Octupolar (C3 and S4) Symmetric Cyclized Indole Derivatives: Syntheses, Structures, and NLO Properties

Article abstract of DOI:10.1021/acs.orglett.5b01912

Several cyclized indole derivatives have been synthesized, and their structures been determined. The C₃-symmetric single-chiral N-phenyltriindole (Tr-Ph3) crystallized in the P1 space group, and the S₄-symmetric saddle-like tetraindo

Full text of DOI:10.1021/acs.orglett.5b01912

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Letter
pubs.acs.org/OrgLett
Octupolar (C and S ) Symmetric Cyclized Indole Derivatives:
3
4
Syntheses, Structures, and NLO Properties
Lei Wang, Qi Fang,* Qing Lu, Shao-jun Zhang, Ying-ying Jin, and Zhi-qiang Liu
†
,‡,†
‡
‡
‡
‡
†School of Chemistry and Chemical Engineering, Shandong University, 250100 Jinan, P. R. China
‡State Key Laboratory of Crystal Materials, Shandong University, 250100 Jinan, P. R. China
*
S Supporting Information
ABSTRACT: Several cyclized indole derivatives have been synthe-
sized, and their structures been determined. The C -symmetric
3
single-chiral N-phenyltriindole (Tr-Ph3) crystallized in the P1 space
group, and the S -symmetric saddle-like tetraindole (TTr) crystal-
4
lized in the I4
̅
space group. The Tr-Ph3 and TTr crystals exhibit
remarkable powder SHG intensities 5 and 11 times that of KH PO
2
4
(
KDP), respectively. TTr is a useful octupolar core to build S4-
symmetric molecules and crystals for second-NLO materials.
n the traditional organic second-order nonlinear optics
The second-NLO active S -symmetric octupolar organic
4
I
(second-NLO), the donor−π−acceptor dipole molecular
compounds are very limited. Several biphenyl compounds were
supposed to be of D2d symmetry without X-ray structures.
1
12
model has acquired great success. However, many 1-D
intramolecular charge-transfer (ICT) dipolar compounds,
which have large molecular second-order polarizability (β),
failed to exhibit macroscopic second-NLO effects (such as the
second harmonic generation, SHG) due to their centrosym-
metric packing in crystals.
The β value of the 3-D ICT compound 4,4′,6,6′-tetrakis(p-
diethylaminostryl)[2,2′]bipyrimidine (see the SI) is large, but
13
its crystal is centrosymmetric (P2/n). A series of 3-D ICT
non-S -symmetric compounds, characterized by so-called
4
through-space delocalization, exhibited large β values but no
14
detectable bulk SHG. Recently, an interesting 3-D charge-
transfer complex bis(phthalocyaninato)lutetium was reported
In the early 1990s, Zyss et al. introduced the concept of
octupole to organic NLO. Since then, numerous second-NLO
active octupolar compounds have been reported, including pure
organic compounds, organic salt, metal−organic complexes,
etc. In a narrow sense, an octupolar molecule is such a
nondipolar and noncentrosymmetric ICT molecule that
contains C or S point group as its subgroup. The octupolar
2
15
to have exceptionally large β value. The only example that has
3
4
5
noncentrosymmetric S symmetry both for the molecule and
4
crystal is a kind of calixarene (see the SI), but it has no
16
macroscopic SHG.
Triindole (Tr, see Scheme 1) is an aromatic and C3h-
symmetric planar octupolar compound. Several novel Tr
3
4
compounds can be classified as a trigonal C3 family
characterized by 2-D ICT) and tetragonal S family (with 3-
(
4
Scheme 1. Synthetic Route
D ICT). For the pure organic octupolar compounds, many
reported works are related to molecular β only, while the
reports concerning the noncentrosymmetric crystal structures
and the useful macroscopic SHG properties are relatively few.
The C -symmetric 1,3,5-triamino-2,4,6-trinitrobenzene
3
(
TATB, see the Supporting Information (SI)) is a pioneer
6
octupolar compound. Its remarkable powder SHG intensity is
not compatible with the centrosymmetric P1 space group but
compatible with the noncentrosymmetric polymorph of the
crystal. 2,4,6-Triphenoxy-1,3,5-triazine (see the SI, point group
C , space group Cc) and 2,4,6-tris(benzyloxy)-1,3,5-triazine
̅
7
8
3
(
see the SI, point group C , space group Pca2 ) exhibited a
3
1
9
derivatives have been synthesized in recent years, including
SHG intensity comparable to that of KDP. 2,4,6-Tris(p-
methylstyryl)-1,3,5-triazine (see the SI, space group Pmn2 )
exhibited a higher powder SHG intensity. As an excellent
example, the quasi-C -symmetric 1,3,5-tricyano-2,4,6-tris(p-
17
18
the σ-bonded piedfort dimer, π-bridged dimer, conjugated
1
1
9
20
10
microporous polymer, and azafullerene precursor. Tr
derivatives have also acquired versatile applications in discotic
3
diethylaminostyryl)benzene (see the SI, P31 space group)
exhibited a powder SHG intensity of 45-fold larger than that of
urea.
Received: July 3, 2015
1
1
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b01912
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
2
1
22
liquid crystals, organic light-emitting diodes, two-photon
the noncentrosymmetry of the Tr-Ph3 molecule (point group
C3) and its crystal (space group P1). In the course of
synthesizing Tr-Ph3, the supposed tetramerized TTr-Ph4 was
not found. Instead, an asymmetric trimerized isomer, named
isoTr-Ph3, has been isolated (yield 3.0%), and its X-ray
structure been determined (see Figure 1 and Table 1). To our
knowledge, it is the first asymmetric triindole compound by
cycling indolin-2-one derivatives. DFT geometry optimizations
at the b3lyp/6-311g(d) level (see the SI) indicates that the
energy (sum of electronic and zero-point energies) of Tr-Ph3 is
slightly lower (0.131 eV) than that of isoTr-Ph3, which can
rationalize the higher yield of Tr-Ph3 relative to isoTr-Ph3.
TTr was treated with a large excess of NaH (>8 equiv) and
methylated with MeI, and TTr-Me4 was successfully
synthesized. When the ratio of NaH:TTr was less than 5:1,
monomethylated tetramer (TTr-Me1) was also isolated.
As shown in Figures 1 and 2, there are two very similar
molecules in the asymmetric unit of the Tr-Ph3 crystal, which
2
3
24
absorption, organic solar cells, etc. The molecular β
25
coefficients of several Tr derivatives are large. However, to
our knowledge, the macroscopic second-NLO properties of Tr
derivatives and the X-ray structure of Tr itself have not been
reported.
The chemistry of tetraindole (TTr, see Scheme 1) can be
thought of as beginning in 2006. TTr was first synthesized in
010 and followed by several tetraindoles of large size. The
only reported X-ray structure is a bromo-substituted TTr with
the S molecular point group and the centrosymmetric Ia3
space group. In this paper, we mainly report the syntheses,
structures, and the second-NLO properties of several C - and
26
2
7
28
2
̅
d
4
2
9
3
S4-symmetric cyclized indole derivatives.
As shown in Scheme 1, we synthesized the Tr (yield 18.2%)
and TTr (yield 5.5%) by trimerizing and tetramerizing the
indolin-2-one monomer in the presence of POCl in a one-pot
3
reaction at 100 °C for 8 h under Ar atmosphere. The low yield
of TTr may be the reason why TTr failed to be noticed in the
30
previous synthesis of Tr. We successfully grew the crystal of
Tr, and the X-ray structure (Figure 1 and Table 1)
demonstrated the C -symmetric octupolar skeleton of the Tr
3h
molecule, but its crystal is centrosymmetric (P2 /c).
1
Figure 2. X-ray structures of Tr-Ph3 and Tr.
take a gearlike structure with the three peripheral phenyl rings
sitting in the same side of the Tr core. The two molecules bear
the same chirality and embed to each other in a face-to-face
manner, forming a noncentrosymmetric dimeric piedfort unit.
The piedfort unit is then translated along the a-axis to form a
columnar structure. In a column, the intermolecular inter-
actions, scaled by the electronic transfer integral t (see the SI),
of the face-to-face pair in the piedfort (t = 22.2 meV) is larger
than that of the back-to-back pair between piedforts (t = 6.4
meV), though the spacing (see Figure 2) between the two
planar Tr cores in the piedfort (5.88 Å) is larger than that
between piedforts (3.65 Å). The interactions between the two
Figure 1. ORTEP plot of the title molecules.
We further synthesized the phenyl-substituted triindole (Tr-
Ph3) by trimerizing N-phenylindolin-2-one with a yield of
19.1%. The X-ray structure (see Figure 1 and Table 1) indicates
Table 1. Structure Data, Linear and Nonlinear Optical Properties
Tr
Tr-Ph3
isoTr-Ph3
TTr
TTr-Me4
297
TTr-Me1
T (K)
296
290
293
P1
295
I4
space group
a (Å)
P2 /c
P1
̅
̅
P2 /c
1
1
5.8395 (2)
12.1606 (4)
23.1234 (9)
90, 94.89, 90
0.075
9.5974 (14)
12.7810 (6)
16.0961 (8)
17.7266 (8)
76.77, 89.91, 68.90
0.087
15.3276 (10)
15.3276 (10)
4.8028 (3)
90, 90, 90
0.037
23.991 (6)
10.921 (3)
23.329 (6)
90, 118.39, 90
0.088
b (Å)
13.3267 (8)
c (Å)
13.4404 (8)
α, β, γ (deg)
R [I > 2σ (I)]
118.42, 93.30, 97.50
0.051
a
λmax, abs (nm)
308s, 322m, 340w
378
314s, 331m, 349w
306m, 353m, 401w
414s, 434s
66
311m
311m
312m
378
3.4
a
λmax, em (nm)
390
379
381
b
Φ (%)
24
24
10.7
5.1
point group
C3h
C3
C1
S4
S4
C1
c
μ (D)
0
1.924
3.073
0
0
−
30
|
|β||, ||β || (10 esu)
15.52, 11.99
35.40, 29.54
5
11.28, 9.46
25.97, 21.17
11
25.04, 21.21
0
d
SHG intensity
1.3
a
In 10−5 M THF solution. In 10 M THF solution (samples) and 10 M quinine sulfate solution in 0.1 M sulfuric (reference). Dipole moment
b
−6
−6
c
d
calculated by DFT/b3lyp/6-311g(d) algorithm. SHG intensity is the ratio of the average intensity of the sample to that of KDP.
B
DOI: 10.1021/acs.orglett.5b01912
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Organic Letters
gears” of the piedfort are chiral-matchable, which is believed to
play a critical role in the noncentrosymmetric structure of the
piedfort and the crystal.
Letter
2d
“
symmetry. All these compounds are transparent for the
frequently used 1064 nm of fundamental light and the 532 nm
light of SHG.
The X-ray structure of TTr (see Figures 1 and 3) indicates
that the 8-membered cyclooctatetraene core of TTr takes a
The fluorescent spectra of these compounds (Figure 4) show
a fine vibronic structure. Similarly to the absorption, the
emission of isoTr-Ph3 again exhibits a significant red-shift (24
nm compared to Tr-Ph3). In addition, isoTr-Ph3 has a high
fluorescence quantum efficiency (Φ = 0.66), which is about 3
times that of Tr-Ph3 (Table 1). This can be attributed to its
relatively rigid structure. For Tr-Ph3, all three phenyl groups
keep the freedom of intramolecular rolling in solution to
inactivate the excited state. For isoTr-Ph3, two phenyl groups
are π−π locked and one phenyl remains relatively free. On the
basis of its intense fluorescence, isoTr-Ph3 may find some
applications.
Figure 3. X-ray structure of TTr (a, top view; b, side view).
The sum-over-states approach was adopted to calculate the
31
βijk values at the level of CIS/6-311g(d) by using G09
programs (see the SI for details). The molecular second-NLO
coefficient can be assessed by the modulus of the β tensor,
nonplanar boatlike conformation. The TTr molecule strictly
belongs to S point group and the TTr crystal also has a strict
4
̅
S4 symmetry (space group I4), exemplifying a perfect
2b
defined by
inheritance of the molecular symmetry to the crystal. In the
c-axis direction, the neighboring saddle-shaped TTr molecules
are totally eclipsed and closely nestled in a concave−convex
2
2
ijk
1/2
|
| β || = || β || = (∑ β )
i,j,k
(1)
̅
fashion to form a 4 symmetric column in which the centroid-to-
centroid spacing between two TTr molecules is 4.80 Å and the
intermolecular plane-to-plane distance between two indole
subunits is 3.77 Å (see the SI).
The ||β|| data in Table 1 are the values at 1064 nm, and the ||β0||
data are the values at 20000 nm, which prove to be virtually
equivalent to the zero-frequency limit (λ → ∞). The S -
4
The two TTr-Me4 molecules (A and B) in the asymmetric
symmetric TTr and TTr-Me4 are pure octupoles (dipole
unit are similar and take a quasi-S conformation. However, the
4
moment μ = 0). As a comparison, the ||β|| and ||β || values of
0
space group (P2 /c) of TTr-Me4 crystal is centrosymmetric,
1
the dipolar molecule p-nitroaniline (pNA) were calculated to be
30
−30
which is very different from the noncentrosymmetric TTr
crystal (I4). What kind of molecular factor makes the difference
̅
1
6.29 × 10− and 13.12 × 10 esu, respectively, by the same
method.
Tr-Ph3 is not a strict octupole disclosed by its small dipole
between these two crystals? We believe that the molecular
dihedral angle between the indole planes is the critical factor. In
TTr crystal, the dihedral angle of the two indole planes in the
same side of the saddle TTr molecule is 103.4° (see Figure 1).
In TTr-Me4 crystal, the molecular dihedral angles are 93.2 and
moment (μ = 1.9 D). By decomposing its β tensor into the
vector (β ) and octupolar (β ) components
j=1
j=3
2
2
2
β = βj=1 + βj=3, ||β|| = ||β || + ||βj=3||
j=1
(2)
9
3.1° (molecule A) or 90.6 and 94.2° (molecule B). The close-
2b
by Zyss’s method, the ratio of ||β ||/||β || of Tr-Ph3 turns
orthorhombic dihedral angle prevents the TTr-Me4 molecules
from parallel superposing one by one like the way in TTr
crystal. On the other hand, the large dihedral angle of TTr
lowers the centroid of the saddle molecule, bringing about the
elegant TTr-saddle way of packing, i.e., the way of concave−
convex−nestled saddles.
j=3
j=1
out to be 6.5. This indicates that the octupolar component is
the dominant and Tr-Ph3 is essentially an octupolar molecule.
The macroscopic powder SHG intensities were measured
32
according to the Kurtz method at 1053 nm of pulses from a
Nd:YLF laser. TTr (powder sample) exhibits a remarkable
SHG (526.5 nm) intensity which is 11 times that of the KDP
reference (Table 1). Tr-Ph3 and TTr-Me1 solids are also SHG
active, with a intensity being 5 and 1.3 times that of KDP,
respectively (Table 1). For Tr and isoTr-Ph3, no SHG signals
were detected because of the centrosymmetry of the crystals.
The molecular second-NLO property of Tr-Ph3 is better than
that of isoTr-Ph3 (larger ||β|| value and better transparency of
The lower lying UV−vis absorption peaks (see Figure 4 and
Table 1) of these compounds are located in the region of λ =
Tr-Ph3), due to the C octupolar symmetry. Though the
3
molecular ||β|| value of TTr is not as large as that of Tr-Ph3,
TTr exhibits the best macroscopic second-NLO property. This
can be attributed to its saddle-like S octupolar symmetry,
4
which results in the excellent concave−convex−nestled way of
Figure 4. Absorption spectra (left) and fluorescent spectra (right) of
the title compounds (10 mol/L in THF).
−5
packing in crystal.
In summary, we have synthesized a series of cyclized indole
derivatives and well determined their X-ray structures. Among
them, the C -symmetric Tr-Ph3 molecules and the S -
3
10−350 nm, except the isoTr-Ph3 with the absorption edge
3
4
being red-shifted about 50 nm relative to that of Tr-Ph3. This
indicates that the UV-transparency of the octupolar Tr-Ph3 is
better than that of the dipolar isoTr-Ph3 (see the SI for
discussion) and can be explained in the viewpoint of molecular
symmetric TTr molecules crystallized into noncentrosymmetric
crystals, which exhibit remarkable second-NLO (SHG)
intensities. If pNA and TATB are the prototypical molecules
in the well-developed 1-D dipolar and 2-D C -symmetric
3
C
DOI: 10.1021/acs.orglett.5b01912
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Organic Letters
Letter
octupolar molecular NLO, there are no suitable prototype
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4
(
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organic NLO. To the best of our knowledge, the saddle-like
TTr is the first well-characterized pure octupolar organic
(
compound which holds strict S -symmetry (at both molecular
4
(
̈
er, D.; Katz, A.
and crystalline levels) and exhibits remarkable macroscopic
second-NLO effect. On the basis of this work, further study to
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TTr derivatives is ongoing in our laboratory.
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ASSOCIATED CONTENT
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b01912.
M.-J.; Yoo, J.; Lee, Y. K.; Lee, G. J.; Kang, T. I.; Cho, M.; Jeon, S.-J. J.
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Crystallographic data for Tr-Ph3 (CIF)
Crystallographic data for TTr (CIF)
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Crystallographic data for isoTr-Ph3 (CIF)
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Experimental details; NMR, MS, computational details
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(
PDF)
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AUTHOR INFORMATION
(
■
*
Corresponding Author
E-mail: fangqi@sdu.edu.cn.
Hummel, G. J.; Harkema, S.; van Veggel, F. C. J. M.; Clays, K.;
Engbersen, J. F. J.; Persoons, A.; van Hulst, N. F.; Reinhoudt, D. N. J.
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The authors declare no competing financial interest.
(
́
́
ACKNOWLEDGMENTS
■
(
This work was supported by the National Natural Science
Foundation of China (Grant Nos. 21472116, 20972089, and
́
́
5
0673054) and by the State Key Laboratory of Crystal
́
Materials.
́
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DOI: 10.1021/acs.orglett.5b01912
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