Synthesis and nonlinear optical properties in the near-IR range of stilbazolium dyes

Article abstract of DOI:10.1016/j.saa.2013.12.048

A series of stilbazolium salts based on donor-π-acceptor (D-π-A) structure have been synthesized and fully characterized. Photophysical properties including linear absorption, one-photon excited fluorescence (OPEF), two-photon absorption (2PA) properties were systematically investigated. The results suggest that increasing electron-releasing character of the terminal group leads to a more pronounced donor-to-acceptor intramolecular charge transfer (ICT). In addition, the dyes possess the largest 2PA cross sections in the near infrared region (NIR) and display maximum two-photon absorption cross sections within the narrow wavelength range from 950 to 970 nm and BL3 exhibits a large nonlinear refractive index coefficient and possesses very large values of the real part of the cubic hyperpolarizability χ⁽³⁾ at 960 nm. Furthermore, the initial density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations provide reasonable explanations for their absorption spectra, meanwhile we used the Lippert-Mataga equation to evaluate the dipole moment changes of the dyes with photoexcitation, the results are corresponding with linear and nonlinear optical properties of the dyes.

Full text of DOI:10.1016/j.saa.2013.12.048

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 46–53
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis and nonlinear optical properties in the near-IR range
of stilbazolium dyes
⇑
Fuying Hao , Dongpo Zhu, Jilong Ma, Lanlan Chai
Department of Chemistry, Fuyang Normal College, Fuyang 236036, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A series of D-p-A type two-photon absorption organic salts based on pyridinium were and synthesized.
ꢀ
The molecular geometry optimization
for calculation results from the
structure.
The dyes exhibit two-photon
absorption in near-infrared range.
The dyes display maximum 2PA cross
sections within the narrow
wavelength range.
Experimental results revealed the dyes possess the largest 2PA cross sections in the near infrared region
(NIR) and display maximum two-photon absorption cross sections within the narrow wavelength range
from 950 to 970 nm. Furthermore, the 2PA cross section values present enhanced trend in turn with the
lengthen of the alkyl chain. In addition, we used the Lippert–Mataga equation to evaluate the dipole
moment changes of the dyes with photoexcitation, the results are corresponding with linear and nonlin-
ear optical properties of the dyes.
ꢀ
ꢀ
ꢀ
ꢀ
The 2PA cross section values
enhanced in turn with the lengthen of
the alkyl chain.
The results of Lippert–Mataga
equation are corresponding with
optical properties.
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of stilbazolium salts based on donor-p-acceptor (D-p-A) structure have been synthesized and
Received 20 September 2013
Received in revised form 18 November 2013
Accepted 5 December 2013
Available online 18 December 2013
fully characterized. Photophysical properties including linear absorption, one-photon excited fluores-
cence (OPEF), two-photon absorption (2PA) properties were systematically investigated. The results sug-
gest that increasing electron-releasing character of the terminal group leads to a more pronounced
donor-to-acceptor intramolecular charge transfer (ICT). In addition, the dyes possess the largest 2PA cross
sections in the near infrared region (NIR) and display maximum two-photon absorption cross sections
within the narrow wavelength range from 950 to 970 nm and BL3 exhibits a large nonlinear refractive
index coefficient and possesses very large values of the real part of the cubic hyperpolarizability v(3) at
960 nm. Furthermore, the initial density functional theory (DFT) and time-dependent density functional
theory (TD-DFT) calculations provide reasonable explanations for their absorption spectra, meanwhile we
used the Lippert–Mataga equation to evaluate the dipole moment changes of the dyes with photoexcita-
tion, the results are corresponding with linear and nonlinear optical properties of the dyes.
Ó 2013 Elsevier B.V. All rights reserved.
Keywords:
Stilbazolium
Donor-p-acceptor
Optical properties
Two-photon absorption
2PA cross section
Near-infrared
⇑
Corresponding author. Tel.: +86 5582596249.
E-mail address: yxf301129@163.com (F. Hao).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.12.048

F. Hao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 46–53
47
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 singlet–
Introduction
Optical materials with striking two-photon absorption effect
have been the focus of intense current research. Such materials
are of great scientific and technologic interest for applications in
the areas of data processing, up-converted lasing, three-dimen-
sional fluorescence microscopy, biological imaging [1–8]. All the
applications depend critically on the availability of new materials
with high two-photon absorption (2PA) cross sections. Therefore,
significant effort has been devoted to the development of them.
The promise of diverse applications has stimulated many stud-
ies with organic 2PA materials over recent years. For those 2PA-
based applications, the demand for rationally designed organic
compounds that exhibit sufficiently large 2PA cross section is con-
sequently escalating. The combination of several structural param-
eters, such as increasing the internal charge transfer and
singlet excitation energies were calculated with
composed of 6-31 G(d) for C N H O atoms.
a basis set
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 photomul-
tiplier tube (HORIBA HuoroMax-4P). The decays were analyzed by
‘least-squares’. The quality of the exponential fits was evaluated by
the goodness of fit (v2).
Two-photon absorption (2PA) cross-sections (d) of the samples
were obtained by two-photon excited fluorescence (TPEF) method
[19] at femtosecond laser pulse and Ti: sapphire system (680–
1080 nm, 80 MHz, 140 fs) as the light source. The samples were
dissolved in DMSO at a concentration of 1.0 ꢂ 10ꢁ3 mol Lꢁ1. The
intensities of TPEF spectra of the reference and the samples were
determined at their excitation wavelength. Thus, 2PA cross-section
(d) of samples was determined by the following equation:
dimensionality of the molecule/extending
p-conjugated system
within a molecule, is closely related to molecular 2PA [9–12]. Stil-
bazolium salts are the best studied amongst such materials [13–
16].
Up to now, there are many molecular design strategies were put
forward to provide guidelines for the development of stilbazolium
salts with large 2PA cross sections. In this context, we launched a
program changing the phenylamine substituents which were
Uref cref nref
F
Fref
d ¼ dref
U
c
n
where the ref subscripts stand for the reference molecule (here
Rh6G in ethanol solution at concentration of 1.0 ꢂ 10ꢁ3 mol Lꢁ1
was used as reference). d is the 2PA cross-sectional value, c is the
concentration of the solution, n is the refractive index of the solu-
tion, F is the TPEF integral intensities of the solution emitted at
introduced to molecules as the donor, and the molecular
p-elec-
tron conjugated system was enlarged by forming ACH@CHA in
4-position of phenylamine. The cationic pyridinium unit as accep-
tor groups were bound on other side of vinyl bond. These dyes
show negative solvatochromism in absorption spectra. Further-
more, they exhibits peak intense two-photon fluorescence-emis-
sion within narrow wavelength range.
the exciting wavelength, and
U is the fluorescence quantum yield.
The dref value of reference was taken from the literature [20].
BL1, BL2, BL3 were synthesized by the following reactions
(shown in Fig. 1).
Synthesis
Experiments
Preparation of 1,4-dimethylpyridinium iodide
General
1,4-Dimethylpyridinium iodide was synthesized according to
the literature method [21]. White powder product 4 was collected.
Yield 90%. Mp: 155 °C. IR (KBr, cmꢁ1) 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 = 6.4 Hz), 8.84
(2H, d, J = 6.4 Hz); M+(MS/ESI), 108.48.
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. Elemental analysis was performed
on Perkin–Elmer 240 instrument. Mass spectra were determined
with a Micromass GCT-MS (ESI source). IR spectra were recorded
on NEXUS 870 (Nicolet) spectrophotometer in the 400–
4000 cmꢁ1 region using a powder sample on a KBr plate.
Preparation of substituted aminobenzaldehydes
4-(N, N-Dialkylamino)benzaldehyde was synthesized according
to the literature method [22]. At room temperature, the com-
pounds are pale yellow oil. For 4-(N, N-diethylamino)benzaldehyde,
IR (KBr, cmꢁ1) 2974 (m), 1665 (s), 1598 (s), 1566 (m), 1528 (s),
1470 (m), 1409 (s), 1357 (s), 1330 (m), 1179 (s), 1002 (w), 839
(m), 591 (w); 1H NMR (400 MHz, d6-acetone), d (ppm): 1.19 (6H,
t, J = 7.0 Hz), 3.51(4H, m), 6.80 (2H, d, J = 8.8 Hz), 7.69 (2H, d,
J = 9.2 Hz), 9.68 (1H, s). For 4-(N, N-dibutylamino)-benzaldehyde, IR
(KBr, cmꢁ1) selected bands: 2958 (s), 2931 (m), 2871 (m), 1669
(s), 1596 (s), 1525 (s), 1464 (w), 1405 (s), 1367 (s), 1313 (w),
1224 (m), 1168 (s), 814 (s), 607 (m), 510 (m); 1H NMR (400 MHz,
d6-acetone), d (ppm): 0.94 (6H, t), 1.38 (4H, m), 3.40 (4H, m),
3.42 (4H, t), 6.76 (2H, d,), 7.65 (2H, d), 9.67 (1H, s).
Optical measurements
The one-photon absorption (OPA) spectra were obtained on a
UV-265 spectrophotometer. The one-photon excited fluorescence
(OPEF) spectra measurements were performed using a Hitachi F-
7000 fluorescence spectrophotometer. OPA and OPEF of BL1–BL3
were measured in solvents of different polarities with the concen-
tration of 2.0 ꢂ 10ꢁ6 mol Lꢁ1. The quartz cuvettes used are of 1 cm
path length. The fluorescence quantum yields (
mined by using Rh6G (in ethanol,
U
) were deter-
U
= 0.94) as the reference
according to the literature method [17]. Quantum yields were cor-
rected as follows:
ꢀ
ꢁ
2s Ds
2r Dr
Preparation of BL1
Ar
As
g
g
Us
¼
Ur
BL1 was synthesized according to the literature method [23],
red solid was collected. Yield: 96%. FT-IR (KBr, cmꢁ1) v: 2922 (w),
1643 (w), 1589 (s), 1521 (m), 1475 (w), 1437 (w), 1373 (m),
1330 (m), 1163 (s), 1037 (m), 708 (m). 1H NMR (400 MHz, d6-CD3-
COCD3): 8.68 (d, 2H), 8.04 (d, 2H), 7.90 (d, 1H), 7.18–7.17 (q, 9H),
6.94–6.90 (t, 8H), 6.80–6.77 (t, 6H), 4.17 (s, 3H), 3.02 (s, 6H); M+
where the s and r indices designate the sample and reference sam-
ples, respectively, A is the absorbance at kexc is the average refrac-
tive index of the appropriate solution, and D is the integrated area
,
g
under the corrected emission spectrum [18].

48
F. Hao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 46–53
Fig. 1. The synthesis routes of target compounds.
(MS/ESI), 239.251. Anal. Calc. for C40H39BN2: C, 86.01; H, 7.04; N,
5.02. Found: C, 86.04; H, 7.08; N, 5.07.
Preparation of BL2
BL2 was synthesized according to the literature method [24],
red solid was collected. Yield: 95%. FT-IR (KBr, cmꢁ1): 2968 (w),
1643 (w), 1584 (s), 1525 (s), 1468 (w), 1434 (w), 1408 (m), 1327
(m), 1172 (s), 1152 (s), 1037 (m), 706 (m). 1H NMR (400 MHz, CD3-
COCD3-d6): 8.62 (d, 2H), 8.04 (d, 2H), 7.89 (d, 1H), 7.59 (d, 2H), 7.35
(s, 8H), 7.14 (d, 1H), 6.94–6.91 (t, 8H), 6.79–6.75 (q, 6H), 4.35 (s,
3H), 3.53–3.48 (q, 4H), 1.22–1.18 (t, 6H); M+ (MS/ESI), 267.283.
Anal. Calc. for C42H43BN2: C, 85.99; H, 7.39; N, 4.78. Found: C,
86.02; H, 7.38; N, 4.82.
Preparation of BL3
Using a 100 mL one-necked flask fitted with a stirrer and a con-
denser, 2.33 g (0.01 mol) 4-(N, N-dibutylamino)-benzaldehyde,
2.35 g (0.01 mol) of compound 1,4-Dimethylpyridinium iodide,
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 tetraphenylbo-
rate was added into the solution. The solution again 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 BL3 was collected. Yield:
90%. FT-IR (KBr, cmꢁ1): 2955 (w), 2927 (m), 2866 (w), 1643 (w),
1587 (s), 1525 (s), 1471 (w), 1404 (w), 1368 (m), 1327 (m), 1213
(m), 1172 (s), 1037 (m), 709 (m). 1H NMR (400 MHz, CD3COCD3-
d6): 9.9 (s, 1H), 7.68 (d, 2H), 6.77 (d, 2H), 3.45–3.41 (t, 4H), 1.65–
1.57 (q, 4H), 1.41–1.36 (q, 4H), 0.97–0.94 (t, 6H). M+ (MS/ESI),
323.391. Anal. Calc. for C46H51BN2: C, 85.96; H, 8.00; N, 4.36.
Found: C, 85.98; H, 8.02; N, 4.38.
Fig. 2. Linear absorption spectra of all compounds in DMSO solution with
2 ꢂ 10ꢁ6 M.
Results and discussion
Structural features
Fig. 3. One-photon excited fluorescence (right) spectra of all compounds in DMSO
solution with 2 ꢂ 10ꢁ6 M.
In these structures, the cationic pyridinium can offer net posi-
tive charge for the molecular, and the large conjugate system
which is composed of phenylamine, vinyl and cationic pyridinium
ensures the large ionic radius. In addition, we can learn from the
crystal data of BL1 in literature [23] and BL2 in our previous work
[24], the CAC bond lengths of BL2 (C5AC7: 1.485 Å, C8AC9:
1.477 Å) which link the benzene ring and pyridine ring are located
between the normal C@C double bond (1.32 Å) and CAC single
strong 2PA response. For another, the linkage between the benzene
ring and pyridine ring for BL2 are shorter than BL1 (C8AC9:
1.513 Å, C4AC7: 1.545 Å). It suggests that dye BL2 possesses a
higher delocalization within the molecule and more charge-trans-
fer features of the ground state in the solid state compared with
those of the dye BL1, which make the dye highly promising candi-
dates for two-photon absorption materials.
bond (1.53 Å), which show that there is a highly
p-electron delo-
calized system in BL2, it is the necessary condition for it bearing

F. Hao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 46–53
49
Table 1
1S–3S, and the relevant photophysical properties of these dyes
are summarized in Table 1.
Photophysical properties of compounds BL1–BL3 in several different solvents.
a
b
c
Compound
Solvents
kmax
(
emax
)
kmax
Ud
s
/pse
Linear absorption
Ethyl acetate
THF
Ethanol
479(2.74)
476(3.92)
475(6.03)
576
583
590
0.0451
0.0203
0.0119
105
289
92
As shown in Table 1, one can observe that absorption bands for
all dyes feature one intense absorption band between 460 nm and
490 nm with the corresponding molar extinction coefficient
BL1
Methanol
Acetonitrile
DMF
DMSO
Ethyl acetate
THF
473(5.22)
465(5.35)
465(5.47)
464(4.80)
490(2.43)
494(4.65)
491(6.39)
587
599
595
601
583
594
590
0.0048
0.0042
0.0043
0.0072
0.0094
0.0137
0.0154
41
3
68
(ꢃ50,000 Mꢁ1 cmꢁ1), which originates from the
p ?
pꢄ or intramo-
lecular charge transfer (ICT) transition. The bands exhibit slightly
blue shifted in high polar solvents such as acetonitrile and DMF
which is a feature of zwitterionic hemicyanine type dyes. This phe-
nomenon is attributed to a relatively high polar mesomeric form,
which is predominant in the ground state leading to a blue shift
of the absorption band [25].
97
152
166
99
Ethanol
BL2
BL3
Methanol
Acetonitrile
DMF
DMSO
Ethyl acetate
THF
483(7.12)
483(6.15)
483(4.76)
479(5.92)
479(2.34)
480(2.75)
491(2.79)
595
606
605
603
596
601
596
0.0034
0.0018
0.0068
0.0126
0.0108
0.0184
0.0127
56
46
89
137
196
231
119
As shown in Fig. 2, for BL1–BL3, with the lengthen of the alkyl
chain, the absorption bands exhibit slightly red shift in turn, this
phenomenon might attributed to the substitution of the n-butyl
(BL3) for methyl (BL1), ethyl (BL2) in phenylamine moiety which
possesses higher electron-donating ability. The results suggest that
increasing electron-releasing character of the terminal group leads
to a more pronounced donor-to-acceptor ICT [26].
Ethanol
Methanol
Acetonitrile
DMF
486(2.69)
485(2.44)
484(2.87)
483(2.57)
598
606
603
605
0.0043
0.0063
0.0116
0.0059
85
77
116
179
DMSO
a
b
c
Peak position of the longest absorption band.
Maximum molar absorbance in 104 molꢁ1 L cmꢁ1
Peak position of SPEF, exited at the absorption maximum.
Quantum yields determined by using Rh6G as standard.
One-photon excited fluorescence (OPEF)
.
One can observe from Table 1, the fluorescence emission peaks
for all dyes incline to red shift as the solvent polarity increase, indi-
cating that the dipole moment of them in the excited states are lar-
ger than that in the ground states, and an increase in the polarity of
the solvent will lower the energy level of the charge transfer ex-
cited states [27]. By comparison of the absorption spectral features
of BL1–BL3, the fluorescence emission bands for them display a
resemble regularity due to the electron-donating ability of the
phenylamine group.
d
Linear optical properties
The UV–vis absorption and one-photon fluorescence spectra of
the dyes in different solvents are shown in Figs. 2 and 3 and
Fig. 4. Frontier molecular orbital diagrams for compounds BL1–BL3.

50
F. Hao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 46–53
Table 2
Excitation energy (E), corresponding wavelength (k), oscillator strength (f) and major contribution of BL1–BL3.
Compound
E (eV)
k (nm)
f
Composition (C)
Character
BL1
BL2
BL3
2.7520
2.7064
2.6710
450.52
465.56
474.51
1.230
1.2005
1.0078
149(H) > 152(L + 2)(0.67982)
157(H-1) ? 159(L)(0.64326)
173(H) ? 175(L + 1)(0.54365)
ICT
ICT
ICT
Meanwhile it can be found that the photophysical properties of
all dyes show low quantum yield (<10%). The behaviour is due to
certain nonradiative decay mechanisms that may arise as a result
of ‘twisted intramolecular charge geometry’ (TICT) [28]. From
Table 1 we can observe that the fluorescence lifetime of these dyes
is short, thereby supporting the assumption that because of the
nonradiative decay arised as a result of TICT enhancing the
quenching efficiency.
solvents where specific solute–solvent interactions are present is
not appropriate. As shown in Fig. 5, the slope of the best-fit line
is related to the dipole moment change between the ground and
excited states (le
8566, 5711, and 6714 cmꢁ1 for dye BL1, BL2, and BL3, respectively.
So the values of le g were calculated as 11.5 D for dye BL1, 16.5 D
–lg). The slopes of all three lines are different:
–l
for dye BL2, and 18.6 D for dye BL3, respectively (Eq. (1)). The large
values of dye BL3 indicates that the molecule in the excited state
has an extremely polar structure, especially for dye BL3, corre-
sponding with their linear and nonlinear optical properties [30].
TD-DFT studies
TD-DFT computational studies were performed to elucidate the
electronic structures of the ground state of BL1–BL3. The sche-
matic representation of the molecular orbitals of them was exhib-
ited in Fig. 4, and Table 2 shows the energy and composition of ICT.
Fig. 4 shows the calculated frontier orbitals of BL1–BL3, the energy
band were all tentatively assigned to ICT. Therefore the initial DFT
and TD-DFT calculations provide reasonable explanations for their
absorption spectra.
Two-photon excited fluorescence
As shown in Fig. 2, there is no linear absorption in the wave-
length range 680–1200 nm for all the compounds in DMSO, which
means that there are no energy levels corresponding to an electron
transition in this spectral range. If frequency up-converted fluores-
cence appears upon excitation with a tunable laser in this range, it
should be mainly attributed to TPEF. The spectra obtained by using
the optical excitation wavelength 960 nm are shown in Fig. 6. In all
the cases, the output intensity of two-photon excited fluorescence
was linearly dependent on the input laser, thereby confirming the
2PA process (shown in Fig. 7).
As shown in Fig. 6, it can be seen clearly that the two-photon
excited fluorescence intensity of BL1–BL3 present enhanced trend
in turn, which may due to the electron-donating ability of the ter-
minal group. The growth of the alkyl chain increases the electron
density of the bridge, favoring the ICT along the extended
from terminal donor to acceptor group.
In addition, as we known, the most widely used to evaluate the
dipole moment changes of the dyes with photoexcitation is the
Lippert–Mataga equation [29],
2Df
2
D
D
m
¼
3 ðle
ꢀ
4pe0hca
ꢁ
lgÞ þ b
ð1Þ
ð2Þ
e
ꢁ 1
n2 ꢁ 1
f ¼
ꢁ
2e
þ 1 2n2 þ 1
p bridge
in which
D
m
=
mabs
ꢁ
mem stands for Stokes shift, mabs and mem are
absorption and emission (cmꢁ1), h is the Planck’s constant, c is
the velocity of light in vacuum, a is the Onsager radius and b is a
ꢀ
TPA cross-sections
constant.
Df is the orientation polarizability, le and lg are the di-
pole moments of the emissive and ground states, respectively and
As shown in Fig. 8, two-photon absorption cross sections (d) of
BL1–BL3 were measured in the wide wavelength range from 840
to 1000 nm. The maximum values in DMSO are 242, 335 and 405
GM respectively (Table 3), which present enhanced trend in turn,
this may due to the electron-donating ability of the terminal group
as well. In addition, from these data, it can be observed that the
2
e0 is the permittivity of the vacuum. (le
the slope of the Lippert–Mataga plot.
ꢁ
lg) is proportional to
Plots of the Stokes shifts as a function of the solvent polarity
factor f are shown in Fig. 5. Only the data involving the aprotic
D
solvents are shown because application of this analysis with
Fig. 5. Lippert–Mataga plots for BL1–BL3.
Fig. 6. The two-photon fluorescence spectra of BL1–BL3 in DMSO (c = 1.0 ꢂ 10ꢁ3).

F. Hao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 46–53
51
Fig. 7. Output fluorescence (Iout) vs. the square of input laser power (Iin) for BL1, BL2, and BL3 excitation carried out at 960 nm, with c = 1.0 ꢂ 10ꢁ3 mol Lꢁ1 in DMSO,
respectively.
with our previous work [31], it may result from that there are
excited-states (S1–Sn) distributing in a much narrow energy band
[32].
TPA coefficient b and v(3)
Considering above, we singled out BL3 which possessed the
largest two-photon absorption cross sections (d), the coefficient b
and v(3) of BL3 were measured by the Z-scan technique [33,34].
In the present experiments, a 1 ꢂ 10ꢁ3 M solution of BL3 in DMSO
contained in a 1.0 mm path-length quartz cell for Z-scan measure-
ments was performed at 960 nm. The TPA coefficient b and v(3) cal-
culation method can be obtained according to literature methods
[35]. Fig. 9 shows the normalized transmittance plotted as a func-
tion of the sample position (z) measured by using open aperture
and closed aperture Z-scan technique at 960 nm. Table 4 shows
the third-order nonlinearity parameters of BL3. From these data,
Fig. 8. Two-photon absorption cross sections of BL1–BL3 in DMSO vs excitation
wavelengths of identical energy of 0.300 W.
it can be seen that BL3 exhibits a large nonlinear refractive index
coefficient and possesses very large values of the real part of the
cubic hyperpolarizability v(3) at 960 nm. This behaviour should
establish the foundation for its device application [15(j)].
Table 3
The largest TPA cross section of BL1–BL3 in DMSO vs. Excitation wavelength 960 nm.
Conclusions
Compound
BL1
BL2
BL3
d/GM
242
335
405
A series of D-p-A type two-photon absorption organic salts
(BL1, BL2, and BL3) based on pyridinium were synthesized. Linear
absorption, one/two-photon excited fluorescence behaviour and
the excited-state lifetimes have been systematically investigated.
dramatically larger two-photon absorption cross sections appear in
the narrow pathlength about 950–970 nm, which is corresponding

52
F. Hao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 46–53
Fig. 9. Normalized open-aperture (left) and closed-aperture (right) Z-scan transmittance of BL3 in DMSO (0.001 M) at 960 nm.
Table 4
Open- and closed-aperture Z-scan measurement data for the third-order nonlinearity parameters of BL3.
n2 Extrapolated to solute
Re (v(3)) extrapolated to
solute (esu)
2PA coefficient
to solute
a
2 (cm/GW) extrapolated Two-photon cross section
(GM)
Im (v(3)) extrapolated to
solute (esu)
(cm2/W)
r
2.60 ꢂ 10ꢁ10
1.44 ꢂ 10ꢁ12
0.024
829.77
1.02 ꢂ 10ꢁ15
[4] J.E. Reeve, H.A. Collins, K.D. Mey, M.M. Kohl, K.J. Thorley, O. Paulsen, K. Clays,
H.L. Anderson, Amphiphilic porphyrins for second harmonic generation
imaging, J. Am. Chem. Soc. 131 (2009) 2758–2759.
[5] J.M. Hales, Design of polymethine dyes with large third-order optical
nonlinearities and loss figures of merit, Science 327 (2010) 1485–1488.
[6] S.A. Haque, J. Nelson, Toward organic all-optical switching, Science 327 (2010)
1466–1467.
[7] S. Sumalekshmy, C.J. Fahrni, Metal-ion-responsive fluorescent probes for two-
photon excitation microscopy, Chem. Mater. 23 (2011) 483–500.
[8] Sheng Yao, Kevin D. Belfield, Two-photon fluorescent probes for bioimaging,
Eur. J. Org. Chem. 17 (2012) 3199–3217.
[9] (a) A. Bhaskar, G. Ramakrishna, Z. Lu, R. Twieg, J.M. Hales, D.J. Hagan, et al.,
Investigation of two-photon absorption properties in branched alkene and
alkyne chromophores, J. Am. Chem. Soc. 128 (2006) 11840–11849;
(b) A. Bhaskar, R. Guda, M.M. Haley, I.I.I. Theodore Goodson, Building
symmetric two-dimensional two-photon materials, J. Am. Chem. Soc. 128
(2006) 13972–13973;
The relationships between the structures and photophysical prop-
erties of them can be understood based on both the experimentally
and theoretically. In the experimental part: it was found that the
dyes showed solvent-polarity-dependent fluorescence in the
visible region. In addition, the dyes possess the largest 2PA cross
sections in the near infrared region (NIR) and display maximum
two-photon absorption cross sections within the narrow wave-
length range from 950 to 970 nm. Furthermore, the 2PA cross
section values present enhanced trend in turn with the lengthen
of the alkyl chain and BL3 exhibits a large nonlinear refractive
index coefficient and possesses very large values of the real part
of the cubic hyperpolarizability v(3) at 960 nm. In the theoretical
part: the initial DFT and TD-DFT calculations provide reasonable
explanations for their absorption spectra, meanwhile the calcula-
tion results of the Lippert–Mataga equation are corresponding with
linear and nonlinear optical properties of the dyes. The results
reported in this paper provide a useful design and synthesis
strategy for the two-photon absorbing materials.
(c) M. Williams-Harry, A. Bhaskar, G. Ramakrishna, I.I.I. Theodore Goodson, M.
Imamura, A. Mawatari, et al., Giant thienylene-acetylene-ethylene
macrocycles with large two-photon absorption cross section and semishape-
persistence, J. Am. Chem. Soc. 130 (2008) 3252–3253.
[10] (a) M. Rumi, J.E. Ehrlich, A.A. Heikal, J.W. Perry, S. Barlow, Z. Hu, et al.,
Structure-property relationships for two-photon absorbing chromophores:
bis-donor diphenylpolyene and bis(styryl)benzene derivatives, J. Am. Chem.
Soc. 122 (2000) 9500–9510;
(b) S.J. Chung, M. Rumi, V. Alain, S. Barlow, J.W. Perry, S.R. Marder, Strong, low-
energy two-photon absorption in extended amine-terminated cyano-
substituted phenylenevinylene oligomers, J. Am. Chem. Soc. 127 (2005)
10844–10845;
(c) S.J. Chung, S. Zheng, T. Odani, L. Beverina, J. Fu, L.A. Padilha, et al., Extended
squaraine dyes with large two-photon absorption cross-sections, J. Am. Chem.
Soc. 128 (2006) 14444–14445.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (20875001), Education Committee of Anhui
Province (KJ2013B205, KJ2013B204).
[11] (a) K.D. Belfield, A.R. Morales, J.M. Hales, D.J. Hagan, E.W.V. Stryland, V.M.
Chapela, et al., Linear and two-photon photophysical properties of a series of
symmetrical diphenylaminofluorenes, Chem. Mater. 16 (2004) 2267–2273;
(b) K.D. Belfield, D.J. Hagan, E.W.V. Stryland, K.J. Schafer, R.A. Negres, New
two-photon absorbing fluorene derivatives: synthesis and nonlinear optical
characterization, Org. Lett. 1 (1999) 1575–1578.
[12] (a) L. Porres, O. Mongin, C. Katan, M. Charlot, T. Pons, J. Mertz, et al., Enhanced
two-photon absorption with novel octupolar propeller-shaped fluorophores
derived from triphenylamine, Org. Lett. 6 (2004) 47–50;
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2013.12.048.
(b) M. Drobizhev, F. Meng, A. Rebane, Y. Stepanenko, E. Nickel, C.W. Spangler,
Strong two-photon absorption in new asymmetrically substituted porphyrins:
interference between charge-transfer and intermediate-resonance pathways,
J. Phys. Chem. B 110 (2006) 9802–9814.
References
[1] G.S. He, L.S. Tan, Q.D. Zheng, P.N. Prasad, Multiphoton absorbing materials:
molecular designs, characterizations, and applications, Chem. Rev. 108 (2008)
1245–1330.
[13] S.R. Marder, J.W. Perry, W.P. Schaefer, Synthesis of organic salts with large
second-order optical nonlinearities, Science 245 (1989) 626–628.
[14] (a) B.J. Coe, J.A. Harris, K. Clays, A. Persoons, K. Wostyn, B.S. Brunschwig, A
[2] M. Pawlicki, H.A. Collins, R.G. Denning, H.L. Anderson, Two-photon absorption
and the design of two-photon dyes, Angew. Chem. Int. Ed. 48 (2009) 3244–
3266.
comparison
of
the
pentaammine(pyridyl)ruthenium(II)
and
4-
(dimethylamino)phenyl groups as electron donors for quadratic non-linear
optics, Chem. Commun. (2001) 1548–1549;
[3] M.J. Therien, How to improve your image, Nature 458 (2009) 716–717.
(b) B.J. Coe, J.A. Harris, I. Asselberghs, K. Clays, G. Olbrechts, A. Persoons, J.T.

F. Hao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 46–53
53
Hupp, R.C. Johnson, S.J. Coles, M.B. Hursthouse, K. Nakatani, Quadratic
nonlinear optical properties of N-aryl stilbazolium dyes, Adv. Funct. Mater.
12 (2002) 110–116;
[18] T.G. Gray, C.M. Rudzinski, E.E. Meyer, R.H. Holm, D.G. Nocera, Spectroscopic
and photophysical properties of hexanuclear rhenium(III) chalcogenide
clusters, J. Am. Chem. Soc. 125 (2003) 4755–4770.
(c) B.J. Coe, J.A. Harris, I. Asselberghs, K. Wostyn, K. Clays, A. Persoons, B.S.
Brunschwig, S.J. Coles, T. Gelbrich, M.E. Light, M.B. Hursthouse, K. Nakatani,
Quadratic optical nonlinearities of N-methyl and N-aryl pyridinium salts, Adv.
Funct. Mater. 13 (2003) 347–357;
(d) K. Clays, B.J. Coe, Design strategies versus limiting theory for engineering
large second-order nonlinear optical polarizabilities in charged organic
molecules, Chem. Mater. 15 (2003) 642–648;
[19] S.K. Lee, W.J. Yang, J.J. Choi, C.H. Kim, S.J. Jeon, B.R. Cho, 2,6-Bis[4-(p-
dihexylaminostyryl)-styryl]anthracene derivatives with large two-photon
cross sections, Org. Lett. 7 (2005) 323–326.
[20] C. Xu, W.W. Webb, Measurement of two-photon excitation cross sections of
molecular fluorophores with data from 690 to 1050 nm, J. Opt. Soc. Am. B 13
(1996) 481–491.
[21] C.F. Zhao, G.S. He, J.D. Bhawalkar, C.K. Park, P.N. Prasad, Newly synthesized
dyes and their polymer/glass composites for one- and two-photon pumped
solid-state cavity lasing, Chem. Mater. 7 (1995) 1979–1983.
(e) E. Stathatos, P. Lianos, Synthesis of
carboxylic groups and its use as
a
hemicyanine dye bearing two
a
photosensitizer in dye-sensitized
photoelectrochemical cells, Chem. Mater. 13 (2001) 3888–3892.
[15] (a) C.F. Zhao, R. Gvishi, U. Narang, G. Ruland, PrasadPN, Structures, spectra,
and lasing properties of new (aminostyryl)pyridinium laser dyes, J. Phys.
Chem. 100 (1996) 4526–4532;
[22] (a) C.F. Zhao, G.S. He, J.D. Bhawalkar, C.K. Park, P.N. Prasad, Newly synthesized
dyes and their polymer/glass composites for one- and two-photon pumped
solid-state cavity lasing, Chem. Mater. 7 (1995) 1979–1983;
(b) C.F. Zhao, C.K. Park, P.N. Prasad, Y. Zhang, S. Ghosal, R. Burzynski,
Photorefractive polymer with side-chain second-order nonlinear optical and
charge-transporting groups, Chem. Mater. 7 (1995) 1237–1242.
[23] S.J. Li, D.C. Zhang, Z.L. Huang, Y.Q. Zhang, K.B. Yu, 4-[2-(4-Methoxyphenyl)
ethenyl]-N-methylpyridinium tetraphenylborate, Chin. J. Struct. Chem. 19
(2000) 263–266.
[24] D.D. Li, R. Li, S.L. Li, 4-(E)-2-[4-(Diethylamino) phenyl] ethenyl-1-
methylpyridin-1-ium tetraphenylborate, Acta Cryst. E68 (2012) o2694.
[25] M. Lal, L. Levy, K.S. Kim, Silica nanobubbles containing an organic dye in a
multilayered organic/inorganic heterostructure with enhanced luminescence,
Chem. Mater. 12 (2000) 2632–2639.
(b) U. Narang, C.F. Zhao, J.D. Bhawalkar, F.V. Bright, P.N. Prasad,
Characterization of a new solvent-sensitive two-photon-induced fluorescent
(aminostyryl)pyridinium salt dye, J. Phys. Chem. 100 (1996) 4521–4525;
(c) B.A. Reinhardt, L.L. Brott, S.J. Clarson, A.G. Dillard, J.C. Bhatt, P.N. Prasad,
et al., Highly active two-photon dyes: design, synthesis, and characterization
toward application, Chem. Mater. 10 (1998) 1863–1874;
(d) M. Lal, L. Levy, K.S. Kim, G.S. He, X. Wang, Y.H. Min, S. Pakatchi, P.N. Prasad,
Silica nanobubbles containing an organic dye in
a multilayered organic/
inorganic heterostructure with enhanced luminescence, Chem. Mater. 12
(2000) 2632–2639;
(e) R. Kannan, G.S. He, L. Yuan, F. Xu, P.N. Prasad, A.G. Dombroskie, et al.,
Diphenylaminofluorene-based two-photon-absorbing chromophores with
various p-electron acceptors, Chem. Mater. 13 (2001) 1896–1904;
(f) Q.D. Zheng, G.S. He, T.C. Lin, P.N. Prasad, 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. 13 (2003) 2499–
2504;
[26] H.M. Kim, B.R. Cho, Two-photon materials with large two-photon cross
sections. Structure-property relationship, Chem. Commun. (2009) 153–164.
[27] X.M. Wang, D. Wang, G.Y. Zhou, W.T. Yu, Y.F. Zhou, Q. Fang, et al., Symmetric
and asymmetric charge transfer process of two-photon absorbing
chromophores:
bis-donor
substituted
stilbenes,
and
substituted
styrylquinolinium and styrylpyridinium derivatives, J. Mater. Chem. 11
(2001) 1600–1605.
(g) R. Kannan, G.S. He, T.C. Lin, P.N. Prasad, R.A. Vaia, L.S. Tan, Toward highly
active two-photon absorbing liquids. synthesis and characterization of 1,3,5-
triazine-based octupolar molecules, Chem. Mater. 16 (2004) 185–194;
(h) T.C. Lin, G.S. He, P.N. Prasad, L.S. Tan, Degenerate nonlinear absorption and
optical power limiting properties of asymmetrically substituted stilbenoid
chromophores, J. Mater. Chem. 14 (2004) 982–991;
[28] C. Rullière, Z.R. Grabowski, J. Dobkowski, Picosecond absorption spectra of
carbonyl derivatives of dimethylaniline: the nature of the tict excited states,
Chem. Phys. Lett. 137 (1987) 408–413.
[29] (a) S.A. Patel, M. Cozzuol, J.M. Hales, C.I. Richards, M. Sartin, J.C. Hsiang, T.
Vosch, J.W. Perry, R.M. Dickson, Electron transfer-induced blinking in Ag
nanodot fluorescence, J. Phys. Chem. C 113 (2009) 20264–20270;
(b) S.M. Ji, J. Yang, Q. Yang, S.S. Liu, M.D. Chen, J.Z. Zhao, Tuning the
intramolecular charge transfer of alkynylpyrenes: effect on photophysical
properties and its application in design of offꢁon fluorescent thiol probes, J.
Org. Chem. 74 (2009) 4855–4865.
(i) Q. Zheng, G.S. He, P.N. Prasad,
p-Conjugated dendritic nanosized
chromophore with enhanced two-photon absorption, Chem. Mater. 17
(2005) 6004–6011;
(j) G.S. He, J. Zhu, A. Baev, M. Samo, D.L. Frattarelli, N. Watanabe, A. Facchetti,
H. Ågren, T.J. Marks, P.N. Prasad, Twisted
p
-system chromophores for all-
[30] M. Irie, K. Sayo, Solvent effects on the photochromic reactions of diarylethene
derivatives, J. Phys. Chem. 96 (1992) 7671–7674.
[31] D.D. Li, D.H. Yu, Q. Zhang, S.L. Li, H.P. Zhou, J.Y. Wu, Y.P. Tian, Synthesis, crystal
structure and third-order nonlinear optical properties in the near-IR range of a
novel stilbazolium dye substituted with flexible polyether chains, Dyes
Pigments 97 (2013) 278–285.
optical switching, J. Am. Chem. Soc. 133 (2011) 6675–6680.
[16] (a) X.J. Zhang, Y.P. Tian, F. Jin, J.Y. Wu, Y. Xie, X.T. Tao, M.H. Jiang, Self-
assembly of an organic chromophore with Cd-S nanoclusters: supramolecular
structures and enhanced, Cryst. Growth Des. 5 (2005) 565–570;
(b) F.Y. Hao, X.J. Zhang, Y.P. Tian, H.P. Zhou, L. Li, J.Y. Wu, 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. 19 (2009) 9163–9169;
[32] M. Johnsen, P.R. Ogilby, Effect of solvent on two-photon absorption by vinyl
benzene derivatives, J. Phys. Chem. A 112 (2008) 7831–7839.
[33] M. Sheik-Bahae, A.A. Said, T.H. Wei, D.J. Hagan, E.W. Van Stryland, Sensitive
measurement of optical nonlinearities using a single beam, IEEE J. Quantum
Electron. 26 (1990) 760–769.
(c) Y.P. Tian, L. Li, Y.H. Zhou, P. Wang, Y.H. Zhou, J.Y. Wu, et al., Design and
synthesis of two new two-photon absorbing pyridine salts as ligands and their
rare earth complexes, Cryst. Growth Des. 9 (2009) 1499–1504;
(d) J.Y. Wu, G.J. Hu, P. Wang, F.Y. Hao, H.P. Zhou, Y.P. Tian, et al., Organic/
polyoxometalate hybridization dyes: crystal structure and enhanced two-
photon absorption, Dyes Pig. 88 (2010) 174–179.
[34] W. Zhao, P. Paltty-Muhoray, Z-scan measurement of v(3) using top-hat beams,
Appl. Phys. Lett. 65 (1994) 673–675.
[35] S.L. Li, J.Y. Wu, Y.P. Tian, Y.W. Tang, M.H. Jiang, H.K. Fun, Preparation,
characterization, two-photon absorption and optical limiting properties of a
novel metal complex containing carbazole, Opt. Mater. 28 (2006) 897–903.
[17] J.N. Demas, G.A. Crosby, The measurement of photoluminescence quantum
yields, J. Phys. Chem. 75 (1971) 991–1024.