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Scheme 27 Original synthesis of 6,12-dihydroindeno[1,2- b ]fluorene (26)[ 26 ]
groups in 97h-j necessitated the addition of a small amount of trifluoroacetic acid
in order for the reaction to proceed in timely fashion.
The absorption spectra for 87a-j displayed a greater variation in
l max values
than 96a-i due to the higher amount of orbital density found on the 6- and
12-positions of the IF core. Cyclic voltammetry data for 87a-j showed that the
diaryl IFs exhibit redox amphoterism, a trait not seen in the previous fully conju-
gated indeno[1,2- b ]fluorenes. This is easily observed with 6,12-diphenyl [1,2- b ]IF
87a, as it exhibited two reduction peaks and three oxidation peaks. The reduction
and oxidation values for 87a-j corresponded to HOMO and LUMO energies that
range between
4.0 eV, respectively. Coincidentally,
87e displayed reduction and oxidation potentials within 0.03 V that of 88.
Single crystal OFETs were fabricated with microcrystals of 87j as the active
channel [ 77 ]. With Au source/drain contacts and Au electrodes, the OFET showed
ambipolar charge transport, with saturation hole and electron mobilities of
7
5.5 to
5.8 eV and
3.6 to
10 -4 and 3
10 -3 cm 2 /V
s, respectively.
Concurrent with the Haley studies, Yamashita et al. reported the synthesis
and properties of 6,12-diarylindeno[1,2- b ]fluorenes 87k-m as well as 87a
(Scheme 26 )[ 78 ]. While their molecules exhibited redox amphoterism as well,
the Japanese authors reported only a single oxidation and reduction wave for each.
Vapor-deposited thin film OFETs of 87k,l also showed ambipolar charge transport
but at diminished mobilities (hole: 1.9
10 -5 and 1.1
10 -5 cm 2 /V
s, respec-
10 -6 and 1.6
10 -6 cm 2 /V
tively; electron: 8.2
s, respectively).
3.4 Other Indeno[1,2- b ]fluorenes
First synthesized by Deuschel in 1951 via dehalogenation of 23 (Scheme 27 )[ 26 ],
6,12-dihydroindeno[1,2- b ]fluorene (26) can be viewed as a planarized p -terphenyl
derivative. As a result of the enforced geometry, there is greater conjugation
between the phenyl rings, which imparts more desirable emissive properties; thus,
this core unit has been utilized in a wide variety of molecular and polymeric
systems - far too many to be reviewed here. Rather, selected, salient examples
will be presented.
Common synthetic routes to 26 typically involve reduction of dione 22. Another
intriguing method, however, involves the use of palladium with N -heterocyclic
carbene ligand L (Scheme 28 )[ 79 ]. Presumably, the cascade begins with oxidative
addition of the aryl halide to the catalyst. Activation of the benzylic C-H bond for
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