Synthesis of pyridinium betaine azo chromophores

Article abstract of DOI:10.1007/s10593-010-0528-8

New chromophores have been synthesized and investigated containing as acceptor a quaternized pyridine unit with mobile bromine in the structure conjugated with two donor units - the indan-1,3-dione anion and the dihydroxyethylamino group through a π-conju

Full text of DOI:10.1007/s10593-010-0528-8

Chemistry of Heterocyclic Compounds, Vol. 46, No. 4, 2010
SYNTHESIS OF PYRIDINIUM BETAINE
AZO CHROMOPHORES
J. Kreicberga1*, L. Laipniece1, G. Bērziņa1, and V. Kampars1
New chromophores have been synthesized and investigated containing as acceptor a quaternized pyridine
unit with mobile bromine in the structure conjugated with two donor units – the indan-1,3-dione anion and
the dihydroxyethylamino group through a -conjugated spacer.
Keywords: azo compounds, chromophore, indane-1,3-dione, pyridine.
Organic nonlinear optical (NLO) materials have the potential for various applications in the field of optical
signal processing, giving the impulse to design more and more chromophores with enhanced NLO properties [1].
Among the most investigated subjects are azobenzene derivatives particularly "push-pull" azobenzenes with
asymmetric electron distribution because of the substitution pattern at the 4 and 4' positions with electron-releasing
(donor, D) and electron-withdrawing (acceptor, A) groups. Due to their large dipole moments and photochemistry
(trans to cis isomerization), they have been investigated as components for NLO devices, lithography, all-optical
switches, and data storage [2]. Incorporation of the π-deficient pyridine moiety as acceptor by noncovalent
interactions (hydrogen bonds) gives additional advantage to obtain liquid crystals [3], liquid crystalline polymers [4],
proton-responsive photo switches [5], and three-dimensional octupolar structures by complexation to transition
metals [6].
Azopyridine-based chromophores could be bound covalently to functionalized surfaces by reaction with
alkyl- or benzyl halides, simultaneously creating the pyridinium group [7, 8]. Quaternization of pyridine provides a
way of enhancing its acceptor capability. Recently, potential NLO and electro-optical properties have been demonstrated
for several pyridinium intramolecular salts [9, 10], including N-(1,3-dioxoindan-2-yl)pyridinium betaine (IPB) [11].
In order to extend the scope of compounds with NLO activity, we aim to synthesize the dual
chromophore 1 by linking the azopyridine-based chromophore with IPB via the common pyridinium moiety.
The dioxyethylamino substituent in compound 1 makes it useful for synthesis of polymers. The constitution of
molecule 1 can be described by two important mesomeric formulas:
CH2CH2OTHP
O
R
+
N
N
CH2CH2OTHP
CH2CH2OTHP
O
R
CH2CH2OTHP
N
N
N
N
+
N
C
N
R1
O
1 a–c
R1
a R = R1 = Br, b R = Br, R1 = CN, c R = R1 = Ph
THP = tetrahydro-2H-pyran-2-yl
O
_______
* To whom correspondence should be addressed, e-mail: jana@ktf.rtu.lv.
1 Riga Technical University, Faculty of Material Science and Applied Chemistry, Riga LV-1048, Latvia.
__________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 551-559, April 2010. Original
article October 20, 2008.
438
0009-3122/10/4604-0438©2010 Springer Science+Business Media, Inc.

Chromophore 1a was synthesized from 4-aminopyridine (2) as shown in the following scheme:
Br
Br
Br
O
P
HNO3/H3PO4
NBS, CCl4
48 h
+
+
N2
N
HO
N
NH2
N
NH2
iso-C5H11ONO
–
NO3
O
Br
Br
4
2
3
N(CH2CH2OH)2
Py
N
N(CH2CH2OH)2
N
N
5a
Br
O
O
N
CN
CN
R
O
Br
Br
N
N(CH2CH2OTHP)2
O
7
N
N
N
N(CH2CH2OTHP)2
N
1a
6 a–d
R1
5b
6a R = Br, R1 = CN; b R = R1 = CN; c R = R1 = Ph; d R = CN, R1 = Ph
Compound 2 was brominated with NBS in CCl4 [12] to give 3,5-dibromopyridine-4-amine (3).
Diazotization of compound 3 was performed by altering the procedure used previously [8] for diazotization of
compound 2, which allowed the formation of amphionic structure 4, possessing a higher electron density at C-4
in comparison with the protonated diazonium salt, and higher stability [13]. Compound 4 was generated by iso-
amylnitrite in a mixture of H3PO4 and HNO3, then coupled with N,N-bis(2-hydroxyethyl)aminobenzene in
pyridine to yield azo compound 5a, which was converted to tetrahydro-2H-pyran-2-yl derivative 5b under
standard conditions.
The bromine atoms in compound 5 activated through azophenyl substitution are capable of gradual
substitution: in the reaction of compound 5b with 1 eq. of Zn(CN)2 under catalysis of CuCN, compound 6a was
obtained as the main product, but with 2 eq. of CuCN – compound 6b. Palladium-catalyzed bis(arylation) of 5b
and 6a with phenylboronic acid using Suzuki's procedure yielded compounds 6c and 6d, respectively.
In order to obtain compound 1, we examined two methods available in the literature for synthesis of
indan-1,3-dione pyridinium betaine [11]: 1) Based on condensation of phthalic anhydride with N-(carboxy-
methyl)pyridinium salts in the presence of Ac2O and triethylamine; 2) Based on reaction of pyridine with
2-dicyanomethyleneindan-1,3-dione oxide (7). To apply the first method, the N-(carboxymethyl)pyridinium
derivative of compound 5 was required. In several attempts to prepare it by the reaction of ethyl or tert-butyl
2-iodo- or 2-bromoacetate with compound 5a, only mixtures of very hygroscopic salts were obtained; therefore
the second method for synthesis of compound 1a was used.
Reaction of compound 5b with oxide 7 proceeded in boiling MeCN for 45 h, and good yields of compound
1a were obtained. Similarly, reaction of oxide 7 with azo compound 6c resulted in chromophore 1c. However,
compound 6b failed to react with oxide 7, which can be ascribed to the low basicity of the pyridyl group due to the
presence of several strong acceptors in the molecule. In the case of the reaction of compound 6a with oxide
439

7, low yields of compound 1b were obtained together with starting material 6a. TLC chromatography allowed
monitoring the partial conversion of compound 6a into compound 1b, which during heating lost the indan-
1,3-dione fragment and gave back compound 6a. No sign of decomposition in solutions of compounds 1a and
1c could be observed at temperatures below 100C in the absence of air oxygen.
With better yield, compound 1b was prepared by treatment of compound 1a with Zn(CN)2 under the
catalysis of CuCN.
Zn(CN)2/CuCN
1b
+
1a
6a
DMF, 90oC
As a side product, compound 6a was formed, which confirmed the low thermal stability of compound
1b in solution. If compound 1a was allowed to react with 2 eq. of CuCN in DMSO at 80C, only compound 6b
was isolated from the reaction mixture.
O
CN
CN
N
N(CH2CH2OTHP)2
CuCN
2 eq. CuCN
+
N
6b
1b
1a
+
C
N
DMSO, 80oC
DMSO, 80°C
O
In the crystalline state compounds 1a-c are stable. In solution they are subjected like IPB [11] to
photochemical oxidation by air.
The new chromophores 1a-c, 5a,b, and 6a-d were characterized by elemental analysis (Table 1),
1H NMR (Table 2 and 3), UV-vis spectra (Table 4 and 5), and MS. All data were consistent with the assigned
structures.
In the visible region of the UV-vis spectrum, compounds 5b and 6a-d, like other D- conjugated
system-A type or "push-pull" chromophores, exhibit an intense charge-transfer band with maxima in EtOH at
427 to 543 nm depending on the electron-releasing or -withdrawing character of substituents in the pyridine ring
and showing a positive solvatochromic effect at 10-20 nm (from PhMe to EtOH).
TABLE 1. Physical and Analytical Data of Azo Compounds 5a,b and 6a-d
and Betaines 1a-c
Found, %
——————
Calculated, %
Com-
pound
Empirical
formula
mp, oC
Yield, %
C
H
N
5a
5b
6a
6b
6c
6d
1a
1b
1c
C15H16Br2N4O2
C25H32Br2N4O4
C26H32BrN5O4
C27H32N6O4
41.09
40.57
49.28
49.03
56.04
55.92
63.95
64.27
72.80
73.24
69.12
69.17
54.08
53.98
59.73
59.83
3.84
3.63
5.23
5.27
6.23
5.78
6.32
6.39
6.23
6.98
6.32
6.71
4.95
4.80
5.08
5.16
12.51
12.62
9.14
9.15
12.03
12.54
16.24
16.66
8.76
9.23
12.43
12.60
7.48
7.41
9.89
9.97
166-169
100-102
108-110
109-111
78-80
69
83
33
45
64
43
48
45
43
C37H42N4O4
C32H37N5O4
68-72
C34H36Br2N4O6
C35H36BrN5O6
C46H46N4O6
184-185
211
73.13
73.58
6.03
6.17
7.36
7.46
157-158
440

TABLE 2. 1H NMR Spectral Data of Synthesized Azo Compounds 5b and 6a-d
R
a
OTHP
a' b'
N
N
N
N
b
R1
a' b'
OTHP
Com-
pound
Chemical shifts of protons (CDCl3), δ, ppm ( J, Hz )
Ha, s
8.59
Hb, s
Ha', d
7.83
Hb', d
6.79
OCHO, t
Other protons
5b
6a
6b
6c
8.59
8.70
8.89
8.59
4.54
(J = 3.4)
3.95-3.38 (12H, m),
1.70-1.47 (12H, m)
(Ja',b' = 9.2)
(Jb',a' = 9.2)
8.84
8.89
8.59
7.92
(Ja',b' = 9.2)
6.80
(Jb',a' = 9.2)
4.53
(J = 3.5)
3.94-3.40 (12H, m),
1.71-1.50 (12H, m)
7.99
(Ja',b' = 9.3)
6.85
(Jb',a' = 9.3)
4.53
(J = 3.7)
4.03-3.46 (12H, m),
1.76-1.47 (12H, m)
7.35
(Ja',b' = 9.3)
6.65
(Jb',a' = 9.3)
4.55
(J = 3.7)
7.72-7.17 (10H, m),
3.92-3.37 (12H, m),
1.76-1.51 (12H, m)
6d
8.81
8.77
7.73
(Ja',b' = 9.2)
6.73
(Jb',a' = 9.2)
4.52
(J = 3.7)
7.45-7.35 (5H, m),
3.91-3.39 (12H, m),
1.76-1.46 (12H, m)
TABLE 3. 1H NMR Spectral Data of Synthesized Compounds 1ac
R
O
O
c
a
+
N
d
d
e
f
O
O
C
N
N
N
b
c
R1
O
f
e
O
Chemical shifts of protons (CDCl3), δ, ppm ( J, Hz )
Com-
pound
OCHO,
br.
Other
protons
Ha
Hb
Hc, m
Hd, m
He, d
Hf, d
1a
1b
1c
10.48 (s) 10.48 (s) 7.53-7.47 7.45-7.39 7.85
6.82
4.53
4.51
4.49
3.96-3.40
(12H, m),
1.68-1.47
(12H, m)
(J = 9.2) (J = 9.2)
10.56
(d,
10.45
(d,
7.55-7.52 7.46-7.41 7.97
6.61
3.96-3.38
(12H, m),
1.68-1.47
(12H, m)
(J = 9.3) (J = 9.3)
J = 1.2)
J = 1.2)
10.02 (s) 10.02 (s) 7.51-7.44 7.42-7.35 7.25
6.60
7.31
(J = 9.2) (J = 9.2)
(10H, br.),
3.87-3.37
(12H, m),
1.69-1.45
(12H, m)
Compounds 1ac could be considered as D A+- conjugated system-D type chromophores. They exhibit
two absorption bands: I) A band near 420-460 nm, which can be assigned [11] to electron transfer from the weak
donor, indan-1,3-dione anion, to pyridinium cation; II) A more intense band near 520-675 nm associated with charge
transfer from the strong dioxyethylamino donor to the pyridinium cation (for example, λmax of 4-dimethylamino-
1-(pyridine-4-ylazo)benzene is 580 nm in benzene [14]). The second maximum is red-shifted by 140 (1a), 169 (1b),
and 103 nm (1c) in comparison with azo compounds 5b, 6a, and 6c, giving evidence of increased charge
441

TABLE 4. UV-vis spectra of synthesized Azo Compounds 5a,b, 6a-d and
Betaines 1a-c
Solvatochromic effect
λmax EtOH  λmax PhMe
Com-
pound

max, nm (log  ), in ethanol

max, nm, in PhMe
5b
6a
6b
6c
6d
1a
269 (3.95)
288 (3.94)
295 (4.07)
270 (4.03)
265 (4.08)
246 (4.66),
284 (4.35)
250 (4.50),
297 (4.29)
246 (4.73),
300 (4.18)
427 (4.34)
506 (4.46)
543 (4.62)
418 (4.43)
484 (4.45)
567 (4.68),
466 (sh, 4.49)
674 (4.71),
418 (4.15)
521 (4.64)
417
10
20
20
11
17
8
486
523
407
467
559, 459 (sh)
1b
1c
657, 426
18
522, 442 (sh)
1
TABLE 5. Solvent Effect on the Visible Absorption Band λmax of
Compounds 1a-c
λmax, nm
Compound
CCl4
PhMe
AcOEt
MeCN
EtOH
1a
1b
1c
459, 557
432, 657
439, 522
459, 559
426, 657
442, 522
459, 559
418, 663
438, 520
462, 562
419, 669
438, 517
466, 567
418, 674
521
delocalization in the pyridinium betaine molecule. The UV-vis spectrum of compound 1b differs from the
spectra of compounds 1a and 1c not only in the position of the absorption bands (the absorption band I is weak
and shows a negative solvatochromic effect), but by the intense positive solvatochromic effect of the absorption
maximum II (+18 nm from PhMe to EtOH). The absorption bands of compound 1a show a solvatochromic
effect (+8 nm from PhMe to EtOH), but both absorption bands of compound 1c do not exhibit solvent
dependence. It can be concluded that incorporation of the cyano substituent in the pyridine unit of the betaine
molecule caused substantial changes in the electronic structure of betaine 1b, possibly due to the contribution of
an additional important mesomeric structure:
N
+
N
O
C
CH2CH2OTHP
CH2CH2OTHP
N
+
N
N
Br
O
The existence of two close negative charges in this structure also explains the observed tendency of
betaine 1b in solution and thermally to split off the indan-1,3-dione fragment.
Studies about the NLO properties of obtained chromophores are in progress.
EXPERIMENTAL
2-Dicyanomethylideneindane-1,3-dione oxide was obtained in accordance with [15]. All other starting
materials were purchased from Acros. The purity of all compounds was checked by TLC on Merck
442

F254 silica plates. The spots were visualized when necessary in UV light and in iodine vapor. Chromatographic
separations were carried out on silica gel (Merck, reinst) or Biotage SP1 HPLC using Biotage silica gel cartridges.
Melting points were taken on an SMP 10 Stuart apparatus and 1H NMR spectra were obtained on a Varian Mercury
BB 200 MHz spectrometer against TMS as internal reference, and elemental analysis on a VARIO EL III CHNOS
Elemental Analyzer. UV-vis spectra were recorded using a Lambda 35 Perkin–Elmer UV/VIS spectrometer.
Waters Alliance 2695 HPLC was used with an XTerra® MS C18 column (2.1×100 mm, particle size 5 µm) and a
Waters EMD 1000 MS detector; mass spectra were obtained in the ESI+ mode, cone voltage 30 V.
The properties of the main products are presented in Tables 1-5.
2,5-Dibromopyridine-4-amine (3). To a suspension of 4-aminopyridine (2) (4.7 g, 0.05 mol)
N-bromosuccinimide (17.8 g, 0.1 mol) is added portionwise with intensive stirring. Stirring is continued at room
temperature in the dark for 48 h, and then the solvent is evaporated under reduced pressure to obtain a mixture
of succinimide, 4-amino-2-bromopyridine, and compound 2. The mixture is extracted with 300 ml of CCl4 in a
Soxhlet extractor for 2 h. After evaporation of the CCl4, a white precipitate (9.33 g) is obtained, which is
recrystallized from 45 ml of EtOH. Yield 6.9 g (55%); mp 167-170C [16]. Found, %: C 23.64; H 2.12; N 11.11.
C5H4Br2N2. Calculated, %: C 23.80; H 1.58; N 11.11.
4-[N,N-Bis(2-hydroxyethyl)amino]-1-(3,5-dibromopyridin-4-ylazo)benzene (5a). Compound 3 (6 g,
0.024 mol) is added to a cooled mixture (-5C) of H3PO4, 85% (19 ml) and HNO3, 65% (13 ml). The mixture is
stirred till complete dissolution (10 to 20 min) and cooled to -10C. Isoamyl nitrite (3.3 ml, 0.024 mol) is added
during 15 min and stirring continued for 45 min at -10C to -5C. The obtained solution is added dropwise with
intensive stirring to the cooled solution of N,N-bis(2-hydroxyethyl)aminobenzene (4.5 g, 0.024 mol) in pyridine
(40 ml). Simultaneously, ice (~80 g) is added in small pieces, and the temperature during addition is kept below
10C. Stirring is continued for 1 h, and the temperature ia allowed to rise to room temperature. The precipitate is
separated by filtration, washed with water, and dried. The crude product is boiled with EtOAc (20 ml), cooled, and
1
the orange crystals filtered off. Yield 7.4 g. H NMR spectrum (DMSO-d6), δ, ppm (J, Hz): 3.55 (8H, br.); 4.82
(2H, br.); 6.85 (2H, d, J = 9.1); 7.72 (2H, d, J = 9.1); 8.73 (2H, s).
4-[N,N-Bis(2-(tetrahydro-2H-pyran-2-yloxyethyl)amino]-1-(3,5-dibromopyridin-4-ylazo)benzene (5b).
To a solution of compound 5a (7.4 g, 0.016 mol) in 15 ml DMF 3,4-dihydro-2H-pyrane (5.2 ml, 0.048 mol) is
added. The solution is stirred at 40 to 45C for 24 h, diluted with water, and extracted with tert-butyl methyl
ether. The organic layer is washed with water, dried over Na2SO4, and evaporated. The crude product is
recrystallized from EtOH. Yield 8.5 g, light-red crystals.
4-[N,N-Bis(2-(tetrahydro-2H-pyran-2-yloxyethyl)amino]-1-(3-bromo-5-cyanopyridin-4-ylazo)-
benzene (6a). To a solution of compound 5b (4 g, 6.5 mmol) in 12 ml of DMF Zn(CN)2 (0.78 g, 6.5 mmol) and
10 mg of CuCN are added under Ar. The mixture is stirred at 90C for 1 h; after cooling, ice is added. The oily
solid phase is separated by filtration below 5C, then dissolved in the minimum amount of solvent mixture
(diisopropyl etheracetone, 3:1) and chromatographed on silica gel using the same solvent mixture as eluent.
Two fractions are obtained  1.2 g compound 6a with Rf 0.73 as violet-black crystals, and 0.7 g compound 6b
with Rf 0.66. After evaporation of the solvent, both are recrystallized from EtOH.
4-[N,N-Bis(2-(tetrahydro-2H-pyran-2-yloxyethyl)amino]-1-(3,5-dicyanopyridin-4-ylazo)benzene
(6b). CuCN (188 mg, 2.1 mmol) is added to a solution of compound 5b (0.6, 1 mmol) in 4.5 ml DMSO, and the
mixture is stirred at 95 to 100C for 4 h under Ar. The reaction mixture is poured onto a mixture of 20 g ice and
ethane-1,2-diamine (140 μl, 2.1 mmol), stirred, and filtered. The crude product is purified by silica gel
chromatography with tert-butyl methyl ether as eluent. Yield 0.22 g, green crystals.
4-[N,N-Bis(2-(tetrahydro-2H-pyran-2-yloxyethyl)amino]-1-(3,5-diphenylpyridin-4-ylazo)benzene
(6c). A mixture of compound 5b (1g, 1.6 mmol), phenylboronic acid (0.6 g, 4.8 mmol), K2CO3 (1.6 g), H2O
(1.25 ml) and Pd(PPh3)4 (115 mg) in 10 ml of toluene is stirred and heated at 110C under Ar for 4 h, cooled to
room temperature, diluted with water, and extracted with toluene. The organic phase is dried (Na2SO4) and
evaporated to yield a yellow oil. Compound 6c is extracted from the oil with boiling hexane (200 ml) and
purified by flash chromatography (hexanetert-butyl methyl ether, 1:5).
443

4-[N,N-Bis(2-(tetrahydro-2H-pyran-2-yloxyethyl)amino]-1-(3-cyano-5-phenylpyridin-4-ylazo)-
benzene (6d) is obtained from compound 6a by an analogous procedure.
2-[4-N,N-Bis(2-(tetrahydro-2H-pyran-2-yloxyethyl)aminophenylazo-3,5-dibromopyridinium-1-yl]-
1,3-dioxo-2,3-dihydro-1H-inden-2-ide (1a). Compound 7 (0.22 g, 1 mmol) is added portionwise to a solution
of compound 5b (0.61 g, 1 mmol) in 5 ml of MeCN at 80C under Ar. The mixture is boiled for 4-5 h, cooled,
and the precipitate is filtered off and recrystallized from MeCN. Yield 0.36 g, green crystals, purity 96% (by
HPLC-MS). This product could be further purified by silica gel column chromatography with a mixture of
dichloromethaneacetone, 20:1 as eluent. Mass spectrum (ESI+), m/z: 723.9 [M+H]+, 726.0 [M+H]+, 727.8
[M+H]+, 580.0 [M-Ind]+, 582.0 [M-Ind]+, 583.9 [M-Ind]+.
2-[4-N,N-Bis(2-(tetrahydro-2H-pyran-2-yloxyethyl)aminophenylazo-3-bromo-5-cyanopyridinium-
1-yl]-1,3-dioxo-2,3-dihydro-1H-inden-2-ide (1b). A mixture of compound 1a (0.48 g, 0.63 mmol), Zn(CN)2
(40 mg, 0.34 mmol), and CuCN (3 mg) in 10 ml of DMF is heated at 90C under Ar for 2 h, cooled, diluted with
H2O, and refrigerated overnight at 0-5C. The crude product is filtered of, dissolved in a mixture of
dichloromethaneacetone, 20:1, and purified by silica gel column chromatography with the same solvent
mixture as eluent. The product (Rf 0.28) is washed with Et2O to remove compound 6a. Yield 0.2 g, green
crystals with gold lustre. Mass spectrum (ESI+), m/z: 702.3 [M+H]+, 704.2 [M+H]+, 558.2 [M-Ind]+, 560.2
[M-Ind]+, 474.1 [M-Ind-THP]+, 476.0 [M-Ind-HP]+.
2-[4-N,N-Bis(2-(tetrahydro-2H-pyran-2-yloxyethyl)aminophenylazo-3,5-diphenylpyridinium-1-yl]-
1,3-dioxo-2,3-dihydro-1H-inden-2-ide (1c). Compound 7 (0.33 g, 1.5 mmol) is added portionwise to a solution
of compound 6c (0.9 g, 1.5 mmol) in 18 ml of MeCN at 80C under Ar. The mixture is boiled for 4-5 h, cooled,
diluted with Et2O, and refrigerated overnight at 0-5C. The crude product is filtered off and purified by silica gel
column chromatography with a mixture of dichloromethaneacetone, 10:1, as eluent. Rf 0.69. Yield 0.48 g,
violet-brown crystals. Mass spectrum (ESI+), m/z: 751.5 [M+H]+, 607.5 [M-Ind]+.
REFERENCES
1.
2.
3.
4.
5.
6.
7.
J. Delaire and K. Nakatani, Chem. Rev., 100, 1817 (2000).
K. Jager and C. Barret, J. Photochem. Photobiol. A: Chem., 182, 250 (2006).
V. Mallia, P. Antharjanam, and S. Das, Liq. Cryst., 30, 135 (2003).
D. Rogness, P. Riedel, J. Sommer, D. Reed, and K. Wiegel, Liq. Cryst., 33, 567 (2006).
J. Otsuki and K. Narataki, Bull. Chem. Soc. Jpn., 77, 1537 (2004).
T. Renouard, H. Le Bozec, S. Brasselet, and I. Ledoux, J. Zyss, Chem. Commun., 871 (1999).
A. Facchetti, L. Beverina, M. Van der Boom, P. Dutta, G. Evmenenko, A. Shukla, C. Stern, G. Pagani,
and T. Marks, J. Am. Chem. Soc., 128, 2142 (2006).
8.
9.
10.
11.
12.
W. Lin, W. Lin, G. K. Wong, and T. J. Marks, J. Am. Chem. Soc., 118, 8034 (1996).
N. Nemoto, J. Abe, F. Miuata, Y. Shirai, and Y. Nagase, J. Mater. Chem., 7, 1389 (1997).
T. Kolev, D. Yancheva, B. Stamboliyska, M. Dimitrov, and R. Wortmann, Chem. Physics, 348, 45 (2008).
O. Neilands and I. Muzikante, Organic Nanophotonics, 447 (2003).
V. Canibao, J. Rodrigues, M. Santos, M. Sanz-Tejedor, M. Carreno, G. Gonzalez, and J. Garcia-Ruano,
Synthesis, 2175 (2001).
13.
14.
A. Razus, L. Birzan, S. Nae, S. Razus, V. Cimpeanu, and C. Stanciu, Synth. Commun., 32, 825 (2002).
H. Li, D. Zhou, C. Huang, J. XU, T. Li, X. Zhao, and X. Xia, J. Chem. Soc., Faraday Trans., 92, 2585
(1996).
15.
16.
I. K. Raiskuma, G. G. Pukitis, and O. Ya. Neilands, Khim. Geterotsikl. Soed., 889 (1978). [Chem.
Heterocycl. Comp., 14, 714 (1978)].
W. Paudler and M. Jovanovic, J. Org. Chem., 48, 1064 (1983).
444