Light-Triggered Metal Coordination Dynamics in Photoswitchable Dithienylethene-Ferrocene System

Article abstract of DOI:10.1021/acs.inorgchem.1c00602

The C2-symmetric photochromic molecule 3, containing dithienylethene (DTE) and ferrocene units connected by an alkyne bridge, represents a unique probe where a metal (Hg2+) binds with the central DTE moiety. Both photoisomerized states of 3 (open, 3o; closed, 3c) are found to interact with Hg2+ ion by the S atoms of the DTE core; however, the binding constants (from a UV-vis study) and DFT calculations suggest that the open isomer (3o) binds with the metal ion more strongly than that of the closed isomer (3c). Notably, the course of metal binding does not perturb the inherent photoisomerization properties of the DTE core and the photoswitchability persists even in the metal-coordinated form of 3, however, with a comparatively slower rate. The quantum yields for photocyclization (φo→c) and photocycloreversion (φc→o) in the free form are 0.56 and 0.007, respectively, whereas the photocyclization quantum yield in the Hg2+ complexed species is 0.068, 8.2 times lower than the photocyclization quantum yield (φo→c) of free 3o. Thus, the rate of photoisomerization can be modulated by a suitable metal coordination to the DTE core. The dynamics of photoswitchability in the metal-coordinated form of DTE has been explored by experimental means (UV-vis and electrochemical studies) as well as quantum chemical calculations.

Full text of DOI:10.1021/acs.inorgchem.1c00602

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Article
Light-Triggered Metal Coordination Dynamics in Photoswitchable
Dithienylethene−Ferrocene System
Manisha Karmakar, Adwitiya Pal, Bijan Mondal, Nayarassery N. Adarsh, and Arunabha Thakur*
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ABSTRACT: The C2-symmetric photochromic molecule 3, containing
dithienylethene (DTE) and ferrocene units connected by an alkyne
bridge, represents a unique probe where a metal (Hg2+) binds with the
central DTE moiety. Both photoisomerized states of 3 (open, 3o;
closed, 3c) are found to interact with Hg2+ ion by the S atoms of the
DTE core; however, the binding constants (from a UV−vis study) and
DFT calculations suggest that the open isomer (3o) binds with the
metal ion more strongly than that of the closed isomer (3c). Notably,
the course of metal binding does not perturb the inherent photo-
isomerization properties of the DTE core and the photoswitchability
persists even in the metal-coordinated form of 3, however, with a
comparatively slower rate. The quantum yields for photocyclization
(Φo→c) and photocycloreversion (Φc→o) in the free form are 0.56 and
0.007, respectively, whereas the photocyclization quantum yield in the Hg2+ complexed species is 0.068, 8.2 times lower than the
photocyclization quantum yield (Φo→c) of free 3o. Thus, the rate of photoisomerization can be modulated by a suitable metal
coordination to the DTE core. The dynamics of photoswitchability in the metal-coordinated form of DTE has been explored by
experimental means (UV−vis and electrochemical studies) as well as quantum chemical calculations.
potential applications in photonic devices: e.g., optical
memories and display devices and optoelectronic devices
such as waveguides, sensors, etc.6 Various photochromic metal
complexes have also been developed by several research groups
for the development of organic optical transistors,7 NLO
switches,8 organic light-emitting diodes (OLED),9 energy
storage systems (batteries),10 and many more. Thus, DTE
derivatives have been of recent interest due to their wide scope
of application.
Several research groups have focused on the study of metal-
coordinated dithienylethene (DTE) backbones in different
fields. A Zn2+ bipyridyl complex attached to a DTE core has
been utilized in the reversible switching of an NLO response,11
and a Cu+ bipyridyl complex with an appended DTE moiety
has been employed in monitoring quadratic NLO properties.12
Also, Pt terpyridine acetylide complexes with pendant DTE
moieties have been used in studying the resultant photophysics
where the two components (Pt complex and DTE) have
different intramolecular energy transfer processes.13 Recently,
INTRODUCTION
■
Stimuli-responsive organic materials with inherent photo-
switchable properties constitute a burgeoning field of research,
especially in materials science1 and structural biology.2 From
the pioneering work of Irie in the late 1980s and early 1990s
photochromic diarylethenes (DAE)3 were primarily associated
with dithienylethene (DTE), which undergoes 6π electrocyclic
ring closing or opening to obtain closed and open isomers
under irradiation of UV and visible light, respectively (Scheme
1).4 The open (o) and closed (c) forms of DTE exist as two
distinctly addressable and thermally stable states. They even
respond independently to electrochemical and photochemical
stimuli.5 Utilizing its open−closed photoswitchable property in
the presence of light, DTE-containing compounds have
Scheme 1. Photoisomerization in DAE/DTE Systems
Received: March 1, 2021
Published: April 8, 2021
https://doi.org/10.1021/acs.inorgchem.1c00602
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Figure 1. Synthetic route of compound 3: (i) NBS, glacial AcOH, rt, 2 h; (ii) n-BuLi in THF, TMSCl, −78 °C to rt, 12 h; (iii) n-BuLi in THF,
octafluorocyclopentene, −78 °C to rt, 18 h; (iv) NBS, glacial AcOH, reflux, overnight; (v) Pd(PPh3)4, CuI, Et3N, 65 °C, 7 h.
Clever and co-workers have developed coordination cages
based on a Pd(II) metal ion and DTE unit, which allow
modulation of guest uptake and release by irradiation with light
of different wavelengths.14 Furthermore, there are many
examples of chemosensors, based on DTE derivatives,15
where heteroatoms containing fluorophores are attached to
the DTE unit and those heteroatom-containing fluorophores
assist or participate in metal coordination.16 All such probes
are designed in a way so that the metal ions do not attach to
the sulfur atoms of the DTE core itself and consequently do
not intervene directly in the photoisomerization process;
rather, they bind with a remote binding unit attached to the
DTE core.
In order to explore the coordinating capability of the DTE
unit toward metal ions, the design of the molecule should be
such that there is a minimal number of other possible binding
sites available for the metal ion. Thus, any additional
heteroatom-containing fluorophore unit should principally be
avoided in the molecular design. In turn, to get access to both
thiophene units at a time, an alkyne moiety (ethynylferrocene)
could be attached to the DTE unit, as an alkyne group is
known to provide linearity in a structure17 and therefore can
impart rigidity and additional stability to the whole structure.
On the other hand, ferrocene is a well-established electro-
chemically responding unit, which gives a pair of reversible
waves in voltammetric studies and is stable under moist and
aerobic conditions. The DTE unit itself is also electrochemi-
cally active by producing irreversible waves,18 but to get to a
conclusive electrochemical study, a standard redox moiety with
reversible waves is beneficial for a comparative output. Also,
there is a negligible scope of binding by the ferrocene unit,
such that it does not compete with the binding experiments of
the DTE unit.
soft Hg2+ ion, in both its open (3o) and closed (3c) forms for
the first time. In this study we are able to show that the
photoisomerization in the DTE core remains unhindered but is
decelerated upon binding with an Hg2+ ion in the same unit.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The synthesis of the
target symmetrical ferrocene-based DTE moiety connected via
an alkyne bridge is designed as depicted in Figure 1. In the
design of our molecular architecture, we wish to combine the
photochromic unit (DTE) with a redox-active ferrocene
moiety via an alkyne (CC) bridge, anticipating that the
alkyne (CC) bridge may impart rigidity to the molecule.
However, a Sonogashira coupling between the diiodo DTE
derivative (I-DTE-I) and ethynylferrocene in the presence of
Pd(PPh3)4 and CuI is known to produce the di-1,4-
ferrocenylbutadiyne (“Glaser product”) by homocoupling of
ethynylferrocene as a byproduct along with the desired product
(by Sonogashira coupling).21 Therefore, to minimize the self-
coupling reaction and to yield the Sonogashira product as the
only product, we have introduced the CH2−O−CH2
functionality as a spacer between the terminal alkyne group
and the ferrocene unit. This derivatization was expected not to
promote the “Glaser” coupling reaction, and indeed it led to
the formation of a Sonogashira coupling product (3) as the
sole product in good yield. Compound 3 was obtained as a
bright yellow solid in 85% yield by a Sonogashira cross-
coupling reaction between Br-DTE-Br (1) and ferrocene-3-
ethoxyprop-1-yne (2), in the presence of Pd(PPh3)4 catalyst,
CuI cocatalyst, and Et3N base at 65 °C (Figure 1 and Figure
1
S1). The molecule 3 was fully characterized by H, 13C, and
19F NMR, HRMS, IR spectra and elemental analysis.
Additionally, the molecular structure of compound 3 was
unambiguously established by a single-crystal diffraction
analysis (Figure 2a). 3o crystallizes in the triclinic space
Unlike the vast majority of DAE or DTE photochromes
containing metal complexes,14,19,20 our designed compound
represents a unique example in which the Hg2+ ion directly
coordinates to the S atoms of thiophene rings of the
photochromic unit, thereby intervening directly in the
dynamics of the photoisomerization process. Herein, we have
designed and synthesized the molecule 3, devoid of any
additional binding site or fluorophore, and it was utilized to
evidence the binding activity of the DTE core itself with the
̅
group P1. In the crystal structure, the compound undergoes
intermolecular hydrogen bonding through C−H of the
ferrocene and an F atom present in the backbone moiety
(C−H···F = 3.248(10) Å, ∠C−H−F = 126.88°), resulting in a
two-dimensional hydrogen-bonded porous network (Figure
2b). The 2D sheets are further assembled on top of each other
through various van der Waals interactions (Figure 2c).
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inclusion of ferrocene into the design of molecule 3 is
necessary as well as beneficial, because it not only serves as an
internal standard in voltammetric studies but also acts as a
chromogenic unit, giving a vivid colorimetric response from
pale yellow to blue (10−3 M CH3CN solution) upon
interaction with the Hg2+ ion (Figure 3b). The color change
perceptible to the naked eye upon metal coordination to the
DTE moiety is always advantageous to comprehend the
photoswitching dynamics of a metal-chelated DTE moiety
more rapidly. The noninteractive nature of compound 1 may
be due to the strong electron-withdrawing property of the two
bulky Br atoms, rendering the S atoms of DTE unit less
nucleophilic, and may generate steric hindrance to accom-
modate the Hg2+ ion.
UV−Vis Absorption Study. The photochromic DTE unit
gives rise to some characteristic undulations in the UV−vis
spectra.21 The UV−vis spectra of many compounds containing
the DTE unit have been reported in the literature; however,
each new orientation of atoms results in a unique property.
The synthesized molecule 3o was designed with the intention
of investigating the reversible photoswitching behavior of the
DTE unit in the presence of chelating metal ions. To
investigate the behavioral absorption fingerprint in our
designed architecture, 3o was exposed to UV irradiation
having a monochromatic wavelength of 365 nm and to visible
light.
As shown in Figure 4a, 3o displayed an absorption peak at
263 nm (ε = 3.66 × 103 M−1 cm−1) due to a π−π* transition
in the open form.22 The irradiation of 365 nm UV light for the
transformation from 3o to 3c was done periodically with 5 s of
irradiation time, and the consequent UV−vis spectra were
recorded. When a CH3CN solution of 3o (3 mL, 5 × 10−5 M)
was periodically irradiated by 365 nm UV light for 5 s, the 263
nm peak showed a slight increment in intensity, whereas a new
shoulder peak at 581 nm (ε = 3.01 × 102 M−1 cm−1),
characteristic of the closed DTE unit,23 appeared. The peak
intensified with gradual irradiation, and after 5 min (300 s) of
irradiation, saturation was obtained, signifying almost full
conversion from 3o to 3c, accompanied by a color change to
violet from pale yellow (Figure 5).
Figure 2. (a) X-ray structure of 3o drawn with thermal ellipsoids at
the 50% probability level. (b) Porous 2D hydrogen-bonded network
structure. (c) Parallel packing of the 2D networks sustained by van
der Waals interactions.
Our objective for synthesizing compound 3 was not to
utilize this molecule as a selective sensing probe for a particular
metal ion; rather, we were interested in monitoring the change
in photoswitching efficiency upon coordination of any metal
ions to the core DTE unit, anticipating some influence on the
dynamics of photoswitching if any metal ions coordinate
directly to the DTE core. However, we have screened several
metal ions (Fe3+, Zn2+, Cu2+, Fe2+, Hg2+, Pb2+, Ag+, Co2+, Ni2+,
Pd2+, Cu+, and Cd2+) to check the binding affinity and
coordination behavior toward the S atoms of the DTE unit and
control the chemistry of photoswitching. Addition of all the
aforementioned metal ions, even in large excess, in CH3CN
solution to the receptor 3 did not impart any color changes
visible to the naked eye except for Hg2+, Fe3+, and Cu2+ ions
(Figure S8a). Interestingly, for the sake of comparison, we have
also screened compound 1 as a reference, containing the S
atoms of DTE, with all the aforementioned metal ions,
anticipating the type of interaction similar to that of compound
3. However, no color change of a colorless acetonitrile solution
of 1 (10−3 M) in the presence of any of the same series of
metal ions (Figure S8b) could be observed by the naked eye
(Figure 3a), which would usually occur for any kind of
interaction between two species. Further, the absence of any
such interaction was validated by UV−vis and electrochemical
studies, which indicate no changes upon interaction of any of
the aforementioned metal ions, including the Hg2+ ion, with
compound 1 (vide supra, Figures S9−S11). Therefore, the
In order to investigate the metal-chelation ability of 3o and
3c along with the dynamics of the phototransformation from
3o to 3c in free and metal-chelated forms, the interactions of
3o with several metal ions were investigated by UV−vis spectra
(Figure S12). Only Hg2+, Fe3+, and Cu2+ ions displayed
positive responses, among which the responses produced by
Fe3+ and Cu2+ ions originate from the oxidation of ferrocene
(Fe2+) to ferrocenium ion (Fe3+), the formation of which was
confirmed by the appearance of a new characteristic absorption
Figure 3. Visible color changes of (a) 1 (open and closed) and (b) 3 (open and closed) in the presence of Hg(ClO4)2 in 10−3 M CH3CN solution.
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Figure 4. (a) Changes in the absorption spectra of 3o by gradual irradiation with 365 nm UV light in CH3CN (inset: variation of absorption
intensity at 581 nm against irradiation time, upon transforming from 3o to 3c). (b) Changes in the absorption spectra of 3o (5 × 10−5 M) upon
gradual addition of Hg2+ ion up to 1 equiv in CH3CN solvent (inset: enlarged portion representing the isosbestic point at 259 nm). (c) Changes in
the absorption spectra of 3c upon gradual addition of Hg2+ up to 1 equiv (inset: variation of absorption intensity of 3c at 581 nm with [Hg2+] and
its comparison with 3o for the same). (d) Changes in the absorption spectra of [3o·Hg2+] upon irradiation with 365 nm UV light (inset: enlarged
portion representing the emergence of a peak at 581 nm).
peak at around 630 nm24 (Figure S13). The back-reduction of
the oxidized ferrocenium ion (Fe3+) to ferrocene (Fe2+) again
was also achieved by adding sodium L-ascorbate, a reducing
agent, to the acetonitrile solution of the oxidized species,
indicating the reversible oxidation−reduction nature of the
interaction (Figure S14). On the other hand, the Hg2+ ion did
not promote any such redox interaction; rather, it induced the
formation of a complex with 3o and 3c. Therefore, the
interactions of 3o and 3c with Hg2+ were carefully monitored
to execute a study of the phototransformation in the metal-
chelated form. Upon stepwise addition of Hg2+ ion (5 × 10−5
M) up to 1 equiv in CH3CN solutions of 3o (5 × 10−5 M)
(Figure 4b) and 3c (5 × 10−5 M) (Figure 4c) separately, the
absorption maximum at 263 nm decreased in intensity with the
emergence of a new peak at 240 nm (Figure S15). Two
distinct isosbestic points at 211 and 259 nm appeared for 3o
along with a color change from pale yellow to blue (Figure 5).
However, no well-defined isosbestic point was obtained for 3c,
and the band at 581 nm diminished slightly by Hg2+ addition,
along with a change in color of the solution from violet to
green (Figure 5). Binding stoichiometries of 1:1 were
determined for both 3o and 3c with the Hg2+ ion from Job’s
plot (Figure S16) and elemental analysis. Binding constant
values of 3o and 3c with Hg2+ were determined from the
equation25 1/[A − A0] = 1/K(Amax − A0)[Hg2+]n+ 1/(Amax
−
A0), where, Amax is the maximum absorbance; A0 is the initial
absorbance of free ligand (3) and A is the absorbance after
addition of Hg2+. The binding constant (K) values for both
isomers (3o and 3c) are 4.779 × 104 and 2.149 × 104 M−1 (log
K = 4.679 and 4.332 for 3o and 3c), respectively (Figure S17),
indicating a slightly stronger binding (2.22 times) of 3o with
Hg2+ in comparison to 3c: i.e., this suggests that 3c is a weaker
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competitor than 3o for binding with Hg2+. To elucidate the
efficiency of electrocyclization and cycloreversion, quantum
yield calculations were performed. The quantum yields for
photocyclization (Φo→c) and cycloreversion (Φc→o) of 3 were
found to be 0.56 and 0.007, respectively (Figure S18 and Table
S3), in hexane with 1,2-bis(2-methyl-5-phenyl-3-thienyl)-
perfluorocyclopentene as a reference.26 The quantum yield
was evaluated from the equation
i
j
j
j
j
j
y
z
z
z
z
z
i
j
j
j
j
y
z
z
z
z
slope(3)
slope(ref)
ε(ref)
ε(3)
Φ(3) =
×
× Φ(ref)
k
{
k
{
where slope is the slope of the linear plot, extracted from A
(absorbance) vs t (irradiation time) plot, ε is the molar
extinction coefficient of the concentration 5 × 10−5 M at 574
nm for the reference and at 581 nm for 3, and Φ is the
quantum yield.
Figure 5. Schematic representation of the photoresponsive switching
process of 3o and 3c in the presence of Hg(ClO4)2 and lights of
different wavelengths.
Moreover, in order to investigate the open to closed
isomerization process in the metal-chelated form, a CH3CN
solution of [3o·Hg2+] was subjected to UV irradiation (λ = 365
Figure 6. (a) Cyclic voltammetry (CV) and (b) differential pulse voltammetry (DPV) of the open form (3o) to closed form (3c) upon UV
irradiation (λ = 365 nm). (c) Cyclic voltammetry (CV) and (d) differential pulse voltammetry (DPV) of 3o (open form) upon addition of Hg2+ up
to 1 equiv in CH3CN solution with [(n-Bu)4N]ClO4 as the supporting electrolyte at a 0.06 V s−1 scan rate.
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Figure 7. (a) Cyclic voltammetry (CV) and (b) differential pulse voltammetry (DPV) of 3c (closed form) upon addition of Hg2+ up to 1 equiv. (c)
Cyclic voltammetry (CV) and (d) differential pulse voltammetry (DPV) of [3o·Hg2+] upon UV irradiation (λ = 365 nm) in CH3CN solution with
[(n-Bu)4N]ClO4 as the supporting electrolyte at a 0.06 V s−1 scan rate.
nm), with 5 s intervals (Figure 4d). The characteristic band for
3c at 581 nm (ε = 4.36 × 102 M−1 cm−1) appeared after 15 s of
irradiation of the [3o·Hg2+] complex, whereas it appeared after
5 s for free 3o. The band at 240 nm (ε = 26.91 × 103 M−1
cm−1), which appeared due to the interaction of 3o with Hg2+,
remained unperturbed even after irradiation of [3o·Hg2+] for a
long time, and the formation of peak at 581 nm is very slow for
the [3o·Hg2+] solution. This slower kinetics of ring closing in
the metal-chelated form, [3o·Hg2+], was also supported by the
lower (8.2 times) quantum yield (Φo→c) of 0.068 (Figure S18
and Table S3), in comparison to that of free 3o. This shows
that, although [3o·Hg2+] gets converted to [3c·Hg2+], the rate
of conversion27 of free 3o to 3c (k = 0.01 s−1) is faster than the
rate of conversion of [3o·Hg2+] to [3c·Hg2+] (k = 0.007 s−1),
as is also confirmed by kinetic studies, using UV−vis spectra
(Figure S19). The binding equilibrium in 3c is not being as
stable as that for the open isomer may be due to the formation
of a strained and puckered five-membered C−S−Hg−S−C
ring, as indicated by DFT calculations (vide infra).
with every aliquot of Na2EDTA addition to a CH3CN solution
of the [3o/3c·Hg2+] complex, the UV−vis spectrum
corresponding to free 3o/3c was generated, whereas upon
Hg2+ addition to the same solution, the UV−vis spectrum
corresponding to [3o/3c·Hg2+] was regenerated. This
experimental cycle could be repeated two times without
much loss of sensitivity; however, in the third cycle the
sensitivity decreased slightly (Figure S20).
Electrochemical Studies. To realize the changes occur-
ring in DTE derivatives in the presence of metal ions by
electrochemical studies was one of the principal objectives of
including a ferrocene unit within the core of the designed
molecule. The electrochemically reversible behavior of
ferrocene28 and irreversible nature of dithienylethene29 in 3o
was observed in cyclic voltammetry (CV) and differential pulse
voltammetry (DPV) experiments, which were carried out at
room temperature in CH3CN solution (1.25 × 10−4 M)
containing 0.1 M [(n-Bu)4N]ClO4 (TBAP) as the supporting
electrolyte.
In order to assess the kind of interaction (in terms of
reversibility) between 3o/3c and Hg2+, an aqueous solution of
Na2EDTA (5 × 10−5 M) was employed as a decomplexing
agent into the corresponding CH3CN solution. The nature of
the interaction of both 3o and 3c with Hg2+ is reversible: i.e.,
To start the electrochemical studies, the CV of 3o was
recorded, which displayed a reversible oxidation wave at 0.426
V corresponding to the ferrocene/ferrocenium couple30 and an
irreversible wave at 1.11 V characteristic of the dithienylethene
(DTE) core.31 Upon UV irradiation of 3o by 365 nm light, an
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1
Figure 8. Comparative H NMR spectra of 3o in the presence of 1 equiv of Hg2+ in CD3CN as a solvent.
increment of the irreversible wave at 1.11 V was observed
along with a slight anodic shift to 1.16 V (ΔE1/2 = 50 mV),
which marked the transformation of 3o to 3c (Figure 6a,b).
The increment of intensity of the irreversible wave upon 3o to
3c conversion may be attributed to the fact that the closed
form is electrochemically more active than the open DTE
core.32
presence of a bonded Hg2+ ion, which is consistent with other
experimental results and DFT calculations. In all of the above
experiments, the ferrocene/ferrocenium oxidation wave
remained almost undisturbed. A tabular depiction of data
(Table S4) and normalized electrochemical measurements
(Figure S21) present the changes more clearly, supporting the
anticipation of participation of S atoms of the DTE core in
both [3o·Hg2+] and [3c·Hg2+] complexes.
An acetonitrile solution of Hg2+(1.25 × 10−4 M) was added
stepwise into CH3CN solutions of 3o and 3c separately, to
conduct binding studies using an electrochemical method. The
oxidation wave at 0.426 V for both 3o (Figure 6c,d) and 3c
(Figure 7a,b) did not exhibit any shift in potential, whereas the
peaks at 1.11 and 1.16 V respectively shifted anodically to 1.17
and 1.19 V (ΔE1/2 = 60 and 30 mV), respectively, and became
flattened with the gradual addition of Hg2+ ion up to 1 equiv.
The significant perturbation of the DTE oxidation wave upon
Hg2+ addition supports the possibility of interaction of 3o and
3c with Hg2+ by the DTE core. In 3o, the wave flattened
readily after addition of a small amount of Hg2+ ion, but in 3c,
the restoration of its own property exceeds the effect of Hg2+
binding for up to a large quantity of Hg2+ ion. The retainment
of the nature of the oxidation wave of DTE core of 3c until
quantitative addition of Hg2+ ion proved the sluggish
interaction of 3c in comparison to 3o with Hg2+ ion.
1
Comparative H NMR and IR Studies. The ring-closure
process was further monitored by 1H NMR study, which
indicated the complete conversion of the open form to the
closed form upon irradiation. 1H NMR was studied thoroughly
to explore the photoisomerization process as well as the
chelation phenomenon. Upon conversion of 3o to 3c, in
CD3CN, the characteristic singlet peak for methyl protons
shifted downfield (from 2.16 to 2.21 ppm) and the signal
assigned to thienyl protons remained unchanged (Figure S7).
To confirm the plausible binding mode of 3o with Hg2+, 1H
NMR spectra were recorded in CD3CN solution. The spectral
changes of 3o and [3o+ 1 equiv Hg2+] are depicted in Figure 8.
They exhibited a singlet at 2.16 ppm which corresponds to Me
protons, peaks at around 4.14 and 4.36 ppm for substituted
ferrocene ring protons and OCH2, respectively, and another
singlet for the aromatic ring proton of the thiophene ring. The
Me protons (Ha), close to coordinating S atoms, shifted
upfield33 from 2.16 to 1.96 ppm (Δδ = 0.20 ppm), whereas no
such displacement was observed for ferrocenyl (Fc), −OCH2,
and thiophene ring (Hb) protons. Hence, the plausible binding
mode in 3o for Hg2+ (soft acid) is via S atoms (soft base)
present in the DTE unit. After the addition of excess Hg2+, no
further change indicated a 1:1 binding stoichiometry. These
Upon irradiation of the [3o·Hg2+] complex with 365 nm UV
light (gradually up to a total of 10 min), a substantial difference
in the irreversible oxidation wave was observed. The
irreversible wave, which had almost flattened due to complex-
ation, was regenerated, but in an indistinct way and to a much
lesser extent (Figure 7c,d). This clearly signifies the resistance
to the conversion of the open form to the closed form in the
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Figure 9. (a) Comparative IR spectra of 3o in the presence of 1 equiv of Hg2+ in the solid state at room temperature. (b) Enlarged image of IR
stretching of the alkyne (−CC−) peak of 3o before and after addition of Hg(ClO4)2.
Figure 10. Energy-minimized structures of 3o ((a) antiparallel and (b) parallel), (c) 3c, (d) [3o·Hg(ClO4)2], and (e) [3c·Hg(ClO4)2].
results suggest that 3o may chelate with Hg2+(soft acid) using
S atoms (soft base) present in the DTE unit and ferrocene
units do not participate in the binding mechanism. This result
also corroborates the CV and DPV studies.
functional theory (DFT) in order to understand the structural
and electronic parameters. The coordinates of the starting
geometry of 3o (Figure 10a) were taken from those of the
solid-state structure (Figure 2a). However, 3c (Figure 10c)
was derived from the optimized geometry of 3o. Various
starting points of [3o·Hg(ClO4)2] and [3c·Hg(ClO4)2] were
finally converged, which led us to conclude their minimized
conformation (Figure 10d,e, respectively).
In Figure 10, we have reported the energy-minimized
conformers for the parallel and antiparallel forms of the open
and closed forms of compound 3. As was determined, in the
antiparallel form (Figure 10a) the ferrocene-containing arms
are well separated. The situation is similar in the closed
structure 3c (Figure 10c) ,which is 66.2 kJ/mol less stable than
the antiparallel open form 3o (Table S5). The energy values
are consistent with the easy conversion from closed to open
conformers. This is also supported by a low energy barrier of
61.6 kJ/mol computed for the closed to open process. This
energy difference between the antiparallel open and closed
isomers in DTE photochromes is expected.36 Further, the
newly formed elongated C−C bond of 1.548 Å suggests 3c to
In order to get further details about the binding behavior, IR
spectra were recorded with Hg2+ (as its perchlorate salt). The
IR spectra of free 3o and [3o + 1 equiv Hg2+] were measured
in the solid state (Figure 9). On addition of Hg(ClO4)2, the
peak at 2220 cm−1, which was assigned to the CC stretching
frequency,34 remained unperturbed and a strong band at 1086
cm−1 due to Cl−O stretching35 appeared, indicating the
inclusion of Hg(ClO4)2 unit in 3o (see the IR sample
preparation below). The above observation indicates that the
alkyne (CC) unit does not participate in coordination to
Hg2+, probably due to the adjacent OCH2 groups which make
the alkyne units less susceptible to react with Hg2+ ion in
1
comparison to the S atoms. Thus, H and IR titrations come
together in unison to suggest a plausible binding mode where
3o coordinates Hg2+ through the S atoms of the DTE core.
Theoretical (DFT) Studies. Solution-phase theoretical
calculations were carried out on the ground of density
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be an unstable product.37 Nevertheless, topological analyses of
the frontier orbitals reveal that the LUMO of 3o indeed has a
bonding character between the two reactive C atoms that
indicates the possible formation of the closed product 3c.
Notably, the parallel conformer (Figure 10b) is nearly
isoenergetic with the antiparallel open form and is unstable
by only 4.06 kJ/mol with respect to the antiparallel open
conformer (this difference, smaller than 4.2 kJ/mol, is
negligible at this level of theory). Therefore, in solution, 3o
exists as a blend of parallel and antiparallel conformers.
However, the energy-minimized geometry of 3o shown in
Figure 10a, which is the antiparallel open form, corresponds to
the SCXRD structure (Figure S22).
The optimized geometries reveal that the free ligands
maintain their core geometries even after binding with the
analyte, Hg2+. The optimized geometries of [3o·Hg(ClO4)2]
and [3c·Hg(ClO4)2] show that in both cases Hg2+ adopts a
distorted-tetrahedral geometry by connecting to two S and two
O atoms. Although the S atoms of the DTE core in the
molecules 3o/3c are taking part in the interactions with the
Hg2+ ion, the Hg−S bonds are relatively weak38 (Wiberg bond
index values of ca. 0.2), as evidenced from their Hg−S bond
lengths (in [3o·Hg(ClO4)2] 3.75, 3.69 Å; in [3c·Hg(ClO4)2]
3.71, 3.69 Å). Notably, [3c·Hg(ClO4)2] is unstable by 79.3 kJ/
mol with respect to [3o·Hg(ClO4)2]. Further, the computed
energy of the reaction suggests that the complexation of 3o to
Hg2+ is 4.7 times more exergonic (−22.7 kJ/mol) than the
formation of the complex [3c·Hg(ClO4)2] (−4.8 kJ/mol). The
calculations also suggest a dynamic coordination process that
exists in the solution phase (Figure 11). The results suggest
HOMO−LUMO gap in [3o·Hg(ClO4)2], whereas the frontier
molecular orbital energies in 3c and [3c·Hg(ClO4)2] are nearly
comparable.
To get more information about the electronic transitions,
TD-DFT (CAM-B3LYP) calculations have been performed on
both the free ligands and the complexes in the solution phase
(acetonitrile). The TD-DFT calculations on 3o show that the
S0 → S1 transitions have a large oscillator strength (f = 1.28)
(Figure S24 and Table S6) and the excitation wavelength to
the S1 state is calculated to be 267 nm (Table S6). This
encompasses the transitions between HOMO-5 to LUMO+1
and HOMO-4 to LUMO+2. These are mainly of ligand to
ligand charge transfer (LLCT) character that is mainly
centered on the ethynyl and thiophene moieties. On the
other hand, the calculated transition at 288 nm for [3o·
Hg(ClO4)2] (Figure S26, and Table S6) involves two major
transitions of MLCT (n(Fe) → π*(C5F6 ring)) and LLCT
character (π orbital centered on ethynyl and thiophene to π*
of the C5F6 ring). Calculations on 3c showed that the
excitation wavelengths to the S1 state are at 302, 344, and 553
nm with oscillator strengths of 0.2, 0.17, and 0.49, respectively
(Figure S25 and Table S6). The transition at 344 nm is due to
an Fe-based lone pair to the delocalized π* orbital of the ligand
(MLCT character), and the transitions at 302 and 553 nm are
due to π → π* transitions of the central part of the ligand
(LLCT character). The TD-DFT calculation on [3c·Hg-
(ClO4)2] showed two major transitions to S1, calculated to be
697 and 572 nm, which are due to a ligand-based π → π*
transition and a ligand-based π → empty orbital centered on
Hg, respectively (Figure S27 and Table S6).
CONCLUSIONS
■
In conclusion, we have reported an unprecedented protocol
where the coordination strength of a metal ion can
discriminate the photoswitchable states of a DTE unit via a
simple naked-eye color change and electrochemical analysis; all
of the experimental results were further supported by DFT
calculations. Moreover, from the dynamics study in terms of
quantum yield and rate constants, it can be established that
metal coordination slows down the inherent photoswitchable
nature of the DTE core. The electrochemical studies by CV
and DPV emphasizes the differential responses of the open and
closed isomers of 3 toward Hg2+, as well as the effect of metal
chelation on photoinduced transformations. The inclusion of a
rigid alkyne moiety and a chromogenic ferrocene unit seems to
be crucial for the successful practical application of this type of
metal-chelated photocontrolled probe. We believe that such
fine tuning by metal chelation and the suitable design of DTE-
based molecular architecture may open up a new avenue for
the development of photocontrolled metal-coordinating smart
molecular switches.
Figure 11. Energy diagram of the reaction of 3c/3o with Hg2+,
calculated at the B3LYP/6-31(d,p)LANL2DZ level of theory.
that [3c·Hg(ClO4)2] either may convert into free 3o by
releasing Hg2+ (which can further combine into [3o·
Hg(ClO4)2]) or may convert directly into the [3o·Hg(ClO4)2]
(“3o-Hg2+”) complex through intramolecular ring opening.
This further shows the instability of the [3c·Hg(ClO4)2]
complex, suggesting a low possibility for the formation of the
complex.
The DFT-calculated MO analyses revealed that, due to
extension in plane conjugation, the LUMO of 3c is stabilized
by 1.11 eV in comparison to that of 3o (LUMO: −1.82 eV
(3o) and −2.92 eV (3c)). This causes a decrease in the
HOMO−LUMO gap in 3c, suggesting a red shift in the
absorption spectra (Figures S23−S25); however, no shifts were
observed experimentally. The complex formation between 3o
and Hg2+ resulted in destabilization of the HOMO of [3o·
Hg(ClO4)2] by 0.97 eV that also resulted in a decrease in the
EXPERIMENTAL SECTION
■
Materials and Measurements. Unless mentioned otherwise, all
of the analytes used for synthesis were purchased from commercially
available suppliers and used without further purification. Perchlorate
salts of metals (Zn2+, Fe2+, Fe3+, Pb2+, Hg2+, Ag+, Cu2+, Ni2+, Pd2+ and
Cd2+), CoCl2·6H2O, and [Cu(CH3CN)4]PF6 were purchased from
Sigma-Aldrich. K2CO3, NBS, and glacial acetic acid were purchased
from local chemical companies. DMF, acetonitrile (HPLC), and THF
were purchased from Merck Chemicals. 2-Methylthiophene and 1.6
M n-BuLi in hexane were purchased from Alfa Aesar. Trimethyl-
chlorosilane, (trimethylsilyl)acetylene, and octafluorocyclopentene
were obtained from TCI Chemicals. Tetrakis(triphenylphosphine)-
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palladium(0), used as a catalyst for Sonogashira coupling, was
purchased from Sigma-Aldrich. All solvents were dried and degassed
prior to use, and the reactions were performed using Schlenk
techniques under a nitrogen atmosphere. The dithienylethene
derivative 139,40 and ferrocenyl precursor 241 were prepared as
previously reported. All of the solvents were either HPLC or
spectroscopic grade in the optical spectroscopic studies. The reactions
were monitored by thin-layer chromatography on silica gel plates, and
column chromatography was conducted on a silica gel (mesh 60−
120) column of 2.5 cm diameter. A conventional three-electrode
configuration setup was used to perform cyclic voltammetry (CV) and
differential pulse voltammetry (DPV) experiments (10−4 M), where
glassy carbon, platinum, and silver/silver chloride (Ag/Ag+) were
used as the working electrode, auxiliary electrode, and reference
electrode, respectively. Tetrabutylammonium perchlorate, [(n-Bu4)-
N]ClO4, was employed as the supporting electrolyte at a scan rate of
0.06 V s−1 for carrying out the experiments.
H, 2.69. Found: C, 40.72; H, 2.81. IR (KBr): ν 2220.84 cm−1 (C
C).
X-ray Crystallographic Analysis. Single crystals of compound 3
were grown by slow diffusion of a EtOAc/hexane (7/3 v/v) solution.
Single-crystal X-ray data of compound 3o were collected using Mo Kα
(λ = 0.71069 Å) radiation at room temperature on a Microfocal D8
venture Bruker APEX 3 diffractometer equipped with a CCD area
detector. The unit cell parameters were refined using all of the
collected spots after the integration process. The structure was solved
by direct methods and refined by full-matrix least squares on F2 using
SHELXL97.42 All of the non-hydrogen atoms were refined with
anisotropic temperature factors. Hydrogen atoms were calculated and
refined in the riding mode.
Computational Details. All of the calculations (DFT and TD-
DFT) were carried out utilizing the Gaussian 09 (Rev. E. 01)43
package and were performed on a parallel cluster system. The ground-
state geometries were optimized without symmetry constraints by
employing the B3LYP functional,44−46 in combination with the basis
set 6-31g(d)47,48 for all nonmetallic atoms (C, H, O, F, S, and Cl) and
the double-ξ-quality basis set LANL2DZ49,50 for the metal atoms (Fe
and Hg). The optimized geometries were confirmed to be local
minima by performing frequency calculations and obtaining only
positive (real) frequencies. The zero-point-corrected energy values are
reported herein. On the basis of the optimized structures, the lowest-
energy vertical transitions were calculated (singlets, 15 states) by TD-
DFT, using the Coulomb-attenuated functional CAM-B3LYP51 at the
aforementioned level of calculation. All of the calculations (geometry
optimizations and frequency calculations, MO calculations, TDDFT)
have been performed including solvent effects through the polarizable
continuum model (PCM) that uses the integral equation formalism
variant (IEFPCM).
Preparation of the Sample for IR and Elemental Analyses of
the [3o·Hg2+] Complex. The pure compound 3o was dissolved in a
minimum amount of CH3CN, to which a 1 molar equiv CH3CN
solution of Hg(ClO4)2 was added. The two solutions were mixed up
progressively, and the solvent mixture was evaporated under reduced
pressure. The resulting blue solid compound was washed with H2O to
remove free perchlorate anion (if any), followed by filtration by
suction, and was dried under vacuum for several hours. The dry solid
complex was directly used for IR and elemental analysis.
1
Instrumentation. H and 13C NMR spectroscopic measurements
were recorded with a Bruker 400 MHz FT-NMR spectrometer using
TMS (SiMe4) as an internal reference at room temperature, and the
chemical shifts are reported in ppm. The shifts were referenced to the
residual solvents as follows: DMSO-d6 2.49 ppm (1H), CD3CN 1.94
ppm (1H), and CDCl3 77.15 ppm (13C). HRMS were collected from
a Waters HRMS Model XEVO-G2QTOF#YCA351 spectrometer.
FT-IR spectra were obtained with a PerkinElmer LX-1 FT-IR
spectrophotometer. UV and visible light irradiations were performed
with a lamp of wavelength 365 nm. A Shimadzu-UV-1900i UV−vis
spectrophotometer was used for the study of absorption spectra at
room temperature. Cyclic voltammetry (CV) was performed on a CH
Instruments electrochemical workstation. CHN analysis was per-
formed on a Vario EL elementar CHNS analyzer.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00602.
■
sı
Experimental details, NMR spectra, XRD data, photo-
physical and electrochemical studies, and additional
theoretical data (PDF)
Caution! Metal perchlorate salts are potentially explosive under certain
conditions. All due precautions should be taken while handling perchlorate
salts!
Accession Codes
CCDC 2039561 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Synthesis of Compound 3. An oven-dried Schlenk flask was
charged with compound 1 (0.271 g, 0.5156 mmol) in distilled Et3N
solvent under a nitrogen atmosphere. The solution was further
degassed by bubbling nitrogen for 45 min to remove dissolved O2.
Then the catalyst Pd(PPh3)4 (0.029 g, 0.0258 mmol) and CuI (0.006
g, 0.0361 mmol) were added under an N2 stream. Predried solid
compound 2 (0.327 g, 1.289 mmol) was added to the solution, and
the reaction mixture was stirred for 7 h at 65 °C. After 7 h of stirring,
the solution was cooled to room temperature and poured into H2O.
The organic layer was extracted by DCM and dried over Na2SO4. The
residue was purified by column chromatography on silica gel (ethyl
acetate/hexane 5/95) to afford the pure compound 3 (85%, yellow
AUTHOR INFORMATION
Corresponding Author
■
Arunabha Thakur − Department of Chemistry, Jadavpur
University, Kolkata 700032, India; orcid.org/0000-0003-
4577-3683; Phone: +919937760940;
1
solid). The synthesized compound 3 was fully characterized by H
Email: arunabha.thakur@jadavpuruniversity.in,
babuiitm07@gmail.com
NMR, 13C NMR, 19F NMR, HRMS, IR spectroscopy and elemental
analysis.
1H NMR (400 MHz, DMSO-d6): δ 7.31 (s, 1H, Hthiophene), 4.34 (s,
2H, OCH2), 4.32 (s, 2H, OCH2), 4.25 (s, 2H, HCp), 4.15 (s, 7H,
HCp), 1.94 (s, 3H, CH3). 13C NMR (100 MHz, CDCl3): δ 143.29
(CDTE), 136.44 (CDTE), 131.98 (CDTE), 124.72 (CDTE), 121.08
(CDTE), 115.74 (CDTE), 113.70 (CDTE), 90.75 (Calkyne), 82.21
(Calkyne), 69.67 (Ccp), 68.83 (Ccp), 68.63 (Ccp), 68.32 (Ccp), 57.42
(OCH2), 57.08 (OCH2), 14.54 (CH3). 19F NMR (400 MHz,
CDCl3): δ −110.29 (s, 4F), −131.87 (s, 2F). HRMS: m/z [M + H]+
calcd for C43H34O2S2F6Fe2 873.0681; found 873.1068. Anal. Calcd for
C43H34O2S2F6Fe2: C, 59.19; H, 3.93. Found: C, 60.8; H, 4.25. Anal.
Calcd for C43H34O10S2Cl2F6Fe2Hg (for [3o + Hg(ClO4)2]): C, 40.60;
Authors
Manisha Karmakar − Department of Chemistry, Jadavpur
University, Kolkata 700032, India
Adwitiya Pal − Department of Chemistry, Jadavpur University,
Kolkata 700032, India
̈
Bijan Mondal − Institut fur Anorganische Chemie, Universität
Regensburg, 93040 Regensburg, Germany; orcid.org/
0000-0002-7359-8926
Nayarassery N. Adarsh − Solid State and Materials Chemistry
Research Group, School of Chemical Sciences, Mahatma
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order nonlinear optical responses of two-dimendional Λ- and W-
shaped polyoxometalate derivatives of dithienylperfluorocyclopen-
tene. J. Phys. Chem. A 2013, 117, 10783−10789.
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A. Multifunctional switching of a photo- and electro-chemilumines-
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Gandhi University, Kerala 686560, India; orcid.org/
0000-0003-2262-8512
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00602
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
M.K. and A.P. are thankful to the STATE and CSIR for JRF
fellowships, respectively. The authors acknowledge a UGC
start up grant (30-421/2018/BSR) for financial support.
(12) Nitadori, H.; Ordronneau, L.; Boixel, J.; Jacquemin, D.;
Boucekkine, A.; Singh, A.; Akita, M.; Ledoux, I.; Guerchais, V.; Bozec,
H. L. Photoswitching of the second-order nonlinearity of a tetrahedral
octupolar multi DTE-based copper(I) complex. Chem. Commun.
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