Syntheses, structures and second-order nonlinear optical properties of octupolar compounds: 2,4,6-Tri-substituted s-triazine

Article abstract of DOI:10.1016/S0009-2614(03)01207-7

Three compounds with three p-substituted styryls attaching to the central s-triazine have been synthesized and two crystals determined to be in non-centrosymmetric Cmc2₁ and Pmn2₁ space groups, respectively. All of them in solid stat

Full text of DOI:10.1016/S0009-2614(03)01207-7

Chemical Physics Letters 377 (2003) 507–511
www.elsevier.com/locate/cplett
Syntheses, structures and second-order nonlinear
optical properties of octupolar compounds:
2
,4,6-tri-substituted s-triazine
a,b
Y.Z. Cui , Q. Fang , H. Lei , G. Xue , W.T. Yu
a,
c
a
a
*
a
State Key Laboratory of Crystal Materials, Shandong University, 250100 Jinan, PR China
Department of Chemical Engineering, Light Industry, College of Shandong, 250100 Jinan, PR China
b
c
School of Information Science and Engineering, Shandong University, 250100 Jinan, PR China
Received 21 April 2003; in final form 11 July 2003
Published online: 6 August 2003
Abstract
Three compounds with three p-substituted styryls attaching to the central s-triazine have been synthesized and two
crystals determined to be in non-centrosymmetric Cmc2 and Pmn2 space groups, respectively. All of them in solid state
are second harmonic generation (SHG) active and two of them show powder SHG intensities one order of magnitude
1
1
that of urea.
Ó 2003 Elsevier B.V. All rights reserved.
1
. Introduction
metric space groups. In early 1990s, Zyss [1,2] in-
troduced the concept of octupolar molecule with
the 1,3,5-triamino-2,4,6-trinitrobenzene (TATB)
as a typical example which is characterized by non-
dipolar and non-centrosymmetric planar C3h or
D3h molecular symmetry. Since then increasing
research attention has been attracted to the NLO
active octupolar molecules, including the extension
from planar two-dimensional octupoles to the
The second-order polarizability b (for micro-
ð2Þ
scopic individual molecule) or v (for macro-
scopic crystal) is a physical property of three-order
tensor, which abide the non-vanishing requirement
of non-centrosymmetry. For years, the molecular
engineering in second-order nonlinear optics
(NLO) has been mainly confined in the D–p–A
type dipole molecule characterized by C
n
or Cnv
D
2
; D
3
d
, or T characterized three-dimensional ones
molecular point groups. Unfortunately most of
these polar molecules crystallize in centrosym-
[3,4]. To interpret the NLO nature of octupolar
molecules, several theoretical models have been
put forward, such as the three-level model [1,5],
the valence-bond three charge-transfer (VB-3CT)
and the valence-bond four charge-transfer (VB-
4CT) models [6,7]. As indicated by the results in
*
Corresponding author. Fax: +86-531-8565403.
E-mail address: fangqi@icm.sdu.edu.cn (Q. Fang).
0
009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0009-2614(03)01207-7

508
Y.Z. Cui et al. / Chemical Physics Letters 377 (2003) 507–511
p-substituted benzaldehyde and trimethyl-s-tri-
azine, and I was prepared referring the literature
22]. Crystals served for structure determinations
[
and powder SHG tests were obtained from ben-
zene/ethanol solutions. Compound II: pale-yellow
1
needle-like crystals. H-NMR (CDCl
3
) d(ppm):
8
.23(d, 3H, J ¼ 15:60 Hz), 7.56 (d, 6H, J ¼ 8:29
Hz), 7.19 (d, 6H, J ¼ 7:80 Hz), 7.09 (d, 3H,
J ¼ 16:09 Hz), 2.38 (s, 9H). Calculated for
Fig. 1. Molecular scheme of compounds I, II and III. I: R ¼ H,
II: R ¼ Me, III: R ¼ OMe.
30 27 3
C H N : C, 83.92; H, 6.29; N, 9.79. Found: C,
83.56; H, 6.55; N, 9.55. Compound III: pale-yel-
1
3
low needle-like crystals. H-NMR (CDCl )
this Letter, an advantage of octupolar molecules is
that they seem to take more probability crystal-
lizing in non-centrosymmetric space groups than
that of dipolar compounds, due to the absence of
ground state dipole moment.
The experimentally investigated octupolar
molecules to date can be categorized into three
groups: (1) the molecules of hexa-substituted
d(ppm): 8.25 (d, 3H, J ¼ 15:60 Hz), 7.64 (d, 6H,
J ¼ 8:78 Hz), 7.07 (d, 3H, J ¼ 15:60 Hz), 6.93 (d,
6H, J ¼ 8:78 Hz), 3.85 (s, 9H). MS (70 eV) m/z
þ
(%): 477 (M ,79.72), 317 (100), 318 (44.45), 303
(93.48), 159 (75.21), 158 (61.89), 160 (45.36), 116
(42.83). Calculated for C H N O : C, 75.47; H,
30
27
3
3
5.66; N, 8.80. Found: C, 75.31; H, 5.80; N, 8.83.
The X-ray diffraction data of II and II were
collected by Bruker P4 four-circle diffractometer
with the MoKa radiation and the two structures
were resolved by SHELXL-97 programs. Crystal
[
8–11] or tri-substituted [12–14] six-membered
aromatic compound with planar trigonal D3h
symmetry, such as TATB; (2) the so-called Ôcen-
tral-atom analoguesÕ with threefold molecular
symmetry such as crystal violet (CV) [15,16],
Guanidinium [17], etc. [11,18]; (3) three-dimen-
sional metallo-organic complexes with Ru(II)
data for I: orthorhombic, space group Cmc2 (No.
1
36), a ¼ 22:689 ð3Þ; b ¼ 12:4638 ð14Þ; c ¼ 7:4086
ꢀ
ꢀ
3
ð11Þ A, V ¼ 2095:1 ð5Þ A , Z ¼ 4; T ¼ 293 ð2Þ K,
À3
D ¼ 1:228 g cm , R ¼ 0:0527; xR ¼ 0:1047 for
c
1
2
[
4,19], Cu(I) [20], and Zn(II) [21] as the metal
I > 2rðIÞ. Crystal data for II: orthorhombic, space
0
centres and 2,2 -bipyridine derivatives as the li-
gands. The majority of reported works about oc-
tupolar compounds are related to their molecular
b properties, while the reports concerning the more
group Pmn2 ; a ¼ 24:218 ð4Þ; b ¼ 6:1945 ð5Þ; c ¼
1
ꢀ
ꢀ
3
8:1438 ð5Þ A, V ¼ 1221:7 ð2Þ A , Z ¼ 2; T ¼ 293
À3
ð2Þ K, D ¼ 1:168 g cm , R ¼ 0:0715; xR ¼
c
1
2
0:1799 for I > 2rðIÞ.
ð2Þ
useful macroscopic v properties are relatively
SHG intensities of the powder-crystals were
measured according to the Kurtz and Perry
method [23] and urea powder was used as refer-
ence. The ps pulses (1064 nm) from Nd:YAG laser
was selected as incident beam.
few. Furthermore, to our knowledge, only a few
crystal structures of NLO active octupolar
compounds have been reported [9,13–15,17]. In
this communication, we report the syntheses,
structures and SHG properties of three planar
octupolar compounds (shown in Fig. 1). The tri-
branched type has been chosen since it may have
less steric hindrance than the hexa-substituted one
3. Results and discussion
[
1,8,12] and tends to be in better planarity.
The crystal I is in non-centrosymmetric polar
1
Cmc2 space group. The bond-lengths and bond-
angles are aromatic characterized and, as shown in
Fig. 2, the molecule has a bent-planar configura-
tion with its three styryl planes slightly curving to
the central triazine. These structural features in-
dicate that molecule I is a p-conjugated system.
2
. Experimental
Crystalline compounds II and III were synthe-
sized by aldol condensation reaction of different

Y.Z. Cui et al. / Chemical Physics Letters 377 (2003) 507–511
509
matched, forming a macro self-polarization in
c-axis direction.
1
Compound II crystallizes in polar Pmn2 space
group which is also non-centrosymmetric (shown
in Fig. 3). Like compound I, the molecular plane
of II is also slightly curved to the triazine plane,
forming a little dipole moment along the normal
of the triazine plane. The dihedral angle be-
tween the triazine plane and any of the three styryl
planes is 5.5° and that between every two of the
styryl planes is 9.5°, indicating a better molecular
planarity and up-graded molecular symmetry
compared with I. The vinyls are also disordered,
which induce a crystallographic symmetric mirror
Fig. 2. Molecular structure and packing diagram of crystal I.
The vinyls in the three styryls are all disordered,
endowing the molecule with a symmetric mirror
passing through N2, C10, C13 and C16, which is
coincident with the crystallographic (1 0 0) sym-
metric plane. The dihedral angle between P
central triazine plane) and P (the styryl plane
containing C16) is 16.7°, and that between P and
(the other styryl plane containing C3) is 6.7°.
Thus the molecule I is strictly C , nearly C3v, and
1
(the
2
1
P
3
s
roughly D3h symmetric. Along c-axis, two neigh-
bouring molecules overlap partly in opposite di-
rection, sharing a same symmetric mirror. All the
near-planar molecules are approximately parallel
to (0 0 1) plane and they are all downwards curved.
Obviously, the slight curvature of the molecule
plane results in a little dipole moment and all of
the molecular dipole moments are basically phase
Fig. 3. Molecular structure and packing diagram of crystal II.

510
Y.Z. Cui et al. / Chemical Physics Letters 377 (2003) 507–511
passing through C10, N2, C14, C17 and C18. Be-
sides, molecule II possesses two more local sym-
metric mirrors, leading to a C3v molecular
symmetry. If the tiny curvature can be neglected,
narity of the molecule becomes better and better.
Though the curvature of the molecular plane is a
positive factor for the dipole moment, it is obvi-
ously a negative factor for the molecular p-con-
jugation. Thus there is a trade-off between
dipolarity and p-conjugation for this kind of near-
planar C3v symmetry. However, there is no such
trade-off between octupolarity and p-conjugation,
for the planarity or p-conjugation are always the
positive factors in the evolution from C3v symme-
try to D3h symmetry. Hence, molecule II must
have larger b than molecule I because of its more
significant charge-transfer character and more
octupolar character embodied by better planarity.
For the D3h symmetry, the molecular b can be
expressed as [3]
the molecular symmetry of II will be D3h
.
Pale-yellow thin platelet crystal of compounds
III was also obtained, but unfortunately its X-ray
diffraction intensities were too weak for its struc-
ture to be determined at present.
All the three compounds are SHG active in
solid state. From I to III, the powder SHG in-
tensity increases significantly with the increase of
donor strength, showing the same tendency as that
in dipolar D–p–A system. Compounds II and III
show the SHG intensities one order of magnitude
that of urea (see Table 1). To make a primary
structure-property analysis, we start from the ori-
ented-gas conjecture that the macroscopic SHG
intensities are dependent on two factors: the mi-
croscopic molecular b and the molecular packing
effect in crystal.
2
2
b ¼ b yðy À 3x Þ:
ð1Þ
At the crystallographic frame, the average mo-
yyy
lecular b can be expressed as [24]
!
N
The second-order polarizability can be decom-
posed into the dipole part b
1 X X
ðsÞ
ðsÞ
ðsÞ
b
IJK
¼
cos hIi cos h cos h
bijk; ð2Þ
and octupolar part
Jj
Kk
J¼1
N
ijk
s¼1
b
[1]
J¼3
in which bIJK is referred to the unit cell nonlinearity
per molecule, N is the number of symmetrically
equivalent molecules in an unit cell. The capital
indices (I; J; K) label the crystallographic axes and
the (i; j; k) label the local molecular axes. Obvi-
ously, expression (2) bridges the microscopic and
macroscopic hyperpolarizability, for bIJK encom-
b ¼ bJ¼1 þ bJ¼3
and
;
kbk ¼ kbJ¼1k þ kbJ¼3k:
The C3v symmetry is compatible with both b
J¼1
vanishes for an object with
and b , while b
J¼3
3h symmetry. With respect to our system, the D3h
J¼1
D
symmetry is the limit of the C3v symmetry as pla-
passes the packing effect in crystal and it is pro-
.
ð2Þ
portional to the macroscopic v
IJK
Both crystal I and II are in mm2 point group
which, under the Kleinman postulate, has three
independent b coefficients, b15; b24; b33. In the
course of tensorial transformation by expression
Table 1
The optical properties and melting points of compounds I–III
Compounds
I
II
III
(
2), the crystallographic a; b; c axes are labeled
Space group
SHG intensity
Cmc2
1.8
0
1
Pmn2
9.1
0
1
a
with X; Y ; Z, respectively. As shown in Figs. 2
and 3, the molecular coordinate system for one
molecule is labeled with (x; y; z) and that for
another twofold screw equivalent molecule with
11.4
b
b
b
k
15
24
33
0.2624byyy
0.0233byyy
)0.3746byyy
)0.1239byyy
b
max (nm)
326
7.2
337
6.7
358
7.4
0
0
0
(
x ; y ; z ). The y-axis is set along the intersecting
4
À1
l
e (ꢀ10 mol
À1
b
line of the molecular least-square plane (LSP)
and the vertical molecular mirror plane, the
x-axis on the least-square plane, and the z-axis
along the normal direction of the central triazine
plane.
cm
)
m.p. (°C)
a
225–227
235–237
226–228
The ratio of SHG intensities between the concerned com-
pound and the urea reference.
b
The solvent was chloroform (C ¼ 1:0 ꢀ 10À5 mol l ).
À1

Y.Z. Cui et al. / Chemical Physics Letters 377 (2003) 507–511
511
Acknowledgements
This work was supported by the National Nat-
ural Science Foundation of China (No. 20172034)
and the grant for State Key Program of China.
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