Selective photodissociation of highly photoactive Bis-2-benzylidenemalononitrile in solution

Article abstract of DOI:10.1016/j.jphotochem.2019.01.034

Photophysical study of highly photoactive bis^-2-benzylidenemalononitrile (BBM) has been investigated in solution using UV–vis, fluorescence and NMR spectroscopic techniques. The seletive photodissociation can be attributed to the presence of hi

Full text of DOI:10.1016/j.jphotochem.2019.01.034

Accepted Manuscript
Title: Selective Photodissociation of Highly Photoactive
Bis-2-benzylidenemalononitrile in Solution
Authors: Sumit Kumar Panja, Suvajit Koley, Haddad
Boumediene
PII:
S1010-6030(18)31683-6
DOI:
Reference:
https://doi.org/10.1016/j.jphotochem.2019.01.034
JPC 11693
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
20 November 2018
24 January 2019
30 January 2019
Please cite this article as: Kumar Panja S, Koley S, Haddad B, Selective
Photodissociation of Highly Photoactive Bis-2-benzylidenemalononitrile in Solution,
Journal of Photochemistry and amp; Photobiology, A: Chemistry (2019),
https://doi.org/10.1016/j.jphotochem.2019.01.034
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Selective Photodissociation of Highly Photoactive Bis-2-
benzylidenemalononitrile in Solution
a
b
c
Sumit Kumar Panja,* Suvajit Koley and Haddad Boumediene
aDepartment of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore
60012, Karnataka, India.
5
bDepartment of Medicinal Chemistry, The University of Kansas, 1251 Wescoe Hall Drive,
Lawrence, Kansas 66045, United States.
cDepartment of Chemistry, Dr. Moulay Tahar University of Saida, Algeria.
Graphical Abstract
Highlights

Solvent specific and temperature sensitive photo dissociative reaction of highly
photoactive bis-2-benzylidenemalononitrile (BBM) is observed under UV irradiation
@
365 nm.


The presence of dicyano groups (acceptor) on the C=C bond, make it more
polarizable, highly active and susceptible for photodissociation reaction.
Product A (2-(4-formylbenzylidene)malononitrile), formed by the selective C=C bond
photodissociation reaction of BBM in H
and identified by the various spectroscopic techniques.
The selective photodissociation reaction followed the pseudo 1 order kinetics and
dipolar mechanism in H O.
2
O under UV irradiation, has been isolated
st

2

Abstract: Photophysical study of highly photoactive bis-2-benzylidenemalononitrile (BBM)
has been investigated in solution using UV-Vis, fluorescence and NMR spectroscopic
techniques. The seletive photodissociation can be attributed to the presence of highly
polarized C=C bond, attached to dicyno groups (CN). Nevertheless, the observed
photodissociation of BBM is highly solvent specific and temperature sensitive. The product
A (2-(4-formylbenzylidene)malononitrile), formed by the selective photodissociation of
BBM under UV irradiation has been isolated and identified by NMR studies. The
st
-3
-4 -1
photodissociation followed the pseudo 1 order kinetics with a rate constant (k) 10 -10 .s .
Moreover the mechanistic study of selective photodissociation of bis-2-
benzylidenemalononitrile (BBM) has been investigated and found to follow the dipolar
mechanism path during photochemical reaction.
KEYWORDS: Photodissociation, Photoactive Material, Dipolar Mechanism, Benzylidene
malononitrile Analogue, Density Functional Theory (DFT)
Introduction
Photochemical behaviours of organic stilbenes have been extensively studied as the model
systems in solutions and biological systems for the better understanding of most fundamental
and elementary physicochemical processes.1-4 Photoinduced trans-cis isomerization about a
C=C double bond is widely occured in various processes such as vision process, phototaxis
etc.5-6 On the other hand trans-stilbene is a very prevalent organic molecule, which shows
7
numerous interesting photo-induced cis-trans isomerization about a C=C bond. The solvent-
induced photoisomerization of trans-stilbene revealed that the trans-cis photoisomerization
occurred at lowest excited singlet state (S
).8-10 The photoisomerization of olefins in solution
1
normally proceeds by the rotation of the double bond, because the reaction coordinate mainly
involves the one bond twist (OBT) mechanism about the central bond.11-13 The Hula-twist
mechanism (HT), which comprises of concerted rotation about a double bond and an adjacent
essential single bond (envisioned as equivalent to a 180° translocation of one CH unit) was
proposed to account for preliminary understanding of photoisomerization in free volume
restricted media.14-16 It is also reported that photoionization of trans-stilbene occurs in
solution to generate the trans-stilbene radical cation.17-19 Generally, diradical isomerization of
olefins (mainly photoisomerization) proceeds either via the* singlet (S
1
) or triplet (T )
1
excited states, depending upon the properties of the molecule and the experimental
conditions.20-21 The ionization mechanism still remains controversial with an intriguing fact

22
that the rise of the radical cation shows a marked delay from the photoexcitation. However,
intersystem crossing (T
1
S
0
+ heat), photosensitization by singlet-singlet or triplet-triplet
7
(
TT) transfer can also lead to isomerization. It is also reported that photoisomeriaztion may
occur simply by the breaking of the double bond, either through a homolytic or a heterolytic
7
process. Arai et al. has reported that exciplex-induced isomerization played a key role in
highly efficient photoisomerization of stilbene-cored derivatives.23-24 Eyring has proposed
that the trans-cis isomerization may happen either through the formation of a biradical (on a
2
5
triplet surface) or in the ground state (S ) by rotation about the C=C bond. An alternative
0
cis/trans isomerization pathway for conjugated dienes was depicted by Buechele and co-
workers which involves the formation of a transient intermediate through tautomerization.26
The effect of substituents on trans-cis photoisomarization of stilbene analogues has been
widely explored and throughly investigated for the better understanding of the mechanism of
photoisomerization process.27-30 Several theoretical and experimentat studies were also
carried out for getting the valuable informations about the ground and excited state of C=C
bond during photochemical process.31-35 Particularly, the effect of donor-acceptor substituents
of asymmetric benzylidenemalononitrile analogues on photoisomerization and formation of
highly polar twisted intramolecular charge-transfer species (TICT) in the excited state has
been investigated widely.36-39 Nevertheless, the solvent and substituent effects on light-
induced rotations of C=C bond of asymmetric benzylidenemalononitrile analogues are also
well reconnoitred.40-41 The solvent and substituent effect on selective photodissociation of
C=C bond of benzylidenemalononitrile analogues have not been investigated yet.
So, in present work, we depicted the solvent dependent photochemical behaviour of C=C
bond in bis-2-benzylidenemalononitrile molecules (BBM, Chart 1). Effects of solute-solvent
interactions on selective photodissociation of BBM are also studied and discussed for better
understanding of the mechanism and kinetics of this chemical process.
Experimental Section
Synthesis of Bis-2-benzylidenemalononitrile (BBM)
Synthesis of BBM was done by our previously work.42 In brief, a mixture of
terephthaldehyde (1.0 mmol), manolonitrile (2.01 mmol) and MP(DNP) (0.05 mmol) in
round bottom flask containing ethanol was stirred at room temperature for appropriate time.
After completion of the reaction (monitored by TLC), solid product was collected by the
filtration and washed with water and ethanol for obtaining pure product.

Characterization of BBM: State: Solid; Colour: White; Melting Point: 205 °C; FTIR (KBr,
-
1
1
cm ): 2937, 2228, 1555, 1565, 1495; H-NMR (300MHz, CDCl
3
): δ (ppm) 8.04 (d, J = 7.0
1
3
Hz, 4H), 7.81 (s, 2H); C-NMR (125 MHz, CDCl
13.2, 85.2; ESIMASS: [M+H] 231.2.
3
): δ (ppm) 160.3, 135.8, 131.3, 114.3,
1
Sample Preparation of BBM in Solution
-
5
For studying kinetics of BBM in solution by UV-Vis spectroscopy, ~10 (M) of BBM is taken
in solvents during experiment. Solution of BBM is kept under UV irradiation at 365 nm for 1
minute before UV-Vis measurement in solvents.
-6
Similarly, for studying kinetics of BBM in solution by fluorescence spectroscopy, ~10 (M)
of BBM is taken in solvents during experiment. This solution of BBM is kept under UV
irradiation at 365 nm for 1 minute before fluorescence measurement in solvents.
Commercially available 4 watt power of UV lamp @ 365 nm is exposed for 1 minute on the
sample for spectroscopic measurement.
For NMR experiment of photodissociated products, photochemical reaction is performed
-2
taking 10 (M) solution of BBM in H
2
O under UV irradiation @ 365 nm for 5 minute. From
1
reaction mixture, crude products are extracted by ethylacetate and further used for H and
13C-NMR in CDCl
solvent.
3
Infrared Spectroscopy
For the study of the vibrational properties, Fourier Transform Infrared (FTIR) spectra was
acquired on a Shimadzu FTIR-8900 spectrophotometer and recorded in the 4000-400 cm-1
region.
Raman Spectroscopy
FT-RAMAN spectra were acquired by means of a Vertex 70-RAM II Bruker FT-RAMAN
spectrometer with a Nd:YAG laser (yttrium aluminium garnetcrystal doped with triply
ionized neodymium) having a wavelength of 1064 nm and a maximum power of 1.5 W. The
measurement accessory is pre-aligned; only the Z-axis of the scattered light is adjusted to set
the sample in the appropriate position regarding the focal point. The RAM II spectrometer is
-1
equipped with a liquid-nitrogen cooled Ge detector and FT-RAMAN spectra (200-3500 cm )
-1
were collected with 1 cm nominal resolution by co-adding 128 scans for each spectrum at
room temperature.
UV-Vis Spectroscopy
Electronic absorption spectrum was measured on Varian-UV-Visible spectrophotometer
(Carry-100-Bio) in various solvents. A 1 cm quartz sample cell at the rate of 0.5 C/min

during the heating and cooling scans using Cary 100-Bio UV-Vis spectrophotometer (Varian
Carry-100-Bio) equipped with a Peltier series II thermostatic cell holder.
Quantum Chemical Calculation
DFT calculation using B3LYP functional with 6-311++G (d, p) basis set is employed to
calculate the optimized geometry of BBM molecule. The electronic absorption spectra is
calculated using the time-dependent density functional theory (TD-DFT) method at
B3LYP/6-311++G(d,p) using the polarisable continuum model (PCM) and ACN as solvent.
The Gaussian 09 suite of programs was used for all the computations, and the GaussView 5.1
program was used to visualize the structures.43
Chart 1: Structural Formula and Abbreviation
Results and discussion
UV-Visible Studies of BBM in Different Solvents
Initially, electronic absorption spectra is measured using UV-Vis spectroscopy in different
solvents (Figure 1) for better understanding the solvent effect on electronic properties of the
BBM molecule. The UV-Vis spectra of BBM does not exhibit a bathochromic shift with the
increase in the polarity of the solvents except H
band of BBM is appeared at 369 nm and * transition is observed at 347 nm in polar
solvents. In H O, the spectral pattern is quite interesting and different from other polar
solvents. For BBM, ICT band is appeared at 400 nm and the * transition is observed at
71 nm seperately with a tailing part which extended up to 900 nm in H O (Figure 1). From
2
O. The intramolecular charge transfer (ICT)
2
3
2
solvent dependent UV-Vis spectra, it is evident that the specific solute-solvent interaction is
not observed in polar protic solvents (Figure 1). The tailing part in UV-Vis spectra is clearly
indicating the solvent specific strong solute-solvent interaction between BBM and H
aborption maxima (max) of BBM in different solvents is tabulated in ESI-Table 1.
2
O. The


Figure 1. Normalized UV-Visible spectra of BBM in different solvents at 25 C
Effect of Solvents On UV-Visible Studies at Room Temperature
For investigating the nature of photodissociation reaction and solute-solvent interaction, time
dependent UV-Vis spectra of BBM is carried out in different solvents. During time dependent
UV-Vis studies, the corresponding UV-Vis spectra of BBM in polar aprotic solvents is not
changed (under UV irradiation at 365 nm for 1 minute before and after UV-Vis measurement)
but significant spectral change is observed in prolar protic solvents (ESI-Figure 1-4) under
same condition. This obsrevation indicates that the photodissociation process is occurred
during UV-Vis spectral measurement in polar protic solvents (Figure 2 and ESI-Figure 1-4).
Decrease of spectral intensity with blue shift of the ICT absorption band (Figure 2-3 and ESI-
Figure 1-4) is observed in polar protic solvents. From the time dependent UV-Vis spectra of
BBM in polar protic solvents, interestingly, the change of UV-Vis spectral pattern in H
with two isobestic points (observed at 598 nm and 283 nm) is different from other polar
protic solvents (ESI-Figure 1-4). The observed unique charactristic spectral change in H O is
O interaction (Figure 2) during photodissociation
2
O
2
due to the presence of strong solute-H
2
process.

Figure 2. Time dependent UV-Visible spectra of BBM in H
2
O at 25 C (External UV
irradiation at 365 nm for 1 minute before and after UV-Vis measurement)
Further, in binary solvent (ACN: H
monitored at room temperature and it is observed that the spectral pattern in binary solvent
ACN: H O= 1:1 (v/v)), is entirely different in nature from pure H O solvent (figure 3 and 4)
at similar reaction condtion. The tail part which is observed in UV-Vis spectra of BBM in
2
O= 1:1 (v/v)), photodissociation process of BBM is also
(
2
2
H
2
H
2
H
2
O, is completely disappeared in case of UV-Vis spectra of BBM in binary solvent (ACN:
O= 1:1 (v/v)) at 25 C. The absorption band at 348 nm of BBM in binary solvent (ACN:
O= 1:1 (v/v)) is decreasing with time and shifted to 315 nm during photodissociation
process at 25 C. During photochemical process at 25 C, two isobestic points are observed at
17 nm and 243 nm in binary solvent (ACN: H O= 1:1 (v/v)). Above experimental results in
O and binary solvent (ACN: H O= 1:1 (v/v)) indicate that nature of spectral pattern is
affected by the nature of solvents during photodissociation process.
3
2
H
2
2

Figure 3. Time dependent UV-Visible spectra of BBM in ACN: H
External UV irradiation at 365 nm)
2
O (1:1 (v/v)), at 25C
(
Figure 4. Time dependent UV-Visible spectra of BBM in H O at 45 C (External UV
2
irradiation at 365 nm)
st
For this photodissociation process, 1 order kinetic model is considered and rate constant (k)
-3
-1
is found to ~10 s in different protic solvents and binary solvent (ACN:H O = 1:1 (v/v)) at
2
st
2
5 C (Table 1). The pseudo 1 order rate constant (k) is calculated during photochemical
process by monitoring the decrease of absorption band at 403 nm with time in H O at 25 C
2
-
3
-1
st
and found to be 2.4x10 .s . Similarly, the pseudo 1 order rate constant (k) is calculated by
monitoring the decrease of absorption band at 348 nm with time in binary solvent (ACN-
-4 -1
H O) and found to be 6.2x10 .s at 25 C. Further, the rate constant (k) is found to be
2
-4
-1
-4 -1
4
.4x10 .s for EtOH and 1.2x10 .s for 2-Pro-OH respectively monitoring the decresing
absorbance at 348 nm (Table 1). The rate constant (k) in polar protic solvents is presented in
Table 1 and ESI-figure 1-4.
Table 1: Rate constant (k) in protic polar solvents at 25 C
H
2.4x10
0.99
2
O
ACN-H
6.2x10
0.99
2
-4
O
EtOH
4.4x10
0.99
2-Pro-OH
1.2x10
-
1
-3
-3
-3
k (s )

ퟐ
0.99
Temperature Effect on Photodissociation Reaction
Temperature effect on photodissociation process of BBM is also investigated in H
ACN-H O solvents seperately. It is observed that the UV-Vis spectral pattern of BBM in H
is significantly changed from 25 C to 55 C during time dependent UV-Vis studies (Figure
-4 and ESI-Figure 5-7). Interstingly the change of UV-Vis spectra is not observed in H O at
2
O and
2
2
O
2
2

1
5C (ESI-Figure 5), but significant change of UV-Vis spectra is observed with increase in
temperature. At higher temperature (> 45 C), a prominent new absorption band is appeared
at 283 nm with three distinct isobestic points at 288 nm and 241 nm in UV-Vis spectra of
BBM, indicates that the photochemical process is occurred at fast rate with higher amount of
photodissociated product A (Figure 4). The most interesting factor in this case is that the
st
pseudo 1 order rate constant (k) in H
2
O solvent is decreased with increase in temperature
(
Table 2). Similarly, effect of temperature on photochemical process of BBM in ACN-H
2
O
(1:1 (v/v)), is also investigated at different temperatures. It is observed that UV-Vis spectral
pattern of BBM in ACN-H
2
O (1:1 (v/v)), is also changed significantly from 15 C to 55 C,
st
during time dependent UV-Vis studies (ESI-Figure 8-11). The pseudo 1 order rate constant
(
k) of BBM in ACN-H
2
O increases with increase in temperature (table 2).
O and ACN-H O systems are also observed at different
Further, rate constants (k) in H
2
2
temperature and presented in table 2. The results clearly indicate that the photochemical
process is influenced by the nature of solvents due to solvent specific solute-solvent
interactions. The rate constant (k), is decreased in H
with with the rising of temperature. This observation can be explained on the basis of
solubility of the product (2-(4-formylbenzylidene)malononitrile) during the
photodissociation process. The product A (2-(4-formylbenzylidene)malononitrile) might has
the greater solublity in ACN-H O binary system compared to pure H O.
Table 2. rate constant (k) for BBM at different temperature
2
O, but increased in binary ACN-H O
2
A
2
2
k
@
obs in H
403 nm
2
O
k
obs in ACN-H O
2
Temperature
@ 348 nm
-
4 -1
1
2
3
4
5
5 C
5 C
5 C
5 C
5 C
a
4.26x10 .s
-3
-1
-1
-1
-4 -1
2.42x10 .s
2.80x10 .s
1.85 x10 .s
1.53 x10 .s
6.21x10 .s
-3
-4 -1
15.80x10 .s
26.63x10 .s
34.61x10 .s
-3
-4 -1
-3
-1
-4 -1
a
Photochemical process is not observed
Fluorescence Studies: Effect of Solvents
Fluorescence spectra of BBM is measured in different solvents and it is observed that the
fluorescence spctra is not influenced much by solvents and their polarity. Fluorescence result
is presented in ESI-Table 1 and ESI-Figure 12. It is observed that fluorescence spectral
4
4
pattern of BBM is similar to previously reported fluorescence results. The photochemical
process of BBM is also monitored by time dependent fluorescence spectra in H O and H O-
ACN solvents seperately at room temperature. For studying kinetics of BBM in solution by
2
2

-6
fluorescence spectroscopy, ~10 (M) of BBM is taken in solvents during experiment. This
solution of BBM is kept under UV irradiation at 365 nm for 1 minute before and after
fluorescence measurement in solvents. Commercially available 4 watt power of UV lamp @
3
65 nm is exposed for 1 minute on the sample for time dependent spectroscopic
measurement. From time dependent fluorescence results, the fluorescence intensity is
decreasing with time in H O solvent at ex = 360 nm (Figure 5). But interesting factor is that
fluorescence intensity in ACN-H O (1:1 (v/v)) solvents is unaltered with time at ex = 340 nm
ESI-Figure 13). The unique observation in ACN-H O (1:1 (v/v)) solvents may be same
emision spectra of BBM and photochemical product A at ex = 340 nm.
2
2
(
2
Figure 5. Time dependent fluorescence spectra of BBM in H
2
O (ex =360 nm) (Exposer of
external UV Irradiation at 365 nm)
From previously reported crystal structure of BBM, it is clearly observed that in solid state
most stable conformer is S-trans (figure 6).44 The intermolecular interactions and
photophysical studies of BBM and its analogues were reported in previous reported work in
43
solid state. It is clear that the C=C bond of BBM is activated under external UV irradiation
and takes part in photodissociation process (Figure 8).
In our presnt work, it is observed that selective photodissociation of BBM is highly feasibile
and occurred at slower rate in specific polar protic solvents under the external exposer of UV
radiation at 365 nm. In presence of dicyano group (acceptor), the C=C bond of BBM
molecule becomes highly polarizable and becomes susceptible for the photodissociation,
even at room temperature.

4
4
Figure 6: ORTEP diagram of BBM (CCDC No. 1056280) (ref. )
Optimization of the geometry at ground state of BBM is carried out at B3LYP/6-311++g(d,p)
level of theory and shown in figure 8. The molecular orbitals involved towards HOMO and
LUMO of BBM at ground and excited state electronic properties, are investigated using the
TD-DFT calculation at the B3LYP/6-311++G (d,p) level of theory. The HOMO-LUMO
calculations are carried out using TD-DFT method at the B3LYP/6-311++G (d,p) level of
theory using the polarisable continuum model (PCM) and ACN as solvent.
From TD-DFT calculations, the higher wavelength absorption band corresponds to
HOMO→LUMO transition is exclusively represents the ICT state and clearly indicates that
the intramolecular charge transfer (ICT) process is taking place from benzene ring to dicyano
group in BBM. This intramolecular charge transfer (ICT) state of the BBM molecule may
results the partially double bond character of C=C bond in BBM. The calculated electronic
absorption spectrum reproduces the experimental spectrum though the intensity of the S
0
S
1
transition (Figure 7). The experimental and theoretical results are well matched and
presented in figure 7.
Figure 7. Optimized structure and HOMO-LUMO diagram of BBM from DFT calculation

From above experimental UV-Vis and fluoresence results, it is observed that
photodissociation process occurs in H O and binary ACN-H O solvent (1:1 (v/v)) (Figure 8)
under UV irradiation via the dipolar state (( C
2
2
δ+
δ-
C ).). The presence of dicyano group in
δ+
δ-
BBM, enhanced the formation of dipolar state ( C C ) and become more stabilized by
4
5
δ+
δ-
polar protic solvents. In presence of H
2
O and UV irradiation, the dipolar state ( C
C )
becomes highly active and favours the photodissociation reaction and forms photodissociated
product A (Figure 8). In presence of dicyano group, activation energy barrier of C=C bond in
BBM may be lower and photodissociation process takes in H O.
2
1
From H-NMR studies, charactristic chemical shift of CH proton in CH=C(CN)
2
is observed
at 8.62 ppm and aromatic CH chemical shift is at 8.10 ppm in solution of pure BBM molecule
(
ESI-Figure 14-15). After UV irradiation@365 nm on solution of BBM in H
ESI-Figure 16-17), we observed new proton peak at 10.09 ppm, 8.66 ppm and 8.11 ppm with
.62 ppm and 8.10 ppm. These new characteristic proton peaks are due to presence of CHO,
H proton (CH=C(CN) ) and aromatic protons of new product A (2-(4-
formylbenzylidene)malononitrile). These new proton peaks are confirmed by the formation of
2-(4-formylbenzylidene)malononitrile) from selective photodissociation reaction of BBM
under UV irradiation in H O. Further, the carbonyl group (C=O) is also observed at 193.0
ppm in C-NMR from product A (2-(4-formylbenzylidene)malononitrile) (ESI-Figure 16-
7). The product A (2-(4-formylbenzylidene)malononitrile) has also showed the
2
O for 5 minute
(
8
2
(
2
1
3
1
characteristic mass at 182 (m/z) in GCMS spectra (ESI-Figure 18).
From above experimental results, it may be stated that the C=C bond is highly polarizable in
δ+
δ-
presence of cyanide groups and shows partial double bond character ( C
C ). This partial
δ+
δ-
double bond ( C
C ) may interact with H
2
O and formed Product A (Figure 8).
Figure 8. Propose photodissociation pathway of BBM in H O solvent
2
Conclusion

In conclusion, photodissociation of selective C=C bond in BBM is investigated in solution
and found to be highly solvent specific and temperature sensitive in nature. The presence of
dicyano group (acceptor) in BBM makes the C=C bond highly polarizable, photoactive and
susceptible for photodissociation process even at room temperature. In presence of dicyano
group, the formation of dipolar state is enhanced and stabilized by polar protic solvents and
followed dipolar mechanism. The product A (2-(4-formylbenzylidene)malononitrile) is
formed exclusively and identified from photochemical reaction under UV irradiation in H
2
O
and H O –ACN solvents. Kinetic study of this photodissociation process was also carried out
2
st
-3
-4 -1
and found to follow pseudo 1 order mechanism with a rate constant (k) 10 -10 .s .
AUTHOR INFORMATION
Corresponding Author
SKP: e-mail: sumitkpanja@gmail.com
ORCID
Sumit K Panja: 0000-0001-5658-0452
H. Boumediene: 0000-0001-9198-9528
ACKNOWLEDGMENT
SKP acknowledges to Prof. Puspendu K. Das for his suggestion and SERB for N-PDF
post-doc fellowship (Scheme: N-PDF/2016/000041) at Department of Inorganic and
Physical Chemistry (IPC), Indian Institute of Science (IISc), Bangalore-560012, India.
Notes
The authors declare no competing financial interest

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