In situ synthesis, photometric and spectroscopic studies of chelating system during the 1,4,7,10,13,16-hexaoxacyclooctadecane charge transfer reaction with different acceptors

Article abstract of DOI:10.1016/j.saa.2011.03.039

Electron donor acceptor complexes (EDA) of the 1,4,7,10,13,16- hexaoxacyclooctadecane (18-crown-6) as a rich donor were spectrophotometrically discussed and synthesized in solid form according the interactions with different nine of usual π-acceptors like

Full text of DOI:10.1016/j.saa.2011.03.039

Spectrochimica Acta Part A 79 (2011) 583–593
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
In situ synthesis, photometric and spectroscopic studies of chelating system
during the 1,4,7,10,13,16-hexaoxacyclooctadecane charge transfer reaction with
different acceptors
Aisha S.M. Hossana, Hanaa M. Abou-Melhaa, Moamen S. Refatb,c,∗
a Faculty of Education of Girls, Scientific Departments, King Khaled University, Saudi Arabia
b Department of Chemistry, Faculty of Science, Port Said University, Port Said, Egypt
c Department of Chemistry, Faculty of Science, Taif University, 888 Taif, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history:
Electron donor acceptor complexes (EDA) of the 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6)
as a rich donor were spectrophotometrically discussed and synthesized in solid form according the inter-
actions with different nine of usual ␲-acceptors like 2,3,5,6-tetrachlorocyclohexa-2,5-diene-1,4-dione
(p-chloranil; p-CHL), tetrachloro-1,2-benzoquinone (o-chloranil; o-CHL), 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ), tetracyanoquinodimethane (TCNQ), 2,6-dichloroquinone-4-chloroimide (DCQ),
2,6-dibromoquinone-4-chloroimide (DBQ), 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (chloranilic
acid; CLA), N-bromosuccinimide (NBS), 2,4,6-trinitrophenol (picric acid; PA). Spectroscopic and phys-
ical data such as formation constant (KCT), molar extinction coefficient (εCT), standard free energy
(ꢀG◦), oscillator strength (f), transition dipole moment (ꢁ), resonance energy (RN) and ionization poten-
tial (Ip) were estimated in chloroform or methanol at 25 ◦C. Based on the elemental analysis and
photometric titrations the CT-complexes were formed indicated the formation of 1:1 charge-transfer
complexes for the o-CHL, TCNQ, DCQ, DBQ and NBS acceptors but 1:3 ratio for p-CHL, DDQ, CLA and
PA, respectively. The charge-transfer interactions were interpretative according to the formation of
dative ion pairs [18C6•+, A•−], where A is acceptor. All of the resulting charge transfer complexes
were isolated in amorphous form and the complexes formations on IR and 1H NMR spectra were
discussed.
Received 3 December 2010
Received in revised form 15 March 2011
Accepted 16 March 2011
Keywords:
Charge transfer complexes
p-CHL
DDQ
TCNQ
CLA
PA
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
surfactant [10] and detergent [11] action and molecular semicon-
ductivity [12]). There is significant interesting in the study of charge
18-Crown-6 is an organic compound with the formula C12H24O6
and the IUPAC name of 1,4,7,10,13,16-hexaoxacyclooctadecane.
Crown ethers coordinate with some metal cations in their cen-
tral cavity. Crown ethers are useful as phase transfer catalysts
[1,2]. Commonly, crown ethers have many interested properties
such as the relative size of the ion and the hole of polyether [3]
the number of O atoms [4] the basicity of O atoms [5] the steric
hindrance in polyether ring [6] the tendency of the ion to asso-
ciate with solvent. Macrocyclic polyethers like crown ether have
received much attention in the last few years both in chemistry
and biology [7]. These compounds act antiulcerogenically against
histamine induced ulcers [8] and as antibacterial agents [9]. Crown
ethers play an important role in non-biological activity (viz. in
transfer complex of crown ethers with different ␲- and ␴-acceptors
[13–16]. The goal of this work is describing nine charge transfer
complexes of 18-crown-6 as a donor with different types of accep-
tors (p-CHL, o-CHL, DDQ, TCNQ, DCQ, DBQ, CLA, NBS, PA) in CT
interactions.
2. Experimental
2.1. Materials
18-Crown-6 (18C6, Aldrich) and ␲-acceptors from Merck Co.
were used without any further purification. The structures of 18-
crown-6 ether and the ␲-acceptors are shown in Formula I.
2.2. Synthesis of 18-crown-6 CT complexes
∗
Corresponding author at: Department of Chemistry, Faculty of Science, Taif Uni-
versity, Mohamed Ali street, 888 Taif, Saudi Arabia. Tel.: +966 561926288.
The solid CT complexes of 18-crown-6 with p-CHL, o-CHL, DDQ,
TCNQ, DCQ, DBQ, CLA, NBS, and PA were prepared by mixing 1 mM
E-mail address: msrefat@yahoo.com (M.S. Refat).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.03.039

584
A.S.M. Hossan et al. / Spectrochimica Acta Part A 79 (2011) 583–593
N
Cl
Cl
Cl
O
O
O
N
Cl
Cl
O
Cl
2,3,5,6-Tetrachlorocyclohexa-2,5-diene-1,4-dione
2,3-Dichloro-5,6-dicyano-1,4-benzoquinone
(p-Chloranil; p-CHL)
(DDQ)
O
Cl
Cl
N
N
N
N
O
Cl
Cl
Tetrachloro-1,2-benzoquinone
Tetracyanoquinodimethane
(TCNQ)
o-Chloranil; o-CHL)
Br
Cl
O
N
O
N
Cl
Cl
Br
Cl
2,6-dibromoquinone-4-chloroimide
2,6-dichloroquinone-4-chloroimide
(DBQ)
(DCQ)
O
N+ O-
O
Cl
OH
-O
O
N+
Br
N
O
O
HO
N+ O-
HO
Cl
O
N-bromosuccinimide
O
2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone
(Chloranilic acid; CLA)
(NBS)
2,4,6-Trinitrophenol
(Picric acid; PA)
O
O
O
O
O
O
Formula I. Structure of 18-crown-6 and ␲-acceptors.
of the 18C6 as a donor in chloroform (30 mL) for all complexes
except for (DCQ and DBQ in methanol) with 1 mM of each acceptor.
The mixture was stirred at room temperature for 1 h, where solid
complexes were obtained. The complexes were filtered off, washed
with chloroform, and dried under vacuum.
2.3. Measurements
Jenway 6405 (Bibby Scientific Limited, Beacon Road, Stone,
Staffordshire ST15 0SA, UK) with quartz cell with a 1.0 cm path
length was used from 800 to 200 nm. A Genesis II FT IR spectropho-

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585
tometer (Mattson Instruments, USA) was used to take the IR spectra
of the reactants and the CT-complexes as KBr discs. 1H NMR spec-
tra of 18C6/DDQ and 18C6/TCNQ complexes were obtained on a
Varian spectrophotometer Gemini 200 MHz (Agilent Technologies,
Inc., USA) using TMS as internal reference and chloroform as the
solvent. The elemental analyses of carbon, hydrogen and nitro-
gen contents were performed by the microanalysis unit at Cairo
University, Egypt, using a Perkin Elmer CHN 2400 (USA).
The 1:1 modified Benesi–Hildebrand equation [18] was used in
the calculations of the 18-crown-6 with o-CHL, TCNQ, DBQ, DCQ,
and NBS charge transfer complexes,
CaoCol
Cao + Cdo
1
Kε
d
=
+
(1)
A
ε
where Cao and Cdo are the initial concentrations of the acceptor
(o-CHL, TCNQ, DBQ, DCQ, and NBS) and the donor 18-crown-6,
respectively, and A is the absorbance of the strong bands at 307 nm
for o-CHL, 406 nm for TCNQ, 295 nm for DBQ, 286 nm for DCQ, and
2.4. Analytical data
280 nm for NBS, respectively. When the CaoCo/A values for each
d
1:1 charge transfer complex are plotted against the correspond-
ing Cao + Cbo values. Straight lines are obtained with a slope of 1/ε
and intercept of 1/Kε.
The spectrophotometric titrations of the intermolecular charge
transfer complexes formed from the reactions of 18-crown-6 with
p-CHL, DDQ, CLA and PA acceptors refereed to formation of 1:2 CT
complexes. The 1:2 Eq. (2) [19] was used in the calculations.
1 [(18C6)(p-CHL)2]; C24H24Cl8O10; yellow color; Mol. wt. = 755.88;
Calcd: %C, 30.10; %H, 3.17, Found: %C, 29.87; %H, 3.13.
2 [(18C6)(o-CHL)]; C18H24Cl4O8; dark red color; Mol. wt. = 510;
Calcd: %C, 42.35; %H, 4.70, Found: %C, 42.11; %H, 4.49.
3 [(18C6)(DDQ)2];
C28H24Cl4N4O10
;
orange
color;
Mol.
wt. = 718.12; Calcd: %C, 46.79; %H, 3.34; %N, 7.80, Found:
%C, 46.33; %H, 3.09; %N, 7.44.
(Cao)2Co
4 [(18C6)(TCNQ)];
wt. = 468.32; Calcd: %C, 61.50; %H, 6.00; %N, 11.96, Found:
%C, 61.21; %H, 5.91; %N, 11.87.
5 [(18C6)(DBQ)]; C18H26Br2ClNO7; dark brown color; Mol.
wt. = 560.94; Calcd: %C, 38.51; %H, 4.63; %N, 2.49, Found: %C,
38.25; %H, 4.31; %N, 2.36.
6 [(18C6)(DCQ)];
wt. = 473.04; Calcd: %C, 45.66; %H, 5.50; %N, 2.96, Found:
%C, 45.44; %H, 5.32; %N, 2.84.
C24H28N4O6; dark green color; Mol.
1
Kε
1
ε
d
=
+
Cao(4Co + Cao)
(2)
d
A
where (Cao)2 and Cdo are the initial concentration of the ␲-acceptor
(p-CHL, DDQ, CLA and PA) and donor (18-crown-6), respectively,
and A is the absorbance of the detected CT-band. The data obtained
Co, (Cao)2, Cao(4Co + Cao) and ((Cao)2Co)/A in chloroform were calcu-
C18H26Cl3NO7; dark brown color; Mol.
d
d
d
lated. By plotting ((Cao)2Co)/A values vs. Cao(4Co + Cao), straight lines
were obtained with a slodpe of 1/ε and an intercept of 1/Kε.
The spectroscopic and physical data such as formation constant
(KCT), molar extinction coefficient (εCT), standard free energy (ꢀG◦),
oscillator strength (f), transition dipole moment (ꢁ), resonance
energy (RN) and ionization potential (Ip) were estimated in chloro-
form or methanol at 25 ◦C, and the different acceptors were found
to have a pronounced effect on the interaction of 18-crown-6 with.
These calculations can be summarized as follows;
d
7 [(18C6)(CLA)2]; C24H28Cl4O14
; red color; Mol. wt. = 679.98;
Calcd: %C, 42.35; %H, 4.12, Found: %C, 42.12; %H, 3.97.
8 [(18C6)(NBS)]; C16H28BrNO8; yellow-orange color; Mol.
wt. = 441.10; Calcd: %C, 43.53; %H, 6.35; %N, 3.17, Found:
%C, 43.40; %H, 6.28; %N, 2.98.
9 [(18C6)(PA)2]; C24H30N6O20; yellow color; Mol. wt. = 722.32;
Calcd: %C, 39.87; %H, 4.15; %N, 11.63, Found: %C, 39.52; %H, 3.99;
%N, 11.55.
The oscillator strength f was obtained from the approximate
formula [20].
3. Results and discussion
f = (4.319 × 10−9)εmaxv1/2
(3)
where ꢂ1/2 is the band-width for half-intensity in cm−1 and εmax
is the maximum extinction coefficient of the CT-band. The oscilla-
tor strength values are given in Table 1. The data resulted reveals
several items. (i) The 18-crown-6/A (where A = p-CHL, o-CHL, DDQ,
TCNQ, DBQ, DCQ, CLA, NBS, and PA) systems show high values
of both formation constant (K) and molar absorptivity (ε). This
high value of (K) reflects the high stability of the 18-crown-6 CT-
complexes as a result of the expected high rich donation of the
18-crown-6 which contains six oxygen atoms. (ii) The different
values of the oscillator strength, f, increases with increasing in the
dielectric constant (D) of the solvent. This result could be explained
on the basis of competitive solvent interactions with the acceptors
[21,22].
The elemental analysis data (C, H, and N) of the resulted
CT-complexes were matched with the molar ratio gained from
spectrophotometric titrations. Spectrophotometric titrations at
292, 307, 286, 406, 295, 286, 308, 280 and 421 nm were per-
formed for the reactions of 18-crown-6 with p-CHL, o-CHL, DDQ,
TCNQ, DBQ, DCQ, CLA, NBS, and PA, respectively, using the Jen-
way 6405 spectrophotometer as follows: A 0.25, 0.50, 0.75, 1.00,
1.50, 2.00, 2.50, 3.00, 3.50 or 4.00 mL aliquot of a standard solu-
tion (5.0 × 10−4 M) of the appropriate acceptor (p-CHL, o-CHL,
DDQ, TCNQ, DBQ, DCQ, CLA, NBS, and PA) in chloroform was
added to 1.00 mL of 5.0 × 10−4 M 18-crown-6 also in chloroform
or methanol. The total volume of the mixture was 5 mL. The con-
centration of 18-crown-6 (Cdo) in the reaction mixture was thus
fixed at 1.0 × 10−4 M while the concentration of ␲-acceptors (Cao)
varied from 0.25 × 10−4 M to 4.00 × 10−4 M. These concentrations
produce donor:acceptor ratios from 4:1 to 1:4. The absorbance of
each CT complexes was measured and plotted as a function with
the ratio of (Cdo):(Cdo) according to a known method [17]. Solid sam-
ples of the 1:1 or 1:2 CT complexes of 18-crown-6 were prepared by
mixing chloroform or methanol solutions of 18-crown-6 (1.0 mM)
and either (p-CHL, o-CHL, DDQ, TCNQ, DBQ, DCQ, CLA, NBS, and
PA) acceptor. It was of interest to observe that the solvent has a
pronounced effect on the spectral intensities of the formed charge
transfer complexes. To study this solvent effect in a quantitative
manner, it was necessary to calculate the values of the equilibrium
constant, K, the molar absorptivity ε, and the oscillator strength, f,
of the 18-crown-6 complexes in the respective solvent.
The transition dipole moment (ꢁ) of the 18-crown-6 CT-
complexes, Table 1, have been calculated from Eq. (4) [23];
ꢀ
ꢁ
1/2
εmaxv1/2
vmax
ꢁ(Debye) = 0.0958
(4)
The transition dipole moment is useful for determining if tran-
sitions are allowed, that the transition from a bonding ␲ orbital to
an antibonding ␲* orbital is allowed because the integral defining
the transition dipole moment is nonzero.
The ionization potential (Ip) of the 18-crown-6 donor in their
charge transfer complexes were calculated using empirical equa-
tion derived by Aloisi and Pignataro Eq. (5) [24,25];
−4
Ip(ev) = 5.76 + 1.53 × 10
v
CT
(5)

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A.S.M. Hossan et al. / Spectrochimica Acta Part A 79 (2011) 583–593
Table 1
Spectrophotometric results of the 18-crown-6 CT-complexes (A) [(18C6)(p-CHL)2], (B) [(18C6)(o-CHL)], (C) [(18C6)(DDQ)2], (D) [(18C6)(TCNQ)], (E) [(18C6)(DBQ)], (F)
[(18C6)(DCQ)], (G) [(18C6)(CLA)2], (H) [(18C6)(NBS)] and (I) [(18C6)(PA)2].
Complex
ꢃmax (nm)
ECT (eV)
K (l mol−1
)
εmax (l mol−1 cm−1
)
f
ꢁ
Ip
D
RN
ꢀG◦ (25 ◦C) kJ mol−1
A
B
C
D
E
F
G
H
I
292
307
286
406
295
286
308
280
421
4.26
4.05
4.35
3.06
4.22
4.35
4.04
4.44
2.95
8.48 × 107
14,334
70,102
10,661
46,694
23,753
34,129
30,211
61,043
8064
60.55
6.14
61.30
20.01
49.51
31.36
36.33
36.35
58.74
22.76
24.54
11.00
10.74
11.11
9.53
10.95
11.11
10.73
11.22
9.39
4.81
4.81
4.81
0.926
0.378
0.845
0.454
0.733
0.719
0.848
0.340
0.305
45,238
23,716
38,083
22,928
22,202
23,936
46,532
23,082
43,494
4.7 × 107
40.33
11.40
21.06
21.75
52.73
8.71
10,430
7783
4.81
32.70
32.70
4.81
4.81
4.81
15,667
1.43 × 108
11,100
4.2 × 107
12,464
6.73
where ꢂCT is the wavenumber in cm−1 corresponding to the CT
band formed from the interaction between donor and acceptor.
The electron donating power of a donor molecule is measured by
its ionization potential which is the energy required to remove an
electron from the highest occupied molecular orbital.
transfer complexation. These shifts are resulted from the building
of higher charge density on the carbonyl groups of the p-CHL accep-
tor in the complex formation. However, the methylene stretching
of the ethoxy groups of 18-crown-6 observed at 2894, 2872 and
2823 cm−1 region considerably affected by shifted and decreasing
the intensities of bands upon complex formation. This is probably
due to the contribution of the ethoxy groups in the interaction with
p-CHL acceptors.
The energy of the charge-transfer complexes ECT of the 18-
crown-6 complexes is calculated using Eq. (6) [23];
1243.667
According to these observations, the following equations are
suggested for the interaction of 18C6 and p-CHL with 1:2 molar
ratios.
ECT = (hvCT) =
(nm)
(6)
ꢃCT
where ꢃCT is the wavelength of the complexation band.
Determination of resonance energy (RN) [26] theoretically
derived from Eq. (7);
+
−
18C6 + p-CHL = 18C6• p-CHL•
7.7 × 10−4
+
−
+
−
18C6• p-CHL• + p-CHL → 18C6• 2p-CHL•
εmax
=
(7)
hvCT/[RN] − 3.5
The conductivity data of 18-crown-6 in the presence of p-CHL
was measured and recorded small increases. This increasing in
conductance after complexation proved that the charge transfer
complexes formed from the formation of dative ion pairs. The
spectral features of the [(18C6•+)(p-CHL•−)2] complex is wholly
consistent with Mulliken’s formulation of weak complexes in
which the new UV–vis absorption relates to the electronic tran-
sition (hꢂCT) corresponding to [28].
where εmax is the molar absorptivity of the CT-complexes at max-
imum CT band, ꢂCT is the frequency of the CT peak and RN is the
resonance energy of the complex in the ground state, which, obvi-
ously is a contributing factor to the stability constant of the complex
(a ground state property). The values of RN for the 18-crown-6
charge transfer complexes under study have been given in Table 1.
The standard free energy changes of complexation (ꢀG◦) were
calculated from the formation constants by the following Eq. (8)
[27];
3.2. o-CHL
ꢀG◦ = −2.303RT log KCT
(8)
The formation of charge transfer complex of o-CHL and 18C6
ether (Fig. 1B) with 1:1 stoichiometry. Here, this paper stud-
ied the stoichiometry of complex o-CHL with 18C6 by the
molar ratio method using both concentration of [o-CHL]/[18C6]
where ꢀG◦ is the free energy change of the CT-complexes
(kJ mol−1), R is the gas constant (8.314 J mol−1 K), T is the tempera-
ture in Kelvin degrees (273+ ◦C) and KCT is the formation constant of
the complexes (l mol−1) in different solvents at room temperature.
Table 2
Characteristic infrared frequencies (cm−1) and tentative assignments of 18C6, p-CHL
and [(18C6•+)(p-CHL•−)2] CT complex.
3.1. p-CHL
18C6
p-CHL
–
[(18C6•+)(p-CHL•−)2]
Assignments(b)
The absorption spectrum of 1 × 10−4 M p-CHL in the presence of
1 × 10−4 M equal concentration of 18-crown-6 is shown in Fig. 1A.
As can be seen, in the formation situation of intermolecular charge
transfer the band appears at 292 nm is hyperchromicity. Therefore,
the observation of this band can be assigned to the formation of
charge transfer complex between 18C6 and p-CHL. 18-crown-6 and
p-CHL in this complex behave as n-donor and ␲-acceptor, respec-
tively. In order to obtain the stoichiometry of the resulting complex,
the molar ratio method [17] was performed. The photometric plot
is shown in Fig. 2A. The plot clearly indicates the formation of a 1:2
(donor:acceptor) complex. From Fig. 3A, the formation constant (K)
and molar absorptivity (ε) were calculated using the relationship
2894
2872
2823
–
1636
1601
ꢂ(C–H)
1680
1583
1555
1528
–
1647
–
ꢂ(C O)
ꢂ(C C)
1471
1469
1454
1432
1361
1345
1285
1279
1248
1235
1229
1108
1058
1471
ı(CH) deformation
–
1351
1285
1249
1108
(Eq. (2)) deduced from plotting ((Cao)2Co)/A values vs. Cao(4Co + Cao)
d
employed the slopes and intercepts ofdthese plots.
ꢂ(C–O)
ꢂ(C–N)
The IR spectral data of 18-crown-6, p-CHL and [(18C6)(p-CHL)2]
complex are presented in Table 2. The stretching of both C O;
p-CHL and –COC– of 18-crown-6 show a drastic shift to lower
frequencies and decreasing the intensities upon molecular charge

A.S.M. Hossan et al. / Spectrochimica Acta Part A 79 (2011) 583–593
587
Fig. 1. Electronic spectrum of (A) 18C6/p-CHL, (B) 18C6/o-CHL, (C) 18C6/DDQ, (D) 18C6/TCNQ, (E) 18C6/DCQ, (F) 18C6/DBQ, (G) 18C6/NBS, (H) 18C6/CLA, and (I) 18C6/PA
systems.
absorption band at 1680 cm−1 which characteristic of the ꢄ(C O)
vibration of the carbonyl group in free o-CHL acceptor was blue
shifted to 1646 cm−1, that is indicative of the involvement of the
and absorbance measurements at ꢃmax = 307 nm. The resulting
data of charge-transfer complex are shown in Fig. 2B. The
Benesi–Hildebrand method [18] Eq. (1) was used for determina-
tion of both ε and K of the [(18C6)(o-CHL)] complex. The straight
line plot of CaoCo/A vs. Co + Cdo is shown in Fig. 4A. Consequently,
the ꢀG◦, f, ꢁ, RNd and Ip vdalues were estimated.
The characteristic bands in the infrared spectra of o-CHL, 18C6
and its CT-complex were selected and as signed in Table 3.
The assignments data of [(o-CHL)(18C6)] complex show that the
C
O group in the charge transfer complexation.
3.3. DDQ
Fig. 1C shows the absorption spectrum of 1.0 × 10−4 M DDQ in
the presence of 1.0 × 10−4 M 18C6 gave the appearance of new band
essentially at 286 nm. Fig. 2C shows the plots of absorbance vs.
18C6/DDQ mole ratio obtained at 286 nm. The plot show a break
at 18C6/DDQ = 2 and further confirm the suggested mechanism;
Table 3
Characteristic infrared frequencies (cm−1) and tentative assignments of o-CHL and
[(18C6•+)(o-CHL•−)] CT complex.
+
−
18C6DDQ = 18C6• DDQ•
o-CHL
[(18C6•+)(o-CHL•−)]
Assignments(b)
–
ꢂ(C–H)
ꢂ(C O)
+
−
+
−
18C6• DDQ• + DDQ → 18C6• 2DDQ•
1682
1593
1557
1514
–
1646
–
ꢂ(C C)
For evaluation of the equilibrium constant and the molar
absorptivity of the resulting 1:2 18C6:DDQ complex (Eq. (2)) were
calculated according to straight line (Fig. 3B) produced.
The IR spectral data of DDQ acceptor, the 1:2 charge transfer
complex are presented in Table 4, as can be seen, the CN and CO
stretching of DDQ show a drastic shift to lower (nearly absence)
1455
1352
1286
1250
1108
ı(CH) deformation
–
ꢂ(C–O)
ꢂ(C–N)

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Fig. 2. Molar ratio curve of (A) 18C6/p-CHL, (B) 18C6/o-CHL, (C) 18C6/DDQ, (D) 18C6/TCNQ, (E) 18C6/DCQ, (F) 18C6/DBQ, (G) 18C6/NBS, (H) 18C6/CLA, and (I) 18C6/PA
systems.
frequencies upon molecular complex formation with 18C6. These
shifts are indicative of a higher charge density on the cyano and car-
bonyl groups of the DDQ acceptor of its transfer complex. However,
the methylene stretching of the ethoxy groups of 18C6 observed
within 2800 cm−1 region was shifted upon the complex formation.
This is probably due to the participation of the ethoxy groups in the
interaction with DDQ acceptors.
The 1H NMR spectrum of the 18-crown-6 CT complex was mea-
sured in CDCl3 (Fig. 5). The spectrum of CT complex was compared
with the free donor, which was found that only one signal was
detected at 3.7 ppm due to protons of 12CH2 of the aliphatic crown
ether, this signal blue shifted indicating the involvement of the
CH2–O–CH2 groups in complexation.
Table 4
Characteristic infrared frequencies (cm−1) and tentative assignments of DDQ and
[(18C6•+)(DDQ•−)2] CT complex.
3.4. TCNQ
DDQ
–
[(18C6•+)(DDQ•−)2]
Assignments(b)
2891
2785
2070
ꢂ(C–H)
Fig. 1D shows the absorption spectrum of 1.0 × 10−4 M TCNQ in
the absence of 1.0 × 10−4 M 18C6. As can be seen, over the range
of 300–600 nm the intensity of TCNQ band at 406 nm within the
[(18C6)(TCNQ)] complex increases comparison with the free TCNQ
acceptor. Such observation is indicative of the formation of charge
transfer complex through an equilibrium reaction. The formation of
TCNQ•− and 18C6•+ radical anions have been recorded frequently
[29–31]. Based on these observations it can be concluded that the
molar ratio between 18C6:TCNQ is 1:12 (Fig. 2D). To make check
for the stability and reactivity of the TCNQ charge transfer com-
2250
2236
1680
1555
1430
–
ꢂ(CN)
1636
1474
ꢂ(C O)
ꢂ(C C)
1455
1353
1286
1251
1105
ı(CH) deformation
ꢂ(C–O)
ꢂ(C–N)
–

A.S.M. Hossan et al. / Spectrochimica Acta Part A 79 (2011) 583–593
589
Fig. 3. Plot of ((Cao)2Co)/A vs. Cao(4Co + Cao) for the (A) 18C6/p-CHL, (B) 18C6/DDQ, (C) 18C6/CLA, and (D) 18C6/PA systems.
d
d
Fig. 4. Plot of CaoCo/A vs. Cao + Cdo for the (A) 18C6/o-CHL, (B) 18C6/TCNQ, (C) 18C6/DCQ, (D) 18C6/DBQ, and (E) 18C6/NBS systems.
d

590
A.S.M. Hossan et al. / Spectrochimica Acta Part A 79 (2011) 583–593
Fig. 5. 1H NMR spectrum of [(18C6•+)(DDQ•−)2] charge transfer complex.
Table 5
Characteristic infrared frequencies (cm−1) and tentative assignments of TCNQ and
[(18C6•+)(TCNQ•−)] CT complex.
Fig. 6. 1H NMR spectrum of [(18C6•+)(TCNQ•−)] charge transfer complex.
TCNQ
[(18C6•+)(TCNQ•−)]
Assignments(b)
3137
3050
2969
2851
2220
2888
2795
2750
ꢂ(C–H)
reactants, 18-crown-6 (1.0 × 10−4 M) and acceptors (1.0 × 10−4 M)
in MeOH or CHCl3 along with those of the formed 1:1 CT-complexes
are shown in Fig. 1E–G, respectively. The spectra demonstrate
that the formed CT-complexes have strong absorption bands
around 295, 286 and 280 nm for [(18C6)(DBQ)], [(18C6)(DCQ)]
and [(18C6)(NBS)] complexes, respectively. These bands do not
exist in the spectra of the reactants in the same situations. The
stoichiometry of the [18C6]-acceptor reactions was shown in all
cases to be of ratio 1:1. This was concluded on the bases of the
obtained elemental analysis data of the isolated solid CT-complexes
as indicated in Section 2, as well as from the complexes infrared
spectra, which indicate the existence of the bands characteristic for
both the [18C6] and the respective acceptors. The stoichiometry of
1:1 is also strongly supported by photometric titration measure-
ments. In these measurements concentration of 18-crown-6 was
kept fixed, while the concentration of the acceptors was varied
over the range of 0.25 × 10−4–4.00 × 10−4 M as described in Sec-
tion 2. Photometric titration curves based on these measurements
are shown in Fig. 2E–G. Accordingly, the formed CT-complexes
upon the reaction of [18C6] as a donor with the ␲-acceptors under
investigation in chloroform or methanol have the general formula
[(18C6)(acceptor)]. The 1:1 modified Benesi–Hildebrand Eq. (1),
was used in calculating the values of the equilibrium constant, K
and the molar absorptivity (ε). Cao and Cdo are the initial concentra-
tions of the ␲-acceptors (DBQ, DCQ and NBS) and the donor 18C6,
respectively, while A is the absorbance of the strong bands around
295, 286 and 280 nm for their CT-complexes. Plotting the values of
2171
2129
1636
1598
1575
1506
1499
1471
1350
1247
1106
ꢂ(CN)
1670
1539
–
ꢂ(C C)
ı(CH) deformation
1352
1285
1205
1118
1044
ꢂ(C–O)
ꢂ(C–N)
ꢂ(C–C)
plex base on the 1:1 modified Benesi–Hildebrand equation [18],
the straight line come from the relationship between CaoCo/A vs.
Cao + Cdo is drawn in Fig. 4B. The spectrophotometric datadof the
TCNQ complex are summarized and inserted in Table 1.
Main band assignments for both TCNQ and its CT-complex
appeared in the spectra are given in Table 5. A comparison of the rel-
evant IR spectral bands of the free donor, 18C6 and TCNQ acceptor
with the corresponding appeared in the IR spectrum of the iso-
lated CT-complex, clearly indicated that the characteristic bands of
18C6 show some shift in the frequencies (Table 5), as well as some
change in their bands intensities. This could be attributed to the
expected symmetry and electronic structure changes upon the for-
mation of the CT-complex. Moreover, in general, the IR spectrum of
the molecular complex of TCNQ with 18C6 indicate the ꢂ(C N) and
ꢂ(C C) of the free TCNQ are shifted to lower wavenumber values
on complexation. Since TCNQ is deprived from any acidic centers,
thus we may conclude that the molecular complexes are formed
through n–␲* charge migration from HOMO of the donor to the
LUMO of the acceptor.
the CaoCo/A against Cao + Cdo values for each acceptor, a straight line
d
is obtained with a slope of 1/ε and intercept of 1/Kε as shown in
Fig. 4C–E. The values of both K and ε associated with these com-
plexes were data obtained throughout these calculations are given
in Table 1. These complexes show high values of both the formation
constant (K) and the extinction coefficients (ε). These high values of
K confirm the expected high stabilities of the formed CT-complexes
as a result of the expected high donation of the 18-crown-6. The
equilibrium constants are strongly dependent on the nature of
the used acceptor including the type of electron withdrawing sub-
stituents to it such as halo groups. For example, Table 1, the value of
equilibrium constant for [(18C6)(DCQ)] is highest value than both
[(18C6)(DBQ)] and [(18C6)(NBS)] complexes in respective solvent.
This value is about twice and one half times higher than the val-
ues of equilibrium constant for the complexes [(18C6)(DBQ)] and
[(18C6)(NBS)], respectively, reflecting the relatively higher electron
acceptance ability for DCQ. The number of donating atoms available
is another important factor that affects the stability of crown-ion
complexes [32]. Since in the process of molecular complexation, it
1H NMR spectrum for the [(18C6•+)(TCNQ•−)] complex (Fig. 6)
showed two main signals around 3.6 and 7.2 ppm due to nCH2 of
18C6 and protons of aromatic groups of TCNQ, respectively. All sig-
nals are downfield shifted compared to signals of free donor and
TCNQ. This shift is due to the ring current influence of the magnetic
anisotropy of oxygen atoms and the field effect of the electronic
dipoles located on the oxygen atoms.
3.5. DCQ, DBQ and NBS
The reactions of 18C6 with the DCQ, DBQ and NBS as a
␲-acceptors were carried out in methanol except for NBS in chlo-
roform as a solvent. The electronic absorption spectra of the

A.S.M. Hossan et al. / Spectrochimica Acta Part A 79 (2011) 583–593
591
Table 6
Characteristic infrared frequencies (cm−1) and tentative assignments of DCQ, DBQ, NBS and their CT complexes.
DCQ
DBQ
NBS
[(18C6)(acceptor)]
DCQ
Assignments(b)
DBQ
NBS
3079
3052
2954
2925
2869
2854
1673
1618
1573
1510
1464
3074
2973
2950
2939
2927
2989
2932
2931
2990
2948
2835
ꢂ(C–H)
1678
1640
1605
1566
1547
1509
1342
1301
1270
1143
1013
1002
902
1716
–
1684
1635
1673
–
1684
1641
ꢂ(C O)
ꢂ(C C)
1426
1414
1542
1471
1438
1398
1368
1316
1267
1227
1180
1144
1078
1023
919
1552
1475
1543
1471
1446
1397
1367
1316
1267
1227
1183
1144
1078
1035
1023
918
␦(CH) deformation
1376
1366
1305
1285
1274
1162
1044
917
1311
1238
1197
1167
1157
1007
–
1369
1318
1226
1147
1080
1036
ꢂ(C–O) + ꢂ(C–N)
ꢂ(C–C) + ꢂ(C–X)
869
918
835
898
837
835
is reasonably assumed that the charge density is donated from the
donor to acceptor, the increased number of crown oxygens in the
ring is expected to increase the crown-acceptor interaction in solu-
tion. The effective overlapping of donor–acceptor orbitals involves
the proper spatial positions of donor and acceptor molecules. This
also needs specific conformation of donor. If the conformation of
donor in the complexes form differs significantly from, it’s most
stable conformation in the Free State. During complexation, some
energy will consume for the conversion of most stable conforma-
tion of free donor to a conformation which is suitable for complex
formation. This will act as a destabiliser factor in the whole process.
Among the donor studied, 18C6 has the most rigid structure. So, the
variation of its conformation involves energy consumption. Based
on this property, the observation of least stability for [(18C6)(DBQ)]
(Table 1) is not unexpected.
The conductance as a function of [acceptor]/[18C6] mole ratios
was measured and the results are shown that there is no increase
of conductance upon acceptor (DBQ, DCQ and NBS) addition
thus, it can be concluded that the complexes are completely
nonionic.
IR spectra of 18C6 and their 1:1 [(18C6)(DCQ)], [(18C6)(DBQ)]
and [(18C6)(NBS)] charge transfer complexes are compared in
Table 6. The bands generally show some shifted upon complexa-
tion for the stretching vibrations of the carbonyl group and C–X
(where X = Cl or Br).
absorption bands associated with the formation of the complex
[(18C6)(PA)2] is observed at 421 nm (red shift) in the spectrum
of 18C6/PA system. Furthermore, photometric titration measure-
ments based on the characteristic absorptions bands of the CT
complexes (Fig. 2H and I) confirmed the molar ratios of the two
mentioned complexes. Photometric titrations between 18-crown-6
and both of CLA and PA in CHCl3 solvent were carried out as fol-
lows: concentration of 18C6 (Cd) were kept fixed at 1.0 × 10−4 M,
whereas that of the acceptor (CLA or PA) Ca was changed over a wide
range of 0.25 × 10−4–4 × 10−4 M. The peak absorbances appeared in
the spectra that assigned to the formed CT complexes were mea-
sured and plotted as a function of the ratio Cd:Ca according to the
known method [17]. The 1:2 modified Benesi–Hildebrand Eq. (2)
was used to calculate the values of the equilibrium constant, K
(l mol−1) and the extinction coefficient, ε (l mol−1 cm−1) for both
complexes. The data obtained throughout this calculation are given
in Table 1. Plotting values of (Cao)2Co/A vs. Cao(Cao + 4Cdo) values of
d
Eq. (2), straight lines are obtained with a slope of 1/ε and intercept
of 1/Kε as shown in Fig. 3C and D. Table 1, have the calculated val-
ues of the spectroscopic data like; ε, K, ꢁ, RN, ꢀG, Ip and f, which
deduced from Eqs. (3)–(8). Generally, these complexes show high
values of both the formation constant (K) and the molar absorptivity
(ε). These high values of K confirm the expected high stabilities of
the formed CT-complexes as a result of the expected high dona-
tion of the 18-crown-6. The equilibrium constants are strongly
dependent on the nature of the used acceptor including the type
of electron withdrawing substituents to it such as nitro and halo
groups.
3.6. CLA and PA
The band assignments of infrared spectra of the donor (18C6)
as well as the formed CT complexes 18C6/CLA and 18C6/PA are
reported in Table 7. However, the appearance of a group of IR
spectral bands in the spectra of the CT complexes supports the
conclusion that a deformation of the electronic environment of
18C6 is occurred by accepting protons from CLA or PA. The IR spec-
tra of the two CT complexes of PA and CLA are characterized by
a group of bands appearing in the region >3400 cm−1, which are
not appearing in the spectra of the free donor and acceptors. These
bands are attributed to the stretching vibration of the intermolec-
Electronic spectra were recorded in the region 200–800 nm.
New detected bands in the UV–visible spectra at 308 and 421 nm
are observed immediately on mixing solutions of 18-crown-6 and
each of the chloranilic and picric acids in chloroform, respectively.
The electronic absorption spectra of the donor (18C6) with accep-
tors (CLA and PA) have molar ratios (donor:acceptor) 1:2. Fig. 1H
and I, revealed strong absorption bands attributed to the CT inter-
action. The spectrum of the 18C6/CLA system shows one strong
absorption band at 308 nm with hyperchromic shift due to the
formation of [(18C6)(CLA)2] CT complex, while the corresponding

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A.S.M. Hossan et al. / Spectrochimica Acta Part A 79 (2011) 583–593
O
Cl
O
O
Cl
O
O
H
O
OH
HO
O
H
O
O
Cl
Cl
O
O
O
Formula II. Suggested structure of [(18C6)(CLA)2] complex.
-O
O
-O
O
N+
N+
O
O
O
O
O
O
-O
O
N+
H
H
N+
O
O
O-
O
N+
N+ O-
O
O-
O
Formula III. Suggested structure of [(18C6)(PA)2] complex.
ular hydrogen bond [33]. These results caused to the protonation
of the +OH group of the donor from two sides, through protons
transfer from the acidic center on the CLA acceptor. On the other
hand the intermolecular hydrogen bond occurs in PA acceptor from
–OH group to the basic center oxygen atoms atom. The shift of the
IR bands of the acceptor part to lower wavenumbers and those
of the donor part to higher values reflects a donor to acceptor
charge transfer of n–␲* interaction, DHOMO → DLUMO transition [34].
Accordingly, the hydrogen bonding between the donor and the
acceptors can be formulated as Formulas II and III. The equivalent
conductance values at 1.0 × 10−4 molar concentrations indicated
that these complexes have a small limit of conductivity. The data of
conductivity confirms that these complexes have a positive (+OH)
and negative charge (O− of acidic in chloranilic or picric acid)
resulted from CT transition. The low conductivity values for the
two CT-complexes may be due to intermolecular hydrogen bond
formation.
Table 7
Characteristic infrared frequencies (cm−1) and tentative assignments of CLA, PA and
their CT complexes.
CLA
PA
[(18C6)(acceptor)]
Assignments(b)
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ꢂ(C–H)
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1664
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1296
1269
1236
1201
1130
1054
981
ꢂ(C O)
ꢂ(NO2)
ꢂ(C C)
–
ı(CH) deformation
1368
1263
1207
1168
981
851
752
690
569
1343
1312
1263
1150
1086
917
829
781
732
703
1434
1337
1264
1237
1159
1075
1053
933
906
866
833
785
ꢂ(C–O) + ꢂ(C–N)
ꢂ(C–C) + ꢂ(C–X)
ı(NO2)
652
522
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