A.S.M. Hossan et al. / Spectrochimica Acta Part A 79 (2011) 583–593
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
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-
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)