Carbocyanines - Derivatives of nitrogen-containing heterocycles with bridging groups in the chromophore

Article abstract of DOI:10.1007/s10593-008-0007-7

Tetramethylene-2,2′-bis(1,3,3-trimethyl-(3H)-indolium), -3-ethylbenzothiazolium,-1-methyl-quinolinium, and-1-methyl-and-1-butylbenz[c,d] indolium salts were synthesized starting from them trimethinecyanines containing an ethylene bridging group in the α, α′-positions of the chromophore have been prepared. These compounds are converted to their analogs containing a vinylene group by the action of benzoquinones. In the case of thiacarbocyanines the effect of the indicated bridging groups on the conformation and protonation route of the carbocyanines has been studied. The chromaticity of the dyes has been investigated.

Full text of DOI:10.1007/s10593-008-0007-7

Chemistry of Heterocyclic Compounds, Vol. 44, No. 1, 2008
CARBOCYANINES – DERIVATIVES
OF NITROGEN–CONTAINING
HETEROCYCLES WITH BRIDGING
GROUPS IN THE CHROMOPHORE
E. I. Mayboroda1, Yu. L. Slominsky1, A. V. Turov2, and A. I. Tolmachev1
Tetramethylene-2,2'-bis(1,3,3-trimethyl-(3H)-indolium), -3-ethylbenzothiazolium, -1-methyl- quinolinium,
and -1-methyl- and -1-butylbenz[c,d]indolium salts were synthesized starting from them
trimethinecyanines containing an ethylene bridging group in the α,α'-positions of the chromophore have
been prepared. These compounds are converted to their analogs containing a vinylene group by the
action of benzoquinones. In the case of thiacarbocyanines the effect of the indicated bridging groups on
the conformation and protonation route of the carbocyanines has been studied. The chromaticity of the
dyes has been investigated.
Keywords: bis-salt, carbocyanine, bridging group, absorption band, chromophore.
Efficient sensitizers of silver halide light-sensitive materials [1] and dyes for the detection of
biopolymers [2] have been discovered amongst carbocyanines with ethylene and vinylene bridging groups in the
α,α'-position of the chromophore. In the case of benzothiazole [3] and benzoxazole [4] derivatives it has been
shown that this type of dye can be synthesized from polymethylene-substituted bisquaternary nitrogen
heterocycle salts by general methods used in the preparation of trimethinecyanines. However a general
procedure for similar bisquaternary salts has not been reported to this time.
We have shown that such salts can be synthesized by treating oxo derivatives of the corresponding
nitrogen heterocycles with the Grignard reagents prepared from α,ω-dibromoalkanes by analogy with the
preparation of polymethylene bispyrilium salts [5, 6].
1.
2.
Br Mg(CH2)4MgBr
HZ
X
X
X
(CH2)4
+
O
+
2
N
N
N
Alk
Alk
Alk
–
2Z
1-3
1 Х = СMe2, Alk = Me, Z = BF4; 2 X= S, Alk = Et, Z = Br;
3 Х = −СН=CH−, Alk = Me, Z= Br
__________________________________________________________________________________________
1Institute of Organic Chemistry, National Academy of Sciences of Ukraine, Kiev 94; e-mail:
bvn1967@rambler.ru. 2Taras Shevchenko National University, Kiev 01033, Ukraine; e-mail:
atver@mail.univ.kiev.ua. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 94-102, January,
2008. Original article submitted April 4, 2003; revision submitted October 13, 2007.
78
0009-3122/08/4401-0078©2008 Springer Science+Business Media, Inc.

The reaction in the cases discussed is similar to that with mono magnesium bromide-substituted alkanes
[7-9] but it occurs at two reactive centers. In this way the reaction of 1,3,3-trimethyloxyindole, 3-ethyl-
benzothiazol-2-one, 1-methylquinolin-2-one, and N-methylnaphthostyril with the Grignard reagent obtained
from 1,4-dibromobutane gave the 2,2'-tetramethylenebis(3H-indolium) 1, benzothiazolium 2, quinolinium 3, and
benz[c,d]indolium (4) salts.
Use of the Grignard reagent prepared from 1,5-dibromopentane gave the bis-salt 5 in the same way.
1. BrMg(CH2)nMgBr
R
R
2. HX
R
(CH2)n
O
+
N
+
N
N
Alk
Alk
Alk
–
2 X
4-6
4, 5 Alk = Me, R = H; 4 n = 4, X = Br; 5 n = 5, X = BF4;
6 Alk = n-Bu, R = n-BuO, n = 4, X = ClO4
In organic solvents the dibromide 4a has a low solubility and it was therefore converted to the di-
p-tolylsulfonate 4b by fusion with excess methyl p-tolylsulfonate. In this way the 6-butoxy-1-butylnaphthostyril
gave the bis quaternary salt 6.
In the electronic spectra of the synthesized bis quaternary salts 1-6 the absorption bands were close in
position and double the intensity when compared with the absorption bands of the corresponding 2-methyl-
substituted monocations (Table 1). The 1H NMR spectra of these salts (Table 2) showed both the signals for the
aromatic protons and the characteristic signals for the protons of the polymethylene chains, the signals for the
protons of the methylene groups in the α,α'-positions being to low field of those in the β,β'-positions. The
accuracy of such an assignment was confirmed by determining the spectrum of compound 4 in CD3OD solution
where the signal for the α,α'-CH2 groups disappears due to deuterium exchange. In the bis(N-methyl-
benz[c,d]indolium) salt of structure 5, which contains a pentamethylene bridge, the signals for the γ-group
protons occur at highest field (α-CH2 3.40, β-CH2 1.98, and γ-CH2 1.79 ppm respectively).
The carbocyanines with a bridging ethylene in the chromophore 7-11 (Table 3) were prepared from the
salts 1-6. Indocarbocyanine 7 was synthesized by condensation of salt 1 with di(ethoxymethyl)acetate in a
mixture of acetic anhydride and pyridine. In the preparation of the carbocyanines 8, 9 (derivatives of
benz[c,d]indole) N-phenylethoxymethaneimine or DMF in acetic anhydride were used as electrophilic agents
and for the quinocyanine 10 and thiacyanine 11, obtained as in [4], the N-phenylethoxymethaneimine was
employed in pyridine medium.
TABLE 1. Electronic Absorption Spectra of Compounds 1-6 in Acetonitrile
λ
max, nm (log ε)
Compound
λmax, nm (log ε)
of monocation
1
2
3
4
5
6
281 (4.58)
280 (3.85)
280 (4.29)
275 (3.91)
323 (4.33)
320 (3.94)
330 (4.30), 374 (4.34)
330 (4.24), 371 (4.23)
268 (4.47), 324 (4.40),
385 (4.24), 449 (4.29)
330 (3.97), 371 (3.94)
330 (3.97), 371 (3.94)
271 (3.08), 324 (3.02),
384 (2.82), 441 (2.85)
79

1
TABLE 2. H NMR Spectroscopic Characteristics for Compounds 1-10,
12-14, 16, 17
Com-
pound
Chemical shifts δ, ppm (J, Hz)
1
1.65 (12Н, s, С(СН3)2); 1.97 (4Н, br. s, β-СН2); 3.24 (4Н, br. s, α-СН2);
4.12 (6Н, s, N–CH3); 7.69 (4Н, m, H-5,5' + H-6,6'); 7.88 (2H, m, H-7,7');
7.97 (2Н, m, H-4,4')
2
1.41 (6Н, t, J = 7.2, N–CH2CH3); 2.03 (4Н, br. s, β-СН2); 3.57 (4Н, br. s, α-СН2);
4.72 (4Н, q, J = 7.2, N–CH2); 7.76 (2H, t, J = 7.5, H-5,5'); 7.86 (2Н, t, J = 7.5, H-6,6');
8.27 (2H, d, J = 8.1, H-7,7'); 8.40 (2H, d, J = 8.1, H-4,4')
3
2.01 (4Н, br. s, β-СН2); 3.57 (4Н, br. s, α-СН2); 4.60 (6Н, s, N–CH3);
8.07 (2Н, t, J = 6.8, H-6,6'); 8.24 (2Н, d, J = 8.7, H-5,5'); 8.31 (2H, t, J = 6.8, H-7,7');
8.47 (2H, d, J = 8.7, H-8,8'); 8.67 (2H, d, J = 9.5, H-3,3'); 9.21 (2H, d, J = 9.5, H-4,4')
4*
5*
6
2.25 (4Н, br. s, β-СН2); 3.52 (4Н, br. s, α-СН2); 4.12 (6Н, s, N–CH3);
7.73 (2Н, t, J = 6.6, H-7,7'); 7.87 (2Н, t, J = 7.2, H-4,4'); 7.95 (2H, d, J = 6.6, H-6,6');
8.11 (2H, d, J = 7.2, H-8,8'); 8.41 (2H, d, J = 7.8, H-5,5'); 8.57 (2H, d, J = 7.8, H-3,3')
1.79 (2Н, m, γ-CН2); 1.98 (4Н, m, β-СН2); 3.40 (4Н, m, α-СН2); 4.02 (6Н, s, N–CH3);
7.71 (2Н, t, J = 6.6, H-7,7'); 7.86 (2Н, t, J = 7.5, H-4,4'); 7.93 (2H, d, J = 6.6, H-6,6');
8.09 (2H, d, J = 7.5, H-5,5'); 8.39 (2H, d, J = 7.5, H-8,8'); 8.51 (2H, d, J = 7.5, H-3,3')
0.88 (6Н, t, J = 7.5, N–C3H6–CH3); 1.00 (6Н, t, J = 7.8, OC3H6CH3);
1.41 (4Н, hex, J = 7.5, N–C2H4–CH2); 1.55 (4Н, hex, J = 7.5, N–CH2–CH2);
1.88 (8Н, m, О–CH2–(CH2)2); 2.16 (4Н, br. s, β-СН2); 3.61 (4Н, br. s, α-СН2);
4.41 (4Н, t, J = 6.0, N–CH2); 4.66 (4Н, t, J = 7.5, O–CH2); 7.37 (2Н, d, J = 8.4, H-8,8');
8.07 (2Н, t, J = 8.1, H-4,4'); 8.49 (2Н, d, J = 8.1, H-5,5'); 8.75 (2Н, d, J = 7.8, H-7,7');
8.95 (2Н, d, J = 6.9, H-3,3')
7
8
1.69 (12Н, s, С(СН3)2); 3.33 (4Н, s, СН2); 3.90 (6Н, s, N–CH3); 7.32 (2Н, m, H-5,5');
7.36-7.50 (4Н, m, H-6,6' + H-7,7'); 7.62 (2Н, m, H-4,4'); 8.34 (1Н, s, meso-Н)
2.22 (3Н, s, Tоs-CH3); 3.22 (4H, s, CH2); 4.01 (6H, s, N–CH3);
7.05 (2Н, d, J = 7. 0, β-Tоs-Н); 7.45 (2Н, d, J = 7.0, α-Tоs-Н);
7.52 (6Н, m, H-8,8' + H-7,7' + H-6,6'); 7.83–8.27 (6Н, m, H-3,3'+ H-4,4' + H-5,5');
8.52 (1Н, s, meso-Н)
9
0.98 (12Н, m, N–C3H6–CH3, O–C3H6CH3); 1.48 (8Н, m, J = 7.5, N–CH2–(CH2)2);
1.81 (8Н, m, О–CH2–(CH2)2); 3.22 (4H, s, CH2); 4.16 (4H, t, J = 6.0, N–CH2);
4.48 (4H, t, J = 7.5 , О–CH2); 7.09 (2Н, m, H-8,8'); 7.63 (2Н, m, H-7,7');
7.88 (2Н, t, J = 8.1 , H-4,4'); 8.25 (4Н, m, H-5,5' + H-3,3'); 8.31 (1Н, s, meso-Н)
10
12
2.28 (3Н, s, Tоs-CH3); 3.15 (4H, s, CH2–СН2); 4.07 (6H, s, N–CH3);
7.16 (2Н, d, J = 8.0, β-Tоs-Н); 7.50 (2Н, m, H-3,3′ + 2Н, α-Tоs-Н);
7.65 (2Н, d, J = 9.0, H-5,5'); 7.78–7.90 (6Н, m, H-6,6'+ H-7,7' + H-8,8');
7.94 (2Н, d, J = 9.0, H-4,4'); 8.15 (1Н, s, meso-Н)
1.70 (12Н, s, С(CH3)2); 4.04 (6Н, s, N–CH3); 7.18 (2H, s, CH=CH);
7.36-7.43 (2Н, m, H-5,5'); 7.46–7.54 (2Н, m, H-6,6'); 7.65–7.73 (4Н, m, H-7,7'+ H-4,4');
7.95 (1Н, s, meso-Н)
13
14
4.41 (6Н, s, N–CH3); 6.88 (2H, s, CH=CH); 7.66 (2Н, d, J = 8.5, H-3,3');
7.85-8.31 (8Н, m, Аr–H); 8.45 (2Н, d, J = 8.5, H-4,4'); 8.50 (1Н, s, meso-Н)
1.56 (6Н, t, J = 7.0, N–CH2CH3); 4.73 (4Н, q, J = 7.0, N–CH2); 6.85 (2H, s, CH=CH);
7.39 (1Н, s, meso-Н); 7.55 (2Н, t, J = 8.0, H-6,6'); 7.68 (2Н, t, J = 8.0, H-5,5');
8.06 (2Н, d, J = 8.0, H-7,7'); 8.16 (2Н, d, J = 8.0, H-4,4')
16
4.20 (6Н, s, N–CH3); 6,86 (2H, s, CH=CH); 7.39 (1Н, s, meso-Н);
7.55 (2Н, t, J = 8.0, H-6,6'); 7.66 (2Н, t, J = 8.0, H-5,5'); 8.02 (2Н, d, J = 8.0, H-7,7');
8.13 (2Н, d, J = 8.0, H-4,4')
17*
4.32 (6Н, s, N–CH3); 4.55 (2H,s, meso-H); 7.81 (2Н, t, J = 9.0, H-6,6');
7.88-7.97 (2Н, m, H-5,5' + 2Н, СН=СН); 8.13 (2Н, d, J = 9.0, H-7,7');
8.26 (2Н, d, J = 9.0, H-4,4')
_______
* Spectra recorded in DMSO-d6 (compounds 1-3, 6-14, 16), deutero-
trifluoroacetic acid (compounds 4, 5) or in deuteroacetonitrile
(compound 17).
80

(CH2)4
+
N
+
N
+
N
N
–
2 An
–
An
1–4, 6
7–11
Me Me
R
S
+
N
=
+
N
,
+
N
+
N
+
N
,
,
Me
Et
Me
Alk
7
8, 9
10
11
–
–
–
–
–
–
–
–
,
= ClO4
, 11 An = Br
= BF4
,
= TosO
7 An
8, 10 An
9 An
We have recently reported the possible dehydrogenation of the ethylene bridging group in
thiacarbocyanine chromophores to vinylene in the presence of quinones [10]. It was found that this reaction is
general to trimethinecyanines of this type containing different heterocyclic residues. The strongly basic
quinocarbocyanine 10 and medium basicity thiacarbocyanine 11 were dehydrogenated using chloranil in
acetonitrile to give the dyes 13 and 14 respectively. In the case of the low basicity indocarbocyanine 7 only the
use of the more powerful oxidant 2,3-dichloro-5,6-dicyanobenzoquinone led to the preparation of the
carbocyanine 12.
Previously a series of carbocyanines, the trimethylene chromophore of which hides a 1,2-vinylene
group, was obtained by treatment of cyclopentadiene with alkylthio- or bromo-substituted quaternary salts of
nitrogen heterocycles [3, 11].
O
R
R
Cl
Cl
O
X
X
R = Cl, CN
+
N
N
–
Alk
Alk
Z
X
X
7, 10, 11
+
N
N
–
Alk
Alk
Z
12–14
7, 12 Х = CMe2, Alk = Me; 7 Z− = BF4−, 12 Z− = ClO4−; 10, 13 Х = −СН=CH−, Alk = Me,
10 Z− = TosО−, 13 Z− = ClO4−; 11, 14 Х = S, Alk = Et, Z− = Br −
With the aim of clarifying the spatial structure of the thiacarbocyanines with ethylene and vinylene
1
bridging groups in the chromophore we have used the nuclear Overhauser effect (NOE). Because the H NMR
spectra of these compounds show signals which are well resolved and virtually nonoverlapped the interpretation
of the results proved unambiguous. In the spectra of the thiacarbocyanine 15 (prepared according to [4]) a cross
peak is observed corresponding to an NOE between the N-alkyl group protons and the proton in
81

the meso position of the chromophore. On the other hand, in the dye with the vinylene bridge 16 a cross peak is
observed between the signals for the N–CH3 groups and the protons of the vinylene bridge. An interaction
between the bridging group protons and the N-ethyl substituents is observed for thiacarbocyanines containing
both ethylene and vinylene bridges and a chlorine atom or nitrile group in the meso position of the chromophore
[10]. Hence it was shown that the introduction of only an ethylene bridge in the thiacarbocyanine is accompanied
by a change in the conformation of the chromophore from all-trans-E,E,E,E (as found in the analog
unsubstituted in the chromophore) to Z,E,E,Z whereas the introduction of the vinylene bridge does not change
the conformation.
S
S
S
S
+
N
+
N
N
N
–
Br
Me
Me
Me
–
Me
Br
E,E,E,E-16
Z,E,E,Z-15
In the case of the thiacarbocyanine 16 it has been shown by 1H NMR that, by contrast with the saturated
bridging group 15, the addition of a proton to the carbocyanines with a vinylene bridge occurs not at the usual α-
position [12] but at the β-position of the chromophore to form the dications with the symmetrical type structure
17 (compound not separated). Protonation was carried out by trifluoromethane-sulfonic acid in CD3CN
solution. As a result, the signal for the vinylene bridge protons (δ = 6.85 ppm) are shifted to low field (δ = 7.40)
and the signal for the CH group in the meso position of the chromophore (δ = 7.38 ppm) disappears. In place a
signal appears at δ = 4.55 pm for the methylene group. Protonation is accompanied by a 64 nm hypsochromic
shift of the electron spectrum absorption maximum. The addition of the proton to the β-position is associated
with the fact that, in the dye with a vinylene bridge in this position, there is localized a partial negative π-charge
equal to -0.059 (value calculated by PPP type quantum-chemical calculation). The addition of a proton to the
β-position of the chromophore also preserves the chain of conjugation.
H
H
+
S
S
H
+
N
+
16
N
Me
Me
–
2 An
17
The spectroscopic data for the dyes synthesized (Table 4) confirms the phenomenon observed previously
of a depression of the color of trimethinecyanines under the influence of an ethylene and its increase with a
vinylene bridge and also the more powerful effect of the latter [10].
EXPERIMENTAL
1H NMR spectra were recorded on a Varian-300 instrument (300 MHz) using HMDS (δ 0.055) as
internal standard. Electronic absorption spectra were taken on a Shimadzu UV-3100 spectrophotometer. The
purity of the products was monitored by TLC on Silufol UV-254 plates with chloroform as eluent. Compounds
11 and 15 were prepared by a known method [4].
82

TABLE 3. Characteristics of Compounds 1, 3-10, 12-14, 16
Found, %
Calculated, %
——————
Com-
pound
Empirical
formula
mp, °С
Yield, %
С
H
Hal
N (S)
1
3
4
С26H34B2F8N2
С24H26Br2N2
С28H26Br2N2
57.20
56.96
57.29
57.38
6.45
6.25
5.36
5.22
5.16
5.11
5.43
5.58
4.92
5.06
227-228
187-188
220-221
56
29
47
28.74
29.03
5
С29H28B2F8N2
С42H54Cl2N2O10
С27H31BF4N2
С36H30N2O3S
С43H51ClN2O6
С32H30N2O3S
С27H29ClN2O4
С25H23ClN2O4
С23Н21BrN2S2
С21Н17BrN2S2
59.94
60.24
4.77
4.88
—
4.80
4.85
3.34
3.22
6.04
5.96
4.91
5.08
3.85
3.83
5.36
5.26
201-203
199-201
217-218
218-220
211-213
175-176
190-192
212-214
153-154
247-249
30
39
66
81
38
47
82
78
87
89
6
7.97
8.15
7
68.89
68.94
75.77
75.90
71.01
71.00
73.54
73.60
67.42
67.27
66.59
66.46
6.60
6.64
5.30
4.97
7.07
7.08
5.79
6.03
8
9
10
12
13
14
16
6.08
6.04
7.37
7.22
7.86
7.85
17.02
16.94
18.11
17.90
—
—
—
—
—
(13.66)
(13.49)
(14.53)
(14.84)
TABLE 4. Characteristics of the Long Wavelength Absorption Bands of
the Dyes 7-14, 16 in Methanol
λ
max, nm
λmax, nm
(log ε)*
Compound
∆λmax, nm
∆ν, cm-1
(log ε)
7
592 (5.00)
814 (5.21)
892 (5.10)
716 (4.90)
601 (5.20)
482 (4.76)
533 (4.87)
472 (4.91)
469 (4.99)
545 (5.08)
755 (5.10)
821 (5.13)
605 (5.25)
558
47
59
1456
960
8
9
71
969
10
11
12
13
14
16
111
43
2562
1282
2398
2232
2885
3271
–63
-72
-76
-85
554
_______
* Characteristics of the analogs unsubstituted in the chain.
Preparation of bis-quaternary salts 1-6 (General Method).
2,2'-Tetramethylenebis(1,3,3-trimethyl-3H-indolium) Ditetrafluoroborate (1). The Grignard reagent
prepared from metallic magnesium (0.5 g, 20 mmol), 1,4-dibromobutane (2.16 g, 10 mmol), and absolute ether
(10 ml) was added with stirring to a solution of 1,3,3-trimethyloxyindole (3.2 g, 20 mmol) in anhydrous ether
(20 ml). After addition of the solution the reaction mixture was heated for 2 h at 40ºC. Solvent was evaporated at
0ºC under reduced pressure and HBF4 (22%, 12 ml) was added. The precipitate was filtered off and washed with
ether. Compound 1 was recrystallized from acetonitrile.
83

Salts 2-4 were prepared similarly but 20% HBr was added with crystallization from ethanol. In the case
of salt 6 perchloric acid was used with crystallization from methanol. The Grignard reagent in the synthesis of
salt 5 was prepared from 1,5-dibromopentane.
1,3,3-Trimethyl-2-[3-(1,3,3-trimethyl-1,3-dihydroindol-2-ylidene)cyclopent-1-enyl]-3H-indolium
Tetrafluoroborate (7). A mixture of salt 1 (0.23 g, 0.42 mmol), di(ethoxymethyl)acetate (0.13 g, 0.84 mmol),
pyridine (0.03 g, 0.42 mmol), and acetic anhydride (2 ml) were refluxed for 7 min. The reaction mixture was
cooled and treated with ether and water. The precipitate was filtered off, washed with ether, and recrystallized
from 2-propanol.
1-Methyl-2-[3-(1-methyl-1,2-dihydrobenzo[c,d]indolylidene-2)-1-cyclopentenyl]benz[c,d]indolium
p-tolylsulfonate (8). Salt 4 (0.28 g, 0.50 mmol) and methyl tosylate (1.25 g, 6.71 mmol) were fused for 1 h at
100-110ºC. The melt was treated with ether and acetone. The ditosylate salt, di(ethoxymethyl)acetate (0.15 g,
1 mmol), pyridine (0.04 g, 0.5 mmol), and acetic anhydride (2.5 ml) were refluxed for 7 min. The reaction
mixture was cooled and treated with ether and water. The precipitate was filtered, washed with ether, and
recrystallized from methanol.
6-Butoxy-1-butyl-2-[3-(1-butyl-6-butoxybenzo[c,d]indolylidene-2)-1-cyclopentenyl]benz[c,d]indolium
Perchlorate (9). A mixture of the bis salt 6 (0.215 g, 0.25 mmol), N-phenylethoxymethaneimine (0.075 g,
0.50 mmol), pyridine (0.025 g, 0.25 mmol), and acetic anhydride (1 ml) was refluxed for 2 min in a gas flame.
The product was cooled and the precipitate formed was washed with water and ether and then recrystallized from
acetonitrile.
1-Methyl-2-[3-(1-methylquinolinylidene-2)-1-cyclopentenyl]quinolinium p-Tolylsulfonate (10) was
obtained from salt 3 similarly to compound 8 but refluxed with N-phenylethoxymethaneimine in pyridine for
3 min. The carbocyanine was recrystallized from acetonitrile.
1,3,3-Trimethyl-2-[3-(1,3,3-trimethyl-1,3-dihydroindol-2-ylidene)cyclopenta-1,4-dienyl]-3H-indolium
Perchlorate (12). The carbocyanine 7 (0.20 g, 0.43 mmol) was dissolved in acetonitrile (3 ml), 2,6-dichloro-
3,5-dicyanobenzoquinone (0.195 g, 0.86 mmol) added, and the product was refluxed for 3 min. The product was
washed with ether, filtered, washed with toluene and ether, and chromatographed on Al2O3 using chloroform as
eluent. The dye was converted to the perchlorate by dissolving in acetonitrile and addition of an aqueous,
saturated solution of sodium perchlorate and was recrystallized from a mixture of ethanol and 2-propanol (1:1).
1-Methyl 2-[3-(1-methyl-1,2-dihydro-2-quinolinylidene)-1,4-cyclopentadienyl]quinolinium Perchlorate
(13). The carbocyanine 10 (0.20 g, 0.38 mmol) was dissolved in acetonitrile (10 ml) and chloranil (0.177 g, 0.72
mmol) was added. After 30 min the precipitate was filtered off, washed with warm benzene, ether, and
recrystallized from acetonitrile. The dye was converted to the perchlorate using aqueous NaClO4 solution.
3-Ethyl-2-[3-(3-ethyl-2,3-dihydro-1,3-benzothiazol-2-yliden)-1,4-cyclopentadienyl]-1,3-
benzothiazolium Bromide (14) was prepared similarly to cyanine 11 but heated at 100ºC for 30 min and
recrystallized from acetonitrile.
3-Methyl-2-[3-(3-methyl-2,3-dihydro-1,3-benzothiazol-2-ylidene)-1,4-cyclopentadienyl]-1,3-benzothia-
zolium Bromide (16) was prepared similarly to compound 14 from 15.
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