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O
D
T, 180 °C
solid state
20
6
Fig. 7 Synthesis of hexacene 6 from monoketone precursor 20 [ 32 ]
Fig. 8 Two molecules of hexacene in the unit cell [ 32 ]
molecular oxygen. However, hexacene is also rather stable towards dimerization in
the solid state [ 32 ], a reaction that was previously observed for silylethynyl
substituted hexacenes in the solid state (see below) [ 33 ]. Watanabe et al. [ 32 ]
explain this finding by different packing motifs: in contrast to parent hexacene,
the silylethynyl substituted hexacenes crystallize in a face to face
ˀ
stacking motif
[ 33 ]. This latter motif is expected to favor dimerization [ 32 ].
Watanabe et al. also constructed field effect transistors from hexacene single
crystals [ 32 ]. The authors report an averaged (14 devices) mobility of
0.88 cm 2 V 1 s 1 with a threshold at 34 V and an on/off ratio of 10 4 -10 6 . The
best hole mobility observed was 4.28 cm 2 V 1 s 1 . The conductivity of crystalline
hexacene (2.21
10 4 Sm 1 ) was slightly higher than that of pentacene [ 32 ]. The
hole mobilities of hexacene thin films have been measured very recently [ 34 ].
2.2 Substituted Hexacenes
2.2.1 Silylethynyl Substitution
One way to increase the persistence of reactive molecules is the introduction of
bulky groups that can exert kinetic stabilization by increasing the barriers for
decomposition reactions. In the case of the acenes, the most important of these
reactions are oxidation via endoperoxide formation and dimerization. The intro-
duction of silylethynyl groups was shown to be advantageous in pentacene
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