Self-Assembled Pd(II) Barrels as Containers for Transient Merocyanine Form and Reverse Thermochromism of Spiropyran

Article abstract of DOI:10.1021/jacs.8b03946

Self-assembly of a cis-blocked Pd(II) 90° ditopic acceptor [cis-(tmeda)Pd(NO₃)₂] (M) with a tetradentate donor L¹ [benzene-1,4-di(4-terpyridine)] in 2:1 molar ratio yielded two isometric molecular barrels MB1 and MB3 in DMSO [tmeda = N,N,N′N′-tetramethylethane-1,2-diamine]. Exclusive formation of the symmetrical tetrafacial barrel (MB1) was achieved when the self-assembly was performed in aqueous medium. The presence of a large confined cavity makes MB1 a potential molecular container. Spiropyran (SP) compounds exist in stable closed spiro form in visible light and convert to transient open merocyanine (MC) form upon irradiation with UV-light or upon strong heating. The transient MC form readily converts to the stable closed SP form in visible light. MB1 has been employed as a safe container to store the planar and unstable merocyanine isomers (MC1/2) of different spiropyran molecules (SP1/2) [SP1/2 = 6-bromo-spiropyran and 6-nitrospiropyran] for several days. The transient MC forms (MC1 and MC2) were found to be stable inside the molecular container MB1 under visible light and even in the presence of different stimuli such as heat and UV light for a long time. Such stabilization of MC forms inside the confined cavity of MB1 is noteworthy. This phenomenon was generalized by utilizing a carbazole-based molecular barrel (MB2) as a host, which also showed a similar stabilization of transient MC form in visible light at room temperature. Moreover, reverse thermochromism was observed as a result of heating of the MC1 ? MB2 complex, which de-encapsulates the guest in the form of SP1 to give a colorless solution. Moreover, both the host molecules (MB1, MB2) were capable of stabilizing transient MC2 even in the solid state. Such stabilization of transient MC forms in the solid state and transformation of SP forms to MC forms in the solid state in the presence of molecular barrel are remarkable, and these properties have been employed in developing a magic ink.

Full text of DOI:10.1021/jacs.8b03946

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Article
Self-Assembled Pd(II) Barrels as Containers for Transient
Merocyanine form and Reverse Thermochromism of Spiropyran
Prodip Howlader, Bijnaneswar Mondal, Prioti Choudhury
Purba, Ennio Zangrando, and Partha Sarathi Mukherjee
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b03946 • Publication Date (Web): 06 Jun 2018
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Self-Assembled Pd(II) Barrels as Containers for Transient Merocya-
nine form and Reverse Thermochromism of Spiropyran
Prodip Howlader,a Bijnaneswar Mondal,a Prioti Choudhury Purba,a Ennio Zangrandob and Par-
tha Sarathi Mukherjeea*
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a Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
b Department of Chemical and Pharmaceutical Sciences, University of Trieste, 34127 Italy.
ABSTRACT: Self-assembly of a cis-blocked Pd(II) 90° ditopic acceptor [cis-(tmeda)Pd(NO3)2] (M) with a tetradentate
donor L1 [benzene-1,4-di(4-terpyridine)] in 2:1 molar ratio yielded two isometric molecular barrels MB1 and MB3 in
DMSO [tmeda = N,N,N′N′-tetramethylethane-1,2-diamine]. Exclusive formation of the symmetrical tetrafacial barrel
(MB1) was achieved when the self-assembly was performed in aqueous medium. The presence of a large confined cavity
makes MB1 as a potential molecular container. Spiropyran (SP) compounds exist in stable closed spiro-form in visible
light and converts to transient open merocyanine (MC) form upon irradiation with UV-light or upon strong heating. The
transient MC form readily converts to the stable closed SP form in visible light. MB1 has been employed as a safe
container to store the planar and unstable merocyanine isomers (MC1/2) of different spiropyran molecules (SP1/2) [SP1/2
= 6-bromo-spiropyran and 6-nitrospiropyran] for several days. The transient MC forms (MC1 and MC2) were found to be
stable inside the molecular container MB1 under visible light and even in presence of different stimuli such as heat and
UV-light for long time. Such stabilization of MC forms inside the confined cavity of MB1 is noteworthy. This
phenomenon was generalized by utilizing a carbazole based molecular barrel (MB2) as a host, which also showed a
similar stabilization of transient MC form in visible light at room temperature. Moreover, reverse thermochromism was
observed as a result of heating of the MC1⊂MB2 complex, which de-encapsulates the guest in the form of SP1 to give a
colourless solution. Moreover, both the host molecules (MB1, MB2) were capable of stabilizing transient MC2 even in
solid state. Such stabilization of transient MC forms in solid state and transformation of SP forms to MC forms in solid
state in presence of molecular barrel are remarkable; and these properties have been employed in developing a magic ink.
Introduction
etc.8 The stable SP form has a non-planar geometry
whereas the metastable form MC acquires a planar Zwit-
terionic structure. This difference in shape and polarity
changes the fate of stability of such isomers in different
solvents, solid matrix, gels, molecular hosts etc.9
Chemists have been inspired by Nature with its ample
repository of dynamic chemical systems in the biological
processes such as heat and touch response in
plants/bacteria; animal camouflage1 and retinal in visual
excitation2 etc. where molecules can change their struc-
tures reversibly under various external stimuli such as
light, pressure, temperature.3-6 Such dynamic materials
have potential applications in molecular devices like mo-
lecular wires, sensors, switches, logic gates, signal nano-
processors etc.4 Among these, photo-induced switches are
the most studied systems in recent years because of the
numerous advantages of light as it can deliver high spatial
and temporal precision.4d,4e,5 Consequently, several photo-
switchable molecules like azobenzenes, stilbenes, spiro-
pyrans, diarylethenes etc. have been investigated in order
to construct photo-responsive smart materials.6 Nonethe-
less, spiropyrans are one of the most inimitable examples
of photo-switches, whose closed-ring stable isomer (SP)
converts into a highly polar, open-ring metastable mero-
cyanine (MC) isomer by cleaving Cspiro-O bond upon ex-
posure to UV light.7 These two switchable isomers have
vastly different properties and the range of stimuli that
induces this switching between isomers include tempera-
ture, pH, redox potential, metal ions, mechanical force
The evolution of coordination-driven self-assembly10 in
the past two decades has enabled chemists to develop
many 3D molecular architectures having defined nano-
cavities.11 Many of these molecular architectures have
been used in host-guest chemistry, organic transfor-
mations12, sensing, drug delivery,13 as well as stabilizing
reactive intermediates.14 Specially, cavity induced stabili-
zation of metastable states of different molecules14h,14j,15
makes such architectures worthy candidates to study the
host-guest interaction with spiropyran systems. Among
several architectures that are reported with square planar
Pd(II)/Pt(II),16 molecular barrels are relatively less ex-
plored. The presence of two large open windows in a bar-
rel may facilitate encapsulation or de-encapsulation of
guest molecules.
Herein, we report the formation of two isomeric self-
assembled Pd(II) barrels (MB1 and MB3) upon 1:2 treat-
ment of L1 [benzene-1,4-di(4-terpyridine)] with cis-
(tmeda)Pd(NO3)2 (M) in DMSO (Scheme 1). The symmet-
rical barrel MB1 was obtained exclusively when the isolat-
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ed mixture of barrels obtained from DMSO solution was used for magic writing. Colourless hexane solution of SP2
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heated in water. Presence of large cavity enabled us to
carry out encapsulation of water insoluble organic guest
molecules in the molecular pocket of MB1.
was used to write letters on a paper coated with the barrel
MB2 barrel. Invisible letters appeared as coloured letters
upon exposure of the paper to sunlight/UV light due to
solid state isomerization in presence of the barrel.
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Scheme 1: Self-assembly of the molecular nano-barrel
MB1 from tetrapyridyl ligand L1 and M in 1:2 molar
ratio.
Treatment of water insoluble bromo-spiropyran (SP1)
with an aqueous solution of MB1 produced a greenish
blue solution due to the presence of merocyanine form
(MC1) encapsulated into the hydrophobic cavity of MB1
(Scheme 2). This coloured solution (MC1⊂MB1) was
found to be stable under various stimuli such as visible
light, UV light and heating over a long period due to sta-
bilization of the transient MC form in confined pocket.
Similar behaviour was also observed with a nitro substi-
tuted spiropyran (SP2). In this case, pink solution of the
host-guest complex (MC2⊂MB1) was obtained and found
to be stable under aforementioned stimuli. To explore
the versatility of molecular barrel for stabilizing metasta-
ble MC forms, a similar study was performed using a
tetraimidazole based molecular barrel (MB2)17 which
leads to the formation of an intense blue solution of
MC1⊂MB2. The presence of wider windows causes a fast-
er encapsulation of MC1 by the barrel MB2. Though the
transient MC1 was stable in the confined pocket of MB2
in visible light, heating of the solution showed reverse
thermochromism as evident from sharp colour change
from blue to colourless. Finally, the encapsulated MC1
was successfully exchanged with polyaromatic hydrocar-
bons like pyrene, coronene and anthracene etc. which
have higher binding ability with the host MB2. The en-
capsulation of MC2 in MB2 also produced an intense
purple solution of (MC2⊂MB2), which was stable under
external stimuli. Solid state transformation of SP2 to its
open MC2 form upon UV-irradiation doesn’t happen.
However, in the presence of the barrels (MB1 and MB2),
we could successfully transform SP2 to MC2 and stabilize
by arresting the metastable merocyanine form (MC2)
even in solid state. Such stabilization of metastable mero-
cyanine form of spiropyran compound both in solid and
solution states using molecular barrels as safe containers
is noteworthy. Unusual solid-state transformation of the
colourless spiro form (SP) to the coloured merocyanine
(MC) form and stabilization of the MC form were finally
Scheme 2: Encapsulation of MC1 & MC2 into the cavi-
ty of nano-barrel MB1 to give greenish blue and pink
solutions, respectively.
Results and Discussion:
The terpyridine based ligand L1 was synthesized following
the literature procedure by treating terephthaldehyde
with NaOH, 4-acetylpyridine and NH4OH in ethanol.18
Reaction of L1 with M in 2:1 molar ratio at 70° C overnight
followed by treatment with excess ethyl acetate yielded
1
off-white solid in quantitative yield. H NMR spectra of
this solid in DMSO-d6 displayed multiple unassignable
peaks at the aromatic region (Figure S3). However, this
solid was soluble in water upon heating at 60° C. 1H NMR
spectrum of the product in D2O at room temperature
showed the presence of four peaks ranging from 9.24 to
7.84 ppm (Figure 1) due to MB1. Due to metal-ligand co-
ordination significant downfield shifts for the pyridyl pro-
tons were noticed. All the aromatic protons were assigned
1
1
with the help of H-1H COSY and H-1H NOESY NMR
spectra (Figures S6,7). The peak position of proton (b)
was observed at 8.53 ppm; however, with an integration
value of four protons, which could be due to the presence
of two different chemical environments of b because of a
slower rotation than the NMR time scale of the pyridyl
1
moiety in the final assembly. The H-1H COSY spectra
(Figure S6) of the cage MB1 indeed revealed the presence
of two peaks for the b protons, the deshielded one resid-
ing close to the N atom has a δ value of 8.53 ppm and the
shielded proton residing close to the c proton shows a δ
value completely merging with that of c protons at 8.25
1
ppm. Therefore, a variable temperature H NMR of MB1
in D2O was carried out in order to understand the behav-
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displayed the presence of isomeric structure MB3. MB1
iour of these protons (Figure 1). With increasing tempera-
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ture, all the aromatic protons showed a gradual downfield
shift; whereas, the peak at 8.53 ppm disappeared in be-
tween 40-50° C, and finally reappeared as a broad peak
after 50° C and sharpening of this peak was observed with
crystallizes in triclinic system with the space group P-1,
containing one molecule in the unit cell located about a
center of symmetry (the asymmetric unit corresponding
to half of the molecule). With a single type of binding
mode of L1-Pd(II), two different shaped windows were
formed containing two and four Pd(II) units. The open
windows formed by four Pd atoms of the tetrafacial barrel
MB1 have a slight distortion in shape which is about 5°
from the perfect square. This produces a rhombus like
geometry of the open faces where the Pd-Pd-Pd angles are
85° and 95° with diagonal Pd-Pd distances of 17.17 Å and
1
increasing temperature. Finally, H-1H COSY spectra at
80° C (Figure S8) revealed the presence of only one type
of correlation between a and b protons. This variable
temperature NMR study explains the presence of two
non-equivalent protons for b.
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−
18.84 Å, respectively (Figure 2). Both PF6 and NO3− ani-
ons were located in the crystal structure with different
disordered solvent molecules including DMSO. All the
counter-anions are present outside the barrel along with
DMSO.
MB3 also was crystallized in P-1 with a single molecule
in the asymmetric unit and thus with two molecules in
the unit cell. MB3 has a distorted barrel shaped geometry
with different binding modes of L1 unlike in MB1. In con-
trast to MB1, two narrow windows containing two Pd(II)
units and four wider windows containing three Pd(II)
units with slight distortion of the faces were observed
making the cage less symmetric. This particular assembly
resembles with a geometry named Gyrobifastigium type
solid J26 (Johnson solids)19. Having regular but not uni-
form faces makes this polyhedra very unique and rare in
the literature20. Formation of these two different geome-
1
Figure 1: H NMR spectra of the ligand L1 (bottom) in CDCl3
1
and variable temperature H NMR spectra of MB1 in D2O
1
(top).
tries in the DMSO solution makes the H NMR spectra
very complicated (Figure S3). However, the presence of
two phenyl peaks at 8.19 ppm confirms the formation of
two distinct geometries. In case of D2O the fairly simple
1H NMR spectra having distinct peaks in the aromatic
region confirms the formation of highly symmetrical MB1.
The formation of the symmetrical barrel in aqueous me-
dium was also asserted from single point energy calcula-
tions of MB1 and MB3, where the total energy of the
asymmetric cage MB3 is much higher than the symmetric
isomer MB1 (Table S2). However, presence of slight
−
In order to record mass spectra, PF6 analogue of the
nano-cage MB1 was prepared by treating an aqueous solu-
tion of MB1 with KPF6 followed by separation of the white
precipitate. The composition of the assembly was accu-
rately ascertained by ESI-MS study in acetonitrile. The
presence of several prominent peaks at m/z = 1107.87,
899.05, 749.89 and 638.17 with isotopic distribution pat-
− +5
− +6
terns corresponding to [MB1⋅11PF6 ] , [MB1⋅10PF6 ] , and
[MB1⋅9PF6−]+7 and [MB1⋅8PF6−]+8 charge fragments respec-
tively, confirmed the formation of a M8L14 species (Figure
S9). Although, the proton NMR and ESI-MS spectroscopy
specifically established the formation of a molecular bar-
rel, further insight about the specific geometry of MB1
was necessary to understand the dimension of the cavity.
Several attempts to obtain single crystals of the nitrate
analogue of MB1 remained unsuccessful. However, single
1
asymmetry in MB1 causes the H NMR peaks in the aro-
matic region to be broader. In addition, the spectrum
shows split peaks at 8.25 ppm for the c protons, which
remain such throughout the whole experimental temper-
ature range.
−
crystals of the PF6 analogue of MB1 were successfully
grown from slow vapour diffusion of dioxane and THF
into a concentrated DMSO solution of MB1 (Figure S4).
Surprisingly, the presence of two different shaped crystals
was observed from the THF diffusion set having different
unit cell parameters. However, in case of the dioxane dif-
fusion set only one type of crystals was observed having
similar cell parameters to one of the crystal obtained from
the THF diffusion set. The single crystal XRD data analy-
sis of the dioxane diffused crystals unambiguously con-
firmed the formation of a tetrafacial barrel type architec-
ture (Figure 2). Nonetheless, XRD analysis of the crystal
having different cell parameters from the THF diffusion
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The solution remains coloured over several weeks under
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visible light. This result indicates that the barrel MB1 can
act as a safe container to arrest and stabilize the metasta-
ble merocyanine conformation (MC1).
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Figure 3: H NMR spectra of MC1⊂MB1 (bottom) in D2O. H
NMR spectra of MB1 (middle) in D2O and SP1 (top) in CDCl3
after extraction with chloroform.
1
Finally, H NMR spectra of that solution confirmed the
formation of a host-guest complex (MC⊂MB1) exhibiting
a complicated spectrum in contrast with the clearer H
NMR spectra of the host MB1 (Figure 3). Multiple peaks
appeared in the aromatic region presumably due to the
lack of sufficient symmetry in the final host-guest com-
plex. Along with these complex multiple aromatic peaks,
several discrete broad peaks for the guest molecule were
Figure 2: (a) Capped stick model of the single crystal XRD
structure of the tetrafacial barrel MB1. (Left; Top view and
right; side view. (b) Capped stick model of the single crystal
XRD structure of the asymmetric barrel MB3. Colour codes:
grey= carbon, blue= nitrogen and brown= palladium. (Hy-
drogen atoms, counter anions, solvent molecules and the
disordered phenyl rings were omitted for better clarity).
1
1
observed from 7.08 to -0.84 ppm. H DOSY NMR con-
The presence of two open windows associated with a
huge barrel shaped hydrophobic cavity with a dimension
of (12.7 Å×17.2 Å) makes MB1 a potential candidate for
encapsulation of water insoluble guest molecules of dif-
ferent shape and size. The photochromic bromo-
spiropyran (SP)22 was then used as guest. This bromo-
spiropyran is colourless, soluble in common organic sol-
vents, and stable under visible light. Upon irradiation
with UV-light or heating at high temperature, the spiro-
form converts to metastable coloured merocyanine form
in solution phase (Scheme 2, direct photochromism). The
metastable merocyanine form (MC) readily reverts back
to colourless spiro-form (SP) under visible light or cool-
ing to room temperature.
firmed the formation of a single host-guest component by
displaying similar diffusion coefficient for the host and
guest peaks. The high upfield shift of the aromatic as well
as of methyl protons is due to a binding with the host
molecule. The stability of cage MB1 as well as of guest
(SP1) was checked by extracting an aqueous solution of
⊂
MC1 MB1 with CHCl3, which transformed the greenish
blue solution into a light-yellow solution. Further investi-
gation (Figure 3) on these two layers reveals that the wa-
ter fraction contains MB1 and chloroform part contains
bromo-spiropyran (SP form). So, if the guest is taken out
of the barrel, it exists in stable spiro (SP) form. The stoi-
chiometry of the host-guest complex was determined as
1:2 (host : guest) from the ratio of integration values be-
tween host and guest peaks of the 1H NMR spectra (Figure
S16). This value was reasserted by extracting the guest
molecule (SP1) with chloroform and removing the solvent
to get solid SP1 followed by determination of molar
equivalent by simply weighing the sample, which also
confirmed the formation of a 1:2 (host : guest) complex
(Table S3). This result was supported by the presence of
two peaks at -0.49 and -0.83 ppm for the dimethyl pro-
tons (in host-guest complex), which otherwise have
equivalent chemical environment in the MC1 form (Fig-
ures S18, 19).
Stunningly, when a light-yellow solution of the nano-
barrel MB1 in D2O was treated with excess amount of
water insoluble solid bromo-spiropyran (SP1), the colour
of the solution gradually turned into greenish blue within
5 hours and the colour got darker over a time period of 6
hours. UV/Vis spectra of the coloured solution exhibited a
broad peak centered at around 590 nm. Generally, the
planar open form (MC1) of bromo-spiropyran shows ab-
sorption peaks at around 520 and 550 nm in chloroform
(Figure S13). Although, SP1 is very poorly soluble in water
1
and cannot be detected by H NMR spectroscopy, over-
night stirring of the organic guest in water produced an
UV-Vis spectrum where the absorption bands are present
only in the UV region (200-350 nm, Figure S12). There-
fore, the new peak generated in the host-guest complex at
590 nm originates due to the presence of open MC1 form.
Multiple attempts to grow single crystals of
MC1⊂MB1by conventional slow evaporation and vapour
diffusion technique were unsuccessful as the guest mole-
cule comes out to the organic phase due to higher solubil-
ity in organic solvent/s. Nevertheless, the geometry of the
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⊂
complex MC1 MB1 was optimized by semi-empirical
method by placing two MC1 molecules as an ion pair
(head to tell) into the cavity of the nano-barrel MB1 (Fig-
ure 4). Finally, to understand the optical property, time
dependent DFT calculation was carried out on this energy
optimized structure, which resulted an UV/Vis spectrum
having a λmax (585 nm) value very close to the experi-
mental one (Figure 4).
is encapsulated and it de-encapsulates from the cage after
addition of an organic solvent such as chloroform and
showing a blue colour. Following the previously described
procedure the MB1:MC2 stoichiometry was calculated to
be 1:2. The geometry was optimized following the previ-
ously described method, which resulted in a similar struc-
ture (Table S2). TD-DFT study also provided UV/Vis ab-
sorption value (545 nm) very close to the experimental
(549 nm) one. Thus, the container MB1 could capture and
stabilize the meta-stable form (MC1/2) of spiropyrans by
means of encapsulation in its cavity (Figure S23).
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To test if this unusual trapping of merocyanine form is
possible by other molecular barrels, we performed similar
study with a different molecular barrel (MB2)17 that was
designed employing a carbazole based tetraimidazole
ligand. Interestingly, when a colourless aqueous solution
of MB2 was treated with excess of solid powder of hydro-
phobic bromo-spiropyran (SP1), the colour of the solution
gradually turned blue within one hour. The colour got
saturated over a time of three hours under visible light at
Figure 4: (a) Comparison of experimental and theoretical
(TD-DFT) UV/Vis spectra. (b) Energy optimized geometry of
MC1⊂MB1.
1
room temperature. H NMR of this blue solution showed
that the hydrophobic spiropyran molecule was encapsu-
lated inside the molecular barrel MB2 and 1H DOSY NMR
confirmed the formation of a single component (Figure
S37). Similar to MB1, when this aqueous blue solution is
treated with a little amount chloroform and stirred for 10
min at room temperature, it gave a colourless bilayer of
Interestingly, this host-guest complex MC1⊂MB1 was
stable under visible light for several weeks. Irradiation
1
with UV light did not alter the colour and the H NMR
spectrum of the complex. Finally, the thermal stability of
MC1⊂MB1 was ensured by heating an aqueous solution of
the complex at 50° C. The complex was stable for 6 hours
1
chloroform and water. Finally, H NMR and its UV-Vis
1
under heating, as confirmed by H NMR spectra (Figure
spectrum (Figures S27, S28) confirm that after extraction
of the guest compound from the encapsulated blue solu-
tion, it is chemically intact. Subsequently, the aqueous
blue solution was analysed by UV-Vis spectroscopy to
confirm the nature of the encapsulated guest bromo-
spiropyran. As can be seen from the UV-Vis absorption
spectra (Figure 5), a new absorption peak at 600 nm ap-
peared.
S20). Similar stabilization of the merocyanine form by
using nano-cavity in solution state was attempted by
Zhou et al.21 where β-CD was used as a molecular host.
However, comparatively narrower cavity than MB1 did
not actually allow the open form to encapsulate into the
cavity and therefore an alkyl substituted SP molecule was
designed. Finally, the open form was stabilized by the
encapsulation of the alkyl moiety of the closed form into
the pocket followed by photo irradiation.
Next, we wanted to explore the possibility of encapsula-
tion of nitro-spiropyran (SP2), which has larger nitro sub-
stituent. The close form (SP2) is stable in aprotic solvents.
Irradiating with UV/sunlight produces blue open form
(MC2) which reversibly transforms to the closed SP2 in a
few seconds in visible light. When the light-yellow pow-
der of SP2 was treated with an aqueous solution of MB1,
formation of a pink solution was noticed after 6 h. The
solution was characterized in the same way to find the
formation of MC2⊂MB1. The 1H NMR spectrum displayed
a complicated spectrum with the dimethyl peaks (b) at -
0.87 and -0.71 ppm (Figure S22). The stability of the nitro-
spiropyran was checked by extracting the pink solution
with chloroform. Both the layers (aqueous and organic)
were characterized by 1H NMR spectroscopy that resulted
in spectra similar to the as-synthesized ones. Interesting-
ly, during the extraction process the pink aqueous layer
started to become light-yellow due to the presence of
MB1 whereas the chloroform layer initially appeared blue
due to MC2, which converted to colourless SP2 in a few
seconds. This certainly explains that the open MC2 form
Figure 5: (a) Comparison of experimental and theoretical
(TD-DFT) UV/Vis spectra. (b) Energy optimized geometry of
MC1⊂MB2.
⊂
The geometry of the complex MC1 MB2 was optimized
(Figure 5) similarly as in the case of MC1⊂MB1 (Figure 4).
Finally, time dependent DFT calculation was carried out
to get the energy optimized structure. The resulted UV-
Vis spectrum exhibits a λmax at 600 nm which is very close
to the experimental one (Figure 5a).
Interestingly, heating the blue aqueous solution of
MC1⊂MB2 for 1.5 h at 60°C resulted in gradual decrease
in blue colour which turns to colourless along with some
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1
insoluble materials at the bottom. The H NMR analysis
ed peaks specially for proton b that appears at -0.84 ppm
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showed that the aqueous solution contains the free host,
compared to MC1⊂MB2, where it appears at around 0.0
ppm. Therefore, it can be concluded that MC1⊂MB1 is
more stable than MC1⊂MB2 and does not show any re-
verse thermochromism.
whereas the insoluble material was bromo-spiropyran in
1
stable SP1 form, which was established by H NMR analy-
sis. Moreover, the absorption spectra of the aqueous solu-
tion and insoluble material confirmed that they contain
MB2 and bromo-spiropyran (SP1 form), respectively.
The rate of encapsulation was very slow in dark. The
rate became fast in daylight and faster in sunlight. From
this it can be concluded that in order to form the host-
guest complex, the formation of the MC isomer is neces-
sary and this can be facilitated by UV light. To confirm
this idea, the encapsulation was carried out under UV
light, which reduced the saturation time from 6 h to 0.5 h.
These observations encouraged us to carry out the encap-
sulation reaction in solid state. To achieve this, MB1 was
thoroughly mixed with a concentrated THF solution of
SP2 and then dried to make a paste followed by UV (365
nm) irradiation. This resulted in the formation of a solid
showing a slow pinkish luminescence over time which
suggested the formation of the host-guest complex
(MC2⊂MB1). After continuing the irradiation for 12 h the
solid mixture turned into a pinkish colour. Finally, the
Upon heating, the MC1 comes out of the barrel in the
form of SP1 as insoluble solid. This colourless solution
turns again blue upon keeping for a while under room
light at ambient temperature because SP1 is encapsulated
in the barrel MB2 in the form of MC1. So, we observed
reverse thermochromism in presence of molecular barrel
MB2.
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resulted solid was characterized in D2O by H NMR,
which exactly matches with the NMR observed from the
solution phase study. To generalize this phenomenon,
similar set of study was carried out with MB2 which pro-
duced a deep purple solid having similar spectroscopic
results to that of solution phase (Supporting Information:
Video_MB2, Figure S40). Although, a solvent was intro-
duced to make the paste, the question arises if that sol-
vent has any role in the encapsulation? To confirm this,
similar experiments were conducted using both the solids
(MB1/2 and SP2). Solid host and guest were mixed thor-
oughly by grinding the mixture followed by irradiation
with UV (365 nm) light which also resulted in the host-
guest formation as described before (Supporting Infor-
mation: Video_SP2inMB2). This experiment unequivocal-
ly confirms the formation of host-guest complex in solid
phase triggered by UV irradiation. Spiropyrans were pre-
viously incorporated into different solid matrixes such as
nano particles22, silica23, polymers and also MOFs24 by the
help of either chemical bond formation or vapour-phase
encapsulation technique. However, the solid state encap-
sulation by simple grinding in the present study including
stabilization of transient merocyanine form in solid state
is noteworthy. This exciting new finding motivated us to
employ this system to develop a magic ink which could be
used to write on a paper coated with molecular barrel. To
do this, a filter paper was soaked in an aqueous solution
of MB2 followed by drying with hot air gun. Concomi-
tantly colourless hexane solution of SP2 was used as an
invisible ink, which was poured into a fountain pen for
writing purpose. As expected, the letters were invisible
after writing with the ink (Figure 7) under room light, but
when this filter paper kept under sunlight/UV for a few
minutes, the coloured letters started to become visible
(Figure 7) and were stable for several weeks. Interestingly,
the letters could be erased by washing the paper thor-
oughly with CH2Cl2, which removes the encapsulated
MC2 from the host molecule to return the blank paper
Figure 6: Schematic representation of reverse thermochrom-
ism of MC1⊂MB2.
A similar study was carried out with SP2, which result-
ed in a purple solution of the host-guest complex
1
MC2⊂MB2. This was characterized by UV-Vis and H
NMR. A broad peak around 598 nm in UV-Vis spectra was
1
observed. H NMR spectrum displayed the usual compli-
cated peaks in the aromatic region with the dimethyl
peaks (b) shifted to -0.48 ppm, while for MC1⊂MB2 it
appears at around 0.0 ppm. This extra deshielding of the
b protons evidently suggests a better binding with the
host molecule for MC2 than MC1. The stoichiometry was
evaluated to be 1:2 (H:G) following similar procedure de-
scribed earlier above (Table S3). However, crystal struc-
ture could not be obtained for this complex either. There-
fore, the geometry was optimized according to the previ-
ously described method followed by TD-DFT calculation
to get the UV/VIS spectra, which displayed a λmax value
(558 nm) very close to the experimental one (558 nm). In
contrast to MC1⊂MB2, this complex was stable even after
heating the solution at 50° C. Thus, MC2⊂MB2 did not
show any reverse thermochromism, which can be ex-
plained in terms of a better binding of MC2 with MB2 as
explained from 1H NMR spectra.
These ambiguity in stability of the host-guest complex-
es can be explained in terms of the cavity size of the re-
spective molecular barrels. As it was observed, MB1 takes
a longer time to form the host-guest complex because of
the presence of narrower windows with a cavity of larger
length with respect to MB2, which comprises wider win-
dows containing a cavity of smaller length. This makes a
significant difference in binding capacity of MB1 and MB2
1
with MC1. H NMR of MC1⊂MB1 reflects highly deshield-
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and deep-purple solutions of host-guest complexes
containing only MB2. Finally, the recovered blank paper
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was again used to write with the invisible ink. This idea
can be used to make a re-writeable paper.
MC1⊂MB2 and MC2⊂MB2, respectively. Although, the
complex MC2⊂MB2 was stable under different stimuli,
the blue complex MC1⊂MB2 showed a gradual decoloura-
tion upon heating due to de-encapsulation of the guest
molecule from the barrel and transformed to the stable
closed form SP1. Thus, SP1 showed unique reverse ther-
mochromism in presence of the barrel MB2. Such unusual
stabilization of transient merocyanine form in visible light
in confined space of molecular barrels was not only re-
stricted to solution phase. Even solid mixture of the barrel
(MB1/2) and spiro compound (SP2) upon irradiation with
UV light produced merocyanine forms that were stable in
solid state for very long period in visible light. The re-
markable solid-state stabilization of the merocyanine in
barrel was finally used for magic writing. A colourless
hexane solution of SP2 was used as invisible ink on a pa-
per that was coated with MB2. The invisible letters ap-
peared coloured upon keeping under UV/sunlight light
for a few minutes due to the solid-state isomerization of
the SP2 to coloured MC2 in solid state.
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Experimental Section:
Methods and Materials:
Reagents used in this study were purchased from com-
mercial sources and used without any purification. NMR
studies were performed using a Bruker-make 400 MHz
spectrometer and the chemical shifts (δ) in the spectra
are reported in ppm relative to tetramethylsilane (Me4Si)
as an internal standard (0.0 ppm) or proton resonance
resulting from incomplete deuteration of the solvents
(CD3)2SO (2.50 ppm), CDCl3 (7.26 ppm) and D2O (4.79
ppm). Electrospray ionization mass spectrometric (ESI-
MS) analyses were carried out on an Agilent 6538 Ultra-
High Definition (UHD) Accurate Mass Q-TOF spectrome-
ter. The nano-barrel MB217, SP125 and SP226 were synthe-
sized according to the reported procedure.
Figure 7: Encapsulation of MC2 in MB1 in solid phase (top).
Writing with hexane solution of SP2 on a filter paper soaked
with aqueous solution of MB2, which can be used as a re-
writable paper.
Conclusions:
In conclusion, self-assembly of a 90° Pd(II) acceptor M
with terpyridine based tetra-pyridyl donors L1 in 2:1 molar
ratio yielded two isometric molecular nano-barrels MB1
and MB3 in DMSO. A single assembly (MB1) was ob-
tained by heating the mixture of barrels in aqueous medi-
um. Spiropyran based compounds are generally stable in
closed spiro-form (SP) in visible light and readily converts
to transient merocyanine (MC) form upon UV irradiation
or strong heating in solution phase. The transient MC
form readily converts back again to stable spiro-form in
visible light. However, such structural transformation of
photochromic/thermochromic spiropyrans in solid state
is very challenging for practical application. The molecu-
lar barrel MB1 was found to stabilize the transient mero-
cyanine form MC1 of a bromo-spiropyran SP1 when the
aqueous solution of the barrel was treated with solid SP1.
This host-guest complex MC1⊂MB1 was found to be very
stable under different external stimuli such as visible/UV-
light and heating. The same barrel was also found to sta-
bilize transient merocyanine isomer (MC2) of a nitro sub-
stituted spiropyran SP2 in the form of a host-guest com-
plex MC2⊂MB1 in aqueous medium. Finally, to explore
this unusual stabilization of transient merocyanine form
of spiropyrans in the confined space of other molecular
barrel, a similar study was carried out with a carbazole
based tetrafacial molecular barrel MB2.15 Treatment of
SP1 and SP2 with aqueous solution of MB2 produced blue
Synthesis of the Ligand L1:
The ligand L1 was synthesized following reported proce-
dure of terpyridine synthesis.16 Terepthaldehyde (670 mg,
5 mmol) was taken into a flame dried 500 mL round bot-
tom flask containing 50 mL of ethanol at 0° C followed by
addition of NaOH (400 mg, 10 mmol) with stirring. To
this cold solution 4-acetyl pyridine (2.42 g, 20 mmol) in 10
mL of ethanol was added dropwise. Finally, 60 mL of
ammonia solution was added followed by stirring at 0° C
for 1 h. The solution was then allowed to reach to room
temperature and refluxed at 80°C for 24 h. Precipitate
formed after refluxing was filtered and washed thoroughly
with ethanol and water to get solid powder of L1 as pure
1
product. Yield 1.5 g (55 %). %). H NMR (400 MHz,
CDCl3): δ = 8.84 (d, 8H), 8.14 (m, 12H), 7.96 (s, 4H).
Synthesis of the Barrel MB1:
cis-(tmen)Pd(NO3)2 (M) (69.2 mg, 0.200 mmol) was dis-
solved in 3 mL DMSO and the yellow clear solution was
added to the solid ligand L1 (54 mg, 0.100 mmol) and
heated at 60 °C with stirring for 6 h resulting in clear
brownish solution. The clear solution was then treated
with 20 mL ethyl acetate to obtain light yellow precipitate
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which was collected by filtration followed by washing
Page 8 of 11
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with acetone and ether. It was then dried under vacuum
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NMR (400 MHz, D2O) δ (ppm): 9.24 (broad s, 8H), 8.53
(broad s, 4H), 8.25 (m, 8H), 7.84 (s, 4H).
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Preparation of MC1/2⊂MB1:
Barrel MB1 (20.0 mg, 0.04 mmol) was dissolved in 1.5 mL
of water by heating at 60° C to get a light-yellow solution.
Solid powder of SP1/2 (10.0 mg) was added to this solu-
tion and stirred at room temperature for 6 h to get green-
ish blue and pink solutions of the host-guest complexes
MC1⊂MB1 and MC2⊂MB1, respectively.
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Preparation of MC1/2⊂MB 2:
Barrel MB2 (18.2 mg, 0.04 mmol) was dissolved in 1.5 mL
of water and to this solution solid powder of SP1/2 (10.0
mg) was added and stirred at room temperature for 3 h to
get blue and purple coloured solutions of the host-guest
complexes MC1⊂MB 2 and MC1⊂MB 2, respectively.
Solid State Encapsulations:
Respective barrel MB1/2 (0.02 mmol) was mixed with SP2
followed by through grinding. The mixture was then irra-
diated with UV light (365 nm), which resulted in a red-
dish fluorescence of the mixture. Over time the fluores-
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yellow to pink and finally deep purple.
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ASSOCIATED CONTENT
Additional NMR, UV-Vis, SCXRD parameters and ESI-MS
spectra. “This material is available free of charge via the In-
ternet at http://pubs.acs.org.”
AUTHOR INFORMATION
Corresponding Author
* psm@ipc.iisc.ernet.in (P. S. Mukherjee)
ACKNOWLEDGMENT
P.S.M. thanks the Science and Engineering Research Board
(New Delhi) for
a
research grant [Grant No.
EMR/2015/002353]. P. H. thanks Milan Kumar Hazra for
meaningful discussion about computational studies.
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Self-Assembled Pd(II) Barrels as Containers for Transient Merocya-
nine form and Reverse Thermochromism of Spiropyran
Prodip Howlader,a Bijnaneswar Mondal,a Prioti Choudhury Purba,a Ennio Zangrandob and Par-
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