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Si
Si
O 2 /h
n
O
O
S i
Si
44
45
Fig. 23 Endoperoxide formation upon irradiation of 44 in presence of oxygen [ 56 ]
from optical spectroscopy (1.35 eV, from the onset of the absorption). The HOMO
and LUMO energies were given as
3.5 eV, respectively.
Qu and Chi added four electron-withdrawing p -trifluoromethylphenyl groups in
addition to two TIPS-ethynyl groups to the heptacene core [ 57 ]. The synthesis of
the required substituted heptacenequinone 49 was achieved by a Diels-Alder
reaction of the transient isonaphthofuran with benzoquinone followed by dehydra-
tion. Addition of the TIPS-acetylide and subsequent oxidation with SnCl 2 /H 2 O
furnished the desired substituted heptacene 50 (Fig. 24 )[ 57 ].
The substituents in 50 are increasing the stability of the heptacene core consid-
erably. Half-lives of 1950, 200, and 100 min were estimated for oxygen-free
toluene solutions of 50 under ambient light, white light (100 W), and UV light
(4 W), respectively [ 57 ]. Even in the presence of oxygen, the heptacene 50 could
still be detected after 47 h in toluene [ 57 ].
From electrochemical investigation of 50 the authors concluded that reversible
reductions to the radical anion and the dianion and a reversible oxidation to the
radical cation are possible [ 57 ]. The electrochemically determined HOMO and
LUMO values are
4.8 and
3.61 eV, respectively. The electrochemical energy
gap of 1.32 eV is in agreement with the optical gap (1.35 eV from the onset of
absorption,
4.93 and
870 nm) [ 57 ]. In summary, the four CF 3 groups considerably
stabilize the heptacene by decreasing the energies of the frontier orbitals.
λ max ¼
3.2.2 Aryl Substituted Heptacenes
The Wudl group investigated the ability of phenyl groups to stabilize the heptacene
core (Fig. 25 )[ 56 ].
Neither the 5,7,9,14,16,18-hexaphenylheptacene 51 nor the 5,9,14,18-
tetraphenyl-9,16-bis[4-(2-ethylhexyloxy)phenyl]heptacene 52 could be isolated
and characterized [ 56 ]. The former is barely soluble and easily oxidized, but the
presence of the characteristic absorption in the NIR region can be considered as
evidence for its (transient) existence, even if the typical vibronic transitions of the
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