The investigation of second harmonic generation from novel molecules [(E)-N-alkyl-4-{2-[4-(dialkylamino)phenyl]ethenyl}pyridazinium iodide]

Article abstract of DOI:10.1039/a707724f

(E)-N-Methyl-4-{2-[4-(dihexadecylamino)phenyl]ethenyl}pyridazinium iodide (MHPd), (E)-N-octadecyl-4-{2-[4-(dimethylamino)phenyl]ethenyl}pyridazinium iodide (OMPd), (E)-N-octadecyl-4-{2-[4-(diethylamino)phenyl]ethenyl}pyridazinium iodide (OEPd), (E)-N-octa

Full text of DOI:10.1039/a707724f

J O U R N A L O F
The investigation of second harmonic generation from novel molecules
(E)-N-alkyl-4-{2-[4-(dialkylamino)phenyl]ethenyl}pyridazinium
iodide]
[
T-R. Cheng,a C-H. Huang,*a L-B. Gan,a C-P. Luo,a A.-C. Yub and X-S. Zhaob
C H E M I S T R Y
aState Key L aboratory of Rare Earth Materials Chemistry and Applications, Peking University,
Beijing 100871, China
bDepartment of Chemistry, Peking University, Beijing 100871, China
(
(
(
(
E)-N-Methyl-4-{2-[4-(dihexadecylamino)phenyl]ethenyl}pyridazinium iodide (MHPd), (E)-N-octadecyl-4-{2-[4-
dimethylamino)phenyl]ethenyl}pyridazinium iodide (OMPd), (E)-N-octadecyl-4-{2-[4-
diethylamino)phenyl]ethenyl}pyridazinium iodide (OEPd), (E)-N-octadecyl-4-{2-[4-
dibutylamino)phenyl]ethenyl}pyridazinium iodide (OBPd) have been designed and synthesized. Their Langmuir–Blodgett film
forming properties and second harmonic generation were studied. According to the results observed, MHPd is the best among the
four congeners. By comparing the NLO properties of OMPd with those of a known compound, OMPy [(E)-N-octadecyl-4-{2-[4-
(dimethylamino)phenyl]ethenyl}pyridinium iodide], the impairing effect of the second nitrogen in the six-membered pyridazine
ring was examined experimentally and theoretically.
Since organic materials have higher nonlinear polarizabili-
ty compared with inorganic ones, research on second har-
monic generation today focuses on finding organic materials
containing donor–p–acceptor conjugation systems.1–5
Experimental Section
Methods and materials
Melting points were determined with an X micromelting-
point apparatus. EI mass spectra were recorded with a VG
ZAB-HS model spectrometer. 1H NMR spectra were measured
using a Bruker ARX400 NMR spectrometer with tetramethyl-
silane as an internal standard (in CDCl ). Elemental analyses
were carried out on a Carlo Erba 1102 and a Heraeus CHN-
Rapid instrument. UV–VIS spectra were recorded using
Shimadzu UV-3100 spectrometer.
N,N-Dihexadecylaminobenzene was synthesized by reacting
aniline with 1-bromohexadecane quantitatively in a mixture
of aqueous sodium hydroxide and toluene. N,N-
Dihexadecylaminobenzaldehyde was synthesized by following
reference 16. 4-Methylpyridazine-3,6-diol, 4-methyl-3,6-dichlo-
ropyridazine and 4-methylpyridazine were synthesized by fol-
lowing reference 17. All of them had satisfactory 1H NMR and
mass spectra.
4
Both experimental observations3,4 and model calculations6–8
have been used in studying the relationship between the
strength of second harmonic generation (SHG) (function) of
organic molecules and their structures. It has been demon-
strated that there is an optimal combination of donor and
acceptor strength required to maximize mb (where m is the
dipole moment of the molecule) for a given connective segment
and beyond that point increasing the donor–acceptor strength
leads to a diminution of the hyperpolarizability.9 The nontrivial
dependence of SHG upon bond length alternation (BLA, peaks
3
˚
at ~0.04 A) is predictable and has also been observed.10,11
Molecules with aromatic ground states tend to have greater
bond length alternation than non-aromatic polyenes of com-
parable length, because of the energy which must be overcome
for the loss of aromaticity while polarizing to a charge separ-
ated state. Heterocyclic rings such as thiazole12 or thiophene13
were thus introduced to replace benzene. Six-membered diazine
rings might give better bond length alternation than five-
membered thiazole or thiophene, but studies on such com-
pounds have not been reported. Also, the function of het-
eroatoms such as sulfur or oxygen studied by other researchers
are directly related to SHG.14 Here, we attempt to study the
function of the additional nitrogen in the title system.
In this work we have designed and synthesized a series of
new compounds: (E)-N-methyl-4-{2-[4-(dihexadecylamino)-
phenyl]ethenyl}pyridazinium iodide (MHPd) and its
congeners (E)-N-octadecyl-4-{2-[4-(dimethylamino)phenyl]
ethenyl}pyridazinium iodide (OMPd), (E)-N-octadecyl-4-{2-
The synthetic methods of the following materials are first
reported in this work. The procedures are as shown in
Scheme 1.
1,4-Dimethylpyridazinium
0.94 g, 0.01 mmol) was dissolved in 5 ml of benzene (dried)
iodide.
4-Methylpyridazine
(
with 1.25 ml (0.02 mol) of methyl iodide. The mixture was
stirred at room temp. for 24 h. The dark brown liquid (1,4-
dimethylpyridazinium iodide) which separated out from the
reaction mixture was washed with diethyl ether twice. Yield:
>
90%; m/z: 108 (M+−1).
[
4-(diethylamino)phenyl]ethenyl}pyridazinium iodide (OEPd)
MHPd. To 3 ml of a tetrahydrofuran solution of 0.190 g
and (E)-N-octadecyl-4-{2-[4-(dibutylamino)phenyl]ethenyl}-
pyridazinium iodide (OBPd). Their nonlinear optics (NLO)
properties were measured and compared with each other. For
the purposes of comparison, a known compound, OMPy
(0.34 mmol) of 4-N,N-dihexadecylaminobenzaldehyde, 2 ml of
an ethanol (dried) solution of 0.08 g (0.34 mmol) of 1,4-
dimethylpyridazinium iodide and 0.04 ml of piperidine were
added to form a homogeneous solution. The mixture was
stirred at room temp. for 5 d. The reaction mixture was then
poured into water to which chloroform was added to extract
the product. The organic phase was dried over magnesium
sulfate and purified on a silica gel column; eluent: chloroform–
methanol (5051). After evaporation of the solvent, a waxy
[
(E)-N-octadecyl-4-{2-[4-(dimethylamino)phenyl]ethenyl}-
pyridinium iodide], was also synthesized and measured.15
*
E-mail: hch@chemms.pku.edu.cn
J. Mater. Chem., 1998, 8(4), 931–935
931

O
OMPd: Calc. for C H N I (%): C, 63.64; H, 8.65; N, 6.94;
2 52 3
Found: C, 63.17; H, 9.04; N, 6.45; d (400 MHz, CDCl ): 0.88
(t, J 6.60, 3H, 1 CH ), 1.16–1.43 (m, 30H, 15 CH ), 2.06 (t, J
3
3
H C
H
3
H N
2
O
3
2
6
2
1
1
.72, 2H, 1 CH ), 3.10 (s, 6H, 2 CH MNR ), 4.57 (t, J 7.48,
2
3
2
H, 1 CH MN+R ), 6.72 (d, J 8.92, 2H, 2 ArH), 6.92 (d, J
O
2
3
5.64, 1H, ArCHN), 7.68 (d, J 8.92, 2H, 2 ArH), 7.97 (d, J
5.64, 1H, 1 ArMCHN), 8.33 (d, J 4.64, 1H, ArH), 9.22 (s, 1H,
ArH), 9.74 (s, 1H, ArH); l /nm (CHCl ): 558.5 (e 31700 dm3
CH
3
max
3
R′
2
N
mol−1 cm−1), 294.5.
Found: C, 66.08; H, 9.11; N, 6.05; d (400 MHz, CDCl ): 0.88
O
OBPd: Calc. for C H N I (%): C, 66.16; H, 9.35; N, 6.09;
3
8 64 3
NH
H
3
O
N
(
3
3
t, J 6.86, 3H, CH ), 0.99 (t, J 7.26, 6H, 2 CH ), 1.26–1.42 (m,
4H, 17 CH ), 1.62 (m, 4H, CH ), 2.00 (t, J 6.83, 2H, 1 CH ),
.38 (t, J 7.4, 4H, 2 R NCH M), 4.61 (t, J 7.21, 2H, 1
H
3
3
2
2
2
2
2
R N+CH M), 6.60 (d, J 9.09, 2H, ArH), 6.94 (d, J 15.67, 1H,
R′
2
N
CHO
CH
3
2
ArCHN), 7.65 (d, J 8.99, 2H, ArH), 7.95 (d, J 15.67, 1H,
ArCHN), 8.43 (d, J 6.75, 1H, ArH), 9.11 (d, J 6.73, 1H, ArH),
3
CH
N
3
CH
3
9
.85 (s, 1H, ArH); l /nm (CHCl ): 573.5 (e 51200 dm3
Cl
max
3
mol−1 cm−1), 321.5.
N
N
N
I–
N
Cl
N
+
p–A isotherm and film deposition
R
The monolayer of MHPd, OMPd, OEPd or OBPd was
obtained by spreading a chloroform solution of MHPd
(OMPd, OEPd or OBPd, 10−3–10−4) on to a pure water
subphase (pH 5.6) in a one-compartment computer-controlled
Nima Technology trough at 25 °C and the surface layer was
then compressed at a speed of 80 cm2 min−1 to a pressure of
for OEPd, 25 mN m−1 for OBPd). The monolayer was trans-
ferred at a rate of 5 mm min−1 on to a hydrophilically pre-
treated quartz slide in the upstroke. The transfer ratios were
N
R′
2
N
CH CH
N
R
I–
+
MHPd: R = CH
OMPd: R = C18
OEPd: R = C18
3
, R′ = C16
H
33
H
H
H
37, R′ = CH
3
3
5 mN m−1 for MHPd (35 mN m−1 for OMPd, 30 mN m−1
2 5
37, R′ = C H
OBPd: R = C18
4 9
37, R′ = C H
Scheme 1
1
.0± 0.1.
purple product was obtained. Yield: 15–20%. MHPd: Calc.
for C H N I(%): C, 68.59; H, 9.98; N, 5.33; Found: C, 68.59;
H, 10.02; N, 5.65; d (400 MHz, CDCl ): 0.72 (t, J 6.6, 6H, 2
SHG and UV–VIS spectra measurement
4
5 78 3
The second harmonic generation measurements were made in
transmission geometry with a Y-cut quartz plate as reference
and with a Q-switched Nd:YAG laser (1.064 mm). A 1/2 l plate
and a Glan–Taylor polarizer were used to vary the polarization
direction of the laser beam. The laser light, linearly polarized
either parallel (p) or perpendicular (s) to the plane of incidence,
was directed at an incident angle of 45° onto the vertically
mounted sample. A set of 1.064 mm filters and a monochroma-
tor were used to ensure that the signal detected by the
photomultiplier was generated by second-harmonic radiation.
The average output signal was recorded on a digital storage
recorder (HP54510A). All the SHG data in this work are
average values of at least three individual measurements.
The second harmonic intensities (I ) obtained from the
H
3
CH ), 1.10–1.27 (m, 52H, 26 CH ), 1.47 (t, J 6.2, 4H, 2 CH ),
3
3
2
2
3
.21 (t, J 7.6, 4H, 2 CH MN), 4.33 (s, 3H, R MN+MCH ),
2
3
6
.50 (d, J 8.8, 2H, ArH), 6.6 (d, J 15.4, 1H, NCHM), 7.43 (d,
J 8.8, 2H, ArH), 7.7 (d, J 15.6, 1H, NCHM), 8.19 (d, J 5.70,
H, ArH), 8.78 (d, J 5.68, 1H, ArH), 9.86 (s, 1H, ArH); l /nm
1
max
(EtOAc)
(
CHCl ): 578 (e 64000 dm3 mol−1 cm−1), 296; l
3
max
5
34, 292.
4
-Methyl-1-octadecylpyridazinium iodide. 4-Methylpyrida-
zine (1.54 ml, 16.4 mmol) and 1-iodooctadecane (6.2 g,
6.4 mmol) were dissolved in 15 ml of ethanol (dried). After
1
the mixture had been refluxed for 2 h, it was cooled. The
precipitate was collected by suction. The product, 4-methyl-1-
octadecylpyridazinium iodide, was recrystallized from ethanol.
Yield: 71%.
2
v
monolayer were analyzed by the general procedure described
by Ashwell.18
The UV–VIS spectra of LB monolayers were recorded on
a Shimadzu UV-3100 spectrometer with uncoated quartz
substrates as reference.
OEPd, OMPd and OBPd (general method). The appropriate
N,N-dialkylaminobenzaldehyde and 4-methyl-1-octadecylpyri-
dazinium iodide (151) were dissolved in methanol. The mixture
was stirred at room temp. (~20 °C) for 2 h after the addition
of piperidine. The purple, waxy product was separated by
Results and Discussion
column chromatography on silica gel using CHCl as eluent.
Yield: ~30%.
OEPd: Calc. for C H N I (%): C, 64.44; H, 8.91; N, 6.63;
3
Pressure–area isotherms
The surface pressure–area (p–A) isotherms for all the com-
pounds are shown in Fig. 1. The sequence of film forming
properties is MHPd>OMPd>OEPd>OBPd.
As shown in Table 1, although their collapse pressures are
about the same, the isotherm of OMPd in the condensed
region is steeper than that of OMPy (slope for OMPd is 2.83,
3
4 56 3
Found: C, 64.22; H, 9.23; N, 6.44; d (400 MHz, CDCl ): 0.88
t, J 6.0, 3H, 1 CH ), 1.11–1.35 (m, 28H, 14 CH ), 1.60 (m,
H
3
(
3
2
8
7
6
7
8
1
2
H, 2CH , 1 CH ), 2.03 (t, J 6.96, 2H, 1 CH ), 3.48 (dd, J
3
2
2
.05, 4H, 2 CH MNR ), 4.65 (t, J 7.24, 2H, 1 CH MN+R ),
2
2
2
3
.70 (d, J 8.9, 2H, 2ArH), 6.82 (d, J 15.64, 1H, 1 ArMCHN),
.60 (d, J 8.92, 2H, 2 ArH), 7.88 (d, J 15.64, 1H, 1 ArMCHN),
.45 (d, J 4.6, 1H, ArH), 8.93 (s, 1H, ArH), 10.04 (d, J 6.08,
H, ArH); l /nm (CDCl ) 569 (e 48900 dm3 mol−1 cm−1),
˚
for OMPy is 2.0 mN mA−2) and the area per molecule for
˚
˚
OMPd (41 A2) is smaller than that for OMPy (55 A2). These
indicate that with one more nitrogen on the conjugation ring,
the hydrophilic property is suitably enhanced, the film forming
max
3
93.
9
32
J. Mater. Chem., 1998, 8(4), 931–935

UV–VIS spectra
Table 2 shows the data from UV–VIS spectra. The maximum
absorption wavelength (l ) of OMPd (558.5 nm) is 62.5 nm
max
longer than that of OMPy (496 nm) and the l
1
of MHPd is
max
of OBPd is longer than
9.5 nm longer than OMPd. Also l
max
of OEPd is longer than that of OMPd.
that of OEPd and l
max
Generally, there are four main factors which may lead to a
red shift of the absorption band: longer conjugation length;
stronger electron accepting and donating power of substituted
groups; the formation of anions or cations; a greater degree of
electron delocalization.19 Between OMPd and OMPy, the only
difference is in their conjugational chromophore. They do not
have the same extent of electron delocalization. Therefore the
lone pair of electrons on the additional nitrogen of OMPd
provides a unique contribution. This function of nitrogen is so
called auxochromation. The fact that 1,4-dimethylpyridinium
iodide is white and 1,4-dimethylpyridazinium iodide is pale
yellow provides further evidence for the existence of auxo-
chromation. In the pyridazine system, the absorption differ-
ences resulted from the fact that they have substituted groups
with different electron donating and accepting strength. Longer
alkyl chains exert stronger electron repulsion power and
weaker electron attracting ability than shorter chains.
Fig. 1 p–A Isotherm of (E)-N-alkyl-4-{2-[4-(dialkylamino)phenyl]-
ethenyl}pyridazinium iodide (a) MHPd; (b) OMPd; (c) OEPd; (d)
OBPd; (e) OMPy
Table 1 Slope of p–A curve in condensed region, collapse pressure,
and area per molecule of (E)-N-alkyl-4-{2-[4-(dialkylamino)phenyl]
ethenyl}pyridazinium iodide (25 °C)
Second harmonic generation
slope in cond.
˚
collapse pressure/ area per molecule/
˚
compound
reg./mN mA−2
mN m−1
A2
The SHG properties of all four compounds are shown in
Table 3. Among these four molecules, x (2) of MHPd is the
highest. Unfortunately the l
is the closest to the second harmonic wavelength (532 nm), so
its resonance enhancement is also the largest.
Although OMPd, OEPd and OBPd also have resonance
zzz
MHPd
OMPd
OEPd
4.12
2.83
1.23
0.91
2.0
54.5
48.5
39.6
38.2
50
60
41
70
115
55
of MHPd in LB films (530 nm)
max
OBPd
OMPy16
enhancement, their x (2) values are comparable. Their l
zzz
max
values in LB films are about the same. By comparing the x (2)
zzz
of these three molecules, we know that there is a x (2)
zzz
property is ameliorated and molecules could be aligned with
each other more closely.
increasing trend from OMPd through OEPd to OBPd. This
result is in accord with the theory that stronger electron
donating substituents on the amino group give larger SHG
signals.16
Significant differences in slope (in the condensed region),
collapse pressure and area per molecule have been observed
between MHPd and OMPd (OEPd, OBPd). The film forming
properties of MHPd are remarkably better than the others
because of its higher collapse pressure and larger isotherm
slope (in the condensed region). Apparently, the attachment of
long hydrophobic alkyl chains to the amino side of the
chromophore is beneficial to the film forming property and
vice versa. Moreover, the area per molecule of MHPd is larger
than that of OMPd, suggesting that MHPd has a greater
volume. The substitution of two long alkyl chains on the
amino group is responsible for this.
Along with the increase in length of alkyl chains substituted
on amino groups, a decline in both slope and collapse pressure
and an increase in area per molecule have been observed in
the series of OMPd, OEPd and OBPd. Obviously attachment
of long hydrophobic alkyl chains onto both sides of the
chromophore will cause the loss of amphiphilic character and
result in worse film forming properties.
The x (2) value, tilt angle in LB films and the area per
zzz
molecule (35 mN m−1) of OMPd and OMPy are about the
Table 3 SHG of (E)-N-alkyl-4-{2-[4-(dialkylamino)phenyl]ethenyl}-
pyridazinium iodide
˚
ca. l/A
compound
x
zzz(2)/pm V−1
w/°
MHPd
OMPd
OEPd
OBPd
OMPya
OMPy16
257(150–400)
75(52–99)
110(100–120)
132(86–200)
87(20–153)
90–150
44
32
39
34
32
41
30
33.3
33.0
36.0
34
aData obtained under the same experimental conditions in this work.
w° is the tilt angle of chromophore in LB films. l is the calculated
length of molecules.
Table 2 UV–VIS data of (E)-N-alkyl-4-{2-[4-(dialkylamino)phenyl]ethenyl}pyridazinium iodide
lmax/nm
compound
in chloroform
in LB films
in ethyl acetate
e/dm3 mol−1 cm−1
MHPd
OMPd
OEPd
OBPd
OMPya
OMPy16
578
530
545
545
540
460
460
534.2
64000
31700
48900
51200
558.5
569.5
573.5
495.6
496
465.6
aData obtained under the same experimental conditions in this work.
J. Mater. Chem., 1998, 8(4), 931–935
933

Fig. 3 The difference between Dr of MMPd and MMPy [D(Dr)=
Dr ]. Each numbered column refers to the D(Dr) of the
correspondingly numbered part of structures in Fig. 2.
−Dr
MMPd
MMPy
charge upon excitation. It is also known that when a molecule
with larger Dm is excited to the first excited state, it loses
eg
more negative charge in the electron donating part and gains
more negative charge in the electron accepting part. Fig. 3
describes the difference between Dr of MMPd and MMPy
(
Dr
−Dr
). It can be seen from Fig. 3 that the dimethyl-
MMPd MMPy
amino group (column 1 and 2) of MMPy loses negative charge
more easily than that of MMPd and the electron accepting
part (columns 11–13) of MMPy gets negative charge more
easily or loses less negative charge than that of MMPd. It is
obvious that the main contribution of MMPd getting less
negative charge in the electron accepting part comes from
column 11 where the additional nitrogen is located. Therefore
because of this contribution of the additional nitrogen, Dm
of MMPd is smaller than that of MMPy. A similar situation
eg
should arise when OMPy is compared with OMPd. So the
negative effect of the second nitrogen in this system may not
be negligible.
Fig. 2 The charge difference Dr between the first excited singlet state
and the ground state of MMPy (top) and MMPd (bottom) (Dr=
r −r ). Each numbered column refers to the Dr of the correspondingly
numbered part of its structure. Dr refers to the sum of charges of all
heavy atoms and hydrogen atoms in each numbered part of the
structure. A positive charge difference indicates loss of negative charge
upon excitation.
e
g
Conclusions
The novel pyridazinium compounds MHPd, OMPd, OEPd
and OBPd were synthesized and characterized for the first
time. Strong SHG was obtained. Among the factors which
affect the second harmonic generation in the pyridazinium
system, besides bond length alternation, aromaticity, resonance
enhancement etc., high electron density around the second
nitrogen in the six-membered diazine ring is the one which
could not be ignored. This last factor should be taken into
account when an aromatic ring with more than one heteroatom
is considered in finding new and high SHG materials.
same. But obviously the resonance enhancement part of OMPd
is greater than that of OMPy, because the l (in LB films)
of OMPd is much closer to 532 nm than that of OMPy. So if
max
this enhancement part is subtracted, the net x (2) of OMPd
should be smaller than for OMPy. This result suggests that
zzz
other than the lower aromaticity, which is beneficial to SHG
enhancement,9 there must be an unknown factor unfavorable
for SHG. Considering that the second nitrogen is the only
difference between OMPd and OMPy, this second nitrogen in
the pyridazine ring may be that unfavorable factor.
In order to get a better understanding on the analysis above,
some theoretical calculations have been carried out by using
MINDO/3 of MOPAC 6.00 on two model molecules, MM-
Pd [(E)-N-methyl-4-{2-[4-(dimethylamino)phenyl]ethenyl}-
pyridazinium iodide] and MMPy [(E)-N-methyl-4-{2-[4-
This work is financially supported by the National Climbing
Program A, The National Natural Science Foundation of
China (29471005, 29671001) and the PhD Program
Foundation (9500115).
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