Heteroatom-bridged tetraphenylenes: Synthesis, structures, and properties

Article abstract of DOI:10.1021/ol501267z

Novel oxygen-, nitrogen-, sulfur-, and selenium-bridged tetraphenylenes were prepared from known tetraphenylene derivatives. Structures of these compounds were unambiguously confirmed by X-ray crystallographic analyses. Photophysical and electrochemical i

Full text of DOI:10.1021/ol501267z

Letter
pubs.acs.org/OrgLett
Heteroatom-Bridged Tetraphenylenes: Synthesis, Structures, and
Properties
Xiao-Dong Xiong,† Chun-Lin Deng,† Xiao-Shui Peng,†,‡ Qian Miao,† and Henry N. C. Wong*,†,‡
†Department of Chemistry, State Key Laboratory of Synthetic Chemistry, Center of Novel Functional Molecules and Institute of
Molecular Functional Materials, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, China
‡Shenzhen Municipal Key Laboratory of Chemical Synthesis of Medicinal Organic Molecules & Shenzhen Center of Novel
Functional Molecules, Shenzhen Research Institute, The Chinese University of Hong Kong, No. 10, second Yuexing Road, Shenzhen
518507, China
S
* Supporting Information
ABSTRACT: Novel oxygen-, nitrogen-, sulfur-, and selenium-bridged
tetraphenylenes were prepared from known tetraphenylene derivatives.
Structures of these compounds were unambiguously confirmed by X-ray
crystallographic analyses. Photophysical and electrochemical investiga-
tions of these heteroatom-bridged tetraphenylenes suggested their
potential applications as electronic materials.
etraphenylene (1) is unique with a saddle-shaped
Scheme 1. Synthesis of 3
T
structure, in which the two opposite pairs of benzene
rings are arranged up and down the plane of the eight-
membered ring (Figure 1).1 Various substituted tetrapheny-
available DMF, which then underwent an intramolecular
coupling to afford the oxygen-bridged tetraphenylene 3.
Yellowish needle-like single crystals of tetraphenylene 3 were
obtained by sublimation of 3 at 295 °C under N2, whose
structure was confirmed by an X-ray crystallographic analysis.
As shown in Scheme 2, upon treatment of 1,8,9,16-
tetrabromotetraphenylene (4)9 with sodium azide and CuI/
Figure 1. Saddle-shaped structure of tetraphenylene 1.
lenes with particular rigidity were employed as asymmetric
catalysts,2 liquid crystals,3 molecular devices,4 and blue organic
light-emitting diodes.5 The extraordinary geometry of 1 has
renewed the interest of several research groups to explore new
strategies to install functional groups on tetraphenylenes or to
embed tetraphenylenes into conjugated systems.6 For instance,
the annulation of small membered rings and rigid planar π-
systems to 1 could planarize its cyclooctatetraene ring. In this
Scheme 2. Synthesis of 5, 6, and 7
domain, Hogberg,7a Nishinaga,7b and Pittelkow6b,7c synthesized
̈
a class of coplanar antiaromatic cyclooctatetraenes with
heteroatom bridges. Herein, we report our own efforts in the
synthesis and studies of dioxo, diaza-, dithio-, and diseleno-
bridged tetraphenylenes in order to explore the chemistry of
these novel heteroatom-bridged tetraphenylenes.
Recently, an efficient synthesis of tetraphenylene triflate 2
has been reported.2a,8 An accidental conversion of 2 to the
oxygen-bridged tetraphenylene 3 in the presence of Pd(PPh3)4
and K3PO4 for 24 h at 140 °C in 68% yield was uncovered, as
depicted in Scheme 1. Presumably, the tetraphenylene triflate 2
could be partially hydrolyzed into a hydroxyl compound in the
presence of a trace amount of water from the commercially
Received: May 3, 2014
© XXXX American Chemical Society
A
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N,N′-dimethylethylenediamine (DMEDA) at 120 °C in
DMSO, nitrogen-bridged tetraphenylene 5 was obtained in
45% yield. Compound 6 was effectively synthesized via a
palladium-mediated double N-arylation with aniline at 120 °C
in 85% yield. Benzylation of 5 allowed us to produce N-benzyl
tetraphenylene 7 in 85% yield. Structures of 6 and 7 were
unambiguously confirmed by X-ray crystallographic analyses.
With oxygen/nitrogen-bridged tetraphenylenes 3, 5, 6, and 7
in hand, we then turned our attention to the syntheses of
sulfur- and selenium-bridged tetraphenylenes 8 and 10.
Although we were unable to convert compound 4 to 8 in a
direct manner employing K2S, treatment of 4 with n-BuLi and
I2,10 followed by a coupling reaction with K2S in the presence of
CuI/DMEDA at 140 °C in CH3CN, nonetheless provided 8 in
69% yield. Upon treatment of 4 with n-BuLi and selenium
powder, compound 9 was obtained in 67% yield. Subsequently,
a mixture of a copper powder and the solid form of 9
underwent thermal deselenation, leading to the desired 10 in a
quantitative yield (Scheme 3). Structures of 8 and 10 were also
confirmed by X-ray crystallographic studies.
regular tetraphenylene 1 (122.5°).2b However, the average bent
angles α of 3, 7, 8, and 10 are 25.6°, 28.8°, 35.3°, and 37.4°,
respectively, which are substantially less than that of 1
(50.7°).2b Thus, these heteroatom-bridged tetraphenylenes
(3, 6, 7, 8, 10) are less puckered than that of 1, and the
heteroatomic bridges effectively force the adjacent two benzene
rings to become nearly coplanar, which was further supported
by the results of smaller dihedral angles between the pentacyclic
and benzene ring (A/B, 7.6°, 14.8°, 12.7°, and 9.4°,
respectively). Investigation of the bond lengths in the
cyclooctatetraene rings of 3, 7, 8, and 10 reveals that although
the average values of the bonds are close to the corresponding
values of 1, the oxygen, nitrogen, sulfur, and selenium bridges
changed the bond length alternation in the eight-membered
core ring. The calculated results with (DFT B3LYP/6-31+G)
correlate well with the experimental results obtained from X-ray
crystallographic studies (Table S2, Supporting Information
(SI)), with the exception of the maximum deviation of the bent
angle α of 6 (26.7° vs 8.9°).
In the crystal structure of 3, the molecule consists of a
crystallographic 2-fold screw axis passing through the center of
an eight-membered ring, including an inversion center (Figure
2). Adjacent molecules are found to be stacked in a concave−
Scheme 3. Synthesis of 8 and 10
As shown in Table 1, X-ray crystallographic data indicate that
these heteroatom-bridged tetraphenylenes (3, 6, 7, 8, 10) are
saddle-shaped. As regarding the inner angles of the central
eight-membered ring, the average values in the crystal structure
of 3, 7, 8, and 10 are found to be 132.2°, 131.1°, 128.7°, and
128.0°, respectively, which are larger than the inner angle of a
Figure 2. Crystal packing in the X-ray crystal structure of 3.
a
Table 1. Selected Structural Data Analysis of Heteroatom-Bridged Tetraphenylenes
a
Determined by X-ray analysis.
B
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convex manner with a π−π distance of 3.82 Å as measured from
the center of one benzene ring to the plane of the neighboring
benzene ring. In comparison, the crystal structure of 8 exhibits
an offset π-stacking with a distance of 3.52 Å between the edges
of adjacent molecules as shown in Figure 3.
Figure 5. Normalized UV/vis absorption (CH2Cl2) of heteroatom-
bridged tetraphenylenes.
Figure 3. Packing observed in the X-ray crystal structure of 8.
unit. In contrast, 6 exhibits the strongest fluorescence among
these heteroatom-bridged tetraphenylenes (Figure S10, SI).
A comparative analysis between the dibenzo analogues
(benzofuran, carbazole, and benzothiophene) and these
heteroatom-bridged tetraphenylenes (3, 5, 8, 10) was also
carried out. The α-protons of the dibenzo units in heteroatom-
bridged tetraphenylenes (3, 5, 8, and 10) show an upfield shift
as compared with the corresponding protons in the relevant
dibenzo compounds. These upfield shifts are principally
ascribed to the change of paratropic ring current in the
heteroatom-bridged tetraphenylenes. The longest absorption
maximum λmax (by ca. 1, 9, and 16 nm, respectively) of 3, 5, and
8 is bathochromically shifted compared with those of
benzofuran, carbazole, and benzothiophene, which may suggest
the existence of π-conjugation between the corresponding two
dibenzo units in the heteroatom-bridged tetraphenylene
framework (Table S1, SI).
In the case of 6, both the bend angle α and dihedral angle
(A/B) values greatly decrease to 8.9° and 5.4° when compared
with that of 7 (28.8° and 9.4°, respectively). The carbazole
units and parent eight-membered ring are even much closer to
planar, probably due to the presence of outstretched sterically
hindered benzene rings. The crystal packing of 6 suggests the
existence of an intermolecular π−π stacking between the π
systems of the closet molecules (Figures 4), and weak aromatic
intermolecular π−π stacking interaction between neighboring
molecules was observed from the crystal packing of 7 (Figure
S3, SI).
As demonstrated in Figure S17 (SI), the electrochemical
behavior of these heteroatom-bridged tetraphenylenes (3, 6, 8,
10) was evaluated employing cyclic voltammetry and DFT
methods. The cyclic voltammogram of 3 shows one irreversible
oxidation wave with a peak potential at 1.21 V and two quasi-
reversible reduction waves with half-wave reduction potentials
at−2.32 V and−2.64 V versus ferrocenium/ferrocene. From
these potentials, the HOMO energy level (EHOMO) and LUMO
energy level (ELUMO) for 3 are estimated as−6.31 eV, −2.78 eV,
respectively.11 The values of ELUMO and EHOMO of 3 are almost
the same as those of 8, indicating both compounds have similar
electrochemical properties. From the oxidation wave of 6, the
HOMO energy level of this compound is estimated as −5.78
eV, which, together with the molecular packing of 6 in crystals,
suggests that 6 is a candidate for a p-type semiconductor.12
Furthermore, the calculated HOMO and LUMO energy levels
are in reasonable agreement with the experimental values
determined by cyclic voltammetry (Table S3 and Figure S17,
SI).
In conclusion, novel heteroatom-bridged tetraphenylenes (3,
5, 6, 7, 8, and 10) were prepared from 2 or 4, whose
configurations were efficiently adjusted using different bridging
atoms. Crystallographic investigations indicate that 3, 6, 7, 8,
and 10 assume a unique saddle-shaped structure with different
packing motifs. The HOMO energy level and π−π stacking in
the crystal structures suggest some of these heteroatom-bridged
tetraphenylenes are potential candidates for electronic materi-
Figure 4. Packing observed in the X-ray crystal structure of 6.
The optical properties of 3, 5, 8, and 10 were investigated by
both UV/vis and fluorescence spectroscopy. When compared
with 1, compounds 3, 5, 8, and 10 exhibit a red shift λmax (by ca.
1, 11, 24, and 26 nm, respectively). The absorptions of these
heteroatom-bridged tetraphenylenes are similar; all of them
show a strong intense absorption band with two shoulders.
Moreover, a weak absorption band in the longer-wavelength
region, ranging from 300 to 420 nm, is observed in 3, 5, 8, and
10. According to theoretical calculations, this long-wavelength
absorption is mainly attributed to the HOMO−LUMO
transition of 3, 5, 8, and 10, which suggests the existence of
n−π conjugation between the lone pair electrons on the
oxygen, nitrogen, sulfur, and selenium atoms and the π-
conjugated biphenylene framework (Figure 5). Compound 5
exhibits blue luminescence with an emission maximum from
410 to 550 nm, which is close to that of the parent carbazole
C
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2010, 132, 1066−1074. (c) Brock-Nannestad, T.; Nielsen, C. B.;
Schau-Magnussen, M.; Hammershoj, P.; Reenberg, T.; Petersen, A.;
Trpcevski, D.; Pittelkow, M. Eur. J. Org. Chem. 2011, 6320−6325.
(d) Hensel, T.; Trpcevski, D.; Lind, C.; Grosjean, R.; Hammershoj, P.;
Nielsen, C.; Brock-Nannestad, T.; Nielsen, B.; Schau-Magnussen, M.;
Minaev, B.; Baryshnukov, G.; Pittelkow, M. Chem.Eur. J. 2013, 19,
17097−17102.
(8) Lin, F.; Peng, H. Y.; Chen, J. X.; Chik, D. T. W.; Cai, Z.; Wong,
K. M. C.; Yam, V. W. W.; Wong, H. N. C. J. Am. Chem. Soc. 2010, 132,
16383−16392.
als. Further investigations of their semiconductor properties are
in progress.
ASSOCIATED CONTENT
* Supporting Information
■
S
Descriptions of experimental procedures for compounds and
analytical characterization. This material is available free of
charge via the Internet at http://pubs.acs.org.
(9) Kabir, S. M. H.; Iyoda, M. Synthesis 2000, 1839−1843.
(10) The relevant iodide intermediate in Scheme 3 was identified as
1,8-dibromo-9,16-diiodotetraphenylene after treatment of compound
4 with n-BuLi and I2.
(11) The commonly used formal potential of the ferrocenium/
ferrocene redox couple in the Fermi scale is −5.1 eV, which is
calculated on the basis of an approximation neglecting solvent effects
using a work function of 4.46 eV for the normal hydrogen electrode
(NHE) and an electrochemical potential of 0.64 V for ferrocenium/
ferrocene versus NHE. See: Cardona, C. M.; Li, W.; Kaifer, A. E.;
Stockdale, D.; Bazan, G. C. Adv. Mater. 2011, 23, 2367−2371.
(12) Tang, M. L.; Reichardt, A. D.; Wei, P.; Bao, Z. J. Am. Chem. Soc.
2009, 131, 5264−5273.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: hncwong@cuhk.edu.hk.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by an Area of Excellence Scheme
established under the Research Grants Council of the Hong
Kong SAR, China (Project No. AoE/P-03/08) and a grant to
the State Key Laboratory of Synthetic Chemistry from the
Innovation and Technology Commission. Grants from the
National Natural Science Foundation of China (NSFC No.
21272199), The Chinese Academy of Sciences−Croucher
Foundation Funding Scheme for Joint Laboratories, the
Shenzhen Science and Technology Innovation Committee for
t h e M u n i c i p a l K e y L a b o r a t o r y S c h e m e
(ZDSY20130401150914965), and the Shenzhen Basic Re-
search Program (JCYJ20120619151721025) are also gratefully
acknowledged. We thank Professor Thomas C. W. Mak and Dr.
Chun-Kit Hau (The Chinese University of Hong Kong) for
helping to carry out X-ray crystallographic analyses.
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