Spectroscopic studies on the intermolecular charge transfer interaction of Fe(II)- and Fe(III)-phthalocyanines with 2,3,5,6-tetrachloro-1,4-benzoquinone and its application in colorimetric sensing of amino acids and amines

Article abstract of DOI:10.1016/j.saa.2011.09.054

The interactions of Fe^IIPc and Fe^IIIPc with π-acceptor 2,3,5,6-tetrachloro-1,4-benzoquinone (p-chloranil, p-CHL) have been investigated spectroscopically (UV/vis and FT-IR) and spectrofluorimetrically at three different temperatures. The stoichiometry of the complexes was found to be 1:1. The results of electronic spectral studies indicated that the formation constant for Fe^IIPc-p-CHL system is found to be higher than that for Fe^IIIPc-p-CHL system. This observation is well supported by the results of fluorescence quenching studies and the association constants calculated for Fe^IIPc-p-CHL system is 4.2 × 10³ mol L^-1 and that for Fe ^IIIPc-p-CHL system is 2.2 × 10³ mol L^-1. The data are discussed in terms of physico-chemical parameters viz. molar extinction coefficient, oscillator strength, dipole moment, ionization potential, dissociation energy and thermodynamic parameters. The results indicated that the formation of π-π CT complex is spontaneous and endothermic. Preliminary studies indicated that the CT complex can effectively be used as a colorimetric agent for sensing amino acids and amines.

Full text of DOI:10.1016/j.saa.2011.09.054

Spectrochimica Acta Part A 86 (2012) 44–50
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Spectroscopic studies on the intermolecular charge transfer interaction
of Fe(II)- and Fe(III)-phthalocyanines with 2,3,5,6-tetrachloro-1,4-benzoquinone
and its application in colorimetric sensing of amino acids and amines
Chellamuthu Balraj, Kuppanagounder P. Elango∗
Department of Chemistry, Gandhigram Rural Institute (Deemed University), Gandhigram 624 302, India
a r t i c l e i n f o
a b s t r a c t
The interactions of FeIIPc and FeIIIPc with ␲-acceptor 2,3,5,6-tetrachloro-1,4-benzoquinone (p-chloranil,
p-CHL) have been investigated spectroscopically (UV/vis and FT-IR) and spectrofluorimetrically at three
different temperatures. The stoichiometry of the complexes was found to be 1:1. The results of electronic
spectral studies indicated that the formation constant for FeIIPc-p-CHL system is found to be higher than
that for FeIIIPc-p-CHL system. This observation is well supported by the results of fluorescence quenching
studies and the association constants calculated for FeIIPc-p-CHL system is 4.2 × 103 mol L−1 and that for
FeIIIPc-p-CHL system is 2.2 × 103 mol L−1. The data are discussed in terms of physico-chemical parameters
viz. molar extinction coefficient, oscillator strength, dipole moment, ionization potential, dissociation
energy and thermodynamic parameters. The results indicated that the formation of ␲–␲ CT complex is
spontaneous and endothermic. Preliminary studies indicated that the CT complex can effectively be used
as a colorimetric agent for sensing amino acids and amines.
Article history:
Received 22 July 2011
Received in revised form
17 September 2011
Accepted 27 September 2011
Keywords:
Charge transfer complex
Iron phthalocyanine
Chloranil
Fluorescence study
Amino acids
Colorimetric sensors
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
spectral properties in the 600–750 nm region which is one of the
two principal features in the electronic spectra of MPcs, i.e. the
Electron-transfer reactions play an important role in biological
process such as photosynthesis and oxidative processes involved
in the degradation of biological material in living cells [1]. Phthalo-
cyanines (Pcs) and Phorphyrins (Por) have been used as electron
donor building blocks in artificial photosynthetic system due to
their strong absorption in solar spectrum for light harvesting, their
redox potentials enabling photoelectron transfer and their struc-
tural tenability by chemical synthesis [2]. Metalophthalocyanines
(MPcs) have also enjoy usage in printing inks, catalysis, display
devices, data storage, chemical sensors, gas sensors, solar cells,
organic light emitting devices, photovoltaic cells, semiconductive
materials and photodynamic therapy [3–9].
Pcs are excellent candidates for electrocatalytic and photocat-
alytic processes because of its extensive redox chemistry coupled
with their high thermal and chemical stability. It is also established
that the redox characteristics of Pcs depend on metal ion, oxidation
state, solvent, substituent, etc. The most colorful phthalocyanine
solutions are obtained during redox chemistry that involves the
ring or the central metal ion [10]. The most significant use of
phthalocyanine complexes to date results directly from their
Q-band [a1u(␲) → eg(␲*)] and the other being the B or Soret band
[a2u(␲) → eg(␲*)] near 325–425 nm [11,12].
Several reports have been published on the study of intramolec-
ular charge transfer of MPcs [13,14] and only very few investigation
have been reported on the intermolecular charge transfer of MPcs
with electron acceptors [15,16]. Thus, a systematic investigation on
the intermolecular charge transfer interaction of Fe(II) and Fe(III)
PCs with a biologically significant quinone viz. p-chloranil (p-CHL)
has been conceived and presented in this article with an aim to
understand the mechanism and utility of such interaction. Elec-
tronic and fluorescence spectroscopic techniques were employed
to evaluate various physico-chemical parameters of the charge
transfer complexes and discussed. Preliminary attempts have also
been made to test the applicability of the charge transfer adduct
formed, in the present study, as a signal magnifier of chloranil for
colorimetric sensing of amino acids and amines.
2. Experimental
2.1. Materials and measurements
The electron donors FeIIPc and FeIIIPc were prepared and
purified by reported procedure [17] and the electron acceptor p-
chloranil (Aldrich, India) was used without further purification.
∗
Corresponding author. Tel.: +91 451 2452371; fax: +91 451 2454466.
E-mail address: drkpelango@rediffmail.com (K.P. Elango).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.09.054

C. Balraj, K.P. Elango / Spectrochimica Acta Part A 86 (2012) 44–50
45
Spectroscopy grade DMF (Merck, India) was used as received. The
3. Results and discussion
structures of the donors are shown below:
3.1. Electronic spectra of the donors
Most of the metallophthalocyanines belong to the point group
D4h [18]. The HOMO orbital is a1u(␲) and next low-lying filled
orbital is a2u(␲). The LUMO is eg(␲*) (Scheme 1) from these
concept to explain the electronic spectra of FeIIPc and FeIIIPc
[Fig. 1S (Supplemental information)]. The strong peak intensity
of the Q-band [a1u(␲) → eg(␲*)] is appeared at 659 nm (FeIIPc)
and 662 nm (FeIIIPc). The peak molar intensity of the Q-band does
depend upon the oxidation state of the central metal ion, increasing
and becoming narrower from FeIIPc to FeIIIPc [12]. The peak molar
intensity of the Q-band of FeIIIPc is comparatively sharper than the
FeIIPc. Transition from a2u(␲) to eg(␲*) is responsible for the Soret
band is appeared at 324 nm (FeIIPc) and 320 nm (FeIIIPc). However,
where empty metal d orbital lie inside the HOMO–LUMO gap, LMCT
transitions of the general type Pc(␲) → d transition, LMCT of both
FeIIPc and FeIIIPc appeared at 424 nm and 426 nm, respectively.
N
X
N
N
N
N
N
N
M
N
1) FeIIPc (M= FeII, X= Nil)
2) FeIIIPc (M= FeIII, X= Cl)
Solutions for the spectroscopic measurements were prepared by
dissolving accurately weighed amounts of donor (D) and acceptor
(A) in the appropriate volume of solvent immediately before run-
ning the spectra. The electronic absorption spectra were recorded
on a JASCO (V630, Japan) UV–vis double beam spectrophotome-
ter using 1 cm matched quartz cells. The temperature of the cell
holder was controlled with a water flow ( 0.1 ◦C). Fluorescence
spectra were recorded in a JASCO (6200, Japan) spectrofluorimeter.
The excitation emission slit width (10 nm) and scan rate (250 nm)
were kept constant for all of the experiments. FT-IR spectra were
recorded in a JASCO (FT-IR 460 Plus, Japan) spectrometer.
3.2. Stoichiometry of the interaction
The stoichiometry of the CT complex, in both the cases, was
determined by applying Job’s continuous variation method [19,20].
In both the cases, the symmetrical curves with maximum at
0.5 mole fraction indicated the formation of a 1:1 (D:A) CT complex
as evidenced from absorption [Fig. 2S (Supplemental Information)]
and emission (Figs. 1 and 2) spectral methods. The photometric
titration measurements were also performed for the determina-
tion of the stoichiometry in these interactions. The results of the
photometric curves [Fig. 3S (Supplemental Information)] also indi-
cated that the stoichiometry of the interaction, in both the cases, is
1:1 (D:A) [21].
2.2. Kinetic procedure
3.3. Kinetics of the interaction
In both FeIIPc-p-CHL and FeIIIPc-p-CHL cases, the kinetics of
the interaction was followed at three different temperatures in
DMF under pseudo-first-order conditions, keeping [A] ꢀ [D] and
with acceptor as the blank. In both the cases, the increasing
absorbance at 682 nm (FeIIPc) and 678 nm (FeIIIPc) was followed as
a function of time. The pseudo-first-order rate constants (k1) were
calculated from the gradients of log(A∞ − At) against time plots,
The electronic spectra of the mixture of the donors and the
acceptor, under pseudo first order condition, i.e. [A]/[D] > 100,
as a function of time were recorded. Representative spectra for
FeIIPc-p-CHL and FeIIIPc-p-CHL systems are given in Figs. 3 and 4,
respectively. On mixing the donors with p-CHL, there was an instan-
taneous formation of CT complex with an absorption maximum at
682 nm for FeIIPc-p-CHL (Fig. 3) and at 678 nm for FeIIIPc-p-CHL
systems (Fig. 4). The intensity of these bands goes on increas-
ing with time. The continuous increase in absorbance of these
where A and At represent the absorbance at infinity and time t,
∞
respectively. The second order rate constants were calculated by
dividing k1 by [A].
Scheme 1. The energy levels in typical metallophthalocyanine, showing origin of the various LMCT, MLCT, Q and Soret bands.

46
C. Balraj, K.P. Elango / Spectrochimica Acta Part A 86 (2012) 44–50
Scheme 2. Interaction of FeIIPc and FeIIIPc with p-CHL.
300
250
200
150
100
product
Fe Pc-p -CHL
2.0
1.6
1.2
0.8
0.4
0.0
every 2 min
C
D
A
0.0
0.2
0.4
0.6
0.8
1.0
400
500
600
Absorbance
700
800
Molefraction of Donor
Fig. 1. Job’s continuous variation plots FeIIPc-p-CHL system in DMF at 298 K.
Fig. 3. Electronic spectra of FeIIPc with p-CHL in DMF at 298 K (curve A: p-CHL; D:
FeIIPc; C: mixture of FeIIPc and p-CHL).
product
1.2
0.9
0.6
0.3
0.0
Fe Pc-p-CHL
100
every 2 min
C
90
80
70
60
D
A
400
500
600
700
800
0.0
0.2
0.4
0.6
0.8
1.0
Molefraction of Donor
Wavelength (nm)
Fig. 2. Job’s continuous variation plots FeIIIPc-p-CHL system in DMF at 298 K.
Fig. 4. Electronic spectra of FeIIIPc with p-CHL in DMF at 298 K (curve A: p-CHL; D:
FeIIIPc; C: mixture of FeIIIPc and p-CHL).

C. Balraj, K.P. Elango / Spectrochimica Acta Part A 86 (2012) 44–50
47
Table 1
Table 3
Effect of concentration of donor and acceptor on the rate of the interaction at 298 K.
Spectral properties of the CT complexes formed between FeIIPc and FeIIIPc with
p-CHL in DMF at 298 K.
[D] (×10−5 M)
FeIIPc-p-CHL system (in DMF at 682 nm)
[A] (×10−3 M)
k1 (×10−4), s−1
k2, s−1 mol−1 dm3
Property
FeIIPc-p-CHL
FeIIIPc-p-CHL
5.5
5.5
5.5
5.5
5.5
5.5
5.5
5.5
5.5
5.5
4.4
3.3
2.2
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1
1
1
1
2.5
3.3
4.1
4.8
5.6
6.3
7.1
7.7
8.3
8.3
8.2
8.0
8.1
1.1
1.1
1.0
1.0
0.9
0.9
0.9
0.9
0.8
ꢀmax (nm)
hꢂCT (eV)
682
1.82
4.41
7770
20,861
0.063
3.03
7.26
4.06
678
1.83
4.42
2221
20,958
0.054
2.79
7.25
4.05
1014 ꢂmax (s−1
)
Formation constant, K (dm3 mol−1
)
Extinction coefficient, ε (dm3 mol−1 cm−1
Oscillator strength, f
)
Dipole moment, ꢃ
Ionization potential (eV)
Dissociation energy (eV)
former CT complex. Negative entropies of activation indicated a
greater degree of ordering during the interaction.
FeIIIPc-p-CHL system (in DMF at 678 nm)
5.5
5.5
5.5
5.5
5.5
5.5
5.5
5.5
5.5
5.5
4.4
3.3
2.2
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1
1
1
1
1.9
2.6
3.1
3.9
4.6
5.2
5.8
6.7
7.3
7.3
7.2
7.2
7.1
0.8
0.8
0.7
0.7
0.8
0.7
0.7
0.7
0.7
3.4. FT-IR studies of the CT complexes
In both FeIIPc-p-CHL and FeIIIPc-p-CHL cases, the CT complexes
were obtained by allowing the reactants to react for 24 h under
equal molar conditions. The FT-IR spectra of the isolated products
were recorded and the results indicated that the shifts in posi-
tions of some of the peaks could be attributed to the expected
symmetry and electronic structure modification in both the donor
and acceptor units in the formed CT complex relative to the free
molecules. The stretching vibration of ␯(C O) of the free p-CHL is
appeared at 1685 cm−1 and the complexation with FeIIPc and FeIIIPc
which is shifted into 1662 cm−1 and 1644 cm−1, respectively. Such
a bathochromic shift could be indicative of a higher charge density
on the carbonyl group of the p-CHL molecule [22].
bands is indicative of the formation of the final interaction prod-
uct (Scheme 2). This is also supported by the fact that the product
isolated exhibited identical electronic spectra (Figs. 3 and 4).
The pseudo-first-order rate constant (k1) values for the forma-
tion of the CT complex as a function of [D] and [A] are also collected
in Table 1. It is evident from the results that the rate is independent
of initial concentration of the donors indicating first order depen-
dence on [D]. The k1 increased with an increase in [A] suggesting
the order with respect to [A] is also unity. The rate constant values
increased with an increase in [A] and a plot of log k1 versus log [A]
[Fig. 4S (Supplemental Information)], is linear in both FeIIPc-p-CHL
(slope 0.94) and FeIIIPc-p-CHL (slope 0.96) systems indicating first
order dependence of rate on [A] and which was further supported
by the constancy in k2 values. These observations indicated that
the interaction of the donors with p-CHL proceed via the following
plausible mechanism.
3.5. Characteristics of the CT complexes
In both FeIIPc-p-CHL and FeIIIPc-p-CHL systems, an attempt
was made to characterize the CT complexes formed. For that
the absorbance of the new bands was measured using constant
donor concentration and varying concentrations of the acceptor,
but always [A] ꢀ [D] [Fig. 5S and 6S (Supplemental Information)].
The formation constants (K) and molar extinction coefficients (ε)
of the CT-complexes were determined spectrophotometrically in
the temperature range 298–313 K using the Scott equation [23].
Representative Scott plots are shown in Fig. 7S (Supplemental Infor-
mation) and the values of K and ε thus determined are given in
Table 3. The observed high values of K suggested that the formed
CT complexes are of strong type [24] and the linearity of the Scott
plots further supports this result. The results indicated that the
FeIIPc-p-CHL CT complex is relatively more stable than the other.
The values of oscillator strength (f) which is a measure of inte-
grated intensity of the CT-band and transition dipole moment
(ꢃ), were calculated as described elsewhere [23] and the values
thus obtained are also given in Table 3. The values of f are rather
relatively large indicating a strong interaction between the donor-
acceptor pairs with relatively high probabilities of CT-transitions
[25]. The magnitude of f is relatively greater for the FeIIPc-p-CHL
complex, indicating relatively higher stability of the complex than
the other. Out of the many applications of CT complex, one impor-
tant application is to calculate the ionization potential of the donor.
The ionization potential (Ip) of the highest filled molecular orbital
of the donor was estimated from CT energies of its complexes with
the acceptor making use of the empirical equations reported by
Aloisi and Pignataro [26].
The above mechanism (Scheme 2) leads to the following rate
law:
Rate = k1[D] [A]
The rate constants as a function of temperature along with
thermodynamic parameters for the FeIIPc-p-CHL and FeIIIPc-p-CHL
interactions are collected in Table 2. It is evident from the results
that the enthalpy of activation, which is a measure of strength of the
interaction between the donor and acceptor molecules in a molec-
ular complex, is relatively higher for FeIIPc-p-CHL system than that
of FeIIIPc-p-CHL system indicating relatively stronger nature of the
Table 2
Kinetic and thermodynamic parameters for the interaction of p-CHL with FeIIPc and
FeIIIPc in DMF.
System
ꢀ, nm
k1 (×10−4), s−1
ꢁH#
−ꢁS#
ꢁG#
298 K
305 K
313 K
Ip (eV) = 5.00 + 1.53 × 10−4ꢂ¯p-CHL(cm−1
)
(1)
FeIIPc-p-CHL
FeIIIPc-p-CHL
682
678
8.3
7.3
9.1
7.7
9.6
8.3
4.9
4.1
287
291
91
90
The calculated Ip values for molecular orbital participating in
CT interaction of the iron phthalocynines are also listed in Table 3.
ꢁH#, kJ mol−1; ꢁS#, J K−1 mol−1; ꢁG#, kJ mol−1.

48
C. Balraj, K.P. Elango / Spectrochimica Acta Part A 86 (2012) 44–50
250
200
150
100
50
1.6
Fe Pc-p-CHL ; r 0.991
Fe Pc-p-CHL; r 0.993
D
a
b
1.5
1.4
1.3
1.2
1.1
c
d
e
0
2
4
6
8
10
12
14
16
350
400
450
500
[Q] 10-5
Wavelength (nm)
Fig. 6. Stern–Volmer plots for the fluorescence quenching of FeIIPc and FeIIIPc with
p-CHL in DMF at 298 K.
Fig. 5. Fluorescence spectra for FeIIPc-p-CHL system in DMF at fixed concen-
trations of [D] = {1.4285 × 10−5 M (curve D)} and variable concentration of [A]
(×10−5 M) = {2.86 (curve a), 5.71 (curve b), 8.57 (curve c), 11.43 (curve d), 14.29
(curve e)} at 298 K.
of 1/(1 − Â) versus [At]/Â was linear (r 0.96 for FeIIPc-p-CHL sys-
tem; r 0.98 for FeIIIPc-p-CHL system) indicating that under the
experimental conditions all the binding sites are equivalent and
independent. The value of Kf obtained from the plots for FeIIPc-p-
CHL and FeIIIPc-p-CHL systems were found to be 4.2 × 103 mol L−1
and 2.2 × 103 mol L−1, respectively. Parallel to the observation
made in the electronic spectral studies, as enumerated earlier,
the FeIIPc-p-CHL complex is found to be relatively stronger than
FeIIIPc-p-CHL complex. The standard Gibbs energy change ꢁG◦ was
calculated from the Kf using the relation ꢁG◦ = −2.303 RT log10 Kf.
The ꢁG◦ values for FeIIPc-p-CHL and FeIIIPc-p-CHL systems were
found to be −20 kJ and −19 kJ, respectively, which indicated that
the interactions between the Pcs and p-CHL are spontaneous [31].
Fluorescence intensity data was also analyzed according to the
Stern–Volmer law:
These values support the interpretation that the new band can be
regarded as the CT band. Further evidence for the nature of CT inter-
action in the present systems is the calculation of the dissociation
energy (W) of the CT excited state of the complex. The dissocia-
tion energies of the complexes were calculated as described earlier
[27] and the calculated values of W (Table 3) suggested that the
investigated complexes are reasonably strong and stable under the
studied conditions with higher resonance stabilization energy [28].
3.6. Fluorescence study
One of the most important binding forces that produce favorable
interaction between the donors and acceptor is the CT interaction.
The nature and magnitude of donor interaction significantly influ-
ences the biological activity of the donor. In the present study, a
preliminary attempt has been made to employ fluorimetric method
to investigate the mode of CT interaction of FeIIPc and FeIIIPc with
p-CHL. The fluorescence spectra of both donors were recorded at
room temperature (298 K) in the range of 230–730 nm upon excita-
tion at 320 nm. It is observed that p-CHL quenched the fluorescence
of both FeIIPc and FeIIIPc through CT complexation. The experimen-
tal results indicated that the quenching efficiency increased with
an increase in the concentration of p-CHL at a fixed concentration
of the donors [Figs. 5 and 8S (Supplemental Information)].
The fraction of p-CHL bound to the FePcs (Â) was determined
using the following equation [29].
F0
= 1 + KSV[Q],
(4)
F
where F0 is emission intensity in the absence of quencher (Q), F
emission intensity at quencher concentration [Q] and KSV is the
Stern–Volmer constant. In both the FeIIPc-p-CHL and FeIIIPc-p-CHL
systems the linear Stern–Volmer relationship observed (Fig. 6) sug-
gested that either static or dynamic quenching is dominant [32–34].
The relationship between the fluorescence quenching intensity
and the concentration of quencher can be described by the follow-
ing equation.
F0 − F
log10
= log KA + n log10[Q],
(5)
F
F0 − F
 =
,
(2)
where KA is the binding constant and
n
is the number of
F0
binding sites per FeIIPc or FeIIIPc [32]. In the present study, a
plot of log10 (F0 − F)/F versus log10 [Q] is linear in both cases
[Fig. 9S (Supplemental Information)]. The binding constant val-
ues of FeIIPc-p-CHL system are 9.9 × 103 mol−1 L (n = 1.1) and
1.3 × 103 mol−1 L (n = 0.9) for FeIIIPc-p-CHL system. The results
showed that the binding constant in FeIIPc-p-CHL system is higher
than that in FeIIIPc-p-CHL system indicating higher stability of the
former than the later. These observations are in corroboration with
the results of absorption spectral, kinetics and thermodynamic
studies as enumerated earlier in this paper. The values of n, for both
the Pcs are nearly constant indicating the presence of equivalent
binding sites.
where F and F0 denote the fluorescence intensities of the donors
in a solution with a given concentration of p-CHL and without p-
CHL, respectively. From the values of  the association constant,
Kf, for the interaction of FePcs with p-CHL was computed using
the method described by Ward [30]. It has been shown that for
equivalent and independent binding sites:
1
[At]
Â
=
− n[DT ],
(3)
(1 − Â)Kf
where n is the number of binding sites, [At] is the total p-CHL
concentration and [DT] is the total donor concentration. The plot

C. Balraj, K.P. Elango / Spectrochimica Acta Part A 86 (2012) 44–50
49
Fig. 8. Electronic spectra of complex FeIIIPc-p-CHL (2.5 × 10−5 M) (blank) and in the
presence of 8.0 × 10−4 M of various amino acids in 80% (v/v) aq. DMF at 298 K. Inset:
solution (a) FeIIIPc-p-CHL, (b) amino acids, (c) (FeIIIPc-p-CHL) + amino acids in 80%
(v/v) aq. DMF.
Fig. 7. Electronic spectra of complex FeIIIPc-p-CHL (2.5 × 10−5 M) (blank) and in the
presence of 4.0 × 10−4 M of various amines in 80% (v/v) aq. DMF at 298 K. Inset:
solution (a) FeIIIPc-p-CHL, (b) amine, (c) (FeIIIPc-p-CHL) + amine in 80% (v/v) aq. DMF.
CT complex even in more polar DMF (Fig. 4). The present study
indicated that the FeIIIPc-p-CHL CT complex is a better candidate
for colorimetric sensing of amino acids and amines. The advantages
of the present sensor are (i) the CT complex can be formed in more
polar solvent like DMF and hence aqueous solutions of amino acids
or amines can easily be mixed with it, (ii) aromatic amines and neu-
tral amino acids can be sensed and (iii) the FeIIIPc-p-CHL CT complex
can also be prepared in solid state by grinding the reactants in 1:1
ratio and which can be dissolved in DMF (This CT complex also
exhibited identical spectral behavior with that prepared in solu-
tion). As a detailed study on the colorimetric application of the
CT complexes is out of the scope of the present study, only this
preliminary report has been presented here.
3.7. Colorimetric application of the ꢄ–ꢄ CT complex
Recently, Liu et al. [35] have reported the interaction
and colorimetric sensing properties of calix[4]pyrrole-chloranil
charge transfer complex with amino acids and amines in
chloroform/ethanol/water medium using UV–vis spectroscopic
technique. The results indicated that the CT complex behaves
like a signal magnifier in sensing basic amino acids and aliphatic
amines. Also no CT complex formation was noticed in more polar
solvents like acetonitrile and DMSO. Hence they have used chloro-
form/ethanol/water mixture to sense amino acids.
A preliminary attempt was made, in the present study, to utilize
the FeIIIPc-p-CHL CT complex to sense amino acids and amines. As
this supramolecular assembly is investigated in more polar DMF,
this can very well be used to sense amino acids in DMF/water mix-
tures. On adding 20% water to the CT complex in DMF, as shown
in Fig. 7, the CT absorption at 669 nm exhibited only slight per-
turbation and retains the blue color indicating the stability of the
complex in DMF/water mixture. (For interpretation of the refer-
ences to color in text, the reader is referred to the web version of
this article.)
4. Conclusion
The stoichiometry of the molecular complexes of FeIIPc and
FeIIIPc with p-CHL was found to be 1:1. The association constants
of both the systems were evaluated by the Scott and spectrofluo-
rimetric methods. The results indicated that FeIIPc-p-CHL complex
is relatively stronger than FeIIIPc-p-CHL complex. This observation
is well supported by the kinetic and thermodynamic studies. The
results showed that the investigated complexes were found to be
stable, endothermic and spontaneous. The results obtained in the
present study would definitely help to understand the mechanism
of interaction of Pc like molecules with quinones in real situa-
tions. The electronic spectral changes of the CT complex solution
observed in the presence of amino acids and amines indicated that
it can be used as a colorimetric sensor.
The colorimetric properties of the FeIIIPc-p-CHL CT complex
in DMF/water (80:20%, v/v) for amines and amino acids were
investigated and the results are shown in Figs. 7 and 8, respec-
tively. FeIIPc-p-CHL CT complex also exhibited similar behavior
but only the results of FeIIIPc-p-CHL CT complex are presented
here. Upon addition of aliphatic amines (ethylene diamine, tri-
ethyl amine), aromatic amines (pyridine, aniline, p-bromoaniline,
p-toluidine, o-phenylene diamine), basic amino acids (arginine)
and neutral amino acids (alanine, leucine, proline), the blue solu-
tion instantly turned to green with new absorption band(s) in
the region of 390–490 nm along with a band at 658 nm corre-
sponds to FeIIIPc. (For interpretation of the references to color
in text, the reader is referred to the web version of this arti-
cle.) This dramatic color change may be due to the formation
of n–␲ CT complex between chloranil and the n-donor amines
and amino acids. The new absorption peaks around 425 and
450 nm is due to chloranil anion radical [35,36]. Such peaks
have not occurred during the formation of ␲–␲ FeIIIPc-p-CHL
Acknowledgement
One of the author (CB) is thankful to the University Grant
Commission, New Delhi, for the award of UGC-BSR Fellowship in
Sciences for Meritorious Students.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2011.09.054.

50
C. Balraj, K.P. Elango / Spectrochimica Acta Part A 86 (2012) 44–50
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