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the exocyclic dicyanovinylene groups contorted the IF core by 13
pointing away
from the central alkyl chains. [1,2-
b
]IF dicyanovinylenes 83a-c and 84a-c were
utilized in thin-film transistors and showed n-type and ambipolar transport. Notably,
83b showed stable n-type transport in air with mobilities as high as 0.16 cm
2
/V
s.
3.3 Fully-Conjugated Indeno[1,2-
b
]fluorene
First mentioned by Deuschel as a possible resonance structure of dione 22, a fully
conjugated indeno[1,2-
b
]fluorene (2, Fig.
5
) might serve as a suitable small
molecule analogue of fullerene due to its low-lying LUMO level, subsequently
shown by Haley et al. [
53
,
60
]. Furthermore, 2 possesses no
s
-
cis
diene linkages in
its structure, meaning that it should be less susceptible to the deleterious cycload-
dition pathways that acenes and fullerenes are susceptible to. Until recently,
however, there has been skepticism of the existence of fully conjugated IF species
for a number of reasons: (1) the formation of 2 should be energetically costly due to
the necessary disruption of the aromaticity of the central benzene ring; (2) IF
2 would possess 20
-electrons, resulting in the formation of an unfavorable
antiaromatic species; and (3) the central rings of 2 would also host a
p
-xylylene
(
p
-quinodimethane) core, a notoriously reactive moiety that typically cannot be
isolated because of its high tendency to oligomerize/polymerize.
As previously mentioned, Swager and coworkers obtained diones 57a-c through
the iodine-induced transannular cyclization of 24 and 56b,c in aerobic conditions
(Scheme
12
)[
49
]. Under anaerobic conditions, however, another class of products
formed that possessed only two aromatic signals in the
1
H NMR spectrum that were
shifted upfield from 57a-c. Furthermore, they found that exposure to air converted
these compounds into 57a-c in near quantitative yield. Swager deduced the
air-sensitive intermediates to be fully conjugated 5,6,11,12-tetraiodoindeno[1,2-
b
]
fluorenes 85a-c, given their highly reactive nature as well as the notable up-field shift
of the proton resonances compared to 57a-c due to their antiaromatic character
(Scheme
23
). The UV-vis spectrum of 85a exhibited low energy absorptions of
534 and 571 nm, nearly 250 nm bathochromically shifted from 57a, attributable to its
fully conjugated state.
Studies by Scherf and coworkers in 1996 on the design and structural properties
of indeno[1,2-
b
]fluorene polymers (86a) came to a similar conclusion [
61
,
62
].
Their argument, based on structurally related 9,9
0
-bisfluorenes, was that in order to
reduce steric hindrance between subunits, the bridging olefinic carbons could
lengthen and hence reduce the overall double bond character (86b, Fig.
6
)[
61
,
63
]. A consequence to such a bond lengthening would be an overall decrease in
band gap energy and geometric distortions. Another consequence would be an
overall change of the bond structure of the subunit itself. As a model system for
86b,IF87a was prepared and reported to exhibit an absorption spectrum
p
l
max
of
543 nm; however, no other characterization was reported.
