Modulation of physical properties of ter(9,9-ditolylfluorene) by backbone-embedded heteroarenes

Article abstract of DOI:10.1021/ol060183f

Novel ter(9,9-ditolylfluorene) analogues containing thiophene and pyridine rings embedded as functional constituents within the parent hydrocarbon backbone have been synthesized. These new molecules exhibit highly efficient photoluminescence and high ther

Full text of DOI:10.1021/ol060183f

ORGANIC
LETTERS
2006
Vol. 8, No. 7
1415-1418
Modulation of Physical Properties of
Ter(9,9-ditolylfluorene) by
Backbone-Embedded Heteroarenes
Ken-Tsung Wong,* Tsyr-Yuan Hwu, Akula Balaiah, Teng-Chih Chao,
Fu-Chuan Fang, Ching-Tien Lee, and Ying-Chu Peng
Department of Chemistry, National Taiwan UniVersity, Taipei 106, Taiwan
kenwong@ntu.edu.tw
Received January 23, 2006
ABSTRACT
Novel ter(9,9-ditolylfluorene) analogues containing thiophene and pyridine rings embedded as functional constituents within the parent
hydrocarbon backbone have been synthesized. These new molecules exhibit highly efficient photoluminescence and high thermal and
morphological stability. The electronic structure of the terfluorene backbone is significantly perturbed, which allows modulation of the backbone
energy levels.
π-Conjugated materials are of great interest for organic
electronics including light-emitting diodes (OLEDs), thin film
transistors, and photovoltaic cells. Although π-conjugated
materials possessing pure hydrocarbon main chains usually
do not exhibit properties that are desirable for their use in
efficient devices, their physical behavior can be modulated
dramatically by incorporating electron-rich or electron-
deficient arenes directly into their conjugated backbones.1
Materials prepared by using such a molecular design strategy
offer many alternatives for device engineering. In this paper,
we demonstrate that implanting π-electron-rich thiophene and
π-electron-deficient pyridine moieties into the backbone of
terfluorene facilitates the ready manipulation of its electronic
structure (HOMO/LUMO levels) and consequent physical
properties.
Among the organic π-conjugated materials, terfluorene 1
(Figure 1) is one of the most promising blue light emitters
for highly efficient OLEDs because of its superior physical
properties.2 Direct modification of the π-conjugated backbone
of terfluorene through functional substitution remains a
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Figure 1. Molecular structures of terfluorene (1) and its het-
eroarene-substituted (2 and 3) and bipolar (4) analogues.
10.1021/ol060183f CCC: $33.50
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Published on Web 03/03/2006

challenge; therefore, it is difficult to manipulate the electronic
properties of its backbone. We anticipated that the new
terfluorene analogues 2 and 3 (Figure 1) would drastically
reconstitute the properties of the main chain of 1 for a couple
of reasons: (1) The π-electron-rich thiophene substitutions
should raise the HOMO level and, thus, offer the resulting
new molecule with better capability for hole injection. In
contrast, the π-electron-deficient pyridine substitutions should
lower the LUMO energy, and thus enhance electron injection
capability. (2) Flattening of the functional arene substitutions
through intramolecular cyclization should reduce their con-
formational disorder, maintain effective coplanarity of the
molecular geometry, and preserve the highly efficient photo-
luminescence of the parent terfluorene chromophore.
The synthesis of the π-electron-rich terfluorene analogue
2 is depicted in Figure 2.
Figure 3. Synthesis of the π-electron-deficient terfluorene analogue
3.
For the synthesis of 3, using a similar synthetic pathway
as for 2 was unsuccessful. Therefore, we applied a neighbor-
ing group-assisted lithiation/cyclization3 strategy to construct
the coplanar azafluorenone skeleton and subsequently in-
troduced the peripheral aryl substituents. Thus, the Suzuki
reaction of 2,5-dibromopyridine with 2-(diisopropylcarban-
oyl)phenylboronic acid (9)4 occurred regiospecifically at the
2-position of the pyridine ring to afford the coupled product
10 in 80% yield. The reaction of 10 with the diboronic ester
(8) gave the diamide 11 (58%), which upon treatment with
LDA at 0 °C furnished the diketone 12 in 80% isolated yield.
The tolyl substituents of the 4-azafluorene units of 3 were
then introduced through the addition of 4-methylphenyl-
lithium to 12 and subsequent acid-catalyzed Friedel-Crafts
arylation to afford 3 in 20% yield.
Previously, we described a feasible molecular doping
strategy for terfluorene 1 through the introduction of a 4,5-
diazafluorene unit as a peripheral substituent.5 This strategy
provides a terfluorene derivative that possesses improved
electron injection ability while retaining the photophysical
properties of the terfluorene chromophore. Combining the
backbone modification and spiro-linked substitution strate-
Figure 2. Synthesis of the π-electron-rich terfluorene analogue 2.
The Pd-catalyzed Negishi coupling reaction of ethyl
2-iodobenzoate with thienyl zinc chloride gave ester 5 in
82% yield. Double alkylation of the ester group of 5 with
p-tolylmagnesium bromide and subsequent acid-catalyzed
intramolecular Friedel-Crafts arylation afforded the ditolyl-
substituted indenothiophene 6 in 73% yield. Bromination of
6 with NBS at 0 °C gave the aryl bromide 7 in high yield
(87%). Subsequently, reaction of 7 with the diboronic ester
(8) provided the desired product 2 in an isolated yield of
75%.
Figure 3 presents the synthesis of the π-electron-deficient
terfluorene analogue 3.
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1416
Org. Lett., Vol. 8, No. 7, 2006

Table 1. Properties of Terfluorene 1 and Analogues 2-4
absa
λmax (nm)
PLb
λmax (nm)
Q.Y.c
(%)
Tg
(°C)
Td
(°C)
HOMO/LUMO
(eV)f
d
e
compd
E1/2 oxd
E1/2 red
1
2
3
4
354
407
359
407
393, 420
448, 477
401, 413
448, 474
85
71
72
70
1.35, 1.60
1.05, 1.22
1.51
-1.99, -2.18
-1.88, -2.06
-1.88, -2.06
201
197
159
nd
411
444
524
463
-5.69/-2.18
-5.39/-2.29
-5.85/-2.29
-5.45/-2.34
1.11, 1.26
-1.83, -2.06, -2.27
a In CHCl3. b In CHCl3, upon excitation at the absorption maximum. c In CHCl3, measured by calibrated integrating sphere. d In CH2Cl2 containing 0.1
M nBu4NPF6 as a supporting electrolyte. e In THF containing 0.1 M nBu4NClO4 as a supporting electrolyte. f HOMO and LUMO levels with respect to zero
vacuum level were estimated directly from CVs.
gies, we synthesized a new bipolar terfluorene analogue 4
(Figure 4). Indenothiophene 6 was converted to boronic ester
rings in 1 with five-membered thiophene rings.6 In contrast,
the analogue 3, which contains 4-azafluorene moieties,
Figure 5. Absorption and photoluminescence spectra of terfluorene
1 and its analogues 2 and 3.
possesses only a slightly extended effective conjugation
length of its backbone, as indicated by the relatively smaller
red shifts in its absorption and emission with respect to those
of terfluorene 1. The bipolar analogue 4, possessing a spiro-
linked 4,5-diazafluorene on the central fluorene ring, exhibits
spectroscopic properties very similar to those of the inde-
nothiophene-containing analogue 2 (see Figure S-1, Sup-
porting Information). This result indicates that the spiro-
linked 4,5-diazafluorene moiety does not perturb the electronic
transitions of the main conjugated unit. In addition, the
terfluorene analogues 2-4 exhibit relatively high photolu-
minescence quantum yields (each g70%, but slightly lower
than that of the parent terfluorene 1) because of the coplanar
molecular configurations of their embedded heteroarene units
with respect to their adjacent phenylene rings, and consequent
conformational inflexibility.
Figure 4. Synthesis of terfluorene analogue 4.
(13) in 70% yield, followed by coupling with 4′,7′-diaza-
2,7-dibromo-9,9′-spirobifluorene (14) to form the desired
coupled product 4 in an isolated yield of 75%.
Incorporating the thiophene and pyridine moieties into the
main conjugated chain and making them into coplanar
configurations through intramolecular annulation with their
neighboring phenylene rings renders the resulting molecules
rather rigid and bulky. These properties are highly beneficial
to the thermal stabilities, as indicated by their high decom-
position (5% weight loss) temperatures (Td) and rather high
glass transition temperatures (Tg) (Table 1), determined by
thermogravimetric analyses (TGA) and differential scanning
calorimetry (DSC), respectively. As a consequence, these
novel analogues can form homogeneous and stable amor-
phous filmssan essential property for any potential device
applicationssupon thermal evaporation.
Cyclic voltammetry (CV) was used to probe the electro-
chemical behavior of 1 and its novel heterocyclic analogues
2-4 (Figure 6; Table 1). Indenothiophene-containing ana-
logue 2 exhibits two reversible oxidation potentials at 1.05
The absorption and photoluminescence of the thiophene-
containing analogue 2 exhibit substantial red shifting relative
to those of the parent terfluorene 1 (Figure 5).
This result clearly indicates the improved π-conjugation
that occurred upon replacing the six-membered phenylene
(6) (a) Lee, S. A.; Hotta, S.; Nakanishi, F. J. Phys. Chem. A 2000, 104,
1827. (b) Apperloo, J. J.; Groenendaal, L. B.; Verheyen, H.; Jayakannan,
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J.-L. Chem. Eur. J. 2002, 8, 2384.
Org. Lett., Vol. 8, No. 7, 2006
1417

reduction potentials relative to those of 2 (Table 1, also see
Figure S-2, Supporting Information), possibly because of the
electronic induction effect of the electronegative diazafluo-
rene substituent. Analogue 4 also exhibits a third reduction
potential at -2.27 V, which is ascribed to the reduction
occuring at the peripheral diazafluorene moiety.5
Table 1 lists the HOMO and LUMO energy levels of 1-4
taken directly from the cyclic voltammagrams (see the
Supporting Information for detail). The electronic structure
of the parent terfluorene 1 is perturbed significantly after its
main chain modifications. Implanting the parent backbone
with thiophene rings extends the π-conjugation effectively,
raising the HOMO level in particular. The improved π-con-
jugation also lowers the LUMO level of 2, which leads to a
smaller energy gap. Upon embedding pyridine rings as
constituent moieties of the parent hydrocarbon backbone, this
modification leads to shifts of both the HOMO and LUMO
levels of 3 in the same direction, i.e., to lower levels, with
respect to those of parent terfluorene 1. The results presented
for the bipolar compound 4 indicate that the modulation of
energy levels can be realized independently through either
main chain electronic perturbation or peripheral substitution
through a spiro-linkage.
Figure 6. Cyclic voltammagrams of terfluorene 1 and its analogues
2 and 3.
and 1.22 V (vs Ag/AgCl); these values are significantly lower
than those of parent terfluorene 1 (1.35 and 1.60 V). This
result clearly indicates the significant HOMO-raising effect
of using π-electron-rich thiophene units as constitutional
moieties. The analogue 3, possessing 4-azafluorene moieties
in its main π-conjugated chain, undergoes only one oxidation,
at a higher potential than those of the first oxidations of 1
and 2, presumably because of the π-electron-deficient
pyridine rings rendering oxidation more difficult.7 Surpris-
ingly, both of the indenothiophene- and 4-azafluorene-
containing analogues 2 and 3, respectively, exhibited two
reversible reductions at the same potentials. The lower
reduction potentials of 2 (-1.88 and -2.06 V), relative to
those of 1 (-1.99 and -2.18 V), are consistent with its higher
degree of π-conjugation resulting from incorporation of the
thiophene moieties into the main backbone. The better
π-conjugation of 2 is also partially attributed to the increased
planarity by the reduced steric interactions between the
embedded thiophene rings and the central fluorene moiety.
For 3, however, the lower reduction potentials arise mainly
from the π-electron deficiency of its pyridine rings. The
electrochemical behavior of 2 was further modified upon
introducing the peripheral 4,7-diazafluorene. The dipolar
molecule 4 displays slightly higher oxidation and lower
In summary, we have established efficient syntheses of
novel heteroarene-embedded (2 and 3) and bipolar (4)
terfluorene analogues. The physical properties of the parent
terfluorene are significantly altered upon introducing het-
eroarene units as constituents. The presence of the hetero-
arenes and their coplanarity with adjacent phenylene rings
provides these novel terfluorene analogues with promising
properties. We are presently fabricating light-emitting devices
that incorporate these molecules as active materials and will
reported our results in due course.
Acknowledgment. This study was supported financially
by the National Science Council and Ministry of Economic
Affairs of Taiwan.
Supporting Information Available: Detailed experi-
mental procedures; spectroscopic characterization and NMR
spectra of new compounds; absorption and emission spectra
and cyclic voltammogram of 4. This material is available
free of charge via the Internet at http://pubs.acs.org.
(7) Wiley, R. H.; Callahan, P. X.; Jarboe, C. H.; Nielsen, J. T., Jr.;
Wakefield, B. J. J. Org. Chem. 1960, 25, 366.
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