PERSPECTIVE www.rsc.org/dalton | Dalton Transactions

p-Conjugated phosphole derivatives: synthesis, optoelectronic functions andcoordination chemistry

Jeanne Crassous* and Regis Reau*

Received 27th June 2008, Accepted 29th August 2008First published as an Advance Article on the web 3rd November 2008DOI: 10.1039/b810976a

p-Conjugated organophosphorus derivatives have recently emerged as valuable building blocks for thetailoring of functional molecular materials. We have focused for ten years on the synthesis ofphospholes bearing aromatic 2,5-substituents (phenyl, thienyl, pyridyl, etc.) and used these compoundsas materials in fields as diverse as OLED’s, WOLED’s, non linear optics. Their properties (such asoptical and electrochemical behaviour, thermal stability) can be finely tuned by functionalization at theP atom. Phosphole-based polymers have also been prepared by electropolymerization and have beenused as sensors. More recently, phospholes substituted by one or two pyridyl groups have proven to actas multidentate ligands with original coordination properties. A diversity of supramolecular rectanglesthat stacked into columns in the solid state has been obtained, opening new perspectives in thecoordination-driven supramolecular organization of p-conjugated systems. Chiral p-conjugatedphospholes having a helical moiety have also been synthesized and spatially organized by usingcoordination to metallic ions, giving rise to sophisticated chiral complexes.

Introduction

Since the works on polyacetylene by Heeger, MacDiarmid andShirakawa, who received the Nobel Prize in 2000,1 organicp-conjugated oligomers and polymers (Fig. 1) have emergedas promising materials for flexible, lightweight and low costelectronic devices.2–4 Field-effect transistors (FETs),5 plastic lasers6

and photovoltaic cells7 are potential appealing applications ofthese p-conjugated molecular materials. Moreover, organic light-emitting diodes (OLEDs)8 are nowadays employed as devicesfor commercialized flat-panel displays in cell phones, digitalcameras, etc. The performance optimisation of these devices isa multi-disciplinary approach involving chemists, physicists andelectronic specialists. The task devoted to chemists is mainly

Sciences Chimiques de Rennes, UMR 6226, CNRS-Universite de Rennes1, 35042, Rennes Cedex, (France). E-mail: regis.reau@univ-rennes1.fr,jeanne.crassous@univ-rennes1.fr; Fax: (+33) 223236939

Jeanne Crassous

Jeanne Crassous is Chargee deRecherches at the CNRS. Sheis interested in many fields ofchirality (fundamental aspects,chiral materials, etc. . .). She hasjoined Regis Reau’s group inSeptember 2005, where she iscurrently studying chiral phos-phorated p-conjugated systems.

Regis Reau

Regis Reau is Professor atthe University of Rennes (Sci-ences Chimiques de Rennes,UMR CNRS 6226, France)since 1997. His current re-search interests mainly involvethe synthesis of organophospho-rus p-conjugated materials foroptoelectronic applications (i.e.OLEDs, NLO), the coordinationchemistry of novel heteroditopicligands, etc.

Fig. 1 Representative structures of p-conjugated systems.

to develop a molecular engineering with the aim to synthesizeconjugated materials with optimised properties. This approach canbe illustrated with the example of single-layer OLEDs. To achievea device, in which a p-conjugated material is sandwiched betweentwo electrodes, the organic derivative has to be able to efficientlytransport injected charges (holes and electrons) and also to behighly luminescent.9 In addition, it should be thermally stable, be

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readily processable to give uniform and pinhole-free thin films,and possess an amorphous morphology.

To satisfy these diverse requirements, the properties of p-con-jugated oligomers or polymers need to be tailored at the molecularlevel. Much attention has therefore been focused on modifyingthe chemical structure and the supramolecular organization oforganic p-conjugated materials in order to control and finelytune their electronic nature (HOMO and LUMO levels, effectiveconjugation length, etc.) that will influence the property of the finalmaterial.3,10 The principal way to influence the physical propertiesof these organic materials is to vary the chemical compositionof the conjugated backbone. For this purpose, aromatic five-and six-membered rings (Fig. 1) have been widely used sincetheir electronic properties (aromatic character, electron-richness/-deficiency, polarisability, etc.) can be considerably varied. Thesebuilding blocks provide not only excellent stability, but also offersignificant synthetic flexibility. However, one potential drawbackof these five- and six-membered ring subunits is their relativelyhigh aromatic character, resulting in a competition betweenp-electron confinement within the ring and delocalisation alongthe conjugated chain.3,12 This point reveals the possible conflictingproperties of a given building block: although aromaticity providesstability and ease of functionalisation, it can hinder access tomaterials having low HOMO–LUMO gap.

It is noteworthy that only a limited number of basic build-ing blocks, namely olefins, acetylenes, aromatic rings (benzene,naphthalene, pyridine) and heterocyclopentadienes (thiophene,pyrrole) (Fig. 1) have been commonly used for the tailoring ofp-conjugated materials.1–10 Therefore, the introduction of novelbuilding blocks furnishes further development of p-conjugatedmaterials. In contrast to sulfur- and nitrogen-based ring systems,which have been widely exploited for decades (Fig. 1), phosphorus-derived building blocks (Fig. 2) have only emerged in the late90s for the construction of p-conjugated materials.10d,11,14a Thissituation is very surprising considering that the chemistry of theseP-containing moieties is now well developed and that they exhibitproperties that are markedly different from those of their S- orN-analogues.13 This chapter intends to give an overview of thesynthesis of p-systems incorporating a phosphole moiety whichhave been developed in our group since 2000.14a The conceptualdesign and specific properties that result directly from the presenceof the P-atom will be illustrated. In parallel to the developmentof phosphole-based p-conjugated systems, phospholes bearing2-pyridyl substituents have emerged as original hetero-multitopicligands, displaying rich coordination chemistry. They have been

Fig. 2 Selected P-based building blocks used for the construction ofp-conjugated systems. See ref. 11.

used to design molecular clips15 which can assemble p-conjugatedsystems into supramolecular metallacyclophanes14b and for thesynthesis of helical ligands which give rise to a variety of nano-scale chiral complexes.37

1 Organic p-conjugated materials based on phospholes

1.1 Electronic structure of the phosphole ring. Up to now,phospholes16 are the most investigated P-derivatives for the designof p-conjugated materials.17 The first phospholes were describedin the 1950s, and nowadays synthetic routes combining high yieldsand diversity of substitution patterns are well established.16 Oneimportant property of the phosphole ring is its very low aromatic-ity which, according to theoretical calculations and experimentaldata, is only slightly superior to that of cyclopentadiene.16,18 Thisproperty is due to the fact that the tricoordinate P-atom adopts apyramidal geometry and that its lone pair exhibits a high degreeof s-character. These two features prevent an efficient interactionof the P-lone pair with the endocyclic p-system. Calculations haveshown that the parent phosphole with a planar P-atom wouldbe more aromatic than pyrrole. However, this stabilization is notsufficient to overcome the high planarization barrier of the P-atom (35 kcal mol-1), but is responsible for the reduced P-inversionbarrier in phospholes (ca. 16 kcal mol-1 versus 36 kcal mol-1 forphospholanes).19 In fact, the aromatic character of the phospholering results from hyperconjugation involving the exocyclic P–R s-bond and the p-system of the dienic moiety.20 This intriguing elec-tronic nature of the phosphole ring has interesting consequencesfor the engineering of p-conjugated materials. Firstly, the P-atomof phospholes exhibits a versatile reactivity since its lone pair isavailable. Many chemical modifications of this centre, includingcoordination to transition metals, can be envisaged to derivatisephosphole-based systems. Secondly, if the chemistry of siloles wastaken as a source of inspiration,21,22 the s–p hyperconjugationassociated with the phosphole ring should allow for the synthesisof multidimensional systems exhibiting s–p conjugation (so-called‘through bond delocalization’23). Lastly, it is noteworthy that thephospholes exhibit a rather high electronic affinity. Calculationsat the HF/6-31 + G*//B3LYP/6-31 + G* level of theory haveshown that the LUMO level of the parent phosphole ring (1.50 eV)is about 0.2 eV lower in energy than that of cyclopentadiene andclose to that of silole (1.39 eV).24 This property is most likely dueto an interaction between the endocyclic dienic p*-orbital and thelow-lying s*(P–R)-orbital, similar to that observed for silole. Inconclusion, these electronic properties (low aromatic character, s–p hyperconjugation, high electronic affinity) set phosphole apartfrom pyrrole and thiophene, and will be fully exploited for theengineering of conjugated materials.

1.2 Synthesis of p-conjugated phosphole derivatives. The firstsystematic investigation of phosphole-containing p-conjugatedsystems, including model molecules as well as conductive polymersand materials suitable for OLED fabrication, was undertakenwith 2,5-di(heteroaryl)phospholes. These compounds are acces-sible via the widely applicable ‘Fagan–Nugent method’25 usingreadily available diynes 1 possessing a (CH2)3 or a (CH2)4 spacer(Scheme 1).20d,24,26 The zirconacyclopentadiene intermediates 2are extremely air and moisture sensitive derivatives that reactin situ with dihalogenophosphanes to give the correspondingphospholes 3a–j in medium to good yields (Scheme 1). Note that

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Scheme 1 Synthesis of di(aryl)phospholes 3a–j by the Fagan–Nugent method.

one zirconacyclopentadiene bearing 2-thienyl substituents wasisolated and characterised by an X-ray diffraction study.24 Thisroute is highly flexible allowing electron-deficient and electron-rich rings to be introduced in the 2,5-positions and also to vary thenature of the P-substituent. This synthetic flexibility is a crucialissue for the development of phospholes as materials buildingblocks since (i) it enables full elucidation of the structure/propertyrelationships, and (ii) their thermal stability as well as their easeof handling and manipulation depends greatly on the nature ofthe P-substituent. For example, parent 1-hydrogeno-phospholecan be observed only at -100 ◦C,27 whereas phospholes bearingphenyl, cyclohexyl, cyano or alkoxy substituents at phosphorusare stable at room temperature.16 In the investigated 2,5-di-(heteroaryl)phosphole series, the substituent on the P-atom isa phenyl group which guaranty that these derivatives are notair- and moisture-sensitive. They can be purified by standardmethods including column chromatography or cristallisation.24

The s3,l3-phospholes 3a26a and 3c24,26b share some importantstructural features in the solid state as established by X-raydiffraction studies. The twist angles between two adjacent ringsare rather small [3a, 7.0◦ and 25.6◦; 3c, 12.5◦ and 16.7◦] and thephosphorus atoms are strongly pyramidalized, as indicated bythe sum of the CPC angles [3a, 299.3◦; 3c, 299.3◦]. The lengthsof the C–C linkages between the rings [1.436(6)–1.467(8) A] arein the range expected for Csp2–Csp2 single bonds. Altogether,these metric data suggest a delocalization of the p-system overthe three heterocycles. It is noteworthy that an X-ray diffractionstudy revealed that the three heterocycles of 3d (Scheme 1) arenot coplanar in the solid state.33c One 3,4-ethylenedioxythiophene(EDOT) substituent lies in the plane of the phosphole ring(dihedral angle, 3.5◦) whereas the second one exhibits a twistangle of 46.2◦. A theoretical study has shown that this rotationaldisorder is due to the steric repulsion of the ether ring of the EDOTmoieties and the phosphole substituents (Ph and C4-bridge). In

the case of compound 3h (Scheme 1), the twist angles betweenthe phosphole ring and the p-substituents (azahelicene, 36.5◦; Ph,37.7◦) are rather high.37 As expected, the naphto[1,2-f]-quinolinepart of 3h displays a helical geometry with a dihedral angle of30.1◦ between the two terminal aromatic rings. The presence of anextended p-conjugated system in phospholes phosholes 3a–j wasalso confirmed by the observation of an absorption maximumin the visible region. The energy of these absorptions, that areattributed to p–p* transitions,24,28 depends dramatically on thenature of the 2,5-substituents. An important bathochromic shiftwas recorded upon replacing the phenyl groups either by 2-pyridyl(lmax = 36 nm) or 2-thienyl rings (lmax = 58 nm) (Table 1).24

Theoretical calculations have confirmed that the HOMO–LUMOseparation gradually decreases in the series 3b/3a/3c.24 A recenttheoretical study has proposed that the more pronounced p-conjugation in 3c is due to the better interaction between theHOMO of the phosphole with the HOMO of the thiophene,compared to that with pyridine.20d,29 It is interesting to notethat the value of lmax recorded for thienyl-capped phosphole 3c

Table 1 Physical properties of representative di(aryl)phospholes andderivatives

lmaxa/nm log e lem

a/nm ffb(%) Epa

c/V Epcc/V

3a 390 4.02 463 1.1 +0.83 -2.453b 354 4.20 466 14.3 +0.69 -2.883c 412 3.93 501 5.0 +0.40 —3j 395 4.36 497 15.2 +1.04 -1.955c 432 3.98 548 4.6 +0.68 -1.956c 442 3.92 593 0.8 +0.92 -1.667c 408 4.04 506 1.3 +0.70 -2.208c 428 4.10 544 12.9 +0.82 -1.75

a In THF. b Relative to quinine sulfate or fluorescein ±15%. c In CH2Cl2,referenced to ferrocene/ferrocenium half cell.

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(412 nm) is considerably more red-shifted than those of related2,5-dithienyl-substituted pyrrole (322 nm), furan (366 nm) orthiophene (355 nm), and very close to that of the 2,5-dithienyl-derivative based on a non-aromatic silole unit (420 nm).30 It isnoteworthy that upon increasing the length of the conjugated pathof phosphole based conjugated systems (3j vs 3e, Scheme 1), thelmax is red-shifted indicating a decrease in the optical HOMO–LUMO energy separations. Likewise, the UV-vis absorptionmaximum of 3h is notably red-shifted (406 nm) compared to thatfor 5-phenyl-2-pyridyl-phosphole 3e (390 nm), clearly indicatingthat the [4]-azahelicene moiety is conjugated with the phospholering.37 The lmax of compound 3i is much shorter (380 nm), probablydue to steric interactions between the azahelicene substituent andthe fused-carbocycle of the phosphole ring resulting in a non-planar structure. Phospholes are indeed valuable building blocksfor the construction of co-oligomers exhibiting low HOMO–LUMO separations. This appealing property is clearly due to theirweak aromaticity that favours an exocyclic delocalization of thedienic p-electrons.

Varying the nature of the 2,5-substituents is also an effectiveway of tuning the emission and electrochemical behaviour ofphosphole-based p-electron systems. A blue-green emission isobserved for diphenyl- and di(2-pyridyl)-phospholes 3b and3a (463–466 nm), respectively, whereas the emission of di(2-thienyl)phosphole 3c is red-shifted (Dlem = 35 nm). The quantumyields also depend on the 2,5-substitution pattern. They aregenerally low (Table 1) with the most efficient fluorophore beingthe s3-phosphole 3j (ff = 15.2%). This value is remarkable aschromophores featuring s3-P centers usually exhibit almost nofluorescence as a result of quenching by the P lone pair (videinfra). This result illustrates the unique properties of phospholescompared to ‘regular’ phosphanes. Cyclic voltammetry (CV)performed at 200 mV s-1 revealed irreversible redox processes withredox potential that are related to the electronic properties of thephosphole 2,5-substituents (Table 1). For example, derivative 3cfeaturing electron-rich thienyl substituents is more easily oxidizedthan compound 3a which possesses electron-deficient pyridylsubstituents (Table 1).24

1.3 Structural diversity exploiting P-derivatisation. The mostappealing property of the phosphole ring is the versatile reactivityof the endocyclic heteroatom that offers a direct access to abroad range of new p-conjugated systems from single P-containingchromophores (Scheme 2).24,31 The chemoselective chemical mod-ifications of the P-moiety proceed in high yields and all thenovel phosphole derivatives are isolated as air-stable solids. X-Raydiffraction studies revealed that the bis(heteroaryl)phospholemoiety remains almost planar in the coordination sphere ofW(CO)5 and AuCl (derivatives 7c and 8c, Scheme 2) and that thebond lengths and angles are unchanged upon coordination.24,31

However, in the case of s4-thioxophospholes the ring conforma-tion in the solid state depends on the substitution pattern. Forexample, crystal structures of derivatives 11–13 (Fig. 3) show thatthe diarylphospholyl moieties exhibit twist angles between thephosphole ring and the aromatic substituents that are similarfor 2,5-diphenylphosphole 11 (50.3◦ and 50.5◦) and 2-phenyl-stylbenylphosphole 12 (44.6◦ and 47.2◦), but much smaller for 2-biphenyl-5-phenylphosphole 13 (35.9◦ and 36.3◦). The smaller ringtwist observed for derivative 13 should favour the delocalization

Scheme 2 Derivatisation of 2,5-arylphospholes 3a–c.

Fig. 3 Thioxophospholes used as materials for OLEDs.

of the p-system since the orbital overlap varies approximatelywith the cosine of the twist angle. Note that the two di(2-thienyl)phospholyl moieties of diphosphaferrocene 9c and 1,1¢-diphosphole 10c exhibit small rotational disorder (twist angles: 9c,0.5◦–12.9◦; 10c, 1.2◦–28.6◦).31d–e In molecules 10b,c through-bondinteractions between the conjugated units can take place due to thehigh polarizability and low s–s* gap of the P–P bond, resultingin a lowering of the HOMO–LUMO gap. This result shows thatphosphole unit can be used for the synthesis of multidimensionalsystems exhibiting s–p conjugation.31e

1.4. Electronic properties of p-conjugated phosphole derivatives.The chemical modifications of the nucleophilic P-centre of thephosphole-based monomers (Scheme 2) have a profound impacton their optical and electrochemical properties as a whole. Forexample, upon modification of the s3-phosphole 3c towards theneutral s4-derivative 5c and the s4-phospholium salt 6c, a redshift in their emission spectra, together with an increase anddecrease of their oxidation and reduction potentials, respectively,is observed (Table 1).29b,29c Several detailed theoretical studies havebeen devoted to rationalising this tuning. One of the most recentone proposes that upon oxidation of the P-centre, the slightlyaromatic phosphole ring becomes slightly antiaromatic.24 It isalso noteworthy that the quantum yields of the gold complex8c is higher than those of the corresponding phosphole 3c orits thioxo-derivative 5c (Table 1), illustrating that the nature of the

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P-modification has a considerable impact on the optical propertiesof these fluorophores.31b This gold-enhancement of the quantumyield is particularly noteworthy for 2,5-thienyl-substituted phos-pholes. Another appealing consequence of the P-modification isthat the quantum yields for s4-thioxophospholes are higher in thesolid state than in dilute solution.31b This rather unusual behavior32

is probably due to the steric protection provided by the substituents(Ph, S) of the tetrahedral P atom which preclude a close cofacialorganization of these P chromophores in the solid state. In markedcontrast, the solution and thin-film emission spectra of the goldcomplexes (i.e. 8c, Scheme 2 and its 2,5-diphenyl analogue 8b)are different. Two broad emission bands are observed for the thinfilms, one at a wavelength similar to that of the solution spectrum,and a second, which is considerably red-shifted. These low-energyluminescence bands observed in the thin films most likely arisefrom the formation of aggregates. These results revealed that thenature of the P-moiety affects the bulk molecular environment andhence the photophysical behaviour of these chromophores in thesolid state; thioxophosphole derivatives are isolated luminophoreswhereas gold(I) phosphole complexes form aggregates leading tobroad emission spectra from both monomer and aggregate states.This dichotomy shows that the presence of reactive phosphorusatoms allows for a unique molecular design to efficiently varyand to control the electronic structures of p-conjugated materials.Exploitation of this method of tailoring p-conjugated systems,which is simply not possible in this variety with pyrrole andthiophene units, has led to the optimization of the properties ofthiophene-phosphole co-oligomers for use as materials suitablefor OLED fabrication.

1.5 Optoelectronic devices. It was really important at thisstage to show that organophosphorus p-conjugated derivativescould be used as functional materials in optoelectronic devicessince one could think that these compounds are not stable enoughfor such uses due to the presence of rather weak P–C bonds.Single layer OLEDs have been prepared through sublimationof phosphole-based conjugated systems onto semi-transparentindium–tin-oxide (ITO) anodes (Fig. 4). Derivatives that arethermally stable enough to be deposited by sublimation are thes3-phosphole 3j (Scheme 1), the thioxo-derivatives 5c (Scheme 2)and 11–13 (Fig. 3), the gold complexes 8b,c (Scheme 2).31a,31b Not

Fig. 4 Single layer OLED obtained by sublimation of thioxophosphole5c. The photoluminescence properties of the molecule are conserved in thedevice.31a,31b

surprisingly, the performances of each device are significantlydifferent depending on the phosphole material employed. TheOLED device using 3j displays greenish electroluminescence (EL)(lEL

max = 510 nm) for a turn-on voltage of 6.0 V. The maximumbrightness attains a rather high value (1000 cd m-2) but the ELquantum yield is low (f = 0.08). With relatively low turn-onvoltages (2.0–3.6 V), devices employing the thioxo-derivatives 5cand 11–13 emit green to yellow light (lEL

max = 524–557 nm). Inall cases, the EL spectra of these devices match those of thesolid-state photoluminescence (PL) spectra of the phospholes,showing that the source of the EL emission band is from theP-derivative. In order to further improve device characteristics,multilayered devices in which the phosphole layer is sandwichedbetween a hole-transport layer (a-NPD) and an electron-transportlayer (Alq3) were investigated.31a,31b The turn-on voltages requiredto drive these multilayer devices are lower or comparable tothose of the corresponding single-layer OLEDs. The externalEL quantum efficiency (EELQ) and brightness of the multilayerdevices are dramatically superior to those of their single-layercounterparts. For example, with 5c the maximum brightness isenhanced by one order of magnitude (3600 vs. 37800 cd m-2)and the maximum EELQ is increased. Phosphole gold complexeswere also employed as emissive materials in single- and the multi-layer OLEDs. As observed for thin-film PL, their EL spectra alsoshow two broad emission bands. The EL spectrum of the single-layer device using complex 8c uniformly covers a broad spectralrange of 500–800 nm due to the well balanced dual monomerand aggregate emissions.32 Overall, these results show that p-conjugated oligomers containing s3- or s4-phosphole moieties,including gold complexes, can be employed as multifunctionalmaterials in OLEDs. They are a basis for the further developmentof P-materials for optoelectronic applications as they show thatthere is no inherent impossibility in using organophosphorusmoieties to construct functional materials. There is clearly manypossibilities to further improve the optoelectronic properties ofthis first generation of phosphole-based p-conjugated derivatives.

1.6. Mixed thiophene-phosphole oligomers and polymers. Theevolution of optical and electrochemical properties with increasingchain length is one of the central principles used for understandingthe characteristics of novel p-conjugated systems.3 This evolutionhas been investigated for a,a’-thiophene-phosphole (Thio-Phos)oligomers since this type of mixed derivatives have shown toexhibit interesting properties (thermal stability, highly delocalizedp-system. . .). To this end, the well-defined oligomers 15a,b wereprepared using the Fagan–Nugent method from the correspondingbis- and tris-diynes 14a,b (Scheme 3).33a Derivatives 15a,b and16a,b are air stable solid which are soluble in common organicsolvents (e.g. THF, CH2Cl2). With respect to their electronicproperties, the longest wavelength absorptions, emission bandsas well as the oxidation potentials all gradually shift to lowerenergies with increasing chain length. This reflects the expecteddecrease in the HOMO–LUMO gap upon increasing the chainlength of the a,a’-(thiophene-phosphole) oligomers. Notably,the lmax-value for Thio-(Phos-Thio)2 15a (490 nm) is consid-erably red-shifted compared to that of quinquethiophene (ca.418 nm), again showing that replacing a thiophene subunit bya phosphole ring induces an important decrease in the opticalHOMO–LUMO gap. As observed with the shorter oligomers,24

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Scheme 3 Synthesis of mixed thiophene-thiooxophosphole derivatives.

oligo(a,a¢-thiophene-phosphole)s incorporating s4-P rings possessmaller HOMO–LUMO gaps than those of their precursorsfeaturing s3-P rings.

The next step was to prepare polymers based on phosp-hole and thiophene subunits. Having in hands thienyl-cappedderivatives, we have investigated their electro-oxidation whichis a general route to polymers using of bis(2-thienyl)heterocyclemonomers.33d,33e

Polymers cannot be directly obtained by electropolymerisationof thienyl-capped s3-phosphole 3c (Scheme 4). This failure isprobably due to the presence of the nucleophilic P-atom whichcan be oxidized, or can trap the intermediate radical cationswhich are key intermediates in the chain growth process.26b

However, poly(a,a¢-thiophene-phosphole)s 18 could be preparedby electropolymerization of the gold complex 8c, followed byreaction of polymer 17 with triphenylphosphine (Scheme 4).33b

This insoluble material was characterised by solid state31P NMRspectroscopy and UV-vis spectroscopy.

It is noteworthy that the P-atoms of this conjugated polymerare reactive. Upon addition of S8 or Se modified polymers 19aand 19b (Scheme 4) with new optical and electronic propertieswere obtained. Consequently, this method gave rise to a new

family of phosphole-containing polymers with tunable propertiesby taking advantage of the reactivity at the P-building block.33b

More recently, the synthesis and electropolymerisation of EDOT-capped s3-phosphole 3d has been investigated. The presenceof the electron-rich EDOT moieties enabled to perform theelectropolymerisation without protection of the s3-P centre of3d (Scheme 4). Here again, the EDOT-phosphole mixed polymerscan have their optical and electronic properties tuned throughchemical modifications of the P-building with either S or AuClgiving rise to new polymers 21a and 21b.33c

2 Coordination chemistry of p-conjugated phospholes

2.1 C 2-Symmetrical complexes from p-conjugated (2-pyridyl)-phosphole moieties. As previously highlighted for bis(aryl)phos-pholes, an important property of phospholes is their ability toform transition metal complexes. Indeed, they behave as classictwo-electron donors towards a wide range of transition metals.24

Phosphole-complexes have been investigated within this contextvia two approaches; the P-ring can be either a part of a conjugatedsystem or either used as a subunit of an assembling ligand to designmolecular clips for coordination-driven self assembly processes.

Scheme 4 Electropolymeristion of thiophene-phosphole-AuCl complex 8c and EDOT-capped phosphole 3d and subsequent functionnalisation of thepolymers.

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According to the first approach, heteroditopic 2-pyridylphospholederivatives have been investigated due to their 1,4-chelatingbehaviour towards transition metals.26c,26d,34 They possess twocoordination centres with different stereo-electronic properties(a “hard” nitrogen and a “soft” phosphorus centres) which,in accordance with Pearson’s antisymbiotic effect,35 can controlthe orientation of a second chelating ligand in the coordinationsphere of a square planar d8-metal centre (Pd2+, Pt2+).26d Thisproperty has been exploited in order to control the in-plane parallelarrangement of 1D-dipolar chromophores with the aim to obtainefficient non-linear optic (NLO) derivatives.36 Phospholes 3f,g(Scheme 5) have a typical ‘D-(p-bridge)-A’ dipolar topology andthey exhibit moderate NLO activities (b 1.9 mm, ª 30 ¥ 10-30 esu)due to the weak acceptor character of the pyridine group.26e Inaccordance with the antisymbiotic effect, these 1,4-P,N chelatesundergo stereoselective coordination leading to a close parallelalignment of the dipoles on the square-planar d8-Pd(II) template.Thus, the trans-effect can overcome the natural anti-parallelalignment tendency of 1D-dipolar chromophores at the molecularlevel. The non-centrosymmetric complexes 22f,g exhibit fairly highNLO activities (b 1.9 mm ca. 170–180 ¥ 10-30 esu), something thatis probably due to the onset of ligand-to-metal-to-ligand chargetransfer (LMLCT), which contributes coherently to the secondharmonic generation.26e

Scheme 5 Stereoselective synthesis of NLO active complexes 22f,g.

The trans effect was also used for controlling the spatial organi-zation of helical phosphole molecules having a 1,4-P,N moiety.The chiral 3-phospholyl-4-aza[4]helicene 3h and 6-phospholyl-5-aza[4]helicene 3i (Scheme 1) reacted with 0.5 eq. of a Pd(II)source yielding the C2-symmetrical chiral complexes 23 and 24 aspairs of enantiomers (Scheme 6).37 It is remarkable that amongstthe numerous possible stereoisomers (26), only one pair of enan-tiomers is obtained. This highly diastereoselective assembly is dueto a combination of electronic (trans-effect) and steric (repulsionof (i) the P–Ph substituents, (ii) the azahelicenes) factors in themetal coordination sphere. For example, the anti-configurationof the P–Ph groups in 23 and 24 is clearly sterically favouredover a possible syn-arrangement. Indeed, phosphole-modifiedazahelicenes appear to be well designed to self-assemble ontometal ions in a highly stereoselective way to afford quantitativelyoriginal chiral p-conjugated metallohelicenes. This result clearlyshows that, in spite of their peculiar structures and the presenceof sterically demanding azahelicene substituents, the heterodi-topic P,N-moieties of derivatives 3h,i dictate their coordinationbehaviour. Likewise, the coordination of helical P,N-chelates 3hand 3i to 0.5 eq. of a Cu(I) source yielded C2-symmetrical

Scheme 6 Synthesis of PdII-complexes 23 and 24 from phospho-lyl-aza[4]helicenes. View of dicationic complex 24 (H atoms, counteranionsand solvent molecules have been omitted). One of the two symmetricallyindependent complexes is shown.

complexes 25 and 26 where two ligands were placed perpendiculararound the tetrahedral Cu(I) metal ion which displaying a spirochirality (Scheme 7).37 These results in the field of NLOphoresand chiral helicoidal derivatives nicely illustrate that heteroditopicconjugated systems based on phosphole-pyridine moiety can bereadily assembled on metal ions in a highly diastereoselective waygiving a straightforward access to sophisticated molecules.

Scheme 7 Synthesis of Cu(I)-complexes 25 and 26 from phospho-lyl-aza[4]helicenes. View of monocationic complexes 25 and 26 (onestereoisomer, H atoms, counteranions and solvent molecules have beenomitted for clarity).

2.2 2,5-Bis(2-pyridyl)-phosphole as N,P,N-pincer with a bridg-ing P-donor. 2,5-Bis(2-pyridyl)phosphole 3a (Scheme 1) is a veryefficient N,P,N-pincer to stabilise bimetallic complexes of metalions such as Pd(I), Pt(I) and Cu(I). In these edifices,38,42 the P-atomof 3a exhibits a bridging phosphane coordination mode, which isa very rare coordination mode for phosphine.39 For example, theX-ray crystal structure of PdI-dimer 27 (Scheme 8) clearly shows

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Scheme 8 Synthesis and view of the crystallographic structure ofcomplexes 27 and 30.

that the Pd(I) dication contains two square-planar metal centrescapped by two 2,5-bis(2-pyridyl)phospholes 3a acting as 6-electronm-1kN:1,2kP:2kN donors with equivalent P–Pd distances [D(Pd–P) = 0,01 A]. This coordination mode is very interesting,40 sincebinucleating ligands potentially allow the synthesis of di- and poly-nuclear complexes that are of great interest in manifold fields suchas catalysis, bio-inorganic and materials sciences.41 Interestingly,the family of metal dimers assembled by N,P,N-pincer 3a hasrecently been extended to heterodimers and metals having dif-ferent coordination geometry. For example, the stepwise methoddepicted in Scheme 8 based on an oxidative addition of the P–Phbound of phosphole on Pd(0), allows for a sequential introductionof the metal centres. According to this approach, a heteronuclearPd–Pt analogue 30 was prepared.38a In this heterodimer, eachm-P atom asymmetrically binds the two metal centres [D(mP–M): 0.083(3) A]. In fact, the m–P centres of the heterobimetalliccomplex 30 exhibit a geometry that is intermediate between asymmetrically bridging and a semi-bridging coordination mode.This result suggests that there is no substantial discontinuitybetween these two coordination modes for m-PR3 ligands. Thisassumption was confirmed by DFT calculations leading to theconclusion that there is little energy cost for a bridging P centre tomove from a symmetrical to a substantially asymmetrical bridgingposition.

The coordination chemistry of ligand 3a with Cu(I) salts havinga tetrahedral coordination sphere was also investigated.42 Thereaction of di(2-pyridyl)phosphole 3a with Cu(CH3CN)4PF6 ina 1:2 ratio afforded quantitatively the dimetallated complex 31(Scheme 9). Its X-ray diffraction study42b revealed that compound31 is a [Cu2(3a)(CH3CN)4]2+·2PF6

- complex in which two Cu(I)atoms are capped by a 2,5-bis(2-pyridyl)phosphole ligand 3aacting again as a 6-electron m-1kN:1,2kP:2kN donor. In thiscomplex, the P atom adopts an almost symmetrical coordinationmode [D(mP–Cu): 0.010(2) A]. Derivative 31 is the first complexin which two d10-metal centres are bridged by a phosphanedonor. Overall, these results suggest that bridging phosphaneligands are able to stabilise a large variety of bimetallic complexes

Scheme 9 Synthesis of dicationic copper(I) complexes 32a,b and 33.

and confirm the prediction of P. Braunstein that “there is noinherent thermodynamic instability associated with this bondingsituation”.40a

The robustness of 3a as a binucleating ligand is nicely illustratedby the fact that the acetonitrile ligands of complex 31 can bedisplaced by different chelates. For example, complex 31 reactseither with one equivalent of phosphole 3a42a or one equivalentof diphenylphosphinomethane (dppm)43 giving rise in very goodyields to the air stable complexes 32a and 32b (Scheme 10).Complex 32a can be further reacted with a third equivalent ofphosphole 3a to afford the air stable bimetallic complex 33 (81%yield).42 X-Ray diffraction studies revealed that these dicationicCu(I)-dimers still feature one 2,5-bis(2-pyridyl)phosphole ligand3a acting as a m-1kN:1,2kP:2kN donor. This series of complexes31–33 gives the unique opportunity to study the evolution of thebridging coordination mode of a phosphane upon decreasing thelocal symmetry of the bimetallic core. It is striking to observethat the difference between the two mP–Cu distances continuouslyincreases in going from complexes 31 to 32a and 33 [D(mP–Cu): 31,0.010(2) A; 32a, 0.094(2) A; 33, 0.295(2) A]. These structural dataconfirm experimentally that there is no substantial discontinuitybetween symmetrical bridging and semi-bridging coordinationmodes for m-PR3 ligands. This situation is reminiscent of that ofCO, the prototypical bridging ligand.40a,41d,44 Finally, these resultsshow that chelates featuring bridging phosphanes are versatilebinucleating ligands and that phosphanes have to be consideredas potentially classical bridging ligands.

2.3. Coordination-driven synthesis of [2.2]-paracyclophane ana-logues. Bimetallic Cu(I) complexes 32a,b possess an unique U-topology, i.e. two cis coordinatively labile sites are closely aligned.In fact, the bridging coordination mode of the phosphole ligand3a imposes a short intermetallic distance [32a, 2.555(1) A; 32b,2.667(1) A] resulting in a close proximity of the two kineticallylabile acetonitrile ligands (N–N distances, ca. 3.2 A). Furthermore,due to the rigidity of the N,P,N-donor 3a and the presence of theadditional P,N- or P,P-chelate, it can be expected that the distancebetween these two labile sites will be conserved in supramolecularassemblies. These properties have been exploited for the syn-thesis of [2,2]paracyclophane analogues45 using supramolecularcoordination-driven chemistry. Indeed, the synthesis of these p-stacked molecular assemblies is of great importance in understand-ing the electronic interactions between individual chromophores.However, straightforward routes to these assemblies as well as fortailoring their structure remain a challenge to chemical synthesis.

Following the concepts of the “directional-bonding ap-proach”,15 the construction of metalloparacyclophanes with p-stacked walls requires a bimetallic clip having the U-topologyof possessing bimetallic Cu(I) complexes 32a,b (Scheme 11).Therefore, complexes 32a,b were reacted with linear homoditopic

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Scheme 10 Synthesis of coordination-driven supramolecular rectangles as 2,2-paracyclophanes analogues.

Scheme 11 Synthesis of coordination-driven supramolecular rectanglesas 2,2-paracyclophanes analogues.

ligands 34–37 incorporating p-conjugated systems affording thenew supramolecular rectangles 38a,b–41a,b (Scheme 12).43

Scheme 12 Synthesis of supramolecular assemblies 38a,b–41a,b.

X-Ray diffraction studies of these supramolecular assemblies(Fig. 5) showed that the metric data of the dimetallic clips32a,b do not change significantly upon their incorporation intothe self-assembled structures demonstrating the conformationrigidity of the Cu(I)-based subunits. In all cases, the four Cu-atoms lie in the same plane defining a rectangle and the aromaticmoieties of the chromophores are parallel as result of a hinderedrotation. Moreover, due to the short Cu(I)–Cu(I) intermetallicdistance imposed by the bridging phosphane coordination mode,

Fig. 5 X-Ray crystal structures of 38b and 39a (H atoms, counteranionsand solvents have been omitted for clarity).

these aromatic moieties participate in face-to-face p-interactions(phenyl centroid-centroid distances: 3.4–3.5 A) with small lateraloffsets. The dimensions of these nano-sized rectangles are fixedby the size of the ditopic ligands and reach 18.1 A for 40a,b and25.2 A for 41a (Cu–Cu distances) with an overall dimension of41a of about 39.0 A in length (Fig. 6).43

These results show that, in spite of the repulsive interactionsbetween the closed-shell p-clouds of the homoditopic ligands, themolecular clips 32a,b can force face-to-face p-stacking of aromaticderivatives upon coordination into well-defined supramolecu-lar metalloparacyclophanes. Indeed, due to their rigidity andunique topology, complexes 32a,b are unique building blocksfor the synthesis of p-stacked molecular assemblies having a[2.2]paracyclophane-like topology. This result opens appealingperspective for the design of multifunctionnal molecular materialvia this supramolecular assembling approach. Moreover, thestacking pattern of the metalloparacyclophanes 40a,b and 41ais remarkable. These supramolecular rectangles self organiseinto columns with short intermolecular distances (ca. 3.6 A).In these columns, the cationic parts of the rectangles have aparallel-displaced arrangement along the a-axes. Unique infinite

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Fig. 6 Views of the crystallographic structures of the tetracationicsupramolecular rectangle 41a and of its packing along the a-axes (H atoms,counteranions and solvent molecules have been omitted for clarity).

columnar stack resulting from intra- and inter-molecular p–pinteractions of (para-phenylenevinylene)-based chromophores 36,37 are thus formed at the macroscopic scale with these novel met-alloparacyclophanes (Fig. 6).43 Organization of the p-conjugatedchromophores is then observed at two hierarchical levels, the firstwithin the metalloparacyclophanes and the second in the infiniteone-dimensional stack of these metalloparacyclophanes.

p-Stacked metalloparacyclophanes have also been incorporatedinto coordination polymers (CP) via a modification of themolecular Cu(I)-clip. A simple replacement of 3a by dppm affordsa bimetallic Cu2(m2-dppm)2 connector 32c (Scheme 13) having anunprecedented F-shape due to the close association (Cu ◊ ◊ ◊ Cudistances, ca. 3.2 A) of a tricoordinated trigonal planar and atetracoordinated tetrahedral Cu(I)-center.46 The propagation ofthe coordination geometry imposed by the “mixed-coordination”connector using p-conjugated linkers such as dicyanostilbene 36gave CP frameworks with p-stacked metalloparacyclophanes asstructural motifs. The fully planar aromatic moieties are parallelwith interplane distances of 3.6–3.7 A revealing p–p interactions.46

Scheme 13 Synthesis of CP 42 using a well-defined Cu(I)-dimer.

The four Cu-atoms define rhombus (endocyclic N–Cu–Cu angles,65.4–66.7◦ and 115.5–116.2◦) and the two p-systems adopt aparallel-displaced arrangement (lateral offset, 1.4–1.5 A), whichis an energetically favoured stacking. This organization differsnotably from the face-to-face p-stacking imposed by the rigidCu(I)-clips 32a,b in discrete rectangles 38a,b–41a,b (Scheme 12).In fact, the flexible mixed-coordination Cu2(m2-dppm)2 connectorcan adapt its structure (shape, Cu ◊ ◊ ◊ Cu distance, etc.) to maximizeweak p-p interactions. Note that these complex CP architecturescould be obtained directly from their basic molecular componentsin a “one-pot, multi-component” approach.

Conclusion

The first phosphole-based p-conjugated systems have appearedin the literature in the 90s with the works of Mathey (smallmolecules) and Tilley (polymers).17 The results obtained in ourgroup have shown that phospholes are valuable building blocksfor the synthesis of p-conjugated systems with optoelectronicor sensing functions. One very important property of thesephosphole-based systems is that simple chemical modificationsof the P-moiety give a straightforward access to a range ofderivatives with different structures and electronic properties.Furthermore, the ability of P-centres to coordinate to transitionmetals offers manifold opportunities to build supramoleculararchitectures in which the p-systems can be organized in a well-defined manner. In this field, the use of pyridyl-phosphole subunitsis particularly fruitful due to their heteroditopic nature. It isnoteworthy that, in very recent years, many other groups haveused phospholes as building blocks for the synthesis of well-defined polymers,47a–c NLO-phores,47d dendrimers,47e small gapoligomers,47f –h macrocyclic ligands,47i–j ladder-type molecules,47k

sensors11t. . . Clearly, the use of phosphole building blocks to designp-conjugated molecular materials is in its infancy and will continueto be the focus of intense research in the future.

Acknowledgements

We thank all coworkers whose work has been presented inthis article. We thank especially Pr. M. Hissler, Dr. C. Lescopand V. Deborde who were mainly involved in these researchtopics. We thank the Ministere de l’Education Nationale, dela Recherche et de la Technologie, the Centre National de laRecherche Scientifique, the conseil regional de Bretagne and theAgence Nationale de la Recherche (ANR) project (PHOSHELIX-137104).

Notes and references

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6876 | Dalton Trans., 2008, 6865–6876 This journal is © The Royal Society of Chemistry 2008

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