Synthesis, crystal structure and third-order nonlinear optical properties in the near-IR range of a novel stilbazolium dye substituted with flexible polyether chains

Article abstract of DOI:10.1016/j.dyepig.2013.01.001

A novel stilbazolium salt with flexible chains, trans- [4-[N,N-bis(2-(2- methoxyethoxy)ethyl)amino]styryl]-N-methylpyridinium tetraphenylborate, was designed, synthesized and fully characterized. Third-order nonlinear optical properties for the new stilba

Full text of DOI:10.1016/j.dyepig.2013.01.001

Dyes and Pigments 97 (2013) 278e285
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis, crystal structure and third-order nonlinear optical
properties in the near-IR range of a novel stilbazolium dye substituted
with flexible polyether chains
*
*
Dandan Li, Daohui Yu, Qiong Zhang, Shengli Li, Hongping Zhou, Jieying Wu , Yupeng Tian
Department of Chemistry, Key Laboratory of Functional Inorganic Materials Chemistry of Anhui Province, Anhui University, Hefei 230039, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel stilbazolium salt with flexible chains, trans- [4-[N,N-bis(2-(2-methoxyethoxy)ethyl)amino]
styryl]-N-methylpyridinium tetraphenylborate, was designed, synthesized and fully characterized.
Third-order nonlinear optical properties for the new stilbazolium salt have been explored. The results
Received 18 October 2012
Received in revised form
2 January 2013
Accepted 3 January 2013
Available online 16 January 2013
show that the novel dye possesses very large values of the real part of the cubic hyperpolarizability c(3)
,
up to 10ꢀ12 esu, and displays maximum two-photon absorption cross sections within the narrow
wavelength range from 950 to 960 nm, while out of the range, it shows a large real part of
(3), which
c
was determined by two-photon induced fluorescence and open/closed aperture Z-scan measurements
using femtosecond pulse laser in near-infrared range, respectively.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Stilbazolium
Flexible chains
Third-order nonlinear optical properties
Two-photon absorption
Z-scan measurements
Near-infrared laser
1. Introduction
reported in some p-aminostyrylpyridinium dyes that synthesized
by Prasad’s group [13] and by us [14e17]. Earlier work on NLO
Molecular materials with nonlinear optical (NLO) properties
have been the focus of intense current research. Such materials are
of great scientific and technologic interest not only for applications
as devices as signal processing, ultrafast optical communication,
data storage, optical limiting, logic devices, all-optical switching,
and bioimaging, but also for the fundamental understanding of
how soft matter interacts with light [1e7]. Research advances in
this field depend critically on the development of new materials
with strong NLO response. Therefore significant effort has been
devoted to building novel NLO molecules. Stilbazolium salts are the
best studied amongst such materials and are particularly attractive
for device applications [8,9].
materials has established that increasing the internal charge
transfer and dimensionality of the molecule, extending -conju-
p
gated system, and assembling inorganic metal ions with organic
units which will improve the NLO response. Recently, new chro-
mophore design included tuning the molecular bond length
alternation characteristics, introducing electron-rich and
electron-deficient hetero-cyclic bridges were proposed, which act
as auxiliary donors and acceptors, into the molecular skeleton.
Those are different from previous NLO strategies. Very recently,
a new chromophore was designed and synthesized to promote the
advances in the field of molecular materials with NLO activity by
Prasad’s group [18].
It was previously studied by Marder’s group that trans-40-
(dimethylamino)-N-methyl-4-stilbazolium tosylate (DAST) ex-
hibits very pronounced bulk quadratic NLO activity [10]. Then
a series of N-aryl stilbazolium salts possessing quadratic NLO
properties were systematically investigated by Coe’s group [11,12].
Recently, efficient TPP (two-photon pumped) lasing has been
In this context, we launched a program aimed in the purpose of
developing soluble dye in high polar solvents, which shows a neg-
ative solvatochromism and the longest excited-state lifetime in
water. Furthermore, it exhibits peak intense two-photon fluo-
rescence-emission within narrow wavelength range, and large
values of the real part of the cubic hyperpolarizability
c
(3). For
another reason, as we know, organic dyes are extensively used as
the signaling units in chemosensor design because of their intense
absorption and emission properties, which are sensitive to external
inputs [19]. In this work, we introduced two methoxyethoxyethyl
* Corresponding authors. Tel.: þ86 551 5108151; fax: þ86 551 5107342.
E-mail address: yptian@ahu.edu.cn (Y. Tian).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.01.001

D. Li et al. / Dyes and Pigments 97 (2013) 278e285
279
Fig. 1. Synthesis of 5.
groups into the molecular skeleton, which may provide the
possibility for acting as an ion probe or sensing to polarity of the
solvents.
TPEF spectrawere measured using femtosecond laser pulse and Ti:
sapphire system (680e1080 nm, 80 MHz, 140 fs, Chameleon II) as the
light source. All measurements were carried out at room temperature.
5 was synthesized by the following reactions (shown in Fig. 1).
2. Experiments
2.2. Synthesis
2.1. General
2.2.1. 2-Methoxyethyl-4-methylbenzenesulfonate (1)
A solution of 2-methoxyethanol (7.36 mL, 0.10 mol) and a cat-
alytic amount of TBAB (tetra-n-butylammonium bromide) in
dichloromethane was stirred, and subsequently NaOH (aq., 30%,
15 mL) was added. A solution of p-toluenesulfonyl chloride in
dichloromethane was added dropwise. The mixture was stirred for
24 h at room temperature, and then washed with distilled water
three times. The product was dried over Na2SO4, the solvent was
removed under reduced pressure to give a yellow oil. Yield 95%. IR
(KBr, cmꢀ1) selected bands: 3468 (m), 2938 (s), 2889 (s), 1597 (m),
1455 (m), 1357 (s), 1184 (s), 1130 (m), 1101 (m), 1018 (m), 919 (m),
All chemicals used were of analytical grade and the solvents
were purified by conventional methods before use. The 1H-NMR
spectra were performed on Bruker 400 MHz spectrometer with
TMS as the internal standard, coupling constants J are given in
Hertz. Elemental analysis was performed on PerkineElmer 240
instrument. Mass spectra were determined with a Micromass GCT-
MS (ESI source) and MALDI-TOF-MS. IR spectra were recorded on
NEXUS 870 (Nicolet) spectrophotometer in the 400e4000 cmꢀ1
region using a powder sample on a KBr plate.
Single-crystal X-ray diffraction measurements were carried
out on a Bruker Smart 1000 CCD diffractometer equipped with
a graphite crystal monochromator situated in the incident beam for
data collection at room temperature. The determination of unit cell
Table 1
Crystal data and structure refinement for 5.
parameters and data collections were performed with MoKa radi-
ꢀ
Empirical formula
Formula weight
C48H55BN2O4
734.75
ation (
l
¼ 0.71073 A). Unit cell dimensions were obtained with
least-squares refinements, and all structures were solved by direct
methods using SHELXL-97. All non-hydrogen atoms were refined
anisotropically. The hydrogen atoms were added theoretically and
not refined. The final refinement was performed by full-matrix
least-squares methods with anisotropic thermal parameters for
non-hydrogen atoms on F2.
Temperature
Wavelength
298(2) K
0.71069 A
ꢀ
Crystal system, space group
Unit cell dimensions
Monoclinic, P21/n
ꢀ
a ¼ 11.498(5) A
ꢀ
b ¼ 20.358(5) A
ꢀ
c ¼ 18.085(5) A
b
¼ 97.436(5)ꢁ
3
Electronic absorption spectra were obtained on a UV-265 spec-
trophotometer. Fluorescence measurements were performed using
a Hitachi F-7000 fluorescence spectrophotometer.
The TD-DFT {B3LYP[LANL2DZ]} calculations were performed on
the optimized structure. All calculations were performed with the
G03 software, the TDDFT calculation of the lowest 25 singlete
singlet excitation energies were calculated with a basis set com-
posed of 6e31 G(d,p) for C N H O atoms.
ꢀ
Volume
4198(2) A
Z, Calculated density
Absorption coefficient
F(000)
4, 1.163 mg/m3
0.073 mmꢀ1
1576
Crystal size
Theta range for data collection
Limiting indices
0.30 ꢂ 0.20 ꢂ 0.20 mm
1.51e25.00ꢁ
ꢀ12 ꢃ h ꢃ 13, ꢀ21 ꢃ k ꢃ 23, ꢀ16 ꢃ l ꢃ 16
24445/6565 [R(int) ¼ 0.0610]
88.8%
Reflections collected/unique
Completeness to theta ¼ 25.00
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
Largest diff. peak and hole
None
For time-resolved fluorescence measurements, the fluorescence
signals were collimated and focused onto the entrance slit of
a monochromator with the output plane equipped with a photo-
multiplier tube (HORIBA HuoroMax-4P). The decays were analyzed
by ‘least-squares’. The quality of the exponential fits was evaluated
0.9856 and 0.9785
Full-matrix least-squares on F2
6565/0/499
1.031
R1 ¼ 0.0604, wR2 ¼ 0.1564
ꢀꢀ3
0.233 and ꢀ0.202 e. A
by the goodness of fit (c
2).

280
D. Li et al. / Dyes and Pigments 97 (2013) 278e285
Table 2
Selected bond lengths (A) and angles ( ) of 5.
mixture was stirred vigorously while a solution of 2 (3.00 g,
ꢁ
ꢀ
0.01 mol) in chloroform (25 mL) was added dropwise, the reaction
mixture was stirred and heated under reflux for 15 h at 65 ꢁC and
then cooled to room temperature, and then the residue was
poured into 500 mL ice water slowly. The NaOH solution was
added to adjust the pH of the solution under vigorous stirring.
After the pH in the solution had reached 8.0, the mixture was
extracted by dichloromethane and dried over by Na2SO4. The
product obtained after evaporation under reduced pressure was
purified by silica gel chromatography column using petroleum/
ethylacetate (3:1 v/v). Yellow oil product 3 was collected. Yield
80%. IR (KBr, cmꢀ1) selected bands: 3494 (m), 2877 (s), 2726 (m),
1667 (m), 1596 (s), 1400 (m), 1315 (m), 1170 (m), 1112 (m), 814 (m),
723 (m), 503 (m). 1H-NMR (400 MHz, CD3COCD3): 3.30 (s, 6H), 3.71
(t, J ¼ 3.6 Hz, 8H), 3.49 (q, J ¼ 3.8 Hz, 4H), 3.58e3.60 (q, J ¼ 3.8 Hz,
4H), 6.88 (d, J ¼ 9.2 Hz, 2H), 7.70 (d, J ¼ 8.4 Hz, 2H), 9.72 (s, 1H).
MALDI-TOF: m/z, cal: 324.59, found: 324.26 (Mþ). Anal. Calc. for
C17H52NO5: C, 62.75; H, 8.36; N, 4.30. Found: C, 62.64; H, 8.50; N,
4.37.
N2eC13
N2eC15
N2eC20
C6eC7
C7eC8
C8eC9
1.368(4)
1.463(4)
1.455(4)
1.445(4)
1.338(4)
1.442(4)
121.0(3)
121.7(3)
117.3(3)
C
C
C
13eN2eC15
13eN2eC20
20eN2eC15
821 (m). 1H-NMR (400 MHz, CD3COCD3): 3.20 (s, 3H), 3.48 (t,
J ¼ 4.6 Hz, 2H), 4.08 (t, J ¼ 4.6 Hz, 2H), 7.71 (d, J ¼ 8.4 Hz, 2H), 7.28
(d, J ¼ 8.4 Hz, 2H), 2.36 (s, 3H). Anal. Calc. for C10H14SO4: C, 52.16;
H, 6.13. Found: C, 52.09; H, 6.17.
2.2.2. N,N-Bis(2-(2-methoxyethoxy)ethyl)benzen amine (2)
A solution of N-phenyldiethanolamine (1.81 g, 0.01 mol) in
acetonitrile was slowly dropped into a solution of NaH (0.48 g,
0.02 mol) in acetonitrile, then 1 (4.60 g, 0.02 mol) was added
dropwise. The reaction mixture was stirred and refluxed for 24 h at
85 ꢁC. The suspension was filtered, washed with acetonitrile, and
the filtration containing product was obtained. The product
obtained after evaporation under reduced pressure was purified by
silica gel chromatography column using petroleum/ethylacetate
(6:1 v/v). Light yellow oil product 2 was collected. Yield 70%. IR
(KBr, cmꢀ1) selected bands: 3436 (m), 2921 (s), 1599 (m), 1505 (m),
1459 (m), 1379 (m), 1195 (m), 1117 (m), 752 (m), 686 (m). 1H-NMR
(400 MHz, CD3COCD3): 3.32 (s, 6H), 3.59 (t, J ¼ 5.2 Hz, 8H), 3.50 (t,
J ¼ 4.6 Hz, 4H), 3.65 (t, J ¼ 6 Hz, 4H), 6.75 (d, J ¼ 8.4 Hz, 2H), 7.18 (t,
J ¼ 8.0 Hz, 2H), 6.63 (t, J ¼ 7.2 Hz, 1H). MALDI-TOF: m/z, cal: 297.25,
found: 297.39 (Mþ). Anal. Calc. for C16H27NO4: C, 64.62; H, 9.15; N,
4.71. Found: C, 64.43; H, 9.50; N, 4.87.
2.2.4. 1,4-Dimethylpyridinium iodide (4)
1,4-Dimethylpyridinium iodide was synthesized according to
the literature method [20]. White powder product 4 was collected.
Yield 90%. Mp: 155 ꢁC. IR (KBr, cmꢀ1) selected bands: 3451 (m),
3023 (m), 1644 (s), 1517 (m), 1517 (m), 1477 (m), 1289 (s), 1182 (s),
1043 (m), 809 (s), 697 (s), 485 (s). 1H-NMR (400 MHz, d6-DMSO):
2.61 (3H, s), 4.29 (3H, s), 7.97 (2H, d, J ¼ 6.4 Hz), 8.84 (2H, d,
J ¼ 6.4 Hz); Mþ(MS/ESI), 108.48.
2.2.5. Trans-[4-[N,N-bis(2-(2-methoxyethoxy)ethyl)amino]styryl]-
N-methylpyridinium tetraphenylborate (5)
Using a 100 mL one-necked flask fitted with a stirrer and
a condenser, 3.25 g (0.01 mol) of compound 3, 2.35 g (0.01 mol) of
compound 4, and 30 mL of absolute ethanol were mixed. Five drops
of piperidine were added to the mixture. Then the solution was
heated to reflux for 4 h. After cooling, 3.46 g (0.01 mol) of sodium
tetraphenylborate was added into the solution. The solution again
2.2.3. 4-(Bis(2-(2-methoxyethoxy)ethyl)amino)benzaldehyde (3)
POCl3 (1.84 g, 0.05 mol) was slowly dropped into a dry 100 mL
flask which contained 0.73 g (0.01 mol) DMF in ice bath. The
Fig. 2. Crystal structure of 5. Hydrogen atoms are omitted for clarity.

D. Li et al. / Dyes and Pigments 97 (2013) 278e285
281
3. Results and discussion
3.1. Structural features
The single crystals of 5, suitable for the X-ray analysis, were
obtained from the slow evaporation of dichloromethane covered
with methanol at room temperature. The crystal data collection and
refinement parameters are listed in Table 1. The selected bond
distances and angles are given in Table 2. The structure of 5,
together with the atom numbering scheme is shown in Fig. 2. For
the molecular structure 5, the sum of the three CeNeC angles
taking nitrogen atom as center (C13eN2eC20, 121.7(3)ꢁ; C13eN2e
C15, 121.0(3)ꢁ; C20eN2eC15, 117.3(3)ꢁ) is 360.0ꢁ, therefore the
trigonal NC3 is practically coplanar. The least-square plane calcu-
lations show that the dihedral angle between the benzene ring and
pyridine ring is 13.5ꢁ, indicating they are nearly coplanar. It can be
seen from Table 2 that all the bond lengths of CeC are located
ꢀ
between the normal C]C double bond (1.32 A) and CeC single
ꢀ
bond (1.53 A), which show that there is a highly
p-electron delo-
Fig. 3. Linear absorption (c ¼ 2 ꢂ 10ꢀ6 mol Lꢀ1) spectra of 5 in different solvents.
calized system in the molecule, which is the necessary condition for
it bearing a strong NLO active.
was heated to reflux for 10 min. A red solid formed after cooling.
The solution was filtered, and the solid was washed twice with
ethanol and water, respectively. Red solid product 5 was collected.
Mp: 130 ꢁC. FT-IR (KBr, cmꢀ1) v: 707 (m),1175 (s),1523 (m),1585 (s),
1644 (w), 3063 (w), 2981 (w), 2924 (w), 2891 (w). 1H-NMR
(400 MHz, CD3COCD3): 8.36 (d, J ¼ 6.4 Hz, 2H), 7.89 (d, J ¼ 6.8 Hz,
2H), 7.83 (d, J ¼ 16.0 Hz, 1H), 7.58 (d, J ¼ 8.8 Hz, 2H), 7.36 (m, 9H),
7.11 (d, J ¼ 16.4 Hz, 1H), 6.93 (t, J ¼ 7.6 Hz, 9H), 6.86 (d, J ¼ 8.8 Hz,
2H), 6.78 (t, J ¼ 7.2 Hz, 4H), 4.16 (s, 3H), 3.69 (s, 8H), 3.58 (t,
J ¼ 5.2 Hz, 4H), 3.48 (t, J ¼ 4.0 Hz, 4H), 3.29 (s, 6H). 13C-NMR
(100 MHz, CD3COCD3): 164.9, 154.7, 151.5, 144.8, 143.3, 136.9, 131.4,
126.2, 123.2, 117.6, 112.8, 72.7, 72.0, 69.2, 58.9, 51.8, 47.2, 44.1. Mþ
(MS/ESI), 415.33. Anal. Calc. for C48H55BN2O4: C, 78.46; H, 7.54; N,
3.81. Found: C, 78.04; H, 7.50; N, 3.87.
3.2. Linear absorption properties and TD-DFT studies
Fig. 3 shows the absorption spectra of 5 (2
mM) in different
solvents. As can be seen from the Fig. 3, by increasing the polarity of
the solvent, the absorption of 5 exhibits a blue shift, from DMF to
water (from 470 to 450 nm, respectively). This behavior is attrib-
uted to a relatively high polar mesomeric form, which is predom-
inant in the ground state. As a result, the hydrogen bond donating
solvents decrease the energy of the ground state, by acting as
proton donors, leading to a blue shift of the absorption band [21].
The linear absorption of 5 in different organic solvents features
one intense absorption band between 450 and 490 nm with the
corresponding molar extinction coefficient (w50,000 Mꢀ1 cmꢀ1),
Fig. 4. Molecular orbital energy diagram of 5.

282
D. Li et al. / Dyes and Pigments 97 (2013) 278e285
Table 3
Table 4
Excitation energy (E), corresponding wavelength (l), oscillator strength (f) and
Single-photon-related photophysical properties of 5 in different solvents.
major contribution of 5.
Fd
D )
v (cmꢀ1
a
b
c
Solvent
lmax
(
3
)
lmax
max
E (eV)
l
(nm)
f
Composition (C)
191(H-6)->198(0.59508)
Character
ICT
Benzene
Ethyl acetate
THF
Ethanol
Methanol
Acetonitrile
DMF
493(4.95)
479(4.10)
490(4.90)
481(5.25)
474(5.20)
469(4.90)
472(5.00)
470(6.00)
450(4.10)
566
590
597
589
592
591
595
599
588
0.0405
0.0549
0.0373
0.0468
0.0187
0.0124
0.0289
0.0363
0.0139
2616
3927
3657
3812
4205
4401
4379
4582
5215
2.7020
458.8
1.2189
which originates from the
(ICT) transition.
p / p* or intramolecular charge transfer
DMSO
H2O
TD-DFT computational studies were performed to elucidate the
electronic structures of the ground state of 5. The schematic rep-
resentation of the molecular orbitals of 5 was exhibited in Fig. 4,
and the energy and composition of ICT are listed in Table 3. The
energy band calculated is at 458.86 nm with oscillator strength
(f ¼ 1.2189). Fig. 4 shows the calculated frontier orbitals of 5, the
energy band was tentatively assigned to ICT. This band mainly
originates from transitions of HOMO-6 to LUMO. Therefore the
initial DFT and TD-DFT calculations provide reasonable explana-
tions for its absorption spectra.
a
Peak position of the longest absorption band.
Maximum molar absorbance in 104 molꢀ1 L cmꢀ1
Peak position of SPEF, exited at the absorption maximum.
Quantum yields determined by using Rh6G as standard.
b
c
.
d
ꢀ
ꢁ
A
A
rhs2Ds
shr2Dr
Fs
¼
Fr
where the s and r indices designate the sample and reference
samples, respectively, A is the absorbance at lexc is the average
3.3. One-photon induced fluorescence
,
h
refractive index of the appropriate solution, and D is the integrated
area under the corrected emission spectra [24].
The single-photon induced emission spectra of 5 (2 mM) in dif-
ferent solvents are illustrated in Fig. 5. It exhibits a weak fluo-
rescence intensity, which is similar to the other the normal
Stilbazolium salts [22,23]. This phenomenon is attributed to that
the excited state of 5 is quenched by the anion. The quenching ef-
ficiency depends on the chemical nature of the anion and the
surroundings [13].
As shown in Fig. 5 (normalized for excitation in benzene and
DMSO), one can see that the fluorescence spectra incline to red shift
as the solvent polarity increase (566 nm in benzene to 599 nm in
DMSO), indicating that the dipole moment of 5 in the excited state
is larger than that in the ground state, and an increase in the po-
larity of the solvent will lower the energy level of the charge
transfer excited state [23].
As shown in Table 4, the photophysical properties of 5 show
low quantum yield (<10%). The behavior is due to certain non-
radiative decay mechanisms that may arise as a result of ‘twisted
intramolecular charge geometry’ (TICT) [25], 5 undergoes an
intramolecular transfer of an electron from the donor to the
acceptor in the excited state, which is accompanied by a twist
around the bond joining the donor and acceptor.
3.5. Fluorescence lifetime
To get more insight into the radiative and nonradiative decay
processes, we also conducted time-resolved lifetime experiments.
As shown in Table 5, 5 has a variable excited-state lifetime (50e
300 ps) in various solvents. It’s worth taking a moment to notice the
fact that the fluorescence lifetime in water is the longest. As we
know, the fluorescence lifetime is a relatively long process on the
time scale of molecular events, and during this time, a high energy
fluorophore can undergo a great variety of transformations, ranging
from electron redistribution and geometric alteration to reorgan-
ization of the surrounding molecules and chemical reactions [26].
In many cases, the energy gained as a result of photon absorption is
lost as a nonradiative processes, collectively called quenching [27],
and inevitably leads to the decrease of the fluorescence lifetime.
From Table 5 we can observe that, with increasing polarity of the
solvent, the fluorescence lifetime of 5 decreased, thereby sup-
porting the assumption that because of the nonradiative decay
arised as a result of TICT enhancing the quenching efficiency.
3.4. Quantum yield determination
The fluorescence quantum yields (F) were determined using
Rh6G as the reference according to the literature method [15].
Quantum yields were corrected as follows:
3.6. Two-photon excited fluorescence (TPEF)
Detailed experiments reveal that there is no significant linear
absorption in the spectral range from 600 to 1200 nm, which in-
dicates that there are no molecular energy levels corresponding to
Table 5
Fluorescence lifetime of 5 in different solvents.
Solvent
Ethyl acetate
255
THF
242
DMF
171
DMSO
47
Water
293
s
/psa
Fig. 5. One-photon induced fluorescence (c ¼ 2 ꢂ 10ꢀ6 mol Lꢀ1) spectra of 5 in dif-
a
ferent solvents.
The fitted fluorescence lifetime.

D. Li et al. / Dyes and Pigments 97 (2013) 278e285
283
3.7. TPA cross-sections
The two-photon absorption (TPA) cross-section
s
was measured
by comparing the TPEF (two-photon excited fluorescence) intensity
of the sample with that of a reference compound by the following
equation:
Fref cref nref
F
d
¼
dref
F
c
n Fref
Here, the subscripts ref stands for the reference molecule
(Rh6G). V is the quantum yield, n is the refractive index, F is the
integrated area under the corrected emission spectrum, c is the
concentration of the solution in mol Lꢀ1. The dref value of reference
was taken from the literature [31].
As shown in Fig. 8, two-photon absorption cross sections (d) of 5
were measured in the wide wavelength range from 800 to
1000 nm. The maximum values in DMF and DMSO are 517 and
668 GM, respectively. Interestingly, from these data, it can be
observed that the dramatically larger two-photon absorption cross
sections appear in the narrow pathlength about 950e970 nm. This
is very different in comparison to previous results [32e37]. Clearly,
the present structural modifications can result in pronounced dif-
ferences in the absorption cross section. Therefore, it is tentatively
suggested that there are excited-states (S1eSn) distributing in
a much narrow energy band [38].
Fig. 6. The two-photon fluorescence spectra of
5
in DMF and DMSO
(c ¼ 1.0 ꢂ 10ꢀ3 mol Lꢀ1). Insert figure: Output fluorescence (Iout) vs. the square of input
laser power (Iin) for 5 excitation carried out at 960 nm, with c ¼ 1.0 ꢂ 10ꢀ3 mol Lꢀ1 in
DMSO.
an electron transition in the spectral range. Therefore, upon exci-
tation from 600 to 1200 nm, it is impossible to produce single-
photon-excited up-converted fluorescence. The linear depend-
ence on the square of input laser power suggests a two-photon
excitation mechanism at 960 nm for 5 (as an insert figure in Fig. 6).
The two-photon excited fluorescence spectra of 5 were inves-
tigated in two organic solvents (Fig. 6, c ¼ 1.0 ꢂ 10ꢀ3 mol Lꢀ1), upon
excitation at the optimal wavelength (960 nm), the TPEF spectra of
5 are presented. From Fig. 7 one can see that a two-photon-excited
fluorescence band appears about 30 nm red-shifted as compared to
its single-photon counterpart. This is due to the re-absorption ef-
fect of 5 solution at high concentration [28].
In addition, a majority of the biological samples possess intrinsic
florescence due to molecules which are inherently fluorescent and
absorb in the ultraviolet and visible region [29]. In our case, the
studied 5 can be excited using an IR beam (960 nm) and emit effi-
ciently in the visible region (620 nm) of the spectrum. This is the most
attractive feature of the dye for its utility as a biological probe [30].
3.8. TPA coefficient
b
and c(3)
To further confirm the TPA performances, the TPA coefficient
and c(3) of 5 were measured by the Z-scan technique [39,40]. Z-
b
scan is one of the commonly utilized methods to perform c(3)
measurements. The advantage of this method is that both the real
and imaginary parts of c(3) can be determined simultaneously or
consecutively, using the so-called closed-aperture and open-
aperture conditions, respectively. In the present experiments,
a 1 ꢂ 10ꢀ3 M solution of 5 in DMSO contained in a 1.0 mm path-
length quartz cell for Z-scan measurements was performed sepa-
rately at 920, 960 and 1000 nm wavelengths.
The TPA coefficient
b
and c(3) calculation method can be
obtained according to literature methods [18,41]. Fig. 9AeC shows
the normalized transmittance plotted as a function of the sample
position (z) measured by using open aperture and closed aperture
Z-scan technique at 920, 960 and 1000 nm, respectively. Table 6
Fig. 7. (a) One-photon excited fluorescence spectrum of
5
in DMSO
(c ¼ 2 ꢂ 10ꢀ6 mol Lꢀ1), (b) Two-photon excited fluorescence spectrum of 5 in DMSO
Fig. 8. Two-photon absorption cross sections of 5 in DMF and DMSO vs. excitation
wavelengths of identical energy of 0.500 W.
(c ¼ 1.0 ꢂ 10ꢀ3 mol Lꢀ1).

284
D. Li et al. / Dyes and Pigments 97 (2013) 278e285
Fig. 9. A. Normalized open-aperture (left) and closed-aperture (right) Z-scan transmittance of 5 in DMSO (0.001 M) at 920 nm. B. Normalized open-aperture (left) and closed-
aperture (right) Z-scan transmittance of 5 in DMSO (0.001 M) at 960 nm. C. Normalized open-aperture (left) and closed-aperture (right) Z-scan transmittance of 5 in DMSO
(0.001 M) at 1000 nm.
shows the third-order nonlinearity parameters of 5. From these
data, it can be seen that 5 exhibits a large nonlinear refractive index
coefficient and possesses very large values of the real part of the
cubic hyperpolarizability c(3) at 920 and 1000 nm, respectively.
However, no TPA was observed under the same conditions. The
results are in good agreement with those determined by two-
photon-induced fluorescence measurement technique.
4. Conclusions
A new stilbazolium salt with flexible polyether chains, 5, was
designed and synthesized. One-photon absorption and emission
spectra, the excited-state lifetime, two-photon excited fluorescence
behavior have been systematically investigated. The relationships
between the structures and photophysical properties of 5 can be
understood based on both the experimentally and theoretically. It
was found that 5 showed a strong solvent-polarity-dependent
fluorescence in the visible region, and its fluorescence lifetime is
the longest in water compared to those in the other solvents. Fur-
thermore, it exhibits peak intense two-photon fluorescence
emission when excited at the narrow wavelength range of a near-
infrared pulse laser and possesses very large values of the real
Table 6
Open- and closed-aperture Z-scan measurement data for the third-order non-
linearity parameters of 5.
Wavelength
920 nm
960 nm
1000 nm
n2 extrapolated to solute
4.08 ꢂ 10ꢀ10 4.29 ꢂ 10ꢀ10 1.45 ꢂ 10ꢀ10
(cm2/W)
Re (
(esu)
c
(3)) extrapolated to solute 2.26 ꢂ 10ꢀ12 2.38 ꢂ 10ꢀ12 8.04 ꢂ 10ꢀ13
2PA coefficient a2 (cm/GW)
Extrapolated to solute
Two-photon cross section
0.039
1368.61
1.68 ꢂ 10ꢀ15
s
Im (
(GM)
c
(3)) extrapolated to solute
part of the cubic hyperpolarizability
how to use the NLO molecular material under selected different
c
(3). The results also suggest
(esu)

D. Li et al. / Dyes and Pigments 97 (2013) 278e285
285
[17] Wu JY, Hu GJ, Wang P, Hao FY, Zhou HP, Zhou AM, et al. Organic/poly-
oxometalate hybridization dyes: crystal structure and enhanced two-photon
absorption. Dye Pigment 2010;88:174e9.
wavelengths. The foregoing behavior should establish the founda-
tion for the development of third-order nonlinear optical molecular
materials.
[18] He GS, Zhu J, Baev A, Samo M, Frattarelli DL, Watanabe N, et al. Twisted
p-
system chromophores for all-optical switching. J Am Chem Soc 2011;13:
6675e80.
Acknowledgments
[19] (a) de Silva AP, Gunaratne HQN, Gunnlaugsson T. Signaling recognition events
with fluorescent sensors and switches. Chem Rev 1997;97:1515e66;
(b) Bissell RA, de Silva AP, Gunaratne HQN, Lynch PLM. Molecular fluorescent
signaling with fluor-spacer-receptor systems: approaches to sensing and
switching devices via supramolecular photophysics. Chem Soc Rev 1992;21:
187e95.
[20] Zhao CF, He GS, Bhawalkar JD, Park CK, Prasad PN. Newly synthesized dyes
and their polymer/glass composites for one- and two-photon pumped solid-
state cavity lasing. Chem Mater 1995;7:1979e83.
[21] Lal M, Levy L, Kim KS, He GS, Wang X, Min YH, et al. Silica nanobubbles
containing an organic dye in a multilayered organic/inorganic heterostructure
with enhanced luminescence. Chem Mater 2000;12:2632e9.
[22] Zhao CF, Gvishi R, Narang U, Ruland G, Prasad PN. Structures, spectra, and
lasing properties of new (aminostyryl)pyridinium laser dyes. J Phys Chem
1996;100:4526e32.
This work was supported by a grant for the National Natural
Science Foundation of China (51142011, 21071001, 21271003,
21271004), the Natural Science Foundation of Anhui Province
(1208085MB22), Ministry of Education Funded Projects Focus on
returned overseas scholar. Program for New Century Excellent
Talents in University (China), Doctoral Program Foundation of
Ministry of Education of China (20113401110004).
Appendix A. Supplementary data
[23] Wang XM, Wang D, Zhou GY, Yu WT, Zhou YF, Fang Q, et al. Symmetric and
asymmetric charge transfer process of two-photon absorbing chromophores:
bis-donor substituted stilbenes, and substituted styrylquinolinium and styr-
ylpyridinium derivatives. J Mater Chem 2001;11:1600e5.
[24] Gray TG, Rudzinski CM, Meyer EE, Holm RH, Nocera DG. Spectroscopic and
photophysical properties of hexanuclear rhenium(III) chalcogenide clusters.
J Am Chem Soc 2003;125:4755e70.
[25] Rullière C, Grabowski ZR, Dobkowski J. Picosecond absorption spectra of
carbonyl derivatives of dimethylaniline: the nature of the tict excited states.
Chem Phys Lett 1987;137:408e13.
[26] Peng XJ, Yang ZG, Wang JY, Fan JL, He YX, Song FL, et al. Fluorescence
ratiometry and fluorescence lifetime imaging: using a single molecular
sensor for dual mode imaging of cellular viscosity. J Am Chem Soc 2011;133:
6626e35.
Crystallographic data for the structural analysis have been
deposited at the Cambridge Crystallographic Data Center, CCDC-
874010 for 5. Copy of this information may be obtained free of
charge via www: http://www.ccdc.cam.ac.uk or from The Director,
CCDC, 12 Union Road, Cambridge CB221EZ, UK (fax: þ44 1223/
336 033; email: deposite@ccdc.cam.ac.uk). Structural factors are
available on request from the authors. Supporting information can
be found in the web version of this paper. Supplementary data
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.dyepig.2013.01.001.
[27] Berezin MY, Achilefu S. Fluorescence lifetime measurements and biological
imaging. Chem Rev 2010;110:2641e84.
References
[28] Ren Y, Fang Q, Yu WT, Lei H, Tian YP, Jiang MH, et al. Synthesis, structures and
two-photon pumpedup-conversion lasing properties of two new organic
salts. J Mater Chem 2000;10:2025e30.
[29] Chorvat Jr D, Chorvatova A. Multi-wavelength fluorescence lifetime spectro-
scopy a new approach to the study of endogenous fluorescence in living cells
and tissues. Laser Phys Lett 2009;6:175e93.
[30] Narang U, Zhao CF, Bhawalkar JD, Bright FV, Prasad PN. Characterization of
a new solvent-sensitive two-photon-induced fluorescent (aminostyryl) pyr-
idinium salt dye. J Phys Chem 1996;100:4521e5.
[1] He GS, Tan LS, Zheng QD, Prasad PN. Multiphoton absorbing materials: molecular
designs, characterizations, and applications. Chem Rev 2008;108:1245e330.
[2] Pawlicki M, Collins HA, Denning RG, Anderson HL. Two-photon absorption
and the design of two-photon dyes. Angew Chem Int Ed 2009;48:3244e66.
[3] Therien MJ. How to improve your image. Nature 2009;458:716e7.
[4] Reeve JE, Collins HA, Mey KD, Kohl MM, Thorley KJ, Paulsen O, et al. Amphi-
philic porphyrins for second harmonic generation imaging. J Am Chem Soc
2009;131:2758e9.
[31] (a) Xu C, Webb WW. Measurement of two-photon excitation cross sections of
molecular fluorophores with data from 690 to 1050 nm. J Opt Soc Am B 1996;
13:481e91;
(b) Albota MA, Xu C, Webb WW. Two-photon fluorescence excitation cross
sections of biomolecular probes from 690 to 960 nm. Appl Opt 1998;13:481.
[32] Iwase Y, Kamada K, Ohta K, Kondo K. Synthesis and photophysical properties
of new two-photon absorption chromophores containing a diacetylene moi-
[5] Hales JM. Design of polymethine dyes with large third-order optical non-
linearities and loss figures of merit. Science 2010;327:1485e8.
[6] Haque SA, Nelson J. Toward organic all-optical switching. Science 2010;327:1466e7.
[7] Sumalekshmy S, Fahrni CJ. Metal-ion-responsive fluorescent probes for two-
photon excitation microscopy. Chem Mater 2011;23:483e500.
[8] Li L, Cui HJ, Yang Z, Tao XT, Lin XS, Ye N, et al. Synthesis and characterization of
thienyl-substituted pyridinium salts for second-order nonlinear optics. Crys-
tEngComm 2012;14:1031e7.
ety as the central
p-bridge. J Mater Chem 2003;13:1575e81.
[33] Kamada K, Iwase Y, Sakai K, Kondo K, Ohta K. Cationic two-photon absorption
chromophores with double- and triple-bond cores in symmetric/asymmetric
arrangements. J Phys Chem C 2009;113:11469e74.
[34] Feng XJ, Wu PL, Bolze F, Leung HWC, Li KF, Mak NK, et al. Cyanines as new
fluorescent probes for DNA detection and two-photon excited bioimaging.
Org Lett 2010;12:2194e7.
[9] Lacroix PG, Carmen Munoz M, Gaspar AB, Real JA, Bonhommeau S,
Rodriguez V, et al. Synthesis, crystal structures, and solid state quadratic
nonlinear optical properties of a series of stilbazolium cations combined with
gold cyanide counter-ion. J Mater Chem 2011;21:15940e9.
[10] Marder SR, Perry JW, Schaefer WP. Synthesis of organic salts with large
second-order optical nonlinearities. Science 1989;245:626e8.
[35] Wang XC, Tian XH, Zhang Q, Sun PP, Wu JY, Zhou HP, et al. Assembly, two-
photon absorption, and bioimaging of living cells of a cuprous cluster. Chem
Mater 2012;24:954e61.
[11] Coe BJ, Harris JA, Asselberghs I, Clays K, Olbrechts G, Persoons A, et al.
Quadratic nonlinear optical properties of N-aryl stilbazolium dyes. Adv Funct
Mater 2002;12:110e6.
[36] Masanta G, Lim CS, Kim HJ, Han JH, Kim HM, Cho BR. A mitochondrial-targeted
two-photon probe for zinc ion. J Am Chem Soc 2011;133:5698e700.
[37] Chen DG, Zhong C, Dong XH, Liu ZH, Qin JG. A new building block, bis(thio-
phene vinyl)-pyrimidine, for constructing excellent two-photon absorption
materials: synthesis, crystal structure and properties. J Mater Chem 2012;22:
4343e8.
[12] Coe BJ, Harris JA, Asselberghs I, Wostyn K, Clays K, Persoons A, et al. Quadratic
optical nonlinearities of N-methyl and N-aryl pyridinium salts. Adv Funct
Mater 2003;13:347e57.
[13] Zheng QD, He GS, Lin TC, Prasad PN. Synthesis and properties of substituted
(p-aminostyryl)-1-(3-sulfooxypropyl)pyridinium inner salts as a new class of
two-photon pumped lasing dyes. J Mater Chem 2003;13:2499e504.
[14] Zhang XJ, Tian YP, Jin F, Wu JY, Xie Y, Tao XT, et al. Self-assembly of an organic
chromophore with Cd-S nanoclusters: supramolecular structures and
enhanced. Cryst Growth Des 2005;5:565e70.
[15] Hao FY, Zhang XJ, Tian YP, Zhou HP, Li L, Wu JY, et al. Design, crystal structures
and enhanced frequency-upconverted lasing efficiencies of a new series of
dyes from hybrid of inorganic polymers and organic chromophores. J Mater
Chem 2009;19:9163e9.
[38] Johnsen M, Ogilby PR. Effect of solvent on two-photon absorption by vinyl
benzene derivatives. J Phys Chem A 2008;112:7831e9.
[39] Sheik-Bahae M, Said AA, Wei TH, Hagan DJ, Van Stryland EW. Sensitive
measurement of optical nonlinearities using a single beam. IEEE J Quantum
Electron 1990;26:760e9.
[40] Zhao W, Paltty-Muhoray P. Z-scan measurement of c(3) using top-hat beams.
Appl Phys Lett 1994;65:673e5.
[41] Li SL, Wu JY, Tian YP, Tang YW, Jiang MH, Fun HK. Preparation, character-
ization, two-photon absorption and optical limiting properties of a novel
metal complex containing carbazole. Opt Mater 2006;28:897e903.
[16] Tian YP, Li L, Zhou YH, Wang P, Zhou YH, Wu JY, et al. Design and synthesis of
two new two-photon absorbing pyridine salts as ligands and their rare earth
complexes. Cryst Growth Des 2009;9:1499e504.