2.1 Synthesis and physical properties

The synthetic route to organophosphorus derivatives 3 employs the one-pot “Fagan–Nugent method” [7] involving the oxidative coupling of functionalized octa-1,7-diynes 1 with zirconocene followed by treatment with a dihalogeneted phosphine (Scheme 1) [8]. This synthetic pathway is rather simple and efficient, allowing gram-scale preparations, since the functionalized octa-1,7-diynes are obtained in good yields by Sonogashira couplings [8]. 1-Phenylphospholes 3a–g (Scheme 1) can be isolated as air-stable solids following flash column chromatography on basic alumina. They are readily soluble in common organic solvents such as CH₂Cl₂, THF or CH₃CN. They exhibit classical NMR data and their structures were supported by high-resolution mass spectrometry and elemental analysis.

It is noteworthy that this synthetic route allows the preparation of symmetrical (Ar = Ar′) and unsymmetrical (Ar ≠ Ar′) 2,5-disubstituted 1-phenylphospholes. This dissymmetry is easily introduced, at the level of the functionalized 1,7-diynes 1 through successive Sonogashira coupling reactions [9]. A large variety of functional groups can be introduced, leading to numerous 1-phenylphosphole derivatives 3a–f with markedly different physical properties [9]. Moreover oligo(heterocycle-phosphole) derivative 3g with precise length has also been obtained according to this synthetic procedure [10].

The X-ray diffraction study performed on the model compounds 3a [8b,d] and 3b [8c,d] (Scheme 1, Fig. 1) shows that the three heterocycles are almost coplanar, with the phosphorus atom being strongly pyramidalised. The lengths of the C–C linkages between the rings are in the range expected for Csp²–Csp² bonds. These solid-state data indicate that an efficient delocalisation of the π-system takes place over the three heterocycles in oligomers 3a,b, as supported by theoretical studies [8d].

The presence of an extended π-conjugated system in phospholes 3a,b was also confirmed by the observation of a broad absorption in the visible region of their UV–vis spectrum. The energy of these absorptions, attributed to π–π^∗ transitions, depends dramatically on the nature of the 2,5-substituents of the phosphole ring [8d]. The values of λ_max and the optical end absorption λ_onset (the solution optical “HOMO–LUMO” gap) [8d], become red-shifted on replacing the 2-pyridyl groups (λ_max = 390 nm) by 2-thienyl rings (λ_max = 412 nm). Theoretical studies showed that this bathochromic shift is due to a better interaction between the HOMO of phosphole with the HOMO of thiophene, compared to that of pyridine [8d,11]. It is noteworthy that the λ_max value recorded for 3a (412 nm) [8d] is considerably more red-shifted than that of the related tert-thiophene (355 nm) [12]. This observation is in agreement with theoretical studies predicting that heterocyclopentadienes with a low aromatic character are optimal building blocks for the synthesis of extended π-conjugated systems with low HOMO–LUMO gap [1d,8a].

Phospholes 3a,b are also fluorophores and their λ_em depend on the nature of the 2,5-substituents. A blue emission is observed for 1-phenyl-2,5-di(2-pyridyl)phosphole 3b (λ_em = 463 nm), whereas the emission of 1-phenyl-2,5-di(2-thienyl)phosphole 3a is red-shifted (Δλ_em = 35 nm). Cyclic voltammetry (CV) performed at 200 mV s⁻¹ revealed that the redox processes observed for all the σ³-phospholes 3 are irreversible and that their redox properties are related to the electronic properties of the phosphole substituents [8d]. For example, derivative 3a featuring electron-rich thienyl substituents is more easily oxidised than compound 3b, which possesses electron-deficient pyridyl substituents [8d].

2.2 Fine tuning of the physical properties via chemical modifications of the phosphole ring

One of the appealing properties of phosphole rings is the versatile reactivity of the endocyclic heteroatom that allows creating structural diversity in these types of systems. This feature offers a direct access to a broad range of new π-conjugated systems from single P-containing chromophores, without the need for additional multi-step syntheses, as illustrated with dithienyl-phospholes 3a (Scheme 2). The chemical modifications of the nucleophilic P centre (Scheme 2) have a profound impact on the optical and electrochemical properties of the phosphole oligomers as a whole. For example, a chemical modification of the σ³-phosphole 3a leading to the neutral σ⁴-derivatives 4–6 (Scheme 2) induces a red shift in their emission spectra and an increase and decrease of their oxidation and reduction potentials, respectively. It is also noteworthy that the quantum yields of the gold complex 5 (Φ = 14%) is much higher than those of the corresponding phosphole 3a (Φ = 5%) or its thioxo-derivative 4 (Φ = 4.6%), illustrating that the nature of the P-modification has a considerable impact on the optical properties of these fluorophores [8d,13].

The comparison of the solution and thin film optical properties of the thioxophosphole 4 shows that this compound does not form aggregates in the solid state. This property, which is supported by an X-ray diffraction study, results in an enhancement of the fluorescence quantum yields in the solid state. In contrast, the solution and thin film emission spectra of the gold complex 5 are different. Two broad emission bands are observed for the thin films, one at a wavelength similar to that of the solution spectrum (550 nm), and a second which is considerably red-shifted (690 nm). The excitation spectra of 5 in thin film, recorded for emission at 550 or 700 nm, are similar to its absorption spectrum, clearly indicating that both emissions are due to the (phosphole)gold complex 5. The low-energy luminescence band obtained in thin film with phosphole–Au(I) complex 5 very probably arises from the formation of aggregates.

The uncommon emission properties of these organophosphorus derivatives prompt us to use them as emissive layer in single- and multilayer OLEDs. In particular, it has been shown that the emission color of the devices and their performances vary with the nature of the P material. Upon sublimation, phosphole 3a decomposed, while the more thermally stable thioxo-derivative 4 formed homogeneous thin films on an indium–tin-oxide (ITO) semi-transparent anode, allowing a simple layer OLED to be prepared [13]. This device exhibits yellow emission for a relatively low turn-on voltage of 2 V. The comparatively low maximum brightness (3613 cd m⁻²) and electroluminescence (EL) quantum yield (0.16%) can be increased by nearly one order of magnitude using a more advanced device, in which the organic layer consisting of 4 was sandwiched between hole- and electron-transporting layers (α-NPD and Alq₃, respectively). The Au(I) complex 5 (Scheme 2) is thermally stable enough to give homogeneous thin films upon sublimation in high vacuum. The corresponding single layer device exhibited an EL emission covering the 480–800-nm domain [13], a result which is encouraging for the development of white-emitting OLEDs based on phosphole–Au complexes. Note that the low-energy emissions are very probably due to the formation of aggregates in the solid state [13]. These results constitute the first application in optoelectronics of π-organophosphorus materials.

In conclusion, co-oligomers with a phosphole core exhibit very interesting properties. They are air-stable, provided that the P-substituent is a phenyl group, they exhibit low optical gap and their electronic properties can be tuned by varying the nature of the 2,5-substituents (2-thienyl or 2-pyridyl) or chemical modification of the phosphole ring. Exploitation of this method of tailoring π-conjugated systems, which is not possible with pyrrole and thiophene units, has led to the optimization of the properties of thiophene–phosphole co-oligomers for their use as materials suitable for OLED fabrication [13].

3.1 Square-planar complexes exhibiting a NLO activity

The pyridylphosphole ligands possess two coordination centres with different stereo-electronic properties, which, in accordance with the Pearson's anti-symbiotic effect [16,17], allows the control of the orientation of a second chelating ligand in the coordination sphere of a square-planar d⁸-metal centre [18]. This property has been exploited in order to control the in-plane parallel arrangement of 1D-dipolar chromophores. Phospholes 3e and 3f (Scheme 1), have a typical “Donor–(π-bridge)–Acceptor” dipolar topology incorporating a 1,4-P,N chelate. They exhibit moderate NLO activities (β_{1, 9 μm}, ca. 30 × 10⁻³⁰ e.s.u.) compared to classical chromophores such as Dispersed Red 1 (DR1) (β, 50 × 10⁻³⁰ e.s.u.) [9a]. These low values are consistent with the weak acceptor character of the pyridine group. However, their potential in NLO is considerably increased by their coordination behaviour toward Pd(II) centres. The reaction of two equivalents of 2-(2-pyridyl)phospholes 3e,f with [Pd(CH₃CN)₄][BF₄]₂ afforded complexes 7e,f as single diastereoisomers (Scheme 3) [9a], with the P and N donor in a mutual trans-arrangement. Clearly, the trans-influence can overcome the natural tendency of 1D-dipolar chromophores to adopt an anti-parallel alignment as the square-planar metal centre acts as a template imposing a non-centrosymmetric assembly of two identical 1D-chromophores 3e,f. Furthermore, the metal centre plays a puzzling role, since a considerable enhancement of the NLO activities is observed upon complexation. For example, complex 7e exhibits fairly high non-linear optical activities with β value of 180 × 10⁻³⁰ e.s.u, a value which is much higher than the sum over the contribution of two sub-chromophores 3e. In a first approach, this effect could be related to an increase of the acceptor character of the pyridine groups [19]. However, it is very likely that the origin of this large β enhancement is due to metal-to-ligand (MLCT) or ligand-to-metal-to-ligand charge transfers (LMLCT) evidenced by UV–vis spectroscopy [9a] that contribute coherently to the second harmonic generation. In conclusion, the coordination chemistry offers a simple synthetic methodology for controlling the in-plane parallel arrangement of P,N-dipoles in a molecular assembly.

3.2 Ruthenium complexes

A great attention has been paid to the synthesis and photophysical properties of dinuclear ruthenium complexes containing bridging π-conjugated ligands [20]. The design of the bridging ligands allows tuning of the electronic properties of these dyads and has led to important progress in the field of photoinduced electron or energy transfer processes. These properties prompted us to investigate a new family of derivatives possessing two terminal (2-pyridyl)phosphole moieties joined by a 2,5-thienyl spacer as potential bridging ligands to build bimetallic ruthenium complexes.

The target complex 8 was obtained by addition of [(p-cymene)RuCl₂]₂ and two equivalents of KPF₆ to the ligand 3g (Scheme 4) [10]. This coordination is diastereoselective, each diastereoisomer of 3g giving one diastereoisomer of 8. The UV–vis spectrum of 3g exhibits an absorption maximum in the visible region at 482 nm attributed to π–π^∗ transitions of the π-conjugated system [10]. This value is considerably red-shifted (Δλ_max = 84 nm) compared to that of 3c (Scheme 1). The coordination of the 2-pyridylphosphole moieties of 3g has almost no influence on the λ_max observed for complex 8. This is probably due to the symmetrical structure of complex 8, which prevents efficient ILCT. It is noteworthy that this broad ligand-centred transition overlaps the possible MLCT transition arising from Ru(II) to π^∗-orbital of the coordinated 2-pyridylphosphole moiety.

4.1 Bimetallic coordination complexes bearing a bridging phosphane ligand

Toward Pd(I) and Cu(I) metal centres, 2,5-bis(2-pyridyl)-phosphole ligand 3b revealed a very original coordination chemistry in comparison to the other 2-(2-pyridyl)-phosphole P,N ligands 3c–f. A family of coordination complexes exhibiting a bridging phosphane coordination mode was thus characterized, a very rare coordination mode for such a commonly used family of ligand.

4.1.1 Pd(I) bimetallic complexes

Ligand 3b was reacted with [Pd₂(CH₃CN)₆][BF₄]₂ and afforded complex 9 in a 85% yield (Scheme 5) [9b,21]. The X-ray crystal structure of complex 9 (Fig. 2) evidences a dimer of square-planar Pd(I) centres gathered by two 2,5-bis(2-pyridyl)phospholes 3b acting as 6-electron μ-1κN:1,2κP:2κN donors.

In this structure, the derivative 3b bridges the two Pd(I) centres via a symmetrically bridging phosphane coordination mode [Δ(Pd − P) = 0.01 Å]. This coordination mode was previously only observed in the case of a family of Rh(I) complexes [22] and constitute a breakthrough in coordination chemistry [23]. The phosphanes acting as binucleating ligands allow the synthesis of di- and polynuclear complexes that are of great interest in manifold fields such as catalysis, bio-inorganic and materials sciences [24].

4.1.2 Cu(I) bimetallic complexes

The coordination chemistry of ligand 3b with Cu(I) salts was also investigated in order to generalize this original bridging phosphane coordination mode to other metal ions [25]. Thus, reaction of di(2-pyridyl)phosphole 3b with Cu(CH₃CN)₄PF₆ in a 1:2 ratio led to the quantitative isolation of the dimetallated complex 10 (Scheme 6).

Its X-ray diffraction study revealed that compound 10 is a [Cu₂(3B)(CH₃CN)₄][PF₆]₂ complex (Fig. 3) in which the two Cu(I) centres are capped by a 2,5-bis(2-pyridyl)phosphole ligand 3b acting as a 6-electron μ-1κN:1,2κP:2κN donor.

In this complex as observed in the derivative 9, the P atom presents an almost symmetrical coordination mode [Δ(μP − Cu) = 0.010(2) Å] and short intermetallic distance [2.568(10) Å]. In addition, derivative 10 is the first complex in which: (i) two d¹⁰-metal centres are bridged by a phosphane donor, and (ii) two metals are held together by one bridging phosphane and no other supporting ligands. Such results suggest that bridging phosphane ligands are able to stabilise a large variety of bimetallic complexes and confirm that “there is no inherent thermodynamic instability associated with this bonding situation” [23a].

Complex 10 can be reacted with one equivalent of phosphole 3b [25] or one equivalent of diphenylphosphinomethane ligand (dppm) [26] affording in very good yields the air-stable complexes 11a and 11b (Scheme 7). In addition, derivative 11a can be reacted with a third equivalent of phosphole 3b to give the air-stable bimetallic complex 12 [25].

X-ray diffraction studies (Fig. 4) showed that these three dicationic Cu(I) dimers possess one 2,5-bis(2-pyridyl)phosphole ligand 3b acting still as a μ-1κN:1,2κP:2κN donor. The fact that the μ-bridging coordination mode of the P centre is retained in this series highlights the robustness of bridging phosphanes as binucleating ligands. The tetrahedral geometry of the Cu(I) atoms in complex 10 prevents the formation of a doubly bridged structure, as observed in the Pd(I) dimers 9 (Fig. 2), which requires square-planar metal centres.

These results demonstrate that chelates exhibiting bridging phosphanes are versatile binucleating ligands and moreover that phosphanes have to be considered as potentially classical bridging ligands.

4.2 Supramolecular organization of π-conjugated chromophores via coordination chemistry: synthesis of analogs of [2,2]-paracyclophanes

Due to their specific topological properties, bimetallic Cu(I) complexes 11a and 11b have been efficiently used as molecular clips [27] in order to organize various π-conjugated systems into rectangular supramolecular assemblies. The synthesis of π-stacked molecular assemblies is indeed of great importance in the understanding the electronic interactions between individual chromophores and thus affords potential new material in the field of molecular devices for optoelectronics. One fruitful approach to probe co-facial π–π interactions involves the assembly of chromophore pairs into well-defined [2,2]paracyclophanes [28], as these molecules provide incisive insights into bulk properties of conjugated systems and are suitable π-dimer models. However, straightforward routes to these assemblies as well as for tailoring their structure are still a challenge to chemical synthesis.

We have investigated a novel synthetic route to analogs of [2,2]paracyclophanes using supramolecular coordination-driven chemistry. Following the concepts of the “directional-bonding approach” [29], the design of metalloparacyclophanes with π-stacked walls requires a bimetallic clip possessing two cis coordinatively labile sites that are closely aligned (Scheme 8).

The two complexes 11a [25] and 11b [26] possess two acetonitrile ligands that have a cisoid arrangement. Furthermore, the bridging coordination mode of the phosphole ligand 3b imposes a short intermetallic distance [11a, 2.555(1) Å; 11b, 2.667(1) Å] and thus a close proximity of the two kinetically labile acetonitrile ligands (N–N distances, ca. 3.2 Å). Such geometrical characteristics are pre-required in order to allow the organization of homoditopic ligands coordinated on 11a,b in new supramolecular assemblies in which electronic interactions could be forced between these homoditopic ligands (see Scheme 9).

Thus, bimetallic derivatives 11a,b have been reacted with linear homoditopic ligands 13–16 incorporating π-conjugated systems affording the new supramolecular rectangles 17a–20b (Scheme 9) [26]. X-ray diffraction studies of these supramolecular assemblies (Fig. 5) evidenced that the metric data of the dimetallic clips 11a,b do not change significantly upon their incorporation into the self-assembled structures. This result demonstrates the conformation rigidity of the Cu(I) based subunits. In all cases, the four Cu atoms are located in the same plane defining a rectangle and the aromatic moieties of the chromophores are parallel as result of a hindered rotation. Moreover, as a consequence of the short Cu(I)–Cu(I) intermetallic distance imposed by the bridging phosphane coordination mode, these aromatic moieties participate in face-to-face π interactions (phenyl centroid–centroid distance: 3.4–3.5 Å) with small lateral offsets. The dimensions of these nano-sized rectangles are fixed by the size of the ditopic ligands and reach 18.1 Å for 19a,b and 25.2 Å for 20a (Cu–Cu distances), with an overall dimension of 26a of about 39.0 Å in length [26].

These results demonstrate that, despite the repulsive interactions between the closed-shell π-clouds of the homoditopic ligands, the molecular clips 11a,b can force face-to-face π-stacking of aromatic derivatives upon coordination into well-defined supramolecular metalloparacyclophanes. This proves that, due to their rigidity and unique topology, complexes 11a,b are unique building blocks for the synthesis of π-stacked molecular assemblies exhibiting a [2,2]paracyclophane-like topology. This result opens appealing perspective for the design of multifunctional molecular material using this supramolecular assembling approach.

Moreover, the stacking pattern of the metalloparacyclophanes 19a,b and 20a is remarkable. In the solid state, these supramolecular rectangles organize into infinite columns with short inter-molecular distances (ca. 3.6 Å). Within these columns, the cationic parts of the rectangles have a parallel-displaced arrangement along the a-axes. Unique infinite columnar stack resulting from intra- and inter-molecular π–π interactions of (para-phenylenevinylene)-based chromophores 15 and 16 are thus formed at the macroscopic scale with these novel metalloparacyclophanes (Fig. 6) [26]. Organization of the π-conjugated chromophores is thus successfully performed at two hieratical levels, the first within the metalloparacyclophanes and the second in the infinite 1D stack of these metalloparacyclophanes. Complexes 11a,b are thus versatile molecular clips that allow one to control the organization of π-conjugated systems in supramolecular assemblies having a [2,2]paracyclophane-topology. Elucidation of the electronic properties of these novel π-stacked molecular assemblies as well as the use of other ditopic conjugated systems to construct novel metallo[2,2]paracyclophane are under active investigations.