Synthesis and photophysical properties of two novel branched pyridinium inner salt dyes

Article abstract of DOI:10.1016/j.dyepig.2012.09.012

Two new branched pyridinium inner salt dyes, 2,6-di[2-(N-ethyl-carbazole-3- yl)vinyl]-N-(3-sulfonatepropyl)pyridinium, inner salt (2) and 2,4,6-tri[2-(N-ethyl-carbazole-3-yl)vinyl]-N-(3-sulfonatepropyl)pyridinium, inner salt (3), with N-ethyl-carbazolyl as electron donor group and pyridine cation as electron acceptor group have been synthesized. Their single- and two-photon related fluorescence properties in various solvents have also been examined. These dyes exhibit stronger fluorescence emission relative to the corresponding linear counterpart and common pyridinium dyes with discrete cation and anion components. Three-branched organic dye 3 has a high fluorescence quantum yield as 0.12 and exhibits strong orange two-photon excited fluorescence at 608 nm in THF with large two-photon absorption cross-section being 606 GM.

Full text of DOI:10.1016/j.dyepig.2012.09.012

Dyes and Pigments 96 (2013) 563e568
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and photophysical properties of two novel branched pyridinium inner
salt dyes
a
b
,
c
,
*, Duxia Cao a , Yunlong Deng , Yunhui Sun , Fuyou Li
,
b
,
a
a
c
*
*
Huihui Chen , Zhiqiang Liu
a
School of Material Science and Engineering, University of Jinan, Jinan 250022, Shandong, China
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, Shandong, China
Department of Chemistry, Fudan University, Shanghai 200433, China
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 August 2012
Received in revised form
Two new branched pyridinium inner salt dyes, 2,6-di[2-(N-ethyl-carbazole-3-yl)vinyl]-N-(3-sulfonate-
propyl)pyridinium, inner salt (2) and 2,4,6-tri[2-(N-ethyl-carbazole-3-yl)vinyl]-N-(3-sulfonatepropyl)
pyridinium, inner salt (3), with N-ethyl-carbazolyl as electron donor group and pyridine cation as
electron acceptor group have been synthesized. Their single- and two-photon related fluorescence
properties in various solvents have also been examined. These dyes exhibit stronger fluorescence
emission relative to the corresponding linear counterpart and common pyridinium dyes with discrete
cation and anion components. Three-branched organic dye 3 has a high fluorescence quantum yield as
16 September 2012
Accepted 19 September 2012
Available online 1 October 2012
Keywords:
0.12 and exhibits strong orange two-photon excited fluorescence at 608 nm in THF with large two-
Pyridinium inner salt
Two-photon absorption
Two-photon excited fluorescence
Synthesis
photon absorption cross-section being 606 GM.
Ó 2012 Elsevier Ltd. All rights reserved.
Carbazolyl
Branched structure
1. Introduction
properties by properly structurally modification, such as the donor/
acceptor strength, the rigidity of the -bridge and introduction of
p
Aminostyryl pyridinium dyes (hemicyanine dyes) are a type of
important optoelectronic functional materials, which exhibit many
outstanding properties such as organic nonlinear optical effect [1],
frequency-upconverted lasing [2,3], optical power limiting [4],
molecular electronics [5] and fluorescence probes [6]. Among them,
two-photon absorption and related frequency-upconverted emis-
sion of these dyes attracted lots of attention. Prasad research group
firstly reported high efficient two-photon excited fluorescence and
lasing properties of a series of new aminostyryl substituted pyr-
idinium dyes [7]. Very recently, Yu group reported the two-photon
fluorescence imaging of DNA and RNA in living tissue with some
vinylcarbazole substituted pyridinium dyes [8,9].
branches. But most of common pyridinium dyes have discrete
cation and anion components, there are a few papers reported
pyridinium inner salts with the cation covalently linked to the
anion, which show enhanced two-photon pumped lasing than
cation/anion discrete analogues [11]. In addition, multi-branched
structure is beneficial to the nonlinear optical properties of mole-
cules because of some cooperative enhancement originating from
inter-branch coupling [12e15].
As part of our ongoing efforts toward developing new two-
photon absorption and fluorescence materials, the synthesis,
single- and two-photon excited fluorescence properties of two new
branched pyridinium inner salt dyes are presented in this work.
Generally, aminostyryl pyridinium dyes are easy to be prepared
and crystallized and purified [10]. Comprehensive researches have
been carried out to finely tune their chemical and physical
2. Experimental
2.1. Chemical and instruments
*
Corresponding author. School of Material Science and Engineering, University of
Jinan, Jinan 250022, Shandong, China. Tel.: þ86 531 89736751; fax: þ86 531
7974453.
* Corresponding author. State Key Laboratory of Crystal Materials, Shandong
1
,3-Propanesulfonate, 2,6-dimethylpyridine
and
2,4,6-
trimethylpyridine were purchased from Aladdin Reagent Inc.
Other reagents were purchased from Shanghai Reagents and were
used as received directly without further purification. All of the
solvents were freshly distilled before using. Nuclear magnetic
8
*
University, Jinan 250100, Shandong, China.
E-mail addresses: zqliu@sdu.edu.cn (Z. Liu), duxiacao@ujn.edu.cn (D. Cao).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.09.012

564
H. Chen et al. / Dyes and Pigments 96 (2013) 563e568
resonance spectra were recorded on a Bruker Avance III 400 MHz
spectrometer. Mass spectra were recorded on an Agilent Q-TOF6510
spectrometer. Infrared spectra were recorded on a Nicolet 380FT-IR
spectrophotometer. Elemental analyses were carried out on a PE
2.2.1. 2,6-Di[2-(N-ethyl-carbazole-3-yl)vinyl]-N-(3-sulfonatepro-
pyl)pyridinium, inner salt (2)
N-ethyl-carbazole-3-aldehyde (1.25 g (0.0056 mol)) and 80 mL
isoamyl alcohol were added to a 250 mL one-necked flask with
a stirrer and condenser, and then 0.32 g (0.0014 mol) S-1 and a few
drops of piperidine were added in turn. The mixture was heated
under reflux for 10 h. After cooling to room temperature, orange
precipitate was filtered and re-crystallized by ethanol to give dye 2
2
400 autoanalyzer.
2.2. Synthesis
1
as orange solid with a yield 56%. H NMR (DMSO, 400 MHz),
The synthesis of linear dye 1 has been previously reported by
d
(ppm): 1.36(t, 6H, J ¼ 7.0 Hz), 2.33(s, 2H), 2.92(t, 2H, J ¼ 5.3 Hz),
our group [16]. The synthetic strategy for dyes 2 and 3 is outlined in
Scheme 1. The dyes were synthesized via two steps. Firstly, 1,3-
propanesulfonate reacted with one equivalent of 2,6-
dimethylpyridine or 2,4,6-trimethylpyridine to afford S-1 and S-2.
Then dyes 2 and 3 were obtained by the reaction of S-1 or S-2 with
4.51(q, 4H, J ¼ 7.1 Hz), 5.17(t, 2H, J ¼ 5.2 Hz), 7.28(t, 2H, J ¼ 7.4 Hz),
7.52 (t, 2H, J ¼ 7.6 Hz), 7.67(d, 2H, J ¼ 8.2 Hz), 7.71(d, 2H, J ¼ 8.6 Hz),
7.87(d, 2H, J ¼ 15.6 Hz), 8.04 (d, 2H, J ¼ 15.6 Hz), 8.06(d, 2H,
13
J ¼ 7.9 Hz), 8.29e8.33(m, 5H), 8.92(s, 2H).
C NMR (DMSO,
100 MHz),
d (ppm): 13.65, 37.14, 47.43, 78.54, 78.87, 79.20, 109.33,
0
N-ethylcarbazole-3-aldehyde. A pyridinium dye 1 with discrete
109.46, 114.70, 119.45, 120.86, 121.67, 122.35, 122.75, 126.24, 127.37,
þ
cation and anion components and similar structure with dye 1 was
also synthesized.
140.10, 140.88, 144.43, 152.93. MS for (MþH) , Calcd exact mass:
ꢀ1
640.2634, found 640.2619. IR (cm ): 3855, 3748, 3054, 2980, 1600,
Scheme 1. Synthetic strategy of dyes 2 and 3.

H. Chen et al. / Dyes and Pigments 96 (2013) 563e568
565
1566, 1483, 1331, 1226, 1193, 1030, 971, 811, 747, 518. Anal. Calcd. for
3. Results and discussion
C
6
40 37 3 3
H N O S: C, 75.09; H, 5.83; N, 6.57. Found: C, 75.25; H, 5.85; N,
.59.
3.1. Linear absorption and single-photon excited fluorescence
properties
2
.2.2. 2,4,6-Tri[2-(N-ethyl-carbazole-3yl)vinyl]-N-(3-sulfonatepro-
The comprehensive photophysical properties of the compounds
in various solvents with different polarity are listed in Table 1.
Linear absorption and single-photon excited fluorescence (SPEF)
spectra of dyes 2 and 3 are shown in Figs. 1 and 2, respectively.
As shown in Table 1 and Fig. 1, one can see that absorption
properties of the dyes show some dependence on the polarity of the
solvents. The dyes exhibit lower linear absorption in toluene than in
other solvents. With increasing polarity of the solvents from THF to
pyl)pyridinium, inner salt (3)
Dye 3 was prepared as a brownish-red powder with a yield
0% following a method similar to the preparation of dye 2. 1
5
H
NMR (CDCl
J ¼ 7.2 Hz), 2.16(s, 2H), 3.20(s, 2H), 4.03(q, 4H, J ¼ 7.1 Hz), 4.15(q,
H, J ¼ 7.1 Hz), 4.55(s, 2H), 6.83(d, 1H, J ¼ 16.0 Hz), 7.08(t, 2H,
J ¼ 7.3 Hz), 7.14e7.24(m, 10H), 7.32e7.47(m, 7H), 7.91(d, 3H,
3
, 400 MHz),
d
(ppm): 1.21(t, 6H, J ¼ 7.2 Hz), 1.31(t, 3H,
2
1
3
J ¼ 8.2 Hz), 8.13(d, 3H, J ¼ 7.5 Hz), 8.19 (s, 1H), 8.24 (s, 2H).
NMR (CDCl , 100 MHz), (ppm): 13.84, 25.48, 29.72, 37.62, 47.80,
6.71, 78.43, 108.62, 109.25, 114.33, 117.62, 119.78, 120.25, 120.88,
21.40, 121.97, 122.97, 123.20, 126.16, 126.64, 140.22, 141.22,
C
3
CH CN, the absorption peak position of dyes 2 and 3 exhibit slight
3
d
blue shift, but the absorption peak position of dye 1 exhibits obvious
blue shift. This characteristic indicates that the ground state of dye 1
has a certain polarity, but dyes 2 and 3 have a weak polarity.
Compared with absorption properties, SPEF properties of dyes 2
and 3 display obvious dependence on the solvent polarity. From
Table 1 andFig. 2, onecansee that thedyes exhibit weakfluorescence
emission in weak polarity solvent (toluene) and fluorescence inten-
sity and quantum yields decrease from medium polarity solvent
7
1
þ
1
43.61. MS for (M þ H) , Calcd exact mass: 859.3682, found
ꢀ1
8
59.3650. IR (cm ): 3867, 3741, 3053, 2980, 1578, 1532, 1485,
327, 1238, 1157, 1128, 1029, 969, 795, 729, 513. Anal. Calcd. for
S: C, 78.29; H, 5.87; N, 6.52. Found: C, 78.43; H, 5.89;
1
56 50 4 3
C H N O
N, 6.53.
3
(THF) to strong polarity solvent (CH CN). The dyes exhibit the
2
.2.3. 2-[2-(N-ethyl-carbazole-3-yl)vinyl]-N-methylpyridinium,
strongest fluorescence emission in THF. The weak fluorescence
emissionintoluenemay bederivedfromthelowabsorptionabilityof
the dyes. The dependence of fluorescence intensity on the polarity of
the solvent may be explained by the twisted intramolecular charge
transfer (TICT) model [19,20]. Upon increasing the solvent polarity,
0
iodide (1 )
0
Dye 1 was synthesized by reaction of N-ethyl-carbazole-3-
aldehyde and 4-methyl-N-methylpyridinium iodide according to
Ref. [10]. H NMR (DMSO, 400 MHz),
4
1
d
(ppm): 1.35(t, 3H, J ¼ 7.2 Hz),
.41(s, 3H), 4.51(q, 2H, J ¼ 7.2 Hz), 7.30 (t, 1H, J ¼ 7.4 Hz), 7.53 (t, 1H,
the fluorescence emission
max
l of dye 3 is slightly red-shifted.
J ¼ 7.6 Hz), 7.61 (d, 1H, J ¼ 15.6 Hz), 7.69 (d, 1H, J ¼ 8.4 Hz), 7.76 (d,
Based on the discussion above, the dyes exhibit the strongest
fluorescence emission in THF. Linear absorption and fluorescence
properties of the three dyes in THF were compared and the spectra
are shown in Fig. 3. As shown in Table 1 and Fig. 3(a), molar
extinction coefficient values of dyes 2 and 3 are more than twice
that of dye 1. From dye 2 to 3, molar extinction coefficient increase
slightly. This may be because the two branches of dye 2 are almost
1
1
H, J ¼ 8.4 Hz), 7.83 (t, 1H, J ¼ 7.2 Hz), 8.01 (d, 1H, J ¼ 8.4 Hz), 8.19 (d,
H, J ¼ 15.6 Hz), 8.23 (d, 1H, J ¼ 7.6 Hz), 8.46 (t, 1H, J ¼ 7.8 Hz), 8.56
13
(
(
1
d, 1H, J ¼ 8.0 Hz), 8.73 (s, 1H), 8.86 (d, 1H, J ¼ 6.0 Hz), C NMR
DMSO, 100 MHz),
13.71, 119.64, 120.52, 121.59, 122.13, 122.67, 123.98, 124.14, 125.94,
26.45, 126.89, 140.14, 141.09, 143.67, 144.60, 145.61, 152.92. Anal.
Calcd. for C22 : C, 60.01; H, 4.81; N, 6.36. Found: C, 60.12; H,
d (ppm): 13.71, 37.23, 45.98, 109.68, 109.71,
1
H21IN
2
parallel and the molecule can be regarded as a linear Dep-A-peD
4
.79; N, 6.35.
type of molecule with lengthened conjugation length. According to
molecular engineering design theory, molar extinction coefficient
increases with the increase of conjugated bridge length. Dye 3 is
can be regarded as a dye 2 molecule imported by a branch.
According to the assumption reported by Blanchard-Desce et al.
that for some multi-branched systems, each branch makes a similar
contribution to the transition from the ground state to the excited
state, which also can explain the change of molar extinction coef-
ficient with the branches [21].
Linear absorption peaks of the dyes are 437 nm (1), 460 nm (2)
and 464 nm (3), respectively. The linear absorption peaks of dyes 2
and 3 are red-shifted obviously relative to that of 1. The red shift of
absorption peak may be derived from the longer molecular
conjugation length.
2.3. Measurements
UV/vis absorption spectra with C ¼ 5.0 ꢁ 10ꢀ mol L
6
ꢀ1
were
recorded on a Shimadzu UV2550 spectrophotometer. Steady-state
fluorescence measurements were performed at room tempera-
ꢀ
6
ꢀ1
ture with C ¼ 5.0 ꢁ 10 mol L
on an Edinburgh Instrument
were
(
FLS920) spectrometer. The fluorescence quantum yields
F
measured by using a standard method [17] with coumarin 307 [18]
as the standard. Two-photon excited fluorescence (TPEF) spectra of
the dyes in THF with C ¼ 1.0 ꢁ 10 mol L were performed with
a femtosecond Ti:sapphire laser (80 MHz, 80 fs pulse width,
Spectra-Physics Inc., Tsunami 3941-M1 BB) over the range 740e
ꢀ
4
ꢀ1
As shown in Table 1 and Fig. 3(b), with the increase of branches,
fluorescence quantum yield increases slightly, but the fluorescence
8
80 nm as pump source.
Table 1
Linear photophysical properties of dyes 1e3 in various solvents with C ¼ 5.0 ꢁ 10ꢀ mol L
6
ꢀ1
.
Solvents
1
2
3
abs
fluo
abs
fluo
abs
fluo
l
ε
max
4
l
F
l
ε
max
4
l
F
l
ε
max
4
l
F
max
max
max
max
max
max
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
[
nm]
[10 cm mol L]
[nm]
[nm]
[10 cm mol L]
[nm]
[nm]
[10 cm mol L]
[nm]
Toluene
THF
Acetone
424
437
425
423
2.06
2.88
2.86
2.88
590
559
565
565
0.06
0.09
0.03
0.03
453
460
458
454
3.51
6.46
6.58
6.24
598
581
578
579
0.08
0.10
0.03
0.03
466
464
462
458
5.08
6.84
7.48
7.28
599
605
610
611
0.09
0.12
0.05
0.05
3
CH CN

566
H. Chen et al. / Dyes and Pigments 96 (2013) 563e568
Fig. 1. Linear absorption spectra of dyes 2 (a) and 3 (b) in various solvents with
Fig. 2. Single-photon excited fluorescence (SPEF) spectra of dyes 2 (a) and 3 (b) in
various solvents with C ¼ 5.0 ꢁ 10 mol L
C ¼ 5.0 ꢁ 10ꢀ mol L
6
ꢀ1
.
ꢀ6
ꢀ1
.
intensity of organic dyes show a substantial increase. The fluores-
cence intensity of two-branched dye 2 is about twice that of dye 1,
which may be derived from the enhanced absorbance. Although
dye 3 exhibits similar molar extinction coefficient relative to 2, its
fluorescence intensity increases smartly, which indicates that dye 3
has stronger fluorescence emission ability. The fluorescence peak
maximum of the dyes in THF are 559 nm (1), 581 nm (2) and
perpendicular to the pump beam. As shown in Fig. 4, the output
fluorescence intensity of dye 3 is proportional to the square of the
input power from 64 mW to 254 mW, which suggests the two-
photon excitation mechanism. Two-photon excitation spectra of
dyes 2 and 3 in THF with excitation wavelength from 730 nm to
860 nm are also shown in Fig. 4. Detailed experiments reveal that
TPEF peak positions of the dyes are independent of the excitation
wavelengths, but TPEF emission intensities are dependent over that
range. From Fig. 4, we can see that the optimal excitation wave-
length of both dyes 2 and 3 is 810 nm, which is smaller than twice
that of the corresponding linear absorption maximum (460 nm (2),
464 nm (3)). TPEF spectra of the dyes 1e3 shown in Fig. 5 were
taken at 810 nm. Dyes 2 and 3 exhibit strong yellow or orange TPEF
emission in THF at 585 nm (2) and 608 nm (3), which are similar to
that of the corresponding SPEF spectra. Based on this similarity and
different linear absorption and two-photon excitation peak, we can
think that the primary excited states obtained by single-photon
absorption and two-photon absorption may be different because
of the different parity selection rules, but they are likely to relax
quickly to the same fluorescence emission states.
6
05 nm (3), respectively, which is red-shifted with the increase of
branches. Larger molar extinction coefficient value and high fluo-
rescence emission of branched molecules relative to the corre-
sponding linear counterpart appears that there are intramolecular
coupling effects between the branches.
0
A pyridinium dye 1 with discrete cation and anion components
was also synthesized. Its absorption and fluorescence spectra in
THF are shown in Fig. 3. Molar extinction coefficient and quantum
yield (
F
¼ 0.03) of 1 are almost one half times and three times,
0
respectively, that of 1 , which indicates that pyridinium inner salt
structure is beneficial to fluorescence emission and exhibits strong
fluorescence emission than common pyridinium dye with discrete
cation and anion components.
Both dyes 2 and 3 show much stronger frequency-upconverted
fluorescence emission than that of 1. Dye 3 exhibits the strongest
TPEF and the intensity is almost four times that of 1. Two-photon
absorption (TPA) cross-sections of the dyes were measured using
two-photon excited fluorescence method with coumarin 307 as
3
.2. Two-photon excited fluorescence properties
Two-photon excited fluorescence (TPEF) properties of the dyes
in THF are also investigated. TPEF was collected at a direction

H. Chen et al. / Dyes and Pigments 96 (2013) 563e568
567
Fig. 5. Two-photon excited fluorescence (TPEF) spectra of dyes 1e3 in THF excited at
10 nm with C ¼ 1.0 ꢁ 10ꢀ mol L
4
ꢀ1
.
8
Ref. [22], which are 133 GM (1), 518 GM (2) and 606 GM (3) at
10 nm, respectively. All the data above indicate that there is some
8
cooperative enhancement from the coupling effects among the
branches connected by the pyridinium group. These two organic
pyridinium dyes 2 and 3 show promising applications in nonlinear
optical area.
4. Conclusion
Two new organic pyridinium inner salt dyes have been synthe-
sized and the single- and two-photon excited fluorescence proper-
ties of the dyes are investigated. In summary, dye 3 has the strongest
fluorescence emission ability and exhibits strong orange two-
photon excited fluorescence at 608 nm in THF with two-photon
absorption cross-section being 606 GM. These two organic pyr-
idinium dyes show promising applications in nonlinear optical area.
Fig. 3. Linear absorption (a) and single-photon excited fluorescence (b) spectra of dyes
ꢀ
6
ꢀ1
.
1e3 in THF with C ¼ 5.0 ꢁ 10 mol L
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (20802026, 50803033), and Natural Science
Foundation of Shandong Province (Q2008F02) and Science and
Technology Star Funds of Jinan (20110314).
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