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.
(3) Fu, J.-M.; Zhao, B.-P.; Sharp, M. J.; Snieckus, V. J. Org. Chem.
1991, 56, 1683.
(2) (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. (b) 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.
(4) (a) Fischer, M.; Shi, Y.; Zhao, B.-p.; Snieckus, V.; Wan, P. Can. J.
Chem. 1999, 77, 868. (b) Laufer, R. S.; Dmitrienko, G. I. J. Am. Chem.
Soc. 2002, 124, 1854.
(5) Wong, K.-T.; Chen, R.-T.; Fang, F.-C.; Wu, C.-C.; Lin, Y.-T. Org.
Lett. 2005, 7, 1979.
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