Macrospirocyclic oligomers based on carbazole and fluorene

Article abstract of DOI:10.1021/ol1025958

Monodisperse macrospirocyclic oligomers were prepared using self-condensation of the Friedel-Crafts reaction. Through the C-9s of the central fluorene units of four surrounding oligofluorenes, four carbazole units are connected in a series to form a macro

Full text of DOI:10.1021/ol1025958

ORGANIC
LETTERS
2011
Vol. 13, No. 2
200-203
Macrospirocyclic Oligomers Based on
Carbazole and Fluorene
Yu Lu,† Yu Zhou,† Yi-Wu Quan,*,† Qing-Min Chen,† Run-Feng Chen,‡
Zhi-Yong Zhang,‡ Qu-Li Fan,*,‡ Wei Huang,*,‡ and Jian-Fu Ding§
Department of Polymer Science & Engineering, State Key Laboratory of Coordination
Chemistry, School of Chemistry and Chemical Engineering, Nanjing UniVersity,
Nanjing 210093, China, Key Laboratory for Organic Electronics & Information
Displays (KLOEID) and Institute of AdVanced Materials (IAM), Nanjing UniVersity of
Posts and Telecommunications, Nanjing 210046, China, and Institute for Chemical
Process and EnVironmental Technology (ICPET), National Research Council of
Canada, 1200 Montreal Road, Ottawa, Ontario, K1A 0R6, Canada
quanyiwu@nju.edu.cn; iamqlfan@njupt.edu.cn; wei-huang@njupt.edu.cn
Received October 26, 2010
ABSTRACT
Monodisperse macrospirocyclic oligomers were prepared using self-condensation of the Friedel-Crafts reaction. Through the C-9s of the
central fluorene units of four surrounding oligofluorenes, four carbazole units are connected in a series to form a macrocyclic core. These
rodlike oligofluorenes form a rigid three-dimensional structure, affording the resulting macrocyclics a high steric hindrance for close interchain
packing.
Monodisperse oligofluorenes have recently become a subject
of intense study for optoelectronic applications, due to their
well-defined conjugation lengths and molecular structures,
high photoluminescence (PL) and electroluminescence quan-
tum efficiencies, ease of purification, characterization, and
solution processing.
Moreover, monodisperse oligofluorenes also have been
employed as ideal models to understand the fundamental
properties of the polydisperse polymeric analogues.1 Recent
rapid development of new synthetic methods makes it
possible to design a variety of monodisperse oligomers,2
permitting efficient color and energy level tuning through
the control of effective conjugation length as well as the
introduction of electron-donating and -withdrawing moieties
into the conjugated systems. A number of monodisperse
oligofluorenes have been synthesized as blue light-emitting
materials.3 Among them, three-dimensional (3D) monodis-
perse oligofluorenes are noted for their specific structure to
suppress undesired chain aggregation, excimer formation, and
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Loy, D. E.; Thompson, M. E. Chem. Mater. 1998, 10, 2235–2250. (d) Zhao,
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† Nanjing University.
‡ Nanjing University of Posts and Telecommunications.
§ National Research Council of Canada.
10.1021/ol1025958  2011 American Chemical Society
Published on Web 12/14/2010

keto defects.4 The core and spiro-linked fluorene moieties
constructed the rigid 3D structures, which formed excellent
amorphous states to render the oligomers better photolumi-
nescence stabilities and lower crystallization tendencies and
excellent optoelectronic properties. However, challenge
remains in achieving balanced charge injection and mobility
in 3D oligofluorene.4d
In this letter, we report the development of a new family
of 3D macrocyclic oligofluorene by a Friedel-Crafts self-
condensation reaction. In the produced macrocyclics, four
carbazole units are connected in a series to form a macro-
cyclic core through the C-9s of the central fluorene units of
four surrounding oligofluorenes, which, like rigid rods,
extend from the ring plane in both sides. Due to nonconju-
gated 3D structure, these multi-H shaped oligomers have the
advantages of (1) higher solubility, (2) reduced interchain
interaction, (3) improved hole injection without sacrificing
fluorene’s electron-injection capability, and (4) easily doped
capability due to hollow space structure.
aromatic compound for the electrophilic substitution, because
of the electron-donating property of the amino group, and
reacts with the fluorenol group, which is a strong alkylating
reagent,2b for this self-condensation by the Friedel-Crafts
procedure. The para position with respect to the nitrogen is
a reactive site, while the ortho position is proved nonreactive
due to steric hindrance effect.5,4c The synthetic route for
(TCAF3)4 was similar to the procedure for (ECAF3)4, where
the starting material was N-(4-methylphenyl)-3-bromocar-
bazole. These reactions also potentially give a series of linear
structural oligomers. Therefore, to optimize the production
of the desired cyclic oligomer, a very low monomer
concentration (1%) has been used for the reaction, which
favors the formation of smaller cyclic oligomers.7,4c A high
purity of the macrocyclics has been proved by a simple color
test. The linear side products contain a fluorenol moiety at
the chain end. In the presence of p-toluenesulfonic acid in
the mesitylene solution, this group will be converted to a
fluorene cation, which has a deep blue color in the solution.
Therefore, the mesitylene solution of the purified (ECAF3)4
or (TCAF3)4 was added with p-toluenesulfonic acid. No color
change was observed in both solutions, confirming that there
is no residual fluorenol in the purified macrocyclics.
The synthetic procedures used to prepare macrocyclic
oligomers (ECAF3)4 and (TCAF3)4 are shown in Scheme 1.
The purity and structure of macrocyclics were also
confirmed by the size exclusion chromatography (SEC),
MALDI-TOF MS, NMR spectroscopy, and element analysis
(see the Supporting Information). After the self-condensation
reaction, the characteristic peak of the C-9 of the central
fluorene units moves from 84.3 to 64.2 ppm in 13C spectra
due to the disappearance of OH group. The SEC curve
displays a narrow peak at 27.95 min for (ECAF3)4 and 27.93
min for (TCAF3)4. Both peaks have a Mn of 3800 Da with a
very low Mw/Mn value (1.03). Because the Mn from SEC
analysis is a polystyrene equivalent value, it is impossible
to assign these peaks on the basis of the SEC data alone.
Therefore, the products were also analyzed by MALDI-TOF
mass spectroscopy. A single peak at m/z 4538.5 for (ECAF3)4
and m/z 4788.3 for (TCAF3)4 corresponds to the macrocyclic
tetramer. All of these results show that the desired compound
could be obtained in high purity and agree well with the
structures of the respective macrocyclic tetramer.
Scheme 1. Synthetic Route of (ECAF3)4 and (TCAF3)4
N-Ethyl-3-bromocarbazole was reacted with n-BuLi in tet-
rahydrofuran at -78 °C, followed by the addition of an
equivalent of 2,7-bis(9,9-dioctylfluorene-2-yl)-9-fluorenone
to result in monomer 1. The oligomer (ECAF3)4 was then
prepared by using self-condensation of the Friedel-Crafts
reaction in mesitylene at 80 °C for 3 h in the presence of
p-toluenesulfonic acid. This polycondensation reaction fol-
lows the aromatic electrophilic substitution mechanism.
Carbazole,5 as well as triphenylamine,4c,6 is a very reactive
The chemical structure of the 3D macrocyclic oligofluo-
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(B3LYP) at the 6-31G(d) level on the model compound of
(MCAF3)4, which simplified the substitutions on the 9-posi-
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Org. Lett., Vol. 13, No. 2, 2011
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tion of fluorenes with methyl to reduce the calculation costs,
since those substitutions have limited influence on the
structural and optoelectronic properties of these compounds.8
The optimized conformation of (MCAF3)4 with energy
minimization shown in Figure 1 suggests that the architecture
Figure 2. (a) UV-vis absorption and PL spectra of (ECAF3)4 and
(TCAF3)4 in cyclohexane; (b) thin film PL spectra of (ECAF3)4;
and (c) thin film PL spectra of (TCAF3)4.
Figure 1. Optimized geometry of macrocyclic oligomer (MCAF3)4,
calculated by B3LYP at the 6-31G(d) level: (left) rear view and
(right) side view (blue ball ) N atom).
in the pure-blue region. The PL efficiencies (ꢀf) of oligomers
were measured in dilute cyclohexane solution with 9,10-
diphenylanthrecene (ꢀf ) 0.90 in cyclohexane) as a refer-
ence. The solution PL quantum efficiency of (ECAF3)4 and
(TCAF3)4 was found to be 0.80 and 0.89, respectively. These
values are higher than those of carbazole-substituted linear
polyfluorene, which showed efficiencies of 0.60-0.80.9
The HOMO and LUMO orbitals of the model compound
(MCAF3)4 were calculated by B3LYP at the 6-31G(d) level
(see Figure 3). Its HOMOs and LUMOs are quite localized
consists of four terfluorene chains as the arms of the H-shape
and four carbazoles as the ring, connecting via completely
rigid spiro linkages with four C-9 sp3 atoms of the central
fluorene units. The arms of terfluorene, like rigid rods, extend
from the ring plane on both sides. We anticipate that this
3D structure would restrict close interchain packing of the
oligomer chains and reduce chain interactions and, conse-
quently, suppress aggregate/excimer formation and enhance
PL efficiency.
The thermal properties of (ECAF3)4 and (TCAF3)4 were
characterized by differential scanning calorimetry (DSC) and
thermal gravimetric analysis (TGA). DSC of the heating/
cooling/reheating scan was measured in nitrogen at a heating
rate of 10 deg/min. The curve reveals no obvious melt
temperature for (ECAF3)4 and (TCAF3)4 below 200 °C, and
only a small glass transition temperature at 132 °C can be
found in the first heating scan in the DSC curve of (ECAF3)4,
indicating that both (ECAF3)4 and (TCAF3)4 have a stable
amorphous structure below 200 °C. TGA analysis indicates
that both oligomers are thermally stable with 5% weight loss
at temperatures over 350 °C.
Figure 3. The computed isodensity surface of HOMO and LUMO
orbitals of (MCAF3)4.
Figure 2 shows the UV absorption and PL spectra of
(ECAF3)4 and (TCAF3)4 in cyclohexane. There is a negligible
shift in absorption and PL spectra for both macrocyclics from
solution to the solid state, indicating the absence of strong
interchain interaction in the solid thin films. The PL spectra
show very little change after annealing in nitrogen at 120
°C for 24 h (see Figure 2). Only relative intensity changes
were observed in the vibronic peaks, whereas the emission
peak positions remain unchanged. Moreover, no additional
PL peak was observed between 500 and 600 nm in the
spectra of (ECAF3)4 and (TCAF3)4 films, as often appear in
the PL spectra of poly(dialkylfluorene) films after thermal
annealing. Both macrocyclics have very strong fluorescence
due to core-shell geometry of the compounds, where the
electron density distribution at HOMOs becomes localized
at the core carbazole units due to the good hole transport
properties of carbazoles, whereas the LUMOs are predomi-
nately determined by H-shaped arms of the terfluorene chain
as shown in Figure 3. In comparison with the frontier obitals
of terfluorene listed in Table 1, the participation of carbazole
in (MCAF3)4 results in increased HOMO, suggesting the
increased hole injection properties of the core-shell com-
pounds. It is expected that these macrospirocyclic oligomers
have a p-type core with high hole injection and transport
properties and a blue light emissive terfluorene shell with
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2004, 16, 2165–2173.
202
Org. Lett., Vol. 13, No. 2, 2011

after several p-doping scans. The decrease in the current on
cycling arises from the fact that (ECAF3)4 and (TCAF3)4 are
not polymers but short oligomers that are somewhat soluble
in their neutral or charged forms in the solvent used.
Traditionally, introduction of carbazole units in conjugated
polymers or organic molecules is found to effectively
enhance the hole-injecting properties of the resulting materi-
als. At the same time, it also causes an increase in the LUMO
energy level, resulting in an increased energy barrier for
electron injection from the metal cathode.9 The existence of
the carbazole units in these macrocyclics does not apparently
show this effect. This result indicates that introducing a
carbazole macrocyclic core into these oligomers promotes
their hole-injection capability, but does not sacrifice their
electron-injection capability. This property should be at-
tributed to the novel structure of the multicarbazole cyclic
core, which is not conjugated with the oligofluorene units
in the oligomers. This property is superior for balancing the
hole and electron injection into the emitting layers.
Table 1. Electrochemical Properties of (ECAF3)4 and (TCAF3)4
compd
EHOMO/eV
ELUMO/eV
Eg/eV
(TCAF3)4
(ECAF3)4
-5.33
-5.28
-5.03
-5.20
-2.28
-2.29
-1.31
-1.36
-3.05
-2.99
-3.72
-3.85
(MCAF3)4
terfluorene11
high geometric separation between the emitters to avoid the
excimer formation, which is one of the important factors that
are responsible for the poor color stability of the fluorene-
based materials.
The HOMO and LUMO energy levels of (ECAF3)4 and
(TCAF3)4 were also investigated by cyclic voltammetry (CV)
(see Figure 4). The ELUMO and EHOMO of a material can be
In summary, two novel macrospirocyclic oligmers with
carbazole and fluorene units were synthesized by using a
simple Friedel-Crafts reaction. The carbazole-based cyclic
core serves as a nonconjugated spacer bearing oligofluorene
arms to form a multi-H shaped structure of the oligomers so
that the optoelectronic properties of the individual oligof-
luorene arm remain relatively unperturbed. Both oligomers
show excellent thermal stability and high photoluminescence
quantum efficiency. Electrochemical analysis and density
functional theory calculations showed that the nonconjugated
carbazole core promotes the hole-injection capability of the
oligomers, but does not sacrifice their electron-injection
capability.
Figure 4. CV curves of (ECAF3)4 and (TCAF3)4.
Acknowledgment. This work was financially supported
by the National Basic Research Program of China under
Grant 2009CB930600, the National Natural Science Founda-
tion of China under Grants 90406021, 20874048, and
20804020, the Fok Ying-Tong Education Foundation under
Grant 111051, the Research Fund for the Doctoral Program
of Higher Education under Grant 20093223110003, and the
Natural Science Foundation of Jiangsu Province of China
under Grant BK2008453. Y.Q. thanks Qingwei Hang of
Nanjing University for NMR analysis and fruitful discus-
sions.
estimated by using the equations EHOMO ) -(Ep′ + 4.38)
eV and ELUMO ) -(En′ + 4.38) eV.9,10 The onset oxidation
potential (Ep′) and onset reduction potential (En′) are 0.90
and -2.09 V for (ECAF3)4 and 0.95 and -2.10 V for
(TCAF3)4, respectively. These values correspond to an EHOMO
of -5.28 eV and an ELUMO of -2.29 eV for (ECAF3)4, and
an EHOMO of -5.33 eV and an ELUMO of -2.28 eV for
(TCAF3)4. Figure 4 also shows that both the p- and n-doping
processes for oligomers are reversible. Under successive
multiple potential scans, the oligomer film only showed a
small decrease in the current intensity in the n-doping
process, and displayed a 15-30% loss of current intensity
Supporting Information Available: Experimental pro-
cedures and NMR, SEC, DSC, and MALDI-TOF-MS
spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
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