The synthesis and third-order nonlinear optical properties of resonance Benzo[a]phenoxazinium salts

Article abstract of DOI:10.1016/j.dyepig.2010.04.014

The third-order nonlinear optical properties of a series of resonance benzo[a]phenoxazinium salts in acetic acid solution were studied using the Z-scan technique employing a Nd:YAG nanosecond laser at 532 nm. The compounds displayed strong reverse absorption with third-order nonlinear optical coefficients of 0.42-1.20 × 10^-11 esu and second hyperpolarizabilities of 2.75-7.29 × 10^-29 esu, respectively. Quantum chemical calculations showed that the compound with more stable highest occupied molecular orbital energy also displayed highest second hyperpolarizability.

Full text of DOI:10.1016/j.dyepig.2010.04.014

Dyes and Pigments 88 (2011) 50e56
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The synthesis and third-order nonlinear optical properties
of resonance Benzo[a]phenoxazinium salts
,
b
,
a
a
a
,
a
Bao-Qiang Liu a, Ru Sun a, Jian-feng Ge a , Najun Li , Xue-Liang Shi , Lihua Qiu , Jian-Mei Lu
*
*
a Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China
b Key Laboratory for Carbon-Based Functional Material and Device of Jiangsu Province, Soochow University, Suzhou 215123, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The third-order nonlinear optical properties of a series of resonance benzo[a]phenoxazinium salts in
acetic acid solution were studied using the Z-scan technique employing a Nd:YAG nanosecond laser at
532 nm. The compounds displayed strong reverse absorption with third-order nonlinear optical coeffi-
cients of 0.42e1.20 ꢀ 10ꢁ11 esu and second hyperpolarizabilities of 2.75e7.29 ꢀ 10ꢁ29 esu, respectively.
Quantum chemical calculations showed that the compound with more stable highest occupied molec-
ular orbital energy also displayed highest second hyperpolarizability.
Received 16 February 2010
Received in revised form
25 April 2010
Accepted 28 April 2010
Available online 13 May 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Benzo[a]phenoxazinium
Nile blue
Third-order NLO
Resonance structure
Z-Scan
NLO material
1. Introduction
p
-bridges [14]. Recently, significant progress has been made in the
development of third-order NLO chromophores based on high
efficient donor and/or acceptor pairs [15e17], although -bridge
Third-order nonlinear optical (NLO) materials have attracted
considerable interest in the last two or so decades, owing to their
potential application in photonic technologies such as ultrafast
optical communications, computing, data storage, all-optical
switching and image processing [1e4]. The synthesis of novel
molecules with large macroscopic optical non-linearity continues
to attract attention [5]. Various types of organic compound have
been shown to display large third-order susceptibility c(3) [6e9]
and it is generally accepted that substantially conjugated, donor/
acceptor compounds of diradical character are important aspects
for third-order NLO materials [6,10e12].
p
structureeproperty relationships have not yet been fully explored.
The third-order NLO properties of di(benzofuranonyl)methanolate
(BM4i4i) indicate that significant coupling between valence bond
(VB) resonance forms was responsible for the observed large
second hyperpolarizability g0 value [12,18,19]. Few examples have
been reported of the resonance structure of NLO materials except
for the resonance structures of nonsymmetric squaraines [20].
More experimental examples are required to validate the selection
of resonance compounds for third-order NLO materials.
Fig. 1 shows the structure of 5,9-diaminobenzo[a]phenox-
However, the relationship between third-order NLO properties
and molecular structure, such as conjugation, charge transfer and
polarization, remains unresolved [13]. Usually, there are two
approaches to achieving good third NLO potency, namely via the
optimization of a p-bridge structure for donor and/or acceptor pairs
and via the optimization of donor and/or acceptor pairs for given
azinium. The delocalized De
azinium 1 can be expressed by the stable resonance structures 2e4
[21e24]. The presence of large polarizable conjugated -electrons
in the aromatic heterocycle and corresponding potential electron
fluctuant ability meet the requirements for a promising third-order
NLO material.
peD structure of benzo[a]phenox-
p
Benzo[a]phenoxaziniums have been utilised as fluorescent
probes for amino acids, heavy metal ions and biomolecules
[25e27], few have been employed as NLO materials [28]. This
paper concerns the synthesis and the third-order NLO properties
of 5-((3-chlorophenyl)amino)-9-(diethylamino)benzo[a]phenox-
azinium salts.
* Corresponding author. Soochow University, No.199, Ren’Ai Road, SuzhouIn-
dustry Park, Suzhou 215123, China. Tel.: þ86 0512 65880368; fax: þ86 0512
65880367.
E-mail addresses: gejianfeng@suda.edu.cn (J.-f. Ge), qiulihua@suda.edu.cn (L. Qiu).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.04.014

B.-Q. Liu et al. / Dyes and Pigments 88 (2011) 50e56
51
N
O
2
N
O
N
O
N
R
R'
R
R'
R
R'
R
R'
N
R
N
H
N
R
N
H
N
R
N
H
N
R
O
N
H
1
3
4
Fig. 1. Resonance structures of benzo[a]phenoxazinium salt.
2. Experimental section
2.3.2. 5-((3-Chlorophenyl)imino)-N,N-diethyl-5H-benzo[a]
phenoxazin-9-amine (7)
2.1. Materials
9-(Diethylamino)benzo[a]phenoxazin-7-ium nitrate (6) (1.00 g,
2.7 mmol) and 3-chloroaniline (1.03 g, 8.1 mmol) were dissolved in
ethanol (40 mL) and the mixture was heated under reflux for 72 h
with stirring. After cooling to room temperature, the residue
obtained by filtration was dissolved in 40 mL water, and aqueous
ammonia was added slowly while stirring until the pH reached
w9.0. The mixture was stirred for 3 h at room temperature and
filtered; the solid was dried under vacuum and purified by silica
column chromatography eluted with chloroform to afford a dark
green powder. Yield: 41%, mp: 171e173 ꢂC; IR (KBr pellet, cmꢁ1):
2972, 2928(alkyl-CH), 1639, 1579, 1446(benzo[a]phenoxazinium
Starting materials were purchased from TCI (Shanghai) Devel-
opment Co., Ltd. or Sinogharm Chemical Reagent Co., Ltd. and used
directly. Synthetic grade solvents were obtained commercially and
used without further purification.
2.2. Instruments
Melting points were measured on an X-4 microscope electro
thermal apparatus (Taike, China) and the results are uncorrected. IR
spectra were recorded on a Nicolet 5200 FT-IR instrument using
solid samples dispersed in KBr pellets. The elemental analyses were
performed with a Vario El-III elemental analyzer. Mass spectra
were obtained on Waters ZQ 2000. UVevis absorption spectra were
recorded at room temperature in quartz cells of 1 cm path length
using a TU-1800 SPC spectrophotometer. Fluorescence spectra were
recorded at a HORIBA Jobin-Yvon Fluorolog 4 Tau-3 spectroscope.
1H NMR spectra were obtained on Bruker-300 or Varian-400 NMR
spectrometer, DMSO-d6 or CDCl3 were used as solvents, and tet-
ramethylsilane was used as the internal standard.
skeleton),1157(CeN); 1H NMR (300 MHz, CDCl3)
dppm: 8.60 (m, 2H,
2 ꢀ Ar-H), 7.65 (m, 2H, 2 ꢀ Ar-H), 7.50 (d, J ¼ 9.0 Hz, 1H, Ar-H), 7.31
(d, J ¼ 7.8 Hz, 1H, Ar-H), 7.08 (d, J ¼ 8.2 Hz, 1H, Ar-H), 6.96 (s, 1H, Ar-
H), 6.84 (d, J ¼ 7.4 Hz, 1H, Ar-H), 6.56 (d, J ¼ 9.1 Hz, 1H, Ar-H), 6.28
(d, J ¼ 11.9 Hz, 2H, Ar-H), 3.41 (q, J ¼ 7.0 Hz, 4H, 2 ꢀ CH2), 1.22 (t,
J ¼ 7.0 Hz, 6H, 2 ꢀ CH3); Calc. for C26H22ClN3O: C 72.97; H 5.18; N
9.82. Found: C 72.71; H 5.21; N 9.85; TOF-MS(EI): m/z 427.1453
[M]þ, Calc for m/z 427.1453 [M]þ.
2.3.3. General procedure for the preparation of benzo[a]
phenoxazinium salts (8aef)
To measure the optical non-linearity, open/close aperture
Z-scan measurements were performed at 532 nm with a pulse
width of 4 ns and a repetition rate of 10 Hz from a Nd:YAG laser
5-((3-Chlorophenyl)imino)-N,N-diethyl-5H-benzo[a]phenoxazin-
9-amine (7) (22.8 mg, 0.053 mmol) was dispersed in CHCl3 (10 mL)
and ethanol (10 mL) and the appropriate acid (0.1 mmol) was added.
The mixture was stirred for 0.5 h at room temperature, the solvent was
removed by rotary evaporation under reduced pressure until 5e7 mL
solvent remained in the flask; diethyl ether (40 mL) was added with
stirring. After filtration, the product was washed with ether and dried
under vacuum overnight.
(EKSPLA), the laser beam with approximately 15 mJ pulse energy
was focused on the solution in 2-mm cell by a lens of 10-cm focal
length, after the sample was collected by a photodiode detector
connected with a Boxcar integrator, the data acquisition was
automated [29e32]. 8a-f were dissolved in acetic acid solution at
the concentration of 1.0 ꢀ 10ꢁ4 mol Lꢁ1, and no aggregation was
observed at this concentration.
2.3.3.1. 5-((3-Chlorophenyl)amino)-9-(diethylamino)benzo[a]phe-
noxazin-7-ium chloride (8a). Green power, yield: 78%, mp:
184e185 ꢂC; IR (KBr pellet, cmꢁ1): 2971, 2867(alkyl-CH), 1639,1574,
1446(benzo[a]phenoxazinium skeleton), 1160(CeN); 1H NMR
(300 MHz, CDCl3) dppm: 9.74 (m, 1H, Ar-H), 8.84 (d, J ¼ 5.9 Hz, 1H,
Ar-H), 7.89 (m, 3H, 3 ꢀ Ar-H), 7.68 (m, 2H, 2 ꢀ Ar-H), 7.43 (m, 2H,
2 ꢀ Ar-H), 7.08 (s, 1H, Ar-H), 6.85 (s, 1H, Ar-H), 6.62 (s, 1H, Ar-H),
3.66e3.57 (m, 4H, 2 ꢀ CH2), 1.36 (t, J ¼ 6.7 Hz, 6H, 2 ꢀ CH3); Calc. for
2.3. Synthesis
2.3.1. 9-(Diethylamino)benzo[a]phenoxazin-7-ium nitrate (6)
The procedure followed the published procedure that used N,N-
diethyl-4-nitrosoaniline hydrochloride (5) and 2-naphthol [33].
The product was directly used in the next stage of the synthesis
without purification.
Fig. 2. Synthesis of benzo[a]phenoxazinium salts 8aef.

52
B.-Q. Liu et al. / Dyes and Pigments 88 (2011) 50e56
Fig. 3. Resonance structures of benzo[a]phenoxazinium salts 8aef.
C26H23Cl2N3O$2.5H2O: C 61.30; H 5.54; N 8.25. Found: C 61.23; H
5.13; N 8.10.
Ar-H), 7.45 (m, 1H, Ar-H), 7.42e7.32 (m, 1H, Ar-H), 7.11 (m, 1H, Ar-
H), 6.89 (s, 1H, Ar-H), 6.63 (s, 1H, Ar-H), 3.64e3.62 (m, 4H, 2 ꢀ CH2),
1.33 (t, J ¼ 6.9 Hz, 6H, 2 ꢀ CH3); Calc. for C26H23ClN4O4$2H2O: C
59.26; H 5.16; N 10.63. Found: C 58.96; H 4.82; N 10.34.
2.3.3.2. 5-((3-Chlorophenyl)amino)-9-(diethylamino)benzo[a]phe-
noxazin-7-ium bromide (8b). Green powder, yield: 83%, mp:
210e212 ꢂC; IR (KBr pellet, cmꢁ1): 2971, 2926, 2867(alkyl-CH),
1639, 1573, 1443(benzo[a]phenoxazinium skeleton), 1160(CeN); 1H
NMR (400 MHz, DMSO-d6) dppm: 8.84 (d, J ¼ 7.8 Hz, 1H, Ar-H), 8.66
(d, J ¼ 9.8 Hz, 1H, Ar-H), 8.01 (m, 3H, 3 ꢀ Ar-H), 7.92 (m, 2H, 2 ꢀ Ar-
H), 7.71e7.60 (m, 2H, 2 ꢀ Ar-H), 7.50 (m, 2H, 2 ꢀ Ar-H), 7.08 (s, 1H,
Ar-H), 6.82 (s, 1H, Ar-H), 3.72e3.68 (m, 4H, 2 ꢀ CH2), 1.23 (t,
J ¼ 6.6 Hz, 6H, 2 ꢀ CH3); Calc. for C26H23ClBrN3O$2.5H2O: C 56.38; H
5.10; N 7.59. Found: C 56.38; H 4.70; N 7.37.
2.3.3.5. 5-((3-Chlorophenyl)amino)-9-(diethylamino)benzo[a]phe-
noxazin-7-ium trifluoromethanesulfonate (8e). Brown powder,
yield: 84%, mp: 203e204 ꢂC; IR(KBr pellet, cmꢁ1): 2972, 2928
(alkyl-CH), 1642, 1580, 1445(benzo[a]phenoxazinium skeleton),
1156(CeN); 1H NMR (300 MHz, DMSO-d6) dppm: 8.88 (d, J ¼ 8.0 Hz,
1H, Ar-H), 8.66 (d, J ¼ 8.0 Hz, 1H, Ar-H), 8.10e7.99 (m, 1H, Ar-H),
7.93 (m, 2H, 2 ꢀ Ar-H), 7.63 (m, 2H, 2 ꢀ Ar-H), 7.52 (m, 3H, 3 ꢀ Ar-
H), 7.09 (s, 1H, Ar-H), 6.83 (s, 1H, Ar-H), 3.72e3.69 (m, 4H, 2 ꢀ CH2),
1.23 (t, J ¼ 6.7 Hz, 6H, 2 ꢀ CH3); Calc. for C27H23ClF3SN3O4$0.5H2O:
C 55.24; H 4.12; N 7.16. Found: C 55.36; H 3.96; N 7.06.
2.3.3.3. 5-((3-Chlorophenyl)amino)-9-(diethylamino)benzo[a]phe-
noxazin-7-ium iodide (8c). Black powder, yield: 76%, mp:
207e209 ꢂC; IR(KBr pellet, cmꢁ1): 2972, 2926(alkyl-CH), 1639,
1573, 1443(benzo[a]phenoxazinium skeleton), 1160(CeN). 1H NMR
(400 MHz, DMSO-d6) dppm: 8.88 (d, J ¼ 7.8 Hz, 1H, Ar-H), 8.67 (d,
J ¼ 7.8 Hz, 1H, Ar-H), 8.02 (d, J ¼ 7.4 Hz, 1H, Ar-H), 7.94 (m, 3H,
3 ꢀ Ar-H), 7.68e7.55 (m, 2H, 2 ꢀ Ar-H), 7.46 (m, 2H, 2 ꢀ Ar-H), 7.06
(s, 1H, Ar-H), 6.81 (s, 1H, Ar-H), 3.73e3.69 (m, 4H, 2 ꢀ CH2), 1.25 (t,
J ¼ 7.0 Hz, 6H, 2 ꢀ CH3); Calc. for C26H23ClIN3O$4H2O: C 49.73; H
4.98; N 6.69, Found: C 49.52; H 4.67; N 6.54.
2.3.3.6. 5-((3-Chlorophenyl)amino)-9-(diethylamino)benzo[a]phe-
noxazin-7-ium hydrosulfate (8f). Green powder, yield: 83%, mp:
130e131 ꢂC; IR(KBr pellet, cmꢁ1): 2974, 2928(alkyl-CH), 1641, 1575,
1445(benzo[a]phenoxazinium skeleton), 1158(CeN); 1H NMR
(300 MHz, DMSO-d6) dppm: 8.92 (d, J ¼ 8.2 Hz, 1H, Ar-H), 8.69 (d,
J ¼ 8.2 Hz, 1H, Ar-H), 8.04 (d, J ¼ 7.8 Hz, 1H, Ar-H), 7.97 (m, 2H,
2 ꢀ Ar-H), 7.71e7.60 (m, 2H, 2 ꢀ Ar-H), 7.59e7.45 (m, 3H, 3 ꢀ Ar-H),
7.13 (s, 1H, Ar-H), 6.88 (s, 1H, Ar-H), 3.71e3.68 (m, 4H, 2 ꢀ CH2), 1.24
(t, J ¼ 6.7 Hz, 6H, 2ꢀCH3); Calc. for C26H24ClSN3O5$4.5H2O: C 51.44;
H 5.48; N 6.92. Found: C 51.73; H 5.20; N 7.01.
2.3.3.4. 5-((3-Chlorophenyl)amino)-9-(diethylamino)benzo[a]phe-
noxazin-7-ium nitrate (8d). Black green powder, yield: 90%, mp:
135e136 ꢂC; IR(KBr pellet, cmꢁ1): 2972, 2928(alkyl-CH), 1640, 1574,
1448(benzo[a]phenoxazinium skeleton), 1159(CeN); 1H NMR
(400 MHz, CDCl3) dppm: 8.91 (m, 1H, Ar-H), 8.82 (d, J ¼ 7.9 Hz, 1H,
Ar-H), 7.91e7.71 (m, 3H, 3 ꢀ Ar-H), 7.60 (m, 1H, Ar-H), 7.54 (m, 1H,
3. Results and discussion
3.1. Synthesis and structure
3.1.1. Synthesis
Fig. 2 shows the synthetic procedure. To remove the zinc chlo-
ride which might affect the following procedure and properties of
the target compound, the 9-(diethylamino)benzo[a]phenoxazin-7-
ium nitrate (6) was prepared from N,N-diethyl-4-nitrosoaniline
hydrochloride (5) and 2-naphthol by two steps. The stable free base
Table 1
Calculated HOMO (eV) and LUMO (eV) energies of 8-III.
Compounds
X
LUMO
(eV)
HOMO
(eV)
DEHL
(eV)
8a
8b
8c
8d
8e
8f
Cl
Br
I
NO3
CF3SO3
HSO4
ꢁ3.523
ꢁ3.541
ꢁ3.670
ꢁ3.560
ꢁ3.498
ꢁ3.678
ꢁ5.249
ꢁ4.844
ꢁ4.559
ꢁ5.379
ꢁ5.884
ꢁ5.814
1.726
1.303
0.889
1.819
2.386
2.136
Fig. 4. Optimized geometryand the total NBO Charge Density of 8a(III) at B3LYP/6-31G*.

B.-Q. Liu et al. / Dyes and Pigments 88 (2011) 50e56
53
5-((3-chlorophenyl)imino)-N,N-diethyl-5H-benzo[a]phenoxazin-
9-amine (7) was obtained by the reaction of 6 and 3-chloroaniline
followed by the treatment of ammonia, then treated with the cor-
responding acids to form 5-((3-Chlorophenyl)amino)-9-(dieth-
ylamino)benzo[a]phenoxazin-7-ium salts (8aef). FT-IR spectra of
aromatic proton of 7 (6.25e8.75 ppm) were fully changed to
6.50e9.75 ppm, it is clear that the proton from the acids fully
changed the properties of the p-conjugated system.
3.1.2. Structure study with DFT calculation
8a-f were exhibited characteristic bands of 1639, 1579, 1446 cmꢁ1
,
As mentioned in the introduction part, there are three reso-
nance structures for the benzo[a]phenoxazinium salts 8aef (Fig. 3).
In order to evaluate the relative stability of the three resonance
structures, their equilibrium geometries were calculated using DFT
method with B3LYP [34]. The basis set used was 6e31G* for C, H, N,
O, F, S, Cl and Br atoms, and LANL2DZ for I atom[35]. Based on the
optimized structures and with the same basis set, vibrational
frequencies were calculated. All predicted vibrational spectra have
no imaginary frequency, which implied that the optimized geom-
etries were locating at the local lowest point on the potential
energy surface [36]. All the calculations were performed by using
Gaussian 98 program package [37]. The results show that 8-III are
the most stable structures, in which the anions are near NeH, and it
which are contributed to the structure skeleton, and CeN stretch-
ing vibrations appear at 1157 cmꢁ1. The 1H NMR chemical shifts of
1.1
A
7 (530nm)
1.0
8a(651nm)
8b(654nm)
8c(660nm)
0.9
0.8
8e
8d(651nm)
8e(655nm)
8f(651nm)
7
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
8a
8c
8d
can be explained by the pe
electron located in nitrogen atom and the
p
conjugation between the lone-pair
electron in 3-chloro-
p
8b
benzene. The total NBO Charge Density with potential of 8a(III) is
given in Fig. 4 together with the optimized geometry. It is clear that
the positive charge is located in the central conjugated heterocy-
cles. Table 1 lists the calculated HOMO and LUMO energies of the
most stable structures, 8aef(III).
8f
300
400
500
600
700
800
3.2. Absorption and emission spectra
Wavenlength(nm)
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
B
The UVevis absorption spectra of 8aef with concentrations of
1.0 ꢀ 10ꢁ5 mol Lꢁ1 in chloroform are shown in Fig. 5. Two
absorption peaks between 500 and 750 nm were present around
530 and 655 nm. According to Fig. 5A, the free base 7 happened to
have the maximum peak at 530 nm, it is obvious that 8aef was
partially dissociated to the free base 7 and corresponding acids in
high diluted neutral solvents. Acetic acid was selected as the
solvent to prohibit the ionization equilibrium, the 530 nm peak was
disappeared as expected and there is only one peak between 500
and 750 nm present at 660 nm (Fig. 5B). The more than 100 nm red
shifts from 7 to 8aef indicated the proton strongly enhanced the
8a
8b
8c
8d
8e
8f
8f
8c
conjugation of delocalized
p electron system. Fig. 5C shows the
emission spectra of these compounds in acetic acid. The emission
peaks were located around 700 nm when the samples were excited
at 500 nm. However, the observed quantum yields were very low.
The absorption and emission properties of 8aef are listed in Table 2.
300
400
500
600
700
800
Wavelength(nm)
3.3. NLO properties
C
8a(698nm)
8b(699nm)
8c(698nm)
8d(698nm)
8e(700nm)
8f(700nm)
100000
80000
60000
40000
20000
0
The nonlinear coefficients of the dyes were measured by the
Z-scan technique [39,40]. In our experiments, the properties of dyes
in acetic acid solution (approximately 1 ꢀ 10ꢁ4 mol Lꢁ1) were
investigated with 532 nm laser pulses using the Z-scan technique.
8e
8a
8f
8b
8c
Table 2
8d
The absorption and emission spectra properties of benzo[a]phenoxazinium salts
8aef.
Compoundsa lmax(abs) lg
3
lmax(em) Stokes shift (nm) Fb
(nm)
(nm)
(L$molꢁ1 cmꢁ1
)
8a
8b
8c
8d
8e
8f
661
661
661
661
661
661
4.87
4.91
4.70
4.93
4.96
4.75
698
699
698
698
700
700
37
38
37
37
39
39
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
650
700
750
800
Wavelength(nm)
a
Fig. 5. Absorption spectra of dyes 8aef in chloroform (A) and acetic acid (B), emission
spectra of dyes 8aef in acetic acid (C).
Acetic acid was used as solvent.
Absolute fluorescence quantum yields were measured by reported method [38].
b

54
B.-Q. Liu et al. / Dyes and Pigments 88 (2011) 50e56
The sample was moved forward or backward along the direction of
the laser beam around the focus (Z ¼ 0). The transmittance was
simultaneously recorded by an energy meter with or without an
aperture in the far field of lens as the function of sample position.
The non-linearity of pure acetic acid solvent was also measured
under the same conditions as the above, but no signal could be
obtained, so the non-linearity of solvent could be ignored.
Fig. 6 shows the normalized transmittances measured with an
open aperture, which is used to describe a third-order NLO
absorptive process. All transmittances are symmetric with respect
to the focus (Z ¼ 0) where they have minimum transmittances,
indicating an induced absorption, i.e. reverse saturated absorption.
Due to the presence of nonlinear absorption, the pure nonlinear
refractive property should be assessed from the division of the
normalized closed-aperture data by the open aperture data. The
NLO refractive properties of compounds 8aef, shown in Fig. 7, were
obtained by the division of the closed-aperture data by the
corresponding open aperture data shown in Fig. 6. We can see
a valley-peak shape, which was representative of positive nonlinear
refractive index(n2 > 0) and a self-focusing effect, as shown in Fig. 7,
and the difference between the normalized transmittance values at
valley and peak positions(
DTp-v) was related to the effective third-
order NLO refractive index n2(m2$Wꢁ1
)
NLO processes are governed by the nonlinear susceptibility
(c
(3)) of NLO materials. The larger the c(3) value, the better the
material’s NLO properties. In accordance to the observed
g and NLO
absorption coefficient(b
) values, the modulus of the effective c(3)
can be calculated [40e43]. The nonlinear susceptibility c(3) of 8d
was determined to be 1.2 ꢀ 10ꢁ11 esu, which is almost six times
larger than that of some metallophthalocyaines [44]. With the
value of
c
(3), the value of second-order hyperpolarizabilities could
be obtained by: g0
¼
c
(3)/Nc[(n20 þ 2)/3]4. Where Nc is the number
density of molecules and n0 is the linear refractive index of solvent.
Here, the solvent is acetic acid and the n0 is 1.3716. All NLO
1.00
1.00
0.95
0.90
0.85
0.80
0.75
0.70
0.65
a
b
0.95
0.90
0.85
0.80
0.75
-40
-20
0
20
40
-40
-20
0
20
40
Z position / mm
Z position / mm
c
1.00
0.98
0.96
0.94
0.92
0.90
0.88
1.00
0.95
0.90
0.85
0.80
0.75
0.70
0.65
0.60
d
-40
-20
0
20
40
-40
-20
0
20
40
Z position / mm
Z possition / mm
1.00
0.95
0.90
0.85
0.80
0.75
e
f
1.00
0.95
0.90
0.85
0.80
0.75
0.70
0.65
0.60
-40
-20
0
20
40
-40
-20
0
20
40
Z position / mm
Z position / mm
Fig. 6. Z-scan data of opened aperture.

B.-Q. Liu et al. / Dyes and Pigments 88 (2011) 50e56
55
b
a
1.2
1.2
1.1
1.0
0.9
0.8
0.7
1.1
1.0
0.9
0.8
0.7
-40
-20
0
Z position / mm
20
40
-40
-20
0
20
40
Z position / mm
c
d
1.10
1.05
1.00
0.95
0.90
1.2
1.1
1.0
0.9
0.8
0.7
-40
-20
0
20
40
-40
-20
0
20
40
Z possition / mm
Z position / mm
e
f
1.2
1.1
1.0
0.9
0.8
1.2
1.1
1.0
0.9
0.8
-40
-20
0
20
40
-40
-20
0
20
40
Z position / mm
Z position / mm
Fig. 7. Z-scan data of refractive part.
properties of compounds 8aef are summarized in Table 3 according
to the formula from previous literature [39].
4. Conclusions
For the compounds 8aef, g0 (8c)< g0 (8b)< g0 (8a)< g0 (8f)< g0
(8d)< g0 (8e), whereas the energies of the highest occupied
molecular orbital (HOMO), EH(8c) > EH(8b) > EH(8a) > EH(8d) >
A series of novel resonance benzo[a]phenoxazinium salts were
designed and synthesized by multi-step reactions, and their
structures were confirmed by FT-IR, UVevis, 1H NMR and elemental
analyses. Z-scan measurements indicated that these salts possessed
large third-order NLO coefficients, and the anions influenced their
EH(8f) > EH(8e), we noted that the increase in g
0 within compounds
8aef is somewhat related to the HOMO energy (Table 1), which
depends upon the anion. Indeed, the more stable HOMO energy is,
the larger g0 value is.
NLO properties, the more stable HOMO energy is, the larger
is. Such novel salts are expected to be potential candidates for
g
0 value
Table 3
NLO properties of 8aef in acetic acid.
Compounds
T0
C (mol$Lꢁ1)10ꢁ4
b
(m$Wꢁ1)10ꢁ11
15.4
9.84
4.88
16.4
n2 (m2$Wꢁ1)10ꢁ18
c(3) (esu)10ꢁ12
c(3)I (esu)10ꢁ12
c(3) (esu)10ꢁ12
g
(esu)10ꢁ29
R
8a
8b
8c
8d
8e
8f
0.73
0.78
0.72
0.74
0.70
0.70
1.08
0.98
0.90
1.00
0.87
0.95
11.6
10.9
4.96
13.7
11.6
11.2
9.05
8.49
3.86
10.7
8.99
8.72
5.06
3.24
1.61
5.39
5.78
3.59
10.4
9.09
4.18
12.0
10.7
9.43
5.71
5.50
2.75
7.12
7.29
5.89
17.5
11.6

56
B.-Q. Liu et al. / Dyes and Pigments 88 (2011) 50e56
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Acknowledgements
We thank the financial support from National Natural Science
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