Azocalixarenes.7: Synthesis and study of the absorption properties of novel mono-azo substituted chromogenic calix[4]arenes

Article abstract of DOI:10.3906/kim-1003-102

Eight new mono-azo substituted chromogenic calix[4]arenes (1-8) were synthesized by diazo-coupling reactions in which 25,26,27-tribenzoyloxy-28- hydroxy-5,11,17-tri-(tert-butyl)calix[4]arene and diazonium salts (2-, 3-, and 4-nitroaniline; 4-phenyl azoani

Full text of DOI:10.3906/kim-1003-102

Turk J Chem
35 (2011) , 87 – 98.
¨
˙
c
ꢀ TUBITAK
doi:10.3906/kim-1003-102
Azocalixarenes.7: Synthesis and study of the absorption
properties of novel mono-azo substituted chromogenic
calix[4]arenes
∗
¨
¨
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˙
Ozlem OZEN KARAKUS¸ and Hasalettin DELIGOZ
Department of Chemistry, Faculty of Science & Arts, Pamukkale University,
20017 Denizli-TURKEY
e-mail: hdeligoz@pamukkale.edu.tr
Received 17.03.2010
Eight new mono-azo substituted chromogenic calix[4]arenes (1-8) were synthesized by diazo-coupling re-
actions in which 25,26,27-tribenzoyloxy-28-hydroxy-5,11,17-tri-(tert-butyl)calix[4]arene and diazonium salts
(2-, 3-, and 4-nitroaniline; 4-phenyl azoaniline; 3- and 4-chloroaniline; or 2- and 4-methylaniline) formed
the corresponding mono-azo substituted derivatives in high yields. The mono-azo substituted calix[4]arenes
were characterized by UV-Vis, FT-IR, and 1 H-NMR spectroscopic techniques. Elemental analysis was also
carried out. In addition, the effects of varying pH levels and solvents upon the absorption ability of azo-
calix[4]arenes substituted with electron-donating and electron-withdrawing groups at their o-, m-, and p-
positions were studied. The results proved that the absorption maxima of the prepared compounds showed
large bathochromic effects in comparison with analog compounds containing aromatic amine residue. Fur-
thermore, concentration effects on the visible absorption maxima of the azocalix[4]arene compounds studied
were also reported.
Key Words: Calixarene, azocalix[4]arenes, mono-azo substituted, diazo-coupling reaction, solvent effect,
substituent effect, absorption properties
Introduction
Calix[n]arenes, which are accessible by the base-catalyzed condensation of para-substituted phenols with
formaldehyde, are ideal frameworks for the development of chromogenic ionophores in the molecular recog-
nition of ionic species of chemical and biological interest, since the calixarene results in a tailored chromogenic
∗Corresponding author
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receptor.1 Selective signaling of heavy metal ions is a very important topic for the detection and treatment
of the toxic metal ions in various chemical systems, including living systems.2 In this field, calixarenes are
currently subjects of interest as chemical sensors3 and selective receptors4 due to their important functional-
ization properties. A variety of compounds based upon calixarene having nitrophenol,5 nitrophenylazophenol,6
azophenol,7 indophenol,8 and indoaniline9 functional groups have been successfully designed and have exhib-
ited a pronounced chromogenic behavior toward Na+ , K+ , Cs+ , Ca2+ , and UO22+ cations, and even chiral
amines.
Shinkai et al. synthesized a calix[4]arene with a 4-(4’-nitrophenyl azo)phenol unit and 3 ethyl ester
residues on the lower rim. They observed the lithium sensitivity of the ionophore.10 Toth et al. reported the
sodium selectivity of similar azophenol derivatives.11 Lu et al. also reported the synthesis of a water-soluble
azocalixarene derivative, detecting highly selective chromogenic ionophores for the recognition of chromium(III)
ions.12
Recently, various derivatives of calix[4]arene13−15 and their complexes,16 polymeric calix[n]arene derivati-
ves17 and azocalix[n]arenes,18−22 have been synthesized by the authors. We examined the selective extraction
of the Fe3+ ion from the aqueous phase into the organic phase, as well as the liquid-liquid extraction of transition
metal ions,23 using diazo-coupled calix[n]arenes. However, there is still an important system that can exhibit
color changes due to ionic or molecular interactions.
We are mainly interested in the development of a new class of chromogenic azocalix[n]arene dyes. The
aim of this study was to synthesize 8 new diazo-coupled mono-azo substituted calix[4]arenes by substituted
aniline derivative subunits 1-8 (Figure 1) and investigate their properties. There have not been any reports on
azocalix[4]arene-based selective chromoionophores or the visible absorption properties of their compounds until
now.
1: 2-nitro
2: 3-nitro
3: 4-nitro
5: 3-chloro
6: 4-chloro
7: 2-methyl
R':
R'
N
N
4: 4-phenylazo 8: 4-methyl
OR
O
C
OR
HO
OR
R:
Figure 1. Diazo-coupled mono-azo substituted calix[4]arene derivatives 1-8.
Experimental
General
All solvents and compounds were commercially graded reagents used without further purification. Melting
points were measured with an Electrothermal IA9100 digital melting point apparatus in capillaries sealed under
nitrogen and are uncorrected. The UV-Vis spectra were obtained with a Shimadzu 1601 UV-Visible recording
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spectrophotometer. The IR spectra were recorded with a Mattson 1000 FT-IR spectrometer as KBr pellets. The
1 H-NMR spectra, referenced to tetramethylsilane (TMS) at 0.00 ppm as an internal standard, were recorded
on a Bruker 400 MHz spectrometer at room temperature (25
1
◦ C). The elemental analyses were performed
¨
˙
at the laboratory of the Scientific and Technological Research Council of Turkey (TUBITAK).
The solvent of crystallization was retained in some of the analytical samples and affected the elemental
analysis results. In such cases, best fits between the analytical values and appropriate fractional increments of
solvents were used.
Preparation of the ligands
p-tert-Butylcalix[4]arene,24 25,26,27-tribenzoyloxy-28-hydroxy-5,11,17,23-tetra-(tert-butyl)calix[4]arene,25 and
25,26,27-tribenzoyloxy-28-hydroxy-5,11,17-tri-(tert-butyl) calix[4]arene25 were synthesized as described by a
previously reported method.
Preparation of mono-phenylazocalix[4]arenes (1-8)
Diazotization of the various carbocyclic amines was effected with concentrated HCl. Compound 1 was as follows:
25,26,27-Tribenzoyloxy-28-hydroxy-5,11,17-tri-(tert-butyl)-23-(3-nitrophenylazo)calix[4]arene (2), 25,26,27-tri-
benzoyloxy-28-hydroxy-5,11,17-tri-(tert-butyl)-23-(4-nitrophenylazo)calix[4]arene (3), 25,26,27-tribenzoyl oxy-
28-hydroxy-5,11,17-tri-(tert-butyl)-23-(4-phenylazophenylazo)calix[4]arene (4), 25,26,27-tri benzoyloxy-28-hy-
droxy-5,11,17-tri-(tert-butyl)-23-(3-chlorophenylazo)calix[4]arene (5), 25,26,27-tribenzoyloxy-28-hydroxy-5,11,
17-tri-(tert-butyl)-23-(4-chlorophenylazo)calix[4]arene (6), 25,26,27-tribenzoyloxy-28-hydroxy-5,11,17-tri-(tert-
butyl)-23-(2-methyl phenylazo)calix[4]arene (7), and 25,26,27-tribenzoyloxy-28-hydroxy-5,11,17-tri-(tert-butyl)-
23-(4-methylphenylazo)calix[4]arene (8) were obtained by applying the described method, and the yield was
60%-96%. The obtained compounds were purified by crystallization using DMF/MeOH and analyzed, respec-
tively.
All of the mono-substituted azocalix[4]arenes were soluble in EtOH, diethyl ether, acetic acid, benzene,
acetone, CHCl3 , and DMSO, and insoluble in water, 10% HCl, and 10% NaOH.
The synthesis of 25,26,27-tribenzoyloxy-28-hydroxy-5,11,17-tri-(tert-butyl)-23-(2-nitrophenylazo)
calix[4]arene (1)
A solution of 2-nitrophenyldiazonium chloride, prepared from 2-nitroaniline (0.21 g, 1.50 mmol), sodium nitrite
(0.12 g, 1.50 mmol), and concentrated HCl (0.35 mL) in water (3 mL), was added slowly to a cold (5 ◦ C) solution
of 25,26,27-tribenzoyloxy-28-hydroxy-5,11,17-tri-(tert-butyl)calix[4]arene (1.0 g, 1.10 mmol) and sodium acetate
trihydrate (0.40 g, 3.00 mmol) in DMF/MeOH (13 mL, 8:5, v/v) to give a yellow suspension. After standing
for 2 h at room temperature, the suspension was acidified with aqueous HCl (50 mL, 0.25%). The resulting
mixture was first warmed to 60 ◦ C and kept at that temperature for 30 min to give 1 (yield: 1.01 g, 87%) as
a yellow solid, and then filtered and washed with water and MeOH, respectively. A typical sample for analysis
was obtained as follows: 1 was dissolved in 100 mL of hot aqueous NaHCO3 (4.20 g) solution, after which 1
g of activated charcoal was added to the solution. After the charcoal was filtered, the filtrate was cooled to
room temperature and acidified with concentrated HCl (1 or 2 mL). The solution was heated to 60 ◦ C and
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kept at that temperature, then cooled down. The resulting solid was filtered, washed with water, and dried.
Recrystallization from the DMF/MeOH mixture gave a yellow product. Yield 0.86 g (87%), mp 265 ◦ C; [Found:
C: 76.02; H: 6.18; N: 3.57]; C67 H63 N3 O9 requires C: 76.33; H: 6.02; N: 3.98. IR (KBr) υ: 3450 cm−1 (-OH),
1720 cm−1 (-C = O), 1490 cm−1 (-N = N), 1255 cm−1 (C-O). 1 H-NMR (DMSO-d6 , 25 ◦ C) δH : 1.0 (27H,
s, t−bu), 3.5-3.9 (8H, d, J = 13.2 Hz, Ar-CH2 -Ar), 5.9 (1H, s, OH), 6.8-7.4 (8H, m, Ar-H), 7.5-7.8 (15H, m,
ArH-CO), 7.9-8.3 (4H, m, ArH-NO2).
The synthesis of 25,26,27-tribenzoyloxy-28-hydroxy-5,11,17-tri-(tert-butyl)-23-(3-nitrophenylazo)
calix[4]arene (2)
Azocalix[4]arene 2 was prepared using 3-nitroaniline in water/HCl and obtained as a pale brown solid, which was
filtered and washed with water and MeOH, and dried as described above. The resulting solid was recrystallized
from the DMF/MeOH mixture, which gave a pale brown product. Yield 0.76 g (65%), mp 269 ◦ C; [Found: C:
76.42; H: 6.12; N: 3.67]; C67 H63 N3 O9 requires C: 76.33; H: 6.02; N: 3.98. IR (KBr) υ: 3450 cm−1 (-OH),
1740 cm−1 (-C = O), 1495 cm−1 (-N = N), 1250 cm−1 (C-O). 1 H-NMR (DMSO-d6 , 25 ◦ C) δH : 1.0 (27H,
s, t−bu), 3.5-4.1 (8H, d, J = 13.3 Hz, Ar-CH2 -Ar), 5.7 (1H, s, OH), 6.6-7.3 (8H, m, Ar-H), 7.5-7.8 (15H, m,
ArH-CO), 7.9-8.2 (4H, m, ArH-NO2).
The synthesis of 25,26,27-tribenzoyloxy-28-hydroxy-5,11,17-tri-(tert-butyl)-23-(4-nitrophenylazo)
calix[4]arene (3)
Azocalix[4]arene 3 was prepared using 4-nitroaniline in water/HCl and was obtained as an ivory solid, which was
filtered and washed with water and MeOH, and dried as described above. The resulting solid was recrystallized
from the DMF/MeOH mixture, which gave an ivory product. Yield 0.92 g (69%), mp 249 ◦ C; [Found: C:
75.92; H: 6.21; N: 3.59]; C67 H63 N3 O9 requires C: 75.92; H: 6.21; N: 3.59. IR (KBr) υ: 3450 cm−1 (-OH),
1700 cm−1 (-C = O), 1490 cm−1 (-N = N), 1250 cm−1 (C-O). 1 H-NMR (DMSO-d6 , 25 ◦ C) δH : 1.0 (27H,
s, t−bu), 3.5-4.2 (8H, d, J = 13.1 Hz, Ar-CH2 -Ar), 5.7 (1H, s, OH), 6.5-7.2 (8H, m, Ar-H), 7.4-7.8 (15H, m,
ArH-CO), 7.9-8.2 (4H, m, ArH-NO2).
The synthesis of 25,26,27-tribenzoyloxy-28-hydroxy-5,11,17-tri-(tert-butyl)-23-(4-phenylazophen-
ylazo)calix[4]arene (4)
Azocalix[4]arene 4 was prepared using 4-phenylazoaniline in water/HCl and was obtained as a dark red solid,
which was filtered and washed with water and MeOH, and dried as described above. The resulting solid was
recrystallized from the DMF/MeOH mixture, which gave a dark red product. Yield 1.16 g (96%), mp 232 ◦ C;
[Found: C: 78.25; H: 6.42; N: 4.85]; C73 H68 N4 O7 requires C: 78.75; H: 6.16; N: 5.03. IR (KBr) υ: 3495 cm−1
(-OH), 1760 cm−1 (-C = O), 1495 cm−1 (-N = N), 1250 cm−1 (C-O). 1 H-NMR (DMSO-d6 , 25 ◦ C) δH : 1.3
(27H, s, t−bu), 3.7-3.9 (8H, d, J = 13.5 Hz, Ar-CH2 -Ar), 5.8 (1H, s, OH), 6.7-6.9 (8H, m, Ar-H), 7.0-7.7 (9H,
m, ArH), 7.7-7.9 (15H, m, ArH-CO).
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The synthesis of 25,26,27-tribenzoyloxy-28-hydroxy-5,11,17-tri-(tert-butyl)-23-(3-chloropheny-
lazo)calix[4]arene (5)
Azocalix[4]arene 5 was prepared using 3-chloroaniline in water/HCl and was obtained as a pale yellow solid,
which was filtered and washed with water and MeOH, and dried as described above. The resulting solid was
recrystallized from the DMF/MeOH mixture, which gave a pale yellow product. Yield 0.80 g (71%), mp 279
◦ C; [Found: C: 76.82; H: 6.12; Cl: 3.15; N: 2.49]; C67 H63 ClN2 O7 requires C: 77.10; H: 6.08; Cl: 3.40; N:
2.68. IR (KBr) υ: 3510 cm−1 (-OH), 1735 cm−1 (-C = O), 1480 cm−1 (-N = N), 1260 cm−1 (C-O). 1 H-NMR
(DMSO-d6 , 25 ◦ C) δH : 1.2 (27H, s, t−bu), 3.5-4.2 (8H, d, J = 13.2 Hz, Ar-CH2 -Ar), 5.7 (1H, s, OH), 6.4-7.5
(8H, m, Ar-H), 6.9-7.6 (4H, m, Ar-Cl), 7.8-8.1 (15H, m, ArH-CO).
The synthesis of 25,26,27-tribenzoyloxy-28-hydroxy-5,11,17-tri-(tert-butyl)-23-(4-chloropheny-
lazo)calix[4]arene (6)
Azocalix[4]arene 6 was prepared using 4-chloroaniline in water/HCl and was obtained as a yellow solid, which was
filtered and washed with water and MeOH, and dried as described above. The resulting solid was recrystallized
from the DMF/MeOH mixture, which gave a yellow product. Yield 0.68 g (60%), mp 280 ◦ C; [Found: C: 77.42;
H: 6.35; Cl: 3.20; N: 2.47]; C67 H63 ClN2 O7 requires C: 77.10; H: 6.08; Cl: 3.40; N: 2.68. IR (KBr) υ: 3500
cm−1 (-OH), 1736 cm−1 (-C = O), 1480 cm−1 (-N = N), 1260 cm−1 (C-O). 1 H-NMR (DMSO-d6 , 25 ◦ C) δH :
1.0 (27H, s, t−bu), 3.2-4.3 (8H, d, J = 13.2 Hz, Ar-CH2 -Ar), 5.6 (1H, s, OH), 6.5-7.5 (8H, m, Ar-H), 6.9-7.6
(4H, m, Ar-Cl), 7.7-8.1 (15H, m, ArH-CO).
The synthesis of 25,26,27-tribenzoyloxy-28-hydroxy-5,11,17-tri-(tert-butyl)-23-(2-methylpheny-
lazo)calix[4]arene (7)
Azocalix[4]arene 7 was prepared using 2-methylaniline in water/HCl and was obtained as an ivory solid,
which was filtered and washed with water and MeOH, and dried as described above. The resulting solid
was recrystallized from the DMF/MeOH mixture, which gave an ivory product. Yield 0.68 g (60%), mp 269
◦ C; [Found: C: 79.42; H: 6.32; N: 2.67]; C68 H66 N2 O7 requires C: 79.82; H: 6.50; N: 2.74. IR (KBr) υ: 3523
cm−1 (-OH), 1735 cm−1 (-C = O), 1481 cm−1 (-N = N), 1268 cm−1 (C-O). 1 H-NMR (DMSO-d6 , 25 ◦ C) δH :
1.1 (27H, s, t−bu), 1.7 (3H, s, CH3), 3.5-4.1 (8H, d, J = 13.2 Hz, Ar-CH2 -Ar), 5.8 (1H, s, OH), 6.6-7.2 (8H,
m, ArH), 7.2-7.9 (4H, m, ArH-CH3) 7.9-8.3 (15H, m, ArH-CO).
The synthesis of 25,26,27-tribenzoyloxy-28-hydroxy-5,11,17-tri-(tert-butyl)-23-(4-methylpheny-
lazo)calix[4]arene (8)
Azocalix[4]arene 8 was prepared using 4-methylaniline in water/HCl and was obtained as an ivory solid,
which was filtered and washed with water and MeOH, and dried as described above. The resulting solid
was recrystallized from the DMF/MeOH mixture, which gave an ivory product. Yield 0.70 g (64%), mp 276
◦ C; [Found: C: 79.30; H: 6.85; N: 2.43]; C68 H66 N2 O7 requires C: 79.82; H: 6.50; N: 2.74. IR (KBr) υ: 3450
cm−1 (-OH), 1740 cm−1 (-C = O), 1495 cm−1 (-N = N), 1250 cm−1 (C-O). 1 H-NMR (DMSO-d6 , 25 ◦ C) δH :
1.1 (27H, s, t−bu), 1.6 (3H, s, CH3), 3.5-4.1 (8H, d, J = 13.2 Hz, Ar-CH2 -Ar), 5.7 (1H, s, OH), 6.5-7.3 (8H,
m, Ar-H), 7.4-8.0 (4H, m, ArH-CH3), 8.1-8.3 (15H, m, ArH-CO).
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Results and discussion
Synthesis and characterizations
In our previous studies, the absorption properties of the carbocyclic and heterocyclic amine substituted azo-
calix[4]arenes, as simple bathochromic and hypsochromic effects, were investigated. In this study, we focused on
calix[n]arene-based diazonium salts as precursors for a new group of macrocyclic oligomeric ligands. Based on
thorough studies of calix[n]arenes as ligands for transition metal complexes,23 the following basic building prin-
ciple should be pursued: A calix[4]arene skeleton may serve as a platform to attach -N = N- ligands. Because
of the typical geometry26 of calix[4]arenes, it is easier to identify different functional sites on the calix[n]arene.
The upper (wide) rim of the macrocycle may serve to place ligands at discrete positions. Variation of both
the number and the relative geometry of the ligands can be easily adjusted to fit the needs imposed by the
catalytic task. Functionalization of the calix[4]arene at the lower (narrow) rim with benzoyl groups fixes the
cone conformation and enhances its solubility in organic solvents.
Recently, we reported the synthesis of various azocalix[4]arenes through diazo-coupling with carbocyclic
and heterocyclic amines.18−22 Azocalix[4]arenes have proven to be versatile substrates in diazo-coupling reac-
tions, allowing the preparation of several new donor-acceptor substituted carbocyclic amine derivatives.
In this work, 8 new azo compounds were synthesized from 2-, 3-, and 4-nitroaniline, 4-phenylazoaniline,
3- and 4-chloroaniline, and 2- and 4-methylaniline, and their properties were investigated. These azocalix[4]arene
compounds (1-8) are outlined in Figure 2. This functionalization was achieved by reacting 25,26,27-tribenzoyloxy-
28-hydroxy-5,11,17-tri-(tert-butyl)calix[4]arene with aryldiazonium salts in DMF/methanol for 2 h at 5 ◦ C.
i
OH
OH
HO
OR
OR
HO
OH
OR
OR
ii
R'
N
N
iii
OR
OR
OR
HO
HO
OR
OR
O
C
R
Figure 2. The synthesis of mono-azo substituted calix[4]arene derivatives 1-8. i) benzoylchloride/pyridine, ii)
AlCl3 /toluene, iii) NaNO2 /HCl and carbocyclic amine derivatives.
The mono-azocalix[4]arene derivatives (1–8) were prepared by coupling 25,26,27-tribenzoyloxy-28-hydroxy-
5,11,17-tri-(tert-butyl)calix[4]arene with diazotized carbocyclic amines in NaNO2 /HCl. The mono-azo substi-
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tuted calix[4]arenes may exist in 4 possible tautomeric forms in solutions, namely the azo-phenol form (A),
the quinone-hydrazone form (B), and their deprotonated anions (C) and (D), respectively. These results are in
agreement with the work of Ertan27 and our recent paper.18−22
Diazo-coupling was accomplished selectively at the p-position of the calix[4]arene ring to give com-
pounds in moderate to excellent yields (60%-96%). These results, which are in accordance with the greater
nucleophilicity of the carbocyclic amine ring, were observed earlier in the case of the formulation of carbocyclic
amine derivatives. The structures of the azocalix[4]arenes were unambiguously confirmed by their analytical
and spectral data.
The infrared spectra of all of the azocalix[4]arenes (in KBr) show broad bands within the range of 3523-
3450 cm−1 , corresponding to υOH . The FT-IR spectra also showed a strong band located at 1495-1480 cm−1
,
which can be assigned to -N = N-. The other υmax values, at 1760-1700 and 1268-1250 cm−1 , can be assigned to
υC=O and υC−O , respectively. It can be suggested that these azocalix[4]arenes do not exist as the keto-hydrazo
form B or its anionic form D in the solid state (Figure 3).
NO2
NO2
N
N
H
N
N
OR
OR
HO
OR
OR
OR
OR
O
azo-phenol
(A)
quinone-hydrazone
(B)
basic solution
basic solution
NO2
NO2
N
N
N
N
OR
OR
O
OR
OR
OR
O
OR
O
(C)
R
C
2420
(D)
Figure 3. The tautomeric forms of diazo-coupled mono-azo substituted calix[4]arene 1.
According to 1 H NMR in DMSO, the structure of azocalix[4]arene derivatives 1-8 not only impose the
cone conformation in the liquid state, but also provide evidence for asymmetric structural features. Each of
the derivatives has 2 pairs of doublets around 3.2 and 4.2 ppm, which indicates a cone conformation with
nonequivalent methylene bridges in the neighboring and remote positions relative to the p-substituted arene
moiety. However, the cone conformation of flexible calixarenes (1-8) could be stabilized by intramolecular H-
bonding involving the OH group. Consecutively, the partial cone conformation may be formed with DMSO by
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breaking the H-bonds, resulting in a similar pattern of Ar-CH2 -Ar protons signals. As previously reported,1,24,25
this can be explained by the presence of chiral substituents and indicates a significant degree of structural rigidity.
In the 1 H NMR spectrum of the azocalix[4]arenes, signals of the aromatic hydroxyl and imine proton shifted
upfield, which suggests that the hydrogen bonds involved in the mono-azo substituted calix[4]arenes are more
important than in azocalix[4]arenes.
The 1 H NMR spectra measured in DMSO-d6 at 25 ◦ C showed a multiplet from 8.3 to 7.7 ppm for
substituted aromatic protons (ArH-X), a multiplet from 7.9 to 7.0 ppm for benzoyl aromatic protons (ArH-
CO), a multiplet from 7.5 to 6.4 ppm for calix[4]arene aromatic protons (Ar-H), a broad peak from 5.9 to 5.6
ppm for hydroxyl protons (-OH), and a singlet at 1.0 ppm for tert-butyl protons. These results indicate that
azocalix[4]arene, rather than the mono-azo substituted one, may exist as a mixture of several tautomeric forms
in DMSO. Such an observation in 1 H-NMR can be easily explained by the C2v symmetry of the bridge protons
in trisubstituted azocalix[4]arenes 1-8. Mono-azo substituted azocalix[4]arenes are in favor of the predominantly
single tautomeric form in DMSO.
Absorption properties
The design and synthesis of organic chromophores as nonlinear optical (NLO) materials has attracted much
attention in recent years.28 They have great potential, especially in optical communication, information pro-
cessing, frequency doubling, and integrated optic fields. Organic NLO materials have many advantages over
inorganic materials, including large nonlinear optical coefficients, great ease of synthetic design, easy preparation,
and low cost. It has been shown that the second order hyperpolarizabilities (β) of heterocyclic chromophores
are often higher than those of carbocyclic amine analogs. Recently, we also demonstrated that donor-acceptor
substituted azocalix[4]arenes have many favorable features as NLO materials.18−22
Electronic absorption spectra of azocalix[4]arenes showed an intense lowest energy charge-transfer ab-
sorption band in the UV-Vis region. Dramatic differences in energy occur upon arylazo substitution of azo-
calix[4]arenes. For example, the wavelength of the lowest energy charge transfer band of azocalix[4]arene 4 was
shifted upfield by 176 nm upon arylazo substitution (azocalix[4]arene 6, 325 nm) (Table). The influence of the
strength of acceptor groups, such as nitro- and chloro-, is demonstrated by comparison of the absorption maxima
of compounds 1, 2, 3, 5, and 6 as the longest wavelengths. The shift in the transition of azocalix[4]arene 1 (404
nm) to azocalix[4]arene 3 (331 nm) in methanol (Table) can be attributed to the stabilization of LUMO by the
electron-withdrawing groups.29 A distinct spectral effect could be also caused by the substituent at the nitrogen
atom of the aromatic amine ring. The influence of the substituent on the nitrogen atom of the aromatic amine
ring was demonstrated by comparison of the absorption maxima of azocalix[4]arenes 2 and 3 as the longest
wavelengths. In addition, a shift in transition was observed from azocalix[4]arene 2 to azocalix[4]arene 3 in
methanol (Table), at 481 and 331 nm, respectively.
Solvent effects
Mono-azo substituted calix[4]arene derivatives 1-8 showed characteristic absorption patterns. The ap-
pearance of the azocalix[4]arene absorption band at a relatively high wavelength may be attributed to the
presence of electron-withdrawing groups, such as nitro- and chloro-, and electron-donating groups, such as azo-
and methyl-. The electronic absorption spectra of azocalix[4]arene derivatives 1-8 exhibited an absorption
94

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Azocalixarenes.7: Synthesis and study of the absorption..., O. OZEN KARAKUS¸, H. DELIGOZ
band and a shoulder in the range of 325-404 nm corresponding to π − π * and n-π * transitions, which are in
accordance with typical diazo spectra.
The visible spectral properties of the mono-azo substituted calix[4]arene derivatives were compared with
respect to positions of substitutions. Electron-withdrawing groups, such as nitro- and chloro- groups, and
electron-donating groups, such as azo- and methyl- groups, exerted a bathochromic effect as substituents in the
aryl ring.
The absorption spectra of azocalix[4]arenes 1-8 were recorded in various solvents at concentrations of
approximately 10−6 to 10−8 M (Table). The visible absorption spectra of these azocalix[4] arenes were found
to exhibit strong solvent dependency, which did not show regular variation with the polarity of the solvents.
Table. Influence of solvent on λmax (nm) of azocalix[4]arenes.
Chloroform
DMSO
conc.
DMF
Acetonitrile + Piperidine
Methanol
Acetic Acid
Compounds
dil.
416
conc.
dil.
411
conc.
401
dil.
399
conc.
397
conc.
404
dil.
404
conc.
398
dil.
397
1
416
411
494
347
2
439
588 s
575s 378
427
435
493
480
480
335
481 s 413s
324
325
567 s 326 s 327 s
3
4
5
380
421
495
375
413
375
412
364
394
321
364
394
324
353
336
331
401
334
331
403
333
361
403
335
357
402
332
495
320s 319 s
355s 355 s
335
489 s
382 s
323 s
323
6
330 s
329 s 363 s 363 s 387 s 380 s
325 s 325 s 326 s 326 s
7
8
347 s
333
347 s 370 s 370 s 330 s 330 s
364 s 364 s
401 403
-
-
333
337
336
333
332
334
403
403
417 s
s: shoulder
2.000
1: DMSO
2: DMF
4
3: Acetonitrile
4: Chloroform
5: Methanol
6: Acetic acid
1
5
6
2
1.000
0.000
3
300
400
500
Wavelength (nm)
600
700
Figure 4. Absorption spectra of azocalix[4]arene 1 in various solvents.
95

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Azocalixarenes.7: Synthesis and study of the absorption..., O. OZEN KARAKUS¸, H. DELIGOZ
Strong evidence for the existence of these azocalix[4]arenes in an equilibrium is provided by the isosbestic
points in the visible spectra of, for example, azocalix[4]arene 1 in different solvents (Figure 4). This equilibrium
may exist among tautomeric forms. The equilibrium depends on the basicity of the solvents used; with proton-
accepting solvents such as DMSO, DMF, acetonitrile, and chloroform, the azocalix[4]arenes displayed a red
shift of λmax
. Moreover, in a proton-donating solvent such as methanol or acetic acid, the λmax of the
azocalix[4]arenes did not significantly change with respect to the absorption spectra in chloroform, with the
exception of azocalix[4]arene 1. Similar approaches and suggestions have also been reported by Ertan28 and
Deligo¨z.18−22
The λmax of azocalix[4]arene 4 in methanol showed bathochromic shifts when 0.1 M KOH was added.
The λmax of this compound in methanol showed both hypsochromic shifts and slight bathochromic shifts when
0.1 M HCl was added. A typical example for azocalix[4]arene 4 is shown in Figure 5. These results indicate
that azocalix[4]arenes 1-8 exist in the anion form.
1.000
1: Methanol
1
2: Methanol + HCl
3: Methanol +KOH
2
3
0.000
300
400
500
600
700
Wavelength (nm)
Figure 5. Absorption spectra of azocalix[4]arene 4 in acidic and basic solutions.
Substituent effects
As shown in the Table, the introduction of electron-withdrawing nitro- and chloro- groups and electron-
donating azo- and methyl- groups into the carbocyclic rings resulted in bathochromic shifts in all solvents.
However, the introduction of electron-donating azo- and methyl- groups into the benzene rings resulted in
hypsochromic shifts in DMSO and DMF, but produced bathochromic shifts in acetonitrile, methanol, acetic
acid, and chloroform. The introduction of nitro- and chloro- groups into the benzene rings imparted the greatest
bathochromic shift of all groups. No group’s position showed a regular variation in all solvents.
The visible absorption maxima (λmax) in all solvents are given in the Table. Inspection of the Table shows
that azocalix[4]arene derivatives 1, 2, 3, 5, and 6, derived from nitro- and chloro- carbocyclic amines containing
electron-withdrawing nitro- and chloro- groups, and azocalix[4]arenes 1-8 did not have uniform bathochromic
shifts of about 48-165 nm relative to the corresponding azocalix[4]arene derivatives 4, 7, and 8, from azo- and
methyl- carbocyclic amines containing one-electron-donating azo- and methyl- groups. Thus, azocalix[4]arenes
1 and 3, with an o- and p- nitro substituent in the diazo component, absorbed at 416 and 378 nm. When the
nitro group was in the m-position (2), the absorption shifted to 439 nm in DMSO.
96

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Azocalixarenes.7: Synthesis and study of the absorption..., O. OZEN KARAKUS¸, H. DELIGOZ
Concentration effects
The effect of concentration on the absorption maxima of compounds 1-8 is given in the Table. As can
be seen, all azocalix[4]arenes had concentration-independent λmax values of compounds, indicating that the
azocalix[4]arenes existed in their tautomeric form in all solvents used. The λmax of no compound changed with
compound concentration.
Conclusions
Eight new calixarene-based receptors with benzoyl groups immersed in a large cavity were synthesized. The
synthesis of all diazo-coupled compounds was achieved using the method of Morita.30 These reactions produced
the corresponding azo compounds in good yield. Diazo-coupling reactions are shown in Figure 2. This pathway
was a convenient way to achieve the best yield of azocalix[4]arene compounds. We studied the diazo-coupling
reactions of calix[4]arene with benzenediazonium chloride; 2-, 3-, and 4-nitroaniline; 4-phenyl azoaniline; 3- and
4-chloroaniline; or 2- and 4-methylaniline.
The o-, m-, and p-substituted groups dissociated progressively in solution with increasing pH, thus
forming the azo-phenol form and quinone-hydrazone form species, with characteristic changes in the visible
absorption spectra.
The azocalix[4]arene 4 derived from calix[4]arene and substituted aniline showed a pronounced color
change from pale yellow to dark red in dry CHCl3 , and could be used as an analytical indicator. As both the
neutral and mono-anionic forms of azocalix[4]arene derivatives were nonfluorescent, these compounds could be
used as pH indicators.
Acknowledgement
We thank the Research Foundation of Pamukkale University of Turkey for the financial support (BAP 2003/023).
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