Encapsulation and Stabilization of a Donor-Acceptor Stenhouse Adduct Isomer in Water Inside the Blue Box: A Combined Experimental and Theoretical Approach

Article abstract of DOI:10.1021/acs.jpcb.1c03890

We synthesized two types of donor-acceptor Stenhouse adducts (DASAs), a new type of photochromic molecules showing dual color in two different isomeric forms in solution phase, using Meldrum acid (DASA-Mel) and barbituric acid (DASA-Bar), along with a naphthalimide derivative to obtain interesting fluorescence properties. DASA-Mel was found to have fast photochromic conversion in comparison to DASA-Bar, evident from ultraviolet-visible (UV-vis) and fluorescence spectroscopic studies. The colored form of DASA-Mel was encapsulated inside the water-soluble Stoddart’s blue box and became soluble in water much faster than DASA-Bar. Interestingly, the competitive encapsulation experiment showed that DASA-Mel was selectively encapsulated inside the blue box in water whereas DASA-Bar was mostly separated out from the solution after centrifugation, and this phenomenon was confirmed by¹H and DOSY NMR and mass spectroscopies. Moreover, we found through density functional theory (DFT) optimization that the open form of DASA-Mel was more stable during the encapsulation reaction in a water medium in comparison to DASA-Bar. The calculated binding energies of encapsulated DASA-Mel and DASA-Bar are ?10.2 and ?9.9 kcal/mol, respectively, clearly showing that the former is more stable by 0.3 kcal. Consequently, the organic macrocycle selectively separating one kind of DASA from a mixture by encapsulation in water is reported for the first time with experimental and theoretical support in the literature.

Full text of DOI:10.1021/acs.jpcb.1c03890

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Article
Encapsulation and Stabilization of a Donor−Acceptor Stenhouse
Adduct Isomer in Water Inside the Blue Box: A Combined
Experimental and Theoretical Approach
Sujay Mukhopadhyay,∥ Arnab Sarkar,∥ Sourav Ghoshal, Pranab Sarkar, Koushik Dhara,*
and Pabitra Chattopadhyay*
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ABSTRACT: We synthesized two types of donor−acceptor
Stenhouse adducts (DASAs), a new type of photochromic
molecules showing dual color in two different isomeric forms in
solution phase, using Meldrum acid (DASA-Mel) and barbituric
acid (DASA-Bar), along with a naphthalimide derivative to obtain
interesting fluorescence properties. DASA-Mel was found to have
fast photochromic conversion in comparison to DASA-Bar, evident
from ultraviolet−visible (UV−vis) and fluorescence spectroscopic
studies. The colored form of DASA-Mel was encapsulated inside
the water-soluble Stoddart’s blue box and became soluble in water
much faster than DASA-Bar. Interestingly, the competitive
encapsulation experiment showed that DASA-Mel was selectively
encapsulated inside the blue box in water whereas DASA-Bar was
mostly separated out from the solution after centrifugation, and this phenomenon was confirmed by 1H and DOSY NMR and mass
spectroscopies. Moreover, we found through density functional theory (DFT) optimization that the open form of DASA-Mel was
more stable during the encapsulation reaction in a water medium in comparison to DASA-Bar. The calculated binding energies of
encapsulated DASA-Mel and DASA-Bar are −10.2 and −9.9 kcal/mol, respectively, clearly showing that the former is more stable by
0.3 kcal. Consequently, the organic macrocycle selectively separating one kind of DASA from a mixture by encapsulation in water is
reported for the first time with experimental and theoretical support in the literature.
viz., E−Z isomerization and thermal electrocyclization
elucidated by Feringa and Beves et al.1,2 Conversion is highly
dependent on solvent polarity, and it is observed that the
colored form is mostly stable in nonpolar solvents, whereas the
colorless form is highly soluble in polar solvents.1−4
Reversibility is observed in polymer matrices, toluene,
dioxanes, etc., while in polar protic solvents, e.g., water and
methanol, irreversible conversion from the open cyclic forms is
identified.1−8 DASAs are now becoming popular in the
scientific community due to their high absorption under
visible light, easy synthesis and purification techniques, and
reversible photoswitching furnishing diverse ultraviolet−visible
(UV−vis) and fluorescence responses under low-energy light
with a low loss of efficiency.1,2,21−23 Recently, theoretical
INTRODUCTION
■
Photochromic molecules photoisomerize under the influence
of visible and UV light and can give rise to different absorption
and emission properties.1,2 Hence, scientists have been
showing great research interest due to the diverse applications
of these smart molecules in drug delivery, photopharmacology,
photoelectronics, receptors, molecular machines, and so
on.3−10 As of now, the major contributors in this field are
azobenzene, diarylethene, stilbene, hemithioindigo, and
spiropyran following mostly E−Z isomerization and electro-
cyclization.11−19 But the major problem with these molecules
is that they require high-energy UV light for their trans-
formation, making them less useful in biological fields.20 In
2014, to overcome this drawback, Read de Alaniz and co-
workers described a low-energy visible light-sensitive photo-
chromic material and termed it as a donor−acceptor
Stenhouse adduct (DASA), named after J. Stenhouse, who
first reported the synthesis of Stenhouse salts upon reacting
furfural with aniline derivatives.3−5 Highly colored linear triene
(open form) photoisomerizes to colorless zwitterionic cyclo-
pentenone, known as the first-generation DASA.1−5 The
process of transformation constitutes two consecutive steps,
Received: April 30, 2021
Revised: June 11, 2021
Published: June 28, 2021
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Scheme 1. Pictorial Presentation of the Selective Encapsulation Phenomenon
analysis has been reported based on H-bonding and the role of
the hydroxyl group, which makes the mechanistic interpreta-
tion easier. In addition, some studies on nanoparticles and
near-infrared (NIR) activities have also been reported, which
strengthens the applicative aspects of DASAs.24−29 Within a
short span of time, they have been utilized in the applicative
fields of polymer and surface chemistry, identification of bullet
impacts in explosives, drug delivery, aggregation-induced
emission and chemosensing, phase transfer catalysis, photo-
reversible photoisomerization and structural changes on the
photoinduced electron transfer (PET) process. It was observed
through UV−vis and fluorescence spectroscopies that the
highly absorbing DASA loses its color and fluorescence in
methanol, indicating “PET on” upon conversion to cyclic
structures. But DASA-Mel was found to be kinetically more
labile than DASA-Bar for irreversible conversion in methanol
as evident from the absorbance and emission behavior. This
kinetic lability was also detected upon encapsulation with BB.
DASA-Mel was encapsulated after stirring with the blue box in
water for 7 days, but DASA-Bar remained unresponsive during
this time, and very light coloration was observed after 30 days
of prolonged stirring. Another amusing result was observed
when we used a mixture of DASA-Mel and DASA-Bar with
stirring for 7 days: only DASA-Mel was selectively encapsu-
lated inside the macrocycle and DASA-Bar mostly separated
out of the solution after centrifugation. This phenomenon is
pictorially presented in Scheme 1. To the best of our
knowledge, the selective separation of one DASA from a
mixture of two DASAs by simple macrocyclic encapsulation
has been reported for the first time in this article.
́
controlled Rayleigh−Benard convection, and liquid crys-
tals.1,2,30−47
As discussed earlier, nonpolar or chlorinated solvents are
suitable for open-chain colored first-generation DASAs, and for
polar protic solvents, the cyclopentenone ring structures
(colorless) are irreversible.1−4,10 To overcome this drawback,
Read de Alaniz and co-workers used secondary aniline
derivatives and the performance, stability, and reversibility of
the photochromic materials in varied solvents have been
augmented.21−23 However, very little attention has been paid
toward tuning the stability and irreversibility of DASAs, i.e., to
stabilize unstable colored isomers as a major fraction in polar
protic solvents such as water. Mukherjee et al. reported for the
first time the unusual stabilization of open-chain analogues,
normally insoluble and unstable in water, through encapsula-
EXPERIMENTAL SECTION
■
Materials and Methods. All solvents and chemicals were
purchased from Sigma-Aldrich, India, and used without further
purification. Spectroscopic-grade solvents were used for all
spectroscopic studies. Absorption and fluorescence spectra
were obtained using a Shimadzu 2450 UV−vis and a Hitachi
F-4600 fluorescence spectrophotometer, respectively. A Bruker
Advance DPX 400 spectrometer was used to obtain NMR
spectra. A Xevo G2-XS QTof mass spectrometer was used for
electron spray ionization (ESI) mass studies.
II
tion of the colored DASAs inside a Pd8 -based water-soluble
molecular container.10 However, the use of the PdII complex as
a vessel is accompanied by the concern of possible toxicity.
Thus, easily synthesizable and cost-effective organic cages,
macrocycles, crown ethers, and cavitands are always better
choices than the heavy metal-based cages, provided that the
former compounds serve the purpose well.48−54 Also, many
previous reports suggest that organic encapsulating agents are
highly efficient toward stabilization of unstable and reactive
species.48−50
General Method of Spectral Studies. DASAs were
dissolved in 2 mL of methanol to obtain stock solutions to
perform time-dependent absorption and fluorometric studies
and check reversibility in toluene. All of the fluorescence
spectra were recorded with an excitation wavelength of 330
nm.
Theoretical Calculation. To understand the stability of
DASA-Mel/Bar inside a macrocycle, we performed individual
geometry optimization of the macrocycle (BB), open DASA-
Mel, open DASA-Bar, and their encapsulated structures in
water medium using the Gaussian-16 suite of programs.58
Implicit solvation was employed to account for the effect of the
solute−solvent interaction through the polarizable continuum
model, using the integral equation formalism variant
(IEFPCM). Time-dependent density functional theory
(TDDFT) calculations were also performed to predict the
absorption spectra of the desired molecule. To compute
excitation energy, we used a long-rage corrected CAM-B3LYP
Since the first report of the blue box (BB) by Stoddart in
1989 having two π-electron-deficient 4,4′-bipyridinium units
linked by two p-xylene spacers in a cyclic manner, numerous
reports of host−guest chemistry have been published,
suggesting the efficacy of the rigid π-electron-deficient host
and the ease of synthesis.55,56 In the case of host−guest
complexes, the matching cavity, guest size, and the electron
density/cloud are the deciding factors for the formation of a
BB⊂host stable topological structure and a molecular
machine.50,55 Keeping these factors in mind, we have
synthesized two comparable DASAs derived from Meldrum
acid (DASA-Mel) and barbituric acid (DASA-Bar) conjugated
with a naphthalimide fluorophore having an N-methylethyle-
nediamine donor.
The naphthalimide moiety was chosen due to its inherent
fluorescence,46,57 which is utilized later to explore the effect of
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functional with a 6-31G** split-valence basis set. The same
level of calculation was also performed for geometry
optimization, as this functional is suitable for non-Coulomb
interactions and provides a faithful representation of charge
transfer excitation.59
was washed several times with cold diethyl ether and finally
dried in vacuo to obtain DASA-Mel as a pure pink compound.
1H NMR (400 MHz, CDCl3): δ 1.70 (s, 6H), 3.25, 3.41 (s,
3H), 3.79−3.86 (m, 2H), 4.49−4.59 (m, 2H), 5.86−5.92 (t,
1H), 6.25−6.34 (m, 1H), 6.94−6.99 (m, 2H), 7.77−7.83 (m,
2H), 8.22−8.30 (m, 2H), 8.55−8.62 (m, 2H), and 11.23 (s,
1H). 13C (100 MHz, CDCl3): δ 27.0, 37.4, 38.2, 44.4, 44.9,
57.1, 64.5, 83.3, 92.7, 102.3, 103.9, 121.9, 127.5, 128.4, 130.0,
132.2, 135.3, 142.0, 145.6, 147.5, 149.7, 156.9, 164.6 and
167.2. HRMS (m/z) found 477.1666, calculated for [M + H+]
477.1662, where M was C26H25N2O7 (see SI Figures S5, S6,
and S7).
Synthesis of DASA-Bar. Briefly, 10 mL of a THF solution
of Nap-N(Me) (0.254 g, 1.0 mmol) was added dropwise into a
suspension solution (15 mL of THF) of DBar (0.206 g, 1.0
mol) under stirring. The mixture was stirred for 10 min at
room temperature, followed by a reduction of the temperature
of the mixture to 0 °C for another 30 min. The reaction
mixture was then filtered to collect the precipitate. The solid
was washed several times with cold diethyl ether and finally
dried in vacuo to obtain DASA-Bar as a pure violet compound.
1H NMR (400 MHz, CDCl3): δ 3.03, 3.10 (s, 3H), 3.62−3.76
(m, 2H), 4.39−4.52 (m, 2H), 5.89−5.95 (t, 1H), 6.40−6.43
(d, 1H), 7.00−7.12 (m, 2H), 7.76−7.80 (m, 2H), 8.23−8.29
(m, 2H), and 8.57−8.64 (m, 2H). 13C (100 MHz, CDCl3): δ
37.0, 38.0, 46.7, 47.9, 58.1, 64.5, 82.7, 91.2, 105.1, 122.4, 126.9,
128.5, 130.7, 131.6, 134.5, 143.9, 148.9, 150.7, 152.9, 158.6,
159.9, 166.5, and 170.5. HRMS (m/z) found 461.1455,
calculated for [M + H+] 461.1461, where M was C26H25N2O7
(see SI Figures S8, S9, and S10).
Synthesis of Compound Nap-N(Me). An ethanolic (10
mL) solution of N′-methyl-ethane-1,2-diamine (96 μL, 1.1
mmol) was added dropwise to 30 mL of an ethanolic solution
of 1,8-naphthalic anhydride (0.198 g, 1.0 mmol) under stirring.
The solution mixture was stirred for 30 min and then allowed
to reflux for another 2 h. After completion of the reaction
(monitored by TLC), the solvent was evaporated using a
rotary evaporator. A white crystalline product was obtained
after washing several times with cold ethanol. The isolated
compound was dried in vacuum to give a pure crystalline
1
compound Nap-N(Me) (isolated yield, 91%). H NMR (400
MHz, CDCl3): δ 2.48 (s, 3H), 2.96−2.99 (t, 2H), 4.33−4.36
(t, 2H), 7.70−7.74 (t, 2H), 8.17−8.19 (d, 2H), and 8.56−8.58
(d, 2H). 13C (100 MHz, CDCl3): δ 36.2, 39.7, 49.8, 122.5,
126.8, 128.1, 131.2, 131.5, 133.9, and 164.4. HRMS (m/z)
found 255.1133, calculated for [M + H+] 255.1126, where M
was C15H14N2O2 (see the Supporting Information (SI) Figures
S2, S3, and S4).
DMel and DBar (Scheme 2) were synthesized by the
reaction of Meldrum acid and barbituric acid with furfural,
respectively, according to reported procedures.3
Scheme 2. Synthetic Routes for DASA-Mel and DASA-Bar
aConditions
and the Isomeric Open Forms
Encapsulation of DASAs. Briefly, 0.005 mmol of BB was
dissolved in 2.0 mL of D2O, and the solution was added to
0.01 mmol of DASA (guest) molecules separately. This
suspension was stirred for 7 days at room temperature.
Then, the resulting solution was centrifuged to obtain a clear
solution. On the other hand, in the competitive encapsulation
experiment, we followed the same procedure as described
earlier, except that we used DASA-Mel (0.01 mmol) and
DASA-Bar (0.01 mmol) together as a mixture.
RESULTS AND DISCUSSION
■
The macrocycle BB was synthesized following the reported
procedure56 and characterized by H NMR spectroscopy to
1
confirm its purity (Figure S1, see the SI). DASA-Mel and
DASA-Bar were synthesized following a stepwise procedure
(Scheme 2). First, naphthalimide and N-methylethylenedi-
amine were mixed in a 1:1.1 molar ratio and refluxed for 2 h in
ethanol. The solvent was removed and washed several times
with ethanol to obtain the pure product as a white powder and
1
was characterized by H, 13C NMR, and high-resolution mass
spectrometry (HRMS) analyses (Figures S2−S4, see the SI).
The furan adducts of Meldrum acid and barbituric acid were
synthesized as yellow and green powders, respectively,
following a previously reported method.3 Finally, the furan
adducts were separately added dropwise to a THF solution of
2-(2-(methylamino)ethyl)-1H-benzo[de]isoquinoline-1,3(2H)
dione (1:1 molar ratio) under stirring at 0 °C to obtain pink
and purple precipitates of DASA-Mel and DASA-Bar,
respectively. The precipitates were filtered and washed thrice
with cold diethyl ether and dried in vacuum. The DASAs were
well characterized by 1H, 13C NMR, and HRMS spectroscopic
studies (Figures S5−S10, see the SI). Both DASA-Mel and
aConditions
used: (a) EtOH and reflux; (b) Meldrum acid, water, and
75 °C for 2 h; (c) barbituric acid, water, and room temperature for 3
h; and (d) Nap-N(Me), tetrahydrofuran (THF), and 0 °C.
Synthesis of DASA-Mel. Briefly, 10 mL of a THF solution
of Nap-N(Me) (0.254 g, 1.0 mmol) was added dropwise into a
suspension solution (10 mL of THF) of DMel (0.222 g, 1.0
mol) under stirring. The mixture was stirred for 10 min at
room temperature, followed by a reduction of the temperature
of the mixture to 0 °C for another 30 min. The reaction
mixture was then filtered to collect the precipitate. The solid
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DASA-Bar are soluble in common organic solvents and
insoluble in water as the colored solid compounds are
hydrophobic in nature.
DASA-Mel and DASA-Bar have intense colors in solution
phases, but the solutions become colorless under visible light
and more so in polar protic solvents with time (Figure 1). The
the close forms supported by the literature reports.3 After 1
day, the higher-wavelength bands were nearly diminished and
solutions became colorless, signifying almost complete
conversion from the open to the close form. Therefore, from
the discussion on UV−vis spectroscopic experiments, we can
conclude that both DASAs show solvent-dependent photo-
isomerization from a highly colored open form to a colorless
zwitterionic form, and reversible conversion from open to close
forms can be observed upon heating in toluene.
We purposely inserted the naphthalimide moiety to observe
the emission behavior of DASA-Mel and DASA-Bar upon
photoisomerization. We observed that exciting the methanolic
solution of both the DASAs at 330 nm gives an emission at 388
nm (Figure 2), since the naphthalimide core is fluorogenic in
Figure 1. Time-dependent UV−vis studies of (a) DASA-Mel and (b)
DASA-Bar in a methanolic solution. The first 80 min data were taken
in 10 min intervals in both cases. The inset color images correspond
to the open and close forms of both the DASAs.
reversibility between open and close forms (Scheme S1, see
SI) was observed in toluene through UV−vis spectroscopic
studies, which clearly show that the absorbance of DASA-Mel
and DASA-Bar at around 525 and 545 nm, characteristic bands
for the open form, respectively, increased upon heating in the
presence of visible light (Figure S11, see the SI). But
irreversible conversions of open−close forms were recorded
for both DASAs in methanol. The bands at higher energy
below 350 nm are due to π−π* transitions, whereas the lower-
energy bands at around 525 nm (DASA-Mel) and 545 nm
(DASA-Bar) may be due to the mixing of π−π* and n−π*
transitions. Also, from a literature survey, it is clear that the
bands at around 340 nm are due to the naphthalimide core,57
whereas bands above 500 nm are due to the highly conjugated
open forms of DASA. As the compounds undergo photo-
isomerization under visible light, we record time-dependent
UV−vis spectra of DASA-Mel and DASA-Bar in methanol
(Figure 1).
Figure 2. Time-dependent fluorescence spectral changes of (a)
DASA-Mel and (b) DASA-Bar in MeOH under excitation at 330 nm.
The first 1 h data were taken in 5 min intervals in both cases.
nature and the open form remains in conjugation. Thus, the
internal charge transfer (ICT) becomes active between the
secondary amine donor and the acceptor units, and thus the
possible photoinduced electron transfer (PET) from the
secondary amine to the naphthalimide fluorophore remains
inactive initially in the methanolic solution. But, with time, a
noticeable decrease in the emission intensity occurs due to the
conversion from the open to the close form, and after 1 day,
more than an eight-fold decrease in intensities was recorded for
both the compounds having ∼12 and ∼4 nm red shift in the
emission wavelength of DASA-Mel and DASA-Bar, respec-
tively. Because the close forms are devoid of conjugation and
thus the ICT process stops, PET becomes active, resulting in
fluorescence quenching. Here, in both the UV−vis and
fluorescence studies, it was evident that the decreasing
intensities were rapid for DASA-Mel in comparison to
DASA-Bar. Therefore, our hypothesis regarding kinetically
fast open−close conversion of DASA-Mel in comparison to
DASA-Bar is reaffirmed from the fluorescence spectral studies.
DASA-Mel was found to be more susceptible to conversion
from the open to the close form, and it can be seen from the
figure that the absorbance at around 525 nm decreases
smoothly with respect to DASA-Bar (the band at 545 nm).
The isosbestic points clearly indicate the reversible involve-
ment of more than one species in solution, and the emergence
of a new band at around 270 nm indicates the generation of
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After observing photoisomerization of DASAs using UV−vis
and fluorescence spectroscopies, our interest shifted toward
stabilizing the open hydrophobic form in water inside a
molecular container. In a previous report, this unusual
stabilization was achieved using a palladium-based cage,10
but our intention was to make the container metal-free so that
its aqueous stability may be utilized in biological fields and
cytotoxicity does not become an issue. The highly absorbing
DASAs coupled with intense fluorescence may be good
candidates for drug delivery and imaging studies. We studied
the encapsulation reaction in methanol, but the solubility of
DASAs in methanol was very high; thus, photoisomerization
suppresses the encapsulation process, and these were
converted to colorless close ring forms rapidly. Based on
these lines, we synthesized Stoddart’s widely used macrocycle
“blue box” (BB) as it has previously shown ability toward
encapsulation of small organic species, and the tetrabromo
analogue is electron-deficient and water-soluble.48,49 Briefly, 2
mL of a solution of BB and excess solid DASAs (1:2) was
stirred in vials for 7 days and centrifuged to obtain a slightly
reddish solution of BB⊂DASA-Mel, but the solution of DASA-
Bar remained colorless and most of the solid DASA-Bar
separated out from the solution after centrifugation, indicating
that encapsulation/stabilization of DASA-Bar (open form)
may be very slow.
Afterward, we checked the preferential encapsulation and
stabilization of DASA-Mel from the mixture of both DASAs in
the presence of BB so that selective separation from the
mixture of the two DASAs could be achieved. Hence, all of the
three types of encapsulation reactions, i.e., (a) DASA-Mel with
BB, (b) DASA-Bar with BB, and (c) both DASAs with BB,
were continued for 7 days with constant stirring in water and
maintaining the same stoichiometry. After 7 days of stirring, we
centrifuged all of the three reaction mixtures to obtain clear
solutions as evident from the inset images in Figure 3.
BB⊂DASA-Mel and BB⊂DASA-Bar were slightly reddish and
colorless, respectively, whereas the mixture solution was found
to have an exactly similar color as that of BB⊂DASA-Mel.
From the results, we suspected that due to the sluggish nature
of DASA-Bar toward encapsulation in comparison to DASA-
Mel, the latter is encapsulated inside BB selectively from the
mixture of the two compounds. Afterward, we studied the
UV−vis spectra of the three aqueous solutions and found that
two prominent absorbance bands appeared near 347 and 263
nm, which undergo a small change after 1 day (Figure S12, see
the SI). Changes at lower wavelengths are due to dilution. All
of the three solutions remained colored for quite a few days,
evident by the naked eye, suggesting the stability of the colored
forms inside the container. Thus, to inspect the origin of color
in the solutions, we minutely studied the region of 400−600
nm and found that a broad low-intensity absorption band
existed in all of the cases due to low-energy π−π* transitions
(Figure 3). The blue shift in absorption is obvious due to the
greater stabilization of bonding orbitals in comparison to
antibonding orbitals, increasing the energy gap in highly polar
solvents (here water); hence, more energy is needed for
electronic absorption causing a blue shift. Again, from the
UV−vis spectra, it is certain that the reversibility between open
and close forms can be attained in the presence of the
molecular container, and the unstable open form in water is
stabilized.
Figure 3. UV−vis spectral changes of (a) BB⊂DASA-Mel, (b)
BB⊂DASA-Bar, and (c) BB⊂Mix (∼10−5 M). The inset images show
the low-energy absorption band at a higher concentration (∼10−4 M)
used for both BB and the corresponding DASAs along with their color
observed with the naked eye after 7 days of the encapsulation reaction
in water and subsequent centrifugation.
reaction. Although the macrocycle shows a very weak emissive
property, the fluorescence intensity of BB⊂DASA systems
indicates significant emissive behavior due to the presence of
the naphthalimide moiety. The emission maximum at around
400 nm for the DASA mixture solution is found to be similar
to that of DASA-Mel and different from the emission
maximum (392 nm) for DASA-Bar, which corresponds to
the selective encapsulation of DASA-Mel inside BB in the case
of mix-experiment. Moreover, the fluorescence spectra of
BB⊂DASA systems were recorded after 1 day of the
encapsulation reaction, which were found to be almost
unchanged. These results again show the high stability of the
encapsulated solutions (Figure S13, see the SI). Since the close
form loses its emission intensity with time, it is understandable
that most of the encapsulated species remaining in open form
have a high emission intensity.
Finally, we examined the encapsulated system, i.e.,
BB⊂DASA-Mel and BB⊂Mix in D2O, using 1H NMR
spectroscopic studies, as presented in Figure 4. The 1H
NMR spectrum of BB⊂DASA-Mel shows the presence of
characteristic peaks in the aromatic and aliphatic regions of BB
Next, we studied the changes of emission behavior of the
aqueous solutions excited at 330 nm after the encapsulation
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and e of BB and DASA-Mel, respectively, in the BB⊂DASA-
Mel species signify that the stoichiometry of the participants is
in a 1:2 ratio (BB/DASA-Mel). Also, DOSY NMR spectra of
BB⊂DASA-Mel (Figure S14, see SI) signify the presence of a
single species in solution, since the diffusion coefficient spots
were in a straight line. On the contrary, if the compounds
remained separate in solution, the straight line in DOSY NMR
would not be obtained.
Moreover, we examined the encapsulation reaction with
both DASAs for 30 days to explore the fate of the DASAs
inside BB. The colors of all of the solutions were found to be
more intense after prolonged stirring (Figure S15, see the SI),
signifying encapsulation of even DASA-Bar, although the rate
was very slow. Then, the encapsulated aqueous solutions were
shaken well with CDCl3 to extract the colored DASAs. The
images in the video (Video S1, see the SI) clearly indicate that
DASA-Mel, DASA-Bar, and the mixture were separated from
the aqueous part to CDCl3. These solutions were also
1
examined using H NMR spectroscopy (Figure S16, see the
SI), where it can be easily seen that the spectra of DASA-Mel
and DASA-Bar match well with the pure spectra recorded
previously. Furthermore, we studied the encapsulated DASA-
Mel form, i.e., [BB⊂DASA-Mel]+, through a HRMS experi-
ment. The observed HRMS isotopic abundance peak ranging
from 1233.1068 to 1240.0727 of the [BB⊂DASA-Mel]+
species corroborates well with the calculated isotopic
abundance peak (1233.1761−1240.1740) (Figure S17, see
the SI). Thus, this experiment established the fact that the
DASAs are stable inside the container and can be stored
without any alteration of properties for a long time.
Figure 4. Comparative 1H NMR plot of the macrocycle (BB),
DASAs, and the encapsulated species (BB⊂DASAs). The peak shift
toward high fields is due to the presence of the electropositive
environment of the macrocycle and the solvent (CDCl3/D2O) effect.
As previously reported in the literature55 and confirmed by
the bromine test (addition of silver nitrate solution to obtain a
precipitate of silver bromide), the four N atoms of the
macrocycle existed in zwitterionic form (N+···Br−) with four
bromide ions. Here, we considered the positive counterpart of
the macrocycle for quantum chemical modeling. Basically, Br−
induces a positive charge on four N atoms resulting in an
overall +4 charge, with two possible spin multiplicities (2S + 1
≡ Ms) either singlet (Ms ≡ 1) or quintet (Ms ≡ 5), depending
upon the complete delocalization of the four unpaired
electrons or spatial separation over the four N atoms.
Optimized geometries and total electronic energies of various
species at different spin states are presented in Figures 5 and
S18 and Table S1 (see the SI), respectively. The optimized
geometry of the quintet macrocycle (Figure S18B) has a
perfect ring structure; however, two pyridine units of the
(peaks a−d) and DASA-Mel (peaks e−k). In the case of
BB⊂DASA-Mix, the peaks corresponding to the macrocycle
are clearly present in the solution, and the characteristic peaks
due to DASA-Mel signify the abundance of only DASA-Mel as
an encapsulated species from the mixture of DASAs in solution
(Figure 4). A closer look at Figure 4 clearly shows that both
the NMR plots I and II are similar, along with the presence of
peaks a−d (BB) and e−k (DASA-Mel) in the case of
BB⊂DASA-Mel and BB⊂DASA-Mix. These findings clearly
indicate the selective encapsulation of DASA-Mel when used in
a mixture of two DASAs inside BB. The areas under peaks b
Figure 5. Optimized structures of BB⊂DASAs, i.e., (a) encapsulated DASA-Mel and (b) encapsulated DASA-Bar.
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macrocycle maintain a propeller-like orientation in singlet spin
configuration. Interestingly, the singlet states of the macro-
cycle, encapsulated DASA-Mel, and DASA-Bar are energeti-
cally far more stabilized than their quintet states.
supported by NMR analyses. Also, we found through the
DFT optimization study that the open form of DASA-Mel has
better stabilization during the encapsulation reaction in water
medium than DASA-Bar, since the binding energy of DASA-
Mel is −0.3 kcal/mol higher than that of DASA-Bar, indicating
more stability of the former inside the macrocycle in
comparison to DASA-Bar. TDDFT showed excellent corrob-
oration of the UV−vis result with the experimental findings.
We proved our claim of selective separation through various
1H NMR and DOSY NMR spectroscopic studies and
theoretical calculations. Hence, this new approach of attaining
unusual stability of DASAs in the presence of an organic
molecular container in water may become popular due to the
avoidance of the toxicity associated with the palladium metal-
based cage reported previously. Also, the selective separation
and high lability toward encapsulation of DASA-Mel over
DASA-Bar may become useful for scientific communities
working in this newly discovered field in understanding
kinetics.
Therefore, here, we mostly highlight the data related to the
singlet spin configuration. Consistent with the experiment, the
open form of DASA-Mel inside the macrocycle is energetically
more stable than that of DASA-Bar. The calculated binding
energies of encapsulated DASA-Mel and DASA-Bar are −10.2
and −9.9 kcal/mol, respectively. The computed inner cavity
breadths of the singlet macrocycle and encapsulated DASA-
Mel and DASA-Bar are 7.392, 7.315, and 7.335 Å, respectively,
which signifies that DASA-Mel inside the macrocycle is better
fitted than DASA-Bar. Again, the computed inner cavity
lengths of the singlet macrocycle and encapsulated DASA-Mel
and DASA-Bar are 10.047, 10.143, and 10.128 Å, respectively,
which signifies that the encapsulation cavity length increases
more in the case of encapsulated DASA-Mel. As both the
theory and the experiment support that DASA-Mel inside the
macrocycle is more favorable than DASA-Bar, we performed
TDDFT calculations on the optimized geometry of only
encapsulated DASA-Mel. It is seen from Figure 5a that the
central alkene subunit of DASA-Mel resides inside the cavity of
the macrocycle and the two end naphthalimide moieties and
Meldrum acid cores are far from the macrocyclic ring. For
singlet spin, the first strong absorption peak appears at 450 nm
with an oscillator strength of f = 0.9050, which corresponds to
the HOMO to LUMO + 2 excitation (Figure S19A, see the
SI). Upon inspection of the MOs involved in this excitation
(Figure S19B, see the SI), it can be assigned as the π−π*
transition (97.7%). The computational band agrees excellently
with the experimental band at 460 nm.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.jpcb.1c03890.
■
sı
Structural presentation of the open forms of DASA-Mel
and DASA-Bar and the close forms of DASA-Mel and
1
DASA-Bar; H, 13C, and DOSY NMR spectra; HRMS
data; UV−vis and fluorescence spectra; images of the
solution after the encapsulation reaction; and total
electronic energies of various species at PCM-CAM-
B3LYP/6-31G** (PDF)
DASA-Mel, DASA-Bar, and the mixture separated from
the aqueous part to CDCl3 (MP4)
CONCLUSIONS
■
Finally, through this report we have depicted the synthesis of
two new DASAs (DASA-Mel and DASA-Bar) from derivatives
of Meldrum and barbituric acid, respectively, characterized well
by several spectroscopic techniques. These compounds show
reversible photo- and thermoisomerization in toluene from the
open-chain conjugated structure (hydrophobic in nature) to a
close-ring nonconjugated structure (zwitterionic and hydro-
philic). But irreversible photoisomerization from the open to
the close form is observable in polar solvents such as methanol
and water. The isomerization process is well understood
through various UV−vis spectroscopic experiments. Also,
judicious choosing of the naphthalimide fluorophore shows
interesting emission properties. The conjugated open form has
a high fluorescent intensity due to the internal charge transfer,
whereas the close form is nonfluorescent due to “ICT off” and
“PET on” states. Afterward, we used an organic macrocycle as
a container for storing the unstable and insoluble open form in
water. DASA-Mel showed high lability toward encapsulation in
comparison to DASA-Bar as evident from the NMR spectra,
UV−vis, and fluorescence spectral analyses of the encapsulated
species. We also encountered the existence of the encapsulated
DASA-Mel form (BB⊂DASA-Mel) through a HRMS experi-
ment. We mixed the two DASAs in water in the presence of
the macrocycle and observed the encapsulation process
thereafter. To our surprise, we noticed through the
competition experiments that DASA-Mel was selectively
encapsulated inside the container in the open form in water.
The color of the solutions was the factor responsible for us to
think about this selective separation process, which is
AUTHOR INFORMATION
Corresponding Authors
■
Koushik Dhara − Department of Chemistry, Sambhu Nath
College, Birbhum 731303, West Bengal, India; orcid.org/
0000-0001-9388-9600; Email: dharachem@gmail.com
Pabitra Chattopadhyay − Department of Chemistry, The
University of Burdwan, Burdwan 713104, West Bengal,
India; orcid.org/0000-0002-5923-7462;
Email: pabitracc@yahoo.com
Authors
Sujay Mukhopadhyay − Department of Chemistry, The
University of Burdwan, Burdwan 713104, West Bengal,
India; orcid.org/0000-0001-9453-0333
Arnab Sarkar − Department of Chemistry, The University of
Burdwan, Burdwan 713104, West Bengal, India
Sourav Ghoshal − Department of Chemistry, Visva-Bharati
University, Santiniketan 731235, West Bengal, India
Pranab Sarkar − Department of Chemistry, Visva-Bharati
University, Santiniketan 731235, West Bengal, India
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.jpcb.1c03890
Author Contributions
∥S.M and A.S. contributed equally.
Notes
The authors declare no competing financial interest.
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Resistant, Photoswitchable Fluorescent Spiropyran-Polyoxometalate-
ACKNOWLEDGMENTS
■
BODIPY Single-Molecule. Chem. Commun. 2015, 51, 16088−16091.
S.M. thanks UGC, New Delhi, India, for the Dr. D.S. Kothari
Postdoctoral Fellowship (Award letter no. F.4-2/2006(BSR)/
CH/18-19/0069). K.D. acknowledges the Department of
Science and Technology (DST), Fast track research grant,
New Delhi (Project no. SB/FT/CS-142/2012), for financial
support. A.S. acknowledges the State Fund, West Bengal, for
offering the fellowship. All authors gratefully acknowledge Dr.
Raghunath Dey, Indian Institute of Technology Ropar, Punjab,
India, for his help in NMR and mass spectral analyses.
̌
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