Synthesis, electronic properties and electropolymerisation of EDOT-capped σ3-phospholes

Article abstract of DOI:10.1039/b801335g

EDOT-capped σ³-phosphole self-rigidifies due to an unprecedented P-O interaction and can be electropolymerised affording functionalizable polymers. The Royal Society of Chemistry.

Full text of DOI:10.1039/b801335g

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Synthesis, electronic properties and electropolymerisation of
EDOT-capped r3-phospholesw
a
a
a
b
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Vincent Lemau de Talance, Muriel Hissler,* Ling-Zhi Zhang, Tamas Karpati,
Laszlo Nyulaszi,* Dolores Caras-Quintero, Peter Bauerle* and Regis Reau*
b
c
c
a
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Received (in Berkeley, CA, USA) 25th January 2008, Accepted 4th March 2008
First published as an Advance Article on the web 4th April 2008
DOI: 10.1039/b801335g
EDOT-capped r3-phosphole self-rigidifies due to an unprece-
dented P–O interaction and can be electropolymerised affording
functionalizable polymers.
Hybrid phosphole-thiophene systems An (Fig. 1) have recently
Fig. 1 Mixed phosphole–thiophene conjugated systems.
emerged as a novel class of p-conjugated material exhibiting
appealing optoelectronic properties.1 The most important
attribute of these derivatives is the presence of reactive
Lewis-basic s3,l3-P atoms, which permit a fine tuning of their
electronic properties and also provide access to other new
functionalised materials.1 For example, phosphole-modified
polythiophenes An are able to detect chalcogens, an unprece-
dented property for organic conducting polymers, due to the
reactivity of their P centers towards S8 and Se (Fig. 1).2
In this context, replacing the thienyl subunits of A1 by 3,4-
ethylenedioxythiophene (EDOT) moieties, which have unique
properties,3a–e represents an appealing method for the preparation
of electronically different materials. Firstly, it was interesting to
elucidate whether the O atom of EDOT could interact with the P
atom of phosphole, inducing a self-rigidification (planarization).
This type of non-covalent interaction is known to take place
between O- and S-atoms in mixed EDOT-thiophene oligomers
and is an invaluable tool for the design of p-conjugated
materials.3c–g Secondly, due to the low oxidation potential and
the high reactivity of EDOT radical cations,3a,e a direct electro-
polymerisation of 4 (Scheme 1) should be possible, even in the
presence of oxidisable/nucleophilic s3,l3-P atoms. There is no
previous precedent for this latter type of process, since s3-P atom
of monomers such as A1 (Fig. 1) acts as a thiophene cation
scavenger.2,4 This synthetic approach, if indeed possible, would be
of great interest since it would give a direct access to functionaliz-
able phosphole–thiophene conjugated copolymers.
equivalent of n-butyl lithium and iodine afforded derivative
2 in 90% yield. This compound decomposes within a few
hours in the solid state at ꢀ5 1C. Therefore, it was rapidly
engaged in a Sonogashira coupling with 1,7-octadiyne, afford-
ing products 3a and 3b (Scheme 1) that were isolated following
purification on column chromatography in 46% and 37%
yields, respectively. The mixed tricyclic derivative 4 was pre-
pared according to a ‘‘zirconocene’’-promoted coupling1a of
3b, leading to an intermediate zirconacyclopentadiene, fol-
lowed by subsequent addition of PhPBr2 (Scheme 1).1a Ac-
cording to NMR analysis of the crude reaction mixture,
phosphole 4 is formed quantitatively showing that the pre-
sence of the electron-donating and bulky EDOT substituents
does not impede the Zr/P exchange. However, derivative 4 was
obtained in 33% yield only following purification by flash
column chromatography and crystallization. Bis-EDOT-
phosphole 4 is soluble in common organic solvents (CH2Cl2,
THF, CHCl3). It is an air stable yellow solid that can be stored
at room temperature under argon for months. Compound 4
exhibits a resonance in its 31P{1H} NMR spectrum at +16.5
ppm, a chemical shift typical for s3,l3-phosphole derivatives,
and was further characterized by high resolution mass spectro-
metry and elemental analysis.
An X-ray diffraction study revealed that the three hetero-
cycles of 4 are not coplanar in the solid state (Fig. 2). One
The synthesis of 2,5-bis{2-(3,4-ethylenedioxy)thienyl}-
phosphole 4 was achieved in three steps from EDOT 1
(Scheme 1). Subsequent treatment of EDOT 1 with one
a
´
Sciences Chimiques, UMR 6226 CNRS-Universite de Rennes 1,
Campus de Beaulieu, Rennes Cedex, France 35042. E-mail:
muriel.hissler@univ-rennes1.fr. E-mail: regis.reau@univ-rennes1.fr
b Department of Inorganic and Analytical Chemistry, Budapest
´
´
University of Technology and Economics, Szt Gellert ter 4,
Budapest, H-1521, Hungary. E-mail: nyulaszi@mail.bme.hu
c Institute of Organic Chemistry II and Advanced Materials, Ulm
University, Albert-Einstein-Allee 11, 89081 Ulm, Germany
w Electronic supplementary information (ESI) available: Experimental
procedures, theoretical calculations, full ref. 5 entry and crystallo-
graphic data (CCDC 671173) in CIF or other electronic format. See
DOI: 10.1039/b801335g
Scheme 1 Synthesis of EDOT-capped phosphole derivatives.
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Fig. 2 Molecular structure of 4 in the solid state. Hydrogen atoms
have been omitted for clarity.
EDOT substituent lies in the plane of the phosphole ring
(dihedral angle, 3.51) whereas the second one exhibits a twist
angle of 46.21. Note that the intramolecular distance between
the P and O atoms of the coplanar phosphole and the EDOT
rings (2.87 A) is markedly smaller than the sum of the van der
Waals radii (3.42 A). To get more insight into the nature of
this unprecedented P–O interaction, density functional calcu-
lations (B3LYP/6-31G*) have been performed.5 Full geometry
optimization results in a structure that is similar to that
obtained in the solid state with a small intramolecular P–O
distance (3.03 A). The presence of a weakly stabilizing inter-
action between these P and O atoms is supported by the Bader
analysis6 of the B3LYP/6-31G* electron density, revealing a
bond critical point (r = 0.01) and also by the Wiberg index
(0.08).7 A B3LYP/6-31G* scan of the rotation about one C–C
bond of 4 (Fig. 3) reveals that the most stable form of 4 is that
obtained by constraint-less optimization, with the fully planar
structure being a local maximum. In marked contrast, for the
Fig. 3 B3LYP/6-31G* rotational potential energy (full lines) and
TD-DFT calculated excitation energy (dotted lines) for bis-EDOT-
0
phosphole 4 (red) and for parent thienyl capped phosphole A1 (blue)
(the valence angles of the bonds in bold are fixed).8
reacted with elemental sulfur and ClAu(tetrahydrothiophene)
to afford thioxophosphole 5 and complex 6, respectively
(Scheme 1). These compounds were purified by column chro-
matography and isolated in good yields (5, 76%; 6, 82%) as air
stable orange powders. Their multinuclear NMR data are
typical and support the proposed structures.1h These com-
pounds have also been characterized by high resolution mass
spectrometry and elemental analysis. Notably, the chemical
modifications of the P-centre have a profound impact on the
properties of the phosphole-based conjugated systems. A
0
model parent compound A1 , the planar structure is a mini-
mum (Fig. 3). Together, these data suggest that the P–O
interactions favour a planar structure for 4, but that they are
not strong enough to overcome the steric repulsion of the ether
ring of the EDOT moieties and the phosphole substituents (Ph
and C4-bridge). The fact that 4 can not reach a planar
structure probably explains that its lmax is blue shifted (Dlmax
= 42 nm) relative to that of its less sterically crowded thienyl-
capped analogue A1 (Fig. 1), which is known to be planar (see
also Fig. 3 for a comparison between 4 and the parent
significant bathochromic shift of the absorption (Dlmax
4
60 nm) and emission (Dlmax 4 40 nm) maxima was observed
along with an increase of the oxidation potentials with respect
to bis-EDOT-phosphole 4 (Table 1). These results nicely
illustrate the ease with which the electronic properties of mixed
EDOT-phosphole co-oligomers can be tuned by taking ad-
vantage of the versatile reactivity of the P-building block.
The next crucial question was to investigate whether the
presence of EDOT-substituents would render electropolymer-
ization of phosphole 4 feasible in spite of the presence of the
1
0
derivative A1 ).
Tuning the electronic properties of EDOT-capped tricyclic
co-oligomers by changing the nature of the central unit,
initially investigated by Reynolds and coworkers,9 is a widely
used and fruitful approach. The most appealing property of
the phosphole ring as a component of conjugated systems is
the versatile reactivity of the P-atom offering straightforward
access to a family of derivatives without the need for multi-
step synthesis.1a–e It was thus of interest to check whether this
possibility was maintained in the presence of the electron-rich
and potentially reactive EDOT moieties. Indeed, phosphole 4
reactive s3-P centre. As expected, the onset (Ep
= 0.10 V)
onset
and peak (Eopx = 0.22 V) potentials vs. Fc/Fc+ (CH2Cl2,
Bu4NPF6 0.2 M) of 4 are more negative than those of
= 0.33 V; Eopx = 0.80 V). Note that
thienyl-capped A1 (Ep
onset
the Ep of bis-EDOT-phosphole 4 is quite low compared to bis-
EDOT-silole (0.29 V) or bis-EDOT-thiophene (0.33 V) and is
in the range of that for ter-EDOT (0.20 V).9a,b The electro-
polymerization experiments were conducted using CH2Cl2
Table 1 31P{1H} NMR, optical and electrochemical data for a,a0-(EDOT-phosphole) oligomers
Compounds
d31P{1H}a/ppm
Yield (%)
lmaxb/nm
log e
lembFc/nm per %
Eonset/Vd
Epa/Vd
EDOT-Phos-EDOT 4
EDOT-Phos(S)-EDOT 5
EDOT-Phos(AuCl)-EDOT 6
+16.4
+55.2
+43.7
c
33
76
82
363
424
433
3.83
3.74
4.20
514/0.5
553/0.1
563/0.7
0.10
0.38
0.35
d
0.22
0.49
0.47
a
b
In CDCl3. Measured in CH2Cl2. Fluorescence quantum yields determined using fluorescein as standard, ꢂ15%. All potentials were
obtained during cyclic voltametric investigations in 0.2 M Bu4NPF6 in CH2Cl2. Platinum electrode diameter 1 mm, sweep rate: 200 mV sꢀ1. All
reported potentials are referenced to the reversible formal potential of the ferrocene/ferrocenium couple.
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to conjugated polymers incorporating functionalizable s3-P
centres via electropolymerisation.
In summary, the study of EDOT-capped phosphole 4 has
revealed novel and important results that open new perspec-
tives for the molecular engineering of P-based conjugated
systems. Favourable s3-P–O interactions able to induce a
self-rigidification have been clearly demonstrated. Moreover,
the presence of the terminal EDOT ring permits the direct
preparation of polymers incorporating reactive s3,l3-P atoms,
a synthetically challenging type of macromolecule.10 Each
heterocyclopentadiene, with its own specific properties, con-
tributes to making this new type of mixed EDOT-phosphole
oligomers and polymers highly versatile.
We thank the Ministere de la Recherche et de l’Enseigne-
ment Superieur, the Institut Universitaire de France, the
´
CNRS, the DFG, the project OTKA T 049258 and the
BALATON program.
Notes and references
1 (a) C. Hay, M. Hissler, C. Fischmeister, J. Rault-Berthelot, L.
Toupet, L. Nyula
(b) C. Fave, T.-Y. Cho, M. Hissler, C.-W. Chen, T.-Y. Luh, C.-C.
Wu and R. Reau, J. Am. Chem. Soc., 2003, 125, 9254; (c) C. Hay,
C. Fave, M. Hissler, J. Rault-Berthelot and R. Reau, Org. Lett.,
2003, 19, 3467; (d) C. Fave, M. Hissler, T. Karpati, J. Rault-
Berthelot, V. Deborde, L. Toupet, L. Nyulaszi and R. Reau, J.
Am. Chem. Soc., 2004, 126, 6058; (e) H.-C. Su, O. Fadhel, C.-J.
Yang, T.-Y. Cho, C. Fave, M. Hissler, C.-C. Wu and R. Reau, J.
Am. Chem. Soc., 2006, 128, 983. For other types of phosphole-
based p-systems, see: R. Reau T. Baumgartner, Chem. Rev., 2007,
szi and R. Reau, Chem.–Eur. J., 2001, 19, 4222;
´ ´
´
Fig. 4 Cyclic voltammograms in dry CH2Cl2 containing Bu4NPF6
(0.2 M): (a) 4 (10ꢀ3 M), 10 sweeps between ꢀ0.3 and 0.6 V, working
electrode: platinum disc (d = 1 mm); (b) Working electrode: platinum
disc (d = 1 mm) coated by poly(bis-EDOT-phosphole) (blue line),
after reaction with Au(tht)Cl (red line). All potentials referred to
ferrocene/ferrocenium.
´
´
´
´
´
´
´
107, 303; M. Hobbs and T. Baumgartner, Eur. J. Inorg. Chem.,
2007, 23, 3611.
2 M. Sebastian, M. Hissler, C. Fave, J. Rault-Berthelot, C. Odin and
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´
solutions of 4 (1 mM) with Bu4NPF6 (0.2 M) as supporting
electrolyte. Upon repeated cycling between ꢀ0.32 V and
0.58 V, a new redox process regularly develops at lower
potential, indicating the deposition of an electroactive film
on the surface of the working Pt electrode (Fig. 4a). The film
formed is insoluble in all common solvents (CH2Cl2, THF,
DMF, acetone, acetonitrile, MeOH). The electrochemical CV
of this material, recorded in monomer-free electrolytic med-
ium, shows an anodic wave that it is stable upon cycling at 0.1
V (Fig. 4b). The shape of the new anodic wave and the regular
growth of the initial anodic peak (no shift to more positive
potentials and no decrease in intensity) along the recurrent
sweeps (Fig. 4) indicate conducting properties of the deposit.
Therefore, it is indeed possible to perform the electropolymer-
isation of s3-phosphole-containing monomer 4 due to the low
oxidation potential of EDOT and the high reactivity of the
corresponding radical cation. This unprecedented result opens
the very appealing perspective toward the synthesis of func-
tionalizable conductive materials. To illustrate this key point,
poly(bis-EDOT-phosphole) deposited on a Pt electrode was
plunged into a toluene solution of (tht)AuCl (0.3 M) for 20
min and was then rinsed with pure toluene. This treatment
resulted in a shift to higher potential (ca. 170 mV) of the
oxidation potential of the polymeric material (Fig. 4b). This
trend, which parallels that observed during the transformation
of An into Dn (Fig. 1), clearly proves that poly(bis-EDOT-
phosphole) contains reactive s3-P centres. Bis-EDOT-phosp-
hole 4 is thus the first monomer offering straightforward access
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4 Note that the synthesis of polymer An requires a multi-steps
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5 Computations were carried out by the Gaussian 03 suite of
programs at the B3LYP/6-31G* level of the theory. All structures
were fully optimized, and the nature of the stationary points
obtained (minima or first order saddle point) was characterised
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6 R. W. F. Bader, Acc. Chem. Res., 1985, 18, 9.
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8 For the rotational potential energy scan of 4, the carbon frame-
work of the phosphole and one of the EDOT rings are fixed
coplanar and the CCCC twist angle was scanned, all the other
structural parameters were allowed to relax.
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