Electroactive dendritic π-conjugated oligothienylenevinylenes

Article abstract of DOI:10.1039/a807810f

Electroactive π-conjugated oligothienylenevinylenes end-capped with dendritic branches of generation one to three have been synthesized.

Full text of DOI:10.1039/a807810f

Electroactive dendritic p-conjugated oligothienylenevinylenes
Isabelle Jestin, Eric Levillain and Jean Roncali*
Ingénierie Moléculaire et Matériaux Organiques, CNRS UMR 6501, Université d’Angers, 2 Bd Lavoisier, 49045
Angers, France. E-mail: jean.roncali@univ-angers.fr
Received (in Cambridge, UK) 7th October 1998, Accepted 30th October 1998
Electroactive p-conjugated oligothienylenevinylenes end-
capped with dendritic branches of generation one to three
have been synthesized.
Reaction between 5 and 3,5-dihydroxybenzyl alcohol in the
presence of K2CO3 and 18-crown-6 afforded the benzylic
alcohol of the first generation (G1-OH) which was then
converted into the bromomethyl derivative (G1-Br) by reaction
with PBr3. Repetition of the procedure followed by re-
crystallization at each step gave successively G2-Br and G3-Br
in high yields. Reaction of G1-Br and G2-Br with diethyl
phosphite in the presence of NaH led to phosphonates G1-W
and G2-W while G3-W was prepared by reaction of G3-Br
with phosphite (yields ! 90%). Compounds 1–3 were obtained
in 50–60% yield by double Wittig–Horner olefination of
dialdehyde 4 with phosphonates Gn-W. The target compounds
were fully characterized by 1H and 13C NMR analyses,
elemental analyses, FAB and MALDI-TOF mass spectrosco-
pies.†
Compounds 1–3 exhibit identical UV–VIS spectra with lmax
at 542 nm and emax = 100000. Owing to the extension of the
conjugation length by the grafting of two styryl groups, these
values are larger than those for the 4TV core and are close to
those for 6TV.1 All spectra exhibit a well-resolved vibronic fine
structure which indicates that the increase of Gn does not alter
the planarity and rigidity of the p-conjugated backbone.
Compounds 1–3 show identical cyclic voltammograms (CV)
with two reversible one-electron oxidation waves correspond-
ing to the formation of the cation radical and dication at redox
potentials E°1and E°2 of 0.53 and 0.65 V (Fig. 1). Both E°1and
E°2 values were fully independent of Gn. Furthermore, the
linear dependence of the intensity of the first anodic peak of
compound 3 vs. the square root of scan rate between 25 and
5000 mV s21 shows that the charge-transfer process between
Thienylenevinylene oligomers (nTVs) have recently emerged
as a new class of extensively p-conjugated systems.1 Whereas
at a molecular level nTVs are interesting models of molecular
wires, they also open interesting potentialities as advanced
materials for electronic and photonic applications. Progress in
these areas implies a supramolecular control of interchain
interactions in order to develop at will either compact materials
with high electron mobility or, in contrast, single molecular
wires.
We report here the synthesis of monodisperse electroactive
nTVs end-capped with dendritic chains2 and preliminary results
on their electrochemical behavior. Previous examples of
incorporation of linear p-conjugated systems within dendritic
structures involved dendrimers with poly(p-phenylene)3 and
oligotriacetylenes4 cores.
In order to analyze the effect of progressive steric shielding of
the nTV system, Fréchet type dendrons5 of generation 1, 2 and
3 have been attached at both ends of a tetrathienylenevinylene
(4TV) derived from 3,4-dihexylthiophene to afford the target
compounds 1–3. While the construction of dendrons from
1-trifluoromethyl-4-bromomethylbenzene 5 is based on the
Fréchet type convergent approach,5 the originality of our
strategy lies in the conversion of each generation of dendron
into a phosphonate (Gn-W) with final assembly of the target
system via a two-fold Wittig–Horner olefination of dialdehyde
4 with phosphonates G1-W, G2-W and G3-W (Scheme 1).
R3
R3
R3
R3
R2
O
R3
O
R3
O
O
O
O
R2
O
R1
O
O
O
O
O
O
O
O
R3
R2
O
O
R3
R3
1
2
3
S
S
S
S
O
O
O
O
R1
R2
O
O
O
O
R3
O
R2
O
R3
O
R3
R3
R3
1 R1 = CF3
2 R1 = H, R2 = CF3
R3
3 R1 = R2 = H, R3 = CF3
Chem. Commun., 1998, 2655–2656
2655

F3C
metal centers encapsulated in dentridic structures for which the
increase of Gn leads to a progressive decrease of the
reversibility of the oxidation and/or reduction process and to a
positive and/or negative shift of the corresponding potential.6
The independence of the electroactivity of the p-conjugated
system on Gn observed here may be related to the length of the
electrophore which prevents complete steric burial within the
dendritic structure. We also observed that the E°1 value for 3
was independent of substrate concentration in the range of 5
O
O
OH
Br
i
F3C
5
F3C
1025 to 5 1023
. This result is consistent with an absence of
M
follow-up reaction of the cation radical such as formation of a p-
dimer;7 however, this hypothesis needs further confirmation.
Work is now underway to extend this approach to longer
nTVs and to analyze in more detail the effects of the dendritic
branches on inter-chain interactions.
G1-OH
G1-Br
ii
i
G2-OH
G2-Br
G2-W
ii
i
iii
G1-W
G3-OH
G3-Br
G3-W
iii
ii
iii
Hex
OHC
Hex
Hex
Hex
S
S
S
S
CHO
Hex
Notes and references
Hex
Hex
Hex
† Selected data for 3: Mp 201 °C; dH(CDCl3) 7.61 (d, 32H, 3J 8.2), 7.50 (d,
32H, 3J 8.1), 7.19 (d, 2H, 3J 15.7), 7.02–6.98 (m, 6H), 6.78 (d, 2H, 3J 15.5,
2), 6.72 (d, 4H, 4J 2.1), 6.68 (d, 8H, 4J 2.0), 6.67 (d, 16H, 4J 2.1), 6.53–6.50
(m, 14H), 5.07 (s, 32H), 4.96 (s, 24H), 2.58 (m, 16H), 1.56–1.29 (m, 64H),
0.94–0.86 (m, 24H); m/z (MALDI-TOF) 5344.56 (M + 2) [Calc. (found): C,
66.39 (66.96); H, 5.25 (5.20); F, 16.85 (17.06); S, 2.32% (2.40%)].
4
iv
1–3
Scheme 1 Reagents and conditions: i, 3,5-dihydroxybenzyl alcohol,
K2CO3, 18-crown-6, acetone; ii, PBr3, toluene; iii, G1-W, G2-W,
HPO(OEt)2, NaH, THF, G3-W, P(OEt)3, reflux; iv, Gn-W, ButOK, THF.
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Chem. Soc., 1998, 120, 8150.
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Fig. 1 Cyclic voltammogram for 3 (1024
electrodes, scan rate 100 mV s21
M) in 0.5 M Bu4NPF6/CH2Cl2, Pt
.
the electrode and the core electrophore is not subject to kinetic
limitation. This behavior contrasts with that of redox active
Communication 8/07810F
2656
Chem. Commun., 1998, 2655–2656