2,2′,5,5′-tetrakis(5′′′-hexyl-2′′,2′′′-bithien-5′′-yl)-1,1′-biphe-
nyl (DHBPT-SC)sare shown in Figure 1.
showing a quasi-reversible oxidation wave in their cyclic
voltammograms. The resulting condensed napthodithiophene
systems could be obtained with a yield of up to 93% by
using FeCl3 in CH2Cl2/CH3NO2 as oxidizing agent (Scheme
1).13 Cao et al. described the cyclization of a tetrathienyl-
Scheme 1. Oxidative Cyclization using FeCl3 as Oxidizing
Agent after (a) Swager et al13 and (b) Cao et al.14
Figure 1. Chemical structures of the investigated swivel-cruciform
oligothiophenes DHBPT-SC and DHPT-SC and the linear oligomer
DHTTPTT.
For future OFET applications, high charge carrier mobili-
ties of the active layers are mandatory. The charge carrier
mobility strongly depends on the orientation of the molecules
in the film and the amount of interactions between their
π-systems. One principle for increasing the π-interaction is
the planarisation of the molecules, as realized and proven in
highly condensed aromatic hydrocarbons like pentacene or
hexabenzocoronene.4-6 But the often observed low solubility
of such highly crystalline substances complicates the pro-
cessing of these substances.7,8
One concept for the synthesis of highly condensed,
aromatic systems is the oxidative cyclization of suitable
oligoarylene precursors. For this purpose, different chemical
and photochemical methods with a variety of oxidizing
agents have been developed. Such reactions have been
successfully applied in ring closure reactions of oligoarylenes
containing thienyl-units.9-14 FeCl3 was established as one
of the most powerful oxidizing agents for this kind of
reactions. Hereby, Swager et al. investigated the intramo-
lecular cyclization of several bisthienylbenzene derivates
dithienobenzene oligomer. They also used FeCl3 in CH2Cl2/
CH3NO2 as the oxidizing agent and achieved the correspond-
ing doubly condensed arylene system with a yield of 83%
(Scheme 1).14 Roncali et al. have reported photochemical
cyclizations of 2-(thien-3-yl)-3-(thien-2-yl)thiophenes to
benzo[1,2-b:3,4-b′:5,6-b′′]trithiophenes with I2/O2 as the
oxidizing agent.12
On the basis of these observations, we also expected the
occurrence of such oxidative ring-closing reactions in our
swivel-cruciform oligothiophenes under formation of higher
condensed aromatic systems. Therefore, the electrochemical
oxidation of the two oligothiophenes DHPT-SC and DPBPT-
SC was first investigated by cyclic voltammetry CV (solvent:
CH2Cl2, electrolyte: n-Bu4NPF6, Ar-atmosphere, electrode:
Pt, Fc/Fc+ as internal standard, scan rate: 0.1 V/s; see Figure
2).15 In the cyclic voltammogram of DHPT-SC two oxidation
peaks (E1 ) 0.415V; E2 ) 0.650V, referenced vs Fc/Fc+)
and the corresponding reduction peaks of oxidized interme-
diates (E3 ) 0.555V; E4 ) 0.335V) are detectable thus
demonstrating a (quasi) reversible oxidation of the swivel
cruciform molecules under formation of fairly stable oxida-
tion products.2
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