Connecting π-Chromophores by σ-P-P Bonds: New Type of Assemblies Exhibiting σ-π-Conjugation

Article abstract of DOI:10.1021/ja0317067

To study the ability of σ-P-P skeleton to mediate interaction between π-chromophores, 1,1′-biphospholes bearing phenyl or thienyl substituents at the 2,2′ and 5,5′-position have been prepared and studied. These air-stable derivatives are readily available via a one-pot synthesis starting from diynes. Theoretical studies and UV-vis data clearly establish that the two π-systems interact via the σ-P-P bridge. This through-bond interaction results in a lowering of the optical HOMO-LUMO gap of the assemblies. The nucleophilic σ³-P centers of these 1,1′-biphospholes allow chemical modifications of the σ-bridge. These modifications offer further tuning of the optical properties of the assembly. Electrooxidation of the thienyl-substituted 1,1′-biphosphole results in electroactive materials characterized by low optical band gap and reversible p-doping.

Full text of DOI:10.1021/ja0317067

Published on Web 04/21/2004
Connecting π-Chromophores by σ-P-P Bonds: New Type of
Assemblies Exhibiting σ-π-Conjugation
Claire Fave,† Muriel Hissler,† Tama´s Ka´rpa´ti,§ Joe¨lle Rault-Berthelot,‡
Vale´rie Deborde,† Loic Toupet,| La´szlo´ Nyula´szi,*,§ and Re´gis Re´au*,†
Contribution from the Institut de Chimie de Rennes, UMR 6509 and UMR 6510, and
the Groupe Matie`re Condense´e et Mate´riaux, UMR 6626, CNRS-UniVersite´ de Rennes1,
Campus de Beaulieu, 35042 Rennes, France, and Department of Inorganic Chemistry,
Budapest UniVersity of Technology and Economics, H-1521 Budapest, Gelle´rt te´r 4, Hungary
Received December 15, 2003; E-mail: regis.reau@univ-rennes1.fr
Abstract: To study the ability of σ-P-P skeleton to mediate interaction between π-chromophores, 1,1′-
biphospholes bearing phenyl or thienyl substituents at the 2,2′ and 5,5′-position have been prepared and
studied. These air-stable derivatives are readily available via a “one-pot” synthesis starting from diynes.
Theoretical studies and UV-vis data clearly establish that the two π-systems interact via the σ-P-P bridge.
This through-bond interaction results in a lowering of the optical HOMO-LUMO gap of the assemblies.
The nucleophilic σ3-P centers of these 1,1′-biphospholes allow chemical modifications of the σ-bridge. These
modifications offer further tuning of the optical properties of the assembly. Electrooxidation of the thienyl-
substituted 1,1′-biphosphole results in electroactive materials characterized by low optical band gap and
reversible p-doping.
been exploited in material science.3 For example, in oligo(1,1′-
siloles) A (Figure 1), effective interaction (σ*-π* conjugation)
Introduction
Understanding and controlling the electronic interactions
between individual chromophoric subunits are of fundamental
importance for the engineering of organic materials relevant to
electronic or optoelectronic applications.1 Assemblies in which
chromophores are connected by π-conjugated spacers,1a-g or
organized on a σ-scaffold to induce through-space interaction,1h-j
have been a subject of intense research. In the late 1960s,
Hoffmann recognized that the interaction between π-chromo-
phores can effectively be mediated by σ-skeletons,2a,b especially
those exhibiting high polarizability and low σ-σ* gap.2 The
Si-Si bond admirably provides these characteristics and has
been used to construct assemblies exhibiting efficient σ-π
conjugation (hyperconjugation). Of particular interest, this con-
jugated path results in original electronic properties that have
takes place between the Si-Si bridges and the butadienic
moieties, making them very attractive chemical sensors for
aromatic molecules3c,d or highly efficient electron-transporting
materials for light-emitting diode materials.3e,f
Considering the unique properties of exhibiting σ-π conju-
gation, the search for new σ-skeletons able to mediate such
unusual electronic interaction is of considerable interest. The
comparison of the properties of disilane B and diphosphane C
(Figure 1) suggests that P-P bonds are likely candidates for
this purpose. The heteroatom-heteroatom bond strength is
somewhat larger in disilane B (74 kcal.mol-1) than in diphos-
phane C (61 kcal.mol-1);4a furthermore, diphosphane C ab-
sorbs at longer wavelengths than disilane B (λmax: B, 200
nm; C, 220 nm).4b-d We have thus explored this possibility with
the design of novel assemblies in which two individual
π-chromophores are connected by a P-P bridge. In this article,
we show that P-P links are excellent σ-scaffolds to mediate
† Institut de Chimie de Rennes, UMR 6509, CNRS-Universite´ de
Rennes1.
‡ Institut de Chimie de Rennes, UMR 6510, CNRS-Universite´ de
Rennes1.
§ Budapest University of Technology and Economics.
| Groupe Matie`re Condense´e et Mate´riaux, UMR 6626, CNRS-Universite´
de Rennes1.
(3) (a) Yamaguchi, S.; Tamao, K. The Chemistry of Organic Silicon Com-
pounds; Rappoport, Z., Appeloig, T., Eds.; Wiley: New York, 2001; Vol.
3, p 647. (b) Yamaguchi, S.; Jin, R.-Z.; Tamao, K. J. Am. Chem. Soc.
1999, 121, 2937. (c) Sohn, H.; Sailor, M. J.; Madge, D.; Trogler, W. C. J.
Am. Chem. Soc. 2003, 125, 3821. (d) Sohn, H.; Calhoun, R.; Sailor, M. J.;
Trogler, W. C. Angew. Chem., Int. Ed. 2001, 40, 2104. (e) Sanji, T.; Sakai,
T.; Kabuto, C.; Sakurai, H. J. Am. Chem. Soc. 1998, 120, 4552. (f) Sohn,
H.; Huddleston, R.; Power, D. R.; West, R. J. Am. Chem. Soc. 1999, 121,
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A. L. J. Am. Chem. Soc. 1996, 118, 10457.
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J. AM. CHEM. SOC. 2004, 126, 6058-6063
10.1021/ja0317067 CCC: $27.50 © 2004 American Chemical Society

Connecting π-Chromophores by σ-P−P Bonds
A R T I C L E S
or, as supported by quantum chemical calculations (vide infra),
a rapid rotation around the P-P bond. An X-ray diffraction
study performed on 3a (Figure 2) confirmed the proposed
structures. The P-P [2.224(1) Å] and P-C [1.802(5)-1.806-
(5) Å] distances are in the range expected for single bonds.6e
The P atoms are strongly pyramidalized [Σ bond angles,
293.7°] (Figure 2), with rather acute C-P-P angles [98.37°-
(15)-105.68°(14)]. The twist angles between two adjacent
thienyl and phosphole rings range from 1.2°(5) to 28.6°(8),
and the bond lengths and valence angles of the organophos-
phorus frame of 3a compare with those of the corresponding
2,5-dithienylphosphole 2a (Table 1). As observed for the sole
other 1,1′-diphosphole characterized by X-ray diffraction study,
namely the octaphenyl-1,1′-diphosphole,6e the two π-conjugated
systems adopt a gauche conformation, with a rather short
distance between two CRP atoms [C1-C28, 3.18 Å] (Figure
2). The sterically disfavored concave topology of 3a might
simply be forced by the packing in the solid state. However,
B3LYP/6-31G* calculations reveal that the C2h anti form of 3a
is a saddle point of the PP rotation, by 10.3 kcal/mol higher
than the C2 gauche form. Note that for 3b, the anti form is also
a saddle point at the B3LYP/6-31G* level, by 9.2 kcal/mol
higher in energy than the gauche form. These data support a
low rotation barrier around the P-P bond in 1,1′-diphospholes
3a,b.
Figure 1. General structure of oligo(1,1′-siloles) A, disilane B, and
diphosphane C.
through-bond conjugation and that the possibility to perform
chemical modification of the P atoms gives an unique way to
tune the electronic properties of the assembly.
Results and Discussion
2,5-Diarylphospholes 2a,b (Scheme 1) exhibit a low HOMO-
LUMO gap due to the weak aromatic character of the P ring
and the low-lying exocyclic σ*P-Ph orbital.5a-e Solid state data
and theoretical studies clearly demonstrate that the extended
π-conjugated system in these chromophores involves the two
aryl substituents and the butadienic moiety of the phosphole
ring.5a-d In the present work, derivative 3a (Scheme 1),
assembling such conjugated units via a P-P bond, was prepared
according to the classical route to 1,1′-biphospholes.6a-c Reduc-
tive cleavage of the P-Ph bond of phosphole 2a afforded an
intermediate phospholyl anion (δ 31P NMR, +91.4 ppm), which
upon oxidation with iodine gave rise to the target derivative 3a
(Scheme 1) in 69% after purification. Interestingly, 1,1′-bi-
phosphole 3a is also accessible according to an expedient one-
pot synthesis from diyne 1a, which is the direct precursor of
phosphole 2a.5c Subsequent addition of “zirconocene” and an
excess of PBr3 to 1a5c afforded an intermediate 1-bromophos-
phole (δ 31P NMR, +54.4 ppm) which, in this media, spontane-
ously provides 1,1′-biphosphole 3a (Scheme 1).7 This “one-
pot” procedure was used to prepare the 2,2′-5,5′-tetraphenyl-
1,1′-biphosphole 3b (Scheme 1). It appeared to be superior to
the classical route to 1,1′-biphospholes since it offers a
straightforward multigram scale route to 3a,b from easily
accessible diyne precursors.
To get more insight into the electronic structure of assemblies
of 3a,b, the frontier MOs of the model molecule 3a′,b′, featuring
no fused carbocycle, were analyzed. They can basically be
derived as the antibonding and bonding combinations of the
π-type HOMO and LUMO of the two 2,5-dithienylphosphole
moieties. The MOs of 3a′are shown in Figure 3. First, note that
the favored C2 gauche conformation might be explained by
considering a secondary orbital interaction between the HOMO
and LUMO at one CR(P) atom of each conjugated subunits.
Second, the splitting in energy of the LUMO and HOMO levels
(Figure 3) clearly shows an interaction between the two
π-systems.2a,b The energy difference between HOMO and
HOMO-1 orbitals is small (0.15 eV), since they have a nodal
surface at the connecting P atoms, which reduces their interac-
tion. The unoccupied LUMO and LUMO+1 have a larger
splitting (0.56 eV) (Figure 3). Furthermore, in contrast to the
occupied MOs, the LUMO is the antibonding combination and
the LUMO+1 is the bonding combination (Figure 3). This
behavior is characteristic for through-bond coupling of two
π-systems over an odd number of σ-bonds,2a,b which can clearly
be seen in the involvement of the σ*PP and the σPP orbitals
(Figure 3). The most important consequence of this through-
bond interaction via the P-P bridge on the electronic properties
of assembly 3a,b is the narrowing of the HOMO-LUMO gap
due the stabilization of the LUMO.
Derivatives 3a,b are air-stable powders, which have been
characterized by high-resolution mass spectrometry and elemen-
tal analysis. Their 31P{1H} NMR spectra consist of a singlet at
classical chemical shift for 1,1′-biphospholes6 (δ: 3a, -0.5 ppm;
3b, -13.6 ppm). The 13C{1H} data of the carbon skeleton of
3a,b and those of the corresponding phospholes 2a,b5c are very
similar, except the PCR signals, which are shielded by ca. 4-5
1
ppm in 3a,b. Only one set of 13C{1H} and H NMR signals
could be recorded in CD2Cl2 solution between room temperature
and -60 °C. These data suggest either a symmetric structure
(5) (a) Hay, C.; Le Vilain, D.; Deborde, V.; Toupet, L.; Re´au, R. Chem.
Commun. 1999, 345. (b) Hay, C.; Fischmeister, C.; Hissler, M.; Toupet,
L.; Re´au, R. Angew. Chem., Int. Ed. 2000, 10, 1812. (c) Hay, C.; Hissler,
M.; Fischmeister, C.; Rault-Berthelot, J.; Toupet, L.; Nyulaszi, L.; Re´au,
R. Chem.-Eur. J. 2001, 7, 4222. (d) Delaere, D.; Nguyen, M. T. Phys.
Chem. Chem. Phys. 2002, 4, 1522. (e) Hissler, M.; Dyer, P.; Re´au, R. Coord.
Chem. ReV. 2003, 244, 1. (f) Fave, C.; Cho, T.-Y.; Hissler, M.; Chen,
C.-W.; Luh, T.-Y.; Wu, C.-C.; Re´au R. J. Am. Chem. Soc. 2003, 125, 9254.
(6) (a) Holand, S.; Mathey, F.; Fisher, J.; Mitschler, A. Organometallics 1983,
2, 1234. (b) Abel, E. W.; Clark, N.; Towers, C. J. Chem. Soc., Dalton
Trans. 1979, 814. (c) Mathey, F. Chem. ReV. 1988, 88, 429. (d) Quin, L.
D.; Quin, G. S. Phosphorus-Carbon Heterocyclic Chemistry: The Rise of
a New Domain; Mathey, F., Ed.; Elsevier Science: Oxford, 2001. (e) Vohs,
J. K.; Wie, P.; Su, J.; Beck, B. C.; Goodwin, S. D.; Robinson, G. H. Chem.
Commun. 2000, 1037.
In accordance with the theoretical results, which predict a
narrowing of the HOMO-LUMO gap and an increased density
of states, the measured absorption spectrum of 1,1′-biphospholes
3a,b differs notably from those of the corresponding phospholes
2a,b. While the spectrum of 2a,b shows only one absorption
above 300 nm due to a π-π* transition (Figure 4), several bands
are recorded for 3a,b with one red-shifted broad shoulder
(λonset: 3a, 560 nm; 3b, 460 nm) (Figure 4). The TD-DFT
simulated spectra of phospholes 2a,b and 1,1′-biphospholes
3a,b, obtained for the most stable gauche conformer, are shown
(7) This transformation has been used to prepare nonfunctionalized 1,
1′-bisphospholes, and probably involves the formation of intermediate
1-bromo-1,1′-biphospholium salts, which are reduced by PBr3: Buzin, F.
X.; Nief, F.; Mathey F. Manuscript in preparation.
9
J. AM. CHEM. SOC. VOL. 126, NO. 19, 2004 6059

A R T I C L E S
Fave et al.
Scheme 1
Figure 2. Single-crystal ORTEP structure of 3a.
Table 1. Comparison of the Solid State Data of Phosphole 2a and
1,1′-Biphosphole 3a
2a5b
3a
P1-C1
1.817(4)
1.366(6)
1.465(7)
1.356(6)
1.818(5)
90.9(2)
110.2(4)
113.9(3)
114.9(4)
109.3(3)
1.806(5)
1.360(7)
1.459(7)
1.360(6)
1.806(5)
90.8(2)
110.6(4)
113.6(4)
114.7(4)
110.0(4)
C1-C2
C2-C7
C7-C8
C8-P1
C1-P1-C8
P1-C8-C7
C8-C7-C2
C7-C2-C1
C2-C1-P1
Figure 3. Frontier MOs of the model tetrathienyl-1,1′-diphosphole 3a′ in
its gauche conformation.
by vertical lines in Figure 4. They fit nicely to the experimental
ones (Figure 4), reproducing the changes attributable to the
through-bond σ-π conjugation. For example, the calculated
spectrum of 3a contains two intense transitions at 381 nm
(LUMO+1 r HOMO-1) and 398 nm (LUMO+1 r HOMO)
(experimental values: 361 and 391 nm) and two lower intensity
transitions at 479 nm (LUMO r HOMO-1) and 512 nm
(LUMO r HOMO), corresponding to the broad shoulder (440-
560 nm) (Figure 4). These experimental and theoretical results
clearly show the crucial role of the σ-P-P bridge in modifying
the optical properties of assembly 3a,b by mediating via
through-bond interaction between the two π-systems.
It is interesting to note that the UV-vis spectrum of the
thienyl-capped 3a is red-shifted compared to that of the phenyl-
substituted 3b, a trend also observed in the phosphole series
2a-b (Figure 4). The DFT calculated HOMO-LUMO gap is
by 0.52 eV smaller in 2a than in 2b, and by 0.53 eV smaller in
3a than in 3b. These theoretical data suggest that the nature of
the substituent on the P ring does not influence significantly
the extent of the PP-mediated through-bond interaction in
assemblies 3a,b.
Figure 4. Absorption spectra of 2a,b and 3a,b in CH2Cl2 and TD-DFT
simulated spectra (vertical lines) of 2a (in blue), 2b (in green), 3a (in red)
and 3b (in black).
Chemical modifications of the P atom of thienyl-capped
chromophore 2a (Scheme 1) offer an unique way to diversify
its properties and are key issues for optoelectronic applications
of phosphole-based materials.5f This fruitful approach has been
extended to 1,1′-biphosphole 3a. Derivatives 4 and 5 are readily
9
6060 J. AM. CHEM. SOC. VOL. 126, NO. 19, 2004

Connecting π-Chromophores by σ-P−P Bonds
A R T I C L E S
Scheme 2
Figure 5. Absorption spectra of 3a, 4 and 5 in CH2Cl2 and TD DFT-
simulated spectra (vertical lines) of 4 (in blue).
obtained according to classical reactions exploiting the nucleo-
philic behavior of the σ3,λ3-P centers of 3a (Scheme 2). These
new derivatives exhibited expected multinuclear NMR data and
high-resolution mass spectrum. The dissymmetric nature of 4
is clearly shown by its 31P{1H} NMR spectrum consisting in
two doublets shielded (-12.5 ppm) and deshielded (+54.5),
respectively, compared to the signal of 3a (-0.5 ppm). The
presence of the P-P bond is indicated by the large value of the
Figure 6. Cyclic voltammograms in dry CH2Cl2 containing Bu4NPF6 (0.2
M) and Al2O3. (a) 3a (5 × 10-3 M), 12 sweeps between -0.5 and 0.8 V,
working electrode: platinum disk (d ) 1 mm). (b) Working electrode:
platinum disk (d ) 1 mm) coated by poly(3a) prepared in (a), CH2Cl2
solution free of the monomer 3a, five scans between -0.5 and 0.32 V.
Potentials refer to ferrocene/ferrocenium.
J
PP coupling constant (234.8 Hz). The downfield 31P{1H} NMR
us to investigate its anodic behavior by cyclic voltammetry (CV)
on a Pt electrode in CH2Cl2 with Bu4NPF6 (0.2 M) as the
supporting electrolyte. The CV recorded during the first anodic
sweep exhibits two irreversible oxidation waves at 0.24 and 0.58
V (ref. ferrocene/ferrocenium, Fc/Fc+), respectively (Figure 6a).
Cycling in a potential range, including only the first wave, does
not lead to any modification of this irreversible oxidation wave.
In contrast, when scanning in a potential range, including the
second wave (i.e., between -0.5 and 0.8 V), the CVs show the
appearance and the regular growth of a new reversible wave
with a threshold potential at about -0.35 V (Figure 6a). These
data indicate that a film is formed on the electrode as a
consequence of electropolymerization involving the terminal
thiophene groups. The shape of the new anodic wave and the
regular growth of the two initial anodic peaks (no shift to more
positive potentials and no decrease in intensity) along the
recurrent sweeps indicate the conductivity of the deposit. The
electrode modified by poly(3a) obtained after the 12th sweep
was rinsed with dichloromethane and then studied in a new
electrolytic solution free of any electroactive species. Poly(3a)
exhibits a stable and reversible p-doping process (five recurrent
coordination shift (∆δ (5/3a), 26.5 ppm) clearly establishes
coordination of the P atoms to the gold center,5f and the
simplicity of the 13C NMR spectrum of 5 favors a symmetric
structure. These chemical transformations allow the family of
phosphorus-bridged chromophore assemblies to be extended
nicely, with three different types of P-P links [(σ3-P)-(σ3-P),
(σ3-P)-(σ4P), (σ4-P)-(σ4-P)] being available. Of particular
interest, modifying the coordination number of the P centers
has an impact on the optical properties of the assemblies.
Oxidation of one P center or complexation of the P atoms with
gold(I) results in a further bathochromic shift of the band onset
compared to 1,1′-biphosphole 3a (Figure 5). These experimental
data are nicely reproduced by TD-DFT calculations performed
on derivative 4 (vertical lines, Figure 5). This narrowing of the
optical HOMO-LUMO gap strongly suggests that the modified
σ-P-P bridges still participate in the interaction of the two
π-systems by the hyperconjugative effect. Furthermore, these
results nicely illustrate the possible tuning of the optical
properties of assembly 3a via chemical modifications of the
P-P σ-skeleton.
One of the most versatile routes toward π-conjugated poly-
mers involves the electropolymerization of thienyl monomers
via an oxidation process involving coupling of radical cations.8
Recently, this process was successfully applied to prepare
π-conjugated materials using thienyl-capped phosphole mono-
mers.5b,c,9 The presence of thiophene termini in 3a prompted
(8) (a) Skotheim, T. A.; Elsenbaumer, R. L.; Reynolds, J. R. Handbook of
Conducting Polymers, 2nd ed.; Dekker: New York, 1998. (b) Nalwa, H.
S. Handbook of ConductiVe Materials and Polymers; John Wiley and
Sons: New York, 1997. (c) Audebert, P.; Catel, J. M.; Duchenet, V.;
Guyard, L.; Hapiot, P.; Le Coustumer, G. Synth. Met. 1999, 101, 642.
(9) Hay, C.; Fave, C.; Hissler, M.; Rault-Berthelot, J.; Re´au, R. Org. Lett.
2003, 19, 3467.
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A R T I C L E S
Fave et al.
CRMPO, University of Rennes. Elemental analyses were performed
by the CRMPO.
X-ray Diffraction Analyses. Single crystals suitable for X-ray crystal
analysis were obtained by crystallization from a dichloromethane/
pentane solution of 3a at room temperature. The unit cell constant,
space group determination, and the data collection were carried out on
a NONIUS Kappa CCD (with graphite monochromatized Mo KR
radiation (λ ) 0.71069 Å)).11a The cell parameters were obtained by
fitting a set of 25 high-θ reflections. After Lorentz and polarization
corrections, absorption corrections with psi scan,11b the structures were
solved with SIR-97,11c which reveals non-hydrogen atoms of the
structure. After anisotropic refinement, all hydrogen atoms were found
with a Fourier difference. The whole structures were refined with
SHELXL9711d by the full-matrix least-squares techniques (use of F
magnitude; x, y, z, âij for C, P, and S atoms), x, y, z in riding mode for
the H atoms. Atomic scattering factors were obtained from International
Tables for X-ray Crystallography.11e ORTEP views were prepared with
PLATON98.11f All calculations were performed on a Silicon Graphics
Indy computer. Crystallographic data (excluding structure factors) for
the structures reported in this article were deposited with the Cambridge
Crystallographic Data Centre as supplementary publication no. CCDC-
225744. Copies of the data can be obtained free of charge on application
to CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (Fax:(+44)-
1223 336-033; e-mail: deposit@ccdc.cam.ac.uk).
Crystal Data for 3a. C64H56P4S8, M ) 1205.45, red blocks (0.12 ×
0.08 × 0.06 mm3). Crystal cell parameters were determined from (ω
values of 10 882 reflections and their equivalents in the range 2.66 <
θ < 27.50°; triclinic, P1h, a ) 10.8103(4) Å, b ) 13.6510 (5) Å, c )
20.8737(10) Å, R ) 94.111(2)°, â ) 91.262(2)°, γ ) 113.140(2)°, V
) 2821.0(2) Å3, Z ) 2; density (calcd) ) 1.419 g cm-3. Of 10 882
measured reflections, 10882 were independent and 6545 were observed
with I > 2θ (l). R1 ) 0.0710, wR2 ) 0.1848 for 686 parameters.
Electrochemistry. Cyclic voltammetries were carried out at room
temperature with a scan rate of 100 mV/s in a 0.2 M tetrabutylammo-
nium hexafluorophosphate (Bu4NPF6) dichloromethane solution. All
electrochemical experiments were performed using a Pt disk electrode
(diameter 1 mm), the counter electrode was a vitreous carbon rod, and
the reference electrode was a silver wire in a 0.1 M AgNO3 CH3CN
solution. Ferrocene was added to the electrolyte solution at the end of
a series of experiments. The Fc/Fc+ couple served as internal standard,
and all reported potentials were referenced to its reversible formal
potential. All solvents were purchased from SDS, contained less than
100 ppm of water, and were stored under argon. Activated Al2O3 was
added in the electrolytic solution to remove traces of moisture. The
three electrode cells were connected to a PAR model 173 potentiostat
monitored with a PAR model 175 signal generator and a PAR model
179 signal coulometer. The cyclic voltammetry traces (CVs) were
recorded on an XY SEFRAM-type TGM 164.
Figure 7. Electronic absorption spectra of 3a in CH2Cl2 and of neutral
poly(3a) deposited on a platinum concave disk by a potentiostatic oxidation
of 3a at Epol (0.65 V), followed by a reduction of the polymer at 0.0 V.
Potentials refer to ferrocene/ferrocenium. Absorbance: arbitrary units.
sweeps between -0.5 and 0.32 V, Figure 6b). The threshold
oxidation potential of poly(3a) (-0.35 V) is the less anodic
encountered in the series of thiophene-phosphole copolymers
generated by electropolymerization.5b,9 Poly(3a) can also be
readily obtained by potentiostatic oxidation at Epol ) 0.65 V. It
is an insoluble material, hence its thin film absorption spectra
was recorded to probe its electronic nature. The material was
deposited at Epol on disks of “mirror-polished” platinum and
then dedoped at 0.0 V. The UV-vis spectra of poly(3a) is
considerably red-shifted compared to that of the monomer 3a
(Figure 7). It exhibits a large band with an unresolved maximum
at about 594 nm (Figure 7) and a high value of λonset (730 nm),
indicating a narrow optical HOMO-LUMO gap. This result
shows that not only molecular species, but also low gap
electroactive materials based on 1,1′-biphosphole units, can be
readily obtained from 3a.
Conclusion
The fact that the P-P skeleton is a powerful σ-scaffold to
establish through-bond electronic interaction between π-chro-
mophores offers promising perspectives. The σ*P-P orbital is
lower in energy than the σ*Si-Si, suggesting that P-P bridges
can be at least as efficient as the extensively studied Si-Si
bridges to mediate σ*-π* conjugation.10 Furthermore, because
of the presence of both reactive P centers and thienyl units,
derivative 3a possesses a high potential in molecular and
materials synthesis.
Experimental Section
Materials and Instruments. All experiments were performed under
an atmosphere of dry argon using standard Schlenk techniques.
Commercially available reagents were used as received without further
purification. Solvents were freshly distilled under argon from sodium/
benzophenone (tetrahydrofuran, diethyl ether) or from phosphorus
Optical Spectra. UV-visible spectra of 2a, 2b, 3a, 3b, 4, and 5
were recorded in CH2Cl2 solutions at room temperature on a UVIKON
942 spectrophotometer. UV-vis spectra of poly(3a) were recorded in
dichloromethane solutions using a Guided Wave model 150 spectro-
photometer with optical fibers. The polymers were previously deposited
on a concave platinum surface that acted as a reflector for the optical
beam.12
1
pentoxide (pentane, dichloromethane). H, 13C, and 31P NMR spectra
were recorded on Bruker AM300, DPX200, or ARX400 spectrometers.
1H and 13C NMR chemical shifts were reported in parts per million
(ppm) relative to Me4Si as external standard. 31P NMR downfield
chemical shifts were expressed with a positive sign, in ppm, relative
to external 85% H3PO4. Assignment of carbon atoms was based on
HMBC and HMQC experiments. High-resolution mass spectra were
obtained on a Varian MAT 311 or ZabSpec TOF Micromass at
(11) (a) Fair, C. K. Molen: An InteractiVe Intelligent System for Crystal Structure
Analysis, User Manual; Enraf-Nonius: Delft, The Netherlands, 1990. (b)
Spek, A. L. HELENA: Program for the Handling of CAD4-Diffractometer
Output, SHELX(S/L); Utrecht University: Utrecht, The Netherlands. 1997.
(c) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Giacovazzo,
C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl.
Crystallogr. 1998, 31, 74. (d) Sheldrick, G. M. SHELX97-2: Program for
the Refinement of Crystal Structures; University of Go¨ttingen: Go¨ttingen,
Germany, 1998. (e) International Tables for X-ray Crystallography; Wilson,
A. J. C., Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands,
1992; Vol. C. (f) Spek, A. L. PLATON: A Multipurpose Crystallographic
Tool; Utrecht University: Utrecht, The Netherlands, 1998.
(10) The B3LYP/6-31G* Kohn-Sham LUMO energies of H3SiSiH3 and
H2PPH2 are +0.55 and at -0.32 eV, respectively. The HOMO-LUMO
gap of the dithienylsilol (2.96 eV) is smaller than that of dithienylphosphole
(3.18 eV). However, the HOMO-LUMO gap of the corresponding 2,2-
5,5′-dithienyl-1,1′-silole (2.80 eV) is comparable to that of 2,2-5,5′-
dithienyl-1,1′-phosphole 3a′ (2.75 eV).
(12) Rault-Berthelot, J.; Questaigne, V.; Simonet, J.; Peslerbe, G. New J. Chem.
1989, 13, 45.
9
6062 J. AM. CHEM. SOC. VOL. 126, NO. 19, 2004

Connecting π-Chromophores by σ-P−P Bonds
A R T I C L E S
2,2′-5,5′-Tetra(2-thienyl)-1,1′-biphosphole 3a. Method A. To a
THF solution (5 mL) of 1-phenyl-2,5-bis(2-thienyl)phosphole 2a5b (0.10
g, 0.26 mmol), Li wire (0.020 g, 2.8 mmol) was added. The solution
was stirred for 12 h at room temperature, the excess of lithium was
removed, and tBuCl (28 µL, 0.26 mmol) was added at 0°. The reaction
mixture was stirred for 1 h at room temperature, and then a THF
solution (5 mL) of iodine (0.067 g, 0.26 mmol) was added. The solution
was stirred for 24 h at room temperature, and the volatile materials
were removed under vacuo. The solid was poured into CH2Cl2 (100
mL), and water (10 mL) and sodium sulfate were added. The organic
layer was extracted, and the solvent removed in a vacuum. After
purification on alumina (CH2Cl2), 3a was obtained as an air-stable red
solid (0.055 g, 69% yield).
126.2 (d, J(P,C) ) 21.5 Hz, PCR), 126.5 (d, J(P,C) ) 4.3 Hz, CH Th),
126.7 (d, J(P,C) ) 5.1 Hz, CH Th), 127.0 (s, CH Th), 127.5 (s, CH
Th), 135.5 (d, J(P,C) ) 17.1 Hz, PCR or PCâ), 138.7 (d, J(P,C) ) 23.9
Hz, C2 Th), 141.4 (d, J(P,C) ) 25.7 Hz, PCR or PCâ), 147.5 (m, PCâ),
one quaternary C2 Th atom was not observed. 31P-{1H} NMR
(CD2Cl2, 121.5 MHz): δ +54.5 (d, J(P,P) ) 234.8 Hz), -12.5 (d,
J(P,P) ) 234.8 Hz). HRMS (FAB, mNBA): (m/z) 619.0591 [M + H]+,
C32H29OP2S4 calcd 619.05765.
Gold Complex 5. To a CH2Cl2 solution (15 mL) of 1,1′-biphosphole
3a (0.074 g, 0.12 mmol), neat AuCl(tetrahydrothiophene) (0.079 g, 0.25
mmol) was added. This mixture was stirred for 1 h at room temperature.
All the volatile materials were removed under vacuo, and the brown
residue was precipated from a CH2Cl2 solution (5 mL) by addition of
pentane (10 mL). The precipate was washed with pentane (4 × 10
mL) and diethyl ether (4 × 10 mL), 5 was obtained as an air-stable
Method B. To a THF solution (70 mL) of 1,8-di(2-thienyl)octa-
1,7-diyne 1a5c (0.752 g, 2.80 mmol) and Cp2ZrCl2 (0.814 g, 2.80 mmol)
was added dropwise, at -78 °C, a hexane solution of 1.6 M n-BuLi
(3.65 mL, 5.80 mmol). The reaction mixture was warmed to room
temperature and stirred for 12 h. To this solution was added, at -78
°C, freshly distilled PBr3 (0.70 mL, 7.50 mmol). The solution was
allowed to warm to room temperature and was stirred for 4 days. The
solution was filtered on basic alumina, and all the volatile materials
were removed under vacuo. The solid was washed with distilled pentane
(3 × 10 mL), and 3a was obtained as a red solid (0.50 g, 60% yield).
1H NMR (300 MHz, CDCl3): δ 1.38 (m, 8H, dCCH2CH2), 2.30 (m,
4H, dCCH2), 2.45 (m, 4H, dCCH2), 6.86 (m, 4H, H3 Th), 6.99 (dd,
3J(H,H) ) 5.2 Hz, 4J(H,H) ) 3.7 Hz, 4H, H4 Th), 7.20 (dd, 3J(H,H) )
5.2 Hz, 4J(H,H) ) 1.1 Hz, 4H, H5 Th). 13C-{1H} NMR (75.46 MHz,
CDCl3): δ ) 22.5 (s, dCCH2CH2), 28.8 (s, dCCH2), 124.6 (s, C5
Th), 125.9 (dd, J(P,C) ) 6.0 and 5.8 Hz, C3 Th), 126.9 (s, C4 Th),
1
brown solid (0.111 g, 87% yield). H NMR (300 MHz, CDCl3): δ
1.49 (m, 8H, dCCH2CH2), 2.45 (m, 4H, dCCH2), 2.56 (m, 4H, d
3
3
CCH2), 7.26 (dd, J(H,H) ) 5.3 Hz, J(H,H) ) 3.6 Hz, 4H, H4 Th),
3
3
7.52 (d, J(H,H) ) 3.6 Hz, 4H, H3 Th), 7.58 (d, J(H,H) ) 5.3 Hz,
4H, H5 Th). 13C-{1H} NMR (CD2Cl2, 75.46 MHz): δ 21.5 (s,
dCCH2CH2), 29.1 (d, J(P,C) ) 10.0 Hz, dCCH2), 128.0 (d, J(P,C) )
16.4 Hz, PCR), 128.2 (s, C4 Th), 128.4 (s, C5 Th), 129.6 (s, C3 Th),
134.5 (m, C2 Th), 150.9 (m, PCâ). 31P-{1H} NMR (CD2Cl2, 121.5
MHz): δ +26.0 (s). HR-MS (mNBA, FAB): (m/z) 1030.9553 [M -
Cl]+; C32H28P2S4ClAu2 calcd 1030.95689. Anal. Calcd for C32H28P2S4-
Cl2Au2: C, 36.00; H, 2.64. Found: C, 35.84; H, 2.41.
Calculations. Computations were carried out by using the Gaussian
98 program package.13 For the density functional calculations, the
B3LYP hybrid functional was used.14 The structures were optimized
at the B3LYP/3-21G(*) level of theory. Subsequent calculation of the
second derivatives showed that real minima (no imaginary frequencies)
or first-order saddle points (only one imaginary frequency) was
obtained. Further optimization was carried out at the B3LYP/6-31G*
level, but at this level no second derivatives were calculated. Time-
dependent DFT calculations were performed at the B3LYP/6-31G*//
B3LYP/6-31G* level of theory. The MOs shown in Figure 3 are Kohn-
Sham MOs, being similar to the canonical HF MOs. The molecular
orbitals were visualized by the Molden program.15
1
131.1 (d, J(P,C) ) 19.3 Hz, PCR), 139.5 (t-like, J(P,C) ) 12.1 and
12.2 Hz, C2 Th), 144.2 (s, PCâ). 31P-{1H} NMR (121.5 MHz, CDCl3):
δ -0.5 (s). HRMS (FAB, oNPOE): (m/z) 602.0549 [M]+, C32H28P2S4
calcd 602.0549. Anal. Calcd for C32H28P2S4: C, 63.76; H, 4.68.
Found: C, 63.56; H, 4.89.
2,2′-5,5′-Tetraphenyl-1,1′-biphosphole 3b. This compound was
prepared according to the above method B using 1,8-di(phenyl)octa-
1,7-diyne 1b5c (0.571 g, 2.20 mmol). 3b was isolated as a yellow solid
1
(0.32 g, 40% yield). H NMR (300 MHz, CD2Cl2): δ 1.42 (m, 4H,
dCCH2CH2), 1.49 (m, 4H, dCCH2CH2), 2.40 (m, 8H, dCCH2), 7.23
Acknowledgment. We thank the CNRS, the MNERT, the
Conseil Re´gional de Bretagne (PRIR n° 99CC10), and OTKA
T 34675 and D 042216. Support from the Scientific and
Technological French-Hungarian Bilateral Cooperation (BALA-
TON program) is also acknowledged.
3
3
(d, 8H, J(H,H) ) 7.3 Hz, H2 Ph), 7.31 (dd, 4H, J(H,H) ) 7.4 Hz,
3
3
H4 Ph), 7.44 (t, 8H, J(H,H) ) 7.3 Hz, J(H,H) ) 7.4 Hz, H3 Ph).
13C-{1H} NMR (75.46 MHz, CD2Cl2): δ ) 22.7 (s, dCCH2CH2),
27.4 (s, dCCH2), 125.9 (s, C4 Ph), 127.9 (s, C3 Ph), 129.3 (s, C2 Ph),
137.3 (dd, J(P,C) ) 9.9 and 8.9 Hz, C1 Ph), 139.7 (m, PCR), 144.4 (s,
PCâ). 31P-{1H} NMR (121.5 MHz, CDCl3): δ -13.6 (s). HRMS (ESI,
CH3OH): (m/z) 601.2200 [M + Na]+, C40H36P2Na calcd 601.2190.
Anal. Calcd for C40HS36P2: C, 83.02; H, 6.27. Found: C, 83.36; H,
6.52.
Supporting Information Available: Crystallographic details
(CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
2,2′-5,5′-Tetra(2-thienyl)-1,1′-biphosphole-1-oxyde 4. To
a
JA0317067
CH2Cl2 solution (15 mL) of 1,1′-biphosphole 3a (0.163 g, 0.27 mmol),
neat Me3SiOOSiMe3 (0.87 mL, 0.27 mmol) was added. The formation
of 4 was monitored by 31P NMR spectroscopy. After stirring for 7 days
at room temperature, the volatile materials were removed under vacuo,
and the residue was washed with diethyl ether (3 × 10 mL). 4 was
obtained as a red-brown solid, which can be stored for months under
(13) Computations were carried out by using the GAUSSIAN 98 program
package. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.,
Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.;
Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford,
S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.;
Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-
Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe,
M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J.
L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision
A.5; Gaussian, Inc.: Pittsburgh, PA, 1998.
1
inert atmosphere (0.143 g, 86% yield). H NMR (CD2Cl2, 300 MHz):
δ 1.75-1.85 (m, 8H, dCCH2CH2), 2.70-2.90 (m, 8H, dCCH2), 7.10
(m, 2H, Th), 7.20 (dd, J(H,H) ) 3.8 and 5.2 Hz, 2H, Th), 7.40 (d,
J(H,H) ) 4.7 Hz, 2H, Th), 7.48 (m, 2H, Th), 7.60 (d, J(H,H) ) 3.8
Hz, 2H, Th), 7.65 (m, 2H, Th). 13C-{1H} NMR (CD2Cl2, 75.46
MHz): δ 20.8 (s, dCCH2CH2), 21.6 (s, dCCH2CH2), 27.0 (d, J(P,C)
) 14.4 Hz, dCCH2), 28.6 (s, dCCH2), 124.8 and 125.2 (s, CH Th),
(14) Becke, A. D. J. Chem. Phys. 1991, 98, 5648.
(15) Schaftenaar, G.; Noordik, J. H. J. Comput.-Aided Mol. Des. 2000, 14, 123.
9
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