N-Methylquinolinium derivatives for photonic applications: Enhancement of electron-withdrawing character beyond that of the widely-used N-methylpyridinium

Article abstract of DOI:10.1016/j.dyepig.2014.07.016

A series of π-conjugated styryl quinolinium push-pull chromophores have been designed and synthesized in order to examine the electron-withdrawing strength of various quinolinium electron acceptor groups, and their influence on the photophysical propertie

Full text of DOI:10.1016/j.dyepig.2014.07.016

Dyes and Pigments 113 (2015) 8e17
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
N-Methylquinolinium derivatives for photonic applications:
Enhancement of electron-withdrawing character beyond that of the
widely-used N-methylpyridinium
Jae-Hyeok Jeong a, Ji-Soo Kim a, Jochen Campo b, Seung-Heon Lee a, Woo-Yong Jeon a,
,
*
Wim Wenseleers b, Mojca Jazbinsek c, Hoseop Yun d, O-Pil Kwon a
a Department of Molecular Science and Technology, Ajou University, Suwon 443-749, South Korea
b Department of Physics, University of Antwerp, Campus Drie Eiken (CDE), Universiteitsplein 1, B-2610 Wilrijk, Belgium
c Rainbow Photonics AG, Farbhofstrasse 21, CH-8048 Zurich, Switzerland
d Department of Chemistry & Department of Energy Systems Research, Ajou University, Suwon 443-749, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of p-conjugated styryl quinolinium pushepull chromophores have been designed and synthesized
Received 1 April 2014
Received in revised form
11 July 2014
Accepted 16 July 2014
Available online 5 August 2014
in order to examine the electron-withdrawing strength of various quinolinium electron acceptor groups,
and their influence on the photophysical properties and in particular on the second-order nonlinear optical
response. The static molecular first hyperpolarizabilities measured by long-wavelength hyper-Rayleigh
scattering are found to follow the order of the electron withdrawing strength of their acceptor groups as
determined by NMR analysis. The quinolinium chromophores based on the strongest electron acceptor
groups (1,2- and 1,4-dimethylquinolinium) exhibit remarkably large first hyperpolarizability values of 233
and 256 ꢀ 10ꢁ30 esu respectively, which is higher than that of the well-known and widely-used pyridinium
analogue 4-(4-(dimethylamino)styryl)-1-methylpyridinium 4-methylbenzenesulfonate with first
hyperpolarizability ¼ 183 ꢀ 10ꢁ30 esu. The dimethylquinolinium electron acceptor groups exhibit increased
electron-withdrawing strength compared to the dimethylpyridinium group used in 4-(4-(dimethylamino)
styryl)-1-methylpyridinium 4-methylbenzenesulfonate, and therefore have a high potential for photonic
applications.
Keywords:
Electron-withdrawing strength
Nonlinear optics
Hyper-Rayleigh scattering
Static first hyperpolarizability
Quinolinium
Pyridinium
© 2014 Elsevier Ltd. All rights reserved.
1. Introduction
groups is obviously very important to obtain the desired optical and
photophysical properties for many applications.
Organic
p
-conjugated molecules with electron donor and
Up to now, a wide range of electron acceptor groups have been
investigated [1e4,16e31]. Among them, heteroaromatic salt-type
electron acceptors exhibit large electron-withdrawing strength
and lead to a large molecular NLO response [25e31]. For example,
the well-known and widely applied styryl pyridinium analogue
acceptor groups are very attractive for numerous photonic appli-
cations including nonlinear optics (NLO) [1e9], electrolumines-
cence [10e13], photovoltaics [14], and molecular electronics [15].
Typically (e.g. for the case of stilbene derivatives), the molecular
second-order NLO response (expressed by the first hyper-
DAST
(4-(4-(dimethylamino)styryl)-1-methylpyridinium
4-
polarizability
b
) increases with increasing electron-withdrawing
methylbenzenesulfonate) [25,26] e having a pyridinium electron
acceptor (see Fig. 1a) e exhibits a large molecular optical nonline-
strength of the electron acceptor group [16], which generally also
results in an increase of the wavelength of maximum absorption
labs [1] and of the wavelength of maximum emission lem [12].
Therefore, the electron-withdrawing strength of electron acceptor
arity with
a high static first hyperpolarizability b0 of 150-
194 ꢀ 10ꢁ30 esu [31,32] and derivatives which are soluble in water
as well as organic solvents have been developed [24]. Recently,
a
quinolinium electron acceptor group has been introduced
[33e38]. The HMQ cation (2-(4-hydroxy-3-methoxystyryl)-1-
methylquinolinium) based on a quinolinium electron acceptor
(see Fig. 1b), exhibits a large molecular nonlinearity and has
moreover led to several bulk materials with a large macroscopic
* Corresponding author.
E-mail address: opilkwon@ajou.ac.kr (O-P. Kwon).
http://dx.doi.org/10.1016/j.dyepig.2014.07.016
0143-7208/© 2014 Elsevier Ltd. All rights reserved.

J.-H. Jeong et al. / Dyes and Pigments 113 (2015) 8e17
9
2. Experimental
2.1. Synthesis of the quinolinium intermediates
To synthesize the chromophores based on quinolinium
electron acceptors, various dimethylquinolinium (DMQ) 4-
methylbenzenesulfonate (DMQ-T) intermediates (see Fig. 3) were
synthesized in a similar manner as reported previously [33e35].
Methanol was used as solvent for DMQ1,6-T and DMQ1,3-T, and 1,2-
dimethoxyethane was used for DMQ1,2-T, DMQ1,7-T, DMQ2,3-T
and DMQ1,4-T.
2.1.1. 1,2-dimethylquinolinium 4-methylbenzenesulfonate
(DMQ1,2-T)
A mixture of 2-methylquinoline (50 mL, 0.351 mol) and methyl
4-methylbenzenesulfonate (54 mL, 0.351 mol) in 1,2-
dimethoxyethane (300 mL) was stirred at 50 ꢂC. After stirring for
6 days, the precipitated powder was filtered and then dried in
vacuum oven at 100 ꢂC for overnight [33]. Yield ¼ 35%. M.p. 164 ꢂC.
Fig. 1. The chemical structure of (a) the styryl pyridinium chromophore DAST and (b)
the previously studied styryl quinolinium chromophore.
1H NMR (400 MHz, DMSO-d6,
d
): 9.07 (d, 1H, J ¼ 8.4 Hz, C5H2N),
8.57 (d, 1H, J ¼ 8.8 Hz, C6H4), 8.38 (d, 1H, J ¼ 8.4 Hz, C5H2N), 8.21 (m,
1H, C6H4), 8.09 (d, 1H, J ¼ 8.4 Hz, C6H4), 7.97 (t, 1H, J ¼ 7.6 Hz, C6H4),
7.44 (d, 2H, J ¼ 8.4 Hz, C6H4SOꢁ3 ), 7.08 (d, 2H, J ¼ 8.4 Hz, C6H4SOꢁ3 ),
4.42 (s, 3H, NCH3), 3.05 (s, 3H, CH3), 2.26 (s, 3H, CH3). Elemental
analysis for C18H19NO3S: Calcd. C 65.63, H 5.81, N 4.25, S 9.73;
Found: C 65.63, H 5.84, N 4.27, S 9.76.
nonlinear optical response [33e35,38]. However, for quinolinium
derivatives the electron-withdrawing strength and its influence on
the photophysical properties have not been fully characterized yet.
Here we designed and synthesized a series of
p-conjugated
styryl quinolinium chromophores based on various quinolinium
electron acceptors (see Fig. 2). We analyzed the electron-
withdrawing strength of the styryl quinolinium chromophores in
relation to their optical nonlinearity and photophysical properties.
Compared to the structurally similar pyridinium-based DAST
chromophore, the quinolinium-based derivatives exhibit large
differences in electron-withdrawing strength, and consequently in
absorption and fluorescence behavior, as well as in NLO response.
2.1.2. 1,7-dimethylquinolinium 4-methylbenzenesulfonate
(DMQ1,7-T)
A mixture of 7-methylquinoline (0.70 g, 4.89 mmol) and methyl
4-methylbenzenesulfonate (0.74 mL, 4.89 mmol) in 1,2-
dimethoxyethane (30 mL) was stirred at 50 ꢂC. After stirring for 4
days, the precipitated powder was filtered and then dried in vac-
uum oven at 65 ꢂC for overnight. Yield ¼ 42%. M.p. 182 ꢂC. 1H NMR
Fig. 2. Chemical structures of the investigated styryl quinolinium chromophores.

10
J.-H. Jeong et al. / Dyes and Pigments 113 (2015) 8e17
NCH3), 2.61 (s, 3H, CH3), 2.26 (s, 3H, CH3). Elemental analysis for
18H19NO3S: Calcd. C 65.63, H 5.81, N 4.25, S 9.73; Found: C 65.66, H
C
5.83, N 4.24, S 9.69.
2.1.5. 2,3-dimethylisoquinolinium 4-methylbenzenesulfonate
(DMQ2,3-T)
A mixture of 3-methylisoquinoline (1.45 g, 9.92 mmol) and
methyl 4-methylbenzenesulfonate (1.53 mL, 9.92 mmol) in 1,2-
dimethoxyethane (40 mL) stirred at 50 ꢂC. After stirring for 5
days, the precipitated powder was filtered and then dried in vac-
uum oven at 50 ꢂC for overnight. Yield ¼ 65%. M.p. 174 ꢂC. 1H NMR
(400 MHz, DMSO-d6, d): 10.00 (s, 1H, C5H2N), 8.41 (s, 1H, C5H2N),
8.37 (d,1H, J ¼ 8.4 Hz, C6H4), 8.17 (d,1H, J ¼ 4.8 Hz, C6H4), 8.16 (t,1H,
J ¼ 6.6 Hz, C6H4), 7.96 (m, 1H, C6H4), 7.44 (d, 2H, J ¼ 8.0 Hz,
C6H4SOꢁ3 ), 7.07 (d, 2H, J ¼ 7.6 Hz, C6H4SO3ꢁ), 4.36 (s, 3H, NCH3), 2.81
(s, 3H, CH3), 2.26 (s, 3H, CH3). Elemental analysis for C18H19NO3S:
Calcd. C 65.63, H 5.81, N 4.25, S 9.73; Found: C 65.62, H 5.81, N 4.24,
S 9.76.
2.1.6. 1,4-dimethylisoquinolinium 4-methylbenzenesulfonate
(DMQ1,4-T)
A mixture of 4-methylquinoline (2.00 g, 33.00 mmol) and
methyl 4-methylbenzenesulfonate (2.62 g, 33.00 mmol) in 1,2-
dimethoxyethane (20 mL) was stirred at 50 ꢂC. After stirring for 5
days, the precipitated powder was filtered and then dried in vac-
uum oven at 70 ꢂC for overnight. Yield ¼ 87%. M.p. 156 ꢂC. 1H NMR
Fig. 3. Chemical structures of the dimethylquinolinium (DMQ) derivatives, as well as
dimethylpyridinium (DMP) as in the DAST reference compound. The asterisk presents
the activated methyl group which forms a double bond with the aldehyde in the
condensation reaction. 1H NMR chemical shifts of the activated methyl group are
indicated as a measure of acceptor strength (see Fig. 4).
(400 MHz, DMSO-d6,
d
): 9.33 (d, 1H, J ¼ 6.0 Hz Hz, C5H2N), 8.52 (d,
1H, J ¼ 7.6 Hz, C6H4), 8.47 (d, 1H, J ¼ 9.2 Hz, C6H4), 8.27 (t, 1H,
J ¼ 15.6 Hz, C6H4), 8.06 (t, 1H, J ¼ 15.2 Hz, C6H4), 8.03 (d, 1H,
J ¼ 4.8 Hz, C5H2N), 7.45 (d, 2H, J ¼ 6.8 Hz, C6H4SOꢁ3 ), 7.09 (d, 2H,
J ¼ 7.6 Hz, C6H4SOꢁ3 ), 4.55 (s, 3H, NCH3), 2.98 (s, 3H, CH3), 2.26 (s,
3H, CH3). Elemental analysis for C18H19NO3S: Calcd. C 65.63, H 5.81,
N 4.25, S 9.73; Found: C 65.64, H 5.82, N 4.27, S 9.77.
(400 MHz, DMSO-d6,
d
): 9.43 (d, 1H, J ¼ 5.6 Hz, C5H2N), 9.22 (d, 1H,
J ¼ 8.0 Hz, C6H4), 8.38 (d, 1H, J ¼ 8.4 Hz, C5H2N), 8.34 (m, 1H, C6H4),
8.10 (t, 1H, J ¼ 8.4 Hz, C6H4), 7.93 (d, 1H, J ¼ 9.6 Hz, C6H4), 7.48 (d,
2H, J ¼ 8.4 Hz, C6H4SO3ꢁ), 7.12 (d, 2H, J ¼ 8.4 Hz, C6H4SOꢁ3 ), 4.60 (s,
3H, NCH3), 2.72 (s, 3H, CH3), 2.29 (s, 3H, CH3). Elemental analysis for
C
18H19NO3S: Calcd. C 65.63, H 5.81, N 4.25, S 9.73; Found: C 65.60, H
5.79, N 4.28, S 9.76.
2.2. Synthesis of the quinolinium-based chromophores (I)
2.1.3. 1,6-dimethylquinolinium 4-methylbenzenesulfonate
(DMQ1,6-T)
The
quinolinium-based
chromophores
with
4-
methylbenzenesulfonate (see Fig. 2a) were synthesized by a
condensation of 4-(dimethylamino)benzaldehyde with the corre-
sponding dimethylquinolinium 4-methylbenzenesulfonate (DMQ-
T) intermediates in a similar manner as reported previously [33].
For comparison, the pyridinium-based DAST chromophore was
synthesized by a condensation of 4-(dimethylamino)benzaldehyde
with 1,4-dimethylpyridinium 4-methylbenzenesulfonate (DMP-T)
in a similar manner as reported previously [27].
A mixture of 6-methylquinoline (5.00 mL, 36.50 mmol) and
methyl 4-methylbenzenesulfonate (5.51 mL, 36.50 mmol) in
methanol (50 mL) was stirred at 50 ꢂC. After stirring for 1 day, a
product was precipitated by evaporating methanol and added 1,2-
dimethoxyethane. The precipitated powder was filtered and then
dried in vacuum oven at 100 ꢂC for overnight. Yield ¼ 81%. M.p.
152 ꢂC. 1H NMR (400 MHz, DMSO-d6,
d
): 9.39 (d, 1H, J ¼ 5.2 Hz,
C5H3N), 9.12 (d, 1H, J ¼ 8.0 Hz, C5H3N), 8.39 (d, 1H, J ¼ 8.8 Hz, C6H3),
8.21 (s, 1H, C6H3), 8.11 (d, 1H, J ¼ 9.2 Hz, C6H3), 8.09 (t,1H, J ¼ 7.0 Hz,
C5H3N), 7.44 (d, 2H, J ¼ 8.0 Hz, C6H4SOꢁ3 ), 7.07 (d, 2H, J ¼ 7.6 Hz,
C6H4SOꢁ3 ), 4.58 (s, 3H, NCH3), 2.60 (s, 3H, CH3), 2.26 (s, 3H, CH3).
Elemental analysis for C18H19NO3S: Calcd. C 65.63, H 5.81, N 4.25, S
9.73; Found: C 65.66, H 5.83, N 4.26, S 9.80.
2.2.1. 2-(4-(dimethylamino)styryl)-1-methylquinolinium 4-
methylbenzenesulfonate (DA-DMQ1,2-T)
The intermediate DMQ1,2-T (5.00 g, 15.18 mmol) and 4-(dime-
thylamino)benzaldehyde (2.26 g, 15.18 mmol) were dissolved in
methanol (100 mL) and the catalyst piperidine (0.5 mL, 5.00 mmol)
was added in the solution. After stirring for 2 days at 70 ꢂC, the
solution was cooled to room temperature and filtered. The final
product was obtained by recrystallization in methanol and dried in
vacuum oven at 100 ꢂC for overnight. Yield ¼ 28%. 1H NMR
2.1.4. 1,3-dimethylquinolinium 4-methylbenzenesulfonate
(DMQ1,3-T)
A mixture of 3-methylquinoline (2.00 mL, 14.78 mmol) and
methyl 4-methylbenzenesulfonate (2.27 mL, 14.78 mmol) in
methanol (50 mL) was stirred at 50 ꢂC. After stirring for 2 days, a
product was precipitated by evaporating methanol and added 1,2-
dimethoxyethane. The precipitated powder was filtered and then
dried in vacuum oven at 100 ꢂC for overnight. Yield ¼ 79%. M.p.
(400 MHz, DMSO-d6,
d): 8.79 (d, 1H, J ¼ 8.8 Hz, C5H2N), 8.49 (d, 1H,
J ¼ 9.2 Hz, C6H4), 8.41 (d, 1H, J ¼ 8.4 Hz, C5H2N), 8.25 (d, 1H,
J ¼ 14.8 Hz, CH), 8.22 (d, 1H, J ¼ 6.4 Hz Hz, C6H4), 8.07 (m, 1H, C6H4),
7.84 (d, 2H, J ¼ 8.8 Hz, C6H4), 7.83 (t,1H, J ¼ 7.6 Hz, C6H4), 7.55 (d,1H,
J ¼ 15.6 Hz, CH), 7.44 (d, 2H, J ¼ 8.4 Hz, C6H4SOꢁ3 ), 7.09 (d, 2H,
J ¼ 7.6 Hz, C6H4SOꢁ3 ), 6.82 (d, 2H, J ¼ 9.2 Hz, C6H4), 4.43 (s, 3H,
NCH3), 3.07 (s, 6H, NCH3), 2.27 (s, 3H, CH3). Elemental analysis for
174 ꢂC. 1H NMR (400 MHz, DMSO-d6,
d): 9.46 (s, 1H, C5H2N), 9.05 (s,
1H, C5H2N), 8.43 (d, 1H, J ¼ 9.2 Hz, C6H4), 8.33 (d, 1H, J ¼ 8.0 Hz,
C6H4), 8.18 (m, 1H, C6H4), 7.99 (t, 1H, J ¼ 8.0 Hz, C6H4), 7.44 (d, 2H,
J ¼ 8.4 Hz, C6H4SOꢁ3 ), 7.07 (d, 2H, J ¼ 7.6 Hz, C6H4SOꢁ3 ), 4.58 (s, 3H,
C
27H28N2O3S: Calcd. C 70.41, H 6.13, N 6.08, S 6.96; Found: C 70.46,
H 6.15, N 6.07, S 6.78.

J.-H. Jeong et al. / Dyes and Pigments 113 (2015) 8e17
11
2.2.2. 3-(4-(dimethylamino)styryl)-2-methylisoquinolinium 4-
methylbenzenesulfonate (DA-DMQ2,3-T)
1,2-dimethoxyethane. For obtaining product, the solution kept
in ꢁ24 ꢂC. The precipitate was filtered and dried in vacuum oven at
105 ꢂC for overnight. The final product was obtained by recrystal-
lization in methanol and dried in vacuum oven at 100 ꢂC for
The intermediate DMQ2,3-T (2.17 g, 6.48 mmol) and 4-(dime-
thylamino)benzaldehyde (1.49 g, 9.72 mmol) were dissolved in
methanol (50 mL). After adding the catalyst piperidine (0.26 mL,
2.6 mmol), the solution was stirred for 12 days at 70 ꢂC. After the
reaction, the solution was cooled to room temperature and a
product was precipitated by evaporating methanol and added 1,2-
dimethoxyethane. The solution was filtered and filtrate was dried in
vacuum oven. The final product was obtained by recrystallization in
methanol and diethyl ether and dried in vacuum oven at 100 ꢂC for
overnight. Yield ¼ 30%. 1H NMR (400 MHz, DMSO-d6,
d): 8.79 (d,1H,
J ¼ 9.2 Hz, C5H2N), 8.49 (d, 1H, J ¼ 8.0 Hz, C6H4), 8.41 (d, 1H,
J ¼ 8.4 Hz, C5H2N), 8.24 (d, 1H, J ¼ 15.2 Hz, CH), 8.22 (d, 1H,
J ¼ 6.8 Hz, C6H4), 8.06 (m, 1H, C6H4), 7.84 (d, 2H, J ¼ 8.8 Hz, C6H4),
7.83 (t, 1H, J ¼ 7.2 Hz, C6H4), 7.57 (m, 2H, C6H5SOꢁ3 ), 7.54 (d, 1H,
J ¼ 14.0 Hz, CH), 7.29 (m, 1H, C6H5SOꢁ3 ), 7.28 (d, 2H, J ¼ 5.6 Hz,
C6H5SOꢁ3 ), 6.83 (d, 2H, J ¼ 8.0 Hz, C6H4), 4.43 (s, 3H, NCH3), 3.07 (s,
6H, NCH3). Elemental analysis for C26H26N2O3S: Calcd. C 69.93, H
5.87, N 6.27, S 7.17; Found: C 69.61, H 6.08, N 6.27, S 6.93.
overnight. Yield ¼ 27%. 1H NMR (400 MHz, DMSO-d6,
d): 9.89 (s, 1H,
C5H2N), 8.84 (s,1H, C5H2N), 8.30 (d,1H, J ¼ 8.4 Hz, C6H4), 8.18 (d,1H,
J ¼ 8.0 Hz, C6H4), 8.11 (t, 1H, J ¼ 8.0 Hz, C6H4), 8.11 (t, 1H, J ¼ 8.2 Hz,
C6H4), 7.67 (d,1H, J ¼ 15.6 Hz, CH), 7.65 (d, 2H, J ¼ 8.8 Hz, C6H4), 7.45
(d, 2H, J ¼ 8.4 Hz, C6H4SOꢁ3 ), 7.33 (d, 1H, J ¼ 15.6 Hz, CH), 7.08 (d, 2H,
J ¼ 8.0 Hz, C6H4SOꢁ3 ), 6.77 (d, 2H, J ¼ 8.8 Hz, C6H4), 4.45 (s, 3H,
NCH3), 3.00 (s, 3H, CH3), 2.26 (s, 3H, CH3). Elemental analysis for
2.3.2. 2-(4-(dimethylamino)styryl)-1-methylquinolinium
naphthalene-2-sulfonate (DA-DMQ1,2-NS)
MQ1-DA-I (1.32 g, 3.17 mmol) and silver(I) naphthalene-2-
sulfonate (Ag-NS, 1.00 g, 3.17 mmol,) were dissolved in methanol
(350 mL and 500 mL, respectively). After completely dissolve each
solutions, Ag-NS solution dropwised into MQ1-DA-I solution and
the solution was stirred. A few hours later, white AgI powder was
precipitated and removed by filtration. The solution was kept
in ꢁ24 ꢂC for 4days for precipitation of product. After precipitation,
the solution was filtered and dried in vacuum oven at 105 ꢂC. The
final product was obtained by recrystallization in methanol and
dried in vacuum oven at 100 ꢂC for overnight. Yield ¼ 47%. 1H NMR
C
27H28N2O3S: Calcd. C 70.41, H 6.13, N 6.08, S 6.96; Found: C 70.44,
H 6.10, N 6.09, S 6.96.
2.2.3. 4-(4-(dimethylamino)styryl)-1-methylquinolinium 4-
methylbenzenesulfonate (DA-DMQ1,4-T)
The intermediate DMQ1,4-T (1.00 g, 3.04 mmol) and 4-(dime-
thylamino)benzaldehyde (0.46 g, 3.04 mmol) were dissolved in
methanol (20 mL). After adding the catalyst piperidine (0.06 mL,
0.61 mmol), the solution was stirred for 2 days at 70 ꢂC. After the
reaction, the solution was cooled to room temperature and filtered.
The final product was obtained by recrystallization in methanol and
dried in vacuum oven overnight. Yield ¼ 14%. 1H NMR (400 MHz,
(400 MHz, DMSO-d6,
d): 8.78 (d, 1H, J ¼ 9.2 Hz, C5H2N), 8.48 (d, 1H,
J ¼ 9.2 Hz, C6H4), 8.41 (d, 1H, J ¼ 8.4 Hz, C5H2N), 8.24 (d, 1H,
J ¼ 15.2 Hz, CH), 8.21 (d, 1H, J ¼ 6.0 Hz, C6H4), 8.11 (s, 1H, C10H7SOꢁ3 ),
8.06 (m, 1H, C6H4), 7.94 (m, 1H, C10H7SOꢁ3 ), 7.87 (m, 1H, C10H7SO3ꢁ),
7.84 (d, 1H, J ¼ 8.8 Hz, C10H7SOꢁ3 ), 7.83 (d, 2H, J ¼ 8.0 Hz, C6H4), 7.82
(t, 1H, J ¼ 7.6 Hz, C6H4), 7.68 (m, 1H, C10H7SOꢁ3 ), 7.54 (d, 1H,
J ¼ 15.6 Hz, CH), 7.50 (m, 1H, C10H7SOꢁ3 ), 7.49 (m, 1H, C10H7SO3ꢁ),
6.82 (d, 2H, J ¼ 8.8 Hz, C6H4), 4.42 (s, 3H, NCH3), 3.07 (s, 6H, NCH3).
The DA-DMQ1,2-NS is also synthesized by a condensation reaction
of 4-(dimethylamino)benzaldehyde with 1,2-dimethylquinolinium
naphthalene-2-sulfonate. The NMR spectrum of condensation is
practically identical with metathesization. Yield ¼ 43%. Elemental
analysis for C30H28N2O3S: Calcd. C 72.55, H 5.68, N 5.64, S 6.46;
Found: C 72.55, H 5.72, N 5.67, S 6.43.
DMSO-d6,
d
): 9.09 (d, 1H, J ¼ 6.8 Hz, C5H2N), 9.01 (d, 1H, J ¼ 8.4 Hz,
C5H2N), 8.32 (d, 1H, J ¼ 6.8 Hz, C6H4), 8.32 (d, 1H, J ¼ 6.8 Hz, C6H4),
8.19 (t, 1H, J ¼ 7.8 Hz, C6H4), 8.16 (d, 1H, J ¼ 15.6 Hz, CH), 8.00 (d, 1H,
J ¼ 15.6 Hz, CH), 7.83 (t, 1H, J ¼ 8.6 Hz, C6H4), 7.85 (d, 2H, J ¼ 8.8 Hz,
C6H4), 7.45 (d, 2H, J ¼ 8 Hz, C6H4SOꢁ3 ), 7.08 (d, 2H, J ¼ 7.6 Hz,
C6H4SOꢁ3 ), 6.81 (d, 2H, J ¼ 9.2 Hz, C6H4), 4.42 (s, 3H, NCH3), 3.05 (s,
6H, NCH3), 2.27 (s, 3H, CH3). Elemental analysis for
C
27H28N2O3S$H2O: Calcd. C 67.76, H 6.32, N 5.85, S 6.70; Found: C
67.97, H 6.26, N 5.78, S 6.85.
The condensation reaction of 4-(dimethylamino)benzaldehyde
with the corresponding DMQ-T intermediates for DA-DMQ1,7-T (7-
(4-(dimethylamino)styryl)-1-methylquinolinium
4-
2.4. Crystal structures analysis
methylbenzenesulfonate), DA-DMQ1,6-T (6-(4-(dimethylamino)
styryl)-1-methylquinolinium 4-methylbenzenesulfonate) and DA-
DMQ1,3-T (3-(4-(dimethylamino)styryl)-1-methylquinolinium 4-
methylbenzenesulfonate) were not successful, as will be dis-
cussed later.
DA-DMQ1,2-B: slow evaporation method in methanol,
C
a
b
26H26N2O3S,H2O, Mr ¼ 464.57, monoclinic, space group P21/n,
¼ 12.5055(12) Å, b
¼
10.3774(8) Å,
c
¼
18.5700(14) Å,
¼ 103.721(3)ꢂ, V ¼ 2341.2(3) Å3, Z ¼ 4, T ¼ 290(1) K,
m
(MoK
a
) ¼ 0.168 mmꢁ1. Of 17898 reflections collected in the
q
2.3. Synthesis of the quinolinium-based chromophores (II)
range 2.99ꢂe25.00ꢂ using an
u scan on a Rigaku R-axis Rapid S
diffractometer, 4116 were unique reflections (Rint
¼
0.0324,
The quinolinium-based chromophores DA-DMQ1,2-B with
benzenesulfonate and DA-DMQ1,2-NS with naphthalene-2-
sulfonate (see Fig. 2b) were synthesized in a similar manner as
the metathesization of styrylquinolinium iodide with silver pre-
cursor [33e37].
completeness ¼ 99.8%). The structure was solved and refined
against F2 using SHELX97 [39] 410 variables, wR2 ¼ 0.2228,
R1 ¼ 0.0624 (Fo2 > 2
s
(Fo2)), GOF ¼ 1.095, and max/min residual
electron density 0.706/-0.427 eÅꢁ3. CCDC-948586.
DA-DMQ1,2-NS (I): rapid cooling method in methanol,
C
30H28N2O3S, Mr ¼ 496.60, triclinic, space group P-1, a ¼ 11.4237(6)
2.3.1. 2-(4-(dimethylamino)styryl)-1-methylquinolinium
benzenesulfonate (DA-DMQ1,2-B)
Å, b ¼ 11.6934(6) Å, c ¼ 12.0951(6) Å,
a
¼ 68.2997(15)ꢂ,
b
¼ 65.327(2)ꢂ,
g
¼ 61.930(2)ꢂ, V ¼ 1264.85(10) Å3, Z ¼ 2, T ¼ 290(1)
2-(4-(Dimethylamino)styryl)-1-methylquinolinium
iodide
K,
m
(MoK
a
) ¼ 0.163 mmꢁ1. Of 9073 reflections collected in the
q
(MQ1-DA-I, 1.00 g, 2.40 mmol) and silver(I) benzenesulfonate
(AgeB, 0.64 g, 2.40 mmol) were dissolved in methanol (280 mL and
60 mL, respectively). After completely dissolve each solutions,
AgeB solution dropwised into MQ1-DA-I solution and the solution
was shaked. A few hours later, white AgI powder was precipitated
and removed by filtration. The solution was evaporated and added
range 3.06ꢂe24.00ꢂ using an
u scans on a Rigaku R-axis Rapid S
diffractometer, 3966 were unique reflections (Rint
¼
0.0171,
completeness ¼ 99.8%). The structure was solved and refined
against F2 using SHELX97 [39], 325 variables, wR2 ¼ 0.1142,
R1 ¼ 0.0373 (Fo2 > 2
s
(Fo2)), GOF ¼ 1.081, and max/min residual
electron density 0.211/-0.274 eÅꢁ3. CCDC-948584.

12
J.-H. Jeong et al. / Dyes and Pigments 113 (2015) 8e17
DA-DMQ1,2-NS (II): slow cooling method in methanol,
analogue, the well-known DAST compound (see Fig. 1a)
C
30H28N2O3S, Mr
¼
496.60, monoclinic, space group P21/n,
[25e27], in which an identical dimethylamino electron donor
group as well as the same 4-methylbenzenesulfonate counter
anion is used.
a ¼ 11.4414(9)Å, b ¼ 19.6315(16)Å, c ¼ 12.0851(11)Å,
b
¼ 113.450(2)ꢂ,
V ¼ 2490.3(4)Å3, Z ¼ 4, T ¼ 290(1) K,
m
(MoK
a
) ¼ 0.17 mmꢁ1. Of 24027
reflections collected in the
q
range 3.20ꢂe27.50ꢂ using an
u
scans on a
To synthesize the quinolinium-based chromophores with 4-
methylbenzenesulfonate, six intermediates of dimethylquinoli-
nium (DMQ) with 4-methylbenzenesulfonate, having a different
position of the nitrogen atom at the acceptor side, were synthesized
first (see Fig. 3). The quinolinium-based chromophores with 4-
methylbenzenesulfonate were synthesized by Knoevenagel
condensation of dimethylamino benzaldehyde with the corre-
sponding DMQ-T intermediates with the weak base, piperidine, as
the catalyst [33e35]. The condensation reactions for DA-DMQ1,2-T,
DA-DMQ2,3-T and DA-DMQ1,4-T were successful, unlike for DA-
DMQ1,7-T, DA-DMQ1,6-T and DA-DMQ1,3-T. The different reactivity
in the condensation reactions with different electron-acceptor
groups is related to the electron-withdrawing strength of the
methylquinolinium group in the DMQ-T intermediates, as dis-
cussed in the following Section 3.2.
Rigaku R-axis Rapid S diffractometer, 5706 were unique reflections
(Rint ¼ 0.122, completeness ¼ 99.7%). The structure was solved and
refined against F2 using SHELX97 [39], 437 variables, wR2 ¼ 0.1866,
R1 ¼ 0.0749 (Fo2 > 2
s
(Fo2)), GOF ¼ 1.02, and max/min residual elec-
tron density 0.43/-0.26 eÅꢁ3. CCDC-948585.
2.5. Hyper-Rayleigh scattering measurement
The molecular first hyperpolarizabilities
b
of the synthesized
compounds DA-DMQ1,2-T, DA-DMQ2,3-T, and DA-DMQ1,4-T, as
well as for DAST were determined in acetonitrile solution at the
long fundamental wavelength of 1550 nm, far from two-photon
resonance, by means of the hyper-Rayleigh scattering (HRS) tech-
nique. The highly sensitive and broadly wavelength-tunable HRS
setup (described in detail in Ref. [40]) consists of a Ti:Sapphire
regenerative amplifier (Spectra-Physics Spitfire) which is pumping
an optical parametric amplifier (Spectra-Physics OPA-800CP, pulse
duration ~2 ps, repetition rate ¼ 1 kHz, pulse energy at the sample
3.2. Electron-withdrawing strengths
Fig. 3 shows the chemical structures of the quinolinium-based
DMQ-T and pyridinium-based DMP-T intermediates. In order to
investigate the electron-withdrawing strength of the various qui-
nolinium electron acceptors, 1H NMR spectra of the DMQ-T and
DMP-T intermediates were measured. Fig. 4 shows the 1H NMR
spectra of the DMQ and DMP derivatives in dimethylsulfoxide
(DMSO)-d6 solution. The asterisks in Fig. 4 present the corre-
sponding proton peaks on the active methyl groups of the DMQ and
DMP intermediates, denoted by the asterisks in Fig. 3. In DMQ
derivatives, the chemical shift of the protons on the active methyl
group is 3.053 ppm for DMQ1,2-T, 2.980 ppm for DMQ1,4-T,
2.806 ppm for DMQ2,3-T, 2.722 ppm for DMQ1,7-T, 2.607 ppm for
DMQ1,6-T and 2.605 ppm for DMQ1,3-T. Therefore, the electron-
withdrawing strength of the derivatives as determined by com-
parison of their chemical shifts follows the order of
DMQ1,2 > DMQ1,4 > DMQ2,3 > DMQ1,7 > DMQ1,6 z DMQ1,3.
The observed relative electron-withdrawing strength of the
quinolinium derivatives agrees very well with the results of the
condensation reactions. In the condensation reaction to synthesize
the styryl quinolinium chromophores, an aldehyde group and an
active methyl group on the DMQ cation form a double bond. First
the proton of the active methyl group of the DMQ cation, denoted
by an asterisk in Fig. 3, is eliminated by a weak base catalyst and
thus a resonance-stabilized anion is formed. Second, the activated
methyl anion attacks the carbon atom on the aldehyde as a
nucleophile. [18,27] Therefore, the reactivity of the activated
methyl group with the aldehyde increases with increasing
electron-withdrawing strength of N-methylquinolinium. As
described in the Experimental section, the DA-DMQ1,2-T, DA-
DMQ1,4-T, and DA-DMQ2,3-T chromophores, which are based on
at 1550 nm ~20 mJ). Parallel detection of a narrow wavelength range
around the second harmonic is achieved by use of a nitrogen cooled
CCD coupled to a spectrograph, enabling fast and complete
correction for broadband multi-photon fluorescence. Such a back-
ground was observed for all four compounds, varying in strength
from (integrated over the central 11 nm wide region) about 20
times the actual HRS signal for DA-DMQ1,4-T, about 4 times for
DAST, about 2.2 times for DA-DMQ1,2-T and down to less than 0.1
times for DA-DMQ2,3-T.
A single significant
b tensor component along the z-axis (i.e. the
conjugated chain) was assumed for all measured molecules.
Acetonitrile itself was used as secondary internal reference stan-
dard. We previously calibrated this solvent extensively against
chloroform [40], adopting the effective
jbzzz
j
value of
0.49 ꢀ 10ꢁ30 esu as determined in Ref. [41]. Specifically, we used a
b
value of 0.599 ꢀ 10ꢁ30 esu at 1550 nm for acetonitrile [40]. The
reported results for
b are expressed in the B* convention [42].
All compounds were found to be very stable in solution, and to
limit effects of local photodecomposition in the laser focus, the
solutions were stirred during the measurement to continuously
refresh the exposed molecules. In these conditions, no decompo-
sition was observed in the linear absorption spectra. As the laser
wavelength was chosen to be far from (one- and) two-photon
resonance, no correction for reabsorption of the HRS light (at
775 nm) was necessary. The experimental error on b, disregarding
the systematic error on the reference value, is estimated to be 5%,
based on the excellent reproducibility of independent
measurements.
3. Results and discussion
DMQ electron acceptors having
a relatively large electron-
withdrawing strength, were successfully synthesized by
a
3.1. Design and synthesis
condensation reaction, while for DA-DMQ1,7-T, DA-DMQ1,6-T, and
DA-DMQ1,3-T the condensation reaction was unsuccessful.
Compared to quinolinium-based DMQ1,7, DMQ1,6 and DMQ1,3,
pyridinium-based DMP having lower chemical shift was reacted
with the aldehyde. Since DMQ and DMP cations are based on
different heteroaromatic characteristics, a fused benzene ring and a
benzene ring, respectively, the reactivity might be related with not
only electron-withdrawing strength, but also steric hindrance and
other effects. Therefore, comparison between DMP and DMQ is
discussed in the following section on hyper-Rayleigh scattering
(HRS) measurements.
In order to examine the electron-withdrawing strength of
different quinolinium electron acceptors and its influence on
physical properties, we designed a rational series of quinolinium-
based chromophores. The chemical structures of the investigated
chromophores are shown in Fig. 2a. They consist of a dimethy-
lamino electron donor group and a methylquinolinium-based
electron acceptor group. All these cationic chromophores are
combined with a 4-methylbenzenesulfonate counter anion. For
comparison, we also included
a
methylpyridinium-based

J.-H. Jeong et al. / Dyes and Pigments 113 (2015) 8e17
13
Fig. 4. 1H NMR spectra of the DMQ and DMP acceptor groups in DMSO-d6. The asterisk denotes the proton peak of the methyl groups which are active in the condensation reaction
(denoted by the asterisk in Fig. 3).
In order to further investigate our hypothesis, the molecular first
hyperpolarizabilities of the successfully synthesized quinolinium
again make use of the TLM [43], which predicts b0 to be inversely
proportional to the (cube of) the transition frequency ueg, and
directly proportional to the difference in dipole moment between
the ground- and electronic excited state Dm and to the oscillator
strength of the transition fosc. The increase of b0 for DA-DMQ1,2-T
compared to DA-DMQ2,3-T having identical conjugation length
can be explained by the strongly red-shifted labs of the former (see
Table 1), caused by its stronger acceptor (1,2-dimethylquinolinium
vs. 2,3-dimethylquinolinium as also determined above by 1H NMR
analysis). Clearly, it is favorable for the fused benzene ring to be
close to the nitrogen to increase the electron accepting abilities of
the quinolinium end group. Note that this increase in labs is over-
compensating for the slight decrease in oscillator strength (see
Table 1).
b
dyes (DA-DMQ1,2-T, DA-DMQ1,4-T and DA-DMQ2,3-T), having
different acceptor strengths and different conjugation lengths
counting between the donor and acceptor nitrogens (but identical
overall size), were directly determined by the hyper-Rayleigh
scattering (HRS) technique (see Table 1) [40]. The HRS measure-
ments were performed at the long laser wavelength of 1550 nm,
well beyond (one- and) two-photon resonance for all three com-
pounds, so that the simple undamped two-level model (TLM) of
Oudar and Chemla [43] can be used to reliably extrapolate the
experimental
b values to the static (‘resonance free’) limit b0 [44],
needed for correct comparison between different molecules.
Remarkably high b0 values of 256 and 233 ꢀ 10ꢁ30 esu are ob-
tained for DA-DMQ1,4-T and DA-DMQ1,2-T respectively, essentially
two times considerably higher than the b0 of DA-DMQ2,3-T
(122 ꢀ 10ꢁ30 esu). To qualitatively interpret these values we can
The further improvement of DA-DMQ1,4-T over DA-DMQ1,2-T
can be attributed to the larger distance between the nitrogen
atoms of the former, resulting in an increase in labs (and Dm), again
Table 1
Measured physical data for the investigated chromophores: the first hyperpolarizability
b measured by HRS and the corresponding b0 values determined by means of the
undamped two-level model; the oscillator strength fosc of the lowest-energy absorption band; the wavelength of maximum absorption labs; the wavelength of maximum
emission lem
.
HRS (10ꢁ30 esu)
fosc
labs (nm)
lem (nm)
b1550nm
b0
AcCN
DCM
Acetone
MeOH
AcCN
H2O
DCM
AcCN
H2O
DA-DMQ1,2-T
DA-DMQ1,4-T
DA-DMQ2,3-T
DAST
475
560
188
320
233
256
122
183
0.65
0.61
0.69
0.66
556
581
441
522
522
540
425
468
523
543
425
474
519
536
422
470
500
510
394
450
645
680
539
608
618
703
530
625
622
698
625a
624
a
This value has a higher uncertainty due to low solubility and low fluorescence quantum yield (<0.05%).

14
J.-H. Jeong et al. / Dyes and Pigments 113 (2015) 8e17
despite a further reduction in oscillator strength. In terms of
conjugation length, DA-DMQ1,4-T can be directly compared to the
pyridinium analogue DAST based on DMP intermediate. Therefore,
we also carried out HRS measurements on DAST, under the exact
same conditions (same non-resonant wavelength, same solvent).
method in methanol solution were found to exhibit monoclinic
space group symmetry P21/n. For the DA-DMQ1,2-NS crystals we
found two polymorphs: the DA-DMQ1,2-NS(I) phase grown by the
rapid cooling method in methanol solution exhibits monoclinic
space group symmetry P-1, while the DA-DMQ1,2-NS(II) phase
grown by slow cooling method in methanol solution from the
saturation temperature of 40 ꢂC exhibits monoclinic space group
symmetry P21/n. In this way we have three examples of molecular
conformation of the DA-DMQ1,2 chromophore in the crystalline
state. Unfortunately, the DA-DMQ1,2 compound crystallizes cen-
trosymmetrically, but many structural variations (side groups,
other counter ions) could be considered to change the crystal
structure without significantly changing the molecular nonline-
arity, as have been used successfully for other related compounds
such as DAST [25] and the HMQ cation from Fig. 1b [33e38].
Our
b
value of 320 ꢀ 10ꢁ30 esu obtained for DAST at 1550 nm in
acetonitrile (see Table 1) is in reasonable agreement with the value
of 367 ꢀ 10ꢁ30 esu reported at 1542 nm in dimethyl sulfoxide
(DMSO) solution by Bosshard and co-workers [45]. Furthermore,
the b0 value of 183 ꢀ 10ꢁ30 esu obtained for DAST is in excellent
agreement with the values obtained from quantum chemical cal-
culations in the gas and solid phases (150-194 ꢀ 10ꢁ30 esu ) as
reported in Refs. [31,32]. The b0 value for DA-DMQ1,4-T is ~40%
higher than for DAST, which can again be explained by the increase
in labs due to the stronger acceptor of the former (again in agree-
ment with the 1H NMR analysis), despite the slightly smaller
oscillator strength. Note that the molecular nonlinearity of the DA-
DMQ1,4 cation has also been previously estimated by Stark elec-
troabsorption spectroscopy in Ref. [46], where a 1.1 times higher b0
was reported for the DA-DMQ1,4 cation than for the DAST cation.
Even the DA-DMQ1,2-T compound with a shorter NeNþ dis-
tance exhibits a 25% higher b0 value than DAST, showing that 1,2-
dimethylquinolinium is a much better electron acceptor than 1,4
dimethylpyridinium (in agreement with the NMR results), so much
that it can even overcompensate for a decrease in conjugation
length of two bonds. DA-DMQ2,3-T however exhibits weaker per-
formance, as the lower acceptor strength of the 2,3-
dimethylquinolinium group combined with the decreased conju-
gation length results in a 35% lower b0 than DAST.
Hence, the molecular optical nonlinearity follows the order of
DA-DMQ1,4-T > DA-DMQ1,2-T > DAST > DA-DMQ2,3-T, and thus,
comparing compounds with the same NeNþ distance, the electron-
withdrawing strength of quinolinium follows the order of
DMQ1,2 > DMQ2,3 and DMQ1,4 > DMP (used in DAST), in perfect
agreement with the results of the 1H NMR measurements. The
further increase in b0 for DA-DMQ1,4-T compared to DA-DMQ1,2-T
is significant (~10%), but less than might be expected considering
the sizeable increase in NeNþ distance alone, which can be
attributed to the larger acceptor strength of DMQ1,2.
3.3. Molecular conformation
The molecular optical nonlinearity of chromophores is strongly
affected by the molecular conformation [47e51]. In general, a
planar conformation of the
delocalization of the -electrons which is beneficial for the optical
nonlinearity [1e4]. Yet, twisting of the -conjugated bridge can
p-conjugated bridge yields efficient
p
p
either lead to a decrease of the optical nonlinearity, through a
breaking of the conjugation [50], or to an increase of it, through
enhanced charge-separation [51].
In order to investigate the molecular conformation of the
quinolinium-based chromophores, the crystal structure of the
quinolinium crystals based on the electron acceptor DMQ1,2 having
the largest electron-withdrawing strength in this work, was
analyzed. Unfortunately, the grown DA-DMQ1,2-T crystals with 4-
methylbenzenesulfonate counter anion did not exhibit sufficient
crystal quality to determine their crystal structure by X-ray anal-
ysis. Instead, two DA-DMQ1,2 analogues with different anions,
benzenesulfonate and naphthalene-2-sulfonate (see Fig. 2b) were
newly synthesized. DA-DMQ1,2-B (2-(4-(dimethylamino)styryl)-1-
methylquinolinium benzenesulfonate) was synthesized by meta-
thesization, while DA-DMQ1,2-NS (2-(4-(dimethylamino)styryl)-1-
methylquinolinium naphthalene-2-sulfonate) was synthesized by
either metathesization or condensation (see Experimental section).
The DA-DMQ1,2-B single crystals grown by the slow evaporation
Fig. 5. Molecular conformation of the DA-DMQ1,2 cation in the crystalline state of (a)
DA-DMQ1,2-B, (b) DA-DMQ1,2-NS (I) and (c) DA-DMQ1,2-NS (II).

J.-H. Jeong et al. / Dyes and Pigments 113 (2015) 8e17
15
Fig. 5 shows the molecular conformation of the DA-DMQ1,2
cation in the crystalline state of DA-DMQ1,2-B, DA-DMQ1,2-NS (I)
and DA-DMQ1,2-NS (II) compounds. The molecular conformation of
the DA-DMQ1,2 cation in these three phases is similar: the plane of
the 4-(dimethylamino)styryl groups is practically planar with the
plane of the quinolinium ring within 8ꢂ (see Fig. 5), ensuring good
Fig. 6 shows absorption spectra of the chromophores in various
solvents (methanol (MeOH), dichloromethane (DCM), acetonitrile
(AcCN), acetone and water). The results are summarized in Table 1.
In addition to the large variation of the wavelength of maximum
absorption labs among the different chromophores in line with the
respective acceptor strengths and NLO results as discussed before,
all of the chromophores (DMQ based as well as DAST), show a
negative solvatochromism (i.e. labs decreasing with increasing
solvent polarity).
Fig. 7 shows the fluorescence spectra of the DA-DMQ1,2-T, DA-
DMQ2,3-T, and DAST chromophores in acetonitrile, excited at
405 nm. All chromophores show weak fluorescence in water
(quantum yield less than 1%). In acetonitrile solution however, the
DA-DMQ1,2-T and DAST chromophores exhibit a significant fluo-
rescence intensity, while it is extremely weak for DA-DMQ2,3-T
delocalization of the p-electrons.
The molecular conformation can be strongly affected by envi-
ronmental conditions. In previously reported DMQ1,2-based chro-
mophores, tilting angles between the planes of the phenyl ring and
quinolinium ring were up to 27ꢂ [33e38], but according to FF-DFT
(finite field-density functional theory) calculations in gas and solid
phase the accompanying change of the molecular nonlinearity with
respect to non-tilted chromophores is relatively small, less than
15% [33e35,38], suggesting that the effects of conjugation and
charge-separation compensate in these DMQ1,2 derivatives. Such
insensitivity of the molecular nonlinearity to variations of the
molecular conformation in the quinolinium based DMQ1,2 electron
acceptor is an important practical advantage towards imple-
mentation in various applications, compared to other electron ac-
ceptors, whose optical nonlinearity may be very sensitive to a
change of molecular conformation.
(see Fig. 7). For quantitative comparison, quantum yields
chromophores in acetonitrile, excited at 405 nm are measured
using coumarin 153 (
¼ 56 %) as a reference standard [52]. The DA-
DMQ1,2-T chromophore exhibits 6 times higher quantum yield
F of
F
(
F
¼ 1.9%) than DAST (
F
¼ 0.32%). Unlike the absorption wave-
length labs, the emission wavelength lem does not show a clear
solvatochromic trend (which may be due to the more complex
behavior typically associated with relaxations in the excited state),
but does correlate well with the NLO results and acceptor strengths
among the different compounds.
3.4. Photophysical properties
While the photophysical properties of pyridinium-based chro-
mophores have been widely investigated [24], those of DMQ1,2-
and DMQ2,3-based chromophores are scarcely known. We inves-
tigated the absorption and fluorescence behavior of the DA-
DMQ1,2-T, DA-DMQ1,4-T and DA-DMQ2,3-T chromophores. For
comparison, the pyridinium-based DAST chromophore was also
investigated under the same experimental conditions.
4. Conclusion
We have investigated a series of
p-conjugated styryl quinoli-
nium chromophores based on various quinolinium electron
acceptor groups. The order of the strength of the different electron
acceptors was determined from 1H NMR analysis to be
Fig. 6. Absorption spectra of the (a) DA-DMQ1,2-T, (b) DA-DMQ1,4-T and (c) DA-DMQ2,3-T chromophores in various solvents.

16
J.-H. Jeong et al. / Dyes and Pigments 113 (2015) 8e17
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the wavelength of maximum absorption labs of the different com-
pounds was indeed found to follow the same order. The static first
hyperpolarizability b0 of the quinolinium compounds, having
different acceptor strengths and different conjugation lengths was
determined by long-wavelength hyper-Rayleigh scattering (HRS)
experiments and, as expected, also the b0 value follows the order of
the electron withdrawing strength among equally long compounds.
The compounds with the strongest acceptor groups (DA-DMQ1,2-T
and DA-DMQ1,4-T) are found to exhibit remarkably large b0 values
of 233 and 256 ꢀ 10ꢁ30 esu respectively, which is higher than the b0
of the widely applied pyridinium-based analogue DAST. This is
attributed to a higher electron-withdrawing capability of the
dimethylquinolinium groups (DMQ1,2 and DMQ1,4) in comparison
to the dimethylpyridinium group (DMP) of DAST. DMQ1,2 is found
to form the strongest electron acceptor group, as it even succeeds in
compensating for the smaller NeNþ separation of the DA-DMQ1,2-
T compound. Therefore, the dimethylquinolinium electron acceptor
groups DMQ1,2 and DMQ1,4 form an interesting potential alter-
native for the widely used pyridinium electron acceptor for various
photonic applications.
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This work has been supported by Mid-career Researcher Pro-
gram (NRF-2013R1A2A2A01007232), Engineering Research Center
of Excellence Program (NRF-2014-009799) and Priority Research
Centers Program (2009-0093826) through the National Research
Foundation of Korea (NRF) funded by the Ministry of Science, ICT
and Future Planning and the Ministry of Education. Financial sup-
port from the Fund for Scientific Research-Flanders (FWO; projects
G.0129.07, G1523913 and G.0206.12) is gratefully acknowledged.
J.C. is a postdoctoral fellow of the FWO.
€
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