Stimuli-Responsive Luminescent Bis-Tridentate Ru(II) Complexes toward the Design of Functional Materials

Article abstract of DOI:10.1021/acs.inorgchem.8b01562

We report here the synthesis, characterization, and photophysics of two bis-tridentate Ru(II) complexes based on a heteroditopic ligand and thoroughly studied their stimuli-responsive behaviors toward the design of functional materials. Both complexes display emission at room temperature having lifetimes in the range of 0.5-70.0 ns, depending on coligand and solvent. Substantial modulations of absorption and emission spectral behaviors of the complexes were done upon interaction with anions, and anion-induced changes in the properties lead to recognition of selected anions in both organic and aqueous media. Photophysical properties of the complexes were also tuned by changing the pH of the medium, and pK_a values in both ground and excited states were determined. The presence of free pyridine-imidazole motifs in the complexes leads to substantial modulation of the optical properties and switching of the emission properties upon interaction with selected cations as well as with protons. Fe²⁺, Co²⁺, Ni²⁺, and Cu²⁺ trigger emission quenching, while Zn²⁺ induces finite enhancement of the emission intensity in the complexes. In essence, modulation of the optical properties and switching of luminescence properties of the complexes were accomplished by a variety of the external stimuli such as anions, cations, protons, and pH, as well as solvent polarity. Importantly, the optical outputs in response to an appropriate set of stimuli were utilized to mimic the functions of two-input IMPLICATION, NOR, and XNOR logic gates.

Full text of DOI:10.1021/acs.inorgchem.8b01562

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Stimuli-Responsive Luminescent Bis-Tridentate Ru(II) Complexes
toward the Design of Functional Materials
Manoranjan Bar, Sourav Deb, Animesh Paul, and Sujoy Baitalik*
Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700032, India
S
* Supporting Information
ABSTRACT: We report here the synthesis, characterization, and
photophysics of two bis-tridentate Ru(II) complexes based on a
heteroditopic ligand and thoroughly studied their stimuli-responsive
behaviors toward the design of functional materials. Both complexes
display emission at room temperature having lifetimes in the range
of 0.5−70.0 ns, depending on coligand and solvent. Substantial
modulations of absorption and emission spectral behaviors of the
complexes were done upon interaction with anions, and anion-
induced changes in the properties lead to recognition of selected
anions in both organic and aqueous media. Photophysical properties
of the complexes were also tuned by changing the pH of the
medium, and pKa values in both ground and excited states were determined. The presence of free pyridine-imidazole motifs in
the complexes leads to substantial modulation of the optical properties and switching of the emission properties upon
interaction with selected cations as well as with protons. Fe2+, Co2+, Ni2+, and Cu2+ trigger emission quenching, while Zn2+
induces finite enhancement of the emission intensity in the complexes. In essence, modulation of the optical properties and
switching of luminescence properties of the complexes were accomplished by a variety of the external stimuli such as anions,
cations, protons, and pH, as well as solvent polarity. Importantly, the optical outputs in response to an appropriate set of stimuli
were utilized to mimic the functions of two-input IMPLICATION, NOR, and XNOR logic gates.
luminescent at room temperature, as the energy difference
between 3MLCT and 3MC states is very close because of their
distorted-octahedral geometries.23 For effective utilization of
Ru(II)-terpyridine complexes as the components of molecular
device, their room-temperature emission behaviors should be
improved. Several synthetic strategies, such as insertion of
electron-donating or -accepting substituents24−27 and extended
π-conjugated systems onto the terpyridine unit,28−32 were
followed for improvement of their emission behaviors by
enlarging the energy difference between MLCT and MC
states. In some cases, replacement of the pyridine moiety in the
tpy unit by another heterocyclic ring yields better results by
generating less distorted octahedral geometries.33−37
In the present work, our aim is to design complexes whose
properties could be modulated/switched by different external
stimuli, as controlling of their stimuli-responsive behavior
represents a complementary step toward increasing the
functionality and utility of these materials. To fulfill our goal,
we report here the synthesis, characterization, photophysics,
and electrochemistry of two bis-tridentate Ru(II) complexes
based on a heteroditopic ligand, dipy-Hbzim-tpy, wherein the
terpyridine moiety is connected with pyridine-imidazole
groups via a phenyl spacer (Chart 1) and thoroughly studied
their stimuli-responsive behaviors. The terpyridine site of dipy-
INTRODUCTION
■
As many aspects of chemical synthesis are mastered to prepare
complex materials, control of their properties upon interaction
with different external stimuli such as light, pH, and chemical
species represents a complementary step toward increasing the
functionality and utility of these materials.1,2 Stimuli-
responsive materials exhibit widespread applications ranging
from sensors3,4 to molecular computing, information displays,
and molecular switches and machines.5−12 Transition-metal-
based materials are much more advantageous over pure
organic frameworks, as they can offer better tunability of the
structural, optical, electrochemical, and electronic properties.
Among various transition metals, coordination complexes
based on Ru(II) metal are considered as potential building
blocks for the design of suitable functional materials, as they
possess outstanding photophysical and optoelectronic proper-
ties which primarily evolve from their metal to ligand charge
transfer excited states.13−15 Generally, bipyridine- and
terpyridine-type chelating units are employed for the synthesis
of Ru(II) complexes. Complexes derived from bipyridine units
(such as [Ru(bpy)3]2+) are better than their terpyridine
counterparts (such as [Ru(tpy)2]2+) as far as the emission
properties are concerned, whereas those obtained from
terpyridine ligands are advantageous with respect to formation
of achiral octahedral complexes but inferior with respect to
their emission behaviors.16−22 [Ru(tpy)2]2+ and most of the
other terpyridine complexes of Ru(II) are usually non-
3
3
Received: June 6, 2018
© XXXX American Chemical Society
A
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external stimuli such as anions, cations, protons, and pH, as
well as solvent polarity. In addition, as a proof of principle of
our motivation for studying this class of complexes, we utilized
absorption and emission spectral responses with a specific set
of ionic inputs and demonstrated that the present complexes
are capable of mimicking several important logic functions
such as those of two-input IMPLICATION, NOR, XNOR,
and OR logic gates. The possibility of information processing
at the molecular level was first explored by de Silva,46 who
thereafter laid the foundation for emerging research field that
ultimately led to the development of molecular comput-
ing.47−53 In this context, the design of suitable molecular and
supramolecular systems capable of mimicking advanced logic
functions is still a great challenge in the field of information
technology.
Chart 1. Drawings of the Compounds under Study
EXPERIMENTAL SECTION
■
Materials. The anions in the form of their tetrabutylammonium
(TBA) salts and the cations in the form of their perchlorate salts were
purchased from Sigma. Solvents and other chemicals were purchased
from local vendors. The Ru(II) precursors, [(tpy-PhCH3)RuCl3] and
[(H2pbbzim)RuCl3] have been prepared by refluxing RuCl3·3H2O
with tpy-PhCH3 and H2pbbzim, respectively in 1:1 molar ratio in
ethanol. The heteroditopic ligand, dipy-Hbzim-tpy and the Ru(II)
complexes have been prepared by slight modification of our previous
literature reports.52,53
Synthesis of [(dipy-Hbzim-tpy)Ru(tpy-PhCH3)](ClO4)2·2H2O
(1). Ru(tpy-PhCH3)Cl3 (100 mg, 0.18 mmol) and dipy-Hbzim-tpy
(130 mg, 0.25 mmol) were suspended in ethylene glycol and refluxed
under argon protection for 5 h. The orange solution was filtered and
poured into an aqueous solution of NaClO4·H2O (1.0 g in 5 mL of
water), whereupon an orange precipitate was obtained. The
compound was collected by filtration and purified by passing through
a silica gel column and eluting with an acetonitrile/toluene (1/1, v/v)
mixture. Upon evaporation, the microcrystalline solid that deposited
was collected by filtration and recrystallized from an acetonitrile/
methanol (1/2, v/v) mixture under slightly acidic conditions. Yield:
135 mg, 62%. Anal. Calcd for C56H44N10Cl2O10Ru: C, 56.57; H, 3.37;
Hbzim-tpy is coordinated to Ru(II), forming heteroleptic
complexes, and leaves two pyridine-imidazole coordination
motifs free which can be utilized for modulation of the optical
properties and switching of the emission properties upon
interaction with selected cations as well as by protons. The
imidazole NH protons in their secondary coordination sphere
were utilized for tuning of properties upon interaction with
anionic guests. Modulation of the photophysical properties was
also made possible by changing the pH of the medium, and the
pKa values in both the ground and excited states of the
complexes were also determined.
Although we have previously published some papers
reporting photophysical and sensing behaviors of related
complexes,38−45 we have not published any paper so far
wherein the optical properties and in particular the emission
properties of bis-tridentate Ru(II) complexes have been
modulated/switched simultaneously by a wide variety of
1
N, 11.78. Found: C, 56.48; H, 3.42; N, 11.65. H NMR (300 MHz,
DMSO-d6, δ/ppm): 13.00 (s, 1H, NH imidazole), 9.57 (s, 2H, 2H3′),
9.47 (s, 2H, 2H3′), 9.17−9.13 (m, 6H, 4H6 + 2H9), 8.74−8.69 (m,
4H, 4H8), 8.62 (d, 2H, J = 8.5 Hz, 2H7), 8.53 (t, 2H, J = 7.9 Hz,
2H11), 8.37 (d, 2H, J = 7.4 Hz, 2H12), 8.11−8.05 (m, 4H, 4H4),
7.91 (t, 2H, J = 6.7 Hz, 2H10), 7.56 (t, 6H, J = 6.4 Hz, 2H7 + 4H3),
7.31−7.25 (m, 4H, 4H5), 2.48 (s, 3H, CH3). ESI-MS (positive,
1
Figure 1. H NMR spectra of the complexes in DMSO-d6.
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Table 1. Photophysical Data for Complexes 1 and 2
luminescence
compound
solvent
DCM
absorption λmax/nm (ε/M−1 cm−1
)
λmax/nm
τ/ns
Φ
kr/s−1
knr/s−1
1
494 (33333), 327 (sh) (65833), 312 (84166),
287 (72250)
493 (21666), 354 (54166), 336 (55833),
317 (64166), 288 (sh) (47500)
502 (29916), 372 (sh) (20750), 317 (60000),
291 (50833)
500 (18333), 348 (sh) (44166), 321 (54166),
293 (45000)
635
τ1 = 1.2,
τ2 = 4.0
τ1 = 7.5, τ2 =
25.0
6.9 × 10−3
4.5 × 10−3
6.0 × 10−3
5.7 × 106,
8.2 × 108
1.7 × 106
9.9 × 108
2
1
2
1
2
674
660
688
642
733
6.0 × 105,
1.8 × 105
1.3 × 108,
3.9 × 107
DMSO
MeOH
τ = 6.0
1.0 × 106
1.6 × 108
τ1 = 29.0, τ2 = 6.6 × 10−3
70.0
τ1 = 0.5, τ2 =
2.2 × 105,
9.4 × 104
3.4 × 107,
1.4 × 107
493 (25000), 311 (58333), 285 (53333)
4.9 × 10−3
1.1 × 10−3
9.8 × 106,
1.9 × 109,
2.5
1.9 × 106
3.9 × 108
508 (32500), 350 (sh) (80000), 317 (101666),
285 (84166)
492 (20000), 313 (45750), 283 (39166)
505 (19916), 349 (sh) (60833), 317 (70000),
288 (57500), 244 (65000)
492 (40320), 351 (br) (48950), 330 (74610),
311 (88770), 282 (72840)
τ1 = 7.0, τ2 =
22.0
τ = 3.5
τ = 30.0
1.6 × 105,
5.0 × 104
1.4 × 108,
4.5 × 107
1
2
H2O
650
677
9.4 × 10−3
5.1 × 10−3
2.6 × 106
1.7 × 105
2.8 × 108
3.3 × 107
1
2
1
2
MeCN
658
678
640
671
τ1 = 2.0
τ = 12.0
15.5
1.8 × 10−3
9.2 × 105
49.9 × 107
8.2 × 107
5.7 × 104
6.2 × 104
490 (16230), 347 (53780), 333 (51360),
315 (49900), 282 (sh) (36570)
14.7 × 10−3 12.2 × 105
MeOH/EtOH
(77 K)
0.12
0.22
7.8 × 103
1.7 × 104
12.5
CH3CN): m/z 477.06 (100%) [(dipy-Hbzim-tpy)Ru(tpy-PhCH3)]2+
and m/z 318.38 (74%) [(dipy-Hbzim-tpy+H)Ru(tpy-PhCH3)]3+.
Synthesis of [(dipy-Hbzim-tpy)Ru(H2pbbzim)](ClO4)2·H2O
(2). Complex 2 was synthesized by adopting the same procedure as
for 1 using [(H2pbbzim)RuCl3] (95 mg, 0.18 mmol) instead of [(tpy-
PhCH3)RuCl3]. Yield: 120 mg, 55%. Anal. Calcd for
C53H38N12Cl2O9Ru: C, 54.92; H, 3.31; N, 14.50. Found: C, 54.85;
H, 3.38; N, 14.44. 1H NMR (300 MHz, DMSO-d6, δ/ppm): 15.03 (s,
2H, NH imidazole, H2pbbzim), 13.03 (s, 1H, NH imidazole), 9.66 (s,
2H, 2H3′), 9.05 (d, 4H, J = 7.6 Hz, 2H6 + 2H9), 8.79 (t, 5H, J = 9.2
Hz, 2H8 + H17 + 2H18), 8.68−8.65 (m, 4H, 2H7 + 2H12), 8.45−
8.43 (m, 2H, 2H11), 7.97 (t, 2H, J = 7.6 Hz, 2H5), 7.85−7.82 (m,
2H, 2H10), 7.66 (d, 2H, J = 7.9 Hz, 2H3), 7.5 (d, 2H, J = 5.0 Hz,
2H19), 7.27−7.25 (m, 4H, 2H5 + 2H20), 7.01 (t, 2H, J = 7.8 Hz,
2H21), 6.08 (d, 2H, J = 8.1 Hz, 2H22). ESI-MS (positive, CH3CN):
m/z 314.46 (100%) [(dipy-Hbzim-tpy+H) Ru(H2pbbzim)]3+ and m/
z 471.18 (4%) [(dipy-Hbzim-tpy)Ru(H2pbbzim)]2+.
singlet at 2.48 ppm (not displayed in Figure 1) accounting for
three protons is assignable to the −CH3 group of the tpy-
PhCH3 unit in 1. The upfield doublet at 6.08 ppm for 2 is
assignable to two H22 protons of the H2pbbzim unit because
of the anisotropic ring current of the adjacent pyridine rings.
The downfield singlet at ∼13.02 ppm in both 1 and 2 is
assignable to the NH proton in tpy-Hbzim-dipy, while the
singlet at 15.03 ppm in 2 is assignable to two NH protons in
H2pbbzim. The singlet at 9.66 ppm corresponds to the H3′
proton of dipy-Hbzim-tpy in 2, while two closely situated
singlets in 1 at 9.57 and 9.47 ppm correspond to the H3′
proton of the tpy unit of tpy-PhCH3 and dipy-Hbzim-tpy,
respectively. Assignments of other protons are also provided in
Figure 1.
Mass Spectra. ESI mass spectra of 1 and 2 acquired in
acetonitrile are presented in Figures S3 and S4 (Supporting
Information), respectively. The isotopic distribution patterns
of experimentally observed peaks correlate well with their
simulated patterns in both cases. 1 exhibits two peaks with (m/
z 318.38 and 477.06) corresponding to tripositive ([(dipy-
Hbzim-tpy+H)Ru(tpy-PhCH3)]3+) and dipositive ([(dipy-
Hbzim-tpy)Ru(tpy-PhCH3)]2+) species, respectively. In the
case of 2, the peak at m/z 314.46 corresponds to the tripositive
ion [(dipy-Hbzim-tpy+H)Ru(H2pbbzim)]3+, whereas the peak
at m/z 471.18 corresponds to the dipositive species [(dipy-
Hbzim-tpy)Ru(H2pbbzim)]2+. Thus, both complexes become
protonated in the matrix of mass spectra.
Electronic Absorption and Emission Spectra. The
photophysical properties of both 1 and 2 have been studied in
a few solvents, and relevant data are given in Table 1, while
selected UV−vis absorption and emission spectra are
presented in Figure 2 and Figure S5 (Supporting Information).
Overall, the spectral features are similar with small variations in
peak position and intensity. By comparison with the data of the
structurally related complexes, the band lying between 491 and
508 nm in the UV−vis spectra is assignable to Ru(dπ) → tpy/
H2pzbzim MLCT and the band located between 311 and 354
nm is assignable to intraligand charge transfer (ILCT), while
the very intense band within 230−293 nm is assignable to
Caution! The complexes as their perchlorate salts are explosive and
should be handled in small amounts with care.
Physical Measurements. The details of different instruments
used and experimental procedures adopted for carrying out various
physicochemical measurements are given in the Supporting
Information.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. Both complexes were
obtained in a straightforward way by reacting dipy-Hbzim-tpy
with the respective ruthenium(II) precursor in ethylene glycol
at elevated temperature and purified by column chromatog-
raphy and recrystallization under weakly acidic conditions to
retain the imidazole NH protons intact within the complexes.
Characterization of the complexes was done through elemental
(C, H and N) analyses and ESI mass and NMR spectroscopic
measurements, and the results are provided in the
Experimental Section.
1
NMR Spectra. H NMR spectra of 1 and 2 acquired in
DMSO-d6 display a fairly large number of peaks with some
overlapping features (Figure 1). Tentative assignment of the
proton resonances was done with the help of their {1H−1H}
COSY NMR spectra (Figures S1 and S2 in the Supporting
Information) and the spectra of related complexes.38−45 The
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solvent polarity as well as increased hydrogen bonding efficacy
(MeOH and DMSO).
Temperature-dependent emission studies were carried out
for an understanding of the excited state deactivation dynamics
of the complexes, and the outcomes are presented in Figure 3.
It is observed that with an increase in temperature the lifetime,
emission intensity, and quantum yield gradually decrease in
both cases. Nonlinear least-squares fitting of lifetime vs
temperature data to eq 1 yields different kinetic and
thermodynamic parameters associated with the excited-state
decay process.13,54
(τ(T))−1 = (k1 + k2 exp[−ΔE/RT])
Figure 2. Overlay of absorption and emission spectra (λex 490 nm) of
the complexes in MeCN at 298 K and in EtOH/MeOH (4/1, v/v) at
77 K.
/(1 + exp[−ΔE/RT])
(1)
where k1 corresponds to the temperature-independent rate
constant which is the summation of radiative (kr) and
3
nonradiative (knr) rate constants from the MLCT state at
π−π* transitions. The MLCT maxima were found to be
slightly red shifted upon an increase in solvent polarity and
hydrogen bond formation capability.
77 K, K2 corresponds to temperature-dependent rate constant
that indicates the rate of population of the 3MC state from the
3MLCT state, and ΔE corresponds to the activation energy
involved in the process.
Excitation at the MLCT band gives rise to a broad emission
band at room temperature with a maximum varying between
635 (DCM) and 660 nm (DMSO) for 1, while for 2, the range
varies between 674 (DCM) and 688 nm (DMSO). The
spectra at 77 K in ethanol/methanol (4/1 v/v) glass exhibit a
small blue shift along with an increase in quantum yield
(Figure 2). Emission maxima at 77 K were utilized to calculate
The outcome of nonlinear fitting gives the values of k1, k2,
and ΔE (Figure 3c,d). The calculated ΔE value is 3402 40
cm−1 for 1, while for 2, the value is substantially higher, 4440
70 cm−1. The value of k1 obtained from 77 K emission data, 6.4
× 104 s−1 for 1 and 8.0 × 104 s−1 for 2, was used for fitting
3
the zero−zero excitation energy (E00) of the MLCT excited
purposes. The estimated k2 value is 2.9 × 1013 −1 for 1 and 8.7
s
state, and the estimated value was found to vary between 1.94
and 1.84 eV. Excited state lifetimes vary between 0.5 and 70.0
ns at room temperature, depending upon the solvent, while
they vary between 12.5 and 15.5 μs at 77 K (Figure S6 in the
Supporting Information). Similarly to the UV−vis absorption,
the luminescence maximum is also red-shifted with increased
× 1013 s−1 for 2. First of all, an enormous increase in ΔE is
observed in both complexes with respect to the parent
[Ru(tpy)2]2+ (ΔE = 1500 cm−1).13 The increase is probably
due to excited state delocalization induced by 2-(2-phenyl-5-
(pyridin-2-yl)-1H-imidazol-4-yl)pyridine onto the tpy moiety
of the dipy-Hbzim-tpy ligand. Second, the energy difference
Figure 3. Decay profiles (λex 450 nm) for 1 (a) and 2 (b) as a function of temperature in MeCN. Temperature-dependent lifetime data along with
the values of different parameters and the corresponding nonlinear fits are shown in (c) and (d) for 1 and 2, respectively.
D
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Figure 4. Absorption (a, b) and emission (λex 490 nm) (c, d) spectral changes of 2 as a function of pH. The insets of (a) and (b) show the change
in absorbance, while the insets of (c) and (d) indicate the change in lifetime with pH.
3
3
among MLCT and MC states is a delicate function of the
electronic environment of the coligand as reflected in the
lifetime of the complexes. It is quite possible that the energetic
absorption and emission spectral measurements as well as
acquired lifetimes of the complexes within the pH range of
2.0−12.0 by using Robinson−Britton buffer.56
3
position of the MC state remains almost constant, while the
Figure 4 and Figure S8 (Supporting Information) display
UV−vis absorption and luminescence spectral change with pH.
Complex 1 exhibits a one-step change while 2 exhibits a two-
step change in the spectral profiles. The single-step change in 1
corresponds to dissociation of the NH proton on tpy-Hbzim-
dipy (Figure S8, Supporting Information). For the two-step
process in 2, the first step change in the pH range of 3.0−6.0
corresponds to dissociation of one of the two benzimidazole
NH protons attached to the H2pbbzim moiety, while the
second step change occurring within the pH range of 6.0−9.0
is due to the dissociation of the second NH proton from
H2pbbzim (Figure 4). The proton associated with the tpy-
Hbzim-dipy moiety in 2 is not dissociated within the studied
pH range. A number of isosbestic points were observed in each
case. Ground-state pKa values have been estimated from
absorbance vs pH data utilizing eq 2.56
3MLCT state is lowered substantially leading to a decrease in
3MLCT → MC surface-crossing efficiency.
3
Both complexes exhibit biexponential decay at room
temperature. The first short-lived component probably
corresponds to deactivation of the MLCT level, while the
second long-lived component probably appears from deacti-
3
3
3
vation of the equilibrated triplet state of MLCT and IL of
dipy-Hbzim-tpy. Emission spectra at 77 K also exhibit
vibrational features with spacings (1216 cm−1 for 1 and 1279
cm−1 for 2) that indicate quite a substantial contribution from
the triplet IL emission in the emission spectra at 77 K (Figure
2). Thus, an important outcome of the present work is that a
new type of luminescent heteroleptic bis-tridentate Ru(II)
complex having moderately long lifetime at room temperature
with dangling pyridine-imidazole motifs has been designed.
Electrochemical Properties. The electrochemical behav-
iors of the complexes have been examined by CV and SWV in
CH3CN solutions (Figure S7 in the Supporting Information).
One reversible oxidation and four reversible/quasi-reversible
reductions were found for both complexes. The couple with
E1/2 = 1.3 V for 1 and E1/2 = 1.0 V for 2 corresponds to the
RuII/RuIII oxidation process, while the couples with their E1/2
values ranging between −0.88 and −2.09 V correspond to the
reductions of the pyridine moieties in the complexes.55
pH = pK − log(A − A0)/Af − A0
(2)
Figure S8b (Supporting Information) shows substantial
quenching of luminescence at 655 nm for 1 with an increase in
pH. For 2, quenching is observed in two consecutive steps
(Figure 4c,d). In the first step, quenching occurs with the
position of luminescence maximum remaining constant, while
quenching is accompanied by a gradual red shift of the
emission maximum in the second step. Excited state decay
curves along with their lifetimes as a function of pH are shown
in the insets of Figure S8b (Supporting Information) and
Figure 4c,d. The lifetimes of both 1 and 2 decrease gradually
with an increase in pH. Lifetime vs pH data were used to
calculate the excited state pKa (pKa*) values.57
pH-Induced Changes in the Photophysical Properties
and Ground- and Excited-State pKa Values of the
Complexes. The imidazole NH protons residing on tpy-
Hbzim-dipy and H2pbbzim groups become acidic and could be
easily removed upon an increase in pH. In order to modulate
the ground and excited state properties, we performed
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Figure 5. UV−vis absorption and emission (λex 490 nm) spectral changes of 1 ((a) and (c), respectively) and 2 ((b) and (d), respectively) in
MeCN upon addition of different anions. The visual color changes are shown in the insets of (a) and (b).
Figure 6. UV−vis absorption and emission (λex 490 nm) spectral changes of 1 in MeCN upon addition of F− ((a) and (c), respectively) and
−
H2PO4 ((b) and (d), respectively). The insets show the fits of the experimental absorbance and luminescence data to a 1:1 binding profile.
Cl−, Br−, I−, AcO−,CN−, SCN−, and H2PO4 were employed
in this study. The spectral changes in the presence of 10 equiv
of anions are shown in Figure 5. Among the anions, only F−
and CN− induce a red shift of the MLCT band in 1. For 2, the
extent of the shift is much greater in comparison to that in 1,
and the observed shift takes place with F−, AcO−, CN−, and
−
pKa* = pH + log τacid/τbase
(3)
where the pH is at the inflection point of the emission intensity
vs pH curve. τacid corresponds to the lifetime of the protonated
form, while τbase corresponds to the lifetime of the
deprotonated form. pKa* values are found to be somewhat
greater with respect to their pKa values, and greater pKa*
values suggest that the MLCT state is primarily located on the
dipy-Hbzim-tpy moiety in the complexes.58−60
Anion-Induced Modulation of the Photophysical
Properties of the Complexes. Both acetonitrile and
aqueous solutions of the complexes and TBA salts of F−,
−
H2PO4 . Interestingly, the anion-induced spectral changes are
also reflected in their visual color changes (Figure 5). The
emission band at 658 nm for 1 undergoes a minor change with
−
the other anions except for F−, AcO−, CN−, and H2PO4 . With
F− and CN−, the emission gets fully quenched, while the extent
of quenching is much less with AcO−. In contrast, H2PO4
−
F
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−
Figure 7. UV−vis absorption ((a) and (b)) and emission (λex 490 nm) ((c) and (d)) spectral changes of 2 in MeCN upon addition of H2PO4 .
The insets show the fit of the experimental absorbance and luminescence data to a 1:1 binding profile.
a b
,
induces substantial emission enhancement in 1. On the other
hand, complete emission quenching occurs in 2 with F−, AcO−,
Table 2. Equilibrium/Binding Constants (K) for 1 and 2
toward Various Anions in MeCN and Water at 298 K
CN−, and H2PO4 .
−
2
To obtain quantitative information, we carried out titration
measurements with selected anions, and the spectral profiles
are presented in Figures 6 and 7 and Figure S9 (Supporting
Information). The MLCT band intensity at ∼500 nm for 1
gradually diminishes and finally saturates with 1 equiv of the
anions in a single step. In contrast, diminution and evolution of
the MLCT band in 2 takes place in two consecutive steps with
a red shift of the band in each step and final saturation occurs
with 10 equiv of the anions. The differences in spectral
behaviors arise due to variation in both the number and extent
of the acidic character of the NH protons in 1 and 2. From the
pH study, it is evident that benzimidazole NH protons of the
H2pbbzim moiety in 2 are more acidic and one of these NH
protons ia removed from the complex initially, while the
second NH proton is dissociated in the second step. The
absorption spectral profile in each step is accompanied by
several isosbestic points. The equilibrium constants of the
complex−anion interaction process have been estimated with
the help of their titration data, and the calculated values lie on
the order of 106 M−1 (Table 2).
On the emission side, gradual emission quenching takes
place in 1 on addition of both F− and CN− and quenching
stops upon reaching 1 equiv of the anions. In contrast to the
case for both F− and CN−, emission enhancement occurs in
the presence of H2PO4− and the enhancement continued up to
addition of 1 equiv of the anion. The observed differences in
spectral responses among the anions arise because of their
differences in charge density, size, and basicity. For 2, emission
quenching occurs in two consecutive steps and the quenching
processes were complete upon addition of ∼10 equiv of the
anions. Equilibrium/binding constants were also calculated
from emission titration profiles, and fairly good correlation was
observed between the values calculated by the two methods
(Table 2).
anion/cation
F−
1 K
K1
K2
From Absorption Spectra (MeCN)
1.63 × 106
1.30 × 106
1.84 × 106
1.68 × 106
2.80 × 106
1.10 × 106
2.12 × 106
2.03 × 106
2.08 × 106
1.55 × 106
1.96 × 106
3.76 × 106
1.13 × 106
2.22 × 106
2.28 × 106
1.22 × 106
1.36 × 106
−
H2PO4
Fe2+
Ni2+
Cu2+
Zn2+
H+
CAN
From Absorption Spectra (Water)
CN−
SCN−
3.40 × 104
4.48 × 104
2.28 × 104
3.71 × 104
From Emission Spectra (MeCN)
F−
H2PO4
2.50 × 106
1.18 × 106
2.70 × 106
1.10 × 106
1.61 × 106
1.27 × 106
2.23 × 106
2.16 × 106
2.02 × 106
1.62 × 106
1.57 × 106
1.06 × 106
1.24 × 106
2.00 × 106
2.07 × 106
1.78 × 106
1.41 × 106
−
Fe2+
Ni2+
Cu2+
Zn2+
H+
CAN
From Emission Spectra (Water)
CN−
SCN−
1.09 × 104
2.51 × 104
1.03 × 104
3.98 × 104
a
tert-Butyl salts of the anions and perchlorate salts of the cations were
used for the studies. Estimated errors were <15%.
b
actions between NH proton(s) and the anions and/or anion-
induced complete proton transfer leading to increasing
electron density at the Ru(II) center.44,45,61,62 The H NMR
1
titration of 2 with H2PO4− in DMSO-d6 shows that the peak at
15.03 ppm due to benzimidazole NH protons on the
H2ppbzim moiety become initially slightly downfield shifted
to 15.06 ppm and thereafter completely vanish (Figure S10,
The shift of the MLCT band to longer wavelength is
probably because of second-sphere hydrogen-bonding inter-
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Table 3. Spectrophotometric and Fluorimetric Detection Limits of 1 and 2 in MeCN and Water
detection limit (M) in water
detection limit (M) in MeCN F−
CN−
SCN−
compd
absorption
emission
absorption
emission
absorption
emission
1
2
9.9 × 10−9
1.0 × 10−9
9.1 × 10−9
2.0 × 10−9
6.6 × 10−8
9.8 × 10−8
9.5 × 10−8
8.7 × 10−8
9.8 × 10−8
8.5 × 10−8
8.7 × 10−8
9.8 × 10−8
Figure 8. UV−vis absorption and emission (λex 490 nm) spectral changes of 2 in water with CN− ((a) and (c), respectively) and SCN− ((b) and
(d), respectively). The insets show the fits of the experimental absorbance and luminescence data to a 1:1 binding profile.
Supporting Information). The signal at 13.02 ppm due to
imidazole NH on tpy-Hbzim-dipy is also removed in the
in anion concentration, in accordance with the steady-state
spectrum, and finally the lifetime decreases from 21.0 to 7.0 ns
−
−
presence of excess H2PO4 . The small downfield shift and
for F− and to 15.0 ns for H2PO4 . Deprotonation of the NH
thereafter complete removal of NH peaks suggest that H2PO4
proton in 1 occurs in the presence of F−, and as the lifetime of
the deprotonated species is shorter than that of its protonated
form, the net result is shortening of lifetime with F−. In
−
interacts with 2 via a hydrogen-bonding interaction with
benzimidazole NH protons and thereafter removal of both
types of NH protons occurs in the presence of excess anions.
−
contrast, H2PO4 induces strong hydrogen bonding with the
−
Emission enhancement of 1 with H2PO4 is probably because
NH proton and the lengthening of the lifetime of 1 in the
presence of H2PO4− is probably because of the formation of a
hydrogen-bonded adduct. A small but finite increase in lifetime
of increased rigidity as well as relatively restricted mobility of
−
the hydrogen-bonded adduct (1-H2PO4 ), which in turn slows
the nonradiative deactivation channels.42,43,63,64 Emission
quenching with other anions is probably because of photo-
induced intramolecular electron transfer from the deproto-
nated imidazole fragment to the excited Ru(II)-terpyridine
unit.
for 2 with F− and H2PO4 , although the decay profiles and the
−
magnitudes of enhancement are somewhat different for the
two ions, could be due to the formation of incipient hydrogen
bonding between the benzimidazole NH protons and oxygen
−
atoms on H2PO4 . However, an excess of both anions leads to
The decay profiles as well as lifetimes of 1 and 2 as a
anion-induced removal of those NH protons and the
substantial lowering of lifetime is probably due to the
formation of the deprotonated species.
−
function of F− and H2PO4 are shown in the insets of Figure
6c,d, Figure 7c,d, and Figure S11 (Supporting Information). In
accordance with the steady-state spectra, the lifetime of 1
Significant modulation of photophysical properties in the
presence of anions can be utilized for recognition of selected
anions in the presence of others. One equivalent of H2PO4
gradually decreases from 2.0 to 1.0 ns with F−, while H2PO4
−
−
induces an increase in lifetime to 18.5 ns. Interestingly, the
−
decay curve of 1 in the presence of H2PO4 shows that the
resulted in almost 2-fold emission enhancement in 1, while 1
equiv of F− and AcO− led to 88 and 50% emission quenching,
respectively. This finding clearly demonstrates the ability of 1
to function as a highly selective “turn on” type of emission
short component remained effectively constant (τ = 2.0−1.5
ns), while the long component gradually increased (from 2.0 to
−
18.5 ns) with the increase in H2PO4 concentration. The
−
−
lifetime change for 2 in the presence of both F− and H2PO4
sensor for H2PO4 , while it is a “turn off” type sensor for F−
takes place in two steps. In the first step, the lifetime increases
from 12.0 to 21.0 ns with both anions, although quenching of
emission was noticed in the steady-state spectrum. In the
second step, the lifetime decreases gradually with the increase
and CN−. In contrast, emission quenching occurs in 2 by F−,
−
CN−, AcO−, and H2PO4 and thus acts as only a “turn off”
type emission sensor for the aforementioned anions without
selectivity. In addition to sensitive emission spectral modu-
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Figure 9. UV−vis absorption and emission (λex 490 nm) spectral changes of 1 in MeCN upon addition of Zn2+ ((a) and (c), respectively) and Ni2+
((b) and (d), respectively). The insets show the fit of the experimental absorbance and luminescence data to a 1:2 binding profile.
Figure 10. UV−vis absorption (a) and emission (λex 490 nm) (b) spectral changes of 2 in MeCN upon addition of Cu2+. The inset to (b) shows
the fit of the experimental luminescence data to a 1:2 binding profile.
lation, optical spectral changes along with visual color changes
of the complexes were also observed in the presence of selected
anions. Detection limits toward selected anions were also
calculated (Figures S12−S15, Supporting Information), and
the calculated values lie in the range of (1.0−9.9) × 10−9 M for
F− (Table 3).
Luminescence decays together with their lifetimes are shown
in the insets of Figure 8c,d and Figure S20 (Supporting
Information). The lifetime of 1 decreases with CN−, while it
increases with SCN−. In contrast, the lifetime of 2 decreases
dramatically in the presence of both CN− and SCN−, although
the extent of decrease is slightly higher for SCN−. The
substantial change in lifetime makes the complexes lifetime-
based sensors for CN− and SCN− in aqueous medium. The
extents of the complex−anion interaction process were
accessed by calculating the equilibrium constants (K), and
the values vary between 1.03 × 104 and 4.48 × 104 M−1 (Table
2). K values in water are less than that of acetonitrile by 2
orders of magnitude. With regard to their practical
applicability, the detection limits have been calculated for
CN− and SCN− (Figures S21−S28 in the Supporting
Information), and the obtained values (lying between 6.6 ×
10−8 and 9.8 × 10−8 M, Table 3) are much lower than the
permissible level (0.2 ppm) for drinking water as recom-
mended by the Environmental Protection Agency (EPA).65
Cation-Induced Modulation of the Photophysical
Properties of the Complexes. Taking advantage of dangling
pyridyl-imidazole groups, we investigated the action of
different cations on the photophysical properties of the
Taking advantage of their water solubility, we also studied
the photophysical behaviors of the complexes in aqueous
medium in the presence of the anions with regard to their
practical applicability. Surprisingly, the selectivity increased
substantially in water in comparison to acetonitrile and both
complexes responsd only toward CN− and SCN− in water,
which was visualized through their color and spectral responses
(Figures S16 and S17, Supporting Information). Titration
experiments were carried out in HEPES buffer solution at pH
7.2 to exclude the interference of hydroxide ion which could be
evolved by hydrolysis of cyanide (Figure 8 and Figures S18 and
S19 in the Supporting Information). In contrast to the case for
acetonitrile, both complexes show a one-step change in water.
Diminution of the MLCT band and emission quenching is
seen with CN− and SCN−, but the extent of change is more
marked for 2 in comparison with 1.
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Figure 11. UV−vis absorption and emission (λex 490 nm) spectral changes of 1 ((a) and (c), respectively) and 2 ((b) and (d), respectively) in
MeCN upon addition of HClO4. The insets show the fits of the experimental absorbance and luminescence data to a 1:2 binding profile.
complexes. Perchlorate salts of Mn2+, Fe2+, Co2+, Ni2+, Cu2+,
Zn2+, Cd2+, and Hg2+ were employed in the study. Spectral
changes in the presence of the cations are shown in Figures
S29 and S30 (Supporting Information). Titration measure-
ments were also carried out to get quantitative insight into the
complex−cation interaction process (Figures 9 and 10 and
Figures S31−S35 in the Supporting Information). The MLCT
absorption bands for both complexes remain almost unaltered
with Mn2+, Co2+, Zn2+, Cd2+, Hg2+, and Pb2+, while they
decrease to some degree with Fe2+, Ni2+, and Cu2+, albeit to
different extents. The intensity of the ILCT and lower energy
π−π* bands increase in general, although the amount varies
with the cations. Among the studied cations, small emission
enhancement is seen only with Zn2+ for both complexes. The
emission profile remains almost unaltered with Cd2+, Hg2+, and
Pb2+, while substantial quenching is observed with Ni2+, Fe2+,
and Cu2+ ions. Equilibrium constants (K) for complex−cation
interaction processes have been calculated from the titration
profiles, and the values are summarized in Table 2.
comparison with 2 (E1/2 = 1.0 V). Moreover, oxidation of 2
by Cu2+ does not occur at all in water. After oxidation of the
Ru(II) center in 2, the reduced Cu(I) becomes stabilized by
the surrounding MeCN, which acts as a good coordinating
ligand to stabilize reduced Cu(I) species.
It is of interest to note that Fe2+, Ni2+, and Cu2+ ions induce
emission quenching, while Zn2+, Cd2+, Hg2+, and Pb2+ ions
having completely filled d shells do not quench. Moreover, a
small emission enhancement was also observed with Zn2+ in
both complexes. This sort of behavior is most likely due to a
photoinduced electron transfer (PET) mechanism, which goes
via an excited-state electron transfer process as shown
below.67,68 1 and 2 both consist of a fluorophore−receptor
combination, where the [Ru(tpy)2]2+ unit in 1 and
[(H2ppbzim)Ru(tpy)]2+ unit in 2 act as fluorophores, while
two dangling pyridine units coupled with the imidazole moiety
act as receptors for incoming cationic guests. Metals with an
incomplete d shell can drag the electron after excitation and
could convert to the lower oxidation state(s).
The titration profile of 2 with Cu2+ deserves special
attention. Upon addition of Cu2+, the MLCT band centered
around 500 nm gradually loses its intensity and at the same
time a broad new band with its maximum centered around 800
nm evolves and gradually intensifies up to 5 equiv of Cu2+
(Figure 10a). The spectral change is accompanied by well-
defined isosbestic points. Cu2+ induces complete emission
quenching in 2, as shown in Figure 10b. The broad absorption
within the 730−800 nm region is probably due to a tpy-
Hbzim-dipy to Ru(III) charge transfer (LMCT) transition. To
obtain support in favor of the assignment of the broad band,
chemical oxidation of 2 was also carried out with the Ce4+ ion
through absorption spectrophotometry (Figure S36 in the
Supporting Information). The close similarity of the spectral
pattern suggests that partial oxidation of Ru(II) in 2 occurs in
the presence of Cu2+.66 In contrast, oxidation of the Ru(II)
center in 1 does not occur by Cu2+. This is probably because of
the higher oxidation potential of 1 (E1/2 = 1.3 V) in
II
II
*
[{Ru (tpy/H2ppbzim)R} −Cu ]
III
I
*
→ [{Ru (tpy/H2ppbzim)R} −Cu ]
In contrast, due to their filled d shells, metals such as Zn2+,
Cd2+, and Hg2+ are unable to consume an electron, and
thereby the PET process is restricted and as a result emission is
not quenched. In line with steady-state spectra, a small lifetime
increase occurs with Zn2+ while substantial lifetime decrease
occurs with Cu2+ for 2 (Figure S37 in the Supporting
Information).
Proton-Induced Changes of the Photophysical Prop-
erties of the Complexes. As already mentioned, two free
pyridine units are present in both complexes. Therefore, there
is a finite possibility of protonation of these nitrogen atoms
with acid. To explore this possibility, we performed titration
experiments with standard HClO4 (Figure 11). Addition of the
acid up to 2 equiv leads to a small increase in intensity of the
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Figure 12. (a) Absorption spectral changes of 2 upon the action of Zn2+ (input 1) and F− (input 2). (b) Schematic representation of an
IMPLICATION gate based on the absorption at 490 nm. (c) Corresponding truth table.
Figure 13. (a) Emission (λex 490 nm) spectral changes of 2 upon the action of F− (input 1) and H2PO4− (input 2). (b) Schematic representation
of a NOR gate based on the emission at 678 nm. (c) Corresponding truth table.
ILCT band at ∼350 nm in both complexes, the MLCT band
remaining almost unaltered. Addition of acid also leads to an
increase in both emission intensity as well as lifetime for both
complexes (Figure 11b,d and Figure S38 in the Supporting
Information). The lone pairs of electrons on pyridine sites
become engaged upon addition of acid and thus the possibility
of PET from free pyridine units to a photoexcited [Ru-
(tpy)2]2+/[(H2ppbzim)Ru(tpy)]2+ moiety is arrested, which in
turn leads to emission enhancement.
Binary Logic Gates. In preceding sections, we noticed that
significant modulation of optical properties of the complexes
were made possible upon interaction with different external
stimuli such as selected anions and cations, as well as protons.
As a proof of principle of our motivation for studying this class
of complexes, we will use absorption and emission spectral
responses as a function of a suitable combination of ionic
inputs to explore the roles that are played by the complexes.
IMPLICATION Gate. The function of the IMPLICATION
logic gate is very important and is also powerful with regard to
“material implication”, and only a limited number of molecular
logic gates with exceptional implication activity (IMP) have
been reported in the literature. Three fundamental logic
functions, viz. AND, OR, and NOT gates, are carried out by an
IMPLICATION gate. In order to mimic the function of an
IMPLICATION gate, one input has to imply the other,
indicating that if the input 1 equals 1, then the input 2 has to
be 0. To construct an IMPLICATION gate, we have chosen
Zn2+ as input 1 and F− as input 2 and both the absorbance
value at 490 nm as well as the emission maximum at 678 nm
for 2 as the output signals (Figure 12). The truth table in
Figure 12 indicates that, in absence of either of the two inputs,
2 exhibits intense absorption at 490 nm corresponding to the
on state (1) of the system. On the other hand, input 2 (F−)
gives rise to a substantial decrease in absorbance, thus
corresponding to the off state (0) of the system. In addition,
all three other possible input combinations as shown in the
truth table give rise to a value of the output signal above the
threshold value and indicate the on state (1) of the system.
Thus, by taking the absorbance change of 2 at 490 nm by the
action of Zn2+ and F−, the function of an IMPLICATION gate
can be mimicked. In a similar manner, by observation of the
change in luminescence intensity at 678 nm in the presence of
the same input combinations, an IMPLICATION logic gate
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Figure 14. (a) Emission (λex 490 nm) spectral changes of 2 upon the action of Fe2+ (input 1) and F− (input 2). (b) Schematic representation of a
XNOR gate based on the emission at 678 nm. and (c) Corresponding truth table.
can also be constructed (Figure S39 in the Supporting
Information).
CONCLUSIONS
■
With regard to our recent interest in the design of suitable
functional materials, we report here two heteroleptic bis-
tridentate Ru(II) complexes derived from a heteroditopic
ligand and thoroughly investigated their stimuli-responsive
photophysical behaviors. The design strategy offers several
acidic imidazole NH protons capable of interacting with anions
as well as vacant pyridine-imidazole rings remote from the
terpyridine site capable of interacting with cations. Substantial
modulation of the optical properties and switching of the
emission properties were made possible upon interaction with
both anions and cations. Anion-induced changes in properties
give rise to multichannel recognition of selected anions in both
organic and aqueous media. Interestingly, 1 acts as a “turn on”
NOR Logic Gate. The NOR gate is an integration of the
NOT and OR logic gates. Importantly, it is a universal gate and
finds its utility in the combinatorial creation of all other
Boolean logic gates. Here, we will monitor the emission
intensity at 678 nm for 2 as the output signal in the presence of
−
two anionic inputs, F− (input 1) and H2PO4 (input 2). The
truth table in Figure 13 indicates that the absence of both
inputs corresponds to the on state (1) of the system, while all
of the other three combinations correspond to the off state (0)
of the system. Thus, the results summarized in the truth table
correspond to the function of a two-input NOR logic gate.
XNOR Logic Gate. To mimic the XNOR logic function in 2,
again the luminescence intensity at 678 nm was monitored as a
function of Fe2+ (input 1) and F− (input 2) ions. The
simultaneous absence or presence of both inputs (1.0 equiv of
Fe2+ and 6.0 equiv of F−) gives rise to high luminescence
intensity and thus corresponds to the on state (1) of the
system (Figure 14). On the other hand, the presence of either
input 1 or input 2 quenches the luminescence intensity
significantly, corresponding to the off state (0) of the system.
Thus, 2 exhibits XNOR logic behavior in the presence of Fe2+
and F− (Figure 14).
OR Logic Gate. Complex 1 has the ability to mimic the
function of an OR gate. The absorbance change at 370 nm was
considered as the output signal in the presence of the same
ionic inputs as those used in the XNOR gate (Fe2+ and F−). A
classical OR logic gate has two input ports and one output
port. In an OR gate, an activated signal should be observed in
the presence of either one or both inputs. The absorbance at
370 nm became low in the absence of both inputs, indicating
the off state (0) of the system (Figure S40, Supporting
Information). In contrast, the absorbance becomes enhanced
in the presence of either one or both of the inputs, indicating
the on state (0) of the system. In essence, complex 1 nicely
mimics the function of a two-input OR logic gate (Figure S40
in the Supporting Information). An OR gate plays an
important role in implementing logical disjunction. It is
important to note that the interactions between complexes and
anions or cations are reversible in all cases.
−
type of emission sensor for H2PO4 , while it is a “turn off” type
emission sensor for F− and CN−. In contrast, 2 functions as
only a “turn off” type emission sensor for F−, CN−, AcO−, and
−
H2PO4 . The selectivity was enhanced greatly in water, and
both complexes act as “turn off” type emission sensors for only
S− and SCN− with a detection limit on the order of 10−8 M.
Modulation of the optical properties was also made possible by
varying the pH of the solution, and pKa values associated with
both ground and excited states were also determined. Finally,
the presence of protons engages the lone pair(s) on the free
nitrogen of the pyridine rings, leading to a substantial increase
in emission intensity in the complexes. Thus, the optical
properties and in particular the emission properties of bis-
tridentate Ru(II) complexes have been modulated/switched
simultaneously by a wide variety of external stimuli such as
anions, cations, proton, and pH, as well as solvent polarity. In
addition, in the context of the design of functional materials,
we utilized both absorption and emission spectral responses as
a function of selected anions and cations and successfully
demonstrated various binary logic operations such as those of
two-input IMPLICATION, NOR, and XNOR logic gates
exhibited by the complexes.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b01562.
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AUTHOR INFORMATION
■
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Coudret, C.; Balzani, V.; Barigelletti, F.; De Cola, L.; Flamigni, L.
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Corresponding Author
*S.B.: e-mail, sbaitalik@hotmail.com, sujoyb@chemistry.jdvu.
ac.in; tel, 91-033-2414-6666.
ORCID
Sujoy Baitalik: 0000-0001-9720-6889
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
S.B. acknowledges financial support received from the SERB-
DST (Grant No. EMR/2015/001163) and CSIR (Grant No.
01(7619)/18/EMR-II), New Delhi, India. M.B. acknowledges
the UGC, S.D. acknowledges the DST-INSPIRE, and A.P.
acknowledges the CSIR for their fellowships. Thanks are also
due to the DST for the TCSPC facility under the PURSE
program at the Department of Chemistry of Jadavpur
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