Strong π-electron donors based on a self-rigidified 2,2′-bi(3,4-ethylenedioxy)thiophene-tetrathiafulvalene hybrid π-conjugated system

Article abstract of DOI:10.1016/S0040-4039(02)02702-8

Strong π-donors based on the association of TTF and 2,2′-bi(3,4-ethylenedioxy)thiophene conjugating unit have been synthesised; optical and an X-ray data show that intramolecular interactions lead to full rigidification of the conjugated system.

Full text of DOI:10.1016/S0040-4039(02)02702-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 649–652
Strong p-electron donors based on a self-rigidified
2
,2%-bi(3,4-ethylenedioxy)thiophene–tetrathiafulvalene hybrid
p-conjugated system
a
a
a
a,
a
Philippe Leriche, Mathieu Turbiez, Vincent Monroche, Pierre Fr e` re, * Philippe Blanchard,
b
a,
Peter J. Skabara and Jean Roncali *
a
Groupe Syst e` mes Lin e´ aires Conjugu e´ s, IMMO CNRS UMR 6501, 2 Boulevard Lavoisier, 49045 Angers, France
b
Department of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, UK
Received 20 November 2002; revised 23 November 2002; accepted 27 November 2002
Abstract—Strong p-donors based on the association of TTF and 2,2%-bi(3,4-ethylenedioxy)thiophene conjugating unit have been
synthesised; optical and an X-ray data show that intramolecular interactions lead to full rigidification of the conjugated system.
©
2003 Elsevier Science Ireland Ltd. All rights reserved.
Organic semi-conductors based on extended linear p-
conjugated systems are a focus of considerable current
interest motivated by their technological applications in
field effect transistors or organic light-emitting
actions (with X=S, O and N atoms) stabilising planar
conformations have been observed in conjugated
8
oligomers based on thiophene units. On the other
hand, for extended TTF analogues, similar S···X inter-
actions between the sulfur atoms of the end dithiaful-
venyl groups and heteroatoms of the spacer strongly
contribute to ensuring the coplanarity of the 1,3-dithi-
ole rings with the central conjugated systems for both
1
,2
diodes.
Tetrathiafulvalene (TTF) is one of the
strongest p-electron donating system widely used for
the elaboration of conducting and superconducting
3
materials. In order to combine these two approaches
6,9
extended TTFs incorporating a large variety of p-conju-
the neutral and oxidised states.
4
gated spacer have been developed. The electronic
properties of the p-conjugated systems are modulated
by the strong p-donor properties of 1,4-dithiafulvenyl
groups and their well-known propensity to strengthen
5
intermolecular interactions. On the other hand, the
overlapping mode of hybrid molecules in the structure
of cation radical salts or neutral compounds is strongly
orientated by the nature of the heteroatoms in the
6
spacer.
In previous studies, we have shown that the covalent
rigidification of p-conjugated systems allows a strong
enhancement of p-electron delocalisation and thus a
decrease of the HOMO–LUMO gap due to the com-
bined effects of planarisation and/or reduction of bond-
We report here the synthesis and characterisation of
extended conjugated systems consisting in a 2,2%-bi(3,4-
ethylenedioxy)thiophene (bi-EDOT) unit end-capped
with dithiafulvalenyl groups (1). Crystallographic, opti-
cal and electrochemical data show that the combination
of the electron-releasing effect of the ethylenedioxy
groups with the full self-rigidification of the molecule
by intramolecular interactions lead to a new series of
extended conjugated systems with strong p-donor
properties.
7
length alternation. An interesting further step in this
direction would consist in the development of full self-
rigidification for extended TTFs through non-covalent
interactions. In this context intramolecular S···X inter-
*
Corresponding author. Tel.: (33)241735063; fax: (33)241735405;
e-mail: pierre.frere@univ-angers.fr
0
040-4039/03/$ - see front matter © 2003 Elsevier Science Ireland Ltd. All rights reserved.
PII: S0040-4039(02)02702-8

6
50
P. Leriche et al. / Tetrahedron Letters 44 (2003) 649–652
The synthesis of compounds 1 is depicted in Scheme 1.
The bi-EDOT unit, obtained by copper–catalyzed cou-
pling of the lithiated derivative of EDOT, was treated by
an excess of Vilsmeier reagent (POCl –DMF) leading to
3
the 5,5%-diformyl-2,2%-bi(3,4-ethylenedioxy)thiophene 2 in
60% yield. The last step consisted in a twofold Wittig–
Horner olefination with phosphonates P. The very low
solubility of dialdehyde 2 led to the adoption of an
unconventional procedure involving dropwise addition of
n
BuLi to a suspension of the dialdehyde in the presence
of an excess of phosphonate P under ultra-sound at room
temperature. Sonication favoured the solubilisation of the
dialdehyde and the fast nucleophilic addition of the anion
onto the carbonyl group afforded the target molecules
1
0
1a–c in 20–35% yield.
Single crystals of 1b, obtained by slow evaporation from
a CHCl –EtOH solution, have been analysed by X-ray
3
1
1
diffraction. The structure of compound 1b (Fig. 1)
reveals the perfect planarity of the conjugated system. The
two EDOT units adopt an anti conformation and short
S···O distances dSꢀO=2.914(5) A
considerably shorter than the sum of the van der Waals
radii of sulfur and oxygen (S: 1.85 A+O: 1.50 A=3.35
), clearly shows that intramolecular interactions rigidify
the central part of the molecule. Further rigidification of
the conjugated system occurs between the dithiafulvalene
groups and thiophene units which adopt a lsyn conforma-
,
are observed. This value,
,
,
A
,
Figure 1. X-Ray molecular structure of compound 1b.
Intramolecular and intermolecular interactions are indicated by
dotted lines.
tion stabilised by S···S intramolecular interactions, as
shown by the short distances dSꢀS=3.098(4) A. The
,
molecules stack with a lateral displacement allowing
minimisation of steric hindrance, hence close S···S con-
tacts (d=3.58 A) between the sulfur atoms of dithiole and
,
thiophene cycles are observed.
The optical and electrochemical properties of compounds
1
gathered in Table 1 are compared to their analogues
12 13
incorporating EDOT (3) or 2,2%-bithiophene (4) as
p-conjugated spacer.
Scheme 1. Synthesis of 1. Reagents and conditions: (i) BuLi,
CuCl ; (ii) POCl , DMF, reflux in ClCH CH Cl; (iii) 4 equiv.
2
3
2
2
of P, BuLi/THF, sonic activation, 20°C.
a
b
Table 1. Optical and electrochemical data for 1–3
Compounds
umax (nm)
u0-0 (nm)
E°1 (V)
E°2 (V)
DE=E° −E° (mV)
Kc
2
1
−
−
−
3
4
3
1
1
1
3
3
4
4
a
b
c
472
474
477
446
446
462
463
505
509
513
474
476
488 (sh)
490 (sh)
0.17
0.16
0.01
0.30
0.08
0.55
0.52
0.34
0.34
0.18
0.51
0.35
0.60
0.57
170
180
170
210
270
50
1.33×10
0.9×10
1.33×10
2.8×10
2.7×10
1.47×10
2.1×10
1
2
−4
−5
−1
−1
a
1
2
c
a
b
1
1
3
3
40
a
b
c
−5
−1
1
5
0
mol L in CH Cl .
2 2
×10−4 mol L−1 in Bu NPF /CH Cl (10−1 mol L−1), scan rate 100 mV s−1, E° versus SCE, determinate from deconvoluted voltammogram.
Calculated from the potential difference with the equation K=exp[(F/RT)×(−DE)].
4
6
2
2

P. Leriche et al. / Tetrahedron Letters 44 (2003) 649–652
651
Compared to derivatives 3, the lengthening of the con-
jugated spacer by insertion of an additional EDOT ring
produces a red shift of umax (31 nm between 3c and 1c)
indicating a decrease of the HOMO–LUMO gap. Com-
parison with compounds 4 with the same number of
carbon atoms in the conjugated chain as 1 shows the
strong effects of the ethylenedioxy groups grafted on
the thiophene cycles. Electronic absorption spectra of
radical and dication. Comparison with data for com-
pounds 1 and 3 bearing the same substituents R on the
1,3-dithiole rings, shows that the insertion of a second
EDOT moiety produces a 40–60 mV negative shift of
E°1 and a decrease of the difference DE=E° –E°
2
1
indicative of a diminution of the intramolecular coul-
ombic repulsion between positive charges in the dica-
tion. It is worth noting that for unsubstituted thiophene
and bithiophene spacers the lengthening of the spacer
from one to two rings leads to the opposite effect due
1
b and 4b are presented in Figure 2. The spectra of 4
present large broad absorption bands with only a slight
fine vibronic structure indicating rotational disorder of
the bithiophene unit. The replacement of bithiophene
by bi-EDOT produces both a red shift of the absorp-
tion bands associated to a shorter gap and a strong
enhancement of the fine structure with the emergence of
two main absorbance bands. Such behaviour, observed
for extended TTF with covalently bridged p-conjugated
spacers, is characteristic of a rigidification of the conju-
13
to the rotational disorder of the conjugated chain.
For compound 1c with the more electron-releasing
propyl groups, E° reaches a value close to 0.0 V versus
1
SCE which is one of the lowest value reported to date
14
for a TTF analogue.
Comparison of the electrochemical data for compounds
1
and 4 shows that, in addition to the expected
7
gated systems. This result indicates that the chalcogen–
enhancement of the p-donor ability, the replacement of
the bithiophene (BT) spacer by bi-EDOT induces a
marked increase of DE from 40–50 to 170–180 mV.
Such evolution corresponds to a lower value of the
equilibrium constant K for the disproportionation of
the cation radical into neutral and dication species.
Thus, the value of K which is two orders of magnitude
lower than that of 4 reveals a strong enhancement of
chalcogen interactions observed in the crystallographic
structure persist in solution, thus contributing to the
full rigidification of the molecule in
a
planar
conformation.
As shown in Figure 3, the cyclic voltammogram (CV)
of compounds 1 exhibits two one-electron oxidation
waves corresponding to the formation of the cation
+
’
the thermodynamic stability of radical cation 1 . This
result, indicative of a dramatic increase of the coulom-
bic repulsion between positive charges in the dication
2+
1
, suggests that replacement of the BT spacer by the
strong electron-donating bi-EDOT leads to a relocalisa-
tion of the positive charges of the dication from the end
dithiafulvenyl group to the middle of the conjugated
system, thus resulting in an increase of the on-site
coulombic repulsion.
To summarise we have shown that the use of bi-EDOT
as p-conjugating spacer in extended TTF analogues
gives rise to the development of intramolecular chalco-
gen–chalcogen interactions which stabilise a fully pla-
nar rigid conformation of the molecule, thus
contributing to strongly enhance the p-donor ability of
the molecule. In view of their electronic properties,
these new extended p-conjugated systems could lead to
interesting active materials for the realisation of field
effect transistors. Work in this direction is now in
progress.
−
5
Figure 2. Electronic spectra of 1b and 4b (10 M in CH Cl ).
2
2
References
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−
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Figure 3. CV of 1c (5×10 M in CH Cl ).
2
2

6
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1
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,
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1
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,
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−
1
2
1
6) from which 4003 (Rint=0.081) were independent,
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8
9
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1
2
a STOE-IPDS diffractometer at 293 K. The structure was
2
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2
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satisfactory results. Selected data:
+
’
2
: white powder; mp >250°C; EI MS M 338 for
−
1
C H O S ; IR (KBr) 1635 cm (strong, CO); solubility
14
10
6 2
too low for NMR experiment.
14. The lowest E° value reported to date is −0.11 V versus
1
1
1
a: red powder; mp 126°C (decomp.); H (CDCl ) 6.61 (s,
SCE: Yamashita, Y.; Kobayashi, Y.; Miyashi, T. Angew.
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3
2
H), 4.37 (br s, 4H), 4.28 (br, s, 4H), 2.82 (s, 6H);