Covalent switching, involving divinylbenzene ligands within 3D coordination polymers, indicated by changes in fluorescence

Article abstract of DOI:10.1039/c8cc02743a

Reversible cycloaddition reactions between 1,4-bis[2-(3-pyridyl)ethenyl]benzene and its photodimers were performed within a pair of three-dimensional coordination polymers with retention of single crystal character. In each case, switching between two crystalline forms could be followed by monitoring the flourescence behaviour.

Full text of DOI:10.1039/c8cc02743a

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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Covalent switching, involving divinylbenzene ligands within 3D
coordination polymers, indicated by changes in fluorescence
Ni-Ya Li,a,b Dong Liu,b Brendan F. Abrahamsc and Jian-Ping Lang*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
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Reversible cycloaddition reactions between 1,4-bis[2-(3- coordination compound may provide an indication of the
pyridyl)ethenyl]benzene and its photodimers were performed chemical form of the ligand.8 In this regard, robust
within a pair of three-dimensional coordination polymers with coordination polymers (CPs) which can undergo reversible
retention of single crystal character. In each case, switching single-crystal-to-single-crystal (SCSC) transformations between
between two crystalline forms could be followed by monitoring two stable states are of particular interest.9
the flourescence behaviour.
Covalent bond rearrangements, involving organic molecules,
commonly result in both electronic and geometrical structural
changes.1-3 In the solid state, reversible covalent conversions
within molecular systems may lead to functionality, allowing
their uses in sensors or their employment within information
storage media as well as optical and optoelectronics switching
devices.4,5 In recent years, the development of optical and
Scheme 1 Possible reversible reactions between in-phase aligned 3,3'-bpeb
optoelectronic devices based on molecular switches, which
and 3,3',3'',3'''-tppcp (a) or out-of-phase aligned 3,3'-bpeb and 3,3',3'',3'''-
bpbpvpcb (b) in coordination compounds.
operate at both molecular and supramolecular levels, has
received considerable attention.6 Whilst there are clear
The reversible [2+2] cycloaddition of olefins to generate a
technological benefits associated with the development of
cyclobutane unit is a reaction that can be exploited in the
systems in which reversible solid state reactions respond to
formation of fluorescent switches.3 Although fluorescent
external stimuli,2-6 at this stage, effective molecular switches
switches based on such a reaction have been reported,8 the
that respond in a controlled manner, are rare.6
utilization of an olefin within
a three-dimensional (3D)
There has recently been considerable interest in
coordination compounds that are able to serve as
multifunctional photoluminescent materials.7 These inorganic–
fluorescent CP, exhibiting complete switching, has not been
described until now. The use of olefin-based ligands in the
generation of switchable CPs is not limited to monovinyl
ligands. Divinyl ligands may also undergo dimerization
reactions to yield a variety of isomers depending upon the
alignment of the vinyl groups prior to UV irradiation. The
organic
hybrid
compounds
take
advantage
of
photoluminescent properties associated with both the
inorganic and organic components.7 In particular, fluorescent
switches based on coordination compounds have received
significant attention as a result of their distinct signal outputs.8
conjugated
divinylbenzene,
1,4-bis[2-(3-pyridyl)ethenyl]-
benzene (3,3'-bpeb) may be used as a bridging ligand to
produce CPs with good photoluminescent properties.7 Whilst a
If
a coordinated organic ligand within a coordination
compound is able to undergo a reversible reaction in response
to certain stimuli, then the photoluminescent properties of the
crystalline sample of pure 3,3'-bpeb is photo-inert,
a
consequence of the large separation between double bonds in
neighbouring molecules (Figs. S1-2, ESI†), it is possible for
coordinated 3,3'-bpeb ligands within CPs to be arranged in a
manner that [2+2] cycloaddtion reactions may occur upon UV
irradiation. Both “in-phase” and “out-of-phase” arrangements
of 3,3'-bpeb ligands are possible as depicted in Scheme 1.
Upon irradiation with UV light, the in-phase 3,3'-bpeb
molecules may be converted to tetrakis(3-pyridyl)-1,2,9,10-
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diethano[2.2]-paracyclophane (3,3',3'',3'''-tppcp) whilst the S6, ESI†). The tetrapyridyl compound, 3,3',3'',3'''-tppcp can be
out-of-phase ligands would lead to the generation of 1,3-bis(3- extracted from using a mixture of aqueous Na2edta (edta =
pyridyl)-2,4-bis[4-{2-(3-pyridyl)-vinyl}phenyl]cyclobutane ethylenediaminetetraacetate) and CH2Cl2. The formation of
2
1
(3,3',3'',3'''-bpbpvpcb) (Scheme 1).10 Of key importance to this 3,3',3'',3'''-tppcp was indicated by its H NMR spectrum which
current work is the potential regeneration of 3,3'-bpeb from showed characteristic signals for cyclobutane protons at 4.79
either 3,3',3'',3'''-tppcp or 3,3',3'',3'''-bpbpvpcb following and 4.67 ppm (Fig. S7, ESI†). The absence of olefinic protons at
thermal cleavage of the cyclobutane units.3 Herein, we report 7.40 ppm was also noted. The cross-polarization magic angle
the synthesis and characterization of two photoactive CPs spinning (CPMAS) 13C NMR spectrum of
2 showed the
involving 3,3'-bpeb ligands and describe their reversible SCSC disappearance of the signals for an olefin at 125.74 and 123.25
transformations and switching behaviours which can be ppm and the emergence of four representative peaks for
followed by monitoring changes in fluorescence.
When Zn(NO3)2·6H2O and 1,4-naphthalenedicarboxylic acid conversion from
(1,4-H2ndc) are combined with 3,3'-bpeb in a 1:1:1 molar ratio that the photodimerization is complete and the bulk sample of
under hydrothermal conditions, crystals of composition is pure (Fig. S4, ESI†). To our knowledge, a photoreactive
[Zn(1,4-ndc)(3,3'-bpeb)]n ( ) are obtained. Single crystal X-ray compound containing 3,3'-bpeb has not been reported
analysis of indicates that Zn atoms are linked through 1,4- previously.
cyclobutane in the 51.53–39.13 ppm region, confirming the
1
to (Fig. S8, ESI†). PXRD patterns indicated
2
2
1
1
ndc ligands to generate a two-dimensional (2D) [Zn(1,4-ndc)]n
layer (Fig. S3, ESI†). Within this network, Zn centres are linked
by a pair of carboxylate (O-C-O) bridges to create Zn2 units.
The axial positions of each Zn octahedral centre are occupied
by two N atoms from two 3,3'-bpeb ligands, that provide a
bridge between adjacent parallel [Zn(1,4-ndc)]n layers to form
an infinite 3D net of composition, [Zn(1,4-ndc)(3,3'-bpeb)]n
(Fig. 1). From a topological perspective, each pair of Zn centres
as depicted in Fig. S3 may be considered as a 6-connected
node in a 3D net that has the point symbol 446108.11 Powder X-
ray diffraction (PXRD) confirmed the purity and highly
crystalline nature of the bulk sample of
Thermogravimetric analysis (TGA) of
1
(Fig. S4, ESI†).
1
showed that the
Fig. 2 Reversible SCSC transformation between 3,3'-bpeb and 3,3',3'',3'''-
tppcp in 1 (a) and 2 (b), respectively. Symmetry code: (A): -x + 2, - y +2, - z +
1. See Fig. 1 for colour codes.
framework is stable up to 300 °C (Fig. S5, ESI†).
Whilst high resolution mass spectroscopy (HRMS) of
3,3',3'',3'''-tppcp shows a molecular ion peak at m/z 569.2698,
corresponding to [3,3',3'',3'''-tppcp + H]+, the spectrum is
dominated by a peak at m/z 285.1387 due to [3,3'-bpeb + H]+
(Fig. S9, ESI†). The presence of 3,3'-bpeb is consistent with
3,3',3'',3'''-tppcp undergoing a cleavage reaction during the
HRMS measurements. This result suggests that the
cycloaddition reaction that occur within
reversible upon heating . TGA of reveals no weight loss up
to a temperature 299°C (Fig. S5, ESI†) indicating that could
1 to yield 2 may be
Fig. 1 View of the 3D framework of 1 along the b axis. The cyan, blue, red
and black balls represent Zn, N, O and C atoms, respectively.
2
2
2
Within the structure of 1, pairs of carboxylate-bridged Zn
be safely heated to temperatures approaching 300 °C in order
to determine if the cycloaddition reaction could be reversed.
centres in a single [Zn(1,4-ndc)]n layer are linked by a pair of
centrosymmetrically arranged, in-phase 3,3'-bpeb ligands to a
symmetry-related pair of Zn centres in the adjacent sheet. As
indicated in Fig. 2a, this arrangement leads to the alignment of
two pairs of olefinic bonds with separations of 3.798 Å
(C6···C15A) and 3.654 Å (C7···C14A) that are within the range
for solid-state photochemical [2+2] cycloaddition to occur.10
When single crystals of
2
were heated to 260°C for 3 h in an
was cleaved to yield
oven, each 3,3',3'',3'''-tppcb molecule in
2
a pair of aligned 3,3'-bpeb molecules. This cycloreversion
reaction occurred with retention of single crystal character and
leads to the regeneration of the parent crystals, i.e.,
compound 1. The formation of 1 from 2 was further verified by
PXRD, CPMAS 13C NMR and solid-state optical absorbance
spectra (Figs. S4, S8 and S10, ESI†).
When single crystals of
1 are irradiated for 4 h with a 30 W LED
lamp (λ = 365 nm), each pair of 3,3'-bpeb ligands in
1
undergoes a dimerization reaction to yield a 3,3',3'',3'''-tppcp
molecule as indicated in Fig. 2b. This reaction results in the
The reversible SCSC transformation between
1 and 2
prompted an investigation of other Zn-based coordination
networks involving the photoactive 3,3'-bpeb ligand. When
3,3',4,4'-biphenyltetracarboxylic acid (3,3',4,4'-H4bptc) was
combined with Zn(NO3)2·6H2O and 3,3'-bpeb under
formation of a new CP, [Zn(1,4-ndc)(3,3',3'',3'''-tppcp)0.5]n (
in 100% yield through a SCSC process. The 3D framework of
is related to that of
in taking the place of a pair of 3,3'-bpeb molecules in
2
),
2
1
with a single 3,3',3'',3'''-tppcp molecule
(Fig.
2
1
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hydrothermal conditions, a CP of composition [Zn2(3,3',4,4'- net has the point symbol (4462)(4284)(4966) (Fig. S12, ESI†).11
1
bptc)(3,3'-bpeb)]n
analysis revealed a more complex structure than that found of the tetrapyridyl ligand extracted from
for . A pair of crystallographically independent Zn centres are spectrum showed the appearance of the cyclobutane proton
(
3
) was obtained. A single crystal X-ray The photoreaction was corroborated by the H NMR spectrum
4
. The 1H NMR
1
linked by a single oxygen atom from one carboxylate group signal at 4.64 ppm. The signal associated with the olefinic
and a O-C-O carboxylate group from a second 3,3',4,4'-bptc proton shifted from 7.40 to 7.17 ppm (Fig. S7, ESI†), consistent
ligand. This pair of Zn centres is linked by additional 3,3',4,4'- with only half of the olefinic undergoing cyclization. CPMAS 13C
bptc ligands to form
[Zn2(3,3',4,4'-bptc)]n that extends in the bc plane (Fig. S11, to
ESI†). Adjacent [Zn2(3,3',4,4'-bptc)]n networks are then linked ppm (Fig. S8, ESI†). The purity of the bulk product was
by 3,3'-bpeb ligands to afford 3D framework with confirmed by PXRD (Fig. S4, ESI†).
a
2D network of composition NMR spectrum of
4 further verified the transformation from 3
4
with the emergence of signals for cyclobutane at ca. 43
a
composition, [Zn2(3,3',4,4'-bptc)(3,3'-bpeb)]n (Fig. 3). In
contrast to 1, in which four 3,3'-bpeb ligands are bound to a
Zn2 unit, only two 3,3'-bpeb ligands are bound to each Zn2 unit,
one extending to the [Zn2(3,3',4,4'-bptc)]n sheet above and one
to the [Zn2(3,3',4,4'-bptc)]n sheet below. From a topological
viewpoint, if the Zn2 units and 3,3',4,4'-bptc ligands are taken
as 6- and 4-connected nodes respectively, the structure of
3
may be described as a 3D (4,6)-connected framework with the
point symbol (4462)(446108) (Fig. S11, ESI†).11 The phase purity
of the bulk sample was indicated by its PXRD patterns which
match the simulated patterns obtained from the structure
determination of
3 (Fig. S4, ESI†). Compound 3 exhibits good
Fig. 4 Reversible SCSC transformation between 3,3'-bpeb and 3,3',3'',3'''-
bpbpvpcb in 3 (a) and 4 (b), respectively. The separation between the
parallel aligned olefinic bonds is represented as striped connections in Fig.
4a . Symmetry code: (A): - x + 3, - y + 2, - z + 1. See Fig. 1 for colour codes.
thermal stability, with TGA indicating no mass loss until 312°C
(Fig. S5, ESI†).
The HRMS spectrum of 3,3',3'',3'''-bpbpvpcb (Fig. S9, ESI†)
closely resembles that of 3,3',3'',3'''-tppcp. Whilst the
spectrum clearly indicates the presence of the dimer, a major
peak at m/z 285.1385 corresponding to [3,3'-bpeb + H]+ is
consistent with the cleavage of the cyclobutane during the
HRMS measurements. As with
possibility that heating crystals of
reaction. When single crystals of
2
, this observation opens up the
will reverse the cyclisation
were heated in an oven for
4
4
a period of 3 h at 260 °C, the transparency of the crystals was
maintained. An X-ray analysis of a heated single crystal
revealed that 3,3',3'',3'''-bpbpvpcb was indeed converted to
Fig. 3 View of the 3D framework of 3 along the c axis. See Fig. 1 for colour
codes.
3,3'-bpeb, resulting in the regeneration of
3 in a SCSC
transformation. The thermally induced cleavage reaction was
confirmed by PXRD, CPMAS 13C NMR and solid-state optical
absorbance spectroscopy (Figs. S4, S8 and S10, ESI†).
In contrast to the in-phase aligned 3,3'-bpeb in
of adjacent 3,3'-bpeb molecules are arranged in an out-of-
phase mode in (Fig. 4a). As a consequence, only one pair of
1, each pair
3
The significant structural differences that exist between
1
olefinic bonds satisfies Schmidt's topochemical criterion for
[2+2] photodimerization10 with a separation between the
aligned olefinic bonds of 3.717 Å (C6···C7A). UV irradiation
and
2, and
3
and
4
offer the prospect that the parent
) could be distinguished from their
and ) on the basis of their
solid state photoluminescent behaviour. Accordingly, the solid
state photoluminescent properties of were investigated
(Fig. S13, ESI†). exhibits green fluorescence emission with a
maximum at 529 nm (λex = 376 nm). Under the same
conditions compound displays a blue emission band at 471
nm. and exhibit green and blue-green photoluminescence
compounds (
1
and
3
respective daughter compounds (
2
4
over single crystals of
3 for 3 h resulted in dimerization of the
two 3,3'-bpeb ligands to afford a new mono-cyclobutane,
3,3',3'',3'''-bpbpvpcb (Fig. 4b). Single crystal X-ray diffraction
1-4
1
analysis of the irradiated sample indicates that
converted to [Zn2(3,3',4,4'-bptc)(3,3',3'',3'''-bpbpvpcb)0.5 n
in a SCSC transformation. Although and share gross
3 was
]
(4)
2
3
4
3
4
structural features, the topologies of the two frameworks are
different owing to the formation of the tetrapyridyl ligand,
3,3',3'',3'''-bpbpvpcb which serves as a 4-connecting node
(Figs. S11-12, ESI†). If the 3,3',3'',3'''-bpbpvpcb ligands,
3,3',4,4'-bptc and the Zn2 units are considered as 4-, 4- and 6-
connected nodes, respectively, the topology of this trinodal
with maxima at 525 and 491 nm, respectively, upon excitation
at 373 nm (Fig. S13, ESI†). These results were verified by laser-
scanning confocal fluorescence microscopy, which clearly
indicated differences in fluorescence between
as and (Fig. 5).
1 and 2 as well
3
4
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transformation. The emission intensity of each state is
maintained over repeated cycles of switching, indicating the
robust nature of these materials. The materials reported
represent the first examples of the employment of a photo-
responsive divinylbenzene ligand in 3D switchable materials
whose states can be identified by fluorescence. The results
described herein offer encouragement with respect to the
incorporation of diethenylbenzene or indeed multi-vinyl
compounds in technologically useful materials such as
rewritable information storage devices.
This work was financially supported by the National Natural
Science Foundation of China (Grant Nos. 21531006 and
21773163), the Priority Academic Program Development of
Jiangsu Higher Education Institutions and the Project of
Scientific and Technologic Infrastructure of Suzhou
(SZS201708). We are grateful to the useful comments of the
editor and reviewers.
Fig. 5 Confocal fluorescence microscopy images for single crystals of 1, 2, 3
and 4, respectively. All samples were excited at 458 nm with an Ar ion laser.
For each image, a bandpass filter was set to separate the light from the
fluorescence of each sample. The range of the bandpass filter is from 443 to
561 nm.
Notes and references
A comparison of the emission bands of the ligands 3,3'-bpeb
(λex = 362 nm, λem = 510 nm), 1,4-H2ndc (λex = 368 nm, λem
=
1
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475 nm) and 3,3',4,4'-bptc (λex = 326 nm, λem = 388 nm) with
the fluorescence emissions of 1 and 3 suggest intraligand and
ligand-to-ligand charge-transfer (LLCT) transitions,12 associated
with 3,3'-bpeb are largely responsible for the fluorescence
exhibited by
shifts of 58 and 34 nm relative to
larger shift in the case of (58 nm) possibly reflects the more
1
and
3. The emission peaks of
2
and
4 show blue
2
1
and
3, respectively. The
2
substantial changes to the 3,3'-bpeb ligand associated with the
loss of both double bonds and the subsequent generation of
two cyclobutane rings to form 3,3',3'',3'''-tppcp.
3
4
5
6
7
Reversibility cycle experiments on powdered crystalline
samples of
1 and 3 were performed to examine the fatigue
resistance of the CPs. These experiments involved repeated
cycles of sample irradiation with UV light followed by heating.
The fluorescence at the characteristic emission bands was
monitored during the cycloaddition-cycloreversion process.
(a) J. Cepeda, G.Beobide, O. Castillo, A. Luque and S. Pérez-
Yáñez, Coord. Chem. Rev., 2017, 352, 83; (b) B. Wang, X. L.
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Zhou, J. Am. Chem. Soc., 2016, 138, 6204; (c) B. Wang, Q.
Yang, C. Guo, Y. Sun, L. H. Xie and J. R. Li, ACS Appl. Mater.
The attenuation of emission intensities of both
1 and 3 were
found to be negligible after eight full cycles (Fig. S13, ESI†).
The time profile of fluorescence switching has recently
attracted much attention because of their specific behaviour in
the solid state and aggregate state.13 Thus we also studied the
fluorescence spectral changes of these compounds against UV
irradiation and heating time. Upon UV irradiation at regular
Inter., 2017, 9, 10286.
8
9
G. K. Kole, T. Kojima, M. Kawano and J. J. Vittal, Angew.
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(a) I. H. Park, A. Chanthapally, Z. Zhang, S. S. Lee, M. J.
time intervals, the fluorescence emission bands of
exhibited blue-shifts gradually (Fig. S14, ESI†). On the contrary,
the emission peaks of and displayed gradual red-shifts with
1 and 3
Zaworotko and J. J. Vittal, Angew. Chem., Int. Ed., 2014, 53
,
414; (b) H. Aggarwal, P. M. Bhatt, C. X. Bezuidenhout and L. J.
Barbour, J. Am. Chem. Soc., 2014, 136, 3776.
2
4
increasing heating time at regular time intervals (Fig. S14,
ESI†). These results indicate that the photophysical properties
of switchable CPs could be strictly controlled by the
stimulating time of these responsive materials.
10 G. M. J. Schmidt, Pure Appl. Chem., 1971, 27, 647.
11 L. Öhrström and K. Larsson, Molecule-Based Materials: The
Structural Network Approach, Elsevier, Amsterdam, 2005.
12 A. Vogler and H. Kunkely, Coord. Chem. Rev., 2002, 230, 243.
13 (a) J. Su, T. Fukaminato, J.-P. Placial, T. Onodera, R. Suzuki, H.
Oikawa, A. Brosseau, F. Brisset, R. Pansu, K. Nakatani and R.
Métivier, Angew. Chem., Int. Ed., 2016, 55, 3662; (b) F. Tong,
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In summary, two examples of crystalline materials that each
exhibits bistability are described. Through UV irradiation and
heating of the samples, it is possible to repeatedly switch
between the two states which can be clearly differentiated on
the basis of their fluorescence behaviour. In each case the
switching process represents an example of
a SCSC
4 | J. Name., 2012, 00, 1-3
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