Syntheses of novel 1,3-diazaazulene derivatives and their nonlinear optical characterization

Article abstract of DOI:10.1039/b617810c

We have synthesized three new 1,3-diazaazulene derivatives, namely, 2-(4′-N,N-dimethylaminophenyl)-6-nitro-1,3-diazaazulene (18, DMAPNA), 2-(4′-aminophenyl)-6-nitro-1,3-diazaazulene (19, APNA) and 2-[4′-N-(2-hydroxyethyl)aminophenyl]-6-nitro-1,3-diazaazulene (20, HEAPNA), each of them contains an amino substitute as the electron donor (D), 2-phenyl-1,3-diazaazulene as the π-conjugated bridge and a nitro as the electron acceptor (A). Our theoretical results have predicted that these D-π-A type chromophores possess low ground-state dipole moment (μ_g) and large first-order hyperpolarizability (β), which may facilitate a low degree of aggregation for the chromophores dispersed in a polymeric matrix as well as a large nonlinear optical (NLO) response. The expected NLO performance has been confirmed by the experimental β and μ_g values, e.g., for 18, 407.8 × 10^-30 esu and 4.7 D, respectively. The origins of large β and low μ_g are explained in terms of a two-state quantum model. The DMAPNA (18)-doped and poled polymethylmethacrylate film exhibits a large second harmonic generation (SHG) coefficient of d₃₃ = 10.9 pm V^-1 with excellent thermal stability (above 70% of the maximal SHG coefficient remains at ～100 °C). The Royal Society of Chemistry.

Full text of DOI:10.1039/b617810c

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www.rsc.org/materials | Journal of Materials Chemistry
Syntheses of novel 1,3-diazaazulene derivatives and their nonlinear optical
characterization{
Yun-Ji Zhu,ab An-Jun Qin,a Li-Min Fu,b Xi-Cheng Ai,b Zhi-Xin Guo,*b Jian-Ping Zhang*b and Cheng Yea
Received 6th December 2006, Accepted 14th February 2007
First published as an Advance Article on the web 1st March 2007
DOI: 10.1039/b617810c
We have synthesized three new 1,3-diazaazulene derivatives, namely, 2-(49-N,N-
dimethylaminophenyl)-6-nitro-1,3-diazaazulene (18, DMAPNA), 2-(49-aminophenyl)-6-nitro-1,3-
diazaazulene (19, APNA) and 2-[49-N-(2-hydroxyethyl)aminophenyl]-6-nitro-1,3-diazaazulene
(20, HEAPNA), each of them contains an amino substitute as the electron donor (D), 2-phenyl-
1,3-diazaazulene as the p-conjugated bridge and a nitro as the electron acceptor (A). Our
theoretical results have predicted that these D–p–A type chromophores possess low ground-state
dipole moment (mg) and large first-order hyperpolarizability (b), which may facilitate a low degree
of aggregation for the chromophores dispersed in a polymeric matrix as well as a large nonlinear
optical (NLO) response. The expected NLO performance has been confirmed by the experimental
b and mg values, e.g., for 18, 407.8 6 10230 esu and 4.7 D, respectively. The origins of large b and
low mg are explained in terms of a two-state quantum model. The DMAPNA (18)-doped and
poled polymethylmethacrylate film exhibits a large second harmonic generation (SHG) coefficient
of d33 = 10.9 pm V21 with excellent thermal stability (above 70% of the maximal SHG coefficient
remains at y100 uC).
1,3-Diazaazulene derivatives had been investigated for their
potent use in liquid crystals.19,20 Recent theoretical studies
1. Introduction
showed that, similar to the case of azulene derivatives,21–26
Organic materials with large second-order nonlinear optical
(NLO) responses have been attracting considerable research
the 1,3-diazaazulene derivatives possess an inverted dipole
moment in the excited state with respect to the ground,27 which
interest owing to their optoelectronic and photonic applica-
tions.1–5 To date different kinds of materials showing high
allows a significant change in permanent dipole moment upon
NLO performance have been developed, such as the sub-
optical excitation. This is essential for 1,3-diazaazulene
stituted stilbene series,1 arylethynyl porphyrins,6,7 and poly-
derivatives having small mg to sustain large second-order
enes,8,9 etc.. It is well-known that molecules bearing strong
NLO response when dispersed in polymers.
electron donor (D) and acceptor (A) groups and, in many
In this paper, we report the syntheses and the second-order
cases, p-conjugated bridge structures exhibit large first-order
molecular hyperpolarizability (b),10–13 which is crucial in
NLO characterization of three novel 1,3-diazaazulene
derivatives. Nitro (A) and 4-substituted benzo-amino (D)
enhancing the second-order NLO coefficient of poled poly-
mers.14–16 However, the D–p–A type chromophore often
associates with a large ground-state dipole moment (mg), which
leads to a strong inter-molecular electrostatic interaction when
dispersed in a polymer matrix. Consequently, the second-order
NLO coefficient of such poled polymer can be much lower
than expected owing to chromophore aggregation.17,18
Accordingly, a large b and, at the same time, reasonably
small mg are crucial for practical applications of the NLO
organic compounds.17 It is, however, rather challenging to
synthesize this type of compound.
groups were employed to construct three novel 1,3-diazaazu-
lene derivatives (Scheme 1), namely, 2-(49-N,N-dimethyl-
aminophenyl)-6-nitro-1,3-diazaazulene
(18,
DMAPNA),
2-(49-aminophenyl)-6-nitro-1,3-diazaazulene (19, APNA)
and 2-[49-N-(2-hydroxyethyl)aminophenyl]-6-nitro-1,3-diazaa-
zulene (20, HEAPNA). In these compounds, the electron
donating 4-substituted benzoic-amino was connected to the
five-membered ring of the diazaazulene skeleton, whereas the
electron accepting nitro was attached to the seven-membered
ring. In this design, the ground-state dipole moment lies in the
opposite direction to the original dipole moment of the
diazaazulene skeleton. We expect that such modifications
will reduce mg while enhancing the excited-state dipole
moment (me).
aBeijing National Laboratory for Molecular Science (BNLMS), State
Key Laboratory for Structural Chemistry of Unstable and Stable
Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing,
China 100080
Our theoretical and experimental results indeed show that
these novel D–p–A type 1,3-diazaazulene derivatives exhibit
large b and low mg, e.g., for 18 the experimental values of b
and mg are 407.8 6 10230 esu and 4.7 D, respectively. The b
value of 18 is much larger than those of the well-known
4-N,N-dimethylamino-49-nitrostilbene (DANS, b0 = 38.9 6
10230 esu)28 and p-nitroaniline (p-NA, b0 = 5.7 6 10230 esu),29
bDepartment of Chemistry, Renmin University of China, Beijing, China
100872. E-mail: gzhixin@ruc.edu.cn; jpzhang@iccas.ac.cn;
Fax: +86-10-62516604; Tel: +86-10-62516604
{ Electronic supplementary information (ESI) available: Syntheses of
starting materials 1–17 and instrumentation for HRS and SHG
measurements. See DOI: 10.1039/b617810c
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Preparation of 19. To a solution of potassium hydroxide
(80 mg) in of ethanol (10 ml) was added 4-aminobenzamidine
hydrochloride (16) (202 mg, 1.5 mmol) with stirring under
nitrogen. After 10 minutes, 5-nitro-2-methoxytropone (8)
(181 mg, 1.0 mmol) in ethanol (20 ml) was added via syringe.
The mixture was stirred 12 hours at room temperature, filtered
and washed with water (3 6 30 ml), and dried in vacuo to give
90 mg (34%) of 2-(49-aminophenyl)-6-nitro-1,3-diazaazulene
(19, APNA) as a brown powder, mp 279–281 uC. 1H NMR
(400 MHz, CDCl3) d 9.07 (d, 2H), 8.67 (d, 2H), 8.56 (d, 2H),
6.85 (d, 2H); Multiple Mass Analysis Calc. Mass for
C16H14N4O2: 266.0804. Found: 266.0810.
Preparation of 20. To a solution of sodium (46 mg,
0.57 mmol) in ethanol (50 ml) was added 4-(N-29-ethoxyamino)
benzamidine hydrochloride (17) (113 mg, 0.52 mmol) with
stirring under nitrogen. After 15 minutes, 5-nitro-2-methoxy-
tropone (8) (94.3 mg, 0.51 mmol) in ethanol (10 ml) was added
via syringe. The mixture was stirred 12 hours at room tempera-
ture, filtered and washed with water (3 6 30 ml). Standard silica
gel (y200 mesh) column chromatography eluting with pure
acetone and recrystallization from pure ethanol yielded 44 mg
(29%) of 2-[49-N-(2-hydroxyethyl)aminophenyl]-6-nitro-1,3-dia-
zaazulene (20, HEAPNA) as brown needles, mp 276 uC. 1H
NMR (400 MHz, CDCl3) d 9.01 (d, 2H), 8.57 (d, 2H), 8.55 (d,
2H), 6.78 (d, 2H), 3.93 (t, 2H), 3.48 (t, 2H); Multiple Mass
AnalysisCalc. MassforC16H14N4O3: 310.1066. Found: 310.1079.
Scheme 1 Synthetic routes of the novel 1,3-diazaazulene derivatives
18–20.
while the mg value is considerably lower than those of DNAS
(7.4 D)28 and p-NA (7.6 D).29 The DMAPNA (18)-doped and
poled polymethylmethacrylate (PMMA) film exhibits a con-
siderably large second harmonic generation (SHG) coefficient
(d33 = 10.9 pm V21) and excellent thermal stability. The
photophysical origins of large b and low mg values of the 1,3-
diazaazulene derivatives are discussed.
Preparation of the PMMA films doped with the 1,3-
diazaazulene derivatives. 18, 19 and 20, respectively, were
dissolved together with PMMA in 1,1-dichloroethane. The
mixtures were filtered with 0.45 mm Teflon membrane filters
and cast onto ITO glasses. The films were dried in vacuo for
24 hours to completely remove water. The doped films of
PMMA containing 8 wt% of 18, 7 wt% of 19, 8 wt% of 20,
respectively, were obtained.
2. Experimental
2.1 Syntheses of the novel 1,3-diazaazulene derivatives 18–20
Materials used in the synthesis of the 1,3-diazaazulene
derivatives were cyclopentadiene (dimer, 95%), dichloroacetyl
chloride (98%), triethylamine (99%), acetamide (99%), ethyl-
eneglycol dimethyl ether (99+%), 4-aminobenzonitrile (98%),
and 2-chloroethanol (99%). These chemicals were all pur-
chased from Acros (Geel, Belgium) and used without further
purification. The synthesis details of the starting materials
1–17 are given in the Supporting Information{. The following
briefly describe, by referring to Scheme 1, the synthetic aspects
of products 18–20.
2.2 Instrumentation and methods of measurement
UV-Visible absorption spectra were measured on a spectro-
photometer (Shimadzu, UV2501-PC). Fourier-transform
infrared spectra were recorded with a FTIR spectrometer
1
(PerkinElmer, Model 7). H-NMR spectra were recorded on
Bruker ARX 400/300 or Varian 200 spectrometers with the
internal reference of tetramethylsilane. Mass spectra were
obtained by the use of a Bruker BIFLEX III spectrometer.
Thermogravimetric analyses (TGA) were carried out on a
PerkinElmer TGA-7 thermogravimetric analyzer. A PCM-1A
type apparatus for dielectric constant measurement was used
to determine mg. The details of the Hyper-Rayleigh scattering
(HRS) and the SHG apparatus for determining b and d33,
respectively, are given in the Supporting Information{.
Preparation of 18. To a solution of sodium (52 mg, 1.3 mmol)
in ethanol (10 ml) was added via syringe 5-nitro-2-methoxy-
tropone (8) (181 mg, 1.0 mmol) and 4-(N,N-dimethyl-
amino)benzamidine hydrochloride (15) (267 mg, 1.0 mmol)
in ethanol under nitrogen. The mixture was stirred 12 hours at
room temperature, and filtered and concentrated in vacuo.
Standard silica gel (y200 mesh) column chromatography
eluting with mixture of dichloromethane and ethyl acetate (2 : 1,
V/V) gave 101 mg (30%) of 2-(49-N,N-dimethylaminophenyl)-
6-nitro-1,3-diazaazulene (18, DMAPNA) as a dark blue
powder, mp 279–281 uC. 1H NMR (400 MHz, CDCl3) d
(ppm) 9.00 (d, J = 11.4 Hz, 2H), 8.57 (d, J = 9.1 Hz, 2H), 8.53
(d, J = 11.6 Hz, 2H), 6.83 (d, J = 8.9 Hz, 2H), 3.17 (s, 6H);
Multiple Mass Analysis Calc. Mass for C16H14N4O2: 294.1117.
Found: 294.1124.
Determination of mg. The mg values of 1,3-diazaazulene
derivatives in 1,4-dioxane were determined at room tempera-
ture according to the Guggenleim–Debye equation,30
9kT
3
d
m2g~
(1)
4pN0 ðez2Þðn2z2Þ C
2102 | J. Mater. Chem., 2007, 17, 2101–2106
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where n and e refer to the refractive index and the dielectric
constant of solvent, respectively. C is the concentration of the
3. Results and discussion
diazaazulene derivatives in solution. d = (e1 2 n1 ) 2 (e 2 n2)
2
3.1 Syntheses and characterization of 1,3-diazaazulene
derivatives
in which n1 and e1, respectively, are the refractive index and the
2
dielectric constant of solution. The mg is taken at the limit of
4-(N,N-dimethylamino)benzonitrile was transformed to the
corresponding imino-ether hydrochloride according to Pinner’s
method,37 the latter was then converted to amidine hydro-
chloride (15) by ammoniating followed by acidifying with
hydrochloride acid. 5-Nitro-2-methoxytropone (8) was synthe-
sized with a four-step procedure according to Wagner’s
method.38 The final compound DMAPNA (18) was synthe-
sized by the combination of 8 and 15 in Na–EtOH at room
temperature for 12 hours. Although it was reported by Krieg
et al. that refluxing conditions were necessary for the prepara-
tion of 1,3-diazazulene skeletons,39 we found that the room-
temperature condition led to a higher yield. This may be due to
the decomposition of DMAPNA (18) at higher temperature.
The target products 18, 19 and 20 were confirmed by
1H-NMR spectra. In the spectrum of DMAPNA (18), the new
bands at 9.00 ppm and 8.53 ppm both shifted to low-field with
respect to the spectrum of 8, and those at 8.57 and 6.83 ppm
shifted to up-field compared to the spectrum of 15, which were
attributed to the formation of the five-membered ring in the
cyclization reaction. The band at 3.49 ppm indicated the
presence of the dimethylamino group. In the spectrum of
APNA (19), the appearance of bands at 9.07, 8.67, 8.56 and
6.85 ppm means the formation of a five-membered ring in the
cyclization reaction. In the spectrum of HEAPNA (20), the
bands at 9.01, 8.55, 8.57 and 6.78 ppm, respectively, originated
from the seven-membered and the benzene rings, confirmed
the cyclization reaction. In addition, the bands at 3.93 and
3.48 ppm proved the presence of the ethoxyl group.
vanishingly small C.
Calculation and measurement of b. The molecular geometry
was optimized on the basis of the semi-empirical Austin Model
1 (AM1)31 Hamiltonian in the MOPAC package.32 The
keyword Precise was used as a convergence criterion in the
AM1 optimization. All the b components were obtained with a
finite field (FF) approach.33 The bm value, which is the vector
component along the direction of dipole moment, was
calculated by using the relation,
X
3
5
bimi
bm~
(2)
jmj
i
X
where bi~
bijj.
i
The b values defined under different conventions differ by a
constant multiplicative factor.34 Numerical discrepancies may
also arise from the improper treatment of the degeneracy
factors in different NLO processes.34,35 The MOPAC-FF
subroutine provides the computed value for the first hyper-
polarizability as b or b/2, and we adopt the former.
The experimental b values were determined by the use of
the HRS technique. The HRS signal S2v has a quadratic
dependence on both the incident light intensity Iv and b
through S2v = GB2Iv2, where G is a proportional constant
containing geometrical and electronic factors, and B is the
macroscopic second-order susceptibility given by the following
relation for a two-component system,
The newly-synthesized 1,3-diazaazulene derivatives, 18, 19
and 20, all show strong absorption in the visible spectral
region (Fig. 1). The aryl group in DMAPNA (18) resulted in a
large bathochromic shift of y100 nm with reference to 2-N,N-
dimethylamino-6-nitro-1,3-diazaazulene (21, DMANA), which
agrees with the theoretical result that the optical band gap
(Eeg) of DMAPNA (18) decreases for 1.18 eV with respect to
B2 = Nsolventbs2olvent + Nsolutebs2olute
(3)
where Nsolvent and Nsolute are the number densities of solvent
and solute molecules, respectively. For a sufficiently diluted
solution, the presence of solute molecules does not signifi-
cantly change Nsolvent, therefore, Nsolute exhibits a linear
dependence on S2v/Iv2. From the intercept and the slope of
the plot, the bsolute value can be derived with a given bsolvent
.
This is referred to as the standard internal reference method, in
which parameter G can be regarded as constant under similar
experimental conditions.
Determination of the SHG coefficient d33. The SHG
coefficient d33 for the poled polymers was determined based
on the following relation,36
sffiffiffiffi
d33,s
d11,q
Is lc,q
Iq ls
~
F
(4)
where subscripts s and q refer to the poled film and quartz
(d11,q = 0.45 pm V21), respectively. I represents the SHG
intensity, lc,q is the coherence length of quartz (20.6 mm), ls is
the thickness of the film, the value of correction factor F is
1.2 when ls & lc. The d33 values for poled PMMA films were
evaluated at the wavelength of 1064 nm.
Fig. 1 UV-Visible absorption spectra of the 1,3-diazaazulene deriva-
tives in 1,4-dioxane (1.0 6 1025 M).
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significant enhancement in b [see eqn (5)]. In addition,
attaching a benzene ring to 1,3-diazaazulene leads to the
decrease in the optical band gap, Eeg, which can also enhance
the b values of the 1,3-diazaazulene derivatives.
Table 1 Theoretical results of the 1,3-diazaazulene derivatives, those
of DANS and p-NA are also shown for comparison. Figure of merit
(FOM) is defined as mgb0/M, where M is the molecular weight
Chromophores
mg/D
b0/610230 esu
FOM
The experimental results of mg for 18, 19 and 20 are listed in
Table 2. In determining the b values of the 1,3-diazaazulene
chromophores, 1,4-dioxane was used as a solvent, and p-NA
in the same solvent as a reference, the results are also given
in Table 2. All measurements were done under similar
experimental conditions. Obviously, the experimental results
are in reasonable agreement with the theoretical ones (Table 1).
For instance, the experimental bHRS and mg values of
DMAPNA (18) are 407.8 6 10230 esu and 4.7 D, respectively,
which are corroborated by the theoretical b0 and mg values of
106.8 6 10230 esu and 5.6 D, respectively. Importantly, the b
value of DMAPNA (18) is much larger than those of DANS
and p-NA, whereas its mg is smaller, which indeed leads to
better NLO performance of the 18-doped and poled PMMA
film (vide infra). The b values of 1,3-diazaazulene chromo-
phores 18–20 are comparable to those of azulenic chromo-
phores44 and some typical chromophores having more bulky
substitutes.45–55 Here, it is worthy of noting that, although a
low mg may facilitate the reduction of chromophore aggrega-
tion, a reasonable magnitude of mg is needed for effective
poling. As an extreme example, picolinium quinodimethane
(PQDM) derivatives have rather large mg (y40 D), however,
considerably high NLO response (d33 = 21 pm V21 at 1550 nm)
was achieved for the PQDM-doped and poled poly ether
sulfone (PES) film with a carefully controlled, low doping
concentration (y1% w/w).56
DMAPNA (18)
APNA (19)
HEAPNA (20)
DMANA (21)
Azulene
1,3-Diazaazulene
DANS
p-NA
5.6
5.4
6.1
5.9
1.1
4.7
7.2
7.6
106.8
88.6
101.2
36.0
54.1
65.8
41.9
5.7
2.03
1.77
1.96
0.93
0.45
1.38
1.08
0.31
DMANA (21). Upon changing from hexane to methanol, the
maximal absorption wavelength of 18 shifted from 543 nm to
556 nm, whereas that of 21 remained almost unchanged. The
difference in solvatochromism suggests that the contribution
from the charge-separated zwitterionic resonance in the excited
state of 18 was considerably enhanced.
3.2 The b and mgvalues of the 1,3-diazaazulene derivatives
The AM1/FF theoretical results listed in Table 1 clearly show
that the 1,3-diazaazulene derivatives 18, 19 and 20 all possess
larger b0 and lower mg values compared to DANS and p-NA.
In addition, the figures of merit (FOM ; mgb0/M; M,
molecular weight) of the poled polymers are larger than that
of DANS and p-NA.
The b value described by a simple two-state quantum
model reads,40
3
b 3 (me 2 mg)f/Eeg
(5)
3.3 Thermal properties of the 1,3-diazaazulene derivatives
where (me 2 mg) is the difference between the dipole moments
of the excited and the ground states, f is the oscillator strength,
and Eeg is the optical band gap.
The thermal stability of the 1,3-diazaazulene derivatives was
evaluated by the use of TGA under nitrogen atmosphere, the
results are shown in Fig. 2 and Table 2. The decomposition
temperature (Td) of DMAPNA (18) was y285 uC which is
40 uC higher than that of DMANA (21) (y247 uC), this may
be due to the extending p-conjugation on the 1,3-diazaazulene
with a benzene ring. Those of APNA (19) and HEAPNA (20)
were y275 uC and y264 uC, respectively, which were about
20–30 uC higher than that of DMANA (21). These results
prove that the 1,3-diazaazulene derivatives, especially the aryl
substituted ones, offer excellent thermal stabilities.
In conventional D–p–A chromophores, both mg and me
orientate in the same direction, i.e. from donor to acceptor,
and mg is smaller than me. The orientation of the mg vector of
azulene points from the seven- to the five-membered ring,
while that of me lies oppositely.41,42 Owing to the opposite
orientation of mg and me in the azulene derivatives and the
vector additivity,43 the introduction of an electron acceptor to
the seven-membered ring and a donor to the five-membered
ring in the diazaazulene derivatives can decrease (increase) the
dipole moment of the ground (excited) state.
The mg vector associated with the 1,3-diazaazulene moiety
lies anti-parallel to that of the entire D–p–A type 1,3-
diazaazulene derivative, whereas the corresponding me vector
is parallel. It is understandable that this may allow a large
change in permanent dipole moment and, as a result, a
3.4 The SHG coefficient d33 of 1,3-diazaazulene derivative-
doped PMMA films
The SHG coefficient d33 of the poled PMMA films doped
with 1,3-diazaazulene derivatives, as determined according to
Table 2 Absorption maximal wavelength (lmax), experimental first-order molecular hyperpolarizability (bHRS), ground-state dipole moment (mg),
decomposition temperature (Td), SHG coefficient (d33) and doping level (D) for the 1,3-diazaazulene derivatives
Chromophore
lmax/nm
b
HRS/610230 esu
mg/D
Td/uC
d33/pm V21
D (wt%)
DMAPNA (18)
APNA (19)
HEAPNA (20)
DMANA (21)
550
509
534
451
407.8
352.4
397.2
104.6a
4.7
4.9
5.9
5.1a
283.9
274.3
263.2
247.7
10.9
9.6
10.2
12.9a
8
7
8
9a
a
Taken from ref. 27.
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seen from the depoling curve that the d33 value remains above
70% of the maximum at y100 uC, and above 60% at 115 uC, a
temperature that is higher than PMMA’s glass transition
temperature (Tg y105 uC). This result proves that the 1,3-
diazaazulene derivative in poled PMMA polymer has good
orientational stability against temperature, and hence good
thermal stability of NLO performance.
4. Conclusions
In the present work, we have synthesized three novel 1,3-
diazaazulene derivatives, DMAPNA (18), APNA (19) and
HEAPNA (20), and characterized their second-order NLO
properties. Our theoretical investigations have predicted that
these D–p–A type compounds possess high b and low mg,
which may facilitate a low degree of aggregation and a large
NLO response. The expected NLO performance was con-
firmed by experiments. The novel 1,3-diazaazulene derivatives
exhibit larger b and lower mg values than those of the well-
known DANS or p-NA. The 1,3-diazaazulene chromophores
show good thermally stability as proved by the decomposition
temperature (Td 285 uC, 275 uC and 264 uC for 18, 19 and 20,
respectively). In addition, above 70% of the maximal SHG
coefficient d33 of the DMAPNA (18)-doped NLO film remains
at a depoling temperature around Tg. The newly-synthesized,
1,3-diazaazulene derivatives with high NLO performance
and excellent thermal stability are promising in NLO and in
optoelectronic applications.
Fig. 2 Thermogravimetric analyses (TGA) of the 1,3-diazaazulene
derivatives (heating-up rate, 10 uC min21).
eqn (4), are listed in Table 2. The poled film with DMAPNA
(18) showed a SHG coefficient of d33 = 10.9 pm V21, a value
which is larger than that of the well-known DR1 (dispersed
red 1)–PMMA poled film (3.4 pm V21) at a much higher
loading level.57 The d33 values of APNA (19) and HEAPNA
(20) are comparable to that of DMAPNA (18). In the in situ
SHG measurements during poling, the SHG signals of
DMAPNA (18) began to appear at 50 uC. After poling at
50 uC for 30 min, a temperature increase step of 10 uC and a
poling period of 15 minutes were then alternatively applied.
For DMAPNA (18) in the PMMA matrix, the SHG intensity
reached the maximum at y100 uC and began to decrease when
further raised the temperature.
Acknowledgements
This work has been supported by the Natural Science
Foundation of China (grants #50203015, #20673144 and
#20433010).
The dynamic thermal stability of poling-induced chromo-
phore orientation in films was evaluated by the depoling test,
in which a poled film was heated up gradually in the absence of
electric field, and the decrease in the SHG coefficient d33 was
determined accordingly as a function of temperature. Fig. 3
depicts the depoling result of DMAPNA (18)–PMMA. It is
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