Synthesis and characterization of highly fluorescent indenofluorenes

Article abstract of DOI:10.1021/ol025972l

(matrix presented) Described here are the synthesis and optical and electrochemical properties of a series of indenofluorenes as new building blocks for electronic and optoelectronic materials.

Full text of DOI:10.1021/ol025972l

ORGANIC
LETTERS
2002
Vol. 4, No. 13
2157-2159
Synthesis and Characterization of
Highly Fluorescent Indenofluorenes
Samuel Merlet, Maria Birau, and Zhi Yuan Wang*
Nortel Networks-Carleton UniVersity Laboratory for AdVanced Materials Research,
Department of Chemistry, Carleton UniVersity, 1125 Colonel By DriVe, Ottawa,
Ontario, Canada K1S 5B6
wangw@ccs.carleton.ca
Received April 4, 2002
ABSTRACT
Described here are the synthesis and optical and electrochemical properties of a series of indenofluorenes as new building blocks for electronic
and optoelectronic materials.
Organic materials such as the semiconducting π-conjugated
compounds (e.g., pentacene)1,2 suitable for applications in
electronics (e.g., thin film transistor) and optoelectronics (e.g.,
organic light emitting diodes, OLED) have received increas-
ing attention during the past years. The research currently
focuses on the development of organic semiconductors with
a good stability to high temperature, air, and chemicals.
Alkylated fluorenes and indenofluorenes are known to
have good fluorescence properties and have also been
investigated for the OLED applications.3-5 A major problem
encountered in fluorescent planar molecules or polymers is
the formation of π-aggregates/excimers, which leads to an
additional emission band in long wavelength and a decrease
in electroluminescence. This problem is mainly solved by
introduction of long or branched side chains (e.g., octyl
groups on the bridging methylene of fluorene and indenof-
luorene) and copolymerization with a suitable bulky comono-
mer. For example, it has been demonstrated that incorpora-
tion of the octyl groups at 6,6′,12,12′-positions and a small
amount of 9,10-anthracendiyl moiety into a rigid-rod inde-
nofluorene polymer can effectively suppress the aggregation
tendency of the indenofluorene segments.6
To increase the steric hindrance, we have designed a new
substituted indenofluorene, 5,11-diphenyl-6,12-dihydroindeno-
[1,2b]fluorene (1). It is also of great interest to synthesize
the monosubstituted and unsubstituted analogues (2 and 3)
to complete the series (Scheme 1). Moreover, it should be
feasible to further functionalize these indenofluorene com-
pounds for thin-film transistor and OLED applications.
Scheme 1
(1) Hide, F.; Diaz-Garcia, M. A.; Schwartz, B. J.; Heeger, A. J. Acc.
Chem. Res. 1997, 30, 430.
(2) Katz, H. E.; Bao, Z.; Giat, S. L. Acc. Chem. Res. 2001, 34, 359.
(3) Reisch, H.; Wiester, U., Scherf, U.; Tuytuylkov, N. Macromolecules
1996, 29, 8204.
(4) Ranger, M.; Rondeau, D.; Leclerc, M. Macromolecules 1997, 30,
7686.
(5) Inaoka, S.; Advincula, R. Macromolecules 2002, 35, 2426.
10.1021/ol025972l CCC: $22.00 © 2002 American Chemical Society
Published on Web 06/06/2002

Scheme 2a
Scheme 3a
a Reagents and conditions: (i) water, K2CO3, Bu4NBr, Pd(OAc)2,
70 °C, 2 h, 98%; (ii) KMnO4, pyridine, water, reflux, 24 h, 96%;
(iii) concentrated H2SO4, 25 °C, 2 h, 90%; (iv) N2H4‚H2O, KOH,
diethylene glycol, reflux, 48 h, 76%.
a Reagents and conditions: (i) dichlorobenzene, reflux, 12 h,
90%; (ii) glacial AcOH, HBr, reflux, 48 h, (1b, 78%; 2b, 78%);
then concentrated H2SO4, 3 h, rt, (1c, 90%; 2c, 88%); (iii)
N2H4‚H2O, KOH, diethylene glycol, reflux, 24 h, (1, 95%; 2, 98%).
λmax and the photoluminescence efficiency are relatively
insensitive to the phenyl substituent and mainly arise from
the indenofluorene moiety. However, the pendent phenyl
groups could influence the emission wavelength, as com-
pounds 1 and 2 with one and two phenyl groups showed a
red-shifted emission compared with that of 3 (Table 1). The
longer emission wavelength of 1 and 2 reveals the contribu-
tion of phenyl substituents to a more conjugated excited state,
in contrast with a twisted ground state. It can also be seen
that the sharpness of emission peak is affected by the phenyl
substituent (Figure 1).
As described in Scheme 2, compound 1 was synthesized
starting with a Diels-Alder addition of the cyclone 1a7 and
diphenylacetylene to give the tetraphenyl diester 1b in 90%
yield. The hydrolysis of 1b was performed in acetic acid
with hydrobromic acid to give the corresponding acid, which
was cyclized quantitatively in the diketone 1c. It was noticed
that the cyclization was regiospecific and gave only one
isomer. The steric hindrance and torsion angles of the
terphenyl moiety probably play a dominant role in this
cyclization. The subsequent Wolf-Kirshner reduction af-
forded 1 in 86% yield.
Cyclic voltammograms of 3 exhibited two quasi-reversible
reduction peaks of respective potential (vs SCE) E1red
)
-2.6 V and Er2ed ) -2.9 V and one quasi-reversible
oxidation peak of potential Eo1x ) 1.25 V (Figure 2 and
Table 1), whereas 1 and 2 exhibited only one quasi-reversible
Compound 2 was synthesized via the same route as 1 using
phenylacetylene instead of diphenylacetylene. In this case
the cyclization gave two isomers, which had to be separated
by fractional recrystallization.
As shown in Scheme 3, compound 3 was synthesized using
a modified Ebel method.8 The aqueous Suzuki coupling
between 2,5-dibromo-p-xylene and phenylboronic acid gave
3a in 98% yield,9 which was oxidized using potassium
permanganate in aqueous pyridine to yield the corresponding
acid. The cyclization and the subsequent reduction afforded
3 in overall 64%.
Figure 1 displays the UV-vis and photoluminescence (PL)
spectra of 1-3 in solution. The absorption spectra of 1-3
are similar, displaying an absorption maximum around 329-
334 nm (Table 1). It is expected that the absorbance from
the indenofluorene moiety remains the same while the peak
at 220 nm decreases as the number of pendent phenyl groups
decreases from 2 to 1 and 0.
Compounds 1-3 are highly fluorescent with a quantum
yield of 73-78%. These results indicate that the absorption
(6) Marsitzky, D.; Scott, J. C.; Chen, J. P.; Lee, V. Y.; Miller, R. D.;
Setayesh, S.; Muellen, K. AdV. Mater. 2001, 13, 1096.
(7) Qi, Y.; Wang, Z. Y. Macromolecules 1994, 27, 625.
(8) Ebel, F.; Deutschel, W. Ber. Dtsch. Chem. Ges.. 1956, 89, 2794.
(9) Badone, D.; Baroni, M.; Cordamone, R. J. Org. Chem. 1997, 62,
7170.
Figure 1. UV-vis absorption (24 µM in cyclohexane for 1 and 2,
in Decalin for 3) and PL of 1 (0.6 µM in cyclohexane), 2 (0.6 µM
in cyclohexane), and 3 (0.6 µM in Decalin) excited at 329-334
nm.
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Org. Lett., Vol. 4, No. 13, 2002

Table 1. Optical and Electrochemical Properties of 1-3
Er1ed (V)b
Er2ed (V)b
Eo1x (V)d
λ
maxabs (nm)/log ꢀ
λ
maxem (nm)
quantum yield (%)a
∆Eabs (eV)
vs SCE
vs SCE
vs SCE
∆Eec (V)
1
2
3
329/4.83
331/4.76
333/4.83
375
364
340, 356
74
78
73
3.77
3.75
3.71
-2.2
-1.8
-2.6
-2.40c
-2.2c
-2.9
1.25
3.85
a Calculated in reference to quinine sulfate (1 µM in 1 N H2SO4). b Performed in 0.1 M solution of n-Bu4NPF6 in DMF, carbon electrode as the working
c
electrode, scan rate of 200 mV s-1, ferrocene as an internal standard. Epc. d Performed in 0.1 M LiClO4 in acetonitrile.
reduction wave of potential Er1ed ) -2.2 and -1.8 V,
It is likely that the radical anion obtained after the first
reduction is transformed into a dianion that is protonated in
DMF, which may contain a trace amount of water.
The presence of the phenyl group obviously made the
electrochemical reduction much easier, as reduction potentials
are increased from -2.6 to -2.2 V for 3 and 1, respectively.
Furthermore, compounds 1-3 have a similar optical band
gap of 3.7 eV (Table 1). It is of interest to compare the
electrochemical band gap with the optical one,10 as the two
∆E values are quite close for 3. Unfortunately ∆Eec cannot
be obtained for both 1 and 2, since their oxidation potentials
are out of the solvent window.
respectively. They both showed an irreversible second
reduction step, suggesting that the ionic species are either
very stable or transformed into the electro-inactive species.
In summary, we have demonstrated efficient methods for
the synthesis of a new series of indenofluorenes. These
indenofluorenes are large band-gap compounds with high
PL in solution and thus show a great potential as optoelec-
tronic materials. Further electrochemical and solid-state
studies are under way and will be reported in due course.
Improvements of physical properties are under investigation,
along with the functionalization of such compounds.
Acknowledgment. This work was supported by NSERC
and Nortel Networks. Financial support was provided by
Nortel Networks for a research studentship to S.M.
Supporting Information Available: Experimental pro-
cedures and characterization data. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL025972L
Figure 2. Cylcic voltammograms of 1-3: reduction peaks for 1
and 2; reduction and oxidation peaks for 3.
(10) Wang, L.; Ng, M. K.; Yu, L. Phys. ReV. B 2000, 62, 4973.
Org. Lett., Vol. 4, No. 13, 2002
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