Flipping the Switch: Fast Photoisomerization in a Confined Environment

Article abstract of DOI:10.1021/jacs.8b02994

Stimuli-responsive materials are vital for addressing emerging demands in the advanced technology sector as well as current industrial challenges. Here, we report for the first time that coordinative integration of photoresponsive building blocks possessing photochromic spiropyran and diarylethene moieties within a rigid scaffold of metal-organic frameworks (MOFs) could control photophysics, in particular, cycloreversion kinetics, with a level of control that is not accessible in the solid state or solution. On the series of photoactive materials, we demonstrated for the first time that photoisomerization rates of photochromic compounds could be tuned within almost 2 orders of magnitude. Moreover, cycloreversion rates of photoresponsive derivatives could be modulated as a function of the framework structure. Furthermore, through MOF engineering we were able to achieve complete isomerization for coordinatively immobilized spiropyran derivatives, typically exhibiting limited photoswitching behavior in the solid state. For instance, spectroscopic analysis revealed that the novel monosubstituted spiropyran derivative grafted to the backbone of the MOF pillar exhibits a remarkable photoisomerization rate of 0.16 s^-1, typical for cycloreversion in solution. We also applied the acquired fundamental principles toward mapping of changes in material properties, which could provide a pathway for monitoring material aging or structural deterioration.

Full text of DOI:10.1021/jacs.8b02994

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Article
Flipping the Switch: Fast Photoisomerization in a Confined Environment
Derek E. Williams, Corey R Martin, Ekaterina A. Dolgopolova, Anton Swifton, Danielle C
Godfrey, Otega A. Ejegbavwo, Perry J Pellechia, Mark D. Smith, and Natalia B. Shustova
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 28 May 2018
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Flipping the Switch: Fast Photoisomerization in a Confined Environ-
ment
Derek E. Williams,‡a Corey R. Martin,‡a Ekaterina A. Dolgopolova,‡a Anton Swifton,b Danielle C. Godꢀ
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frey,a Otega A. Ejegbavwo,a Perry J. Pellechia,a Mark D. Smith,a and Natalia B. Shustovaa*
aDepartment of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States
bDepartment of Mathematics, University of South Carolina, Columbia, South Carolina 29208, United States
ABSTRACT: Stimuliꢀresponsive materials are vital for addressing emerging demands in the advanced technology sector as well as
current industrial challenges. Here, we report for the first time that coordinative integration of photoresponsive building blocks
possessing photochromic spiropyran and diarylethene moieties within a rigid scaffold of metalꢀorganic frameworks (MOFs) could
control photophysics; in particular, cycloreversion kinetics with the level of control, which is not accessible in the solid state or
solution. On the series of photoactive materials, we demonstrated for the first time that photoisomerization rates of photochromic
compounds could be tuned within almost two orders of magnitude. Moreover, cycloreversion rates of photoresponsive derivatives
could be modulated as a function of the framework structure. Furthermore, through MOF engineering we were able to achieve
complete isomerization for coordinatively immobilized spiropyran derivatives, typically exhibiting limited photoswitching behavior
in the solid state. For instance, spectroscopic analysis revealed that the novel monosubstituted spiropyran derivative grafted to the
backbone of the MOF pillar exhibits a remarkable photoisomerization rate of 0.16 sꢀ1, typical for cycloreversion in solution. We
also applied the acquired fundamental principles towards mapping of changes in material properties, which could provide a pathꢀ
way for monitoring material aging or structural deterioration.
ed as (left) a side group on the organic linker, (middle) as a
framework backbone, and (right) as a capping linker77
INTRODUCTION
Photochromic building blocks capable of switching beꢀ
tween two discrete states upon external stimuli are vital for
applications such as “smart” windows, transition lenses, optiꢀ
cal switches, or controlled drug delivery systems mainly due
to the drastic difference in the photoisomer properties.1–14 For
instance, upon changing excitation wavelength, temperature,
pressure, or pH, a material absorption/emission profile, dielecꢀ
tric constant, or reduction potential could be drastically moduꢀ
lated.15–21
with the size similar to the framework pocket. A grey
“photoswitch” symbol indicates a position of a photo-
chromic unit.
Due to the tunability and multifaceted modularity of metꢀ
alꢀorganic frameworks (MOFs),22–44 photochromic derivatives
could be embedded into a framework through different pathꢀ
ways: inside the pores or as linkers (or a part of a linker).45–53
In this work, we developed a novel synthetic methodology in
combination with recent advances in the MOF field54–80 to
probe photophysics of photochromic compounds in a confined
environment defined by framework topology (Scheme 1). We
attempted for the first time to gain fundamental knowledge of
cycloreversion kinetics (i.e., transition between two states
upon radiation with visible light) of two distinct classes of
photochromic compounds with spiropyranꢀ and diarylethene
cores in the solid state, solution, and coordinatively immobiꢀ
lized inside the rigid matrix. For that, we prepared novel spiꢀ
ropyran derivatives with anchors for metal coordination,
which resulted in the first example of a spiropyranꢀbased MOF
with a photochromic unit as a side group on the linker
backbone. Such coordinative immobilization of photochromic
derivatives inside a scaffold allowed us to study the possibility
of tuning photoisomerization rates as a function of framework
topology with a level of control that is not accessible in the
solid state or solution. We have investigated five systems,
which provide us different pathways for anchoring of photoꢀ
chromic compounds to allow for tunability of rigidity and
photoswitching capacity. In our studies, we were also able to
investigate the effect of steric hindrance on photoswitching
ability. As a result, the acquired fundamental principles were
applied towards mapping the changes of material properties,
which provide a pathway for utilization of the introduced conꢀ
cept towards development of stimuliꢀresponsive markers.
Scheme 1. A schematic representation of coordinatively
immobilized photochromic derivatives inside the metal-
organic rigid scaffold. The photochromic moiety integrat-
RESULTS AND DISCUSSION
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The Result and Discussion section includes the following
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subsections: (1) synthesis and study of photochromic comꢀ
pounds; (also includes characterization of diastereoisomer
equilibration kinetics for one of the novel ligands bearing two
photoswitchable units) (2) design and preparation of frameꢀ
works for integration of synthesized photochromic comꢀ
pounds, highlighting benefits of framework structural tunabilꢀ
ity (Scheme 1) including structural analysis; (3) photoisomeriꢀ
zation kinetics of photochromic molecules in the solid state
and solution as well as after their incorporation inside a rigid
scaffold; (4) mapping changes of material properties through
utilization of a prepared photochromic scaffold.
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Comprehensive analysis of the frameworks and their preꢀ
cursors includes singleꢀcrystal and powder Xꢀray diffraction
(PXRD), solidꢀstate and solution nuclear magnetic resonance
(NMR), Fourier transform infrared (FTIR), steadyꢀstate fluoꢀ
rescence, diffuse reflectance, UVꢀvis spectroscopies, and mass
spectrometry, which will be discussed for each respective sysꢀ
tem.
Synthesis of Photochromic Linkers
Figure 1. (a) Photoisomerization of the diaryletheneꢀbased and
To build novel stimuliꢀresponsive frameworks with coorꢀ
dinatively immobilized photoswitchable units, we prepared
four different photochromic molecules belonging to two disꢀ
tinct classes (Figures 1–3). Photochromic linkers with a diaꢀ
rylethene core were chosen due to their fatigueꢀresistant phoꢀ
tochromic performance, thermal stability, and rapid response
in the solid state (Figure 1).78–81 For coordinative immobilizaꢀ
tion inside the framework, the photoswitchable core was modꢀ
ified with pyridyl groups or carboxylic acid arms to facilitate
coordination to metal centers. The diarylethene derivatives,
1,2ꢀbis(2ꢀmethylꢀ5ꢀ(pyridinꢀ4ꢀyl)thiophenꢀ3ꢀyl)cyclopentꢀ1ꢀ
ene (BPMTC) and 4,4'ꢀ(cyclopentꢀ1ꢀeneꢀ1,2ꢀdiyl)bis(5ꢀ
methylthiopheneꢀ2ꢀcarboxylic acid (H2BCMTC) are shown in
Figure 3.82,83
(b) spiropyran derivatives. (c) For simplicity, structural differꢀ
ences of “inꢀin”, “inꢀout”, and “outꢀout” diastereoisomers are
shown on the example of the brominated precursor (Scheme 2)91
of HDDB. Displacement ellipsoids drawn at the 60% probability
level. Gray, blue, and red spheres represent C, N, and O atoms,
respectively.
harness full potential of spiropyran derivatives in adꢀ
vanced technologies.
To overcome such steric hindrance observed for spiropyꢀ
ran groups in the solid state, we synthesized two novel photoꢀ
chromic linkers with spiropyran groups attached to the ligand
backbone (Figure 1 and Scheme 2). Such derivatization of
photochromic units will allow for their integration inside the
rigid core of a porous framework, where spiropyran phoꢀ
toisomerization could occur within the pore aperture. With this
in mind, we prepared 1',3',3'ꢀtrimethylꢀ6ꢀnitroꢀ4',7'ꢀdi(pyridinꢀ
4ꢀyl)spiro[chromeneꢀ2,2'ꢀindoline] (TNDS, Scheme S1), and
1',1''',3',3',3''',3'''ꢀhexamethylꢀ6,6''ꢀdinitroꢀ4',4''' di(pyridinꢀ4ꢀ
yl)ꢀ7',7'''ꢀbispiro[chromeneꢀ2,2'ꢀindoline] (HDDB, Scheme S2)
via a multistep synthetic procedure as shown in Scheme 2 (six
steps for TNDS and seven steps for HDDB, Schemes S1 and
S2, respectively). These linkers with oneꢀ and twoꢀ
photoresponsive moieties on one backbone can be also used as
a model compound to probe the influence of steric hindrance
on spiropyran isomerization.
As the second class of photochromic linkers, we chose
spiropyran derivatives due to the different pathway of the phoꢀ
toinduced cyclization reaction in comparison with diarylethene
derivatives (Figure 1). In contrast to diarylethenes, which
isomerization could be described by covalent bond formation
between the methylthiophene groups,84 spiropyranꢀbased comꢀ
pounds undergo relatively large structural changes centered on
the sp3 “spiro”ꢀcarbon, rotating from orthogonal to planar geꢀ
ometry (Figure 1).3,85,86 Moreover, spiropyran is hydrophobic
and uncharged, whereas its colored isomer, merocyanine, is a
hydrophilic zwitterion (Figure 1).87 Furthermore, diaryletheneꢀ
based compounds are capable of 100% reversible photoisomꢀ
erization in the solid state,88 while spiropyran derivatives do
not typically lead to solidꢀstate reversible photoisomerization
due to the necessary large structural reorganization.89 In genꢀ
eral, solidꢀstate spiropyran photoisomerization has been exꢀ
plored for derivatives tethered to solid supports.90 Therefore,
there is a demand to systematically create solidꢀstate spiropyꢀ
ranꢀbased materials to
All the prepared photochromic linkers and their precurꢀ
1
sors underwent comprehensive analysis by H and 13C NMR,
FTIR spectroscopies, singleꢀcrystal Xꢀray diffraction, and
mass spectrometry. Detailed description can be found in the
Supporting Information (SI) or the experimental section below
(Figures S1–S6). The compound, HDDB (Schemes 2 and S2),
containing two spiropyran groups attached to the linker skeleꢀ
ton, is of a particular interest to study the enantiomerization
process of spiropyran units due to resolvable diastereoisomers.
As it was previously shown on the example of its precursor,91
the HDDB linker can exist in three unique diastereomers:
HDDBinꢀin, HDDBinꢀout, and HDDBoutꢀout (Figures 1 and S5).
We studied the thermal equilibration process of HDDB in
solution to estimate the effect of steric
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Scheme 2. Reaction scheme for the synthesis of TNDS
(black) and HDDB (grey)a
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a i) Cu powder, DMF, 120 °C 4 h; ii) SnCl2, HCl, EtOH, reflux 12
h; iii) NaNO2, HCl, 0 °C 1 h / CO(NH2)2, 0 °C 10 min / SnCl2,
0 °C, 4 h; iv) 3ꢀmethylꢀ2ꢀbutanone, reflux 3 h / H2SO4, EtOH,
reflux 3 h; v) Et2O, hexanes, methyl trifluoromethanesulfonate,
room temperature, 8 h; vi) 2ꢀhydroxyꢀ5ꢀnitrobenzaldahyde, EtOH,
piperidine, reflux 8 h; vii) pyridineꢀ4ꢀboronic acid, PdCl2(PPh3)2,
CuCl, Na2CO3, DMF, 120 °C, 48 h.91–93
Figure 2. (top) Single crystal Xꢀray structures of HDDBinꢀin and
HDDBinꢀout. Two orientations of each diastereoisomer are shown.
Displacement ellipsoids drawn at the 60% probability level. Gray,
blue, and red spheres represent C, N, and O atoms, respectively.
1
(bottom) A region of the H NMR spectra for thermal equilibraꢀ
tion of HDDBin−in to HDDBin−out and HDDBout−out in CD2Cl2 is
shown at t = 0, 9, 24, 48, and 72 h. The full spectra for the HDDꢀ
Binꢀin conversion are shown in Figure S22.
hindrance of spiropyran groups on the possibility of
isomerization.
The thermal equilibration of the system was studied using
a similar procedure utilized by Klajn and coꢀworkers.94
A
which were used to model the concentration profiles. A
good agreement was achieved between the experimental and
calculated results. The obtained rate constant values were simꢀ
ilar to those of the HDDB precursor (VII’, Scheme S2).
Therefore, we can hypothesize that the extension of ligand
backbone containing spiropyran moieties with pyridyl group
does not significantly affect isomerization rates.
thermal equilibration process between the three isomers,
HDDBinꢀin, HDDBinꢀout, and HDDBoutꢀout was represented as a
cyclic reversible reaction involving three components with six
firstꢀorder reactions described by six rate constants (where k =
rate constant, C =concentration).
∂Cinꢀin/∂t = ꢀk1Cinꢀin ꢀ kꢀ3Cinꢀin + kꢀ1Cinꢀout + k3Coutꢀout
∂Cinꢀout/∂t = ꢀk1Cinꢀout ꢀ k2Cinꢀout + k1Cinꢀin + kꢀ2Coutꢀout
∂Coutꢀout/∂t = ꢀk3Cin Coutꢀout ꢀ kꢀ2Coutꢀout + kꢀ3Cinꢀin + k2Cinꢀout
Synthesis of Photoresponsive 1, 2, 3, 3’, and 4 MOFs
The prepared linkers were used for coordinative immobiꢀ
lization inside the porous scaffolds. We used two distinct apꢀ
proaches for linker incorporation. One approach relies on coꢀ
ordination of linkers containing pyridyl groups to unsaturated
metal sites of a framework. Three photochromic ligands
TNDS, HDDB, and BPMTC (Figure 3) contain terminal
pyridyl groups, which make them suitable for incorporation
between twoꢀdimensional (2D) MOF layers. For instance,
BPMTC, has been previously utilized as pillars in a MOF, in
which 2D layers consist of a tetracarboxylate linker, DBTD4ꢀ
To reduce the number of unknown variables, the steady
state95 was described with following equations:
kꢀ1 = k1(Cinꢀin, eq/Cinꢀout, eq
kꢀ2 = k2(Cinꢀout, eq/Coutꢀout, eq
kꢀ3 = k3(Coutꢀout, eq/Cinꢀin, eq
)
)
)
To be able to monitor thermal isomerization process, we
isolated two pure diastereoisomers HDDBinꢀin and HDDBinꢀout
(see the SI) to track changes of chemical shifts in the NMR
spectra for Nꢀbound methyl group protons (Scheme 2 and Figꢀ
ure 2), which allow to distinguish between three diastereoisoꢀ
mers. In order to estimate equilibration kinetics between diaꢀ
stereoisomers in solution, HDDBinꢀin was dissolved in diꢀ
chloromethaneꢀd2, and its spectra were monitored by 1H NMR
spectroscopy. As shown in Figure 2, the equilibration between
the three isomers was reached after three days. We solved the
rate equations numerically (see the SI for more details) and
obtained six rate constants (k1 = 0.214, kꢀ1 = 0.122, k2 = 0.228,
kꢀ2 = 0.402, k3 = 0.481, kꢀ3 = 0.481),
(H4DBTD = 3’,6’ꢀdibromoꢀ4’,5’ꢀbis(4ꢀcarboxyphenyl)ꢀ
1[1,1’:2’,1’’ꢀtetraphenyl]ꢀ4,4’’ꢀdicarboxylic acid, Figure 3).29
We anticipated that our novel spiropyran pillars could also
bind axially to the zinc paddlewheel secondary building units
(SBUs). Indeed, integration of TNDS and HDDB were perꢀ
formed between 2D layers, consisting of DBTD4ꢀ connected
by
Zn2(O2Cꢀ)4,
and
resulted
in
formation
of
Zn2(DBTD)(TNDS) (1), Zn2(DBTD)(HDDB) (2), and
Zn2(DBTD)(BPMTC) (3, Figures 3, S7–S14, see experimental
details in the SI).
As discussed above, the structures consist of stacked layꢀ
ers of DBTD4ꢀ connected in the equatorial planes of planar
zinc paddlewheel SBUs (Figure 3). Both 1 and 2 consist of a
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Figure 4. (top) Singleꢀcrystal structures of 1 and 2 with simulated
spiropyran moiety located in the pores. (bottom) The simulated
and experimental PXRD patterns of MOFs 1 (red) and 2 (blue).
The colored text shows the increase of interlayer distance for each
MOF, which is in agreement with the sizes of the installed photoꢀ
chromic linkers.
crystal Xꢀray diffraction showed the presence of twoꢀfold
interpenetrated frameworks (Figure S15 and Table S3).
In the case of 4, we applied a different method for coordiꢀ
native immobilization of H2BCMTC inside the porous scafꢀ
fold. We integrated a photochromic compound as a capping
linker in an already synthesized framework using a previously
developed synthetic methodology.96 For that, we used a Zrꢀ
based framework, Zr6(Me2BPDC)4,77 consisting of 2,2′ꢀ
dimethylbiphenylꢀ4,4′ꢀdicarboxylate, Me2BPDC2ꢀ, linkers
Figure 3. (top) The TNDS, HDDB, and BPMTC linkers were
used to synthesize 1, 2, and 3 (3’), respectively, in the presence of
H4DBTD (H2BPDC). (bottom) The linker, H2BCMTC, was inꢀ
stalled as a capping linker into an existing MOF framework,
Zr6(Me2BPDC)4,77 to make 4. Orange, red, teal, and gray spheres
correspond to zinc, oxygen, zirconium, and carbon atoms, respecꢀ
tively.
8+
coordinated to Zr6O4(OH)8(H2O)4 metal nodes (Figure 3).
The H2BCMTC integration occurred through replacement of
the terminal OH– and H2O groups in the SBU. The schematic
representation of such installation is shown in Figure 3 and
more details about suitable linkers for such coordination as
well as calculation of the appropriate linker size could be
found elsewhere.97 However, it is important to note that
Zr6(Me2BPDC)4 is flexible enough to accommodate structural
changes associated with capping linker insertion. The powder
pattern of prepared Zr6(Me2BPDC)4 was also in good agreeꢀ
ment with the simulated pattern,77 and framework crystallinity
was retained after incorporation of BCMTC2ꢀ (Figure S18). To
remove any traces of residual (nonꢀcoordinated) linkers, we
applied an extensive washing procedure using a Soxhlet appaꢀ
ratus. After that, the installation of BCMTC2ꢀ into 4 was conꢀ
pillar backbone with three and four sixꢀmembered rings,
respectively, which are anchored to the axial positions of the
SBUs with twoꢀfold disorder (for more details see the crystalꢀ
lographic section of the SI, Table S3). The bulk samples of 1
and 2 were also characterized using PXRD, and the acquired
data are in agreement with their simulated patterns (Figure 4).
1
The H NMR spectroscopic studies of digested 1 (i.e., deꢀ
stroyed in the presence of hydrochloric acid) also confirm the
presence of the photochromic pillars (Figure S9). The 13C{1H}
CPꢀMAS NMR spectroscopic analysis confirmed integration
of photochromic moieties in 2 (Figure S12). The FTIR spectra
of 1 and 2 are shown in Figures S8 and S11, respectively.
MOF 3 contains the BPMTC linker coordinatively anꢀ
chored between the axial zinc paddlewheel SBUs and the 2D
layers of DBTD4ꢀ, as it was previously reported (Figure 3).29
The powder pattern of the bulk sample of 3 was in good
1
firmed by H NMR spectroscopy of digested 4 (Figure S19).
In combination with experimental spectroscopic studies, the
geometrical constraints placed by the linker size and pocket
dimension of the framework suggest that the capping linker
can occupy both axial and equatorial positions of Zrꢀnodes, ca.
50% occupancy of available sites.
1
agreement with the simulated pattern (Figure S13). The H
NMR spectrum of the digested sample shows the presence of
both BPMTC and H4DBTD (Figure S14). The structure 3’
consisting of the same BPMTC linker integrated between 2D
sheets composed of the Znꢀbased paddleꢀwheel connected by
BPDC2ꢀ linkers (Table S3, Figures S15–S17). Singleꢀ
Photophysics of Photochromic Compounds
The photoswitching behavior of photochromic molecules
with spiropyran and diarylethene cores are vastly distinct
mainly due to the nature of structural changes associated with
photoinduced isomerization. The expansion of the photoswitch
from the spiropyran form to the merocyanine form is accomꢀ
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panied with significant structural changes, and therefore, can
severely impact the photoisomerization rate.4,98–100 In general,
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spiropyran derivatives consist of two moieties: indoline and
chromene linked together through a single sp3 “spiro” chiral
carbon. Once the photoswitch is irradiated with UV light (λex
<
400 nm), the photoswitch undergoes a cyclization reaction and
generates a highly colored, zwitterionic form (merocyanine),
shown in Figure 1.85
Diarylethene photoswitches also have two isomeric
forms, colorless (“open”) and colored (“closed”, Figure 1).
The colorless “open” form of diarylethene can be irradiated
with UV light, which causes formation of a bond between the
two heterocyclic thiophene moieties, generating the colored
diarylethene form (Figure 1).80 However, the diarylethene
photoswitch, unlike spiropyran, is thermally stable in either
form and does not undergo such a drastic structural change
during isomerization. In fact, relatively little movement of
diarylethene “arms” within the same plane is necessary to
achieve rapid photoisomerization, particularly in the solid
state.
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To probe photoisomerization kinetics of the prepared spiꢀ
ropyran and diarylethene derivatives for the first time, we perꢀ
formed photophysical measurements in solution and in the
solid state. For solution studies, a 3 mM solution of the phoꢀ
toswitch was loaded into a 8 mm × 1 mm frontꢀfacing quartz
sample cell. For solidꢀstate studies, 4 µmol of photoswitch was
mixed with 100 mg of potassium bromide, which was then
pressed into the same 8ꢀmm sample cell.
Figure 5. Normalized absorption plots of TNDS, HDDB,
BPMTC, H2BCMTC in the solid state, solution, and coordinativeꢀ
ly immobilized inside MOF upon irradiation with UV and visible
light.
up to 0.06 sꢀ1 during conversion of HDDBinꢀin to HDDBinꢀ
out and HDDBoutꢀout. Coordinative immobilization of TNDS and
formation of 1 significantly affects its photophysics. Indeed,
once TNDS was integrated into a rigid scaffold, it demonstratꢀ
ed solutionꢀlike photoisomerization behavior, even in the solid
state. Integrated TNDS (1) showed a 100% conversion from
merocyanine to spiropyran with a rate of 0.16 sꢀ1 (Figures 5
and 7, Table 1). Therefore, a framework provides a pathway
for photoswitch isomerization in the solid state through the
spatial separation of photochromic linkers in contrast to the
close packing observed for free ligand in the solid state.
Spiropyran Derivatives in the Solid State, Solution, and
Coordinatively Immobilized in Scaffolds1 and 2
The photochromic cycloreversion kinetics of both the
TNDS and HDDB linker were investigated in the solid state,
solution, as well as coordinatively inside scaffolds (Figures 3,
4, and S7–S12). Figure 5 shows photoisomerization of TNDS
in the solid state and solution with an excitation wavelength of
365 nm. As expected, the TNDS powder was not fully reversiꢀ
ble due to the necessity of significant structural changes reꢀ
quired for the photoisomerization process. As a result, the
TNDS derivative displayed limited photoconversion to the
colorless form in the solid state, whereas in solution, complete
photoinduced reversion was successfully achieved.
Based on the results observed for 1, we integrated the
bulkier HDDB inside the rigid scaffold resulting in preparation
of 2. However, HDDB does not exhibit photophysical behavꢀ
ior similar to TNDS. In contrast, HDDB exhibits limited phoꢀ
toisomerization more typical for the solid state, rather than that
in solution. Such difference between integrated HDDB and
TNDS could be attributed to the presence of two photoswitchꢀ
able moieties and their close proximity in a framework, which
impede the complete cycloreversion process. Thus, compariꢀ
son of integrated TNDS versus coordinatively embedded
HDDB possessing the same photoresponsive moieties, but
dissimilar in their amount (1 vs 2, respectively), allowed us to
demonstrate the crucial role of steric hindrance in the phoꢀ
toisomerization process.
The photophysical studies of TNDS in N,Nꢀ
dimethylformamide (DMF) showed a linear correlation beꢀ
tween ln(A) versus time of irradiation, which is in line with
the data previously reported for other spiropyranꢀderivatives in
solution.90,101–103 The rate constant, k, was found to be 0.12 sꢀ1.
Cycloreversion plots and experimentally determined rates are
shown in Figure 6 and Table 1, respectively.
In comparison with TNDS, HDDB also showed limited
photoisomerization in the solid state, while demonstrating
100% photoconversion in solution. However, measurements of
photoisomerization rates in solution were complicated by the
Diarylethene Derivatives in the Solid State, Solution, and
Coordinatively Immobilized in 3, 3’, and 4 Scaffolds
presence of three isomers, HDDBinꢀin
, HDDBinꢀout, and
HDDBoutꢀout (Figure 1), each of which possess a different phoꢀ
toisomerization rate. For the isolated HDDBinꢀin isomer, k, was
found to be 0.20 sꢀ1 in DMF, which decreases
In the case of diarylethene photoswitches, we found that
both BPMTC and H2BCMTC were fully reversible in the solid
state and solution (λex (cyclization) = 365 nm, λex (cycloreversion) ≥ 400
nm, Figure 1). In solution, the rate constants, k, for the
BPMTC cycloreversion reaction varied in a narrow range
Table 1. The rate constants of cycloreversion process (k, s-
5
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Table 2. The rate constants of cycloreversion process of
BPMTC in different solvents (C = 3 mM).
Journal of the American Chemical Society
Page 6 of 24
k × 102, s-1
1) of TNDS, HDDB, BPMTC, and H2BCMTC, in the solid-
state, solution, and coordinatively immobilized in a MOF.
solvent
1
2
3
4
5
6
7
8
N,Nꢀdimethylformamide
toluene
1.0
2.8
2.7
2.9
2.4
2.4
1.0
1.9
solid state
n/aa
n/aa
solution
1.2 × 10ꢀ1 b
(6.0–20) × 10ꢀ2 b n/a (2)a
MOF
1.6 × 10ꢀ1 (1)
tetrahydrofuran
chloroform
TNDS
HDDB
2.8 × 10ꢀ3 (3)
2.0 × 10ꢀ2 (3’)
4.1 × 10ꢀ2 (4)
dichloromethane
acetonitrile
2.0 × 10ꢀ1
1.0 × 10ꢀ2 b
BPMTC
4.8 × 10ꢀ2
2.6 × 10ꢀ2 c
ethanol
9
H2BCMTC
a Photoisomerization was not fully reversible
b C = 3 mM in DMF
c C = 3 mM in MeOH
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methanol
to a high degree of framework flexibility in 3’ in comparison
with 3. Notably, both 3 and 3’ frameworks were able to acꢀ
commodate structural changes associated with the photoisomꢀ
erization process of BPMTC coordinatively immobilized beꢀ
tween metal nodes, which was confirmed by PXRD. Thus, a
rigid framework allowed us to tune the photoisomerization
rate mimicking solution or solidꢀstate behavior as a function of
its topology.
from 0.01 to 0.03 sꢀ1 (Figures 5–7, Tables 1 and 2). The
obtained k values for BPMTC are in line with literature data
reported for other diarylethene derivatives and support the
previous hypothesis that solvent polarity does not exhibit a
significant effect on the cycloreversion process due to irreleꢀ
vance of ringꢀopening process to changes in a molecular conꢀ
formation.79,104–108 The photoisomerization rate for H2BCMTC
in methanol (2.6 × 10ꢀ2 sꢀ1) was also comparable with the rate
of BPMTC in the same solvent (1.8 × 10ꢀ2 sꢀ1, Figure 6, Tables
1 and 2). However, in contrast to spiropyran derivatives, diaꢀ
rylethene photoswitches underwent fast cycloreversion in the
solid state, which could be explained by the accommodation of
strain energy generated by the geometrical structure change
during the cyclization reaction as well as the low activation
energy of ringꢀopening process.105–107 For instance, photoinꢀ
duced isomerization of BPMTC occurs with a rate of k = 0.20
sꢀ1. However, this tendency was not observed for coordinativeꢀ
ly immobilized BPMTC inside a rigid framework. In 3, where
the BPMTC linker is coordinatively immobilized as a pillar
between DBTD4ꢀꢀcontaining layers (Figure 3), the recovery of
the openꢀform isomer was found to be 0.0028 sꢀ1, which is ~70
times slower in comparison with the “free” compound in the
solid state. Such a drastic rate decrease could be associated
with the fact that the BPMTC pyridyl arms are tethered beꢀ
tween the twoꢀdimensional layers of DBTD4ꢀ, which impede
the possibility of pillar photoisomerization since photoinduced
transformations should occur through the linker skeleton. By
changing framework topology, we were able to enhance the
photoisomerization rate by almost four times. Thus, BPMTC
in framework 3’, connected to the skeleton of ZnꢀBPDC units,
consisting of more flexible dicarboxylic units, BPDC2ꢀ, phoꢀ
toisomerizes with the rate k = 0.01 sꢀ1. Remarkably, the obꢀ
tained k value for 3’ correlates with behavior of the linker in
solution (Tables 1 and 2, Figure S17) and could be attributed
In the case of H2BCMTC, the rate constant in the solid
state was found to be 4.8 × 10ꢀ2 sꢀ1, which is four times slower
in comparison with BPMTC (Figures 5 and 7, Table 1) as well
as other diarylethene derivatives.79,104–108 Such difference
could be explained by the fact that H2BCMTC could result in
formation of a “hydrogenꢀbonding” framework with behavior
similar to a MOF matrix (Figure S23). We hypothesize that
hydrogenꢀbonding interactions in the solid state could restrict
molecular dynamics in the similar fashion as a MOF and poꢀ
tentially be a reason for the similar rate constants (4.1 × 10ꢀ2 sꢀ2
(4) and 4.8 × 10ꢀ2 sꢀ1 (solid state, Table 1).
Figure 6. a) The cycloreversion kinetics of photochromic MOFs
upon irradiation with visible light. b) The cycloreversion kinetics
plots for BPMTC linker in solution (circles, brown ꢀ ethanol, grey
ꢀ methanol, purple ꢀ acetonitrile, light blue – tetrahydrofuran, blue
– toluene, and orange – chloroform) and coordinatively immobiꢀ
lized inside MOFs 3 (dark green diamonds) and 3’ (light green
diamonds).
6
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Page 7 of 24
Journal of the American Chemical Society
Figure 7. The cycloreversion kinetics of TNDS, BPMTC, and H2BCMTC as a solid (red), solution (blue: TNDS and BPMTC in DMF and
H2BPMTC in methanol; the solvent was chosen according to solubility and photochromic behavior), and immobilized in a MOF (green)
upon irradiation with visible light.
1
2
3
4
5
6
7
8
9
purple to green) allowed detection of scaffold degradation by
the naked eye. Moreover, these changes were local, i.e., ocꢀ
To summarize this subsection, we showed the effect of a
curred only at the exposed area. As a control experiment, the
rigid scaffold on photoisomerization kinetics for two distinct
nonꢀcoordinated linker, TNDS, was exposed to HCl vapors
under
classes of photochromic molecules. Based on the comparison
of spiropyranꢀ and diaryletheneꢀbased linkers possessing difꢀ
ferent photoisomerization pathways, we promoted photoinꢀ
duced isomerization of spiropyran derivatives in the solid state
through their integration inside a porous matrix. Moreover,
coordinatively immobilized spiropyran derivatives can mimic
solutionꢀlike photophysical behavior. However, steric hinꢀ
drance still could impede photoisomerization dynamics as it
was shown on the example of the photochromic linker posꢀ
sessing two spiropyran moieties located in close proximity to
each other. On the example of diaryletheneꢀbased frameworks
with different topologies and structural flexibilities, we
showed the possibility to tune and control the photoisomerizaꢀ
tion rate of incorporated photochromic linkers with a level of
control, which cannot be achieved in the solid state or soluꢀ
tion. To the best of our knowledge, the presented studies are
the first example of extensive photophysical studies demonꢀ
strating tunability of photoisomerization rates within two orꢀ
ders of magnitude for the same organic linker (Figure 6). We
hypothesize that such drastic difference in behavior could be
potentially tuned as a function of framework topology, which
provides a pathway for development of new “smart” phoꢀ
toswitchable materials with a controllable photoresponsive.
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48
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50
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53
54
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60
Figure 8. (top) The surface of 1 was subjected to HCl vapors and
shows evidence of MOF degradation at the point of contact. (botꢀ
tom) The fluorescence spectra of 1 before (purple) and after
(green) exposure to HCl vapors.
Photochromic Compounds to Map Changes in Material
Properties
similar conditions (Figure S20). In that case, we observed
similar emission with λmax = 550 nm (λex = 345 nm) to that of
the “exposed” framework. The same protonation of TNDS
was observed in solution by bubbling HCl vapors (Figure
S21). Furthermore, due to framework degradation (monitored
by PXRD studies), the “released” photochromic TNDS highꢀ
lighted the corroded areas permanently, which provided a
pathway to use such changes as a good indicator for necessary
material treatment or replacement. To summarize, the presentꢀ
ed concept could be applied towards invasive monitoring of
material surfaces with a good spatial resolution. These studies
could potentially provide an avenue to monitor material aging
or structural deterioration, which are key points for a number
of industrial processes or applications including tests of airꢀ
craft reliability or steel corrosion.
The concept of photoisomerization inside a confined enꢀ
vironment discussed above could be potentially translated
towards mapping of changes in a profile of a material. We
tested the hypothesis that prepared photochromic porous scafꢀ
folds, overcoming steric hindrance for photoisomerization
processes, can be utilized as a marker for material degradation.
The advantage of a porous framework is that it promoted phoꢀ
toisomerization under irradiation (or daylight exposure), and
the changes in the photophysical profile of a material became
“invisible” due to a relatively rapid photoisomerization when
the scaffold is intact, i.e., its integrity is maintained.
In our studies, we used the TNDSꢀcontaining MOF (1),
which was mixed with potassium bromide, leading to forꢀ
mation of the slightly colored homogeneous mixture (Figure 8
and S24). After deposition of the obtained mixture on filter
paper, nitrogen gas was bubbled through a 6 M HCl solution.
Notably, exposure to just nitrogen did not lead to any surface
changes. Hydrochloric acid vapors were released through a
narrow tip (i.d. = 4 mm) to localize their effect. As shown in
Figure 8, such treatment resulted in local framework degradaꢀ
tion, which is accompanied with linker release, i.e., formation
of nonꢀcoordinated photochromic TNDS. Surface changes
were monitored by photoluminescence spectroscopy while
structural ones by PXRD. Figure 8 demonstrates that before
acid vapor exposure, the emission profile has a maximum of
λmax = 680 nm (λex = 350 nm) but framework degradation
causes a hypsochromic shift of more than 100 nm, and thereꢀ
fore the emission maximum became similar to that of a “free”
photochromic linker. These drastic changes in emission (from
CONCLUSION
The presented studies focus on photoisomerization kinetics
of two distinct classes of photochromic derivatives with spiroꢀ
pyran and diarylethene cores in the solid state, solution, and
coordinatively immobilized inside the rigid scaffold. To study
such dependence as well as the possible effect of steric hinꢀ
drance, we prepared two novel spiropyran compounds, TNDS
and HDDB with one and two photoswitching moieties, which
resulted in preparation of the first examples of MOFs (1 and 2)
with spiropyran moieties embedded as a part of the organic
linker. Due to the presence of different HDDB isomers
(HDDBinꢀin, HDDBinꢀout, and HDDBoutꢀout), we were also able to
study thermal equilibration process of HDDB in solution to
estimate the effect of steric hindrance of spiropyran groups on
isomerization.
7
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Journal of the American Chemical Society
Page 8 of 24
Preparation of [4,7-dibromo-1,2,3,3-tetramethyl-3H-indol-1-
For the first time, through utilization of five distinct frameꢀ
works (1, 2, 3, 3’ and 4), we also showed that (i) the solidꢀ
state spiropyran metalꢀorganic materials with capability of
reversible photoinduced isomerization could be prepared; (ii) a
photochromic linker could be integrated not only as pillars but
as well as postsyntheticallyꢀinstalled capping linkers; (iii) the
photoisomerization rate could be tuned as a function of
framework structure; (iv) coordinatively immobilized spiropyꢀ
ran derivatives inside a MOF could possess a high isomerizaꢀ
tion rate mimicking solution behavior; and (v) the acquired
fundamental principles could be translated towards mapping
changes in material properties to develop stimuliꢀresponsive
markers.
ium][trifluoromethanesulfonate] ([C12H14Br2N]+ [CF3O3S]ꢀ, V,
Scheme S1).
1
2
3
4
5
6
7
8
Compound V (Scheme S1) was prepared based on a modified
literature procedure.91 Methyl trifluoromethanesulfonate (4.13 g,
25.2 mmol) was added to synthesized IV (Scheme S1) (2.00 g,
6.31 mmol) in 120 mL of diethyl ether and 80 mL of hexanes.
After the resulted mixture was stirred vigorously for 8 hours at
room temperature, a pale pink powder was collected by filtration
and washed with diethyl ether (3 × 10 mL). After drying under
vacuum, V (Scheme S1) (2.70 g, 5.61 mmol) was isolated in 89%
yield and used without further purification.
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Preparation
of
4’,7’-dibromo-1’,3’,3’-trimethyl-6-
nitrospiro[chromene-2,2’-indoline]
Scheme S1).
(C19H16Br2N2O3,
VI,
To conclude, the presented studies not only address some
aspects of photophysics of photochromic compounds integratꢀ
ed in a confined environment, but also provide a pathway for
development of a new generation of photoactive materials.
Under a nitrogen atmosphere, piperidine (1.41 g, 16.6 mmol) was
added to a solution of V (2.00 g, 4.15 mmol, Scheme S1) and 2ꢀ
hydroxyꢀ5ꢀnitrobenzaldehyde (1.25 g, 7.47 mmol) in 80 mL of
ethanol and heated at reflux for 8 h. After cooling to 0 °C, the
resulting lightꢀyellow precipitate was filtered and washed with
cold ethanol. After drying under vacuum, VI (0.971 g, 2.02 mmol,
Scheme S1) was isolated in 48% yield. Crystals of VI (Scheme
S1) were obtained upon cooling of the crude reaction mixture in
ethanol at 0 °C over several days. The detailed description for the
crystallographic data collection and refinement details are given
in the SI (Table S1). The determined structure of VI (Scheme S1)
EXPERIMENTAL SECTION
Materials. Zinc(II) nitrate, hexahydrate (technical grade, Ward’s
Science), tin(II) chloride, anhydrous (98%, Beantown Chemicals),
copper(I) chloride, anhydrous (97%, Strem Chemicals),
bis(triphenylphosphine)palladium(II) dichloride (96%, Oakwood
Chemical), copper powder (99.9%, Alfa Aesar), sodium carbonate
(ACS grade, Ameresco), magnesium sulfate, anhydrous (USP,
ChemꢀImplex, International Inc.), sodium sulfate, anhydrous
(99.5%, Oakwood Chemical), sodium hydroxide (ACS, Oakwood
Chemical), 2,5ꢀdibromonitrobenzene (99%, Oakwood Chemical),
sodium nitrite (98%, Oakwood Chemical), 3ꢀmethylꢀ2ꢀbutanone
(98%, Beantown Chemicals), methyl trifluoromethanesulfonate
(97%, Matrix Scientific), 2ꢀhydroxyꢀ5ꢀnitrobenzaldehyde (98%,
Oakwood Chemical), pyridineꢀ4ꢀboronic acid (95%, Matrix Sciꢀ
entific), hexabromobenzene (>99%, TCI America), pꢀ
tolylmagnesium bromide (0.5 M in diethyl ether, Acros Organics),
hydrochloric acid (ACS, Fisher Chemical), sulfuric acid (ACS
grade, Fisher Chemical), nitric acid (ACS reagent, Sigmaꢀ
Aldrich), fluoboric acid (48%, Oakwood Chemical), ethanol (200
proof, Decon Laboratories, Inc), methanol (ACS grade, Fisher
Scientific), methylene chloride (99.9%, Fisher Chemical), acetone
(ACS grade, Sigma Aldrich), ethyl acetate (99.9%, Fisher Chemiꢀ
cal), chloroform (99.9%, Fisher Chemical), diethyl ether (ACS
grade, J. T. Baker® Chemicals), hexanes (ACS, BDH), N,Nꢀ
dimethylformamide (ACS grade, Oakwood Chemical), piperidine
(99%, SigmaꢀAldrich), carbon tetrachloride (99.9%, Sigmaꢀ
Aldrich), bromine (99.8%, AcrosꢀOrganic), tetrahydrofuran
(HPLC grade, Beantown Chemicals), chloroformꢀd (Cambridge
Isotopes), acetoneꢀd6 (Cambridge Isotopes), methylene chlorideꢀ
d2 (Cambridge Isotopes), dimethyl sulfoxideꢀd6 (Cambridge Isoꢀ
topes) were used as received.
1
is shown in Figure S1. H NMR (acetoneꢀd6, 300 MHz): δ = 1.27
(3H, s), 1.49 (3H, s), 3.17 (3H, s), 6.00ꢀ6.04 (1H, d, J = 10.4 Hz),
6.92ꢀ6.95 (1H, d, J = 8.55 Hz), 6.96ꢀ6.99 (1H, d, J = 9.00), 7.26ꢀ
7.29 (1H, d, J = 8.62 Hz), 7.30ꢀ7.33 (1H, d, J = 10.4 Hz), 8.09ꢀ
8.13 (1H, dd, J = 2.83, 8.96 Hz) ppm (Figure S2). 13C NMR (aceꢀ
toneꢀd6, 400 MHz): δ = 20.61, 22.46, 32.91, 54.59, 101.15,
108.00, 116.27, 118.15, 119.48, 121.36, 123.81, 126.69, 126.75,
130.59, 135. 93, 136.86, 142.31, 147.37, 160.18 ppm (Figure S2).
FTIR (neat, cmꢀ1): 3076, 2968, 2934, 2868, 2545, 2162, 2041,
1980, 1910, 1736, 1654, 1614, 1577, 1510, 1478, 1466, 1443,
1396, 1379, 1361, 1331, 1272, 1255, 1228, 1175, 1145, 1127,
1088, 1061, 1017, 955, 927, 915, 904, 877, 834, 811, 792, 771,
745, 717, 703, 666. HRMS (ESI, m/z) calculated for
C19H16Br2N2O3 [M+H]+ 478.9601, found 478.9600.
Preparation of 1’,3’,3’-trimethyl-6-nitro-4’,7’-di(pyridin-
4-yl)spiro[chromene-2,2’-indoline]
Scheme S1).
(C29H24N4O3,
TNDS,
Using a Schlenk technique, 25 mL of dry DMF was transferred
into a flask containing VI (300 mg, 0.624 mmol, Scheme S1),
Na2CO3 (240 mg, 2.26 mmol), PdCl2(PPh3)2 (88.0 mg, 0.125
mmol), CuCl (134 mg, 1.35 mmol), and pyridineꢀ4ꢀboronic acid
(460 mg, 3.74 mmol). The obtained mixture was stirred for three
days at 120 °C. After cooling to room temperature, the reaction
mixture was transferred into a 250ꢀmL separatory funnel, followꢀ
ing dilution with deionized water (75 mL). The product was then
extracted with ethyl acetate (3 × 50 mL). The organic layers were
combined and washed with brine (50 mL), dried over sodium
sulfate, followed by filtration and solvent removal under vacuum.
The resulting red/brown oil underwent several recrystallizations in
ethyl acetate and hexanes. The obtained orange powder was disꢀ
solved in ethyl acetate (20 mL) and hexanes (30 mL), and the
solvent was removed under vacuum until a suspension formed.
The suspension was filtered to collect a lightꢀyellow powder.
After drying on vacuum, TNDS (44.0 mg, 93.8 µmol) was isolatꢀ
ed in 17% yield. Single crystals of TNDS were obtained by slow
evaporation of saturated acetone solution at 0 °C. The detailed
description for the crystallographic data collection and refinement
details are given in the SI (Table S2).
The compounds 2,5ꢀdibromoaniline (II, Scheme S1),92 (2,5ꢀ
dibromophenyl)hydrazine (III, Scheme S1),93 4,7ꢀdibromoꢀ2,3,3ꢀ
trimethylꢀ3Hꢀindole (IV, Scheme S1),93 4,4’ꢀdibromoꢀ2,2’dinitroꢀ
1,1’ꢀbiphenyl (II’, Scheme S2),91 4,4’ꢀdibromoꢀ[1,1’ꢀbiphenyl]ꢀ
2,2’ꢀdiamine (III’, Scheme S2),91 (4,4’ꢀdibromoꢀ[1,1’ꢀbiphenyl]ꢀ
2,2’ꢀdiyl)bis(hydrazine) (IV’, Scheme S2),91 4,4’dibromoꢀ
2,2’,3,3,3’,3’ꢀhexamethylꢀ3H,3’Hꢀ7,7’ꢀbiindole (V’, Scheme
S2),91
3’,6’ꢀdibromoꢀ4’,5’ꢀbis(4ꢀcarboxyphenyl)ꢀ1[1,1’:2’,1’’ꢀ
tetraphenyl]ꢀ4,4’’ꢀdicarboxylic acid (H4DBTD, Figure 3),109 1,2ꢀ
bis(2ꢀmethylꢀ5ꢀ(pyridinꢀ4ꢀyl)thiphenꢀ3ꢀyl)cyclopentꢀ1ꢀene
(BPMTC, Figures
1
and 3),82 4,4’ꢀ(cyclopentꢀ1ꢀeneꢀ1,2ꢀ
diyl)bis(5ꢀmethylthiopheneꢀ2ꢀcarboxylic acid) (H2BCMTC, Figꢀ
ures 1 and 3),83 2,2′ꢀdimethylbiphenylꢀ4,4′ꢀdicarboxylic acid
(H2Me2BPDC),110 and Zr6(Me2BPDC)477 were prepared according
to the reported procedures.
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Page 9 of 24
Journal of the American Chemical Society
1
The Xꢀray structure of TNDS is shown in Figure S3. H NMR
acetate (20 mL) and hexanes (30 mL) and the solvent was reꢀ
moved under vacuum until a suspension formed. The suspension
was filtered to collect a lightꢀtan powder. After drying on vacuum,
HDDB (50.0 mg, 625 µmol) was isolated in 15% yield. In order
to obtain crystals of a specific diastereomer of HDDB, it was
acetoneꢀd6, 400 MHz): δ = 0.81 (3H, s), 1.33 (3H, s), 2.43 (3H, s),
6.00–6.03 (1H, d, J = 10.5 Hz), 6.69–6.71 (1H, d, J = 7.9 Hz),
6.85ꢀ6.88 (1H, d, J = 9.0 Hz), 7.12–7.14 (1H, d, J = 7.9 Hz),
7.18–7.22 (1H, d, J = 10.6 Hz), 7.39–7.45 (1H, m), 8.01–8.05
(1H, dd, J = 2.8 Hz and 9.0 Hz), 8.08 (1H, d, J = 2.8 Hz ), 8.64
(4H, m) ppm (Figure S4). 13C NMR (acetoneꢀd6 400 MHz): δ =
20.61, 22.46, 32.91, 54.59, 101.14, 108.00, 116.27, 118.15,
119.49, 121.36, 123.81, 126.74, 130.59, 135.93, 136.86, 142.31,
147.37, 160.18 ppm (Figure S4). FTIR (neat, cmꢀ1): 3746, 2965,
2344, 2163, 2049, 1981, 1596, 1543, 1522, 1494, 1464, 1438,
1392, 1363, 1337, 1286, 1253, 1216, 1145, 1129, 1072, 1055,
1011, 980, 939, 905, 873, 827, 803, 749, 729, 681 (Figure S8).
HRMS (ESI, m/z) calculated for C29H24N4O3 [M+H]+ 477.1918,
found 477.1921.
1
2
3
4
5
6
7
8
subjected to preparatory TLC using
a 5:95 (v:v) methaꢀ
nol/dichloromethane, in which the product was extracted with
acetone followed by slow evaporation of the solvent overnight,
and crystals of HDDBinꢀin were isolated. Single crystals of HDDꢀ
Binꢀout were obtained from a solution of HDDB in 50:50 (v:v)
ethyl acetate/hexanes that had been cooled to 0 °C for several
days. The detailed description for the crystallographic data collecꢀ
tion and refinement details are given in the SI (Table S2). The
determined structure of HDDBinꢀout and HDDBinꢀin are shown in
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Figure S5. H NMR (dichloromethaneꢀd2, 400 MHz): δ = 0.74
(1H, s), 1.24 (6H, s), 2.58 (6H, s), 5.79–5.83 (2H, d, J = 10.5 Hz),
6.23–6.26 (2H, d, J = 9.06 Hz), 6.55–6.58 (2H, d, J = 7.81 Hz),
6.93–6.96 (2H, d, J = 10.7 Hz), 7.05–7.08 (2H, d, J = 7.91 Hz),
7.29–7.31 (4H, d, J = 5.81 Hz), 7.52–7.57 (2H, dd, J = 2.70, 8.99
Hz), 7.94–7.95 (2H, d, J = 2.69 Hz), 8.58ꢀ8.60 (2H, d, J = 5.23
Hz) ppm (Figure S6). 13C NMR (acetoneꢀd6, 400 MHz): δ =
21.00, 25.65, 31.66, 52.51, 108.23, 114.92, 118.80, 120.65,
120.89, 121.17, 122.99, 125.13, 125.64, 129.69, 131.38, 132.90,
136,40, 141.08, 145.72, 149.54, 160.00 ppm (Figure S6). FTIR
(neat, cmꢀ1): 3327, 2972, 2934, 2364, 2343, 2288, 2191, 2150,
2043, 1984, 1920, 1648, 1613, 1594, 1576, 1519, 1467, 1386,
1340, 1282, 1230, 1180, 1149, 1125, 1090, 1074, 1059, 1020,
951, 928, 906, 833, 803, 747, 706, 682, 661 (Figure S11). HRMS
(ESI, m/z) calculated for C48H40N6O6 [M+H]+ 797.3088, found
797.3094.
Preparation of [4,4’-dibromo-1,1’,2,2’,3,3,3’,3’-octamethyl
-3H,3’H-[7,7’-biindole]-1,1’-diium]bis-[trifluoromethane-
sulfonate] ([C24H28Br2N2]2+ [(CF3O3S)2]2ꢀ, VI’, Scheme S2).
Compound VI’ (Scheme S2) was prepared based on a modified
literature procedure.91 Methyl trifluoromethanesulfonate (1.50 g,
10.1 mmol) was added to synthesized V’ (Scheme S2) (1.00 g,
2.11 mmol) in 90 mL of diethyl ether and 60 mL of hexanes. Afꢀ
ter the resulted mixture was stirred vigorously for 8 h at room
temperature, a pale pink powder was collected by filtration and
washed with diethyl ether (3 × 10 mL). After drying under vacuꢀ
um, VI’ (1.57g, 1.96 mmol, Scheme S2) was isolated in 93%
yield and used without further purification.
Preparation of 4’4,’’’-dibromo-1’,1’’’,3’,3’’’-hexamethyl-
6,6’’-dinitro-7’,7’’’-bispiro[chromene-2,2’-indoline]
(C38H32Br2N4O6, VII’, Scheme S2).
Preparation of 1 (Zn2C63H40N4O11Br2, Zn2(DBTD)(TNDS)).
MOF 1 was prepared using a slightly modified literature proceꢀ
dure.109 In a oneꢀdram vial, Zn(NO3)2·6H2O (18 mg, 59 µmol),
H4DBTD (5.4 mg, 7.5 µmol), and TNDS (5.0 mg, 10.5 µmol)
were dissolved in 0.8 mL of DMF with 0.1 µL of HCl, followed
by sonication. The resulting solution was placed in a preheated
oven at 80 °C for 24 h, then cooled down to room temperature
over 2 h. Red almondꢀshaped crystals of 1 (13.1 mg, 9.4 µmol)
were isolated in 90% yield. The detailed description for the crysꢀ
tallographic data collection and refinement details are given in the
SI (Table S3). The determined structure of 1 is shown in Figure
S7. FTIR (neat, cmꢀ1): 3445, 3069, 2931, 2880, 1642, 1609, 1558,
1495, 1460, 1435, 1385, 1338, 1275, 1255, 1220, 1178, 1151,
1092 (Figure S8). The PXRD pattern of 1 is shown in Figure S8.
Compound VII’ (Scheme S2) was prepared based on a modified
literature procedure.91 Using a Schlenk technique, piperidine (713
mg, 8.37 mmol) was added to a solution of VI’ (Scheme S2) (1.50
g, 1.87 mmol) and 2ꢀhydroxyꢀ5ꢀnitrobenzaldehyde (706 mg, 4.38
mmol) in 60 mL of ethanol and heated at reflux for 8 h. After
cooling to 0 °C, the resulting lightꢀyellow precipitate was filtered
and washed with cold ethanol. After drying under vacuum, VII’
1
(761 mg, 948 µmol, Scheme S2) was isolated in 51% yield. H
NMR (acetoneꢀd6, 400 MHz): δ = 1.30 (6H, s), 1.43 (6H, s), 2.59
(6H, s), 5.96–5.99 (2H, d, J = 10.4 Hz), 6.34–6.37 (2H, d, J =
9.00 Hz), 6.87–6.90 (2H, d, J = 8.18 Hz), 6.97–7.00 (2H, d, J =
8.25 Hz), 7.24–7.28 (2H, d, J = 10.4 Hz), 7.59–7.54 (2H, dd, J =
2.77 and 9.01), 8.10 (2H, d, J = 2.73 Hz). FTIR (neat, cmꢀ1):
3556, 3076, 2976, 2934, 2873, 2258, 2162, 2040, 1980, 1895,
1654, 1614, 1572, 1513, 1473, 1447, 1435, 1397, 1363, 1336,
1274, 1255, 1232, 1179, 1148, 1126, 1088, 1062, 1022, 1001,
950, 923, 905, 863, 823, 799, 766, 747, 709, 697.
1
The H NMR spectra and massꢀspectrometry data of digested 1
are shown in Figure S9. The experimental procedure utilized for
MOF digestion can be found in the SI.
Preparation of 2 (Zn2C82H56N6O14Br2, Zn2(DBTD)(HDDB)).
MOF 2 was prepared using a slightly modified literature proceꢀ
dure.29 In a oneꢀdram vial, Zn(NO3)2·6H2O (8.8 mg, 30 µmol),
H4DBTD (2.5 mg, 3.5 µmol), and HDDB (4.8 mg, 6.0 µmol)
were dissolved in 1 mL of DMF and 0.2 µL of HBF4, followed by
sonication. The resulting solution was placed in a preheated oven
at 85 °C for 18 h, then cooled down to room temperature over 2 h.
Deep red oval plate crystals of 2 (9.6 mg, 5.8 µmol) were isolated
in 97% yield. The detailed description for the crystallographic
data collection and refinement details are given in the SI (Table
S3). The determined structure of 2 is shown in Figure S10. FTIR
(neat, cmꢀ1): 3450, 2934, 2870, 1660, 1558, 1463, 1436, 1385,
1337, 1316, 1274, 1255, 1221, 1178, 1152, 1090 (Figure S11).
PXRD pattern and 13C{1H} CPꢀMAS NMR spectrum of 2 are
shown in Figures S11 and S12, respectively.
Preparation of 1’,1’’’,3’,3’,3’’’,3’’’-hexamethyl-6,6’’-dinitro-
4’,4’’’-di(pyridin-4-yl)-7’,7’’’-bispiro[chromene-2,2’-indo-
line] (C48H40N6O6, HDDB, Scheme S2).
Using a Schlenk technique, 20 mL of dry DMF was transferred to
a flask containing VII’ (336 mg, 420 µmol, Scheme S2), Na2CO3
(84.0 mg, 793 µmol), PdCl2(PPh3)2 (47.0 mg, 670 µmol), CuCl
(168 mg, 1.69 mmol), and pyridineꢀ4ꢀboronic acid (206 mg, 1.68
mmol). The resulting mixture was stirred for two days at 120 °C.
After cooling to room temperature, the reaction mixture was
transferred in a 250 mL separatory funnel, following dilution with
deionized water (75 mL). The product was then extracted with
ethyl acetate (3 × 50 mL). The organic layers were combined and
washed with brine (50 mL), dried over sodium sulfate, followed
by filtration and solvent removal under vacuum. The obtained
red/brown oil was recrystallized several times using ethyl acetate
and hexanes. The resulting orange powder was dissolved in ethyl
Preparation of 3 (Zn2C59H44N2O8Br2, Zn2(DBTD)(BPMTC)).
9
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Journal of the American Chemical Society
Page 10 of 24
1
MOF 3 was prepared using a slightly modified literature proceꢀ
dure.29 In a oneꢀdram vial, Zn(NO3)2·6H2O (35.0 mg, 120 µmol),
H4DBTD (10.0 mg, 13.9 µmol) and BPMTC (9.0 mg, 22 µmol)
were dissolved in a solution of 2 mL DMF and one drop of HBF4,
followed by sonication. The resulting solution was placed in a
preheated oven at 85 °C for 18 h, then cooled down to room temꢀ
perature over 2 h. Colorless plate crystals of 3 (13 mg, 10 µmol)
were isolated in 75% yield. FTIR (neat, cmꢀ1): 3485, 3067, 2925,
2870, 1665, 1639, 1610, 1556, 1502, 1435, 1385, 1255, 1222,
1177, 1150, 1091 The PXRD pattern of 3 is shown in Figure S13.
62.5 kHz field on the 13C channel were used. H dipolar decouꢀ
pling was performed with SPINAL64 modulation and 147 kHz
field strength. Free induction decays (2048–5000 transients) were
collected with a 27 ms acquisition time over a 400 ppm spectra
width with a relaxation delay of 2.0 s.
1
2
3
4
5
6
7
8
Other Physical Measurements. FTIR spectra were obtained on a
PerkinꢀElmer Spectrum 100. NMR spectra were collected on
Bruker Avance IIIꢀHD 300 and Bruker Avance III 400 MHz
NMR spectrometers. 13C and H NMR spectra were referenced to
1
1
1
The H NMR spectra and massꢀspectrometry data of digested 3
natural abundance 13C peaks and residual H peaks of deuterated
9
are shown in Figure S14.
solvents, respectively. Powder Xꢀray diffraction patterns were
recorded on a Rigaku Miniflex II diffractometer with accelerating
voltage and current of 30 kV and 15 mA, respectively. The Waꢀ
ters QTOFꢀI quadrupole timeꢀofꢀflight and Thermo Scientific
Orbitrap Velos Pro massꢀspectrometers were used to record the
massꢀspectra of the prepared compounds.
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Preparation of 3’ (Zn2C53H30N2O4S2, Zn2(BPDC)2(BPMTC)).
In a 20 mL vial, Zn(NO3)2·6H2O (7.0 mg, 24 µmol), H2BPDC
(6.0 mg, 25 µmol), and BPMTC (10 mg, 24 µmol) were dissolved
in a solution of 2 mL DMF, followed by sonication. The resulting
solution was heated at 110 °C in an isothermal oven. After 24 h,
the reaction mixture was cooled down to room temperature over 2
h. Light brown rectangular block crystals of 3’ (9.2 mg, 8.9 µmol)
were isolated in 74% yield. The detailed description for the crysꢀ
tallographic data collection and refinement details are given in the
SI (Table S3). The determined structure of 3’ is shown in Figure
S15. FTIR (neat, cmꢀ1): 3410, 3080, 2932, 2866, 1660, 1604,
1545, 1500, 1384, 1253, 1176, 1140, 1095, 1080, 1022, 1005,
859, 840, 800, 770, 704, 681, 660 (Figure S16). The PXRD patꢀ
tern of 3’ is shown in Figure S16.
ASSOCIATED CONTENT
SUPPORTING INFORMATION
The supporting information is available free of charge on the ACS
publications website at DOI: Xꢀray structure refinement data of
1
prepared MOFs, PXRD patterns, FTIR, H, 13C, and 13C{H} CP
MAS NMR spectra. Crystallographic information for
C19H16Br2N2O3, VI, CCDC 1829891 (CIF); Crystallographic
information for TNDS, CCDC 1829888 (CIF); Crystallographic
information for C38H32Br2N4O6, VII’, CCDC 1829895 (CIF);
Crystallographic information for HDDBinꢀin, CCDC 1829894
Preparation of 4 (Zr6C81H78O34S2, Zr6(Me2BPDC)4(BCMTC)0.5).
MOF 4 was prepared using a modified synthetic route.96 Crystals
of Zr6(Me2BPDC)4 (30 mg, 16 µmol) were added to solution of
H2BCMTC (42 mg, 0.12 mmol) in 4 mL of DMF in a oneꢀdram
vial. The vial was placed in a preheated oven at 75 °C for 24 h,
then cooled to room temperature over two hours. Light tan crysꢀ
tals of 4 (35 mg, 14 µmol) were isolated in 64% yield. FTIR (neat,
cmꢀ1): 3450, 2935, 2855, 1657, 1586, 1544, 149,5 1411, 1384,
1255, 1208, 1139, 1091. The PXRD pattern of 4 is shown in Figꢀ
(CIF); Crystallographic information for HDDBinꢀout
, CCDC
1829893 (CIF); Crystallographic information for 1,
(Zn2(DBTD)(TNDS)), CCDC 1829890 (CIF); Crystallographic
information for 2, (Zn2(DBTD)(HDDB)), CCDC 1829889 (CIF);
Crystallographic information for 3’, (Zn2(BPDC)2(BPMTC)),
CCDC 1829892 (CIF).
1
ure S18. The H NMR spectra of digested 4 is shown in Figure
AUTHOR INFORMATION
S19.
Corresponding Author
UV-vis, diffuse reflectance, and fluorescence spectroscopies.
Steadyꢀstate emission spectra were acquired on an Edinburgh FS5
fluorescence spectrometer equipped with a 150 W Continuous
Wave Xenon Lamp source for excitation. Emission measurements
on solid samples were collected on the powders of the appropriate
materials placed inside a 0.5 mm quartz sample holder using the
frontꢀfacing module. The Ocean Optics JAZ spectrometer was
also used for absorbance and diffuse reflectance measurements.
An Ocean Optics ISPꢀREF integrating sphere was connected the
spectrometer using a 450 µm SMA fiber optic cable. A 400 nm
long pass filter was placed between an 8.0 mm quartz sample cell
with cover and the integrating sphere to filter any UV light from
the internal tungstenꢀhalogen lamp. An Ocean Optics WSꢀ1 Specꢀ
tralon® reflectance standard was placed on the sample cell
throughout the measurements. A mounted highꢀpowered LED
(M365L2, Thorlabs), λ = 365 nm, was used for in situ irradiation
of the samples. Before timeꢀresolved UVꢀvis and diffuse reflecꢀ
tance measurements, the sample background was subtracted to
obtain the spectra corresponding to photophysical behavior of the
photochromic derivatives.
*shustova@sc.edu
ORCID
Natalia B. Shustova: 000ꢀ0003ꢀ3952ꢀ1949
Author Contributions
The manuscript was written through contributions of all authors. /
All authors have given approval to the final version of the manuꢀ
script. / ‡These authors contributed equally.
Notes
This author declares no competing financial interests.
ACKNOWLEDGMENT
N.B.S. gratefully acknowledges support from the NSF CAREER
Award (DMRꢀ1553634), the Sloan Research Fellowship provided
by the Alfred P. Sloan foundation, and a Cottrell Scholar Award
from the Research Corporation for Science Advancement.
13C CP-MAS NMR Spectroscopy. Solidꢀstate NMR spectra (13C
CPꢀMAS) were collected on a Bruker Avance IIIꢀHD 500 MHz
spectrometer fitted with a 1.9 mm MAS probe. 13C{1H} CPꢀMAS
NMR spectra (125.79 MHz) were collected at ambient temperaꢀ
ture with a sample rotation rate of 20 kHz. For cross polarization,
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A schematic representation of coordinatively immobilized photochromic derivatives inside the metal-organic
rigid scaffold. The photochromic moiety integrated as (left) a side group on the organic linker, (middle) as a
framework backbone, and (right) as a capping linker86 with the size similar to the framework pocket. A grey
“photoswitch” symbol indicates a position of photochromic unit.
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Scheme 2. Reaction scheme for the synthesis of TNDS (black) and HDDB (grey)a
a i) Cu powder, DMF, 120 °C 4 h; ii) SnCl2, HCl, EtOH, reflux 12 h; iii) NaNO2, HCl, 0 °C 1 h / CO(NH2)2, 0
°C 10 min / SnCl2, 0 °C, 4 h; iv) 3-methyl-2-butanone, reflux 3 h / H2SO4, EtOH, reflux 3 h; v) Et2O,
hexanes, methyl trifluoromethanesulfonate, room temperature, 8 h; vi) 2-hydroxy-5-nitrobenzaldahyde,
EtOH, piperidine, reflux 8 h; vii) pyridine-4-boronic acid, PdCl2(PPh3)2, CuCl, Na2CO3, DMF, 120 °C, 48 h.
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Figure 1. (a) Photoisomerization of the diarylethene-based and (b) spiropyran derivatives. (c) For simplicity,
structural differences of “in-in”, “in-out”, and “out-out” diastereoisomers are shown on the example of the
brominated precursor (Scheme 2) of HDDB. Displacement ellipsoids drawn at the 60% probability level.
Gray, blue, and red spheres represent C, N, and O atoms, respectively.
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Figure 2. (top) Single crystal Xꢀray structures of HDDBinꢀin and HDDBinꢀout. Two orientations of each
diastereoisomer are shown. Displacement ellipsoids drawn at the 60% probability level. Gray, blue, and red
spheres represent C, N, and O atoms, respectively. (bottom) For simplicity, a methyl region of the 1H NMR
spectra corresponding to thermal equilibration of HDDꢀBin−in to HDDBin−out and HDDBout−out in CD2Cl2
is shown at t = 0, 9, 24, 48, and 72 h. The rest of the spectra of the HDDBinꢀin conꢀversion are shown in
Figure S22.
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Figure 3. (top) The TNDS, HDDB, and BPMTC linkers were used to synthesize 1, 2, and 3 (3’), respectively,
in the presence of H4DBTD (H2BPDC). (bottom) The linker, H2BCMTC, was in-stalled as a capping linker into
an existing MOF framework, Zr6(Me2BPDC)4,86 to make 4. Orange, red, teal, and gray spheres correspond
to zinc, oxygen, zirconium, and carbon atoms, respectively
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Figure 4. (top) Single-crystal structures of 1 and 2 with simulated spiropyran moiety located in the pores.
(bottom) The simulated and experimental PXRD patterns of MOFs 1 (red) and 2 (blue). The colored text
shows the increase of interlayer distance for each MOF, which is in agreement with the sizes of the installed
photochromic linkers.
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Figure 5. Normalized absorption plots of TNDS, HDDB, BPMTC, H2BCMTC in the solid state, solution, and
coordinatively immobilized inside MOF upon irradiation with UV and visible light.
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Figure 6
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Figure 7. The cycloreversion kinetics of TNDS, BPMTC, and H2BCMTC as a solid (red), solution (blue), and
immobilized in a MOF (green) upon irradiation with visible light.
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Figure 8
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