Photoresponsive porous materials: The design and synthesis of photochromic diarylethene-based linkers and a metal-organic framework

Article abstract of DOI:10.1039/c3cc49666j

The synthesis and characterization of novel photochromic diarylethene-based linkers for use in metal-organic frameworks is described including crystal structure analysis of nearly all reaction intermediates. The bis-carboxylated dithien-3-ylphenanthrenes can be prepared under relatively mild conditions in high yield and were subsequently used to create a photoresponsive metal-organic framework, UBMOF-1. While the photochromism of the ligand TPDC in solution is fully reversible, the cycloreversion reaction is suppressed when this linker is incorporated into the crystalline framework of UBMOF-1. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3cc49666j

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Photoresponsive porous materials: the design and
synthesis of photochromic diarylethene-based
linkers and a metal–organic framework†
Cite this: Chem. Commun., 2014,
50, 2653
Received 20th December 2013,
Accepted 21st January 2014
Dinesh G. (Dan) Patel,‡a Ian M. Walton,‡b Jordan M. Cox,b Cody J. Gleason,b
David R. Butzerb and Jason B. Benedict*b
DOI: 10.1039/c3cc49666j
www.rsc.org/chemcomm
The synthesis and characterization of novel photochromic diarylethene- photochromic groups are covalently attached to the framework
based linkers for use in metal–organic frameworks is described including leading to MOFs with well-defined stoichiometry. To date,
crystal structure analysis of nearly all reaction intermediates. The photoswitchable MOF linkers consist entirely of conventional
bis-carboxylated dithien-3-ylphenanthrenes can be prepared under MOF linkers with an appended azobenzene group capable of
relatively mild conditions in high yield and were subsequently used exhibiting photoinduced cis–trans isomerization.17–19,22 In this
to create a photoresponsive metal–organic framework, UBMOF-1. report, we introduce a new class of photoswitchable linkers based
While the photochromism of the ligand TPDC in solution is fully upon the highly popular diarylethene photochromes.10,23–25
reversible, the cycloreversion reaction is suppressed when this
linker is incorporated into the crystalline framework of UBMOF-1.
The prevalence of diarylethenes in photochromic research is
due in large part to their excellent thermal stability, large optical
differences between ring open and ring closed forms, and resistance
Constructed from transition metal building blocks bridged by to fatigue.10 The core of the multi-functional diarylethene linker
organic linkers, metal–organic frameworks (MOFs) remain an extre- consists of a bis-thien-3-yl substituted phenanthrene (BT3P) as
mely promising area of research given their enormous potential shown in Fig. 1. A linker suitable for the preparation of light-
impact on separation science, chemical sensing, gas storage, and responsive MOFs is obtained by incorporating carboxylic acid groups
drug delivery.1–5 While MOFs have traditionally been employed as on either the phenanthrene or the thiophene units, although other
passive materials, next generation active materials will be capable of binding groups are also possible (Fig. 1). Carboxylation of the 2,7
undergoing structural reorganization upon application of external positions of the phenanthrene unit is a unique especially promising
stimuli including light, heat, or changes in pH.6–8
route as this leads to a structurally rigid ditopic linker which strongly
Photochromic molecules, compounds that change their resembles several common MOF linkers and ultimately leads to
molecular and electronic structure upon application of light, photo-responsive MOFs which are isostructural with the MOFs
remain ideal candidates for sensors, switches, and optical data which utilize non-photochromic linkers.
storage media and have been reviewed extensively.9–15 More
Using the work of Geng26 as a starting point, we set out to
recently, photochromic technologies have been pursued for use utilize the pinacol rearrangement of a thien-3-yl substituted
in MOFs in hopes of affording photonic control over the physical phenanthrene diol in a one-pot synthesis to give the desired
properties of the crystalline host.16–19
diarylethene (Scheme 1). Geng and coworkers explored the one-
Although several instances of MOFs containing non-covalently pot rearrangement and reduction of a thien-2-yl substituted
attached photochromic molecules have been reported,16,20,21 the phenanthrene diol (Scheme 1, top) by including both a strong
lack of control over guest orientation and concentration diminishes acid to promote rearrangement and zinc dust to afford the
the utility of this method. By using photoactive linkers, the
a Department of Chemistry, Pennsylvania State University, 76 University Drive,
Hazleton, PA 18202, USA
b Department of Chemistry, University at Buffalo, The State University of New York,
Buffalo, New York 14260-3000, USA. E-mail: jbb6@buffalo.edu
† Electronic supplementary information (ESI) available: Synthetic details, digestion
experiment results, additional UV/Vis spectra, crystallographic data, crystallo-
Fig. 1 Molecular structures of the prototypical diarylethene, pFDTE, of
parent compound in this study, BT3P, and of potential metal binding
substituent groups suitable for MOFs.
graphic refinement details. CCDC 975076–975080. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c3cc49666j
‡ These authors contributed equally to this work.
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Fig. 2 UV visible spectra (1.4 ꢀ 10ꢁ5 M in methanol) of the open (solid
blue line) and closed form (dashed red line) of 7a. The absorbance has
been multiplied by a factor of 5 (pale blue and red line) to emphasize the
absorption bands above 350 nm. Inset: the difference absorption spectrum
(before and after irradiation at 365 nm) of a UBMOF-1 single crystal.
Scheme 1 A one pot reaction allows for the double pinacol rearrange-
ment of a thien-2-yl substituted phenanthrene diol (top) while the thien-
3-yl substituted diol explored in this study does not react (bottom).
in sulfuric acid.28 The crystal structure of diol 3a was found to
be the trans isomer as predicted by Cram’s rule.29
Key diol 3 is subjected to the first of two pinacol rearrangements
by refluxing in toluene containing 10 mol% p-toluenesulfonic acid
(p-TsOH) to give ketone product 4. While mineral acids are typically
used for such rearrangements and dehydration, p-TsOH is known
to avoid oxidizing sensitive substrates while other acids are not.30
Water is removed by means of a Dean–Stark trap. Reduction of 4 to
the corresponding alcohol 5 is achieved in a 1 : 1 solvent system of
THF and methanol using sodium borohydride added in portions over
several hours. The alcohol obtained is spectroscopically pure but can
be further purified by column chromatography. The last pinacol
rearrangement, giving photochromic bis-thien-3-yl substituted
phenanthrene 6, proceeds as before by treating alcohol 5 with
p-TsOH in refluxing toluene with concomitant removal of water.
While photochromes 6a and 6b are interesting in their own
right, in order to make them suitable for MOF inclusion they
must be lithiated and then treated with carbon dioxide followed
by protonation with dilute acid. The resulting dicarboxylic acids
maintain photochromic activity; the electronic absorption spectra
for the open and closed forms of 7a are presented in Fig. 2.
Initial attempts to synthesize a framework using 7b have been
unsuccessful, potentially due in part to the conformational flexibility
and non-linearity of the carboxylate binding groups. The solvothermal
reaction of 7a, which is a rigid linear ditopic linker, with zinc nitrate
in dimethylformamide yields large photochromic single crystals. In
these crystals, deprotonated 7a (9,10-bis(2,5-dimethylthiophen-3-yl)-
phenanthrene-2,7-dicarboxylate, TPDC) coordinates to zinc metal
centers to form frameworks of Zn4O(TPDC)3, which we label as
UBMOF-1 (University at Buffalo MOF-1). The structure of UBMOF-1,
which was determined by single crystal XRD, is isomorphous with
the previously reported frameworks IRMOF-10 and IRMOF-14.31
That the three structures are isomorphous is not surprising as
all three structures possess an identical oxide-centered Zn4O
tetrahedron, and the known structures utilize the linkers
biphenyl-4,40-dicarboxylate and pyrene-2,7-dicarboxylate, which
are similar in length to the phenanthrene-dicarboxylate back-
bone of TPDC.
Scheme 2 Conversion of 3-iodo-2,5-dimethylthiophene to the corres-
ponding Grignard reagent followed by reaction with benzil affords the
corresponding diol. The pinacol rearrangement of this diol is achieved by
treatment with acetyl chloride in refluxing benzene–acetic acid.
necessary reduced product. However, several attempts at
forcing this rearrangement for our model system consisting
of a 2,5-dimethylthien-3-yl substituted diol led only to the
recovery of starting material (Scheme 1, bottom).
To our knowledge, only one report exists where a 2,5-dimethyl-
thien-3-yl substituted diol was used as a substrate in the pinacol
rearrangement, and harsh conditions were required to carry out this
transformation (Scheme 2).27 The use of refluxing acetyl chloride is
not compatible with many functional groups including alcohols and
amines, both of which could find use as ligands.
To preclude the need for the harsh conditions required to
achieve a pinacol rearrangement with our systems, we chose a
stepwise synthetic route which starts with the formation of the
requisite phenanthrene diol (Scheme 3). A 3-halosubstituted
thiophene 1 is lithiated and then treated with the appropriate
phenanthrene-9,10-dione 2 to afford diol 3. Compound 2b is available
commercially while compound 2a is prepared in a single step and
with reasonable yield by treating 2b with N-bromosuccinimide (NBS)
The high site symmetry (mm2) of the photochromic linker
(which itself possesses no internal symmetry) in UBMOF-1 gives
rise to strong crystallographic disorder of the low symmetry ligand
and prevents reliable refinement of the dimethylthiophene groups.
Scheme 3 Synthetic route to bis-carboxylated phenanthrene dithienyl-
ethenes 7a and 7b. Single crystal XRD structures were obtained for 3a, 4a,
6a, and 7b.
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which is photochemically inert.10,32,33 Dissolving irradiated UBMOF-1
crystals provides an opportunity to distinguish between these
two possibilities. A photochemically inert fatigue product will
not revert back to the colorless ring open form, whereas, a
solution of the ligand will be photochromic.
Digestion of a sample of UV irradiated UBMOF-1 yields a
pale red solution that turns pale yellow upon exposure to visible
light (Fig. 5). Exposing the pale yellow solution to 365 nm light
causes the sample to revert back to a red color consistent with
Fig. 3 (a). Image of the superposition of eight distinct confirmations of TPDC
resulting from the high site symmetry of the ligand. The crystallographically
refined portion of the ligand is boxed in red. Images of the packing of UBMOF-1 the ring-closed photoproduct. As the solutions of digested
viewed along [110] (b) and [100] (c). Color scheme of atoms is as follows: zinc
UBMOF-1 retain their reversible photochromism, this suggests that
the persistent coloration is a consequence of the local chemical
(blue), oxygen (red), carbon (grey), and sulfur (yellow).
environment as opposed to the formation of a photochemical
fatigue product. Similar stabilization of the photoproduct has been
observed in the azobenzene derivatized MOFs,16,17 although the
precise origin of stabilization has not been determined.
In conclusion, the synthesis and characterization of a new
class of photochromic metal–organic framework linkers based
Fig. 4 Images of a UBMOF-1 single crystal: (a) before exposure to UV
on diarylethenes is described. The first framework synthesized
light, (b) after irradiation with 365 nm light, and (c) after irradiation with
with the photoswitchable ligands, UBMOF-1, is isostructural
visible light for >2 hours.
with the previously reported IRMOF-10 and -14. This result
reinforces the supposition that additional functionality, such as
It should be noted that crystallographic symmetry imposed
upon the ligand causes the phenanthroline backbone of TPDC
to resemble pyrene (see Fig. 3 and discussion in the ESI†).
To verify that no significant ligand decomposition occurs
during the course of the solvothermal synthesis, a sample of
UBMOF-1 was digested using DCl–DMSO-d6 and analyzed by
1H-NMR spectroscopy. As the spectrum of the digested UBMOF-1
is virtually identical to that of 7a in DMSO-d6, we conclude that
the ligand remains intact throughout the solvothermal synthetic
process and subsequent digestion.
photoswitching, can be designed into a ligand without affecting
the topology of MOFs created using the new ligands. The
suppression of the ligand ring-opening reaction in the MOF
relative to solution highlights the challenges and opportunities
present in this new class of materials.
The authors declare no competing financial interests.
Partial support of this work was provided by the University at
Buffalo, The State University of New York.
Notes and references
While the photochromism of 7a is fully reversible (the red
solutions of the closed photoproduct return to the initial colorless
state upon application of visible light), crystals of UBMOF-1 do not
completely revert back to their initial state upon irradiation with
visible light (Fig. 4). After more than two hours of irradiation
with visible light, the red coloration persists. Two possibilities
exist for the persistent coloration: the local chemical environ-
ment of the TPDC in the framework is suppressing the ring-
opening reaction, or the TPDC has undergone an undesirable
photochemical reaction leading to a colored fatigue product
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