Vol. 45 No. 3
SCIENCE IN CHINA (Series B)
June 2002
SHG from centrosymmetric supermolecular crystal
GUO Wensheng ()1, GUO Fang ( )1, WEI Chunsheng ()1,
LIU Qitao ()1, ZHOU Guangyong (
)2, WANG Dong ( )2
& SHAO Zhongshu ()2
1. Institute of Chemical Science & Engineering, Liaoning University, Shenyang 110036, China;
2. Institute of Crystal Materials, Shandong University, Jinan 250100, China
Correspondence should be addressed to Guo Wensheng (email: [email protected])
Received February 10, 2002
Abstract Supermolecular crystal composed of p-nitrophenol, hexamethylenetetramine, phosphoric acid and water was obtained by supermolecular assembly. The crystal has a centrosymmetric
structure with space group P21/c. The relative intensity and integral value of green frequency doubling of light from the crystal powder compared with that from KDP powder measured by
pico-second Nd:YAG laser are 757.3 (KDP: 183.4) and 1771.7 (KDP: 423.98), respectively. In supermolecular crystal, due to the arrangement of centrosymmetric p-nitrophenol molecules in the
inorganic-organic host framework formed by hexamethylenetetramine, phosphoric acid and water,
the distances between the p-nitrophenol molecules with centrosymmetry are changed (N1A-O1A:
0.8158 nm, N1Bh
-O1B: 1.4450 nm, N1A-N1Ah
: 0.5837 nm, N1Bh
-N1B: 0.6898 nm), and the interaction between the dipoles becomes weak, which is easy for the ground state to turn into the
excited state charge separated form, and generate the stable asymmetrical distribution of
π-electron cloud density of quinonal structure, thus exhibiting SHG effect.
Keywords: supermolecular crystal, SHG effect, centrosymmetry, p-nitrophenol.
Second harmonic generation (SHG) has close correlation with the nonlinear electrical susceptibility χijk. For centrosymmetric crystals, based on the Kleinman approximation symmetry and
centrosymmetry of crystal, χijk exists no independent irreducible component of tensors, thus exhibiting no SHG effect. As a result, noncentrosymmetric molecules are usually chosen when preparing organic systems for the study of SHG. However, for many noncentrosymmetric dipole
molecules, such as p-nitrophenol, and p-aminoaniline, anti-parallel arrangement is often formed
due to the dipoles interaction in crystal, which can result in centrosymmetric space group and
make the bulk material exhibit no SHG [1,2]. In order to obtain noncentrosymmetric crystal, the
approaches, such as attaching chiral carbon atoms to the noncentrosymmetric molecules, introducing the hydrogen bonding, and decreasing ground state dipole moments of the molecules, are
adopted to increase the probability of the formation of the noncentrosymmetric crystals[3]. Recently, two-dimension charge-transfer molecules, for example, type, X type, Y type[4,5] , are
furthermore developed, which show more increase in probability of formation of noncentrosymmentric crystal compared with the system of one-dimension dipole molecule. In addition, the
methods, using guest-host inclusion complexation to assemble bulk materials by polar directional
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alignment of the chromophore dipoles in host channel for nonlinear optics[6], and changing the
orientation of βvalue molecules in crystal and/or the symmetry of supermolecular crystal to obtain noncentrosymmetric crystals with SHG[7,8], are also adopted.
It is known that a series of centrosymmetric molecules with C3v, D3h and Td symmetry, such
as trinitrotriaminobenzene (TATB), crystal violet (CV), and ruthenium trisbipyridine (RuTB),
possess nonzero β value. Zyss[9] put forward octopolar molecules theory. The nonlinear optical
properties of octopolar molecules provide a new approach for improving the ratio of performance
of nonlinearity to transparency for crystal. However, owing to the absence of permanent dipole
moment in octopolar molecule, how to incorporate octopolar molecules into materials with
macro-noncentrosymmetry has become a challenging question[10].
Bis(4-alkylaminophenyl) squaraine dyes crystallize in centrosymmetric structure. Ashwell[11]
used it as Langmuir-Blodgett (LB) monolayer films, which exhibited strong SHG effect. In the LB
films, due to that the centrosymmetric framework of squaraine molecules is influenced by the
charge distortion, the LB films assembled become noncentrosymmetric. The mechanism for SHG
still involves intramolecular charge transfer[12]. Consequently, from the viewpoint of crystal engineering, it has become common practice to analyze that the noncentrosymmentry of the crystal is
one of the necessary conditions of the exhibition of SHG effect.
In recent years, the SHG effect from C1v cubic centrosymmetric crystal with vicinal faces has
been studied[13]. Considering the SHG arising from bulk electric quadrupole and magnetic dipole
effects and a surface dipole source, Lüpke presented a phenomenological theory[14]. However, the
SHG effect from centrosymmetric organic crystal by intramolecular charge transfer mechanism
has not been reported.
We report here SHG effect from centrosymmetric supermolecular crystal composed of
p-nitrophenol, hexamethylenetetramine, phosphoric acid and water. The experimental results show
that it can exhibit strong SHG effect. This phenomenon, which is incompatible with the traditional
viewpoint, although, cannot be taken as being general, at least, it reflects supramolecular particular architecture rules in relation to the SHG effect in the supermolecular crystals.
1
Experimental
1.1
Preparation of supermolecular crystal
Put 1.40 g (0.01 mol) hexamethylenetetramine and 1.5 mL water into a 25 mL Erlenmeyer
flask and shaken for dissolving, and 0.8 g phosphoric acid was added and allowed to stand for a
while, then 1.40 g (0.01 mol) p-nitrophenol and 68 mL DMF were added. The mixture was
heated to dissolve. After standing for 1 or 2 d, a great deal of pale yellow, transparent crystal was
obtained, then filtered for the measurement of SHG and X-ray diffraction analysis.
1.2
SHG effect measurement
SHG effects of the powder samples were measured by lock-model pico-second Nd:YAG la-
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SHG FROM CENTROSYMMETRIC SUPERMOLECULAR CRYSTAL
269
ser (CONTINUUM in USA ), and irradiated by extra-short pulse with the output pulse width 40
ps, repetition frequency 10 Hz, center wavelength 1064 nm and scattering angle 0.4 mrad. After
grinding the tested sample and the consulted sample (KDP) into the same granularity power, they
were put into the glass color comparison vessel and pressed solid. Pump beam focuses on the vessel via the lens which the focus is 15 cm (allow focus of beam to depart a little from the surface of
the vessel to prevent the damage of the vessel) and the incidence angle between the surface of
vessel and pump beam is 45. The frequency doubling of light was collected by two lens perpendicular to the beam and the intensity and wavelength were measured by homochrometer
(Hamamatsu, C5094) and stripe camera (Hamamatsu, C5680-01). During the procedure of the test,
remain the steadiness of the light way and instrument, and guarantee that the samples were in the
same position. The experiment error is less than 5%.
1.3
X-ray diffraction analysis
X-ray single-crystal analysis was performed on a pale yellow and transparent crystal of the
inclusion compound with approximate dimensions of 0.30 mm0.40 mm0.40 mm at room
temperature: monoclinic, space group P21/c, a = 0.6036(1) nm, b = 0.7986(8) nm, c = 3.8868(13)
nm, β = 89.90(2), V = 1.8788(3) nm3, Z = 4, Dc = 1.525 gcm−3. All measurements were made
on Bruker Axs P4 four-circle diffractometer with graphite monochromated Mo Kα radiation (λ =
0.071073 nm). The diffraction data were collected by ω scan mode in the range 3.52θ 50.The number of unique data is 2945, and the observed [I 2σ (I )] is 2430. The structure was
solved by direct method and refined using least squares on a personal computer with Shelxs-97
program. All non-hydrogen atoms were refined anisotropically, and the crystal was refined to
convergence R = 0.0640, s = 1.038.
2 Results and discussion
2.1
Measurement results of SHG effect
The wavelength which detects the frequency doubling of light of supermolecular crystal is
ranged from 508 to 563 nm, 650 data of the relative intensity were collected, and a group of main
data were selected (see table 1).
It can be seen that the green light center detected is in 532 nm, which shows the exhibition of
the SHG effect. Moreover, bulk crystal samples also exhibit SHG effect, illustrating that the SHG
is not induced by the surface of the crystal. From table 1, we see also that the maximums of
relative intensity of KDP and supermolecule crystal are 183.4 and 757.3, respectively, the integral
values are 423.98 and 1771.7, respectively, and SHG effect is approximately 3.1 times larger than
that of KDP.
2.2
X-ray single crystal structure analysis
The structural stereodrawing of supermolecular crystal composed of p-nitrophenol, hexamethylenetetramine, phosphoric acid and water is shown in fig. 1. The unit cell and the packing 270
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Table 1 The wavelength and relative intensity of frequency doubling of light from supermolecule crystal
Intensity
Wavelength (λ)/nm
KDP
11.13
76.73
133.13
172.467
176.20
176.40
169.60
147.93
166.07
145.53
73.87
27.33
530.0164
531.0556
531.4886
531.9216
532.0082
532.0948
532.1814
532.3546
532.5278
533.0474
533.567
534
test sample
12.87
336 .00
657.47
722.73
706.27
685.73
694.40
757.33
678.47
551.87
278.00
99.67
diagram are shown in figs. 2 and 3, respectively.
In supermolecule crystal, the molar ratio of
p-nitrophenol, hexamethylenetetramine, phosphoric acid and water is 1
1
1
3 (fig. 1). It
is a centrosymmetric structure with space group
P21/c. In the unit cell, p-nitrophenol molecules 1
and 1
, as well as 2 and 2
, are centrosymmetric
and form an anti-parallel architecture (fig. 2).
Moreover, 1and 1A in its neighbor unit cell (x,
y, 1+z) are also anti-parallel and centrosymmetric each other (fig. 3). The phosphoric acid
Fig. 1.$tructural stereodrawing of supermolecular crystal.
molecules are hydrogen bonded (O1O2 (2−x,
Fig. 2. Diagram of the unit cell of supermolecular crystal.
y + 0.5, 0.5− z)) and form phosphoric acid dimer, in which the length of the hydrogen bonding
between O1O2 is 0.28455 nm (H-O2: 0.10317 nm, O1H: 0.18285 nm) and the bond angle of
O1H-O2 is 167.90.In addition, O4 of phosphoric acid is hydrogen bonded to O1w (1+x, y, z)
(O4O1w: 0.28473 nm, H1-O1w: 0.08581 nm, O4H1: 0.2083 nm, the bond angle is 148.01).
N1 of p-nitrophenol and O1w (x, y−1, z) also have hydrogen bonds (N1O1w: 0.29172 nm,
H2-O1w: 0.08328 nm, N1H2: 0.22102 nm, the bond angle is 142.78). N2 of p-nitrophenol and
O3w (x, y, z) are also connected with hydrogen bonds (N2O3w: 0.27958 nm). Since one
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SHG FROM CENTROSYMMETRIC SUPERMOLECULAR CRYSTAL
271
Fig. 3. The packing diagram of supermolecular crystal along a axis.
hydrogen atom of O3w has not absolute peak position in the electron density map, the corresponding hydrogen bonds cannot be demonstrated. But the disorder of hydrogen atoms apparently
indicates that there is strong hydrogen bonding between N2 and O3w. Thus, the inorganic-organic
host framework, a wide layer, is formed by phosphoric acid dimer, hexamethylenetetramine, and
water through hydrogen bonding network. This framework is then associated with O1w and O1a
(1+x, y, z) of p-nitrophenol (the hydrogen bond lengths O1aO1w: 0.30502 nm, O1a-H: 0.10279
nm, HO1w: 0.20265 nm, the angle is173.61), thus forming the architecture of supermolecule
system, in which the orientation and position of p-nitrophenol are fixed in the unit cell.
2.3 Discussion
In the crystalline, p-nitrophenol is a unique chromophore molecule. Since p-nitrophenol
molecules are isolated and fixed by the inorganic-organic framework composed of hexamethylenetetramine, phosphoric acid and water, the arrangement distances between p-nitrophenol molecules are lengthened. The distances between N1A of 2 and O1A
, N1Aof 2are 0.8158 and
0.5837 nm, respectively (the middle in fig. 4). In addition, the distances between N1B
of 1
and
O1B, N1B of 1A in the neighbor unit cell are 1.4450 and 0.6898 nm, respectively (the right in fig.
4).
The dimer (PNA)2 of p-nitroaniline is performed in H-typed centrosymmetric cofacial arrangement (the left in fig.4). When the distance between NO2 and NH2 is 0.38 nm, the interaction
of two dipoles possesses the strongest attraction, and the potential energy of the system becomes
minimum. For the distance less than 0.38 nm, the interaction of dipoles presents the strongest repulsion and the potential energy increases rapidly. While for larger (0.38 nm) distance, the attraction becomes weak, and the potential energy increases slowly again[15].
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Fig. 4. (PNA)2 dimer spacing arrangement (left) and the spacing arrangement of p-nitrophenol (right).
On the comparison of (PNA)2, one molecular of p-nitrophenol dimer is situated obliquely
above the other one, thus showing J-typed anti-parallel, centrosymmetric architecture (the middle
and right in fig. 4). The distances between p-nitrophenol molecules ((N1AO1A
: 0.8158 nm,
N1AN1A
: 0.5837 nm) or (N1B
O1B: 1.4450 nm, N1B
N1B: 0.6898 nm)) are much
longer than the interaction distance 0.38 nm of dimer (PNA)2, which make the dipoles interaction
be remarkably weakened.
Using a two-state model derived from perturbation theory, molecular susceptibility β is given
by the following
β ∝ (µe − µg )
2
µeg
2
Eeg
,
(1)
where µg and µe denote the dipole moments of ground state and CT excited state, respectively, µeg
is transition dipole moment between the two subscripted states, and E is transition energy.
Eq. (1) represents that the molecular susceptibility β is dominated by the change in dipole
moment between the ground and first CT excited state, i.e. the value of β lies on stability of
resonance form of CT excited state. The resonance form of ground state of p-nitrophenol is
aromatic, and CT excited state has quinonal resonance form. When the ground state wave-function
is perturbed by applied strong light field, because of the loss of aromaticity of excited state, the
aromaticity of ground state will inhibit molecular polarization of ground state. The spacial
arrangement of dimer in the crystal formed only by p-nitrophenol molecules is similar to that of
(PNA)2 model in fig. 4 (left). In this crystal, due to the strong dipoles interaction, the ground state
2
and 1/E2 are deenergy is decreased and transition energy is increased, so that the items µeg
creased. It is difficult for electron transition from ground state to first excited state. In addition,
dipole moment µe of excited state of molecule 2 exhibits strong counteraction with dipoles
moments µe′ of centrosymmetric molecules around, and the charge separated form becomes very
unstable, therefore,p-nitrophenol takes priority of the resonance form with aromatic, and ∆µ is
nearly zero, so the crystal of p-nitrophenol exhibits no SHG. When p-nitrophenol molecules are
incorporated in the supermolecular crystal, due to the distance between p-nitrophenol molecules 2
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SHG FROM CENTROSYMMETRIC SUPERMOLECULAR CRYSTAL
273
and 2his lengthened by the host framework, the dipoles interaction between p-nitrophenol mole)[15], and decreases
cules is weakened greatly, which increases the energy of ground state 2 (or 2h
difference of energy levels between the ground state and first excited state, thereby leading to the
2
and 1/E2, which is easy for electron transition from the ground state to first
increase of µeg
excited state and for the aromatic resonance form of ground state 2 to turn into the quinonal
resonance form. As we know, the quinonal structure is not as stable as the benzene structure,
however, since donor group OH of quinonal form is hydrogened strongly with water in the host
framework, and the quinonal form is stabilized by the whole hydrogen bonding network of
supermolecular crystal besides water,
, therefore the stability of quinonal
form is increased. In addition, by the hydrogen bonding, the stability of charge separated form and
asymmetrical distribution of π
electron cloud density of quinonal structure are enhanced, thus,
∆µ2, ∆µ 2 ≠ 0. Furthermore, 2 and 2h
, as well as 1hand 1A, are isolated by the host framework
′
and form the mono-molecules layer architecture, respectively, which is remarkably different from
,
the crystal architecture formed by p-nitrophenol alone. The dipoles interaction between 2 and 1h
as well as 2hand 1A, almost cannot exist. Molecule 2 is only related with two up-down
p-nitrophenol molecules around, which possess reverse dipole moment. Since the distance between two molecules is lengthened, counteract effection between µe2 of the excited state of molecule 2 and that of two molecules of excited state is weakened, and the charge separated form of
excited state is also stablized, so ∆µ2 and ∆µ2 are not equal to zero. Thus, p-nitrophenol mole′
cules that are centrosymmetric can keep independent component of tensors of hyperpolarizability
β. Similarly, ∆µ1 of 1h(or ∆µ1A of 1A) is also not equal to zero. Due to that the distance be′
2
2
tween 1 h and 1A is longer than that between 2 and 2 h, so µeg1
> µeg2
, 1/ E12 > 1/ E22 ,
′
′
∆µ1 > ∆µ2 , therefore, β1 > β2 .
′
′
As the above, due to the stable effect of supermolecular crystal on quinonal structure, the
stability of charge separated form of quinonal structure is increased and forms the stable
asymmetrical distribution of πelectron cloud density, ∆µ0, thus making p-nitrophenol molecules with centrosymmetry keep independent component of tensors of hyperpolarizability β and
exhibiting SHG.
The formation of crystal is a process of the decrease of the energy, so is the supermolecular
crystal. When dipoles interaction of p-nitrophenol molecules is weakened greatly, the architecture
energy of p-nitrophenol may be increased, which is unfavorable for the decrease of the crystal
energy. However, p-nitrophenol, as one component of the supermolecular assembly, besides having the individual properties such as the nonlinear optical property, must have the supremolecular
population properties such as stability of the assembly entity. Due to that inorganic-organic host
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framework, composed of hexamethylenetetramine, phosphoric acid and water, is formed through
the association of the hydrogen bonding network, the decreasing energy of the host framework
dominates the crystal energy. In addition, the p-nitrophenol assembles the supremolecular entity
with the host framework by hydrogen bonding, in which the decreasing energy induced by the
hydrogen bonding compensates appreciably for the increasing energy caused by p-nitrophenol
architecture.Therefore, the crystal energy of the supermolecular system is low and the entity is
stable.
Different from the crystals formed by p-nitrophenol itself, the orientation and distances of
p-nitrophenol molecules in cocrystalline are changed with the need of supermolecular system architecture, in which the change of the orientation can result in the change of the symmetry of the
crystal [7,8], and the variation of the distances leads to the change of nonlinear optical property of
chromophore with centrosymmetry. Thus, it can be seen that, to increase the distances of chromophores by the supermolecular assembly and inhibit the counteraction between dipoles of centrosymmetric molecules, even the supermolecular crystal with centrosymmetry can also exhibit SHG
effect.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No.
69778022) and the Natural Science Foundation of Liaoning Province (Grant No. 9810301001).
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