the hydroxide ion. The
pyridyl nitrogen atom and a hydroxide ion are axially coordinated
to the octahedral zinc centres while the equatorial sites consist of
three oxygen atoms from three carboxylate groups and an addi-
tional hydroxide ion (Fig. S3, Supplementary Material).
isomerisation occurred rapidly, with significant changes in the
band intensities observed within 1 min and the isomerisation
was complete within 20 min.
Raman studies were also undertaken to observe the isomer-
isation of tbazip in situ. As cis and trans azobenzenes possess
different Raman spectra, it is possible to identify the ratio of
[
50]
isomers present.
A 325 nm laser was employed as this
wavelength lies in the p-p* excitation band of tbazip. Spectra
were recorded before and after irradiation of the ligand with UV
light for 60 s (Fig. 2). Upon excitation, all peaks in the Raman
The core of the zinc secondary building unit (SBU) is
reminiscent of a typical paddlewheel SBU where the two metal
centres are bridged equatorially by four carboxylate groups. In
the present work, the metal centres form an ‘extended paddle-
wheel’ unit in which the ends of the SBU feature four carboxylate
groups bridging two zinc centres. The ‘extension’ of the paddle-
wheel arises from the insertion of monodentate carboxylate
groups that hydrogen bond to hydroxide ions located in the
interior of the SBU (Fig. 3). The hydroxide ions in the centre of
the cluster form an open parallelogram structure as they bridge
between two octahedral and one of the tetrahedral zinc atoms.
This M-OH bonding is similar to that described in several recent
ꢂ1
spectrum decreased relative to the peak at 1605 cm , which
also appeared to broaden significantly. Several studies of
asymmetric azobenzenes have suggested that the decrease in
ꢂ1
relative intensity of peaks between 1400 and 1550 cm is
[
indicative of an isomerisation process.
51,52]
Crystal Structure and Thermophysical Properties of
[
Zn (tbazip) (bpe) (OH) ] ꢀ bpe ꢀ {solvent}
4
3
2
2
The solvothermal reaction of tbazip, bpe, and zinc acetate
under basic conditions yielded large orange crystals of
[53,54]
papers.
[
Zn (tbazip) (bpe) (OH) ]ꢀbpeꢀ{solvent}. The asymmetric unit
4
3
2
2

Photoactive Frameworks
E
N
N
O
O
M
M
M
O
O
N
N
O
O
M
M
M
O
O
M
Fig. 4. Schematic showing the two binding modes of tbazip in [Zn
4
(tbazip) (bpe)
3
2
(OH)
2
]ꢀbpeꢀ{solvent}.
(a)
(b)
(c)
Fig. 5. (a) [Zn
layers. bpe molecules coordinated to Zn are shown in a ball and stick representation. Uncoordinated bpe are coloured pink. Hydrogen atoms and the minor
component of the refined disorder species are omitted for clarity. [Zn (tbazip) (bpe) (OH)
]ꢀbpe as viewed slightly off the b-axis showing (b) the two
disordered solvent spaces in the unit cell (green) and (c) the extended structure which illustrates the connectivity of these spaces along the a-axis (orange).
4
(tbazip)
3
(bpe)
2
(OH)
2
]ꢀbpeꢀ{solvent} as viewed down the a-axis, with alternate layers of organic ligand and mixed metal cluster/organic ligand
4
3
2
2
The low symmetry 3D framework exhibits three distinct
bonding motifs along three different directions in the lattice
Connectivity along the b-axis occurs through the Zn-N bonds
of a bpe molecule coordinated to two zinc atoms. This forms
alternate layers of organic ligand and a mixed layer of the zinc
cluster and bridging ligands (Fig. 5). The organic ligands feature
extensive p-stacking interactions within the organic layers, and
the free bpe molecule is held within the pores by p-stacking with
nearby tbazip ligands. The crystal lattice contains regions
occupied by disordered solvent molecules, the structures of
which could not be reliably refined. The spaces occupied by
these solvent molecules are connected to neighbouring solvent
spaces to form channels along the a-axis. The observed
porosity in [Zn (tbazip) (bpe) (OH) ]ꢀbpeꢀ{solvent} is likely
(Fig. S4, Supplementary Material). Along the a- and c-axes,
the metal SBUs are bridged by three crystallographically unique
tbazip ligands coordinated to zinc atoms. Along the c-axis, both
carboxylate groups of one of the unique tbazip ligand bridge two
zinc atoms. The two remaining unique linkers exhibit identical
connectivity along the a-axis. In both linkers, one of the carbox-
ylate groups is bridging while the other exhibits monodentate
binding. The oxygen atom of the monodentate carboxylate group
that is not coordinated to a metal centre exhibits hydrogen
bonding to the hydroxide group of the SBU (Fig. 4).
4
3
2
2

F
J. S. Caddy et al.
(a)
2.5
2.0
1.5
1.0
0.5
0
H2
N2
0
200
400
600
800
1000 1200
Absolute pressure [mbar]
(b)
(c)
3.0
2.5
2.0
1.5
1.0
0.5
0
3.0
2.5
2.0
1.5
1.0
0.5
0
CO2
CO2
O2
N2
H2
0
200
400
600
800
1000 1200
0
200
400
600
800
1000
1200
Absolute pressure [mbar]
Absolute pressure [mbar]
Fig. 6. (a) N
bpe of N (black), H
process and open squares show the desorption process.
2
(black) and H
2
(blue) adsorption isotherms for [Zn
4
(tbazip)
3
(bpe)
2
(OH)
2
]ꢀbpe at 77 K. (b) 195 K isotherms for [Zn
4
(tbazip)3(bpe)
2
(OH)
2
]ꢀ
2
2
(blue), O
2
(purple), and CO (red). (c) CO isotherm of [Zn
2
2
4
(tbazip)
3
(bpe) (OH)
2
2
]ꢀbpe at 298 K. Filled squares show the adsorption
2
ꢂ1
attributable to the partial retention of these channels upon
evacuation of the solvent molecules.
PXRD analysis was employed to confirm the bulk purity of
area calculated at 30 m g . Hydrogen gas is able to diffuse into
the structure, although the very slow adsorption/desorption
kinetics cause substantial hysteresis. This behaviour is consis-
tent with the narrow one-dimensional channels observed in the
[
Zn (tbazip) (bpe) (OH) ]ꢀbpeꢀ2H O (Fig. S5, Supplementary
4
3
2
2
2
˚
Material). Additionally, it was found that the PXRD pattern
undergoes only relatively minor changes upon desolvation of
the framework and upon subsequent exposure to 750 mbar CO2
crystal structure (between 2.68 and 6.65 A between adjacent
hydrogen atoms on tbazip ligands).
At 195 K, the nitrogen gas uptake is greater (Fig. 6b),
suggesting ‘gate-opening’ processes driven either by expansion
of the channels or thermal excitation increasing the diffusion
(
Fig. S9, Supplementary Material).
As the solvent could not be refined satisfactorily by SCXRD
[55]
[56]
the SQUEEZE function of OLEX2 was used), thermogravi-
(
rate. Oxygen gas is adsorbed, and CO condenses in the micro-
2
metric analysis (TGA) was used both to determine the amount of
solvent in the sample and to estimate the porosity of the frame-
work. TGA indicates that there is ,6.4% w/w mass loss below
pore channels in a typical type I isotherm. The H uptake is
2
relatively lower than that at 77 K due to the higher temperature.
The CO isotherm at 298 K (Fig. 6c) also demonstrates that
2
1
308C; this is attributed to the loss of the disordered chemical
the framework admits this guest, with a maximum uptake of
ꢂ1
3
˚
species residing within the 594.8 A of solvent accessible void
space per unit cell (13.0 % v/v). The mass loss over the range
2.9 mmol g , or 12 % w/w at 1100 mbar. This uptake could be
explained by the free bpe molecules within the pores, since
2
within the pores of the framework, which drives decomposition of
00–3208C is attributed to the loss of the bpe molecule occluded
accessible nitrogen atoms have been reported to bind CO
[
2
12]
effectively. Additionally, simulations on the azo functional
group suggest that interactions with CO are improved, com-
the material. Supercritical CO drying was undertaken in an
2
2
[25]
attempt to remove this ligand without damaging the framework;
however, TGA analysis following the process still indicated the
presence of excess ligand (Fig. S6, Supplementary Material).
pared with N2.
Photoactivity of the Framework
The UV-vis diffuse reflectance
spectrum
of
Gas Adsorption
[
Zn (tbazip) (bpe) (OH) ]ꢀbpeꢀ2H O strongly resembles that of
4
3
2
2
2
At 77 K, the desolvated form of the framework,
the tbazip ligand. The main difference between the two spectra
is the hypsochromic shift of the n-p* and p-p* bands by 10 nm
[
Zn (tbazip) (bpe) (OH) ]ꢀbpe, was found to be essentially non-
4
3
2
2
ꢂ1 ꢂ1
(520 cm ) and 18 nm (1360 cm ), respectively (Fig. 7a).
porous to nitrogen gas (Fig. 6a), with a nominal BET surface

Photoactive Frameworks
G
(a)
(b) 0.25
p–p∗
1.0
0.8
0.6
0.4
0.2
0
0.20
0.15
0.10
0.05
0
n-p∗
15000
20000
25000
30000
20000
22000
24000
]
Wavenumber [cmϪ1
]
Wavenumber [cmϪ1
(c)
(d)
4
0
43.0
42.5
42.0
41.5
41.0
40.5
40.0
39.5
In the absence of visible light
In the presence of visible light
39
38
37
3
6
5
3
0
10
20
30
40
50
60
0
20
40
60
80
100
120
Resting time [min]
Resting time [min]
Fig. 7. (a) UV-vis spectra of [Zn
4
(tbazip)
(bpe)
3
(bpe)
(OH)
2
(OH)
2
]ꢀbpe ꢀ 2H
2
O (black) and the free tbazip ligand (red). (b) UV-vis spectra showing the
evolution of photoexcited [Zn (tbazip)
4
3
2
2
]ꢀbpe with time. The blue spectrum represents the photoexcited framework after irradiation with
UV light for 120 min. The green spectrum represents the framework after resting for 60 min in ambient light. The grey intermediate spectra
demonstrate the relaxation of the framework. Graphs showing the relative intensity of the n-p* band compared with the p-p* band with time:
(c) upon resting in the absence of visible light and (d) upon exposure to ambient light after being left in the dark.
Irradiation ofthe frameworkwithUV light leads toanincrease
in the intensity of the n-p* band relative to the p-p* band,
suggesting that trans-cis isomerisation is occurring (Fig. 7b). The
change in the ratios begins to plateau after 120 min of irradiation,
suggesting that a photostationary state was attained. Similarly,
there are strong indications of the reverse cis-trans reaction
occurring when light irradiation is ceased, through the observa-
ꢂ1
tion of an isosbestic point at ,390 nm (25700cm ), in addition
to the decrease in intensity of the n-p* band and increase of the
p-p* band. There was a small change in intensity as the
framework was rested in the dark for 110 min (Fig. 7c), but when
exposed to ambient light, changes occurred more rapidly
(Fig. 7d). It must be noted that the photochromic behaviour
observed is likely to be confined to the surface of the particles.
Raman studies were also performed on the framework both
before and upon irradiation with UV light at 325 nm for up to
800
1000
1200
1400
1600
1800
2000
Wavenumber [cmϪ1
]
1
20 s (Fig. 8). As was observed with the discrete tbazip, all peaks
ꢂ1
Fig. 8. Raman spectra of [Zn (tbazip) (bpe) (OH)
4
3
2
2
]ꢀbpe ꢀ 2H
2
O prior to
decreased relative to the peak at 1605 cm , which underwent
broadening. This may suggest that surface isomerisation could
be occurring; however, this peak change could also be a sign of
crystal degradation.
irradiation (black) and upon irradiation with a 325 nm laser for 60 s (red) and
1
20 s (blue).
Note that in situ light dependent gas adsorption measure-
ments were attempted; however, the changes in adsorption were
not outside experimental uncertainty, and are therefore not
reported here. Presumably, the densely packed nature of
Conclusions
The new coordination framework [Zn (tbazip) (bpe) (OH) ]ꢀ
4
3
2
2
bpeꢀ{solvent} containing the novel tbazip ligand was synthe-
sised. UV-vis studies of both the ligand and its MOF following
photoirradiation with UV light revealed an isomerisation
[
Zn (tbazip) (bpe) (OH) ]ꢀbpe precludes the effect of isomer-
4
3
2
2
isation on the gas adsorption properties.

H
J. S. Caddy et al.
process within the material. This process was found to be
reversible in the solid state, with trans-cis isomerisation
occurring after 15 min exposure to light. Upon the removal of
light, the reverse cis-trans isomerisation was observed.
[15] D. M. Thomas, J. Mechery, S. V. Paulose, Environ. Sci. Pollut. Res.
Int. 2016, 23, 16926. doi:10.1007/S11356-016-7158-3
[
16] Z. Zhang, Z.-Z. Yao, S. Xiang, B. Chen, Energy Environ. Sci. 2014, 7,
2
868. doi:10.1039/C4EE00143E
[
17] J. W. Brown, B. L. Henderson, M. D. Kiesz, A. C. Whalley, W. Morris,
S. Grunder, H. Deng, H. Furukawa, J. I. Zink, J. F. Stoddart, O. M.
Yaghi, Chem. Sci. 2013, 4, 2858. doi:10.1039/C3SC21659D
18] C. B. Fan, Z. Q. Liu, L. L. Gong, A. M. Zheng, L. Zhang, C. S. Yan, H. Q.
Wu, X. F. Feng, F. Luo, Chem. Commun. 2017, 53, 763. doi:10.1039/
C6CC08982H
Gas adsorption studies at 77, 195, and 298 K revealed that the
ꢂ1
2
MOF exhibited a nominal BET surface area of 30 m g , but was
foundtoadsorbCO despite its modest N uptakeandsurfacearea.
2
2
[
It is postulated that the presence of the azo groups in the material
aid in the uptake CO , due to their positive calculated interaction
2
[33]
with CO2 and their suggested ‘N -phobicity’.
This work
[19] A. V. El’tsov, Organic Photochromes 1990 (Consultants Bureau: New
2
demonstrates some of the interesting effects of azo-incorporation
into frameworks, and aids in the future development of novel
multifunctional materials featuring functionalised azobenzene
ligands.
York, NY).
[20] S.-L. Huang, T. S. A. Hor, G.-X. Jin, Coord. Chem. Rev. 2017, 346,
1
12. doi:10.1016/J.CCR.2016.06.009
21] J. M. Cox, I. M. Walton, J. B. Benedict, J. Mater. Chem. C 2016, 4,
028. doi:10.1039/C6TC00131A
[
[
[
4
22] I. M. Walton, J. M. Cox, C. A. Benson, D. G. Patel, Y.-S. Chen, J. B.
Benedict, New J. Chem. 2016, 40, 101. doi:10.1039/C5NJ01718A
23] I. M. Walton, J. M. Cox, J. A. Coppin, C. M. Linderman, D. G. Patel,
J. B. Benedict, Chem. Commun. 2013, 49, 8012. doi:10.1039/
C3CC44119A
[24] K. Healey, W. Liang, P. D. Southon, T. L. Church, D. M.
D’Alessandro, J. Mater. Chem. A 2016, 4, 10816. doi:10.1039/
C6TA04160D
Supplementary Material
Additional data for the crystallographic analysis, powder dif-
fraction, and thermogravimetric analysis, as well as infrared and
UV-vis-NIR data for the ligand and framework are available on
the Journal’s website.
Conflicts of Interest
[
[
[
[
[
[
25] Y. Dou, Y. Hu, S. Yuan, W. Wu, H. Tang, Mol. Phys. 2009, 107, 181.
doi:10.1080/00268970902769497
26] H. Koshima, N. Ojima, H. Uchimoto, J. Am. Chem. Soc. 2009, 131,
The authors declare no conflicts of interest.
Acknowledgements
6
890. doi:10.1021/JA8098596
This work was supported by the Australian Research Council and the Uni-
versity at Buffalo, the State University of New York. We gratefully
acknowledge A/Prof. Matthew Hill and Dr Richelle Lyndon for their
assistance in trialling the in situ light irradiated gas adsorption measurements
on the framework, and Dr Samuel Duyker for his assistance with performing
Le Bail refinements.
27] G. S. Hartley, R. J. W. Le Fevre, J. Chem. Soc. (Res.) 1939, 531.
doi:10.1039/JR9390000531
28] S. Eligehausen, S. M. Sarge, G. Ohlschl a¨ ger, H. K. Cammenga,
¨
J. Therm. Anal. 1989, 35, 515. doi:10.1007/BF01904453
29] G. D. Jaycox, J. Polym. Sci. Part A: Polym. Chem. 2004, 42, 566.
doi:10.1002/POLA.10806
30] J. Shi, B. Wang, H. Du, Colloid Polym. Sci. 2014, 292, 1217.
doi:10.1007/S00396-014-3173-4
[31] E. Vaselli, C. Fedele, S. Cavalli, P. A. Netti, ChemPlusChem 2015, 80,
References
[
[
[
[
[
[
1] K. J. Gagnon, C. M. Beavers, A. Clearfield, J. Am. Chem. Soc. 2013,
135, 1252. doi:10.1021/JA311613P
1547. doi:10.1002/CPLU.201500179
2] S. Nagata, K. Kokado, K. Sada, Chem. Commun. 2015, 51, 8614.
doi:10.1039/C5CC02339D
3] J. Yu, Y. Cui, C.-D. Wu, Y. Yang, B. Chen, G. Qian, J. Am. Chem. Soc.
[32] X.-P. Qiu, E. V. Korchagina, J. Rolland, F. M. Winnik, Polym. Chem.
2014, 5, 3656. doi:10.1039/C3PY01776A
[33] H. M. D. Bandara, S. C. Burdette, Chem. Soc. Rev. 2012, 41, 1809.
doi:10.1039/C1CS15179G
[34] N. Yanai, T. Uemura, M. Inoue, R. Matsuda, T. Fukushima,
M. Tsujimoto, S. Isoda, S. Kitagawa, J. Am. Chem. Soc. 2012, 134,
4501. doi:10.1021/JA2115713
[35] D. Hermann, H. Emerich, R. Lepski, D. Schaniel, U. Ruschewitz,
Inorg. Chem. 2013, 52, 2744. doi:10.1021/IC302856B
[36] A. Modrow, D. Zargarani, R. Herges, N. Stock, Dalton Trans. 2011,
40, 4217. doi:10.1039/C0DT01629B
2015, 137, 4026. doi:10.1021/JA512552G
4] F.-X. Coudert, Chem. Mater. 2015, 27, 1905. doi:10.1021/ACS.
CHEMMATER.5B00046
5] X. Meng, B. Gui, D. Yuan, M. Zeller, C. Wang, Sci. Adv. 2016, 8,
e1600480. doi:10.1126/SCIADV.1600480
6] Z. Wang, A. Knebel, S. Grosjean, D. Wagner, S. Br a¨ se, C. W o¨ ll,
J. Caro, L. Heinke, Nat. Commun. 2016, 7, 13872. doi:10.1038/
NCOMMS13872
[7] R. Huang, M. R. Hill, R. Babarao, N. V. Medhekar, J. Phys. Chem. C
[37] A. Modrow, D. Zargarani, R. Herges, N. Stock, Dalton Trans. 2012,
2016, 120, 16658. doi:10.1021/ACS.JPCC.6B03541
41, 8690. doi:10.1039/C2DT30672G
[
8] R. Lyndon, K. Konstas, B. P. Ladewig, P. D. Southon, P. C. J. Kepert,
[38] A. Modrow, M. Feyand, D. Zargarani, R. Herges, N. Stock, Z. Anorg.
Allg. Chem. 2012, 638, 2138. doi:10.1002/ZAAC.201200048
[39] J. Park, D. Yuan, K. T. Pham, J. R. Li, A. Yakovenko, H. C. Zhou,
J. Am. Chem. Soc. 2012, 134, 99. doi:10.1021/JA209197F
[40] C. L. Jones, A. J. Tansell, T. L. Easun, J. Mater. Chem. A 2016, 4, 6714.
doi:10.1039/C5TA09424K
M. R. Hill, Angew. Chem. Int. Ed. 2013, 52, 3695. doi:10.1002/ANIE.
201206359
[
9] R. Lyndon, K. Konstas, A. W. Thornton, A. J. Seeber, B. P. Ladewig,
M. R. Hill, Chem. Mater. 2015, 27, 7882. doi:10.1021/ACS.CHEM
MATER.5B02211
[
10] L. T. M. Hoang, L. H. Ngo, H. L. Nguyen, H. T. H. Nguyen, C. K.
Nguyen, B. T. Nguyen, Q. T. Ton, H. K. D. Nguyen, K. E. Cordova,
T. Truong, Chem. Commun.2015, 51, 17132. doi:10.1039/C5CC05985B
11] X. Sun, Q. Yu, F. Zhang, J. Wei, P. Yang, Catal. Sci. Technol. 2016, 6,
[41] A. Schaate, S. D u¨ hnen, G. Platz, S. Lilienthal, A. M. Schneider,
P. Behrens, Eur. J. Inorg. Chem. 2012, 790. doi:10.1002/EJIC.
201101151
[
[
[
[
[42] B. Chen, S. Ma, E. J. Hurtado, E. B. Lobkovsky, H.-C. Zhou, Inorg.
3
840. doi:10.1039/C5CY01716E
12] D. M. D’Alessandro, B. Smit, J. R. Long, Angew. Chem. Int. Ed. 2010,
9, 6058. doi:10.1002/ANIE.201000431
13] A. Das, D. M. D’Alessandro, CrystEngComm 2015, 17, 706.
doi:10.1039/C4CE01341G
14] E. S. Rubin, J. E. Davison, H. J. Herzog, Int. J. Greenh. Gas Control
Chem. 2007, 46, 8490. doi:10.1021/IC7014034
[43] L. L. Gong, X. F. Feng, F. Luo, Inorg. Chem. 2015, 54, 11587.
doi:10.1021/ACS.INORGCHEM.5B02037
[44] C. M. Nagaraja, R. Haldar, T. K. Maji, C. N. R. Rao, Cryst. Growth
4
Des. 2012, 12, 975. doi:10.1021/CG201447C
[45] H. A. Patel, S. H. Je, J. Park, D. P. Chen, Y. Jung, C. T. Yavuz,
A. Coskun, Nat. Commun. 2013, 4, 1357. doi:10.1038/NCOMMS2359
2015, 40, 378. doi:10.1016/J.IJGGC.2015.05.018

Photoactive Frameworks
I
[
[
[
[
[
[
46] H. E. Gottlieb, V. Kotlyar, A. Nudelman, J. Org. Chem. 1997, 62,
[52] Y. B. Zheng, J. L. Payton, C.-H. Chung, R. Liu, S. Cheunkar, B. K.
Pathem, Y. Yang, L. Jensen, P. S. Weiss, Nano Lett. 2011, 11, 3447.
doi:10.1021/NL2019195
7512. doi:10.1021/JO971176V
47] B. Priewisch, K. R u¨ ck-Braun, J. Org. Chem. 2005, 70, 2350.
doi:10.1021/JO048544X
48] B. H. Toby, R. B. Von Dreele, J. Appl. Cryst. 2013, 46, 544.
doi:10.1107/S0021889813003531
49] H. M. D. Bandara, S. Cawley, J. A. Gasc o´ n, S. C. Burdette, Eur. J. Org.
[53] Y. C. Ou, D. S. Zhi, W. T. Liu, Z. P. Ni, M. L. Tong, Chem. – Eur. J.
2012, 18, 7357. doi:10.1002/CHEM.201200226
[54] G. A. Senchyk, A. B. Lysenko, H. Krautscheid, E. B. Rusanov, A. N.
Chernega, K. W. Kr a¨ mer, S.-X. Liu, S. Decurtins, K. V. Domasevitch,
Inorg. Chem. 2013, 52, 863. doi:10.1021/IC3020157
[55] A. Spek, J. Appl. Cryst. 2003, 36, 7. doi:10.1107/S0021889802022112
[56] O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard,
H. Puschmann, J. Appl. Cryst. 2009, 42, 339. doi:10.1107/S0021889
808042726
Chem. 2011, 2916. doi:10.1002/EJOC.201100216
50] C. M. Stuart, R. R. Frontiera, R. A. Mathies, J. Phys. Chem. A 2007,
111, 12072. doi:10.1021/JP0751460
51] B. K. Pathem, Y. B. Zheng, J. L. Payton, T.-B. Song, B.-C. Yu, J. M.
Tour, Y. Yang, L. Jensen, P. S. Weiss, J. Phys. Chem. Lett. 2012, 3,
2388. doi:10.1021/JZ300968M