Synthesis and photophysical properties of a novel ethynyl zinc(II) phthalocyanine and its functionalized derivative with click chemistry

Article abstract of DOI:10.1142/S1088424613500260

The synthesis of novel, symmetrical zinc(II) phthalocyanine (ZnPc) bearing four ethynylcyclohexyloxy terminal moieties was achieved by cyclotetramerization of novel 4-(2-ethynylcyclohexyloxy) phthalonitrile in pentanol in the presence of DBU and zinc acetate without any protective/deprotective chemistry. Subsequently, this new zinc(II) phthalocyanine derivative was reacted with 6-azido-hexanoic acid under ?click-chemistry? conditions to give phthalocyanine-hexanoic acid conjugates linked by 1,2,3-triazole units. The new compounds have been characterized by using elemental analyses, UV-vis, FTIR, ¹H NMR and mass spectroscopic data. The aggregation properties of the compounds were investigated in different concentrations. General trends are also described for fluorescence quantum yields and lifetimes of novel zinc derivatives in tetrahydrofuran. The fluorescence of the tetrasubstituted zinc(II) phthalocyanine complexes is effectively quenched by 1,4-benzoquinone (BQ) in THF.

Full text of DOI:10.1142/S1088424613500260

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2013; 17: 540–547
DOI: 10.1142/S1088424613500260
Published at http://www.worldscinet.com/jpp/
Synthesis and photophysical properties of a novel ethynyl
zinc(II) phthalocyanine and its functionalized derivative
with click chemistry
◊
˙
Altug˘ Mert Sevim, Ibrahim Özçes¸meci and Ahmet Gül*
Technical University of Istanbul, Department of Chemistry, Maslak, Istanbul TR-34469, Turkey
Dedicated to Professor Özer Bekaroğlu on the occasion of his 80th birthday
Received 5 February 2013
Accepted 5 March 2013
ABSTRACT:Thesynthesisofnovel, symmetricalzinc(II)phthalocyanine(ZnPc)bearingfourethynylcy-
clohexyloxyterminalmoietieswasachievedbycyclotetramerizationofnovel4-(2-ethynylcyclohexyloxy)
phthalonitrile in pentanol in the presence of DBU and zinc acetate without any protective/deprotective
chemistry. Subsequently, this new zinc(II) phthalocyanine derivative was reacted with 6-azido-hexanoic
acid under “click-chemistry” conditions to give phthalocyanine-hexanoic acid conjugates linked
by 1,2,3-triazole units. The new compounds have been characterized by using elemental analyses,
1
UV-vis, FTIR, H NMR and mass spectroscopic data. The aggregation properties of the compounds
were investigated in different concentrations. General trends are also described for fluorescence quantum
yields and lifetimes of novel zinc derivatives in tetrahydrofuran. The fluorescence of the tetrasubstituted
zinc(II) phthalocyanine complexes is effectively quenched by 1,4-benzoquinone (BQ) in THF.
KEYWORDS: phthalocyanine, fluorescence, click chemistry, aggregation, quantum yield.
follows the classical route of cyclotetramerization of
INTRODUCTION
suitably substituted phthalonitriles and in this context
Phthalocyanines (Pcs) are tetrabenzo[5,10,15,20]-
most synthetic efforts have focused on the preparation
tetraazaporphyrinsknownfortheirveryinterestingindust-
of substituted phthalonitriles with different reactive end
rial applications due to their high stability, architectural
groups like hydroxy, amine, azide or alkynyl functional
flexibility, diverse coordination properties, and good spec-
groups on the aromatic system [8–11].
troscopic characteristics [1]. The properties and effects
In the last decades, Cu(I)-catalyzed 1,3-dipolar cyclo-
of phthalocyanines are diverse and cover many important
addition reactions between alkynes and azides, known
hi-tech applications, including photodynamic therapy,
as ‘‘click reactions,” have been recognized as a facile
optical data storage, reverse saturable absorbers and solar
synthetic methodology due to their fast, reproducible,
screens [2]. The functions of phthalocyanine derivatives
quantitative nature and being resistant to side-reactions
are usually based on their electron transfer reactions,
and performing under quite mild reaction conditions
becauseoftheirp-electronconjugatedringsystem[3].The
[12–15]. Although ‘click’ chemistry has been thoroughly
Pcs with different functional groups situated at peripheral
explored in recent years and applied considerably in
positions may have increased solubility and enhanced the
preparation and functionalization of new materials,
spectral properties and applications [4–7]. Synthesis of
substituted Pc systems with different functional groups
relatively few examples of Pc functionalization via this
versatile reaction can be found most likely due to the
limited solubility of most Pcs [16–22].
◊SPP full member in good standing
In the present study, we have synthesized tetra cyclohe-
xylalkynyloxy-substituted zinc phthalocyanine without
*Correspondence to: Ahmet Gül, email: ahmetg@itu.edu.tr,
tel: +90 212-285-6827, fax: +90 212-285-6386
protective/deprotective chemistryvia cyclotetramerization
Copyright © 2013 World Scientific Publishing Company

SYNTHESIS AND PHOTOPHYSICAL PROPERTIES OF A NOVEL ETHYNYL ZINC(II) PHTHALOCYANINE
541
of novel 4-(2-ethynylcyclohexyloxy) phthalonitrile in
dichloromethane and then THF/hexane (1:1) as the
eluent. After click reaction, compound 3 became much
more soluble in a wide variety of solvents including
chloroform, DCM, THF, and acetone as compared to
the precursor 2, which was only soluble in THF, DMSO,
and DMF.
pentanol in the presence of DBU. Additionally, we have
applied Cu(I)-catalyzed click reaction to the synthesis of
soluble tetratriazole-functionalized zinc phthalocyanine
under very mild reaction conditions. We also report
spectroscopic characterization, aggregation behavior as
well as photophysical (fluorescence quantum yields
and lifetimes) and quenching properties of novel ZnPc
complexes.
All the synthesized compounds were fully charact-
erized by NMR, IR, and UV-vis spectral methods and the
purity was confirmed by elemental analysis. In the FT-IR
spectra of the cycloalkynyl substituted phthalonitrile 1,
the characteristic H–C and –C≡C– stretching vibrations of
alkynyl moiety were observed at 3289 cm-1 and 2107 cm-1,
respectively. Cyclotetramerization of the dinitrile 1 was
confirmed by the disappearance of the sharp C≡N vibration
at 2230 cm-1 after phthalocyanine formation. Besides, the
characteristicbandsofcycloalkynylgroupsmaintainedtheir
positions. After the click reaction, the characteristic C–H
vibration at high wavenumbers disappeared completely
and a broad band around 2500–3300 cm-1 (COOH) and
an intense sharp band at 1730 cm-1 (C=O) corresponding
to carboxylic acid moiety was revealed. Additionally,
increasing intensity of aliphatic C–H vibrations proved the
assigned structure of 3 (Fig. 1).
RESULTS AND DISCUSSION
Synthesis and characterization
The new zinc phthalocyanine with terminal cyclo-
alkyne groups was synthesized in two steps. In the first
step, the 4-(2-ethynylcyclohexyloxy)phthalonitrile 1 was
synthesized by nucleophilic displacement reaction of
4-nitrophthalonitrile with 2-ethynylcyclohexanol at 50 °C
under nitrogen atmosphere in dry dimethylsulphoxide
in the presence of potassium carbonate as a base. Then,
phthalocyanine 2 was prepared by the standard method
of cyclotetramerization of 4-(2-ethynylcyclohexyloxy)
phthalonitrile in the presence of DBU and anhydrous
Zn(CH3COO)2 in pentanol for 24 h (Scheme 1). Product
2 was purified by column chromatography using silica
gel as the stationary phase and hexane/THF (2:1) mixture
as the eluent. The zinc phthalocyanine obtained is soluble
only in certain organic solvents such as DMF, THF and
DMSO.
The 1H NMR spectra of the phthalonitrile derivative 1
and phthalocyanines 2 and 3 are in excellent agreement
with the proposed structures. The dominant features in the
NMR spectra of (1–3) are the aliphatic group resonances.
1
Besides, in the H NMR spectrum of compounds 2 and
3, as a consequence of the presence of four constitutional
isomers and supramolecular association, the signals
are very broad and the integration values of the many
assignable signals are consistent with the expected values
[9]. The molecular ion peak of phthalonitrile derivative
1 was observed at m/z = 250 [M + 1]+ by using GC-MS
(ESI) technique. The peaks for the molecular ion by using
MALDI-TOF technique were found at m/z = 1066.47 [M]+
for 2, and m/z = 1693.82 [M + 1]+ for 3.
The 1,3-dipolar cycloaddition of compound 2 with
6-azido-hexanoic acid was carried out in THF/H2O (3:1)
at room temperature using CuSO4∙5H2O as a catalyst and
sodium ascorbate as the reducing agent (Scheme 2). The
crude product was purified by column chromatography
with silica gel as the stationary phase and first
NC
NC
NO2
O
HO
O
N
K2CO3
dry DMSO
N
N
N
N
N
Zn
N
Zn(CH3COO)2
n-pentanol
N
O
CN
O
O
CN
1
2
Scheme 1. Synthetic route of 4-(2-ethynylcyclohexyloxy) phthalonitrile and related zinc(II) phthalocyanine 2
Copyright © 2013 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2013; 17: 541–547

542
A.M. SEVIM ET AL.
OH
O
HO
O
N
N
O
N
O
O
O
N
Zn
N
N
N
N
N
N
N
N
N
N
N
N
N
N
6-azido-hexanoic acid
N
Zn
N
N
N
N
N
CuSO4·5H2O
sodium ascorbate
THF/H2O (3:1)
N
O
O
N
O
N
O
N
O
2
3
HO
O
HO
Scheme 2. Preparation of compound 3 via “click” reaction
Fig. 1. FTIR spectra of synthesized compounds 1–3
Ground state electronic absorption
and fluorescence spectra
temperature [27]. As anticipated, the UV-vis absorption
spectra of 2 and 3 have no evidence of aggregation as
shown by the position and the appearance of the intense
Q-bands (Figs 2 and 3). In this study the aggregation
behavior of 2 and 3 was studied at different concentrations
in THF. As depicted in Figs 2 and 3, the appearance of the
Q-band absorption maxima remained unchanged as the
concentration increases and its apparent molar extinction
coefficient remains almost constant (insert figures
in Figs 2 and 3) indicating the existence of a purely
monomeric form which obeyed the Beer–Lambert Law
in the outlined range of concentration [28]. By evaluating
these observations, it can be clearly concluded that the
2 and 3 did not show any aggregation behavior in THF
between 2 × 10-5 M and 1.25 × 10-6 M [29].
The UV-vis spectra of the studied phthalocyanines
2 and 3 in THF were given in Figs 2 and 3. The p–p*
transitions for the Q-band absorptions were observed at
694nmfor2and706nmfor3 (Table1).Whenwecompare
the Q-band absorption bands of 2 with Q-band absorption
bands of 3, after click reaction, a significant red shift
(12 nm) and increased intensity in Q-band wavelengths
was observed [23–26]. Aggregation is usually depicted as
a coplanar association of phthalocyanine core progressing
from monomer to dimer and higher order complexes. It
is dependent on the concentration, nature of the solvent,
nature of the substituents, complexed metal ions and
Copyright © 2013 World Scientific Publishing Company
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SYNTHESIS AND PHOTOPHYSICAL PROPERTIES OF A NOVEL ETHYNYL ZINC(II) PHTHALOCYANINE
543
Table 1. Absorption, excitation and emission spectral data for zinc phthalocyanine complexes (2 and 3) in THF
Comp.
Q-band lmax, nm (log e) Excitation lEx, nm Emission lEm, nm Stoke shift DStokes, × 105 cm-1
2
694
706
666a
4.73
5.04
5.19a
691
707
666a
708
719
673a
7.14
7.69
14.28a
3
ZnPc
a Reference 32.
Fig. 2. Aggregation behavior of 2 in THF at different
concentrations
Fig. 3. Aggregation behavior of 3 in THF at different
concentrations
Fig. 4. Absorption, excitation and emission spectra for
compound 2 (a) and 3 (b) in THF. Excitation wavelength = 623
nm for 2 and 635 nm for 3 in THF
The fluorescence behavior of zinc phthalocyanine
complexes (2 and 3) was studied in THF. Figure 4 shows
the absorption, fluorescence emission and excitation
spectra for complex 2 and 3 in THF. Fluorescence
emission peaks were observed at 708 nm for 2 and
719 nm for 3 (Table 1). The observed Stokes shifts
were within the region ~10–15 nm observed for zinc
Pc complexes and they were typical of phthalocyanine
complexes [30]. Both studied zinc Pc complexes (2 and
3) showed similar fluorescence behavior in THF. The
excitation spectra were similar to absorption spectra and
both were mirror images of the fluorescent spectra for
Copyright © 2013 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2013; 17: 543–547

544
A.M. SEVIM ET AL.
both zinc Pc complexes. The proximity of the wavelength
of each component of the Q-band absorption to the
Q-band maxima of the excitation spectra for both zinc Pc
complexes suggested that the nuclear configurations of
the ground and excited states are similar and not affected
by excitation.
Fluorescence quantum yields and lifetimes
The fluorescence quantum yields (FF) of zinc
phthalocyanines (2) and (3) were studied in THF.
Fluorescence quantum yields (FF) were determined by
the comparative method (Equation 1). Unsubstituted
ZnPc was used as standard in DMF (FF = 0.17) [31]
and both the sample and the standard were excited at
the same wavelength. The fluorescence quantum yields
were calculated as 0.21 for 2 and 0.15 for 3. FF values
of zinc phthalocyanine complexes (2 and 3) are lower
than unsubstituted ZnPc (FF = 0.25) [32] and typical
of phthalocyanine complexes (Table 2) in THF. This
implies that the presence of peripheral cycloalkynyloxy
and triazole hexanoic acid substituents caused
some fluorescence quenching of the parent 2 and
3. According to these results we can say that after
click reaction additionally triazole hexanoic acid
substituents in 3 caused more fluorescence quenching
of complex 3 compared to 2 as shown in Table 1. In our
study, the fluorescence lifetimes (tF) were calculated
using the Strickler–Berg equation (Equation 2). The tF
values of the Pc complexes 2 and 3 (3.37 and 0.26 ns,
respectively) are lower compared to unsubstituted ZnPc
complexes in THF [31], suggesting more quenching
by substitution. The natural radiative lifetime (to)
and the rate constants for fluorescence (kF) values are
also shown in Table 2. The kF values of studied zinc
phthalocyanine complexes (2 and 3) are higher than
unsubstituted ZnPc in THF.
Fig. 5. Fluorescence emission spectral changes of 2 (a)
(4 × 10-6 M) and 3 (b) (4 × 10-6 M) on addition of different
concentrations of BQ in THF. [BQ] = 0, 0.008, 0.016, 0.024,
0.032 and 0.040 M
Fluorescence quenching studies
by 1,4-benzoquinone (BQ)
The fluorescence quenching of zinc phthalocyanine
complexes (2 and 3) by benzoquinone (BQ) in THF
was found to obey Stern–Volmer kinetics, which
is consistent with diffusion-controlled bimolecular
reactions. Figure 5 shows fluorescence emission
spectral changes of 2 and 3 on addition of
different concentrations of BQ in THF. The
Stern–Volmer plots for studied complexes (2
and 3) gave straight lines, depicting diffusion
controlled quenching mechanisms (Fig. 6).
Table 2. Photophysical and photochemical parameters along with
fluorescence quenching data of zinc phthalocyanine complexes (2 and 3)
in THF
Comp.
FF
tF, ns to, ns kF, s-1 (× 108)a KSV kq, s-1 (× 1011)
The slope of the plots shown at Fig. 6 gave
KSV values, listed in Table 2. The bimolecular
quenching constant (kq) values for the BQ
quenching of phthalocyanine complexes in THF
are also listed in Table 2. The KSV values of the
phthalocyanine complexes 2 and 3 are higher
than Std-ZnPc in THF.
2
3
0.21
0.15
0.7
3.37
0.26
2.96
38.2
0.92b
66.5
0.95
18.13
1.78b
0.04
72.5
48.48b
ZnPc
0.25b 2.72b 10.9b
a
kF is the rate constant for fluorescence. Values calculated using
kF = FF/tF. b Reference 32.
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SYNTHESIS AND PHOTOPHYSICAL PROPERTIES OF A NOVEL ETHYNYL ZINC(II) PHTHALOCYANINE
545
quantum yields (FF) according to the following equation
[34], utilizing unsubstituted ZnPc in DMF as the standard
(FF = 0.17) [31]:
FAStd h2
FStd AhS2td
(1)
∅F = ∅F (Std)
where F and FStd are the areas under the fluorescence
curves of the zinc phthalocyanines and the standard,
respectively. A and AStd are the respective absorbances of
sample and standard at the excitation wavelengths (which
was around 0.04–0.05 in all solvents used), and h and hStd
are the refractive indices of solvents used for the sample
and standard, respectively.
Radiative or natural lifetimes (to) were estimated using
Photochem CAD program which uses the Strickler–Berg
equation [35]. Finally, the fluorescence lifetimes (tF)
were calculated using the following equation:
tF
tO
∅F =
(2)
Fluorescence quenching by 1,4-benzoquinone. Fluore-
scence quenching experiments on zinc phthalocyanines
werecarriedoutbytheadditionofdifferentconcentrations
of BQ to a fixed concentration of the complex, and the
concentrations of BQ in the resulting mixtures were
0.008, 0.024, 0.032, 0.040 and 0.048 M, respectively.
The fluorescence spectra of compound 2 and 3 at each
BQ concentration were recorded, and the changes in
fluorescence intensity related to BQ concentration by the
Stern–Volmer (SV) equation [36]:
Fig. 6. Stern–Volmer plots for benzoquinone (BQ) quenching
of 2 (a) [ZnPc] = 4.00 × 10-6 M and 3 (b) [ZnPc] = 4.00 × 10-6 M
in THF. [BQ] = 0, 0.008, 0.016, 0.024, 0.032 and 0.04 M
EXPERIMENTAL
Materials
IR spectra were recorded on a Perkin-Elmer Spectrum
One FT-IR spectrometer with ATR capability, electronic
spectra were recorded on a Scinco SD 1000 single-
beam ultraviolet-visible (UV-vis) spectrophotometer
using 1 cm path length cuvettes at room temperature.
Fluorescence spectra were recorded on a Perkin-Elmer
I0
(3)
= 1 + kSV [BQ]
I
where I0 and I are the fluorescence intensities of
fluorophore in the absence and presence of quencher,
respectively. [BQ] is the concentration of the quencher
and KSV is the Sterne–Volmer constant which is the
product of the bimolecular quenching constant (kq) and
the tF and is expressed in Equation 4:
1
LS55 fluorescence spectrophotometer. H NMR spectra
were recorded on a Bruker 250 MHz spectrometer
using TMS as internal reference. Melting points were
determined on a Büchi Melting Point B-540 apparatus.
Elemental analyses were performed by the Instrumental
Analysis Laboratory of the TUBITAK Marmara
Research Centre. Mass spectra were performed on
Bruker Microflex MALDI-TOF/MS and Perkin-Elmer
Clarus 500 mass spectrometers. All reagents and solvents
were of reagent grade quality obtained from commercial
suppliers. The homogeneity of the products was tested in
each step by TLC (SiO2). All reactions were carried out
under nitrogen atmosphere in dried solvents. The solvents
were stored over molecular sieves. 4-nitrophthalonitrile
was synthesized according to published methods [33].
KSV = kq × tF
(4)
The ratios of I0 /I were calculated and plotted against
[BQ] according to Equation 4, and KSV is determined
from the slope.
Synthesis
4-(2-ethynylcyclohexyloxy)phthalonitrile(1). 2-ethy-
nylcyclohexanol (0.124g, 1mmol) and 4-nitrophthalo-
nitrile (0.173g, 1mmol) were added successively with
stirring to dry DMSO (6mL). After they were dissolved,
anhydrous K2CO3 (0.250g, 11.56 mmol) was added
portion wise for 2h and the mixture was stirred vigorously
at 50 °C for 72h under nitrogen. Then the solution
Photophysical parameters
Fluorescence quantum yields and lifetimes. The
comparativemethodwasusedtodeterminethefluorescence
Copyright © 2013 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2013; 17: 545–547

546
A.M. SEVIM ET AL.
was poured into ice water (200 mL). The solution was
acidified with conc. HCl until precipitation occured (pH =
1–2). The yellow brownish product was collected by
centrifugation and rinsed with ethanol. Finally the solid
was dissolved in minimum chloroform and then added to
n-hexane dropwise. The brownish crystals were filtered
and dried. Yield 0.133 g (53.2%); mp 69.4 °C. IR: n,
cm-1 3289 (–C≡C–H), 3079 (Ar–H), 2940–2850 (alkyl
CH), 2230 (C≡N), 2107 (C≡C), 1244 (C–O–C). 1H NMR
(250 MHz; CDCl3; Me4Si): dH, ppm 7.68 (1H, s, Ar–H),
7.63 (1H, d, J = 8.20 Hz, Ar–H), 7.52 (1H, d, J = 7.90 Hz,
Ar–H), 2.78 (1H, s, –C≡CH), 2.10 (1H, q, –O–CH), 1.89
(1H, q, –C≡C–CH), 1.68–1.56 (4H, m, CH2), 1.40–1.33
(4H, m, CH2). MS (GC-MS, scan EI+): m/z 250 [M + 1]+.
Anal. calcd. for C16H14N2O: C, 76.58; H, 5.64; N, 11.19%.
Found C, 76.14; H, 5.62; N, 11.01%.
Anal. calcd. for C88H100N20O12Zn: C, 62.35; H, 5.95; N,
11.52%. Found C, 62.44; H, 5.92; N, 11.47%.
CONCLUSION
We have been planning to prepare sufficiently soluble
Pcs with carboxylic acid substituents applicable in
biomolecule interaction or dye sensitized solar cells. In
the presented work, the synthesis of new terminal alkynyl
group-substituted zinc phthalocyanine without protective/
deprotective chemistry was described. In addition, we
have applied click chemistry to the synthesis of more
soluble tetratriazole-functionalized zinc phthalocyanine.
Thecharacterization, aggregationbehavior, photophysical
and photochemical properties of these novel zinc
phthalocyanines have been also investigated. It was also
observed that solubility of compound 3 is increased after
click reaction. The click product 3 is very soluble in a
number of organic solvents, such as chloroform, DCM,
THF and acetone. The photophysical fluorescence
properties of these zinc(II) phthalocyanine complexes (2
and 3) were investigated in THF. The phthalocyanines
2 and 3 show lower fluorescence quantum yields and
shorter lifetimeswhen compared to unsubstituted zinc
phthalocyanine and it was concluded that after click
reaction additional triazole hexanoic acid substituents in
3 caused more fluorescence quenching. The fluorescence
of 2 and 3 is effectively quenched by 1,4-benzoquinone
in THF.
Tetrakis(2-ethynylcyclohexyloxy)phthalocyani-
natozinc(II) (2). A mixture of dinitrile 1 (0.15 g, 0.6 mmol),
anhydrous Zn(CH3COO)2 (0.033 g, 0.24 mmol) and a
catalyticamountofDBUinn-pentanol(1.5mL)washeated
at 145 °C with stirring for 24 h. The reaction mixture was
cooled to room temperature. The precipitate was filtered
off and washed with methanol.After extraction of the crude
product with chloroform, the purification was carried out
by column chromatography on silica gel using hexane/
THF (2:1) as the eluent to afford 2 as a green solid. Yield
0.032 g, 20%. IR: n, cm-1 3289 (–C≡C–H), 3075 (Ar–H),
1
2928–2853 (alkyl CH), 2222 (C≡C), 1254 (C–O–C). H
NMR (250 MHz; CDCl3; Me4Si): dH, ppm 7.61–7.48 (8H,
b, Ar–H), 6.96 (4H, s, Ar–H), 3.46 (4H, s, –C≡C–H), 2.25
(4H, b, –O–CH–), 1.59 (4H, b, –C≡C–CH–), 1.41–0.93
(32H, m, CH2). UV-vis (in THF): l, nm (log e) 350 (4.72),
694 (4.73). MALDI-TOF MS: m/z 1066.47 [M]+. Anal.
calcd. for C64H56N8O4Zn: C, 72.07; H, 5.25; N, 10.59%.
Found C, 72.14; H, 5.32; N, 10.65%.
Acknowledgements
This work was supported by the Research Fund of
the Technical University of Istanbul. AG thanks Turkish
Academy of Sciences (TUBA) for partial support.
Preparation of 3 using click chemistry. A mixture
of 2 (0.075 g, 0.07 mmol), 6-azido-hexanoic acid (0.055
g, 0.35 mmol), CuSO4.5H2O (0.070 g, 0.28 mmol) and
sodium ascorbate (0.056 g, 0.28 mmol) were dissolved in
4 mL of THF/H2O (3:1). The mixture was stirred at room
temperature for 24 h under nitrogen. The solvent was
removed under reduced pressure, and the residue was
taken-upindichloromethane,washedwithwateranddried
over MgSO4. After the filtration and evaporation of the
solvent, the product was washed with methanol to remove
unreacted 6-azido-hexanoic acid. Final purification of the
product was accomplished by column chromatography
with silica gel using first dichloromethane and then THF/
hexane (1:1) as eluent. Yield 0.065 g, 55%. IR: n, cm-1
3400–2650 (broad, carboxylic acid OH), 2924–2853
(alkyl CH), 1716 (C=O), 1243 (C–O–C). 1H NMR
(250 MHz; CDCl3; Me4Si): dH, ppm 7.79–7.64 (8H, b,
Ar–H), 7.52 (4H, s, Ar–H), 7.39 (4H, s, Ar–H), 5.03
(8H, b, N–CH2), 4.28(4H, b, –O–CH–), 2.87–2.73 (8H,
b, –CH–C=O), 2.25–0.94 (60H, m, –CH2). UV-vis (in
THF): l, nm (log e) 351 (4.78), 706 (5.04). MALDI-
TOF MS: m/z 1693.82 [M + 1]+, 1810.11 [M + 3K]+.
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