Tuning the intramolecular charge transfer (ICT) process in push-pull systems: Effect of nitro groups

Article abstract of DOI:10.1039/c6ra17521j

The intramolecular charge transfer (ICT) process in donor-acceptor systems has tremendous importance in various physical and biological systems. Three nitrophenolate salts were synthesized and studied here. The ICT and π → π? transition processes were identified in these derivatives using UV-Vis spectroscopy and theoretical calculations. It was observed that by simple substitution with nitro groups, one can generate and control the ICT process by regulating the charge distribution over the molecule. While for a monosubstitute nitro derivative, only one ICT band was observed, additional ICT processes can be generated at will by introducing a second nitro group. The intensity of this second ICT channel can be regulated with introduction of a third nitro group. Further, the association constants and solvation processes for these potassium nitrophenolate derivatives were found to be drastically dependent on the number of ICT channels present in the molecule. Theoretical studies (MEP analysis) support the experimental observations presented here. The results show that by simply introducing additional acceptor groups to the system, one can tune the ICT band efficiently in a conjugate system.

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Tuning the Intramolecular Charge Transfer (ICT) Process in Push-Pull
Systems: Effect of Nitro group
Sumit Kumar Panja, Nidhi Dwivedi and Satyen Saha*
Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi-221005, India
Graphical Abstract
Keywords: Donor-acceptor, Intramolecular Charge Transfer (ICT), DFT calculation.
Abstract: ICT process in donor acceptor system has tremendous importance in various physical and biological systems.
Three nitrophenolate salts have been synthesized and studied here. Intramolecular Charge Transfer (ICT) and π→π* tran-
sition processes are identified in these derivatives with UV-Vis spectroscopy and theoretical calculation. It is observed
that by simple substitution with nitro group, one can generate and control the ICT process by regulating the charge dis-
tribution over the molecule. While for monosubstitute nitro derivative, only one ICT band is observed, additional ICT
process can be generated at will by introducing another nitro group. The intensity of this second ICT channel can be
regulated with a further introduction of a nitro group. Further, the association constants and solvation processes for these
potassium nitrophenolate derivatives found to be drastically dependent on number of ICT channels present in the mole-
cule. Theoretical studies (MEP analysis) supports the experimental observations presented here. The results show that by
simply introducing additional accepting group in the system, one can tune the ICT band efficiently in a conjugate system.
.
interesting photophysics. Hyperpolarizable organic
molecules that display intramolecular charge transfer
INTRODUCTION
The ICT process is basics for conversion of solar energy
transition (ICT) are important building blocks for
into chemical energy allowing of photosynthesis for
advance optical materials.2 The degree of charge transfer
(CT) character of an electronic transition differs
significantly between chromophores and is modulated by
crucial charge-separation processes.1 Many protein
biochromophores and bioluminophores have shown

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a nearby environment. One such class of species is the
nitrophenolates that serve as donor-acceptor molecules,
or so-called push-pull chromophores, since an electron is
transferred from the negatively charged phenolate to the
nitro group upon photoexcitation, with a significant
change in the charge distribution in the molecule. The
ICT process depends on both donor and acceptor
strength i.e. the energy of the HOMO of donor and
LUMO of acceptor, as well as extent of electronic
coupling between orbitals determined by spacer. The
spacer can act as an insulator or a conductor depending
on the magnitude of coupling by the spacer between the
two groups. Both the transition energy and oscillatory
strength are linked to the coupling between the donor
and acceptor group: The weaker coupling, the smaller the
excitation energy.3
is highly efficient for charge transfer complex formation.10
Kirkrterp et.al. have reported the effect of electronic
coupling between two orbitals conveyed by spacer group
on ICT process of nitro phenolate ions in vacuum and in
solution.11
In our present work, effect of nitro group on these two
different ICT process have been studied by experimental
and theoretical methods. We have shown that it is
possible to creat two distinct ICT patways by simple
incorporation of nitro groups at different positions in
phenyl ring. Solvent effect on these two ICT bands has
been monitored and explained on the basis of standard
solvent polarity model.
EXPERIMENTAL SECTION
Chemicals and Solvents
In this regard, it is advantageous to know the intrinsic
absorption of the isolated molecule or ion to shed light on
the electronic perturbation by a microenvironment. The
degree of CT character of an electronic transition differs
significantly between chromophores and is modulated by
nearby environment. Ionophores are not easily dissolved
in nonpolar solvents and even then there are field effects
from counter ions. In polar solvents, the ground and
excited states are stabilized to different extents, which
results in solvatochromic shifts. When electron-
withdrawing groups which can stabilize the phenoxide
ion are present, a substantial change in acidity of the OH
group can be changed significantly and highly sensitive
towards environment.4 Moreover, the hydroxyl group is a
weak electron donating group whereas the deprotonated
hydroxyl group, i.e, O- is a much stronger donating group
with a negative charge that can be delocalized along a
All starting materials were used as received from Sigma
Aldrich without further purification. Ultrapure water,
acetonitrile, and dichloromethane, ethanol, DMSO, and
others solvents are of analytical grade and found to be
completely
transparent
in
the
range
UV/Vis
spectrophotometric studies.
Instrumentation
Electronic absorption spectrum was measured on Varian-
UV-Visible spectrophotometer (Carry-100-Bio) in various
solvents. The FT-IR spectrum of the investigated
compounds was recorded in the 4000-400 cm-1 region
with a Shimadzu FTIR-8900 spectrophotometer using KBr
pellet.
Quantum Chemical Calculations
DFT calculations with a hybrid functional B3LYP and
CAM-B3LYP functional at 6-311G (d, p) basis set by using
Berny method were performed with the Gaussian 09W
software package.12 The electronic absorption spectra were
calculated using the time-dependent density functional
theory (TD-DFT) method at B3LYP/6-311G (d, p) and
CAM-B3LYP/6-311G (d,p) using the polarisable continuum
model (PCM) and ACN as solvent. The NBO and MEP are
calculated using same method.
conjugated path involving the benzene ring. As
a
consequence, the phenol and the corresponding
phenolate ion display very different absorption optical
properties.
Phenolic compounds, specifically, nitro phenolic
derivatives are extensively used as model compounds for
structural studies, extraction and transport through
membranes. The polarizable and soft nitrophenolate ion
are often used to facilitate and increase the extraction and
transport of alkaline metal cations through solvents of
low polarity.5 Interesting nonlinear optical effects
extended to optical parametric amplifiers, optical
parametric oscillators, Q-switched intracavity second
harmonic devices, high optical damage threshold and
other electro-optical applications are observed.6 It was
reported that nitro phenolic derivatives have shown
significantly changed in spectral pattern and maxima of
absorption spectra when complex is formed with different
amine in solution.7 So far, few experimental and
theoretical studied have been reported related to
nitrophenolate derivatives.8 El-Dossoki has measured the
ion-pair association of sodium and potassium picrate in 2-
butanone by conductometric and spectroscopic studies.9
Ahmed et.al. have reported that nitro phenolic derivative
Chart 1: Structural Formula and Abbreviations for In-
vestigated Molecular Systems
Results and Discussions
Tuning the ICT bands:
The electronic properties of nitrophenolate derivatives
have been investigated depending upon the nitro groups.
At first the effect on ICT process in various
nitrophenolate derivatives has been investigated. Fig. 1
shows that while only one strong and highly feasible ICT
2

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process is observed at 420 nm in UV-Vis spectra for KNP
in ACN. The number and strength of the ICT band can be
tuned by increasing the nitro group.
two ICT processes are therefore can be regulated by
varying the number of nitro group.
As can be seen from the Fig. 1 (red line) that the
incorporation of second nitro group, makes two ICT band
appeared at 421 nm and 371 nm (for KDNP in ACN).
Among these two ICT bands in KDNP: one higher
wavelength is due to channel I and lower wavelength is
due to channel II respectively (schematically depicted in
Fig. 2). Due to longer conjugation and presence of the
channel 1, absorption band comes in lower energy, i.e., at
higher wavelength.
Solvent Effect on ICT process:
The solute-solvent interaction is investigated by the
variation of polarity and nature of the surrounding
solvent molecules. The spectral shift, intensity and shape
are influenced by environment and polarity of solvents
depending upon number of nitro groups. KNP and KDNP
have shown larger spectral shift of ICT band at higher
wavelength and found to be highly sensitive towards
solvent polarity (Fig. 3 and 4). On the other hand, KTNP
is comparatively less sensitive towards solvent polarity
and nature of the solvents (Fig. 5). As the Fig. 3 shows
that in nonpolar solvents like dioxane and THF, there is
no ICT band and the ICT band becomes prominent in
polar solvent. Among polar solvents, DMSO shows the
largest red shift. Low energy ICT band for KDNP is also
regulated by the solvent polarity and decreases with
decrease of solvent polarity. But in case of KTNP, the
lower energy ICT band is clearly separated from higher
energy ICT band with decrease of solvent polarity. Mainly
red shift of lower energy ICT band is observed in polar
aprotic and blue shift in polar protic solvents (~45 nm for
KNP, ~40 nm for KDNP and ~38 nm for KTNP from
methanol to DMSO solvent systems). As seen in Fig. 3,
KNP has shown strong solvent sensitivity of ICT band (for
channel 1) in different solvent which is absent in nonpolar
solvent such as THF and dioxane. However, interestingly
the ICT band for channel 1 is clearly observed for KTNP
even in THF and dioxane. From the solvent dependent
studies (Fig. 3 to 5) of these nitrophenolate salts it can be
stated that the different sensitivity of ICT bands (lower
energy ICT band compared to higher enery ICT band)
towards the solvent polarity is observed.
Fig. 1 Absorption spectra of KNP, KDNP and KTNP in ACN
Further when we incorporate third nitro group, new ICT
channel (channel III) is generated (making a one donor-
three acceptor system) but overlapped with the channel
II (at 375 nm) because of the similarity of the position
(i.e., both are at ortho position in KTNP, Fig. 1, blue line).
Push
Push
Push
I
I
Donor
I
I
l
e
C
h
l
e
n
n
n
n
a
a
n
n
Donor
a
h
C
h
e
C
l
l
l
Donor
l
I
l
u
I
u
I
P
P
l
l
u
P
Acceptor
ICT
Channel I
ICT
Channel I
Acceptor
Acceptor
Acceptor
Cha
nne
l I
Acceptor
Acceptor
Pull
Pull
Pull
KNP
KDNP
One Donor-Two Acceptor
KTNP
One Donor-One Acceptor
One Donor-Three Acceptor
Fig. 2 Schematic representation of ICT channels depending upon
nitro group.
The highly feasible and intense ICT band in KNP are due
to delocalized charge over molecules. Similarly highly
feasible two ICT bands (one ICT at higher wavelength due
to channel I and another ICT at lower wavelength due to
channel II) in KDNP (one donor and two acceptor
system) are due to two different channel of ICT process.
Interestingly in KTNP the band at lower wavelength
becomes much stronger due to combine contributions of
channel II and III. The intensity and feasibility of these
Fig. 3 Normalized absorption spectra of KNP in various solvents
3

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solvents and solvates ions separately resulting the ‘ion
solvation’.
Further we have also correlated the solvatochromic
behavior of nitro phenolate derivatives using empirical
solvent polarity parameter, ET (30). The ET (30) scale has
found wide-spread applications in studying chemical
processes in solvents. This parameter is usually measured
by UV-Vis spectrophotometric measurements of the
longest wavelength ICT absorption band of Reichardt's
pyridinium-N-phenolate betaine dye (known as ET (30)
dye) dissolved in the solvent or solvent mixture of
interest.13 The ET (30) values, empirically derived from
solvatochromic measurements, are simply defined as the
molar transition energies (in kcal mol-1) of the standard
betaine dye 30, measured in solvents of different polarity
at room temperature (25 °C) and normal pressure (1 bar),
according to equation (1):
Fig. 4 Normalized absorption spectra of KDNP in various solvents
ET (30)/ (kcalmol-1) = 28591/λmax …………..
(1)
where λmax is the wavelength of the maximum of the
long wavelength intramolecular CT absorption band of
the betaine 30.
Using equation 1, we have calculated the molar transition
energies (in kcal mol-1) of nitrophenolate derivative,
measured in different solvents and presented in Fig. 6 and
Tab. 2. The ET value of nitrophenolate derivatives in THF
is much higher than that of ET (30) value, is ~31.6 kcalmol-
Fig. 5 Normalized absorption spectra of KTNP in various solvents
1
.
Similarly difference between the ET value of
nitrophenolate derivatives and ET (30) value in H2O is
~7.0 kcalmol-1. From our experimental results, it is
observed that the different between the ET value of
nitrophenolate derivatives and ET (30) value decreases
gradually with the increase of ET (30) value. The above
results indicates that the ICT band of nitrophenolate
derivatives is not influenced by the lower polarity of
solvents (ET value in THF = 68.0 kcalmol-1, ET (30) value in
THF = 37.4 kcalmol-1) but more effected by higher polarity
of solvent due to the comparable ET value of
nitrophenolate derivatives ET value in H2O = ~ 70.0
kcalmol-1, ET (30) value in H2O = 63.1 kcalmol-1. Further
the transition energy is not changed drastically depending
upon solvents and is quit higher than that of ET (30) value
with respect to solvents (Tab. 2).
In addition, for KNP, while π-π* transition is clearly
visible in EtOH and MeOH, but completely absent in
water. The ICT band (for channel 1) is quite sensitive with
polarity as evident from the ~ 45 nm shift of peak maxima
from DMSO to methanol. Tab. 1 shows the variation of
wavelength maxima of ICT band with solvent polarity.
Tab. 1 Absorption maxima (λmax) of nitrophenolate deriv-
ative
λ
max (nm)
Solvent
KNP
435
420
398
410
KDNP
431
KTNP
435
430
416
DMSO
ACN
421
H2O
400
403
398
395
416
PrOH
EtOH
MeOH
THF
420
415
402
390
310
410
421
Dioxane
302
417
421
After detailed analysis of UV-Vis spectra in different
solvent for K-salts, we are confident that the ‘ionic couple’
solvation is observed in polar but aprotic solvents like
ACN, DMSO and ‘ion solvation’ is observed in polar protic
solvents like H2O, EtOH and MeOH. In later case, the ‘ion
couples’ are completely disrupted by the protic polar
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Fig. 6 Solvatochromism of nitrophenolate derivatives. The absorp-
band is observed which is due to π→π* transition.
However, on formation of KTNP from HTNP, ICT
absorption band shifted to 375 nm from 338 nm (Fig. 9).
The π→π* transition band is also blue shifted. It is
observed that low energy ICT band is significantly
reduced while that of at high energy one increases with
increasing the number of nitro group on phenolate
moiety.
tion spectra were measured in H2O (ET (30): 63.1 kcal mol-1), MeOH
(55.4), EtOH (51.9), ACN (45.6), DMSO (45.1), THF (37.4). ET (30)
values of different solvents are taken from literature.14
Tab. 2 Solvatochromic parameter ET value of nitropheno-
late derivative
ET value (λmax (nm))
Solvent
KNP
65.7
nm)
68.0
nm)
KDNP
KTNP
65.7
nm)
66.4
nm)
68.7
nm)
68.0
nm)
68.8
nm)
69.7
nm)
67.9
nm)
67.9
nm)
(435
(420
(435
(430
(416
(420
(415
(410
(421
(421
DMSO
ACN
66.3 (431 nm)
67.9 (421 nm)
H2O
71.8 (398 nm) 71.4 (400 nm)
69.7
nm)
(410
PrOH
EtOH
MeOH
THF
70.9 (403 nm)
71.8 (398 nm)
71.1 (402 nm)
73.3 (390 nm) 72.3 (395 nm)
Fig. 7 UV-Vis titration for the formation of KNP. KOH is added
-
-
68.7 (416 nm)
68.5 (417 nm)
successively (0 to 2.28 x 10-4 M) in HNP (3.35x10-5 M in ACN).
Dioxane
Equilibrium Constant (Keq) from Benesi Hilde-
brand Spectroscopic Method
Here, strong donor-acceptor systems have been created
from weak donor-acceptor using strong based like KOH
to investigate ICT process in nitro phenolic derivatives.
Our investigated systems having weak donor and strong
acceptor groups, are converted to strong donor-acceptor
systems using strong alkaline base as activator to
investigate combining effect of donor and acceptor on
ICT process. Initially HNP (4-Nitrophenol) has shown
only two characteristic bands due to presence of strong
ICT band at 308 nm and π→π* transition at 225 nm in
ACN (Fig. 7). But when HNP is converted to KNP by
addition of KOH, a new strong ICT band at 420 nm in
ACN generated while the original ICT band disappeared
(Fig. 7). The figure also clearly indicates that, it is the ICT
band at 308 nm which has shifted to 420 nm. A much less
and insignificant blue shift of the π→π* transition band
(234 nm to 225 nm) is also observed. In case of HDNP
(2,4-Dinitrophenol), two absorption band due to ICT and
π→π* transition are observed at 331 nm and 258 nm
respectively in ACN (Fig. 8). During formation of KDNP
from HDNP on addition of KOH, a new but similar
characteristic strong ICT absorption band at 421 nm is
found to appear along with a 371 nm band (shifted from
331 nm) (Fig. 8). Several isosbestic points are
consequently observed (at 315, 232, 264 and 351 nm). The
π→π* transition band is shifted to 219 nm from 258 nm
on KDNP formation with two clear isosbestic points at
240 nm, 330 nm. Further in case of HTNP (2,4,6-
Trinitrophenol), ICT band at 435 nm along with stronger
ICT band at 338 nm are observed. At 235 nm another
Fig. 8 UV-Vis titration for the formation of KDNP. KOH is added
successively (0 to 6.14x10-5 M) in HDNP (5.06x10-5 M in ACN).
Fig. 9 UV-Vis titration for formation of KTNP. KOH is added succes-
sively (0 to 7.60x10-5 M) in HTNP (4.65x10-5 M) ACN.
It is clearly indicated that modified ICT process is
observed due to the presence of strong base like KOH in
ACN (Aq. KOH solution). In presence of strong base
(KOH), phenolic O-H (weaker donor) is converted into
phenolic O- (strong donor), as a results, strong donor-
acceptor system is generated and highly feasible ICT
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process is observed. Above experimental results suggest
that relative charge density on the oxygen atom of C-O
group is altered in presence of number of nitro groups.
The charge density on oxygen atom is higher and acts as
electron donor. This charge density can be easily be
delocalized to benzo-nitro moiety causing the strong ICT
process. Similarly, due to presence of two nitro groups (in
KDP) charge density on oxygen atom (C-O) is
redistributed over the molecule and delocalized primarily
to two nitro groups, resulting in two strong ICT processes.
However, in presence of three nitro groups (KTP), ICT
process at higher wavelength is suppressed due to doubly
favourable ICT process at lower wavelength. The above
observations are supported by DFT calculations as
discussed in the following section.
From spectral shift of solvent dependent studies it
appears that different solvated state exists depending
upon the nature and polarity of the solvents. The ion-pair
solvation increases with the variation of solvent polarity
and feasible ICT process (red shift of ICT band with
increasing the solvent polarity with respect to dielectric
constant). The results were interpreted to support the
existence of strong ionic O•••K structure. Experimentally
the red shift of the ICT band is observed in aprotic polar
solvent {(MeOH (32.7) to DMSO (46.7)} but blue shift of
ICT band (~ 40 nm) for protic polar solvent system.
Further, Benesi-Hildebrand method has been employed
to determine equilibrium constant (Keq) by UV-Vis
spectroscopy. It is an important parameter to understand
the nature of ion-pair in solution. On the basis of Benesi-
Hildebrand (BA) spectroscopic method, the equilibrium
constant (Keq) has been determined by monitoring the
ICT bands and calculated according to the equation 2 for
1:1 system.
Tab. 3 Equilibrium Constant (Keq) of nitrophenolate de-
rivatives in ACN. χ2 values indicate the goodness of the
fittings.
KA (cm3 mol−1)
χ2
Intercept
(1/[A-Ao]
0.10
Slope
(1/KA[A-Ao])
1.68x10-7
KNP
0.99
0.99
0.99
5.95
(@420 nm)
x105
x103
KDNP 0.43
KTNP 0.17
5.69x10-5
6.90x10-5
7.55
(@421 nm)
2.46
x103
(@435 nm)
UV-Vis studies of these nitrophenolate derivatives help us
to predict a quantitative electronic energy level diagram
to understand alternation of energy levels depending
upon number of nitro group. ICT-1state is for channel I
and ICT-
Further it is also observed that the ratio of spectral areas
of two ICT bands {Area (channel- ) / Area (Channel II/
2
is for channel II and channel III both.
1
Channel III)} decreases with increase of number of nitro
groups as expected, indicates the strong interaction
between benzene ring electron and electron acceptor
groups (Table 4). The ration of A421 (channel-1)/ A369
(Channel II) is 0.98 and the ration of A421 (channel-
1)/A369 (Channel II and Channel III) is 0.20. The above
result indicates that incorporation of channel III
(presence of third nitro group), drastic change of spectral
area in UV-Vis spectra of KTNP derivatives, results less
probable ICT-1 state compared to ICT-2 state.
Tab. 4 Spectral band shape and area and full width half
maxima (FWHM).
Peak
Position (a.u.)
Area
FWHM Height Ratio
Systems
KNP
ꢀ
ꢀ
ꢀ
(nm)
(a.u.)
=
+
[ꢃ
ꢇꢈꢉ
ꢂꢃ ]…..
2
ꢆ[
]ꢊ
ꢂꢃ [ꢅꢋꢌ]
[ꢁꢂꢃ ]
ꢅ
ꢁ
ꢃ
ꢄ
ꢇꢈꢉ
ꢄ
ꢄ
52.95
0.26
-
420 nm
421 nm
14.68
9.82
50.62
51.33
0.18
0.19
A421/A3
where Ao = absorbance without KOH, A = absorbance on
addition of KOH, Amax = absorbance at the end of titration
(final KOH addition), Keq = equilibrium Constant.
KDNP
KTNP
69
369 nm
435 nm
375 nm
10.42
27.12
= 0.98
35.37
61.30
0.60
1.72
A
435/A3
75
Spectrophotometric titrations allows us to determine ion
equilibrium constant using the above equation. Fig. 6-8
depicts the B-H plot obtained using the equation 1 from
the UV-Vis titration of all. The B-H plot is carried out at
particular wavelength (420 nm for KNP formation, 421 nm
for KDNP and 435 nm for KTNP respectively). In every
cases B-H plot is fitted linearly with equation 1(vide ESI-
Fig. 1 (a-c)). The linearity of B-H plot indicates that
experimental result is acceptable for 1:1 system.
Equilibrium constant (Keq), intercept and slope are
obtained from linear fit curve (data presented in Tab. 3).
The Keq of KNP is 5.95x105 cm3 mol−1 and is higher by 100
times that of KDNP (Keq =7.55x103 cm3 mol−1) and KTNP
(Keq = 2.4x103 cm3 mol−1). The equilibrium constant values
of KDNP and KTNP are comparable with the previously
reported work.9
132.29
= 0.20
Quantum Chemical Calculation
Quantum chemical calculations has been performed for
these nitrophenolate derivatives to obtain useful
structural and electronic information from optimized
geometry and TD-DFT calculation respectively. The
hybrid functional B3LYP and CAM-B3LYP (Becke’s three
parameter hybrid functional using the LYP correlation
functional) at 6-311G (d, p) basis is used. From optimized
geometry of KNP, it reveals that potassium ion is directly
and linearly attached to the oxygen atom of hydroxyl
group of KNP (ESI-Fig. 2a). But in KDNP, potassium ion
is acting as bridging atom between oxygen of hydroxyl
group with oxygen of nitro group at ortho position. It
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helps to make an overlap with oxygen atom of neighbour
309 nm) due to HOMO to LUMO (+1) transition orbitals.
Similarly the two ICT transitions in KDNP are appeared at
365 nm (experimentally 421 nm) and 330 nm
(experimentally 371 nm) due to HOMO to LUMO and
HOMO to LUMO (+1) transition orbitals respectively.
Further two ICT transitions in KTNP are calculated at 388
nm (experimentally at 435 nm) and 357 nm
(experimentally at 375 nm) due to HOMO to LUMO (+1)
and HOMO to LUMO (+1) transition orbital respectively.
From TDꢀDFT calculation using the polarisable continuum
model (PCM) and ACN as solvent, we clearly observed that
the charge of anion is distributed two different channel from
orbital contribution during electronic transition for DNP and
TNP anion from HOMOꢀLUMO calculation using CAMꢀ
B3LYP method. For DNP anion, 1st electronic transition
(HOMO to LUMO; f = 0.259; ICTꢀChannel I) results for the
participation orbital of para nitro group. The 2nd transition
(HOMO to LUMO (+1); f = 0.3052; ICTꢀChannel II) results
due to higher contribution of orbital of one ortho nitro group.
For TNP anion, 1st electronic transition (HOMO to LUMO; f =
0.0154; ICTꢀChannel I) is due to involvement of orbital of
para nitro group for charge distribution process, results in very
weak ICT process for ICT channel I. The 2nd electronic
transition (HOMO to LUMO (+1) transition; f = 0.69258; ICTꢀ
Channel II)) results mainly for the participation orbital of two
ortho nitro group. It is observed that the energy difference
between 1st and 2nd excited state energy level are gradually
increased with increase of nitro group. After performing the
orbital contribution analysis, it is observed that mainly the π→
π* transition is involved during electronic transition. The
electron density of the π* orbitals are altered depending upon
position and no. of nitrogroups during electronic transition
(Table 6).
nitro group having bond angle 66.9 Å with 2.50 Å and
2.42 Å respectively (ESI-Fig. 2b). Prominent bridging
activity of K+ ion is observed in KTNP derivative, where K+
ion makes bridge with three oxygen atom (One oxygen
atom from hydroxyl group and two oxygen atoms from
adjacent nitro groups) with bonds 2.44 Å (hydroxyl
group) and 2.91 Å (oxygen atoms from adjacent nitro
groups) respectively along with bond angle of 61.4°. (ESI-
Fig. 2c). The transformation from single bond to partial
double bond character of the oxygen atom of hydroxyl
group in nitrophenolate derivatives is observed with
increase of nitro group. The bond distance of hydroxyl
group (C-O) in KNP is found to be 1.29 Å, quit shorter
than that of pure C-O bond (1.43 Å). Similarly the bond
distance of hydroxyl group (C-O) in KDNP and KTNP is
1.26 Å and 1.24 Å respectively and comparable with the
pure C=O bond (1.20 Å). Above theoretical calculation
suggests that the hydroxylic group from nitrophenolate
derivatives becomes partial double bond character due to
charge transfer from oxygen atom of hydroxylic group to
the benzene ring in presence of nitro group.
TD-DFT calculation at B3LYP/6-311G (d, p) and CAM-
B3LYP/6-311G (d, p) using the polarisable continuum
model (PCM) and gas phase is also performed to better
understand the electronic properties of these
nitrophenolate derivatives. TD-DFT results at CAM-
B3LYP/6-311G (d, p) using PCM model and ACN as solvent
are reported in Table 5 and detailed in ESI-Table 1.
Tab. 5 HOMOꢀLUMO diagram obtained from TDꢀDFT calculaꢀ
tion
KNP
KDNP
KTNP
Table 6: Electronic transition from TD-DFT calculation
CAM-B3LYP/6-311G(d,p)
B3LYP/6-311G(d,p)
GAS phase
468 nm
ACN
GAS phase
363 nm
ACN
KNP
375 nm
354 nm
(f = 0.0005)
(f = 0.5156) (f = 0.0008)
418 nm 352 nm
(f = 0.1987) (f = 0.1650)
415 nm 348 nm
(f = 0.1606) (f = 0.0994)
* f = Oscillator strength; nm = nanometre
(f = 0.5597)
365 nm
(f = 0.2593)
388 nm
KDNP 402 nm
(f = 0.1154)
KTNP 406 nm
(f = 0.1222)
LUMO (+1)
(ꢀ0.19eV)
LUMO (+1)
(ꢀ2.07 eV)
LUMO (+1)
(ꢀ1.96 eV)
(f = 0.0154)
It is observed that CAM-B3LYP functional provides better
correlation compared to B3LYP functional. From the
involvement of orbital in ICT transition process, it can be
stated that involvement of benzene π electron with nitro
group play important role for ICT process. In case of KNP
benzene π electron is strongly interacted with orbital of
nitro group. This interaction is decreased with increase of
nitro group at ortho position.
LUMO
(ꢀ1.11 eV)
LUMO
(ꢀ2.71 eV)
LUMO
(ꢀ2.65 eV)
Distributions and energy levels of HOMO and LUMO
orbitals computed with CAM-B3LYP/6-311++G (d, p) level
have been shown in Tab. 6. From experimental and
theoretical results, we have drawn a schematic energy
level diagram for our investigated nitro phenolate
derivatives in Fig. 10.
HOMO
(ꢀ7.06 eV)
HOMO
(ꢀ6.22 eV)
HOMO
(ꢀ8.08 eV)
It shows the existance of a ICT and one π→π transition in
KNP. The ICT transition in KNP appeared at 354 nm due
to HOMO to LUMO transition. The π→π transition in
KNP is calculated at 268 nm (experimentally observed at
7

RSC Advances
Page 8 of 9
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DOI: 10.1039/C6RA17521J
Delocalization of electron density between occupied
Lewis type (bond or lone pair) NBO orbitals and formally
unoccupied (anti bond or Rydberg) non-Lewis NBO
orbital correspond to
a stabilizing donor-acceptor
interaction. One deprotonated hydroxyl group (electron
donor) in the aromatic ring and lone pair on oxygen atom
interacte with π system of aromatic ring and nitro group.
The intramolecular interaction is formed by the orbital
overlap between LP(O)→BD*(1)C1-C6 and →BD*(1) C5-C6
bond orbital, which results intra-molecular charge
transfer and causing system stabilization. The one of the
most important interactions in this molecule having lone
pair LP (3) O14 with that of BD*(1) C5-C6 results the
stabilization of 56.3 kcal/mol in KNP. The interaction
between lone pair of LP(2)O13 with BD*(1)C5-C6 results a
stabilization energy 13.86 kcal/mol. Further the
interaction between lone pair of LP (2) O12 with BD*(1) C5-
C6 results a stabilization energy 17.49 kcal/mol in KTNP.
Above results clearly stated that delocalization of charge
is highly effective for KNP and lowest for KTNP as shown
in Table 7.
The molecular electrostatic potential (MEP) is related to
the electronic density and is a very useful descriptor in
understanding sites for electrophilic attack and
nucleophilic reactions as well as hydrogen-bonding
interactions. The electrostatic potential V(r) is also well
suited for analysing processes based on the ‘recognition’
of one molecule by another, as in drug–receptor, and
enzyme-substrate interactions, because it is through their
potentials that the two species first ‘‘see’’ each other.
Fig. 10 Comparison of theoretically calculated (CAM-
B3LYP) energy levels with experimentally obtained energy
levels.
Natural Bond Orbital (NBO) analysis provides the
accurate possible natural Lewis structure because all
orbitals are mathematically chosen to include the highest
possible percentage of the electron density. Interaction
between both filled and virtual orbital spaces information
correctly explained by the NBO analysis, it could enhance
the analysis of intra- and inter-molecular interactions.
The second-order Fock matrix was carried out to evaluate
donor (i) - acceptor (j) i.e. donor level bonds to acceptor
level bonds interaction in the NBO analysis. The result of
interaction is a loss of occupancy from the concentration
of electron NBO of the idealized Lewis structure into an
empty non-Lewis orbital. For each donor (i) and acceptor
(j), the stabilization energy E(2) associates with the
delocalization i→j is estimated as
ꢔꢆꢑ, ꢒꢊꢎ
ꢍꢆꢎꢊ ꢏ ∆ꢐꢑꢒ ꢏ ꢓꢑ
ꢒ ꢕ ꢑ
where qi is the donor orbital occupancy, are εj and εi
diagonal elements and F(i,j) is the off diagonal NBO Fock
matrix element. The larger E(2) value, the more intensive
is the interaction between electron donor and acceptor
i.e. the more donation tendency from electron donors to
electron acceptors and the greater the extent of
conjugation of the whole system.
Table. 7 Second-order perturbation energies E(2) (donor
→acceptor).
(a) KNP
(b) KDNP
Donor
NBO
Acceptor
NBO
E(2)a
∆Eb
=
F(ij)c
(a.u.)
(kcal/mol)
E(i)-
E(j)
(i) elec- (j) electron
tron
(a.u.)
KNP
12.02
12.02
56.39
KDNP
1.91
LP(2)O14
LP(2)O14
LP(3)O14
BD*(1)C1-C6
BD*(1)C5-C6
BD*(2)C5-C6
0.79
0.79
0.29
0.088
0.088
0.118
(c) KTNP
Fig. 11: Molecular electrostatic potential (MEP) of various
molecular complexes studied here.
LP(1)O13
LP(1)O13
LP(2)O13
LP(2)O13
BD*(1)C1-C6
BD*(1)C5-C6
BD*(1)C1-C6
BD*(1)C5-C6
1.17
0.043
0.040
0.100
0.093
1.70
1.20
0.73
0.76
16.75
13.86
KTNP
1.91
Being a real physical property V(r) can be determined
experimentally by diffraction or by computational
methods. To visually consider the most probable sites of
LP(1)O12
LP(1)O12
LP(2)O12
BD*(1)C1-C6
BD*(1)C5-C6
BD*(1)C1-C6
1.17
1.17
0.73
0.043
0.043
0.102
1.91
8
17.49
LP(2)O12
BD*(1)C5-C6
17.49
0.73
0.102

Page 9 of 9
RSC Advances
View Article Online
DOI: 10.1039/C6RA17521J
Hindu University, Varanasi-221005, India, for providing in-
strumentations and infrastructure facilities. Authors also
acknowledge the CMSD facility at HCU, Hyderabad for com-
putation.
the nitrophenolate derivatives for an interaction with
electrophilic and nucleophilic species, MEP was
calculated at the DFT/B3LYP/6-311++G (d,p). MEP of
nitrophenolate derivative in presented in Fig. 11.
While electrophilic reactivity visualized by yellow colour
indicative of negative regions of the molecule, the
nucleophilic reactivity coloured by blue, indicating the
positive regions of the molecule. From MEP of KNP, it is
shown that colour of nitro group at para position is yellow
colour over the nitro group and potassium atom is blue
colour. Similarly it is shown that colour of nitro group at
para position is light yellow colour over the nitro group
along with appearance of very light yellow colour of nitro
group at ortho position and potassium atom from MEP of
KDNP. We also observed that colour of nitro group at
para position is yellow colour along with appearance of
very light yellow colour of nitro group at ortho position
(both) and potassium atom is blue colour in KTNP.
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Conclusion
In conclusion, we have investigated intramolecular charge
transfer (ICT) process in
a series of synthesized
nitrophenolate derivatives. It is found that depending on
the number of nitro (acceptor) moiety present, the
electron density redistribute via multiple ICT process.
Highly feasible strong ICT process at 420 nm (designated
as channel 1 ICT process) in ACN is observed in KNP (one
7
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donor (O-) and one acceptor (NO3 ) system). When an
additional nitro group is introduced in the KDNP system
(one donor - two acceptor system), a new additional ICT
channel (designated as Channel II) opens up at 371 nm.
Introduction of third nitro group in ortho position leads
to the increase of intensity 371 nm band considerably
while the 421 nm band intensity diminishes and no new
band appears. The ICT band (Channel I) for KNP is highly
influenced by solvent polarity while the same is much less
for KTNP. The equilibrium constant (Keq) for these
complexes are also found to be severely depend on nitro
group (~105 for KNP, ~103 for KDNP and KTNP in ACN).
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14
ASSOCIATED CONTENT
F. Shakerizadeh-Shirazi, B. Hemmateenejad and A. M.
Supporting Information This material is available free of
charge via the Internet at http:// www.rsc.org/advances
Mehranpour, Anal. Methods, 2013, 5, 891-896.
AUTHOR INFORMATION
Corresponding Author
E-mail: satyen.saha@gmail.com
Department of Chemistry, Institute of Science, Banaras Hin-
du University, Varanasi-221005, India.
Present address of SKP is Department of Inorganic and Phys-
ical Chemistry, Indian Institute of Science, Bangalore, India.
ACKNOWLEDGMENT
We acknowledge CSIR, New Delhi (code: 01(2792)/14) for
research grants and also Department of Chemistry, Banaras
9