Synthesis of polymethine dyes possessing nonlinear optical properties and preparation of photoconducting systems based thereupon

Article abstract of DOI:10.1023/B:RUGC.0000025515.86256.8e

Reactions of 2,3,3-trimethyl-3H-indolium and 2-methylquinolinium quaternary salts with o- and p-hydroxybenzaldehydes afforded polymethine dyes (styryls) which were isolated for the first time as the corresponding tetrafluoroborates. The dyes can redily be

Full text of DOI:10.1023/B:RUGC.0000025515.86256.8e

Russian Journal of General Chemistry, Vol. 74, No. 2, 2004, pp. 271 275. Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 2, 2004,
pp. 304 308.
Original Russian Text Copyright
2004 by Khokhlova, Lebedeva, Lukoshkin, Bykova.
Synthesis of Polymethine Dyes
Possessing Nonlinear Optical Properties and Preparation
of Photoconducting Systems Based Thereupon
L. V. Khokhlova, G. K. Lebedeva, V. A. Lukoshkin, and L. M. Bykova
St. Petersburg State Institute of Technology, St. Petersburg, Russia
Institute of High-Molecular Compounds, Russian Academy of Sciences, St. Petersburg, Russia
Ioffe Physical and Technical Institute, Russian Academy of Sciences, St. Petersburg, Russia
Received December 27, 2002
Abstract Reactions of 2,3,3-trimethyl-3H-indolium and 2-methylquinolinium quaternary salts with o- and
p-hydroxybenzaldehydes afforded polymethine dyes (styryls) which were isolated for the first time as the
corresponding tetrafluoroborates. The dyes can redily be deprotonated by the action of alkali to obtain neutral
merocyanine species. Chemical, photophysical, and optical properties of the products were studied. All the
obtained dyes were found to exhibit second-order nonlinear optical activity; in combination with 2,3-dimethyl-
indole, they are also capable of photochemically generating mobile charge carriers.
Organic photorefractive compounds constitute
a new promising type of materials for quantum
electronics and nonlinear optics. First representatives
of such compounds have been reported in early 1990s.
Their functioning is based on photorefraction, i.e.,
modulation of the refractive index of a substance
(medium) under irradiation [1]. Polymeric compounds
exhibit photorefractive properties when the following
conditions are met simultaneously: nonlinear optical
activity and photoconductivity [2]. As a rule, this is
achieved by introduction into a polymeric matrix of
chromophoric units possessing second-order nonlinear
optical activity and units capable of effecting photo-
induced charge generation and transfer. Just the above
approach to design of photorefractive materials is
employed in most studies and publications on this
topic [3, 4]. According to published data, the most
widely used nonlinear optical chromophores are
4-nitrobenzazole derivatives, while photoconducting
properties are attached using compositions on the
basis of carbazole derivatives [e.g., poly(vinylcarba-
zole)] [5, 6]. We tried to obtain a chromophore pos-
sessing both the properties required. As subjects for
study we selected polymethine dyes (cyanines) which
are characterized by an extended delocalized -elec-
tron system, high polarizability, and bond leveling [7]
and are capable of forming charge-transfer complexes.
While our synthetic work being in progress, data in
support of our expectations have been published.
Hwang et al. [8] and Jung et al. [9] reported on photo-
refractive properties of materials containing a single
chromophore, namely 6-[3-(6-nitrobenzoxazol-2-yl)-
indol-1-yl]-1-hexanol [8] and 1-triphenylamino-2-
(p-nitrophenyl)ethylene [9].
Our present work reports on the synthesis of poly-
methine dyes based on 2,3,3-trimethylindolium and
2-methylquinolinium quaternary salts and their
chemical, photophysical, and optical properties.
Quaternary 2,3,3-trimethylindolium and 2-methyl-
quinolinium salts were prepared by alkylation of the
corresponding free bases, 2-methylquinoline and
2,3,3-trimethyl-3H-indole, with methyl p-toluene-
sulfonate and methyl benzenesulfonate. 1,2,3,3-Tetra-
methyl-3H-indolium benzenesulfonate was converted
into previously unknown tetrafluoroborate IV. The
use of quaternary salts as one of the components in
the synthesis of dyes based on 2-methylquinoline is
a common matter, while they are not used typically in
the synthesis of dyes on the basis of 1,2,3,3-tetra-
methylindolium. In the latter case, Fischer’s aldehyde
and 2-methylenedihydroindole [10, 11] are generally
used instead. We have synthesized dyes V VIII by
condensation of quaternary salts I and IV with o- and
p-hydroxybenzaldehydes (Scheme 1).
All the obtained dyes exhibit solvatochromic pro-
perties. Table 1 contains their electronic absorption
spectra. The dyes with a hydroxy group in the para
position of the benzene ring (compounds II and VI)
and their merocyanine forms III and VIII are charac-
terized by distinct splitting of the long-wave absorp-
tion band. In the spectrum of III, two absorption
1070-3632/04/7402-0271 2004 MAIK Nauka/Interperiodica

272
KHOKHLOVA et al.
Scheme 1.
OHC Ph OH
+
+
CH3
CH=CH
BF4
OH
N
N
CH3PhSO3
CH3
CH3
I
II
NaOH
O
CH CH
N
CH3
III
CHO
R1
H3C
H3C
CH3
CH3 +
BF4
CH3
R2
R1
CH=CH
BF4
CH3
+
N
+
N
R2
CH3
IV
V, VI
H3C
H3C
CH3
CH CH
N
O
CH3
NaOH
VII
CH3
O
CH CH
N
CH3
VIII
V, R1 = H, R2 = OH; VI, R1 = OH, R2 = H.
maxima of approximately equal intensities are present.
The origin of these maxima is a matter of discussion.
Some authors [12] believe that the observed two long-
wave maxima belong to two stereoisomers (cis and
trans). According to the other authors [13], the corre-
sponding dimers are formed in solution. However, the
formation of dimers seems to be hardly probable, for
the spectral pattern does not depend on the dye con-
centration. On the other hand, the formation of stereo-
isomers in the transformation of merocyanine struc-
ture III into dipolar (upon absorption of a quantum of
light) is quite possible. Insofar as the band intensities
are similar, the formation of both the cis and trans
isomer is equally probable. In the electronic absorp-
tion spectrum of cationic dye II, one long-wave maxi-
mum is more intense; this syggests that one of the iso-
mers prevails. Presumably, the major isomer is trans,
for trans isomers usually absorb at longer wave-
lengths as compared to the respective cis isomers [14].
Table 1. Spectral parameters of 2,3-dimethylindole and
compounds I IV and VI VIII
Compound (no.)
Solvent
max, nm (log )
2,3-Dimethyl-
2-Propanol
230 (4.49), 280 (3.87)
indole
I
320
(4.95)
II
III
430 (3.9), 582 (4.5)
437 (4.83), 580 (4.86)
447 (4.75), 580 (4.79)
Both cationic dye II and its merocyanine form III
show luminescence properties independendly at each
of the absorption maxima. The absorption maxima at
447 and 580 nm in the spectrum of III correspond
to the luminescence maxima at 532 and 630 nm,
Chloroform
2-Propanol
IV
VI
VII
VIII
280
515 (3.52), 548 (3.88)
455 (3.14)
(5.1)
515 (4.69), 547 (5.05)
510 (4.57), 544 (4.98)
respectively, while the absorption maxima at
and 570 nm in the spectrum of II correspond to the
luminescence maxima at 484 and 622 nm. The
420
Chloroform
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 74 No. 2 2004

SYNTHESIS OF POLYMETHINE DYES
273
luminescence efficiency is low. The luminescence
quantum yields reduced to the corresponding absorp-
Table 2. Light sensitivity at the absorption maximum of
polymeric films containing charge-transfer complexes
3
3
tion intensity are 5.3 10 and 6.4 10 (III) and
2
3
2.1 10
and
1.0 10
(II).
Independent
max, nm
(log )
(2-propanol)
S0.1 105,
luminescence at each absorption maximum of dyes II
and III provides an additional support for the
existence of stereoisomeric structures.
Molecular complex
cm2/J
1,2,3,3-Tetramethyl-3H-
indolium tetrafluoroborate
2,3-dimethylindole
1,2-Dimethylquinolinium
p-toluenesulfonate 2,3-di-
methylindole
1-Methyl-2-(4-hydroxystyryl)- 563 (2.13)
quinolinium tetrafluoroborate
2,3-dimethylindole
550 (2.94)
500 (3.32)
1
All the prepared dyes in crystal possess second-
order nonlinear optical activity with the following
(2)
9
coefficients (
10 , SGSE): 0.22 (II), 2.40 (III),
0.5
1.2
0.98 (VI), 0.16 (VII), 1.50 (VIII). It is seen that
the activity of merocyanine dyes is higher by an order
of magnitude and is comparable with the efficiecy of
(2)
second harmonic generation of urea (
[15].
= 3.40 109)
It is known that a chromophoric group responsible
for photoconductivity of a polymer should be capable
of effectively generating mobile charge carriers upon
absorption of a quantum of light [16]. Such an ability
is typical of charge-transfer complexes which are
EXPERIMENTAL
1
The H NMR spectra were recorded on a Bruker
AC-200 spectrometer (200.1 MHz); the chemical
shifts were measured relative to signals from residual
protons in the solvent. The electronic absorption spec-
tra were determined on a Specord M-400 spectrophoto-
meter. The luminescence spectra were measured using
a setup described in [21]. Second harmonic generation
was effected using an YAG-Nd3+ pulse laser operat-
ing in the modulated quality factor mode (wavelength
1.06 m, pulse width 10 ns). The pulse energy was
varied over a wide range (up to 30 mJ). The laser
beam was adjusted to a Gauss profile via selection
of transversal modes with the aid of a diaphragm built
into the resonator. A part of the irradiation energy
(4%) was split with a beam splitter and was passed
to an FD-24K photodiode to monitor the exciting
radiation energy. The main part of p-polarized radia-
tion was focused onto a sample with a lense (F =
100 mm). The second-harmonic radiation was directed
to an MDR-2 gitter monochromator and then to
an FEU-106 multiplier. Electric signals from the two
photoreceivers were sent to integrating AD trans-
ducers and (via CAMAC interface) to a PC for further
processing.
formed between
-electron-rich and
-electron-
deficient systems [17]. Using quaternary salts I and
IV and dyes derived therefrom as electron-acceptor
components and 2,3-dimethylindole as donor, we
obtained molecular charge-transfer complexes which
gave rise to electronic absorption bands in the visible
region (no such absorption was observed in the
spectra of the initial compounds). The ability of these
complexes to generate mobile charge carriers was
estimated by studying electrophotographic sensitivity
of polymeric films [on the basis of poly(butyl
methacrylate)] containing the complexes. Table 2
contains absorption maxima of the charge-transfer
complexes and light sensitivities of an S0.1 of
polymeric layers at the absorption maximum, which
were determined from the decrease in the surface
potential (by 10%) on exposure to white light. The
quantum yield of photogeneration of charge carrier
for all the examined charge-transfer complexes was
0.1 0.2. The rate of surface potential drop in the dark
ranges from 1 to 3%/s, indicating good dielectric
properties of the polymeric layers. It should be noted
that the light sensitivity of polymeric films containing
the most popular donor acceptor complexes based on
carbazole derivatives of fluorenone (e.g., trinitro-
fluorenone) is about 105 106 cm2/J, while the
maximal experimental value of is 0.4 [18 20].
Photoinduced surface potential drop was measured
as follows. Polymeric films were prepared from solu-
tions of poly(butyl methacrylate) (M = 2.5 106) and
charge-transfer complexes in cyclohexanone, which
were applied to glass plates coated with a transparent
conductor layer (ITO In2O3/SnO2). The concentra-
tion of the complexes was about 15 wt %. The glass
plate-supported films were dried for 24 h at 50 C and
kept for 12 h at 90 C. The light sensitivity and
quantum yields for generation of charge carriers were
measured in the electrophotographic mode according
Thus, the synthesized polymethine dyes exhibit
second-order nonlinear optical properties and are
capable of generating mobile charge carriers on
exposure to light, which makes them promising as
chromophoric species for photorefractive materials.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 74 No. 2 2004

274
KHOKHLOVA et al.
to the procedures described in [19, 20] at the Vavilov
State Optical Institute (St. Petersburg, Russia).
ethanol was heated for 2.5 h under reflux. The precip-
itate was filtered off and dispersed in 100 ml of
a 0.2 M solution of KOH, the suspension was stirred
at 70 72 C until the dye dissolved, and the precipitate
was filtered off and washed with water until neutral
washings. Yield 0.82 g (85%), mp 205 C; published
data [26]: mp 208 C.
2,3-Dimethylindole was prepared by the procedure
reported in [22]. Yield 91%, mp 109 C [23].
1,2-Dimethylquinolinium p-toluenesulfonate (I)
was prepared by fusion of equimolar amounts of
2-methylquinoline and methyl p-toluenesulfonate at
120 C (reaction time 5 min). Yield 81%, mp 56 C.
6-[2-(1,3,3-Trimethyl-2,3-dihydro-1H-indol-2-
ylidene)ethylidene]-2,4-cyclohexadienone (VII).
Yield 83%, mp 163 C. 1H NMR spectrum
(DMSO-d6), , ppm: 1.2 d [6H, C(CH3)2], 4.1 s (3H,
NCH3), 5.7 d (1H, CH), 6.6 d (1H, CH ), 6.8 m
(2H, quinone ring), 6.9 7.3 m (4H, indole), 7.6 m
(2H, quinone ring).
1,2,3,3-Tetramethyl-3H-indolium tetrafluoro-
borate (IV). A mixture of 10.0 g of 2,3-dimethyl-
indole and 14.7 g of methyl benzenesulfonate was
heated for 4 h at 170 C. The mixture was dissolved
in water, the solution was treated with charcoal on
heating to the boiling point and filtered, and anhy-
drous sodium tetrafluoroborate was added to the warm
filtrate. The precipitate was filtered off and purified
by reprecipitation from ethanol with diethyl ether.
4-[2-(1,3,3-Trimethyl-2,3-dihydro-1H-indol-2-
ylidene)ethylidene]-2,5-cyclohexadienone (VIII).
Yield 89%, mp 62 C [27].
1
Yield 8.6 g (71%), mp 198.5 C. H NMR spectrum
ACKNOWLEDGMENTS
(DMSO-d6), , ppm: 1.5 s [6H, C(CH3)2], 2.8 s (3H,
CCH3), 4.0 s (3H, NCH3), 7.6 s (2H, Harom), 7.8 d
(2H, Harom).
The authors are grateful to E.L. Aleksandrov (Vavi-
lov State Optical Institute, St. Petersburg, Russia) for
measuring the electrophotographic sensitivities of
polymeric films and N.I. Rtishchev (St. Petersburg
State Institute of Technology, St. Petersburg, Russia)
for studying luminescence properties of the dyes.
Compounds II, V, and VI were synthesized follow-
ing the general procedure described in [24].
2-(4-Hydroxystyryl)-1-methylquinolinium tetra-
fluoroborate (II). A mixture of equimolar amounts
(4.08 mmol) of compound I (1.0 g), p-hydroxybenz-
aldehyde (0.49 g), and piperidine (0.35 g) in 5.5 ml
of ethanol was heated for 80 min under reflux. Dye II
(tetrafluoroborate) was isolated by adding anhydrous
sodium tetrafluoroborate to a solution of the reaction
mixture in water. Yield 1.06 g (67%), mp 181 C.
1H NMR spectrum (DMSO-d6), , ppm: 3.9 s (3H,
NCH3), 6.2 d (2H, Harom), 6.35 s (1H, OH), 6.58 d
(1H), 7.32 t (1H), 7.5 d (2H, Harom), 7.6 7.95 (6H,
quinoline ring).
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