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C. Moussallem et al. / Tetrahedron Letters 56 (2015) 5116–5119
Figure 1. Frontier orbitals of compound 1b.
destabilization both of the HOMO and LUMO levels. For 1c and 1d,
the cyano and ester groups are in the plane defined by the benzod-
ifuran units (Fig. S1 in Supplementary data). The comparison of the
HOMO and LUMO levels of 1b–d reveals the effect of the electron
acceptor characters of the substituent grafted on the benzodifuran
unit. Cyano groups procure higher stabilization of the HOMO and
LUMO levels than ester and pentafluorophenyl groups.
Nevertheless, the replacement of the electron donor groups on
thiophene units and electron acceptor groups on the central BDF
moiety leads to a translation of the HOMO and LUMO levels with-
out modifying the gap between the two levels about 2.7 eV.
As shown in Figure 2 for 1d, the UV–vis absorption spectra of
1a–d in CH2Cl2 exhibit an absorption band with vibronic fine struc-
tures that are characteristic of rigid conjugated systems. The sub-
stituents grafted on the benzodifuran or thiophene units have a
limited effect on the absorption band with kmax in 476–480 nm
range. Films of 1a–c have been deposited on glass by the spin coat-
ing technique using chloroform, dried on anhydrous Na2CO3, as the
solvent. The UV–vis absorption spectra of the films present a 90 nm
red shift of the absorption bands compared to the solution spectra
and an enhancement of the vibronic bands with the apparition of
well distinct maxima. Such behavior is characteristic of strong
the supporting electrolyte. The various substituents induce strong
modifications of the electrochemical properties. Although diamin-
oderivative 2a presents two reversible oxidation waves, compound
1a shows an irreversible oxidation peak at 1.18 V and compounds
1b–d with dialkoxythiophene units a reversible oxidation peak at
potential inferior to 1V (Fig. 3). Thus, the electron donor effect of
alkoxy groups favors the oxidation of 1b–d and strongly stabilizes
the oxidized states. A comparison of the data of compounds 1b–d
shows that the oxidation potentials ranging from 0.71 V for 1b
(R1 = AC6F5) to 0.75 V for 1d (R1 = ACOOBut) then to 0.91 V for
1c (R1 = ACN), follow the electron withdrawing character of the
substituents grafted on the central BDF unit.
Moreover the analysis of the oxidation processes shows that the
difference
peaks of the reversible oxidation waves is 70–80 mV for 2a and
decreases to 40–50 mV for 1b–d. Thus for 2a, the values of Ep
DEp between the anodic (Eaox) and the cathodic (Ecox)
D
are superior to 60 mV indicating that the two oxidation waves cor-
respond to mono electronic process with the successive formation
of radical-cation and dication. For 1b–d, the
DEp values are smaller
than 60 mV expected for a mono-electronic process and we can
stipulate that the oxidation of compounds 1b–d gives directly
the dication state.28,29
intermolecular interactions without presenting strictly parallel
p
-
By comparison with the diamino benzodifuran derivatives, the
lengthening of the conjugated chain with the azomethine junctions
isoelectronic of ethylenic bonds17 leads to a large electronic delo-
calization.30,31 Thus thiophene–imine–benzodifuran–imine–thio-
stacking between the molecules. Annealing tests of the films have
been performed at temperatures varying from 70 to 150 °C during
10 min. Except for 1b that presented a rapid degradation of the
films from 80 °C, the other derivatives did not show any evolution
of the spectra of the films before 130 °C.
phene systems correspond to
a large extended conjugated
system favouring the access to the dication state. On the other
hand, as already observed for extended oligothiophenes based on
ethylene-dioxythiophene (EDOT) units,32 alkoxy groups grafted
The electrochemical properties of 1a–d have been studied by
using cyclic voltammetry in CH2Cl2 in the presence of Bu4NPF6 as
Figure 2. Normalized UV–vis absorption spectra of 1d in solution in CH2Cl2 (solid
line) and film of 1d deposited on glass (dashed line).
Figure 3. Voltamperogramms of 2a (red), 1b (blue) and 1c (black), 10ꢀ3 molꢁlꢀ1 in
0.1 M Bu4NPF6/CH2Cl2, v = 100 mVꢁsꢀ1, E (V) versus SCE.