Synthesis and metal ion-sensing properties of fluorescent PET chemosensors based on the 2-phenylimidazo[5,4-a]anthraquinone chromophore

Article abstract of DOI:10.1039/a900424f

Novel fluorescent chemosensors in which an aza-crown is linked to a 2-phenylimidazoanthraquinone fluorophore by a methylene spacer have been synthesized for the sensing of metal cations. A dramatic fluorescence enhancement at 515 nm was observed upon bind

Full text of DOI:10.1039/a900424f

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Synthesis and metal ion-sensing properties of fluorescent PET
chemosensors based on the 2-phenylimidazo[5,4-a]anthraquinone
chromophore
Katsuhira Yoshida,*a Takayoshi Mori,a Shigeru Watanabe,a Hideki Kawaib and Toshihiko
Nagamurab
a Department of Chemistry, Faculty of Science, Kochi University, Akebono-cho,
Kochi 780-8520, Japan
b Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8011,
Japan
Received (in Cambridge) 15th January 1999, Accepted 21st January 1999
Novel fluorescent chemosensors in which an aza-crown is linked to a 2-phenylimidazoanthraquinone fluorophore
by a methylene spacer have been synthesized for the sensing of metal cations. A dramatic fluorescence enhancement
at 515 nm was observed upon binding of the metal ions, which was interpreted in terms of the control of a
photoinduced electron transfer (PET) process. This system can recognize the charge and the size of alkali and
alkaline-earth metal cations and can transform its information sensitively and precisely into the magnitude of the
enhanced fluorescence.
O
NH2
CH2Cl
Introduction
NH2
The design and synthesis of fluorescent sensing systems for
metal cations and a variety of biologically important species are
areas of growing interest because of their potential applications
to biomedical and environmental chemosensors1 or molecular
logic devices.2 Various fluorescent sensors and switches have
been reviewed and classified into several types in recent
literature.3
+
O
COCl
1
2
i
Cl
Chemosensors which apply the principle of photoinduced
electron transfer are called PET chemosensors. Fluorescent
aromatic hydrocarbons such as anthracene, pyrene, and their
derivatives were previously employed for the construction
of PET chemosensors with fluorophore–spacer–receptor
systems.1–3 Moreover, fluorescent chromophores such as
coumarin,4 benzimidazobenz[de]isoquinolinone,4 or pyrido-
imidazopyrazine (PIP)5 etc., were also found to be valuable
resources for the construction of PET chemosensors emitting
visible signals. In these cases, the sensing abilities strongly relied
on the photophysical and electrochemical properties of the
fluorophore. Therefore, research for new fluorophores is
indispensable for further development of PET chemosensors.
During the course of our research to develop new fluoro-
phores,6 we have found that 2-phenylimidazoanthraquinone
(PIAQ) emits strong fluorescence at around 515 nm in the
visible region. As the chromophoric PIAQ is an effective
electron acceptor, it was expected that the PIAQ moiety could
be used to construct new PET chemosensors. Hence, we have
designed and investigated the metal ion-sensing ability of new
fluorescent PET chemosensors in which an aza-crown (AC) is
linked to an electron accepting 2-phenylimidazoanthraquinone
(PIAQ) fluorophore using a methylene spacer.
O
O
NH2
NH
3
O
ii
O
O
N
O
n
O
O
NH2
O
O
NH
4a n = 1, aza-15-C-5
4b n = 2, aza-18-C-6
iii
O
O
N
n
O
O
O
O
HN
N
Results and discussion
The synthesis of novel fluorescent chemosensors (PIAQ–AC)
is outlined in Scheme 1. The reaction of 1,2-diamino-
anthraquinone 1 with 4-(chloromethyl)benzoyl chloride 2 in the
presence of triethylamine gave the acylation product 3. The
displacement of the chloro atom of 3 by aza-15-crown-5 and
5a n = 1, aza-15-C-5
5b n = 2, aza-18-C-6
Scheme 1 Reagents: i, Et3N–1,4-dioxane, 60 ЊC, 7 h; ii, aza-crown,
Et3N–1,4-dioxane, reflux, 25 h; iii, NaOH, 95% EtOH, reflux, 1.5 h.
J. Chem. Soc., Perkin Trans. 2, 1999, 393–397
393

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aza-18-crown-6 ethers in 1,4-dioxane–triethylamine gave the
corresponding aza-crowned products 4a and 4b, respectively. In
order to get the final designed PIAQ–AC chemosensors, 5a and
5b, we investigated the intramolecular condensation of 4a and
4b under various conditions and found that it proceeds success-
fully under basic conditions using sodium hydroxide in reflux-
ing ethanol. The structures of 5a and 5b were confirmed on the
1
1
basis of IR, H NMR, and elemental analysis. The H NMR
spectra exhibited a broad singlet signal at around δ = 11.25
ppm, which supports the formation of intramolecular hydrogen
bonding between the NH of the imidazole ring and the
neighboring quinone carbonyl group. The absorption spectra
of PIAQ–AC chemosensors which have a maximum at 405 nm
due to a π–π* transition are similar to those of the parent PIAQ
chromophore.
The thermodynamic driving force (∆GPET) calculated by appli-
cation of the Rehm–Weller equation [eqn. (1)]7 is conveniently
∆GPET = Eox Ϫ Ered Ϫ e2/εr Ϫ E00
(1)
used to design PET chemosensors with fluorophore–spacer–
receptor systems.4
The possibility of intramolecular photoinduced electron
transfer from the aza-crown ring to the excited imidazo-
anthraquinone moiety can be estimated from the ∆GPET value.
Hence, we measured the redox potentials of the compound
5a; cyclic voltammetry was performed in a 1 × 10Ϫ3 mol dmϪ3
Fig. 1 Absorption spectra of 5a: before and after addition of metal
salts in acetonitrile ([5a] = 2.5 × 10Ϫ5 mol dmϪ3, [metal salt] = 1.25 ×
Ϫ
solution of 5a in acetonitrile–0.1 mol dmϪ3 NEt4ϩClO4 at a
10Ϫ4 mol dmϪ3).
glassy carbon electrode. The cyclic voltammograms for the
reduction of the imidazoanthraquinone moiety showed two
reversible one-electron steps and the first and second reduction
potentials vs. Ag/AgCl were Ϫ0.94 and Ϫ1.16 V, respectively.
With the aza-crown ether moiety, one irreversible oxidation
wave was observed at ϩ0.84 V (vs. Ag/AgCl). The PIAQ–AC
chemosensor 5a exhibits an absorption band at λmax = 405 nm
(εmax 14,500) and a weak fluorescence emission band at 515 nm.
The singlet energy (E00) of the PIAQ chromophore can be
calculated as 2.70 eV from the average of the energies of the
spectral maxima in absorption and emission. The attractive
potential between the radical ion pair Ϫe2/εr is approximately
Ϫ0.1 eV.8 Application of these values into eqn. (1) gives the free
energy (∆GPET) as ca. Ϫ98.4 kJ molϪ1, which shows that
the photoinduced electron transfer (PET) reaction is highly
possible.
The sensing abilities of PIAQ–AC chemosensors, 5a and 5b,
for alkali and alkaline-earth metal cations have been investi-
gated. Fig. 1 and 2 show the absorption and fluorescence
spectral changes upon addition of various metal salts to an
acetonitrile solution of 5a. The patterns of the absorption and
emission spectral changes upon binding of the metal ions are
fully consistent with those of other PET chemosensors reported
in previous papers;9 that is, the absorption and emission
wavelengths and the absorption intensity are substantially
unaffected but the emission intensity is significantly changed.
Especially, in the case of PIAQ–AC chemosensors, a highly
sensitive and dramatic enhancement of fluorescence intensity
at 515 nm upon recognition of metal cations was observed.
Fig. 3 and 4 show plots of the fluorescence intensity vs. the
molar ratio of [Metal ion]/[Ligand (5a and 5b)], which indicate
1:1 complex formation and strong metal ion binding ability
of the chemosensors. The magnitude of the fluorescence
enhancement was greatly dependent on both the charge and the
size of metal ions with respect to the cavity size10 of the crown
Fig. 2 Fluorescence spectra of 5a: before and after addition of metal
salts in acetonitrile ([5a] = 2.5 × 10Ϫ5 mol dmϪ3, [metal salt] = 1.25 ×
10Ϫ4 mol dmϪ3, λem = 515 nm, λex = 400 nm).
charge. These results confirm that our sensors can recognise the
size and the charge of the metal ions and can transform their
information precisely into the magnitude of the enhanced
fluorescence. Fluorescence quenching was observed upon
complexation with metal thiocyanates in the case of PIP–crown
chemosensors,5 which is explained by the PET process from the
thiocyanate counter anion to the PIP chromophore. In the case
of PIAQ–AC chemosensors, fluorescence quenching by thio-
cyanate counter anions was not observed. As shown in Fig.
2–4, the fluorescence sensing of the PIAQ–AC chemosensors
for metal cations was not disturbed by thiocyanate counter
ring. The orders of the fluorescence intensity were Ca2ϩ > Ba2ϩ
Mg2ϩ ӷ Naϩ > Liϩ > Kϩ for 5a and Ba2ϩ > Ca2ϩ > Mg2ϩ
>
ӷ
Kϩ > Naϩ > Liϩ for 5b, respectively. Alkaline-earth metal
cations induce much larger enhancement of fluorescence
intensity than alkali metal cations. The cation whose size is
closer to that of the cavity of the aza-crown seems to exhibit a
larger fluorescence enhancement among cations of the same
394
J. Chem. Soc., Perkin Trans. 2, 1999, 393–397

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anions. The oxidation potential of thiocyanate anion measured
by means of cyclic voltammetry shifted from ϩ0.89 to ϩ0.69 V
(vs. Ag/AgCl) upon complexation of PIAQ–AC with Ca(SCN)2
in acetonitrile solution. From the values, an intermolecular
PET from thiocyanate anion to the excited PIAQ chromophore
quenching by the intermolecular PET process from thiocyanate
anion to the excited PIAQ is thermodynamically possible but
would be kinetically suppressed.
Using changes in the fluorescence intensity with an increase
in the metal ion concentration, the association constants (log K)
were determined according to the procedure reported by
Bourson et al.11 Table 1 summarizes the log K values and
fluorescence quantum yields (ΦF) for 5a and 5b and their metal
complexes. The log K values are reasonable in comparison with
those known for original aza-crown ethers, aza-15-crown-5
(A15C5), aza-18-crown-6 (A18C6), and diaza-18-crown-6
(A218C6), with various alkali and alkaline-earth metal cations
in the literature12 as follows: 4.4–4.8 (A15C5–Liϩ in MeCN),
4.7–5.7 (A15C5–Naϩ in MeCN), 3.5–4.5 [A18C6–Naϩ in
MeOH (anhydrous)], 4.9–6.0 [A18C6–Kϩ in MeOH
(anhydrous)], 5.9–6.1 (A218C6–Ba2ϩ in MeOH), 7 < (A218C6–
Ba2ϩ in MeCN). The log K values for the alkali metal ions are
smaller than those for the alkaline-earth metal ions, suggesting
that the higher stability of the latter is due to the double charge
of the former. A large difference in the fluorescence quantum
yields (ΦF) between the alkali metal ions and the alkaline-earth
metal ions would suggest that a stronger electrostatic inter-
action between the bivalent metal ions and the lone pair of the
nitrogen atom of the aza-crown suppresses more efficiently the
photoinduced electron transfer process.
was considered to be possible, because the free energy (∆GPET
)
for the PET reaction was estimated to be highly negative, ca.
Ϫ113 kJ molϪ1. However, in the case of PIAQ–AC chemosen-
sors, fluorescence enhancement was observed upon addition of
metal thiocyanates. The above results suggest that fluorescence
Fig. 5 shows the fluorescence decay curves of PIAQ-But at
529 nm and PIAQ–AC at 515 nm in acetonitrile solution
Fig. 3 Fluorescence intensity of 5a vs. molar ratio of [metal salt]/[5a].
([5a] = 2.5 × 10Ϫ5 mol dmϪ3, λem = 515 nm, λex = 400 nm); ᮀ: LiClO4, ᭿:
KSCN, ᭝: NaSCN, ᭺: Ca(SCN)2, ᭡: Mg(ClO4)2, ᭹: Ba(SCN)2.
Fig. 5 Fluorescence decay curves of PIAQ-But at 529 nm and 5a with
Fig. 4 Fluorescence intensity of 5b vs. molar ratio of [metal salt]/[5b].
([5b] = 2.5 × 10Ϫ5 mol dmϪ3, λem = 515 nm, λex = 400 nm); ᮀ: LiClO4, ᭿:
KSCN, ᭝: NaSCN, ᭺: Ca(SCN)2, ᭡: Mg(ClO4)2, ᭹: Ba(SCN)2.
metal salts at 515 nm in acetonitrile solutions, excited with a 400 nm
femtosecond laser pulse ([PIAQ-But] = [5a] = 2.5 × 10Ϫ5 mol dmϪ3
[Ca(SCN)2] = [Ba(SCN)2] = 1.25 × 10Ϫ4 mol dmϪ3).
,
Table 1 Association constants (log K) and fluorescence quantum yields (ΦF) of 5a and 5b and their complexes in acetonitrile
Complexes
PIAQ–AC
Liϩ
Naϩ
1.96
Kϩ
Mg2ϩ
1.32
Ca2ϩ
1.98
Ba2ϩ
2.68
Ionic diameter/Å
5aa
1.36
2.66
log K
4.62 0.16
0.088
4.82 0.04
0.207
4.32 0.08
0.068
5.52 0.29
0.596
6.51 0.04
0.679
6.01 0.12
0.590
ΦF × 102
5ba
0.021
0.019
log K
3.62 0.30
0.085
4.96 0.07
0.100
5.22 0.02
0.131
5.78 0.14
0.439
7.16 0.02
0.507
7.24 0.03
0.601
ΦF × 102
a Cavity size9 of the crown: aza-15-crown-5-ether (1.7–2.2 Å), aza-18-crown-6-ether (2.6–3.2 Å).
J. Chem. Soc., Perkin Trans. 2, 1999, 393–397
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(2.5 × 10Ϫ5 mol dmϪ3) under five times excess of metal salts,
excited with a femtosecond laser at 400 nm. PIAQ-But was
used as a model compound for PIAQ without AC. The decay
for PIAQ-But is almost a single exponential with a lifetime of
420 ps. The fluorescence lifetime of PIAQ–AC in metal free
conditions was too fast to be measured by our system. The
fluorescence lifetimes of PIAQ–AC with Ca2ϩ and Ba2ϩ ions
were estimated from Fig. 5 as 234 and 228 ps, respectively.
These results strongly suggest that the fluorescence quenching is
due to the photoinduced intramolecular electron transfer from
AC to the excited PIAQ and that it is sensitively controlled by
the electrostatic interactions between metal cations and the lone
pair electrons in the aza-crown.
The time-resolved absorption spectra upon excitation with a
femtosecond laser were measured in order to directly prove the
above mechanism, the details of which will be reported
elsewhere.13 PIAQ-But exhibited transient absorption with a
lifetime of 470 ps due to the excited singlet state. PIAQ–AC
exhibited transient absorption of the excited singlet state with a
lifetime of only 5.8 ps and a new absorption with a peak of 610
nm which was attributed to PIAQ anion radicals. From these
results the fluorescent sensing of metal ions by PIAQ–AC due
to the PET process was directly proved.
In conclusion, we have developed new fluorescent PET
chemosensors based on a 2-phenylimidazo[5,4-a]anthraquin-
one (PIAQ) chromophore. The PIAQ chromophore is a good
electron acceptor and exhibits absorption and emission bands
in the visible region, which draw out favorable characteristics of
the PIAQ–AC chemosensors. Marked fluorescence quenching
by an intramolecular PET reaction from the AC to the excited
PIAQ chromophore was directly proved. The changes in the
association constants (log K) and fluorescence quantum yields
(ΦF) upon binding of metal ions to the aza-crown shown in
Table 1 revealed that PIAQ–AC chemosensors can recognize
the charge and the size of metal cations and precisely transform
their information into the magnitude of the enhanced
fluorescence. Thus, the PIAQ chromophore is a useful molec-
ular part for designing new fluorescent chemosensors emitting
visible signals.
Synthesis of 1-amino-2-[4-(chloromethyl)benzoylamino]anthra-
quinone (3)
A solution of 4-(chloromethyl)benzoyl chloride (0.43 g, 4.62
mmol) in 1,4-dioxane (30 ml) was added dropwise to a solution
of 1,2-diaminoanthraquinone (1.00 g, 4.2 mmol) and triethyl-
amine (0.43 g, 4.2 mmol) in 1,4-dioxane (180 ml) with stirring at
60 ЊC. After being stirred for 7 h at 60 ЊC, the solvent was
removed under reduced pressure, and the residue was washed
with 50% aqueous ethanol, dried, and chromatographed on
silica gel (CHCl3–AcOEt = 3:1 as eluent) to give 3 (0.958 g,
60%). mp 256–257 ЊC. IR (KBr)/cmϪ1 3422 (NH), 3304, 3229
1
(NH ), 1655 (C᎐O), 1287, 718; H NMR δ (DMSO-d6) 4.85
᎐
2
H
(2H, s), 7.55–8.3 (12H, m), 9.93 (1H, b); (Found: C, 67.48; H,
3.86; N, 7.23. C22H15N2O3Cl requires C, 67.61; H, 3.87; N,
7.17%).
Synthesis of 1-amino-2-[4-(aza-15-crown-5-ethermethyl)-
benzoylamino]anthraquinone (4a)
A mixture of the compound 3 (0.64 g, 1.54 mmol), 1-aza-15-
crown-5-ether (0.405 g, 1.85 mmol), and triethylamine (0.47 g)
in 1,4-dioxane (180 ml) was heated under reflux with stirring for
25 h. After the reaction, the solvent was removed under reduced
pressure and the residue was extracted with CH2Cl2. The
organic extract was washed with water and evaporated, the
residue was chromatographed on silica gel (CHCl3–AcOEt =
3:1 as eluent) and was further purified by recrystallization
from 95% ethanol to give 4a (0.58 g, 68%). mp 167–168 ЊC. IR
(KBr)/cmϪ1 3416 (NH), 3302 (NH ), 2870 (CH ), 1658 (C᎐O),
᎐
2
2
1
1283, 1105, 720; H NMR δH (CDCl3) 2.77 (4H, t), 3.5–3.8
(18H, m), 7.08 (2H, b), 7.45 (2H, d), 7.6–8.3 (9H, m); (Found:
C, 66.77; H, 6.01; N, 7.17. C32H35N3O7 requires C, 67.00; H,
6.15; N, 7.33%).
Synthesis of 2-[4-(aza-15-crown-5-ethermethyl)phenyl]imidazo-
[5,4-a]anthraquinone (5a)
A mixture of the compound 4a (1.0 g, 1.74 mmol), NaOH (0.07
g, 1.75 mmol) in 95% ethanol (350 ml) was heated under reflux
with stirring for 1.5 h. After cooling to room temperature, the
reaction mixture was neutralized with aqueous HCl, and the
solvent was evaporated, then the residue was washed with
water. The crude product was chromatographed on silica gel
(ethanol as eluent), and was further purified by recrystallization
from 95% ethanol to give 5a (0.420 g, 43%). mp 141–142 ЊC. IR
(KBr)/cmϪ1 3422 (NH), 2874 (CH ), 1664 (C᎐O), 1296, 1092,
Experimental
Mps were measured with a Yanaco micro melting point
1
apparatus MP-500D and are uncorrected. H NMR spectra
were taken on a Hitachi Model R-90H spectrometer with
tetramethylsilane (TMS) as an internal standard. IR spectra
were recorded on a JASCO FT/IR-5300 spectrophotometer for
samples in KBr pellet form. Elemental analyses were measured
on a Perkin-Elmer 2400 II. Absorption spectra were measured
using a JASCO U-best 30 spectrophotometer. Fluorescence
spectra and quantum yields were measured with a JASCO FP-
777 spectrophotometer. A solution of quinine sulfate in sulfuric
acid (0.05 M) was used as the standard for quantum yield (ΦF)
measurements and taken to have ΦF = 0.55. Cyclic voltammetry
was performed by means of a BAS CV-50W. A three-electrode
system, containing of a glassy carbon working electrode and
platinum wire counter electrode and a Ag/AgCl electrode as
reference, was adopted. The electrolytic solvent was acetonitrile
containing 0.1 mol dmϪ3 tetraethylammonium perchlorate
(TEAP) as supporting electrolyte. Prior to measurements, all
solutions were thoroughly degassed with argon for at least 20
min. The association constants (K) were obtained by a non-
linear least-squares analysis reported by Bourson et al.11 The
fluorescence lifetimes were measured with an imaging spectro-
graph (Hamamatsu, C5094) and a streak scope (Hamamatsu,
C2830) with a high speed streak unit (Hamamatsu, M2547).
Acetonitrile for use in absorption and fluorescence spectro-
scopy or cyclic voltammetry was dried and distilled before
use.
᎐
2
720; 1H NMR δH (CDCl3), 2.84 (4H, t), 3.5–3.9 (18H, m), 7.57
(2H, d), 7.7–7.9 (2H, m), 8.0–8.4 (6H, m), 11.25 (1H, b);
λmax(MeCN)/nm 405, εmax/dm3 molϪ1 cmϪ1 14,300; (Found: C,
68.93; H, 5.95; N, 7.47. C32H33N3O6 requires C, 69.17; H, 5.98;
N, 7.56%).
Synthesis of 1-amino-2-[4-(aza-18-crown-6-ethermethyl)benzoyl-
amino]anthraquinone (4b)
A mixture of the compound 3 (0.50 g, 1.28 mmol), 1-aza-18-
crown-6 (0.404 g, 1.54 mmol) and triethylamine (0.389 g) in
1,4-dioxane (150 ml) was heated under reflux with stirring for
25 h. After the reaction, the solvent was removed under reduced
pressure and the residue was extracted with CH2Cl2. The
organic extract was washed with water and evaporated,
the residue was chromatographed on silica gel (CHCl3–
AcOEt = 3:1 as eluent), and was further purified by recrystal-
lization from 95% ethanol to give 4b (0.506 g, 66%). mp 167–
168 ЊC. IR (KBr)/cmϪ1 3418 (NH), 3302 (NH2), 2876 (CH2),
1658 (C᎐O), 1281, 1109, 720; 1H NMR δ (CDCl ) 2.78 (4H, t),
᎐
H
3
3.5–3.8 (22H, m), 7.10 (2H, b), 7.46 (2H, d), 7.6–8.3 (9H, m);
(Found: C, 65.83; H, 6.30; N, 6.74. C34H39N3O8 requires C,
66.11; H, 6.36; N, 6.80%).
396
J. Chem. Soc., Perkin Trans. 2, 1999, 393–397

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Synthesis of 2-[4-(1,4,7,10,13,16-pentaoxaazacyclooctadecin-16-
yl)phenyl]imidazo[5,4-a]anthraquinone (5b)
Acknowledgements
The present work was supported by a Grant-in-Aid for
Scientific Research (B) No. 08455418 from the Ministry of
Education, Science, Sports and Culture.
A mixture of the compound 4b (0.704 g, 1.14 mmol), NaOH
(0.06 g, 1.15 mmol) in 95% ethanol (250 ml) was heated under
reflux with stirring for 1.5 h. After cooling to room temper-
ature, the reaction mixture was neutralized with aqueous HCl,
and the solvent was evaporated, then the residue was washed
with water. The crude product was chromatographed on silica
gel (ethanol as eluent), and was further purified by recrystalliz-
ation from 95% ethanol to give 5b (0.325 g, 48%). mp 143–
144 ЊC. IR (KBr)/cmϪ1 3435 (NH), 2870 (CH ), 1666 (C᎐O),
References
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᎐
2
1
1296, 1109, 716; H NMR δH (CDCl3) 2.84 (4H, t), 3.5–3.9
(22H, m), 7.57 (2H, d), 7.7–7.9 (2H, m), 8.0–8.4 (6H, m), 11.25
(1H, b); λmax(MeCN)/nm 405, εmax/dm3 molϪ1 cmϪ1 14,500;
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68.10; H, 6.22; N, 7.01%).
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Synthesis of 2-(4-tert-butylphenyl)imidazo[5,4-a]anthraquinone
(PIAQ-But)
4-tert-Butylbenzoyl chloride (1.69 g, 8.60 mmol) was added
dropwise to a solution of 1,2-diaminoanthraquinone (2.00 g,
8.40 mmol) and Na2CO3 (0.89 g, 8.40 mmol) in pyridine (30 ml)
with stirring. After being stirred for 1 h at 60 ЊC, the solvent was
evaporated and the residue was washed with hot water and
dried. The resulting orange powder was dissolved in pyridine
(60 ml), and NaOH (0.40 g, 10.0 mmol) dissolved in water (10
ml) was added to the solution. After the mixture had been
stirred for 1 h at 70 ЊC, cooled to room temperature, neutralized
with aqueous HCl, and the solvent was evaporated, the residue
was washed with water. The crude product was chrom-
atographed on silica gel (CHCl3–AcOEt = 9:1 as eluent), and
was further purified by recrystallization from ethyl acetate to
give PIAQ-But (2.94 g, 92%). mp 245–245.5 ЊC. IR (KBr)/cmϪ1
10 C. J. Pedersen and H. K. Frensdorff, Angew. Chem., Int. Ed. Engl.,
1972, 11, 16.
11 J. Bourson, J. Pouget and B. Valeur, J. Phys. Chem., 1993, 97, 4552.
12 R. M. Izatt, K. Pawlak, J. S. Bradshaw and R. L. Bruening, Chem.
Rev., 1991, 91, 1721.
13 H. Kawai, T. Nagamura, T. Mori and K. Yoshida, J. Phys. Chem.,
submitted.
3432 (NH), 2961, 1665 (C᎐O); 1H NMR δH (CDCl3), 1.39 (9H,
᎐
s), 7.5–7.8 (4H, m), 7.9–8.3 (6H, m), 11.19 (1H, bs); λmax
(MeCN)/nm 403, εmax/dm3 molϪ1 cmϪ1 15,300; (Found: C,
79.21; H, 5.31; N, 7.15. C25H20N2O2 requires C, 78.93; H, 5.30;
N, 7.36%).
Paper 9/00424F
J. Chem. Soc., Perkin Trans. 2, 1999, 393–397
397