Facile synthesis of complicated 9,9-diarylfluorenes based on BF 3·Et2O-mediated Friedel-Crafts reaction

Article abstract of DOI:10.1021/ol061268j

BF₃·OEt₂-mediated Friedel Crafts reaction of 9-phenyl-fluoren-9-ol with electron-rich aromatic substrates to prepare a new family of complicated 9,9-diarylfluorenes is described. The 9,9-diarylfluorenes tethered with various functional substituents, e.g., bulky spiro units, hole-transporting moieties, and fluorescent dyes, will be promising building blocks for the construction of optoelectronic materials.

Full text of DOI:10.1021/ol061268j

ORGANIC
LETTERS
2006
Vol. 8, No. 17
3701-3704
Facile Synthesis of Complicated
9,9-Diarylfluorenes Based on
BF3
Reaction
‚Et2O-Mediated Friedel−Crafts
Ling-Hai Xie,†,‡ Xiao-Ya Hou,† Yu-Ran Hua,† Chao Tang,†,‡ Feng Liu,†
Qu-Li Fan,†,‡ and Wei Huang*,†,‡,§
Institute of AdVanced Materials (IAM), Fudan UniVersity, 220 Handan Road,
Shanghai 200433, People’s Republic of China, Institute of AdVanced Materials (IAM),
Nanjing UniVersity of Posts and Telecommunications, 66 New Mofan Road,
Nanjing 210003, People’s Republic of China, and Faculty of Engineering,
National UniVersity of Singapore, 9 Engineering DriVe 1,
Singapore 117576, Republic of Singapore
wei-huang@fudan.edu.cn; chehw@nus.edu.sg
Received May 25, 2006
ABSTRACT
BF3‚OEt2-mediated Friedel−Crafts reaction of 9-phenyl-fluoren-9-ol with electron-rich aromatic substrates to prepare a new family of complicated
9,9-diarylfluorenes is described. The 9,9-diarylfluorenes tethered with various functional substituents, e.g., bulky spiro units, hole-transporting
moieties, and fluorescent dyes, will be promising building blocks for the construction of optoelectronic materials.
Recently, oligofluorenes and polyfluorenes have been rec-
ognized as very promising classes of organic semiconductors
in the field of organic electronics, especially in OLEDs and
PLEDs.1 9,9-Diarylfluorenes are of great importance because
aryl moieties at the C9 position of fluorene can afford various
functionalities, e.g., improvement of solubility,2 increase of
charge carrier mobility,3 tunable electronic structures,4 and
aggregate suppression to improve morphological stability.5
Two synthetic routes have been utilized to introduce aryl
substituents into the C9 position of fluorene starting from
fluorenone in the literature, as shown in Scheme 1 (route I
Scheme 1. Two Synthetic Routes for the Preparation of
9,9-Diarylfluorene
† Fudan University.
‡ Nanjing University of Posts and Telecommunications.
§ National University of Singapore.
(1) (a) Bernius, M. T.; Inbasekaran, M.; O’Brien, J.; Wu, W. AdV. Mater.
2000, 12, 1737-1750. (b) Scherf, U.; List, E. J. W. AdV. Mater. 2002, 14,
477-487. (c) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem.,
Int. Ed. 1998, 37, 403-428. (d) Pei, Q.; Yang, Y. J. Am. Chem. Soc. 1996,
118, 7416-7417.
(2) Setayesh, S.; Grimsdale, A. C.; Weil, T.; Enkelmann, V.; Mullen,
K.; Meghdadi, F.; List, E. J. W.; Leising, G. J. Am. Chem. Soc. 2001, 123,
946-953.
(3) Wu, C.-c.; Liu, T.-L.; Hung, W.-Y.; Lin, Y.-T.; Wong, K.-T.; Chen,
R.-T.; Chen, Y.-M.; Chien, Y.-Y. J. Am. Chem. Soc. 2003, 125, 3710-
3711.
and route II). Route I provides an effective method to
introduce the two same substituents into the C9 position of
fluorene. Mu¨llen has employed synthetic route I to introduce
two bulky triphenylamine groups into the C9 position of
10.1021/ol061268j CCC: $33.50
© 2006 American Chemical Society
Published on Web 07/26/2006

fluorene.6 Chou has also exploited synthetic route I to achieve
coupling of “Fre´chet-type” dendrons and POSS to fluorene.7
Route II mainly involves an addition reaction of the Grignard
reagent to fluorenone and a Friedel-Crafts reaction of
tertiary alcohol. Wong has achieved a series of efficient large-
gap blue-light emitters and hole-transporting materials with
highly morphological stability via route II.8 However,
Friedel-Crafts reaction has not been fully exploited for the
preparation of 9,9-diarylfluorenes. For example, an alterna-
tive pathway could be utilized for the carbazole derivative
in the literature,9 as shown in Scheme 2.
Table 1. Reaction of Tertiary Alcohol with Different
Substratesa
Scheme 2. Alternative Synthetic Route to the Carbazole
Derivative9
In this contribution, we focus on the BF3‚OEt2-mediated
Friedel-Crafts reaction for the preparation of various opto-
electronic building blocks based on 9,9-diarylfluorenes via
route II. Using our strategy, we have designed and synthe-
sized a series of wonderful functional building blocks by
incorporating bulky spiro structures, hole-transporting car-
bazole, and fluorescent dye pyrene into the C9 position of
fluorene. In addition, multibrominated diarylfluorenes with
nonplanar conformations will be useful monomers for the
hyperbranched architectures and potential interrupting mono-
mers for semiconjugated polymers.
Our expectation that the BF3‚OEt2 complex10 as a Lewis
acid may be an effective catalyst for the Friedel-Crafts
reaction of 9-phenyl-fluoren-9-ol (tertiary alcohol 1) with
aromatic substrates was experimentally confirmed. A 1:2
mixture of thiophene and tertiary alcohol 1 in CH2Cl2 did
(4) (a) Wu, F.-I.; Reddy, D. S.; Shu, C.-F.; Liu, M. S.; Jen, A. K.-Y.
Chem. Mater. 2003, 15, 269-274. (b) Shu, C.-F.; Dodda, R.; Wu, F.-I.;
Liu, M. S.; Jen, A. K.-Y. Macromolecules 2003, 36, 6698-6703.
(5) (a) Yu, W.-L.; Pei, J.; Huang, W.; Heeger, A. J. AdV. Mater. 2000,
12, 828-831. (b) Zeng, G.; Yu, W.-L.; Chua, S.-J.; Huang, W. Macro-
molecules 2002, 35, 6907-6914. (c) Chao, T.-C.; Lin, Y.-T.; Yang, C.-Y.;
Hung, T. S.; Chou, H.-C.; Wu, C.-c.; Wong, K.-T. AdV. Mater. 2005, 17,
992-996.
(6) Ego, C.; Grimsdale, A. C.; Uckert, F.; Yu, G.; Srdanov, G.; Mu¨llen,
K. AdV. Mater. 2002, 14, 809-811.
(7) (a) Chou, C.-H.; Shu, C.-F. Macromolecules 2002, 35, 9673-9677.
(b) Chou, C.-H.; Hsu, S.-L.; Dinakaran, K.; Chiu, M.-Y.; Wei, K.-H.
Macromolecules 2005, 38(3), 745-751.
(8) (a) Wong, K.-T.; Chien, Y.-Y.; Chen, R.-T.; Wang, C.-F.; Lin, Y.-
T.; Chiang, H.-H.; Hsieh, P.-Y.; Wu, C.-C.; Chou, C. H.; Su, Y. O.; Lee,
G.-H.; Peng, S.-M. J. Am. Chem. Soc. 2002, 124, 11576-11577. (b) Wong,
K.-T.; Wang, Z.-J.; Chien, Y.-Y.; Wang, C.-L. Org. Lett. 2001, 3, 2285-
2288. (c) Wong, K.-T.; Liao, Y.-L.; Lin, Y.-T.; Su, H.-C.; Wu, C.-c. Org.
Lett. 2005, 7, 5131-5134. (d) Wong, K.-T.; Chen, R.-T.; Fang, F.-C.; Wu,
C.-c.; Lin, Y.-T. Org. Lett. 2005, 7, 1979-1982.
a Reaction conditions: 0.6, 1.2, or 4.8 mmol of tertiary alcohol, 0.5 mmol
of aryl derivatives, and 0.6, 1.2, or 4.8 mmol of the BF3‚Et2O complex in
100 mL of CH2Cl2 at room temperature for 30 min to 10 h. b The main
products for entries 1, 2, 6, 7, and 8 are 2:1 adducts, and the main products
for entries 4, 5, 9, and 10 are 1:1 adducts.
(9) Wong, K.-T.; Chen, Y.-M.; Lin, Y.-T.; Su, H.-C.; Wu, C.-c. Org.
Lett. 2005, 7, 5361-5364.
react smoothly at room temperature, as shown in Table 1
(entry 1). To our delight, the efficiency of BF3‚OEt2 could
(10) Mitschke, U.; Ba¨uerle, P. J. Chem. Soc., Perkin Trans. 1 2001, 740-
753.
3702
Org. Lett., Vol. 8, No. 17, 2006

be further confirmed by the following reaction of thiophene
derivatives with tertiary alcohol 1. The reaction of 3,3-
bithiophene with 4 equiv of 1 gave the congested tetrasub-
stituted products P3 with a high yield of 69% (entry 3). The
electrophilic substitution reaction illustrated that steric
hindrance showed little impact on the reactivity of tertiary
alcohol 1, which was useful information for the complicated
optoelectronic system, e.g., dendrimers. The fact was also
demonstrated by entry 4, a reaction of a highly sterically
hindered substrate with tertiary alcohol 3 in a good yield of
70%. In addition, this reaction reveals that the dibromo
substituents hardly have an impact on the reactivity of tertiary
alcohol 3. We also prepared the spiro-containing 9,9-
diarylfluorenes P5 and P6 in good yields (entries 5 and 6),
which are potential building blocks for the construction of
stable amorphous molecular glasses.
We then examined Friedel-Crafts reaction of tertiary
alcohol 1 with other electron-rich substrates, e.g., carbazole
and phenyl ether, which indeed exhibits the excellent
reactivity with yields of 74% and 70% (entries 7 and 8),
respectively. We have also attempted the reaction with
weakly electron-rich substrates, such as pyrene and tert-
butylbenzene (entries 9 and 10). As a result, a low yield of
30% was observed for the monosubstituted product P9, and
no P10 was obtained. The main byproduct, 9-phenylfluorene,
in entry 10 was obtained in a yield of 70%, in which the
mechanism is still unknown. Therefore, the BF3‚OEt2-
mediated Friedel-Crafts reactions are more favorable for
strongly electron-rich substrates.
Finally, a stable blue-light emitter 2,5-bis(2,7,-bis([1,1′-
biphenyl]-4-yl)-9- phenyl-fluoren-9-yl) thiophene (TBPFTF)
with nonplanar conformation was constructed by a Suzuki
coupling reaction of 4 equiv of biphenyl boronic acid with
the tetrabrominated P11, which was obtained from the
reaction of thiophene with tertiary alcohol 3 (Scheme 3).
Figure 1. ORTEP drawing (30% probability) showing the X-ray
structure of P9 and the crystal packing.
the two pairs of protons in the fluorene ring. The splitting
phenomenon is probably attributed to nonequivalent magnetic
environments due to steric hindrance among the 9-phenyl-
fluorene groups.
TBPFTF has poor solubility in common organic solvents
except for CHCl3 and THF but has much higher solubility
than the dispiro analogue.12 The optical spectra of TBPFTF
in chloroform are shown in Figure 3. TBPFTF exhibits a
strong π-π* absorption with λmax at 341 nm and two well-
resolved emission peaks at 386 and 405 nm with an optical
band gap (3.19 eV). The thermal properties of TBPFTF were
investigated by differential scanning calorimetry (DSC) and
thermogravimetric analysis (TGA). TBPFTF showed a high
Scheme 3. Divergent Synthetic Route to TBPFTF
The structures of all the above products were confirmed by
1H and 13C NMR, MALDI-TOF-MS, and elemental analysis.
The monosubstituted pyrene (P9) was characterized by X-ray
crystallographic analysis (Figure 1).11 The sterically con-
1
gested P3 shows special splitting signals in the H NMR
spectrum, as shown in Figure 2. It was found that, unexpect-
edly, two double peaks with an integrated ratio of 2:2 instead
of a double peak with four protons are assigned to one of
Figure 2. 1H NMR spectra of P1 and P3 in CDCl3.
Org. Lett., Vol. 8, No. 17, 2006
3703

Figure 3. UV and PL spectra of TBPFTF in CHCl3 solution.
Figure 4. CV curve of TBPFTF.
decomposition temperature up to 470 °C (5% loss weight
temperature) and a high Tg up to 247 °C. Cyclic voltammetry
was performed on the film of TBPFTF deposited on a glassy
yl-fluoren-9-ol and its derivatives with several aromatic
substrates. The results show that the BF3‚OEt2 complex is
an effective catalyst for the Friedel-Crafts reaction of
strong electron-rich substrates. We also prepared a series
of complicated 9,9-diarylfluorenes with nonplanar confor-
mations bearing various functional substituents, and they
are promising candidates for construction of organic semi-
conductors. Further attempts to synthesize special dendr-
imers on the basis of Friedel-Crafts reaction could be
achieved via a rational design, which is currently being
pursued.
+
carbon electrode measured in Bu4 PF6- (0.1 M)/acetonitrile
with a platinum counter electrode and a Ag+/Ag reference
electrode (Figure 4). On the basis of the onset potentials of
the oxidation and reduction, the HOMO and LUMO energy
levels of TBPFTF were estimated to be -5.82 and -2.28
eV, respectively, with regard to the energy level of the
ferrocene reference (4.8 eV below the vacuum level).
Therefore, the electrochemical band gap (3.54 eV) does not
closely match the optical band gap (3.19 eV). In addition,
the HOMO energy level of the TBPFTF is lower than that
reported for POF (-5.8 eV). The results indicate that the
conjugation length of TBPFTF is shorter than that of POF.
In conclusion, we have utilized the BF3‚OEt2 complex
as a Lewis acid catalyst to examine the reactivity of 9-phen-
Acknowledgment. This work was financially supported
by the NNSFC under Grants 60325412, 90406021, and
50428303, the Shanghai Commission of Science and Tech-
nology under Grants 03DZ11016 and 04XD14002, and the
Shanghai Commission of Education under Grant 03SG03.
(11) Crystal data for P9: Crystals were grown from CDCl3. The structure
was solved on a Bruker SMART CCD diffractometer using Mo KR
radiation. C35H22, (Mr ) 442.5): monoclinic, space group P21/c, Dc ) 1.296
g cm-3, a ) 14.922(4), b ) 8.735(2), c ) 17.450(5) Å, R ) 90°, â )
94.143(4)°, γ ) 90°, V ) 2268.5(11) Å3, Z ) 4, λ ) 0.71073 Å, µ )
0.073 mm-1, T ) 297(2) K, R ) 0.0402 for 12 903 observed reflections [I
> 2σI] and Rw ) 0.1017 for all 4460 unique reflections.
(12) Xie, L.-H.; Hou, X.-Y.; Tang, C.; Hua, Y.-R.; Wang, R.-J.; Chen,
R.-F.; Fan, Q.-L.; Wang, L.-H.; Wei, W.; Peng, B.; Huang, W. Org. Lett.
2006, 8(7), 1363-1366.
Supporting Information Available: The experimental
1
procedures for the new compounds and H and 13C NMR,
MS spectra, TGA, DSC thermograms, and an X-ray crystal-
lographic file (CIF) for P9. This material is available free
of charge via the Internet at http://pubs.acs.org.
OL061268J
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Org. Lett., Vol. 8, No. 17, 2006