Novel cyanine dyes: Synthesis, characterization and photosensitization- structure correlation

Article abstract of DOI:10.1002/jccs.200200153

Novel furo, thieno and pyrrolo[2,3-b]pyrazole cyanine dyes were synthesized. The structure-photosensitization properties correlation of the dyes were examined in 95% ethanol solution by absorption spectroscopy. The chemical structure of the starting biheterocyclic compounds and their derived cyanine dyes were confirmed by elemental analysis, IR and ¹H NMR spectroscopy.

Full text of DOI:10.1002/jccs.200200153

Journal of the Chinese Chemical Society, 2002, 49, 1061-1068
1061
Novel Cyanine Dyes: Synthesis, Characterization and
Photosensitization-Structure Correlation
H. A. Shindy*, M. A. El-Maghraby and F. M. Eissa
Department of Chemi s t r y, Aswan Faculty of Science, Aswan, Egypt
Novel furo, thieno and pyrrolo[2,3-b]pyrazole cyanine dyes were synthesized. The structure-photo-
sensitization properties correlation of the dyes were examined in 95% ethanol solution by absorption spec-
troscopy. The chemi c a l structure of the starting biheterocyclic compounds and their derived cyanine dyes
were confirmed by elemental analysis, IR and 1H NMR spectroscopy.
INTRODUCTION
Many cyanine dyes and specially heterocyclic cyanines
salt in ethanol containing piperidine as a basic catalyst to give
the 3[2(4)] trimethine cyanine dyes (7a-e) and 4[2(4)]-di-
3[2(4)]-tri-mixed methine cyanine dyes (8a-e), Scheme I, Ta-
ble 2. Chemical confirmation takes place through reaction of
(7a-e) with equimolar ratios of iodomethane quaternary salt
of -picoline, quinaldine, -picoline in ethanol catalysed by
piperidine to achieve the same 4[2(4)]-di-3[2(4)-tri-mixed
methine cyanine dyes (8a-e), characterized by melting points,
hold an important position in the chemistry of dyes and pig-
ments. Cyanines are intensely coloured dyes whose phot o -
chemical and photophysical properties are of great practical
significance because of their applications in laser technolo-
gies1 and in colour photographic processes.2,3 Cyanine dyes
have found prominent use as membrane-staining probes,4 and
more recently as reagents in biomedicine.5,6 In addition, they
have been widely employed as model chromophores in
monolayer assemblies.7,8
1
mixed melting points, same IR and H NMR spectra data,
Scheme I, Tables 2, 3.
The structure of the new prepared compounds were
characterized by elemental analy s i s , Tables 1, 2, IR16 and 1H
NMR17 spectroscopy, Table 3.
RESULTS AND DISCUSSION
Characterization
The new prepared cyanine dyes are highly coloured
compounds; they are partially soluble in non polar organic
solvents but easily soluble in polar organic solvents exhibit-
ing coloured solutions, accompanied by pale to intensed fluo -
rescence. The intensity and colour of the fluorescence depend
upon the type of dye and solvents used. They are soluble in
concentrated H2SO4 liberating iodine vapour on warming.
They are pH sensitive dyes, so their ethanolic solutions gave
changeable colour on acid/base media giving yellow or
colourless on acidification and restored their permanent col-
our on basification. Their intense colour is due to the two sug-
gested mesomeric structures A[B(B)] producing delocalized
positive charge over the conjugate system, Scheme II.
Synthesis
In relation to the former introduction, and in the course
of our research on the synthesis and properties of cyanine
dyes,9-13 4-bromo-3-methyl-1-phenyl-pyrazole-5-one (1)14,15
was reacted with equimolar ratios of ethyl acetoacetate,
mercapto ethyl acetoacetate and imino ethyl acetoacetate
(2a-c) in ethanol containing pyridine and afforded 4-ethoxy-
carbonyl-3,5-dimethyl-1-phenyl-furo, thieno and pyrrolo-
[2,3-b]pyrazole (3a-c). Quaternization of (3a-c) usi n g excess
of iodomethane led to their 2-methyl iodide quaternary salt
(4a-c). Interaction of equimolar ratios of (4a-c) with iodo-
methane quaternary salt of pyridine, quinoline and isoquino-
line in ethanol containing a few drops of piperidine gave the
3[4(1)]-monomethine cyanine dyes (5a-e), Scheme I, Table 1.
Additionally, ethanolic solution of the quaternized
compounds (4a-c) were reacted with equimolar ratios of
triethyl orthoformate in presence of piperidine and led to the
formation of the intermediate compounds (6a-c). The inter-
mediate (6a-c) reacted with equimolar (bimolar) ratios of
N-methyl (2-picolinium, quinaldinium, 4-picolinium) iodide
Photosensitization-Structure Correlation
Structure-photosensitization properties correlation of
the new prepared cyanine dyes were determined by studying
their electronic visible absorption spectra in 95% ethanol so-
lution. The dyes are thought better photosensitizers when
they absorb light at long e r wavelengths (bathochromic and/or
red shifted dyes), and consequently photosensitization of the

1062 J. Chin. Chem. Soc., Vol. 49, No. 6, 2002
Shindy et al.
Scheme I
OEt
O
OEt
OEt
H3C
H
Br
O
H3C
Br
H3C
H
H
H
X
O
EtOH/Pyridine
- HBr; - H2O
O
N
N
N
+
N
N
N
X
CH3
OH
HX
CH3
CH3
ph
ph
ph
(3a-c)
(1)
(2a-c)
A
OEt
OEt
A
H3C
H3C
N
I
O
O
CH3
EtOH/Pip.
CH3I
N
N
N
I
I
H3C
N
X
CH3
N
X
CH3
CH3
HC(OEt)3
EtOH/Pip.
(4a-c)
ph
ph
(5a-e)
A
OEt
OEt
H3C
A
I
N
EtO
O
O
I
CH3
N
N
OEt
H3C
N
I
EtOH/Pip.
CH3
H3C
N
X
CH3
N
X
CH3
ph
ph
(7a-e)
(6a-c)
A
A
H3C
H3C
OEt
A
I
N
N
A
I
I
1 Mole
2 Mole
CH3
Route ( 2 )
CH3
Route ( 1)
N
CH3
N
N
I
CH3
H3C
N
X
CH3
ph
(8a-e)
Substituents in scheme (1):
(2a - c); (3a - c); (4a - c); (6a - c): X = O (a); S(b); NH (c).
(5a - e): X = O, A = 1-methyl pyridinium-4-yl salt (a); X = O, A = 1-methyl quinolinium-4-yl salt (b); X = O, A = 2-methyl isoquinolinium-1-yl salt (c);
X = S, A = 1-methyl quinolinium-4-yl salt (d); X = NH, A = 1-metyl quinolinium-4-yl salt (e).
(7a - c); (8a - c): X = O, A = 1-methyl pyridinium-2-yl salt (a); X = O, A = 1-methyl quinolinium-2-yl salt (b); X = O, A = 1-methyl pyridinium-4-yl salt (c);
X = S, A 1-methyl quinolinium-2yl salt (d); X = NH, A = 1-methyl quinolinium-2-yl salt (e).
dyes decrease in the dyes which absorb light at shorter wave-
lengths (hypsochromic and/or blue shifted dyes).
linkage position of the quinolinium residue from 4-yl salt to
1-yl salt passing from dye 5b to dye 5c resulted in a hypso-
chromic shift of 5 nm accompanied by decreasing the inten-
The electronic absorption spectra of the monomethine
cyanine dyes (5a-e) showed absorption bands in the visible
region 471-501 nm. The bathochromic and hypsochromic
shifts depend highly on the nature of the heterocyclic quater-
nary salts, their linka g e position and the type of the bihetero-
cyclic ring. For instance the absorption spectra of the mono-
methine cyanine dye 5a, X=O, A = 1-methyl pyridinium-4-yl
salt showed an absorption spect r a band located at max 471
nm ( max 7950 mol-1cm2). Substituting A by 1-methyl quino-
linium-4-yl salt to give dye 5b causes bathochromic shift of
sity of the absorption band, 5c,
476 nm (
8120
max
max
mol-1cm2), Scheme I, Table 1. This is due to decreasing con-
jugation in the latter dye. Additionally, the monomethine
cyanine dye 5b containing the furo-pyrazole nuclei showed
hypsochromically shifted band by 9 nm, 20 nm if compared
by their analogous dyes 5d, 5e which contains the thieno-
pyrazole and pyrrolo-pyrazole nucleus. The thieno-pyrazole
monomethine dye 5d showed hypsochromically shifted band
by 11 nm if compared by the pyrrolo-pyrazole monomethine
dye 5e, Scheme I, Table 1. This is due to that the furan ring in
dye 5b has a more electron attracting character than thio-
phene and pyrrole rings in dyes 5d, 5e, and the thiophene ring
10 nm with intensification of the absorption band, 5b,
max
481 nm ( max 9190 mol-1cm2). This is due to the extra conjuga-
tion in quinoline, Scheme I, Table 1. In contrast, changing the

Methine Cyanine Dyes: Synthesis and Properties
J. Chin. Chem. Soc., Vol. 49, No. 6, 2002 1063
Table 1. Characterization of 4-Ethoxycarbonyl-3,5-dimethyl-1-phenyl-furo, Thieno and Pyrrolo[2,3-b]pyrazole (3a-c); 4-
Ethoxycarbonyl-2,3,5-trimethyl-1-phenyl-furo, Thieno and Pyrrolo[2,3-b]pyrazolium Iodide (4a-c) and 4-Ethoxycarbonyl-2,5-
dimethyl-1-phenyl-furo, Thieno and Pyrrolo[2,3-b]pyrazolium Iodide-3[4(1)]-monnomethine Cyanine Dyes (5a-e)
Absorption spectra in
Nature of products
Colour Yield M.P.
Analysis
Molecular
Formula
(M. Wt.)
95 ethanol
Comp.
No.
Calculated
H
found
H
max
max
(mole-1cm2)
%
C
C
N
C
N
(nm)
3a
3b
3c
4a
4b
4c
5a
5b
5c
5d
5e
Bright red crystals
Dark red crystals
23
144
C16H16O3N2
(284)
C16H16O2N2S 64.0 5.3
(300)
C16H17O2N3
(283)
C17H19O3N2I 47.9 4.5
(426)
67.6 5.6
9.9
67.4 5.7 10.0
____
____
____
____
____
____
____
7950
34
162
160
158
9.3
64.2 5.0 9.4
____
____
____
____
____
471
Yellowish red crystals 50
67.8 6.0 14.8 67.8 5.9 14.6
Pale red crystals
Brown red crystals
Red crystals
76
62
60
6.8
6.3
9.9
8.1
7.4
7.4
7.2
9.9
47.8 4.7
46.3 4.1
48.2 4.5
53.0 4.2
56.8 4.8
56.9 4.7
55.5 4.3
57.3 4.9
6.7
6.7
9.9
8.6
7.3
7.1
7.5
9.7
155 C17H19O2N2IS 46.2 4.3
(442)
170
177
178
181
C17H20O2N3I 48.0 4.7
(425)
Reddish brown crystals 43
Brown crystals 37
Reddish brown crystals 36
C23H24O3N3I 53.4 4.6
(517)
C27H26O3N3I 57.1 4.6
481
9190
(567)
C27H26O3N3I 57.1 4.6
(567)
476
8120
Dark brown crystals
Pale brown crystals
57
63
191 C27H26O2N3IS 55.6 4.5
(583)
168
490
10010
8550
C27H27O2N4I 57.2 4.8
(566)
501
in dye 5d is more electron attracting than the pyrrole ring in
dye 5e.
shifts for the absorption bands. This is due to the high electro-
negativity of the oxy g e n atom in dyes 7b, 8b compared with
sulphur and nitrogen ato m s in dyes 7d, 7e, 8d, 8e. For in-
stance, substituting thiophene ring in dyes 7d, 8d by pyrrol
rings in dyes 7e, 8e causes a red shift for the absorption
bands. This is due to the more electron accepting character of
sulphur atom in dyes 7d, 8d compared to nitrogen atom in
dyes 7e, 8e, Scheme I, Table 2. Generally, comparing the
electronic absorption spectra bands of the trimethine cyanine
dyes 7a-e with their corresponding di-tri mixed methine
cyanine dyes 8a-e disclosed that the latter mixed methine
dyes have red shifted bands. This can be explained by the
presence of two charge transf e r pathways in the mixed
Also the trimethine cyanine dyes 7a-e and the di-tri
mixed methine cyanine dyes 8a-e showed electronic absorp-
tion spectra in 95% ethanol in which their bands positions
and molar extinction coefficients are influenced by the nature
of the quaternary heterocyclic nuclei, their linkage position
and the electronegativity of hetero ato m s in the bihetero-
cyclic nuclei. So, substituting A = 1-methyl pyridinium-2-yl
salt in the trimethine and di-tri mixed methine cyanine dyes
7a, 8a by A = 1-methyl-quinolinium-4-yl salt to give dyes 7b,
8b causes bathochromic and intensification for the absorp-
tion bands. This is attributed to a more extensive -delo-
callization that leads to an easier charge transfer in the later
dyes, Scheme I, Table 2. Otherwise, substituting A = 1-
methyl pyridinium-2-yl salt in dyes 7a, 8a by A = 1-methyl
pyridinium-4-yl salt to give dyes 7c, 8c causes bathochromic
shifts for the absorption bands. This is related to the extended
conjugation in the latter dyes, Scheme I, Table 2. Add i -
tionally, substituting furan ring in dyes 7b, 8b by thiophene
and pyrrole rings to obtain dyes 7d, 7e, 8d, 8e causes red
methine dyes from N-pyrazole to the heterocyclic quarter-
nary salt (left side of the dye molecule) and from either O-
furan, S-thiophene, NH-pyrrole to the heterocyclic quater-
nary salt (right side of the dye molecule), Schemes I, II, Table
2.

1064 J. Chin. Chem. Soc., Vol. 49, No. 6, 2002
Shindy et al.
Table 2. Characterization of 3-(2-Diethoxy ethyl)-4-ethoxycarbonyl-2,5-dimethyl-1-phenyl-furo, Thieno and Pyrrolo[2,3-b]pyrazolium
Iodide (6a-c); as Intermediate Compounds; 4-Ethoxycarbonyl-2,5-dimethyl-1-phenyl-furo, Thieno and Pyrrolo[2,3-b]pyrazole-
3[2(4)]-trimethine Cyanine Dyes (7a-e) and 2,5-Dimethyl-1-phenyl-furo, Thieno and Pyrrolo[2,3-b]pyrazole-4[2(4)]-di-
3[2(4)]-tri-mixed Methine Cyanine Dyes (8a-e)
Absorption spectra in
Nature of products
Analysis
Calculated
Molecular
Formula
(M. Wt.)
95 ethanol
Comp.
No.
Colour
Yield M.P.
Found
H
max
max
(mole-1cm2)
%
C
C
H
N
C
N
(nm)
____
6a
6b
6c
7a
7b
7c
7d
7e
8a
8b
8c
8d
8e
Brown crystals
Pale brown crystals
Reddish brown crystals
Deep brown crystals
Deep violet crystals
Deep brown crystals
Deep violet crystals
Violet crystals
40
173
C22H29O5N2I 50.0 5.5 5.3 50.3 5.2 5.5
(528)
169 C22H29O4N2SI 48.5 5.3 5.1 48.3 5.4 5.3
(544)
____
37
69
35
46
37
66
53
39
41
34
53
51
____
____
____
190
179
184
177
C22H30O4N3I 50.1 5.7 8.0 50.0 5.4 8.2
____
(527)
C25H26O3N3I 55.2 4.8 7.7 55.0 5.0 7.5
498, 382
509, 388
499, 389
513
13150, 18660
17170, 10300
13060, 18400
18270
(543)
C29H28O3N3I 58.7 4.7 7.1 58.5 4.9 7.3
(593)
C25H26O3N3I 55.2 4.8 7.7 54.9 5.1 7.8
(543)
181 C29H28O2N3SI 57.1 4.6 6.9 57.0 4.8 6.7
(609)
C29H29O2N4I 58.8 4.9 9.5 58.5 5.0 9.6
(592)
177 C32H34O2N4I2 50.5 4.5 7.4 50.3 4.7 7.4
203
515
12030
Intense brown crystals
Intense violet crystals
Intense brown crystals
Intense violet crystals
Deep violet crystals
507, 389
519, 397
509, 391
523
12540, 23020
16080, 8420
11990, 22990
18690
(760)
184 C40H38O2N4I2 55.8 4.4 6.5 55.7 4.3 6.7
(860)
207 C32H34O2N4I2 50.5 4.5 7.4 50.6 4.2 7.6
(760)
190 C40H38ON4SI2 54.8 4.3 6.4 54.5 4.5 6.5
(876)
185
C40H39ON5I2 55.9 4.5 8.1 55.7 4.7 8.3
(859)
529
16170
were carried out at the Microanalytical Center of Cairo Uni-
versity by an automatic analyzer (Heraeus). IR (KBr pellets)
spectra were determined on a Perkin Elmer Infrared 127
spectrophotometer (Cairo University), and the 1H NMR spec-
tra were obtained usi n g 400 and 300 MHz NMR spectropho-
tometers (Assiut and Cairo Universitis). Electronic visi b l e
absorption spectra were carried out on a Shimadzu UV-
Visible recording spectrophotometer (South Valley Univer-
sity, Faculty of Science, Aswan).
CONCLUSION
Based on our studies, we can conclude that the phot o -
sensitization of the cyanine dyes are very markedly depend-
ent on the nature of the dyes structures. Photosensitization of
the dyes increase (decrease) by increasing (decreasing) con-
jugation in the dye molecule. Also the photosensitization of
the dyes increase (decrease) by decreasing (increasing) the
electronegativity of the hete r o atom in the biheterocyclic ring
of the dye. Increasing number of methine groups gives better
photosensitizers dyes, and dyes which have two conjugated
charge transf e r pathways inside the dye molecule are better
photosensitizers than their analogues which have only one
conjugated charge transfer pathw a y.
Synthesis of 4-ethoxycarbonyl-3,5-dimethyl-1-phenyl-
furo, thieno and pyrrolo[2,3-b]pyrazole (3a-c)
A mixture of 4-bromo-3-methyl-1-phenyl-5-pyrazolone
(1) (0.01 mol) and ethylacetoacetate, mercapto ethylaceto-
acetate, Imino ethylacetoacetate (2a-c) (0.01 mol) was
refluxed in ethanol (50 mL) containing pyridine (20 mL) for 8
h. The reaction mixture changed from a reddish colour to
red-dark brown at the end of refluxing. It was filtered while it
was hot, precipitated by ice-water and neutralized by concen-
trated hydrochloric acid. The precipitated products were col-
EXPERIMENTAL
General
All melti n g points are uncorrected. Elemental analyses

Methine Cyanine Dyes: Synthesis and Properties
J. Chin. Chem. Soc., Vol. 49, No. 6, 2002 1065
Table 3. IR and 1H NMR Spectral Data of the Prepared Compounds
Comp. No.
IR spectrum (KBr), cm-1
1H NMR spectrum (DMSO),
3a
692, 757 (mono substituted phenyl)
1028, 1121 (C – O – C cyclic)
1317 (C – N)
1.05-1.28 (t, 3H, CH3 of ethoxy)
4.32-4.61 (q, 2H, CH2 of ethoxy)
3.6-3.9 (m, 6H, 2CH3 of C3 and C5)
6.76-7.72 (m, 5H, aromatic).
1497 (C = N)
1597 (C = C)
1715 (C = O)
3b
406, 502 (Thiophene ring)
690, 756 (mono substituted phenyl)
1029, 1190 (C – S – C cyclic)
1364 (C – N)
0.97-1.18 (t, 3H, CH3 of ethoxy)
4.31-4.64 (q, 2H, CH2 of ethoxy)
3.29-3.53 (m, 6H, 2CH3 of C3 and C5)
6.76-7.86 (m, 5H, aromatic).
1496 (C = N)
1596 (C = C)
1714 (C = O)
3c
690, 755 (mono substituted phenyl)
1090, 1172 (C – N – C cyclic)
1201 (C – O ether)
1324 (C – N)
1.00-1.27 (t, 3H, CH3 of ethoxy)
4.27-4.68 (q, 2H, CH2 of ethoxy)
3.6-3.9 (m, 6H, 2CH3 of C3 and C5)
6.10 (br, 1H, NH)
1372 (pyrrole ring stretching)
1541 (C = N)
7.17-8.10 (m, 5H, aromatic).
1597 (C = C)
1716 (C = O)
3356 (NH)
4a
688, 756 (mono substituted phenyl)
1068, 1122 (C – O – C cyclic)
1273 (C – O ether)
1359 (C – N)
1.14-1.30 (t, 3H, CH3 of ethoxy)
3.96-4.48 (q, 2H, CH2 of ethoxy)
3.38-3.65 (m, 9H, 3CH3 of N2, C3 and
C5)
1494 (C = N)
7.1-7.9 (m, 5H, aromatic).
1616 (C = C)
1713 (C = O)
2919 (quaternary salt)
5b
692, 755 (mono substituted phenyl)
829, 909 ( . di substituted phenyl)
1091, 1163 (C – O – C cyclic)
1235 (C – O ether)
1.14-1.31 (t, 3H, CH3 of ethoxy)
3.96-4.25 (q, 2H, CH2 of ethoxy)
3.30-3.80 (m, 9H, 3CH3 of N2, C5 and
N-quinoline)
1350 (C – N)
1487 (C = N)
7.1-8.1 (m, 12H, aromatic + heterocyclic
+ CH=).
1596 (C = C)
1716 (C = O)
2920 (quaternary salt)
6a
691, 756 (mono substituted phenyl)
820, 906 ( . di substituted phenyl)
1029, 1068 (C – O – C cyclic)
1121 (C – O ether)
0.77-0.95 (t, 3H, CH3 of ethoxy)
4.03-4.21 (q, 2H, CH2 of ethoxy)
1.17-1.32 (t, 6H, 2CH3 of diethoxy ethyl)
3.78-4.03 (q, 4H, 2CH2 of diethoxy
ethyl)
1313, 1360 (C – N)
1496 (C = N)
1595 (C = C)
1714 (C = O)
2920 (quaternary salt)
3.32-3.53 (m, 6H, 2CH3 of N2 and C5)
4.60-4.77 (t, 1H, CH of diethoxy ethyl)
3.65-3.72 (d, 2H, CH2 of diethoxy ethyl)
6.95-7.32 (m, 5H, aromatic).

1066 J. Chin. Chem. Soc., Vol. 49, No. 6, 2002
Shindy et al.
7b
695, 757 (mono substituted phenyl)
1.10-1.31 (t, 3H, CH3 of ethoxy)
3.91-4.24 (q, 2H, CH2 of ethoxy)
3.21-3.62 (m, 9H, 3CH3 of N2, C5 and N-
quinolinium)
828, 906 ( . di substituted phenyl)
1001, 1160 (C – O – C cyclic)
1362 (C – N)
1496 (C = N)
1597 (C = C)
6.72-7.60 (m, 14H, aromatic +
heterocyclic + - CH=CH-CH=).
1712 (C = O)
2922 (quaternary salt)
8b
694, 757 (mono substituted phenyl)
822, 905 ( . di substituted phenyl)
1002, 1161 (C – O – C cyclic)
1361 (C – N)
1.31-1.45 (t, 3H, CH3 of ethoxy)
3.93-4.12 (q, 2H, CH2 of ethoxy)
3.21-3.81 (m, 12H, 4CH3 of N2, C5 and
N-quinolinium)
1496 (C = N)
1596 (C = C)
6.57-7.2 (m, 21H, aromatic +
heterocyclic + =CH-).
2922 (quaternary salt)
Scheme II
OEt
OEt
A
A
A
CH CH CH
H3C
O
n
N
N
CH3
N
N
N
I
CH3
N
X
CH3
H3C
XN
I
CH3
CH3
I
I
ph
ph
(B)
(B)
OEt
OEt
A
A
A
CH CH CH
H3C
O
n
N
N
N
I
CH3
N
N
I
CH3
I
XN
CH3
H3C
XN
XN
CH3
CH3
ph
ph
(8a-e)
(A)
(5a-e), n = 0
(7a-e), n = 1
(A)
OEt
OEt
A
I
A
A
CH CH CH
H3C
O
n
N
N
N
N
N
I
CH3
N
X
CH3
H3C
CH3
CH3
I
CH3
ph
ph
(B)
(B)
lected, washed with water several times, dried and crystal-
lized from etha n o l . The results are listed in Table 1.
lized from etha n o l . The data are shown in Table 1.
Synthesis of 4-ethoxycarbonyl-2,5-dimethyl-1-phenyl-
furo, thieno and pyrrolo[2,3-b]pyrazolium iodide-3[4(1)]-
monomethine cyanine dyes (5a-e)
Synthesis of 4-ethoxycarbonyl-2,3,5-trimethyl-1-phenyl-
furo, thieno and pyrrolo[2,3-b]pyrazolium iodide (4a-c)
A pure sample of (3a-c) and excess methyl iodide were
heated under reflux by a water bath for 2 h. The reaction mix-
ture was cooled, and the precipitated products were crystal-
A mixture of equimolar ratios (0.01 mol) of (4a-c) and
methyl iodide quaternary salt of pyridine, quinoline, iso-
quinoline were refluxed in ethanol (30 mL) containing a few

Methine Cyanine Dyes: Synthesis and Properties
J. Chin. Chem. Soc., Vol. 49, No. 6, 2002 1067
drops (3-5) of piperidine for 8-10 h. The reaction mixture,
which changed from reddish brown to deep brown at the end
of refluxing, was filtered while it was hot, concentrated,
cooled, neutralized with glacial acetic acid and precipitated
by the addition of cold water. The precipitate was collected
and crystallized from etha n o l . The relevant data are given in
Table 1.
ratios (0.01 mol) of iodomethane quaternary salt of -pico-
line, quinaldine, -picoline were dissolved in ethanol (30
mL), to which piperidine (3-5 drops) was added. The reaction
mixture was refluxed for 3-5 h. until it attained a permanent
colour. It was filtered while hot, concentrated to half its vol-
ume, cooled, neutralized by acetic acid and precipitated by
addition of cold water. The precipitates were collected and
crystallized from ethanol to give the same compounds (8a-e)
obtained by route (1), characterized by melti n g points, mixed
melting points, same IR and 1H NMR spectra data, Tables 2,
3.
Synthesis of 3-(2-diethoxy ethyl)-4-ethoxycarbonyl-2,5-
dimethyl-1-phenyl-furo, thieno and pyrrolo[2,3-b]-
pyrazolium iodide (6a-c) as intermediate compounds
Equimolar ratios (0.01 mol) of the quaternary com-
pounds (4a-c) and triethyl orthoformate were dissolved in
ethanol (30 mL), then piperidine (3-5 drops) was added. The
reaction mixture was refluxed for 8 h. and attained a dark
brown mixture at the end of refluxing. It was filtered while it
was hot, concentrated to half its volume, cooled and neutral-
ized by acetic acid. The separated products were collected
and crystallized from etha n o l . The results are summarized in
Table 2.
Absorption Spectroscopy
The electronic visible absorption spectral behaviour of
the prepared cyanine dyes were examined in 95% ethanol so-
lution and recorded usi n g 1 cm cells on Shimadzu UV-visible
recording spectrophotometer. A stock solution (10-3 M) of the
dyes was diluted to a suitable volume in order to obtain the re-
quired concentrations.
Synthesis of 4-ethoxycarbonyl-2,5-dimethyl-1-phenyl-
furo, thieno and pyrrolo[2,3-b]pyrazole-3[2(4)]-
trimethine cyanine dyes (7a-e)
Received October 24, 2001.
Key Words
Iodomethane quaternary salt of -picoline, quinaldine,
-picoline was heated und e r reflux in equimolar ratios (0.01
mol) with (6a-c) in ethanol (30 mL) containing a few drops
(3-5) of piperidine for 8-10 h. The reaction mixture attained
dark brown-dark violet colours at the end of refluxing. It was
filtered off hot, concentrated, neutralized by acetic acid and
precipitated by addition of cold wat e r. The separated cya-
nines were filtered, washed with water and crystallized from
ethanol. The results are listed in Table 2.
Cyanine dyes: Synthesis; Spectral behaviours;
Absorption spectra.
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