Synthesis and properties of novel C3-symmetric coplanar chromophores

Article abstract of DOI:10.1021/ol201466z

New synthetic pathways for novel C₃-symmetric molecules featured with a thiophene-fused six-five-five carbon-bridged coplanar core structure have been established. The incorporation of thiophene as the constituent of a C₃-symmetric c

Full text of DOI:10.1021/ol201466z

ORGANIC
LETTERS
2011
Vol. 13, No. 16
4168–4171
Synthesis and Properties of Novel
C3-Symmetric Coplanar Chromophores
Jia-Hong Chen, Shao-An Wang, Yi-Hung Liu, and Ken-Tsung Wong*
Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan
kenwong@ntu.edu.tw
Received May 31, 2011
ABSTRACT
New synthetic pathways for novel C3-symmetric molecules featured with a thiophene-fused six-five-five carbon-bridged coplanar core structure
have been established. The incorporation of thiophene as the constituent of a C3-symmetric core provides effective modulation of the physical
properties and imparts extra flexibility for π-conjugated functionalization stemming from either the embedded thiophene or pendant
aryl substitutions.
C3-symmetric molecules intrigue organic chemistry
scientists for their new synthetic pathways and optoelec-
tronic applications.1 A major goal for C3-symmetric ma-
terial chemistry is to design new families of π-conjugated
star-shaped molecules with novel core structures, and to
investigate their structureÀproperty relationships. Among
the reported C3-symmetric polycyclic aromatics, truxene
(Figure 1) is one of the most promising and versatile
building blocks for the synthesis of fascinating molecules
with various applications such as organic field-effect tran-
sistors (OFETs), solar cells, and electroluminescent
devices.2 In addition to truxene, C3-symmetric coplanar
analogues with bridging heteroatoms (N, O, S; Figure 1)
have also been explored,3 making possible direct manip-
ulations on the electronic structure (HOMO/LUMO
levels) and consequent physical properties of the truxene
analogues. For example, triazatruxene-based materials
have been exploited as the precursor of a triaza-analogue
of crushed fullerenes4 and have been utilized as liquid
crystals,5 as two-photon absorption materials,6 and in
OLED applications.7 To the best of our knowledge, the
reported C3-symmetric coplanar structures are generally
composed of six-five-sixfused rings. To date, an analogous
C3-symmetric coplanar structure featuring six-five-five
fused rings has not been reported. In this communication,
we report for the first time the syntheticroutes and physical
properties of a novel class of thiophene-embedded C3-
symmetric star-shaped coplanar molecules. The incor-
poration of electron-rich thiophene rings as the π-conju-
gated constituent of this new C3-symmetric coplanar
structure drastically perturbs the electronic properties of
(1) (a) Martineau, C.; Blanchard, P.; Rondeau, D.; Delaunay, J.;
Roncali, J. Adv. Mater. 2002, 14, 283. (b) De Bettignies, R.; Nicolas, Y.;
Blanchard, P.; Levillain, E.; Nunzi, J.-M.; Roncali, J. Adv. Mater. 2003,
15, 1939. (c) Cao, X.-Y.; Liu, X.-H.; Zhou, X.-H.; Zhang, T.; Jiang, Y.;
Cao, Y.; Cui, Y.-X.; Pei., J. J. Org. Chem. 2004, 69, 6050. (d) Nicolas, Y.;
Blanchard, P.; Levillain, E.; Allain, M.; Mercier, N.; Roncali, J. Org.
Lett. 2004, 6, 273. (e) Kanibolotsky, A. L.; Berridge, R.; Skabara, P. J.;
Perepichka, I. F.; Bradley, D. D. C.; Koeberg, M. J. Am. Chem. Soc.
2004, 126, 13695. (f) Lai, W.-Y.; Xia, R.; He, Q.-Y.; Levermore, P. A.;
Huang, W.; Bradley, D. D. C. Adv. Mater. 2009, 21, 355. (g) Shang, H.;
Fan, H.; Liu, Y.; Hu, W.; Li, Y.; Zhan, X. Adv. Mater. 2011, 23, 1554.
(2) (a) Kimura, M.; Kuwano, S.; Sawaki, Y.; Fujikawa, H.; Noda,
K.; Taga, Y.; Takagi, K. J. Mater. Chem. 2005, 15, 2393. (b) Sun, Y.;
Xiao, K.; Liu, Y.; Wang, J.; Pei, J.; Yu, G.; Zhu, D. Adv. Funct. Mater.
2005, 15, 818. (c) Luo, J.; Zhou, Y.; Niu, Z.-Q.; Zhou, Q.-F.; Ma, Y.; Pei,
J. J. Am. Chem. Soc. 2007, 129, 11314. (d) Xia, H.; He, J.; Xu, B.; Wen,
S.; Li, Y.; Tian, W. Tetrahedron 2008, 64, 5736. (e) Wang, J.-L.; Yan, J.;
Tang, Z.-M.; Xiao, Q.; Ma, Y.; Pei, J. J. Am. Chem. Soc. 2008, 130, 9952.
(3) (a) Bergman, J.; Egestad, B. Tetrahedron Lett. 1978, 34, 3143. (b)
Bergman, J.; Egestad, B. Tetrahedron 1986, 42, 763. (c) Boccbl, V.; Palia,
C. Tetrahedron 1986, 42, 5019.
ꢀ
(4) Gomez-Lor, B.; Echavarren, A. M. Org. Lett. 2004, 6, 2993.
ꢀ
(5) (a) Gomez-Lor, B.; Alonso, B.; Omenat, A.; Serrano, J. L. Chem.
Commun. 2006, 5012. (b) Luo, J.; Zhao, B.; Shao, J.; Lim, K. A.; Chan,
H. S. O.; Chi, C. J. Mater. Chem. 2009, 19, 8327.
(6) (a) Ji, L.; Fang, Q.; Yuan, M.-S.; Liu, Z.-Q.; Shen, Y.-X.; Chen,
H.-F. Org. Lett. 2010, 12, 5192. (b) Shao, J.; Guan, Z.; Yan, Y.; Jiao, C.;
Xu, Q.-H.; Chi, C. J. Org. Chem. 2011, 76, 780.
(7) (a) Lai, W.-Y.; Zhu, R.; Fan, Q.- L.; Hou, L.-T.; Cao, Y.; Huang,
W. Macromolecules 2006, 39, 3707. (b) Lai, W.-Y.; He, Q.-Y.; Zhu, R.;
Chen, Q.-Q.; Huang, W. Adv. Funct. Mater. 2008, 18, 265.
r
10.1021/ol201466z
Published on Web 07/21/2011
2011 American Chemical Society

the core, namely raising the HOMO level as compared to
the pure hydrocarbon homologues.
1,3,5-tribromobenzene with 3-(ethoxycarbonyl)-2-thienyl
zinc cholride. Before carrying out the nucleophilic addition
on the ester groupsof 1 with aryllithium, a triisopropylsilyl
(TIPS) group was introduced to block the reactive sites of
the R-thienyl groups. Thus, deprotonation of 1 with LDA
followed byquenching with TIPS-Cl affordedcompound2
in good yield (76%). Subsequent addition of phenyl-
lithium, followed by intramolecular FriedelÀCrafts cycli-
zation, gave the desired product 3a in 60% yield. Finally,
the hexaphenyl substituted coplanar molecule 4 was iso-
lated with an excellent yield of 90% after protodesilylation
with tetrabutylammonium fluoride (TBAF). Instead of
using TIPS as the reactive site blocking group, bromina-
tionoftriester1 withNBS gavethe versatile tribromide5 in
high yield. The regioselective bromination indicates that π-
conjugated derivatives stemming from the new thiophene-
fused coplanar core structure could be feasibly achieved.
Along this line, a Pd-catalyzed Suzuki coupling reaction of
intermediate 5 with 4-tert-butylphenyl boronic acid gave
triester 6 with an isolated yield of 82%. The addition of an
excess amount of phenyllithium onto the ester groups of 6
gave the corresponding triaryl alcohol. Without further
purification, the crude triaryl alcohol was subjected to
acid-mediated triple cyclization to afford the coplanar
C3-symmetric product 7 with an isolated yield of 63%.
Scheme 2 depicts an alternative pathway for the synthesis
of thiophene-fused C3-symmetric coplanar molecules.
Figure 1. Chemical structures of truxene and its heteroho-
mologues.
Scheme 1 depicts our synthetic approaches of the target
C3-symmetric chromophores. Based on our previously
established synthesis of sp3-carbon bridge coplanar ring
systems,8 a double addition of aryl organometallics onto
an ortho ester group of biaryls followed by intramolecular
FriedelÀCrafts cyclization is generally involved.
Scheme 1. Synthesis of the Thiophene-Fused C3-Symmetric
Coplanar Molecules
Scheme 2. Alternative Synthetic Route of the Thiophene-Fused
C3-Symmetric Coplanar Molecules
The crucial triester intermediate 1 was furnished in
42% yield by the Negishi cross-coupling reaction of
For this approach, the lithiated core was utilized to add
onto the diarylketones for generating the tertiary alcohols
used for the proton-promoted intramolecular cyclization.
The synthesis started from the bromination of 1,3,5-
trithienylbenzene 8 toyield hexabromo intermediate9 with
a yield of 78%. Treatment of compound 9 with n-BuLi
atÀ78 °C regioselectively gave the trilithiated intermediate,
(8) (a) Wong, K.-T.; Hwu, T.-Y.; Balaiah, A.; Chao, T.-C.; Fang, F.-
C.; Lee, C.-T.; Peng, Y.-C. Org. Lett. 2006, 8, 1415. (b) Wong, K.-T.;
Chi, L.-C.; Huang, S.-C.; Liao, Y.-L.; Liu, Y.-H.; Wang, Y. Org. Lett.
2006, 8, 5029. (c) Wong, K.-T.; Chao, T.-C.; Chi, L.-C.; Chu, Y.-Y.;
Balaiah, A.; Chiu, S.-F.; Liu, Y.-H.; Wang, Y. Org. Lett. 2006, 8, 5033.
Org. Lett., Vol. 13, No. 16, 2011
4169

which was quenched with TIPS-Cl and 1-iodohexane,
giving TIPS and hexyl-substituted tribromo compounds
10a and 10b, respectively. A subsequent Li/Br exchange
reaction of tribromo compound 10 with n-BuLi gave the
reactive lithiated intermediate, which was then quenched
with various diarylketones to afford the corresponding
tertiary alcohols. Without further purifications, these al-
cohols underwent acid-mediated intramolecular cycliza-
tion to give the coplanar cores 3aÀe in moderate yields.
After protodesilylation of 3a with TBAF followed by
regioselective NBS bromination, the resulting tribromine
intermediate can be directly exploited for a Suzuki cou-
pling reaction with 4-tert-butylphenyl boronic acid to give
molecule 7 in 68% yield (three steps). This synthetic
strategy provides extra flexibility for the functionalization
of the aryl substitutions, as we can envision that further
decoration on the peripheral bromo-substituted fluorene
moieties of 3e would give rise to new structural features
arranging perpendicularly relative to the coplanar C3-
symmetric core.
The molecular structure of thiophene-fused C3-sym-
metric molecules 3a and 3d can be verified by X-ray
crystallography (Figure 2). The averaged dihedral angles
of the pivotal phenylene ring and peripheral thiophene
rings of molecules 3a and 3d were calculated to be 6.67°
and 6.15°, respectively, suggesting slight deviations from a
perfect coplanar plane due to the manner of fused six-five-
five rings. The aryl substitutions on the saturated carbon
bridge position on the top and bottom faces of the
molecular plane imparts a low tendency for interchromo-
phore interactions. In addition, the rigid and coplanar
spirofluorene substituents are orthogonal relative to the
molecular plane of 3d, giving an interesting tripod-like
molecule.
materials exhibit no obvious glass transition temperatures
(Tg) except for 3d (120 °C). Thermogravimetric analysis
(TGA) revealed the high heat tolerance of these new
compounds with decomposition temperatures (Td) (5%
weight loss) ranging from 365 to 433 °C. The data are
summarized in Table 1.
The absorption (UVÀvis) and photoluminescent (PL)
sepecta of molecules 4 and 7 are compared to those of pure
hydrcarbon parent counterpartT1(Figure3). TheUVÀvis
and PL spectra of the thiophene-fused coplanar molecule 4
exhibited substantial red shifts relative to those of truxene
derivative T1. This result clearly indicates the effect of
reducing the optical gap whenthiophene was introduced as
the constitutent of the C3-symmetric coplanar core. An
evident Stokes shift (6832 cmÀ1) of 4 was observed which
may be a result of the first two excitations being symmetry-
forbidden. If so, the main absorption peak could result
from the electronic transition to higher states, followed by
internal conversion to the S1 state where emission
originates.9 The PL quantum yields (QYs) of these C3-
symmetric molecules are comparable as compared to that
of the parent truxene T1 (Table 1). In addition, the
extension of the π-conjugation from the thiophene-fused
core can pronouncedly alter the photophysical behavior as
evidenced by the significant red shifts in UVÀvis and PL
spectra and improved PL efficiency of compound 7.
Figure 3. UVÀvis absorption spectra (solid symbol) and photo-
luminescence spectra (open symbol) of T1, 4, and 7.
Cyclic voltammetry (CV) was used to probe the elec-
trochemical behavior of thiophene-fused C3-symmetry
coplanar molecules and parent molecule T1 (Figure 4).
The electrochemical data are summarized in Table 1.
The oxidation potentials of the thiophene-embedded
molecules are significantly lower than that of truxene
T1. For example, the oxidation onset of 4 is 1.28 V,
which is evidently lower than that of T1 (1.44 V). This
result clearly indicates the considerable HOMO-raising
Figure 2. (a) Top view of molecular structure of 3a. (b) C3-
symmetric core structure of 3a. (c) The tripod-like 3d with
orthogonal spirofluorene substituents; the alkyl side chains are
omitted for clarity.
The introduction of aryl substitutions on the top and
bottom faces of the thiophene-fused C3-symmetric copla-
nar core makes the resulting molecules rather bulky and
rigid. Differential scanning calorimetry (DSC) analysis
indicated that these new thiophene-fused C3-symmetry
(9) Kao, M.-T.; Wong, K.-T.; Chen, J.-H.; Tseng, K.-P.; Hsu, C.-H.;
Chang, C.-W.; Hsu, C.-P.; Liu, Y.-H. Org. Lett. 2011, 13, 1713.
4170
Org. Lett., Vol. 13, No. 16, 2011

Table 1. Properties of Truxene T1 and the Thiophene-Embedded Truxene Analogues
compd
absaλmax (log ε) [nm]
PLbλmax [nm]
Q.Y.c [%]
Tgd [°C]
Tdf [°C]
E1/2 oxdg [V]
HOMOi [eV]
LUMOi [eV]
T1
3a
3b
3c
3d
4
292, 308 (4.85)
377
410
408
411
409
403
423
12
12
9
n.d.e
n.d.
n.d.
n.d.
120
n.d.
n.d.
409
339
365
365
380
373
433
1.52
1.25
1.22
1.16
1.14
1.36h
1.15
À5.85
À5.58
À5.55
À5.49
À5.47
À5.69
À5.48
À2.10
À2.11
À2.06
À2.00
À1.93
À2.07
À2.39
326 (4.79), 342 (4.80)
324 (5.00), 341 (4.99)
324 (4.86), 339 (4.86)
321 (4.92), 337 (4.85)
316 (4.48), 330 (4.44)
367 (4.95)
11
6
10
30
7
a In CHCl3. b In CHCl3, upon excitation at the absorption maximum. c In CH2Cl2, measured by calibrated integrating sphere system (Hamamatsu
C9920) d Analyzed using DSC. e n.d. = not detected. f Analyzed using TGA (5% weight loss). g In CH2Cl2 containing 0.1 M nBu4NPF6 as a supporting
electrolyte; potentials were recorded vs Ag/AgCl. h Epa, due to irreversible CV. i HOMO and LUMO were estimated from E1/2 oxd and absorption onset.
effect upon the implantation of electron-rich thiophenes
as the constitutional moieties of the C3-symmetric co-
planar core. It should be noted that the reduction peak
observed at 0.89 V of 4 is due to the electrochemically
dimerized species as the electroactive sites are unpro-
tected (Figure S-2, Supporting Information). The ex-
a thiophene-fused six-five-five carbon-bridged coplanar
structure have been successfully established. The versatile
synthetic pathways provide the flexibility for further π-
conjugated functionalization stemming from either the
constituted thiophene ring or the pendant aryl substitu-
tions. The molecular core structures were confirmed to be
nearly coplanar by X-ray crystallography. The incorpora-
tion of thiophene as the constituent of a C3-symmetric core
provides effective modulation of the physical properties,
particularlythe HOMO-raisng effect, which was evidenced
by the lower oxidation potential in CV and red-shifted
absorption and emission in photophysics. With great
thermal stability as a property, we could anticipate the
great opportunity of these novel C3-symmetric chromo-
phores to serve as potential materials for optoelectronic
devices.
Acknowledgment. We thank for the financial support
from the National Science Council (98-2119-M-002-007-
MY3), Taiwan.
Supporting Information Available. Detailed experi-
mental procedures; spectroscopic characterization; the
crystal data and cif files of compounds 1, 3a, and 3d; the
X-ray crystal structure of compound 1; cyclic voltam-
mograms of compounds T1, 4, and 7; UVÀvisible
absorption and photoluminescence spectra of 3aÀd;
1H, 13C NMR spectra of new compounds. This material
is available free of charge via the Internet at http://
pubs.acs.org.
Figure 4. Cyclic voltammograms of compounds T1, 4, and 7.
tended π-conjugation is beneficial for giving a higher
HOMO level and quasi-reversible oxidation and reduc-
tion potentials (Figure S-3).
In summary, for the first time, inventive synthetic
pathways for novel C3-symmetric molecules featuring
Org. Lett., Vol. 13, No. 16, 2011
4171