Synthesis and nonlinear optical properties of rod-like luminescent materials containing Schiff-base and naphthalimide units

Article abstract of DOI:10.1039/b109384n

Two series of novel rod-like compounds containing Schiff-base and naphthalimide units were synthesized. The liquid crystalline and amorphous phases of these compounds have been studied by differential scanning calorimetry (DSC), polarizing microscopy and

Full text of DOI:10.1039/b109384n

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Synthesis and nonlinear optical properties of rod-like luminescent
materials containing Schiff-base and naphthalimide units{
Yingchao Zhang,a Weihong Zhu,a Wenjun Wang,b He Tian,*a Jianhua Sua and
Wencheng Wangb
aInstitute of Fine Chemicals, East China University of Science and Technology, Shanghai
200237, P. R. China. E-mail: tianhe@ecust.edu.cn; Fax: 186-21-64248311
bState Key Joint Lab for Materials Modification by Laser, Ion and Electron beams,
Department of Physics, Fudan University, Shanghai 200433, P. R. China
Received 15th October 2001, Accepted 27th February 2002
First published as an Advance Article on the web 26th March 2002
Two series of novel rod-like compounds containing Schiff-base and naphthalimide units were synthesized. The
liquid crystalline and amorphous phases of these compounds have been studied by differential scanning
calorimetry (DSC), polarizing microscopy and X-ray diffraction. It was found that compounds 1A, 2A and 3A
with a bridge of benzene show an amorphous glassy state, whereas compounds 1B, 2B and 3B with 2,5-
dioctyloxy-1,4-bis(phenylenevinylene)benzene as a bridge exhibit smectic F and nematic mesomorphic phases.
These rod-like compounds emit yellow–green light with the photoluminescent peak at about 505 nm and have
relatively high quantum yields. The nonlinear optical properties of the compounds have been investigated by
using a second harmonic generation (SHG) technique on a Langmuir–Blodgett monolayer. Compound 3B
exhibits SHG properties with a susceptibility x(2) of about 1.51 6 1029 esu.
is a prerequisite for SHG materials.15,17 The LB-technique was
first applied to the alignment of emission layers of conjugated
light-emitting polymers by Neher and co-workers.18
Introduction
Research into organic functional materials is an expanding
area because of their interesting optical and electronic proper-
ties as well as their industrial applications in many fields
including displays, electroluminescent (EL) devices, transistors,
batteries, sensors and photoreceptors.1–4 Materials possessing
both self-organizing capability and fluidity properties can be
used as active components in photorefractive materials,5 liquid
crystalline (LC) semiconductors,6,7 and molecular wires and
fibers.8,9 Meanwhile, a great deal of light has been shed on
amorphous materials due to their unique morphology.10,11
The synthesis and application of novel materials which form
stable amorphous glasses above room temperature have
attracted considerable interest due to their excellent flexibility,
transparency, non-existence of grain boundaries and isotropic
properties.12,13 Consequently, there is a strong interest in the
application of organic light-emitting diodes (OLEDs) as light
sources in the fast developing field of liquid crystal display
(LCD) technologies. An approach to constructing OLEDs that
emit linearly polarized light is the alignment of the emitting
moieties in ultra-thin layers.14 Alternatively, liquid crystalline
fluorescent materials can be aligned on suitable alignment
layers.3
Conventional Schiff-base compounds have limited use in
many fields due to their low stability and poor solubility.19 But
Schiff-base compounds can offer compounds a highly polari-
sable p-electron conjugation system. It is known that crucial
prerequisites for achieving large bulk second-order nonlinear
optical (NLO) properties are that the individual constituents
have large molecular NLO properties.20 In addition, naphthal-
imide derivatives are well known to have a wide range of
applications, such as, brilliant green–yellow dyes for synthetic
polymer fibers,21 supermolecular moieties for the study of
photo-induced electron transfer,22,23 and electroluminescent
(EL) materials.24–26 In this study, we incorporated a benzene-
1,4-dicarboxaldehyde or a 2,5-dioctyloxy-1,4-bis(4-formylphe-
nylenevinylene)benzene unit into a naphthalimide chromophore
and synthesized six novel rod-like Schiff-base chromophores
(shown in Scheme 1). In these compounds, Schiff-base units
maintain liquid crystalline properties and potentially offer
NLO properties, and the long terminal hydrocarbon chains
improve the solubility and film-forming ability. With the
alignment of these rod-like LCs the naphthalimide moiety
could been utilized to orient fluorescent mesogens.
The phenomenon of second-harmonic generation (SHG),
whereby a material under illumination generates light at twice
the incident frequency, is finding increasing use in the deve-
lopment of photonic devices.15 Compared with traditional
inorganic nonlinear optical materials, organic functional
materials have several advantages, such as large birefringence
and high dispersion of the refractive index.16The Langmuir–
Blodgett (LB) technique is of interest because it permits control
of the materials properties at the molecular level and it can
make self-organized, non-centrosymmetric thin films , thus
offering a way to create a non-centrosymmetric structure which
Results and discussion
1. Synthesis of rod-like Schiff-base molecules
The synthetic route to all these rod-like Schiff-base chromo-
phores (shown in Scheme 1) is a modification of conventional
preparation of Schiff-base compounds. According to the litera-
ture,27,28 Schiff-base compounds can be prepared by conden-
sing an amino group with an aldehyde group under reflux. The
method we use here includes longer reaction times to allow the
reaction to go further towards completion, using a catalyst
and a stoichiometric excess of the amine moiety. In this way,
after several recrystallizations from DMF we can obtain pure
{Electronic supplementary information (ESI) available: DSC of 2A and
3B, and a picture of the smectic F phase for 3B. See http://www.rsc.org/
suppdata/jm/b1/b109384n/
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DOI: 10.1039/b109384n

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Scheme 1 Synthetic route of the target molecules.
(¢99.5%) isolated target compounds with yields in the range of
72–88%.
depicted in Scheme 2. 1,4-Dioctyloxybenzene and 2,5-bis(bro-
momethyl)-1,4-dioctyloxybenzene could be similarly prepared
according to the literature31 with yields of 86% and 85%,
respectively. As for the target intermediates, minor modifica-
tions31 were made, such as using more lithium ethoxide solu-
tion and recrystallizing the desired compound from absolute
ethanol instead of column chromatography. All of this sim-
plified the procedure and resulted in good yields.
The corresponding substituted naphthalimides were pre-
pared via two steps from commercially available 4-bromo-1,8-
naphthalic anhydride. Since 4-bromo-1,8-naphthalic anhydride
has two reactive positions (the bromo and anhydride reactive
centers), it has two possible reaction tendencies (either the
bromo group is displaced or there is imidization) when it reacts
with a primary amine. Fortunately, the reaction could be well
controlled by choosing different solvents and reaction tem-
peratures. According to the literature,29,30 4-bromo-1,8-
naphthalic anhydride firstly reacts with hydrazine in a protic
solvent (ethanol) at room temperature to give N-amino-4-
bromo-1,8-naphthalimide with excellent yield, then N-amino-
4-bromo-1,8-naphthalimide reacts with excess aliphatic amine
in an aprotic solvent, N-methylpyrrolidin-2-one, at high tem-
perature (110 uC) to give the desired products with a yield of
72–85%.
2. Spectral properties
As indicated in Table 1 and Fig. 1, the absorption peaks at
about 435 nm correspond to the absorption of the naphthal-
imide moiety and the peaks of about 273 nm belong to the
benzene moiety. It is evident that each of the compounds 1B,
2B, 3B has an additional absorption peak of about 347 nm
compared with compounds 1A, 2A, 3A, which is attributed to
the absorption of the conjugation system of phenylenevinylene.
As found in Table 1, the fluorescent wavelength peaks of all these
compounds show approximately the same value regardless of
Based upon Wittig-type ethylenic group formation metho-
dology, the intermediate 2,5-dioctyloxy-1,4-bis(4-formylphe-
nylenevinylene)benzene could be prepared by the process
Fig. 1 Absorption spectra of compounds 1A, 2A, 1B and the
fluorescence spectra of compound 2A (excited at 274 nm and
434 nm) in CHCl3 (2.5 6 1025 mol L21).
Scheme 2 Preparation of 2,5-dioctyloxy-1,4-bis(4-formylphenylene-
vinylene)benzene.
Table 1 The absorption and fluorescent spectral data of the compounds in CHCl3 (2.5 6 1025 mol L21
)
Compounds
1A
1B
2A
2B
3A
3B
lmaxab/nm (loge)
274 (4.72)
273 (4.81)
346 (4.55)
436 (4.98)
503.2 (346)
499.8 (436)
76.4
274 (4.79)
273 (4.80)
348 (4.54)
436 (4.97)
498.2 (347)
497.3 (435)
70.7
275 (4.86)
273 (4.78)
348 (4.53)
438 (4.97)
501.0 (347)
502.3 (437)
78.3
434 (4.27)
505.6 (274)
507.5 (434)
77.9
434 (4.47)
506.2 (274)
506.7 (433)
94.6
433 (4.56)
501.7 (275)
501.1 (433)
68.8
lemmax/nm (lex/nm)
Wfl*a (%)
aThe relative fluorescence quantum yields (Qfl) of these compounds were measured at room temperature by comparison with the standard of
rhodamine 6G.
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Table 2 Calorimetric data for compounds 1B, 2B, 3B
Compounds Phase T/uC Phase T/uC Phase T/uC Phase
1B
2B
3B
Cr
Cr
Cr
113
106
94
S
S
S
150
148
127
N
N
N
238
220
205
Is
Is
Is
the excited wavelength. A case in point is the spectrum of
compound 2A (shown in Fig. 1). All of these compounds
emitted strong yellow–green fluorescence with high quantum
yields. For example, compound 2A has a high fluorescent
quantum yield of 94.6% compared with a standard solution of
rhodamine 6G. These materials are expected to find potential
applications as functional materials, e.g. luminescent materials
for photoreceptors and electroluminescent (EL) devices. The
EL properties of these compounds are being investigated and
the potential application of compounds 1B, 2B and 3B in
polarized electroluminescent devices will be studied in due
course.
Fig. 3 Nematic phase texture of compound 3B.
smectic F (Sf) phase, however, this conclusion can not yet be
exactly confirmed in our lab by other measurements such as
temperature-dependent wide- and small-angle X-ray patterns.
Thermotropic behaviors were evaluated by means of DSC with
at least two cycles (heating and then cooling refers to one
cycle). Fig. 2 shows the DSC thermogram of compound 3B on
its second cooling and Fig. 4 is the thermogram of compound
2A on its second heating.
3. Mesomorphism and amorphous properties
Compounds 1B, 2B and 3B exhibit liquid crystalline phases
as evidenced by differential scanning calorimetry (DSC) and by
the optical textures observed on a polarizing microscope
equipped with a hot-stage apparatus. Table 2 depicts the
calorimetric data for compounds 1B, 2B and 3B. The symbols
Cr, S, N and Is refer to the crystalline solid, smectic phase,
nematic phase and isotropic liquid phase, respectively.
Representative DSC thermogram and nematic phase texture
of compound 3B are shown in Fig. 2 and 3, respectively. We
observe three sharp exothermic peaks at 94 uC, 127 uC and
205 uC as shown in Fig. 2. It seems that the exothermic peaks at
94 uC and 127 uC refer to the LC phase transitions and another
peak at 205 uC is due to the melting point (Tm). Referring to the
Schlieren texture, the phases could be assigned to a nematic
phase transition at 127 uC and a smectic phase transition at
94 uC. Compounds 1A, 2A, and 3A have phase transitions
evidenced by their TGA analysis and DSC thermograms.
However, we can not observe their liquid crystalline phases
under a polarizing microscope equipped with a hot-stage
apparatus, but, when they were heated, melted, then cooled,
they changed into the amorphous glassy state. Fig. 4 shows the
DSC thermogram of 2A on the second heating. The glass
transition temperature (Tg) was observed at 32 uC, followed by
an exothermic peak owing to crystallization at around 98 uC,
then another crystallization occurred at 132 uC and the crystal
melted at 295 uC. The formation of an amorphous glassy state
is further evidenced by the X-ray diffraction pattern. As
depicted in Fig. 5, compound 2A showed no sharp signals but
only broad halos at 20 uC, whereas several sharp signal peaks
appeared at 120 uC which indicates that 2A is in a crystalline
state. Therefore compound 2A has no liquid crystalline phase
but an amorphous state.
As seen in Table 2, compounds 1B, 2B and 3B possess two
kinds of mesophase, S and N phases. These mesogenics possess
mesophases that exist over a large temperature range. For
example, the nematic phase of compound 3B was observed
from 205 to 127 uC. This range is much larger than common
Schiff-base LCs. This phenomenon is similar to the LC pro-
perties of some compounds containing a perylene core.6,32 It is
probably more accurate to say that, as the p-surface become
larger, the enhanced stacking interactions increase stability,
widening the temperature range. We have noticed that the
transition temperature for these compounds is dependent on
the chemical structure. The clearing temperature drops
significantly as the number of carbons on the terminal group
is increased. A case in point is that the clearing point of
compound 3B is 205 uC; 33 uC lower than that of compound
1B. The phenomenon was also observed in other LC phase
temperatures, such as the S phase temperature of compound 3B
which is 23 uC lower than that of compound 1B. Within the
temperature range of 127–94 uC for compound 3B, a liquid
crystal phase with high viscosity was observed by a polarizing
microscope equipped with a hot-stage apparatus. Based on the
knowledge concerning LC phases from the literature and our
experience,19 the observed phase should be referred to as a
From the molecular optimization structure calculated by
Fig. 2 Differential scanning calorimeter (DSC) traces of compound 3B
on the second cooling; the scan rate was 10 uC min21
Fig. 4 Differential scanning calorimeter (DSC) traces of compound 2A
on the second heating. Scan rate was 10 uC min21
.
.
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Fig. 7 p–A isotherm of compounds 3A and 3B.
Fig. 5 X-Ray diffraction patterns of 2A at 20 uC and 120 uC.
solid phase part and the monolayer collapse pressure is over
40 mN m21. It was also assumed that the limiting area is larger
for 3B than for 3A. Consequently, the molecules stand up on
the surface as the surface pressure increases and the molecular
area decreases, and the film eventually collapses.
In the SHG experiment, the macroscopic second-order
susceptibility x(2) of the LB film of 3B was determined by the
method developed by Ashwell et al.17
using a semiempirical molecular orbital package (MOPAC
AM1 1997 release version) as shown in Fig. 6, it can be seen
that these compounds are nonplanar. In this study we think
that the nonplanar molecular conformation of 1A, 2A, and 3A,
together with the introduction of the long hydrocarbon chains
at the 4-position of the naphthalimide prevent the easy spacial
reorientation of the molecules, and consequently crystallization
when cooled from the melting state. It thus favors the
formation of a stable amorphous glassy state. For compounds
1B, 2B and 3B, the increasing of the conjugation system and the
introduction of long hydrocarbon chains to the 1,4-bis(4-
formylphenylenevinylene)benzene moiety can decrease the
strength of the steric hindrance, thus making such molecules
more flexible. Hence they show liquid crystalline phases as
conventional Schiff-base compounds.
(x(2)lIv)2
n2vn2v
I2v
~
(1)
Where l is the film thickness, Iv is the incident beam intensity,
and nv and n2v are the refractive indices at 1064 nm and
532 nm, respectively. The SHG signal from the 3B monolayer
was compared with the SHG signal from the reference quartz
crystal for which x(2) is known (1.20 6 1029 esu). The SHG
measurements for 3B and the reference were done under the
same conditions for all controllable variables, taking nv ~ 1.51
and n2v ~ 1.52 for the reference quartz plate. The refractive
indices for 3B were taken to be nv ~ 1.57 and n2v ~ 1.49. Thus
the second-order molecular hyperpolarizability (b) was then
derived using the equation developed by Lupo et al.33
4. Second-order nonlinear optical properties
Fig. 7 shows the surface pressure–area (p–A) isotherm of 3A
and 3B. The steeply inclined part which corresponds to the
formation of the solid monolayer, together with the high
surface pressure of the collapse point of the monolayer indicate
the good film-forming behavior of these compounds. From the
surface p–A isotherm of 3B, the monolayer collapse pressure
x(2)
2v(f v)2s
Here fv,2v ~ [(nv,2v)2 1 2]/3 is a local field factor, and s is the
surface density of the monolayer. Hence we can obtain x(2)
l
b~
(2)
f
is about 30 mN m21 and the limiting area per molecule is
2
estimated to be 180 A . This indicates that the configuration of
˚
~
compound 3B at the air–water interface is highly ordered. As
for 3A, the molecules at the air–water interface have a distinct
1.51 6 1029 esu and b ~ 2.3 6 10230 esu by comparison with
the signal from the quartz reference and using the values above.
The measured P-in/P-out SHG intensity vs. incident angle is
shown in Fig. 8(a) for the monolayers on both sides of the
substrate. A maximum in intensity was obtained at an incident
angle of 60u, which was close to results observed in previous
work.33,34 Notice that a clear fringe pattern was observed in
Fig. 8(a). It is not a Maker-fringe because the sample thickness
was much less than the SHG coherence length of the 3B thin
film. This pattern was caused by the interference between two
internal SHG beams coming from the film on the front and
back of the substrate. On the other hand, such a periodic fringe
pattern disappears when a monolayer is on only one side as
shown in Fig. 8(b). This phenomena gives the statement further
evidence.
The SHG intensities of 3A were also measured in the
Langmuir–Blodgett films for pure 3A Z-type 10 layers. But no
SHG signal was observed. It is well known that an effective
second-order NLO material needs to be noncentrosymmetric,
which is a prerequisite for SHG materials. Comparing the
optimization structural configuration between 3A and 3B, as
shown in Fig. 6, we find that compound 3A is almost centro-
symmetric and 3B is obviously noncentrosymmetric. We can
assume that the origin of the SHG signals of 3B is attributed to
such noncentrosymmetry resulting from the incorporation of
the 1,4-bisphenylenebenzene unit.
Fig. 6 Optimized molecular structure of compounds 3A and 3B
calculated by using MOPAC AM1 methods.
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J ~ 8.3 Hz, 1H), 8.45(d, J ~ 7.3 Hz, 1H), 8.70(d, J ~ 8.5 Hz,
1H).
Synthesis of 1,4-dioctyloxybenzene
A suspension of 1,4-hydroquinone (2.75 g, 25 mmol), 1-bromo-
octane (14.48 g, 75 mmol), and K2CO3 (10.4 g, 75 mmol) in
acetonitrile (50 mL) was heated at reflux for 2 days under an
argon atmosphere before being poured into water (50 mL). The
precipitates were firstly collected by suction and then dissolved
in a minimum of hot hexane. Subsequently, the resulting hot
solution was poured into methanol (50 mL) to give a white
solid in 86% yield, mp 55–57 uC.
Synthesis of 2,5-bis(bromomethyl)-1,4-dioctyloxybenzene
To a suspension of 1,4-dioctyloxybenzene (0.7 g, 1.85 mmol)
and paraformaldehyde (0.11 g, 3.80 mmol) in acetic acid
(10 mL) was added hydrobromic acid (1.7 mL), then heated to
65 uC with stirring for 2 h. After cooling to room temperature,
this suspension was poured into water (25 mL), the precipitate
was filtered and dissolved in hot chloroform. Reprecipitation of
the resulting solution in methanol then gave a white solid in
85% yield. Mp 74–76 uC. 1H-NMR (CDCl3): d(ppm) 0.88(t,
6H, CH3-), 1.28(m, 16H, -(CH2)4-), 1.48(m, 4H, -OCH2CH2-
CH2-), 1.81(m, 4H, -OCH2-CH2-), 3.98(t, 4H, -O-CH2-), 4.52(s,
4H, CH2Br), 6.85(s, 2H, aromatic ring).
Fig. 8 SHG intensities as a function of incident angle for monolayers of
compound 3B: (a) monolayer on both sides of the substrate; (b)
monolayer on one side of the substrate.
Synthesis of 2,5-dioctyloxy-1,4-bis(4-
formylphenylenevinylene)benzene
A suspension of 2,5-bis(bromomethyl)-1,4-dioctyloxybenzene
(0.52 g, 1.0 mmol) and triphenylphosphine (0.55 g, 2.1 mmol) in
toluene (30 mL) was heated at reflux for 3 h under an argon
atmosphere. The solvent was then removed under reduced
pressure. The resulting residue, along with benzene-1,4-
dicarboxaldehyde (0.27 g, 2 mmol), was dissolved in methylene
chloride (50 mL). To this solution was added lithium ethoxide
solution (3 mL, 1.0 mol L21 in ethanol) dropwise via a syringe
at room temperature. The base should be introduced at such a
rate that the transient red–purple color produced upon the
addition of the base should not persist. The resulting solution
was stirred for 20 min after the addition of the base and was
then poured into a dilute aqueous hydrochloric acid. The
organic layer was extracted, washed with water and dried over
anhydrous sodium sulfate. The residue, after removal of
solvent, was then crystallized from absolute ethanol to give a
Experimental section
Infrared spectra were recorded on a Nicolet FTR-20sx, and 1H-
NMR spectra on a Brucker AM (500 MHz) NMR spectro-
meter using TMS as an internal standard. Absorption spectra
were measured on a Shimadzu UV-260 spectrophotometer, and
fluorescence spectra on a Hitachi-850 fluorescent spectrometer.
Differential scanning calorimetry (DSC) was performed with a
˚
TA DSC 2910 instrument. Solvent toluene was dried over 4 A
molecular sieves for more than two days.
Synthesis of N-amino-4-octylamino-1,8-naphthalimide
N-Amino-4-bromo-1,8-naphthalimide can be readily prepared
according to reference 24 with a yield of 92%, mp 218–220 uC
(literature: 218–220 uC). A mixture of N-amino-4-bromo-1,8-
naphthalimide (0.58 g, 2 mmol), 1-aminooctane (0.52 g,
4 mmol) and N-methylpyrrolidin-2-one (10 mL) was heated
to 110 uC and stirred for 6 h under an argon atmosphere. After
cooling, the mixture was poured into a mixture of ice and
water, the solid was filtered and recrystallized from chloro-
benzene, yielding bright yellow needles in 87% yield, mp 214–
216 uC. 1H-NMR (DMSO): d(ppm) 0.88(t, 3H, CH3-), 1.30(m,
12H,-(CH2)6-), 3.30(t, 2H, -NHCH2-), 5.72(2H, NH2), 6.80(d,
J ~ 8.6 Hz, 1H), 7.70(d, J ~ 7.8 Hz, 1H), 8.32(d, J ~ 8.5 Hz,
1H), 8.46(d, J ~ 7.3 Hz, 1H), 8.72(d, J ~ 8.5 Hz, 1H).
Other N-amino-4-substituted 1,8-naphthalimide compounds
were synthesized in a similar way. N-Amino-4-dodecylamino-
1,8-naphthalimide: yield 92%. Mp 202–204uC. 1H-NMR
(CDCl3): d(ppm) 0.88(t, 3H, CH3-), 1.18(m, 16H, -(CH2)8-),
1.56(m, 2H, -NHCH2CH2-CH2-), 1.75(m, 2H, -NHCH2-CH2-),
3.45(t, 2H, -NH-CH2-), 5.60(2H, NH2), 6.80(d, J ~ 8.6 Hz,
1H), 7.68(d, J ~ 7.8 Hz, 1H), 8.20(d, J ~ 8.4 Hz, 1H), 8.45(d,
J ~ 7.3 Hz, 1H), 8.70(d, J ~ 8.5 Hz, 1H). N-Amino-4-
octadecylamino-1,8-naphthalimide: yield 95 %. Mp 194–196 uC.
1H-NMR (CDCl3): d(ppm) 0.88(t, J ~ 6.9 Hz, 3H, CH3-),
1.20(m, 28H, -(CH2)14-), 1.60(m, 2H, -NHCH2CH2-CH2-),
1.70(m, 2H, -NHCH2-CH2-), 3.50(t, 2H, -NH-CH2-), 5.65(2H,
NH2), 6.80(d, J ~ 8.6 Hz, 1H), 7.72(d, J ~ 7.8 Hz, 1H), 8.15(d,
1
red, fluorescent solid in 81% yield. Mp 114–116 uC. H-NMR
(CDCl3): d(ppm) 0.89(t, 6H, CH3-), 1.30(m, 16H, -(CH2)4),
1.55(m, 4H, -OCH2CH2- CH2-), 1.86(m, 4H, -OCH2-CH2-),
4.05(t, 4H, -O-CH2-), 7.15(s, 2H, central-phenyl-H), 7.42(d,
J ~ 8.25 Hz, 2H, vinyl-H), 7.48(d, J ~ 8.28 Hz, 2H, vinyl-H),
7.75(m, 4H, phenyl-H), 7.85(m, 4H, phenyl-H), 10.10(s, 2H,
CHLO).
General synthetic procedures of the target compounds. Into a
50 mL flask equipped with a Dean and Stark water distillation
trap and a condenser were added the substituted naphthalimide
(1 mmol), the dialdehyde (0.45 mmol), p-toluenesulfonic acid
(0.05 mmol) and toluene (20 mL). The stirred reaction mixture
was refluxed for 24 h and monitored by thin-layer chromato-
graphy (TLC). The yellow precipitate from the cooled reaction
mixture was collected by suction, washed with toluene and
dried, then crystallized from DMF to give bright yellow
crystals.
Compound 1A. Mp w280 uC, yield 80%. IR (KBr): 3340,
2930, 2850, 1680, 1650, 1630, 1580, 1550, 1450, 1340, 1240, 820,
1
760, 720 cm21. H-NMR (CDCl3): d(ppm) 0.88(t, 6H, CH3-),
1.30(m, 20H, -(CH2)5), 1.65(m, 4H, -NHCH2-CH2-), 3. 45(t,
4H, -NH-CH2-), 6.70(d, J ~ 8.7 Hz, 2H), 6.85(d, J ~ 8.6 Hz,
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2H), 7.40(m, 4H, central-phenyl-H), 7.70(d, J ~ 8.4 Hz, 2H),
8.28(d, J ~ 7.3 Hz, 2H), 8.45(d, J ~ 7.3 Hz, 2H), 8.75(s, 2H,
CHLN). MALDI-TOF-MS: [M1] 775.49. Elemental analysis:
Calc. for C48H52N6O4, C 74.22; H 6.70; N 10.82; Found: C
74.17; H 6.65; N 10.80%.
Determination of nonlinear optical properties
Surface pressure–molecular area (p–A) isotherm measurement
and LB film deposition were all carried out on a KSV 5000 two-
compartment Langmuir trough (made in Finland). Com-
pounds 3A and 3B in different concentration toluene solutions
were spread onto the aqueous subphase surface, then after
waiting for 40–50 min for the solvent to evaporate, the float-
ing film was compressed. The subphase was de-ionized with
doubly distilled water at 20 uC. The surface pressure–area
(p–A) isotherms were recorded at a barrier moving speed of
5 mm min21. Substrates (quartz) of size 30 mm 6 18 mm 6
2 mm were treated to obtain a hydrophilic surface. 3A and
3B were deposited during upstrokes at a dipping speed of
5 mm min21. Z-type multilayer films were deposited at a
constant pressure of 15 mN m21. It took 5 minutes to dry the
substrate after each deposition cycle. The transfer ratio could
be displayed and automatically recorded by the computer
during deposition.
Compound 1B. Mp 238–240 uC, yield 76%. IR (KBr): 3350,
2930, 2850, 1680, 1650, 1630, 1570, 1540, 1450, 1340, 1240, 970,
.
910, 820, 760, 740, 720 cm21 1H-NMR (CDCl3): d(ppm)
0.88(m, 12H, CH3-), 1.36(m, 40H, -(CH2)5), 1.75(m, 8H,
-NHCH2-CH2- and -OCH2-CH2-), 3.40(t, 4H, -NH-CH2-),
4.10(t, 4H, -O-CH2-), 6.70(d, J ~ 8.5 Hz, 2H), 6.90(d, J ~
8.4 Hz, 2H), 7.10(d, J ~ 8.2 Hz, 2H, vinyl-H), 7.15(d, J ~
8.5 Hz, 2H, vinyl-H), 7.35(s, 2H, central-phenyl-H), 7.50(m,
4H, phenyl-H), 7.65(m, 4H, phenyl-H), 7.75(d, J ~ 8.3 Hz,
2H), 8.24(d, J ~ 7.9 Hz, 2H), 8.45(d, J ~ 8.2 Hz, 2H), 8.70(s,
2H CHLN). Elemental analysis: Calc. for C80H96N6O6, C
77.67; H 7.77; N 6.80; Found: C 77.57; H 7.75; N 6.83%.
The experimental setup used for SHG measurements has
been described in reference 35. The incident fundamental beam
of 35 ps pulse-width, 10 Hz repetition rate, 1.5 mJ pulse21
energy at a wavelength of 1.064 mm from a mode-locked
Nd : YAG laser was directed onto the sample through a low
filter which removed any second harmonic component in the
incoming beam. The SHG signal at 532 nm was detected in
transmission using a photomultiplier tube (PMT) and a boxcar
averager, and recorded using a personal computer. The ratio of
the SHG intensity from the samples to that from a z-cut quartz
reference plate was taken to eliminate measurement errors
caused by laser power fluctuations. The dependence of SHG
intensity on the incident angle a was measured by rotating the
samples in the incident plane.
Compound 2A. Mp w280 uC, yield 85%. IR (KBr): 3350,
2920, 2850, 1680, 1640, 1620, 1580, 1550, 1450, 1350, 1240, 820,
1
760, 720 cm21. H-NMR (CDCl3): d(ppm) 0.88(t, 6H, CH3-),
1.60(m, 36H,-(CH2)9), 2.62(m, 4H, -NHCH2-CH2-), 3.40(t, 4H,
-NH-CH2-), 6.70(d, J ~ 8.6 Hz, 2H), 7.20(d, J ~ 8.1 Hz, 2H),
7.45(m, 4H, central-phenyl-H), 7.60(d, J ~ 8.1 Hz, 2H), 8.25(d,
J ~ 8.2 Hz, 2H), 8.50(d, J ~ 8.3 Hz, 2H), 8.72(s, 2H CHLN).
MALDI-TOF-MS: [M1] 888; [M1 1 3], 891. Element analysis:
Calc. for C56H68N6O4, C 75.68; H 7.66; N 9.46; Found: C
75.60; H 7.63; N 9.44%.
Compound 2B. Mp 220–222 uC, yield 74%. IR (KBr): 3350,
2920, 2850, 1680, 1650, 1620, 1570, 1540, 1460, 1340, 1240, 970,
1
910, 890, 820, 770, 740, 720 cm21. H-NMR (CDCl3): d(ppm)
Acknowledgement
0.89(m, 12H, CH3-), 1.40(m, 56H, -(CH2)5- and, -(CH2)9-),
1.80(m, 8H, -NHCH2-CH2- and -OCH2-CH2-), 3.35(t, 4H,
-NH-CH2-), 4.05(t, 4H, -O-CH2-), 6.75(d, J ~ 8.5 Hz, 2H),
6.95(d, J ~ 8.4 Hz, 2H), 7.10(d, J ~ 8.2 Hz, 2H, vinyl-H),
7.15(d, J ~ 8.4 Hz, 2H, vinyl-H), 7.30(s, 2H, central-phenyl-
H), 7.50(m, 4H, phenyl-H), 7.65(m, 4H, phenyl-H), 7.75(d, J ~
8.2 Hz, 2H), 8.25(d, J ~ 7.8 Hz, 2H), 8.40(d, J ~ 8.2 Hz, 2H),
8.70(s, 2H CHLN). Elemental analysis: Calc. for C88H112N6O6,
C 78.34; H 8.31; N 7.12; Found: C 78.27; H 8.35; N 7.10%.
This project was financially supported by NSFC/China
(29836150) and Shanghai Education Committee. Authors
thank Mr Min Zhao and Dr Dong Shen in ECUST for their
help in making liquid crystal boxes.
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J. Mater. Chem., 2002, 12, 1294–1300
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