Hierarchical construction of SHG-active polar crystals by using multi-component crystals

Article abstract of DOI:10.1039/c6cc07544d

Organic salts composed of chiral amines and sulfonic acid with high hyperpolarizability allowed the construction of polar crystals with incorporated guest molecules. The polarity of the crystals was precisely regulated by employing suitable guest molecules. As a result, the crystals generated a strong second harmonic generation property.

Full text of DOI:10.1039/c6cc07544d

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DOI: 10.1039/C6CC07544D
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Hierarchical Construction of SHG-active Polar Crystals by Using
Multi-component Crystals
Tetsuya Miyano, Tatsuya Sakai, Ichiro Hisaki, Hideki Ichida, Yasuo Kanematsu and Norimitsu
Tohnai*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Organic salts composed of chiral amines and sulfonic acid with high centrosymmetric space group, were prepared from chiral
hyperpolarizability allowed the construction of polar crystals with components.8 However, in the field of crystal engineering using
incorporated guest molecules. The polarity of the crystal was molecular design, it remains a significant challenge to control
precisely regulated by employing suitable guest molecules. As a the molecular arrangement and space group of crystals.
result, the crystals generated strong second harmonic generation
property.
Recently, our group regulated the molecular arrangement of
functional molecules in a crystalline state using strong charge-
assisted hydrogen bonds (CAHBs) between sulfonic acids and
amines in organic salts. For example, an arrangement of
anthracene sulfonic acids were regulated by changing the
counter amines to affect the fluorescent properties.9,10
Furthermore, the fluorescent properties of inclusion crystals of
organic salts were indirectly responsive to the guest
molecules.11, 12 Namely, the arrangement of supramolecular
unit composed of organic salt was regulated by guest molecules.
On the basis of previous studies, we planned to prepare polar
crystals with SHG capability by using organic salts composed of
chiral amines with guest molecules. As a building block core, we
designed 4-aminoazobenzene-4-sulfonic acid (AAS) (Scheme.
1a). Here in, we report the detailed crystal design and SHG
signals of the crystals.
Organic crystals with a large non-linear optical (NLO) effect
have attracted considerable attention because of their
application in high frequency electro-optic modulation,
frequency conversion, and so on.1 Compared to that of
inorganic crystals, the NLO properties of organic crystals,
especially their second harmonic generation (SHG), can be
easily increased by molecular design. Polar π-conjugated
molecules with donor and acceptor groups such as p-
nitroaniline derivatives have been studied as potential to NLO
materials.2,3 Along with molecular design, regulation of
molecular arrangement is essential for strong SHG. In other
words, molecules must crystallize into a non-centrosymmetric
space group and should be arranged preferably with high
polarity. A strategy based on design of supramolecular synthons
plays an important role in crystal engineering.4 Until now,
various polar arrangements have been constructed using
hydrogen bond,5 halogen bond,6 and coordination bond.7 And
some of them were effectively constructed in multi-component
crystals like co-crystals and salts.6(a, c), 7, 8 In terms of crystal point
group, 10 point groups (C1, C2, Cs, C2v, C4, C4v, C3, C3v, C6, C6v)
have polarity among all of 32 point groups. Focused on 11
enantiomorphic point groups (C1, C2, D2, C4, D4, C3, D2, C6, D6, T, O),
5 point groups have polarity. Judging from this stochastic
classification, chiral molecules tend to crystallize into a non-
centrosymmetric space group with high probability. In previous
studies of other research groups, crystals, belonging to a non-
AAS was prepared according to Scheme S1. Sodium (E)-4-
((4-aminophenyl)diazenyl)benzenesulfonate was dissolved into
Scheme 1. Molecular structures of (a) AAS, (b) chiral amines, (c) organic salts,
and (d) guest molecules.
a.Department of Material and Life Science, Graduate School of Engineering, Osaka
University, 2-1 Yamadaoka, Suita, Osaka 565-0871.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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DOI: 10.1039/C6CC07544D
Fig. 3 (a, c) Crystal structures of the 1-GF crystal. Colours: orange (AAS);
magenta (amine). (b, d) Hydrogen-bonding networks between AAS and
chiral amines.
chlorophenyl) ethylamine and (R)-1-phenylethylamine to
prepare organic salts 1 and 2 (Scheme. 1b, c). Salt 1 was
recrystallized from methanol to yield a 1-GF crystal, with the
crystallographic parameters shown in Table S1. 1-GF belongs to
the P21 space group, with the polar direction parallel to 21
helical axis. In this crystal structure, AAS does not form
intermolecular hydrogen bonds with other AAS (Figure. 3a,c),
but AAS does form CAHBs with chiral amines to form 21 helices
(Fig. 3b, d). Therefore, the intended polar sheet structure was
not constructed, but the AAS was arranged in a polar manner
that partially cancelled the polarity. The failure to construct a
polar sheet structure was probably caused by discrepancy of
molecular length between the AAS and the amines, which are
12 Å and 5 Å respectively (Scheme. 1a-b). This discrepancy
prevented AAS from constructing polar sheet structure.
Therefore, we attempted to efficiently regulate the
arrangement by incorporating guest molecules. We employed
guest molecules (Scheme. 1d) that can form hydrogen bonds
with chiral amines to ensure their incorporation into the crystals.
Furthermore, the total length of the amines and guest
molecules were close to AAS. Therefore, AAS sheets and
amine/guest sheets were expected to be alternately laminated
through CAHBs to yield highly polar crystals (Fig. 1d-e).
Fig. 1 Schematic representation of (a) AAS, amine, and gust molecules, (b)
the polar sheet structure of AAS, (d) laminated AAS and amine/guest sheets.
Hydrogen-bonded networks in (c) the polar sheet structures and (e) the
laminated polar AAS and amine/guest sheets.
water and a 0.5 M aqueous solution of HCl was added to yield a
precipitate, which was filtered and dried in vacuo to give AAS.
AAS has a sulfonyl group as an electron acceptor and an
amine as an electron donor, giving it a large first order
hyperpolarizability. Furthermore, AAS should form head-to-tail
linkage through strong intermolecular hydrogen bonding
between the sulfonic acid and amine groups, yielding a polar
sheet structure (Figure. 1a-c).
Recrystallization from water provided a crystal with the
crystallographic parameters shown in Table S1. Single X-ray
diffraction measurements revealed that AAS formed a polar
sheet structure, as intended, by CAHBs between sulfonic acids
and amines (Fig. 2a, b). However, the AAS crystal belongs to the
P-1 space group and the polar sheets are laminated in an
antiparallel manner, completely cancelling the polarity (Fig.2c,
d). Because it is difficult to arrange polar sheets in a non-
centrosymmetric manner from only achiral molecules, as
described above, AAS was mixed with chiral amines to introduce
a chiral component into organic crystals. We employed (R)-1-(4-
Guest inclusion crystals were prepared by recrystallization
from mixture of methanol and various guest molecules. The
crystallographic parameters of the inclusion crystals: 1-DMSO
, 1-
,
1-DMF 1-DMA 1-DEF 2-Dioxane 2-DMA, and 2-DEF are shown in
,
,
,
,
Dioxane
Table S1. Interestingly, the crystal system of all of the guest
inclusion crystals were in polar P21 or P1 space group. In these
crystals, AAS certainly form a polar sheet structure by hydrogen
bonds (i) in Fig. 4a. Moreover, guest molecules were
incorporated through hydrogen bond (ii) with chiral amines.
Subsequently, polar AAS and amine/guest sheets were
alternately laminated through CAHBs (iii) between the sulfonic
acid of AAS and chiral amines without exception (Figure. 4c-j).
In inclusion crystals in which AAS was arranged in a polar
sheet structure, AAS also formed a 21 helix in polar sheet
structures except for 1-DMF, which belongs to the P1 space group.
The degree of polarity of these crystals depends on the angle
(θ) between the polar direction (21 helical axis) and the long
molecular axis direction of AAS (Fig. 4b and Table 1). To increase
Fig. 2 (a) Polar sheet structures in AAS crystal. (b) Hydrogen-bonded networks in
the polar sheet structures. (c) Crystal structure of AAS crystal. (d) Hydrogen-
bonded networks in the polar sheet.
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Table. 2 SHG signal of inclusion crystals
DOI: 10.1039/C6CC07544D
SHG signal[a]
Laser radiation
(μm)
1-Dioxane
11.8U
18.0U
12.1U
1-DMF
2-Dioxane
1.96U
2.60U
3.14U
2-DEF
1.4
1.5
1.6
19.4U
38.5U
26.9U
21.9U
58.8U
30.7U
[a] 1U=SHG of urea.
23° and 0° from 37°, respectively, whereas the distance of the
guests increases to 2.8 Å and 3.6 Å from 2.7 Å. Furthermore,
the larger molecules, DMA and DEF, increased the angle to 21°.
Based on molecular arrangements of AAS, 1-DMF is expected to
result in the strongest SHG signal in inclusion crystals of salt 1.
In inclusion crystals of salt 2, the angle also depends on the size
of the guest molecule. The θ value in inclusion crystals of salt 2
varies from 32° (2-Dioxane) to 18° (2-DMA). These results strongly
suggest that crystal design by using multi-component crystals is
effective methods of precisely regulating molecular
arrangements.
Powder SHG measurements were carried out for 1-Dioxane, 1-
DMF, 2-Dioxane, 2-DEF, and urea using the Kurtz−Perry method13 at
wavelength of 1.4, 1.5, 1.6 μm. At 1.5μm, the resulting SHG
signals were approximately 18, 39, 3, and 59 times that of urea.
This difference of signal strength corresponded to expectations
based on crystallographic analysis of crystals in each salts.
Furthermore, the results at 1.4 and 1.6μm also corresponded to
the expectations. These results indicate that effective
regulation of AAS molecular arrangement by guest molecules
drastically increased the SHG signal. This approach is useful to
yield most suitable molecular assemblies not only for SHG but
also various functional materials.
In summary, we designed AAS with large first-order
hyperpolarizability and was presumable to construct polar
sheet structure. Then, AAS was combined with chiral amines to
prepare organic salts to form non-centrosymmetric
arrangement. Finally, we succeeded in controlling the molecular
arrangement of the salts by using guest molecules as a
component to regulate the molecular arrangement. Moreover,
the adoption of guest molecules of adequate molecular length
enabled us to control the crystal polarity and thereby increase
SHG signal. These results indicate that our strategy of regulating
molecular arrangement by means of organic salts and guest
molecules have promising potential for functional materials.
This work was supported by "JSPS KAKENHI Grant Number
JP16K13979" and "JSPS KAKENHI Grant Number JP16H01027 in
Precisely Designed Catalysts with Customized Scaffolding". We
thank Prof. Dr. M.Miyata (Osaka University) for his support. T.
M. thanks the Interactive Materials Science Cadet Program in
Osaka University.
Fig. 4 (a) Hydrogen-bonding network between (i) AAS, (ii) AAS and amine, and (iii)
amine and guest in 1-Dioxane. (b) Definition of angle (θ) between the polar
direction and the long molecular axis of AAS. Inclusion crystal structures: (c) 1-
DMSO, (d) 1-Dioxane, (e) 1-DMF, (f) 1-DMA, (g) 1-DEF, (h) 2-Dioxane, (i) 2-DMA, and (j) 2-DEF
.
Colours: orange (AAS); magenta (amine); light green (guest).
the polarity, a small angle is desirable, and an arrangement
parallel to polar direction (θ = 0) is best suited for SHG materials.
We controlled the angle using guest molecules in inclusion
crystals. The angle depends on the molecular size and distance
between the oxygen atom and the farthest carbon atom in the
guest molecules (Fig. 1a and Table. 1). In comparison with 1-
DMSO, for example, the angle in 1-Dioxane and 1-DMF decreases to
Table. 1 Crystal structural features in inclusion crystals
Guest length
(Å)
Guest volume
(Å3)
Crystals
Space group
θ (°)
AAS
1-GF
P-1
P21
P21
P21
P1
P21
P21
P21
P21
P21
-
-
-
-
79
94
-
-
1-DMSO
1-Dioxane
1-DMF
1-DMA
1-DEF
2-Dioxane
2-DMA
2-DEF
2.7
2.8
3.6
3.6
4.5
2.8
3.6
4.5
37
23
0
16
21
32
23
18
86
104
124
94
104
124
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