Introduction

Triggered by the pioneering work of Ian Manners and others, significant research efforts have recently been focused on the synthesis and investigation of metallopolymers [1, 2]. In fact, due to their potential usefulness in a plethora of applications that range from catalysis to light-emitting devices to sensory materials, metallopolymers have become a most prominent research theme at the interface of metal-organic chemistry and polymer science. Main-chain conjugated metallopolymers are an interesting class of materials with desirable electronic and mechanical properties [3]. One important subset of metallopolymers is the class of metallo-supramolecular polymers (MSPs) [4], which encompasses metal-containing polymers in which the metal–ligand interaction is dynamic in nature and thus acts as the supramolecular motif. Utilizing a variety of non-covalent interactions, several recent studies have demonstrated that virtually any type of building block can be assembled with supramolecular motifs, yielding polymer materials that span a broad range of structures and properties [5]. Metal–ligand interactions represent a particularly useful and well-studied means of preparing supramolecular polymers (Scheme 1) [6–10]. A wide variety of metal–ligand binding motifs is available that offers a broad range of binding characteristics (e.g., thermodynamic and kinetic stabilities) [11], which in turn can be utilized to tune the nature of the resulting supramolecular materials [12]. Interestingly, however, reports of metallo-supramolecular conjugated polymers with appreciable mechanical properties appear to be rare. With the objective to develop new organic/inorganic hybrid materials, which combine good mechanical properties and other attractive properties (e.g., high stability at elevated temperatures, specific opto/electronic functions) with ease of processing, we embarked on the exploration of various new classes of metallo-supramolecular polymers. The general design approach attempts to merge the structure of known polymer systems, for example thermally stable hydrocarbon polymers or semiconducting polymers, with the advantages of a dynamic (reversible) polymerization process [13], to allow access to materials that are otherwise difficult to process [14]. We herein summarize our recent activities focused on the synthesis and investigation of metallo-supramolecular conjugated polymers and high-temperature stable poly(arylene alkylene)s prepared by this approach [15–18]. Our study made use of the following building blocks (Scheme 2): (i) ditopic macromonomers based on low-molecular-weight poly(2,5-dialkoxy-p-phenylene ethynylene) (PPE) (1b) or poly(2,5-dioctyloxy-p-xylylene) (PPX) (2b) segments; (ii) low-molecular model compounds mimicking either the basic p-phenylene ethynylene (1a) or p-xylylene unit (2a); (iii) the 2,6-bis(1′-methylbenzimidazolyl)pyridine (Mebip) ligand as the binding unit; and (iv) Zn(ClO4)2, Fe(ClO4)2, and/or La(ClO4)3, which cause linear chain extension (Zn2+ and Fe2+) or branching/crosslinking (La3+). The new metallopolymers prepared by these low-molecular-weight building blocks can be readily solution-processed and represent illustrative examples for metallo-supramolecular polymers with good mechanical properties.

Scheme 1.
scheme 1

Schematic representation of the metallo-supramolecular polymerization of ditopic ligands with transition and lanthanide metal ions.

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Scheme 2.
scheme 2

Synthesis and molecular structures of the Mebip end-capped PPE and PPX (macro)monomers 1 and 2.

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Supramolecular Poly(p-phenylene ethynylene)s

Since the discovery of electrical conductivity in π-conjugated polymers three decades ago [19], (semi)conducting polymers have become the focus of major research and development activities around the globe [20]. The excitement for this new generation of polymeric materials reflects their potential to combine the processability and outstanding properties of polymers with the exceptional, readily tailored electronic and optical properties of functional organic molecules. In particular their potential utilization as synthetic metals [21] and as organic semiconductors in light-emitting diodes [22], field-effect transistors [23], photovoltaic cells [24], sensors [25], and other devices have motivated the development of synthesis and processing methods of conjugated polymers with unique electronic properties. Unfortunately, high-molecular-weight conjugated polymers typically display high thermal transition temperatures, limited solubility, and high solution viscosity, and processing of these materials is therefore often intricate and time-consuming [26]. Furthermore, it is sometimes difficult to avoid structural defects in these macromolecules and/or to appropriately purify the materials, and hence, defective sequences disrupting the conjugation are not uncommon [27]. Low-molecular-weight conjugated polymers or oligomers are therefore frequently employed [28], but these materials lack of chain entanglements preventing the expression of “typical polymer properties” [29]. As a result, the mechanical properties of these (semi)conductors are inferior compared to conventional plastics. The framework of dynamic (reversible) polymerization is an attractive approach to solve this dilemma, since it allows one to assemble high-molecular weight macromolecules from well-defined, easy-to-process precursors. To explore this opportunity, we embarked on the metallo-supramolecular polymerization of conjugated macromonomers that were derived by functionalizing a low-molecular weight poly(2,5-dialkoxy-p-phenylene ethynylene) core – as a representative of an intensely investigated class of conjugated polymers [30] – with 2,6-bis(1′-methyl-benzimidazolyl)pyridine (Mebip) ligands on the two terminal positions (Scheme 2, 1b) [16]. The Mebip moiety was chosen as it has been demonstrated to be a most versatile metal ion binding motif in supramolecular polymerizations [31] and also offers high thermal stability [32]. Reference materials using the ditopic model monomer 1a, which mimics an isolated p-phenylene ethynylene unit, were also studied [15].

The model monomer 1a and the Mebip end-capped PPE macromonomer 1b are readily synthesized by the Sonogashira coupling of 2,5-dicotyloxy-1,4-diiodobenzene, 1,4-diethynyl-2,5-bis(octyloxy)benzene with 2,6-bis(1′-methylbenzimidazolyl)-4-ethynylpyridine as an end capper (Scheme 2) [15, 16]. In view of the low solubility of high-molecular-weight PPEs with octyloxy side-chains [33], the number-average degree of polymerization, X n , of 1b was limited to 20–25 by slightly off-setting the molar ratio of the two bifunctional monomers and the use of appropriate amounts of the ligand end capper.

Optical Properties of Monomer 1a and Macromonomer 1b

Model monomer 1a exhibits electronic properties that are significantly different from those of Mebip ligands, which are not further conjugated [15, 31, c]. The UV-vis absorption spectrum of 1a in solution (Fig. 1a) shows a band associated with the Mebip moiety (λabs max = 321 nm) and additionally displays transitions at 352 and 390 nm, which appear to originate from the chromophore constituted by the Mebip ligands and the 1,4-diethynylphenylene (DPE) bridge [7, 34, 35]. Monomer 1a displays a much stronger fluorescence than electronically isolated Mebip ligands. The emission associated with the isolated Mebip moieties (around 368 nm) is virtually absent; instead, an emission band at 448 nm is observed, which (through excitation scans) can be linked to the electronic transition (presumably π–π*) that is also responsible for the lowest-energy absorption (Fig. 1a).

Fig. 1.
figure 1

UV-vis absorption (solid lines) and PL emission (dashed lines) spectra of (a) model compound 1a and (b) macromonomer 1b measured in CHCl3.

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The solution absorption spectrum of macromonomer 1b exhibits a main band centered at 448 nm and a weaker band around 319 nm (Fig. 1b). These transitions are associated with the π–π* and n–π* transition of the polymer backbone, respectively [33]. Optical excitation of 1b causes intense photoluminescence. The emission spectrum displays two well-resolved bands near 484 and 510 nm and exhibits features that are consistent with π–π* fluorescence. A comparison between the optical data of 1b and a similar poly(2,5-dioctyloxy-p-phenylene ethynylene) without the Mebip end groups [33] reveals that these materials display virtually identical optical characteristics and that the Mebip end groups do not significantly alter the electronic properties of the PPE backbone.

Supramolecular Polymerization of Monomer 1a and Macromonomer 1b

The formation of metallo-supramolecular polymers of the type [1a · MX2] n [15] and [1b · MX2] n [16] is readily achieved by the addition of an acetonitrile solution of the metal ion salt to a chloroform solution of (macro)monomer 1a or 1b. A variety of metal ions display appropriate interactions (i.e., large equilibrium constant and relatively rapid complexation kinetics) that allow for supramolecular polymerization utilizing the terdentate Mebip motif [31]. In our study, we employed Zn2+ or Fe2+, with the intent to probe the influence of the different electronic characteristics of these metals on the resulting metallopolymers. Zn2+ features a fully occupied d-orbital (3d10) and is a prototype for metals that show hardly any tendency for metal-to-ligand charge transfer (MLCT) with imine-type ligands [36]. By contrast, Fe2+ is well known to form pronounced MLCT complexes and to act as a strong fluorescence quencher [37].

With the goal of attaining insight into the mechanism of the metal-ion-mediated self-assembly processes of these ditopic ligands, we titrated Zn(ClO4)2 into solutions of 1a and examined the resulting products by means of UV-vis absorption and PL spectroscopy (Fig. 2) [15]. At a concentration of 10 μM the titration of 1a with up to 1 eq. of Zn2+ rendered the originally colorless solution orange. Concomitantly, the lowest-energy absorption of the monomer is red-shifted to ca. 421 nm (Fig. 2a), consistent with either an intra-ligand charge transfer (CT) occurring from the electron-rich arylethynylene core to the electron-poor metal-bound Mebip moieties [38, 39], or a narrowing of the π–π* transition upon metal binding on account of stabilization of the π* molecular orbital [40]. Irrespective of the mechanism, the electron density on the ligand is a key factor for the large bathochromic shift [18]. The titration data further reveal a shift of the two other absorption bands to 404 and 339 nm (Fig. 2a); the appearance of three isosbestic points suggests equilibria between a finite number of spectroscopically distinct species. Upon addition of Zn2+, the intensities of the new peaks increase in linear fashion, until a metal-to-monomer ratio of 1:1 is reached (Fig. 2a). Beyond this point, the subsequent addition of Zn2+ causes new spectral changes (Fig. 2b). The lowest-energy absorption is further red-shifted to 441 nm, while the 404 peak shifts to ca. 390 nm and the 339 nm peak appears to split into two bands at ca. 345 and 317 nm. The spectral changes level off at a metal-to-monomer ratio of 2:1. Isosbestic points are again observed, but they occur at different wavelengths than seen for the titration at metal-to-monomer ratios of below 1:1, indicating equilibration between a different set of spectroscopically distinct species. Thus, these data are consistent with the fact that at a 1:1 Zn2+:1a ratio a metallo-supramolecular complex has formed, in which each Zn2+ is complexed with two monomers, and the resulting [1a · Zn(ClO4)2] repeat unit is the origin of the observed dominating electronic transitions. At metal:1a ratios of above 1:1, depolymerization occurs, driven by the formation of the chain-terminating 1a–Zn2+-solvent complexes. Insets in Figs. 2a and 2b show the absorption changes at two selected wavelengths, highlighting the different nature of the predominant species involved in the self-assembly process at Zn2+:1a ratios of below and above 1:1. It should be noted that the high extinction coefficient of 1a dictated rather low concentrations (10 μM) for the above discussed optical experiments. As a result, these conditions clearly favor the formation of oligomeric species, as opposed to high-molecular weight macromolecules. In fact, as discussed above, the UV data nicely reflect a linear combination of absorption spectra associated with three distinctly different chromophores that we assign as free ligand, 1:1 Zn2+:1a and 1:2 Zn2+:1a complex. The relative contribution of these species was found to depend on the concentration of Zn2+ in the expected manner.

Fig. 2.
figure 2

UV-vis absorption spectra acquired upon titration of 1a (10 μM) with Zn(ClO4)2. Shown are spectra at selected Zn2+:1a ratios ranging from 0:1 to 1:1 (a) and from 1:1 to 2:1 (b). The insets show the normalized absorption at 317 nm (a) and 423 nm (b) as function of Zn2+:1a ratio. (c) PL emission spectra (at selected Zn2+:1a ratios of 0:1–2:1) acquired upon titration of 1a (10 μM) in CH3CN:CHCl3 (1:9 v/v) with Zn(ClO4)2. The inset shows the normalized emission intensity at 484 nm as function of Zn2+:1a ratio.

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Metal-binding also exerts a pronounced influence on the emission characteristics of 1a, as is evident from the PL spectra observed upon titration of Zn(ClO4)2 into a solution of 1a (Fig. 2c). The emission maximum is strongly red-shifted from 448 nm to ca. 594 nm and experiences significant broadening. Similar to the absorption titration experiment for the same system, the emission spectra gradually change as the metal-to-1a ratio is increased. At a ratio of 1:1 the emission of the uncomplexed monomer has completely disappeared (Fig. 2c inset). Interestingly, in contrast to the absorption data, subsequent addition of Zn2+ results in no further spectral changes. This behavior is consistent with the fact that essentially irrespective of the relative amount of Zn2+, emission occurs always from the same low-energy electronic states, presumably the 1a–Zn2+-solvent complexes referred to above.

Evidence for the formation of metallo-supramolecular polymers at higher concentrations comes from viscosity data (Fig. 3) for a series of solutions with different Zn(ClO4)2:1a ratios. The data (acquired at constant total solute concentration) show a steady increase of the reduced viscosity up to a Zn2+:1a ratio of 1. Beyond this point a decrease in the reduced viscosity is observed, confirming the depolymerization of the supramolecular polymer. As expected, macromonomer 1b also polymerizes readily if exposed to Zn2+ or Fe2+ and pronounced visual changes are observed [16]; the addition of an equimolar amount of Zn2+, dissolved in CH3CN, to a solution of 1b in CHCl3 (38.5 mg/mL) resulted in an instantaneous and significant increase of the solution’s viscosity. To ensure processability of the resulting metallopolymer by attenuating its molecular weight, a slightly higher than stoichiometric amount (1.04 equiv) of Zn2+ relative to the Mebip units present was employed. Evaporation of the solvent led to a red solid, which displayed appreciable mechanical properties. This metallopolymer could be re-dissolved in CHCl3 and the resulting solutions were employed to produce (i) thin films for optical experiments by spin-coating (thickness <1 μm), (ii) free-standing films (thickness ca. 60 μm) for mechanical experiments by solution-casting, and (iii) monofilaments by simple solution spinning. These samples, shown in Fig. 4, visualize unequivocally that [1b · Zn(ClO)4] n - very much in contrast to the neat macromonomer 1b and a poly(2,5-dioctyloxy-p-phenylene ethynylene) of similar X n but without the Mebip end groups [33] – offers considerable mechanical strength and flexibility. Metallopolymers based on 1b and Fe2+ were prepared in a fashion similar to those of Zn2+. This led to the instantaneous formation of a green mixture and a significant increase of the solution’s viscosity. In this case, the solution was directly processed into thin films for optical experiments by spin-coating and freestanding films (thickness ca. 110 μm, Fig. 4c) for mechanical experiments by solution-casting.

Fig. 3.
figure 3

Reduced viscosities of a series of dimethylacetamide solutions comprising Zn(ClO4)2 and 1a at different molar ratios. The total concentration of solute was kept constant at 35 mg mL−1.

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Fig. 4.
figure 4

(a) A film and (b) fiber of the metallo-supramolecular polymer [1b · Zn(ClO4)2] n and (c) a film of the metallo-supramolecular polymer [1b · Fe(ClO4)2] n .

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The solid-state optical absorption spectrum of a [1b · Zn(ClO4)2] n film prepared by spin-casting from a CHCl3 solution (not shown), exhibits essentially the same features as a thin film of the neat macromonomer 1b, i.e., broad bands around 453 and 321 nm and a narrow aggregation band at 484 nm, suggesting that the band gap of the conjugated PPE sequences remains essentially unchanged upon binding of 1b with Zn2+. In order to compare the Zn2+-mediated self-assembly process of 1b with that of 1a, Zn(ClO4)2 was titrated into a solution of 1b, and the resulting products were analyzed by means of PL spectroscopy (Fig. 5). It should be noted that the precipitation of high-molecular-weight macromolecules at increased concentrations dictated that the PL titration experiment be performed at rather low concentrations (22 μM). As such, these conditions clearly favor the formation of oligomeric species as opposed to high-molecular-weight aggregates. Figure 5 reveals that the emission associated with the PPE moieties is strongly quenched upon addition of Zn2+. The inset of Fig. 5, which displays the relative emission intensity at λabs max (484 nm) as a function of Zn2+, nicely shows that the effect depends on the concentration of Zn2+ in a nonlinear fashion and levels off at a Zn2+:1b ratio of 1:1, at which point the emission observed for the neat 1b is almost completely quenched. In the light of the above-discussed behavior of model monomer 1a, these titration data are indicative of the formation of a metallosupramolecular complex of the form [2 · Zn(ClO4)2] n .

Fig. 5.
figure 5

PL emission spectra acquired upon titration of 1b (0.022 mM) with Zn(ClO4)2 (excitation at 400 nm). Shown are spectra at selected Zn2+:1b ratios of 0:1 to 2:1. The inset shows the normalized emission intensity at 484 nm as function of Zn2+:1b ratio.

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A comparison of the solid-state optical properties of [1b · Fe(ClO4)2] n and the neat macromonomer 1b reveals the expected similarities and differences. The absorption spectrum (Fig. 6) shows that the transitions associated with the PPE backbone – broad bands around 445 and 320 nm and a narrow aggregation band at 476 nm – remain essentially unchanged. A characteristic [41] metal-to-ligand charge-transfer absorption band centered at 635 nm can be observed, which is absent in case of [1b · Zn(ClO4)2] n . This MLCT band is indicative of the formation of the 2:1 Mebip:Fe2+ complex, which, of course, is the basis for the formation of the targeted supramolecular polymer [7]. It is well known that the latter complex is prone to dissipate excited-state energy through non-radiative processes; concomitantly [1b · Fe(ClO4)2] n does not fluoresce.

Fig. 6.
figure 6

UV-vis absorption spectra of metallopolymer [1b · Fe(ClO4)2] n measured on a spin-cast thin film (dashed) and macromonomer 1b (solid).

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Thermomechanical Properties of [1b · Zn(ClO4)2] n and [1b · (Fe(ClO4)2] n

The thermal and mechanical properties of the supramolecular polymers [1b · Zn(ClO4)2] n and [1b · (Fe(ClO4)2] n and the parent macromonomer 1b were investigated by means of thermogravimetric analysis (TGA) and dynamic mechanical thermoanalysis (DMTA) [16]. TGA traces of the neat macromonomer 1b (Fig. 7a) acquired under N2 reveal an onset of significant weight loss (2%) at 363 °C, corresponding to the thermal degradation of the polymer side chains. This experiment suggests outstanding thermal stability for the PPE building block, but it should be noted that thermal cross-linking of the ethynylene units, which cannot be discerned by TGA, typically occurs at temperatures above 150 °C. The Zn2+ and Fe2+ metallopolymers (Fig. 7a) show a rather similar thermal behavior, but the 2% weight loss occurs at somewhat lower temperatures (313 and 250 °C, respectively), particularly in the case of Fe2+. We ascribe this situation to the thermal degradation of the perchlorate counterions. We speculate that the significantly lower 2% weight loss temperature of the Fe2+ metallopolymer (compared to the Zn2+ system) may be due to a catalytic effect of the metal on the thermal decomposition of the perchlorate ions [42]. With the objective to maximize the thermal stability, we are considering investigating the influence of the counterion in more detail.

Fig. 7.
figure 7

(a) Thermogravimetric analysis (TGA) traces of 1b (solid), [1b · Zn(ClO4)2] n (dashed), and [1b · Fe(ClO4)2] n (dotted). The experiments were conducted under N2 at a heating rate of 20 °C/min. (b) Dynamic mechanical thermoanalysis (DMTA) traces of [1b · Zn(ClO4)2] n (dashed) and [1b · Fe(ClO4)2] n (solid). The experiments were conducted under N2 at a heating rate of 3 oC/min and a frequency of 1 Hz.

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Gratifyingly, both [1b · Zn(ClO4)2] n and [1b · (Fe(ClO4)2] n films display sufficient mechanical strength to allow for characterization by DMTA (Fig. 7b). Experiments were conducted in a temperature range of −20 to 100 °C. The moduli of [1b · Zn(ClO4)2] n and [1b · (Fe(ClO4)2] n were determined to be ca. 160 and ca. 140 MPa at −20 °C and ca. 100 and ca. 83 MPa at 25 °C, respectively. A decrease of the modulus of both materials was observed over the experimental temperature range; [1b · (Fe(ClO4)2] n exhibits a distinguishable transition at ∼40 °C, which, in view of the striking similarity to the DMTA trace of a high-molecular-weight poly(2,5-dialkoxy-p-phenylene ethynylene) (M n ∼83,000 g mol−1) reported before [43], is tentatively assigned to a glass transition. The moduli of [1b · Zn(ClO4)2] n and [1b · (Fe(ClO4)2] n are similar to that of the latter polymer, and appear to be among the highest reported for any metallo-supramolecular conjugated polymer to date.

Supramolecular Poly(p-xylylene)s

As another example, we recently investigated metallo-supramolecular poly(arylene alkylene)s, which were targeted because of their potential usefulness as high-temperature stable materials [17]. Poly(p-xylylene) (PPX) [44] is the most prominent representative of the poly(arylene alkylene) family. This polymer and its derivatives are well-known for an appealing combination of high thermal stability, excellent solvent resistance, high degree of crystallinity, low dielectric permittivity, and outstanding barrier properties [45, 46]. In addition, PPX exhibits excellent mechanical properties with a Young’s modulus and a tensile strength of ca. 2.4 GPa and 47 MPa, respectively [45]. Thus, PPX is an attractive material for many applications, including packaging of electronic components, medical device fabrication, and artifact conservation [45]. Unfortunately, broad industrial exploitation of PPX has been stifled on account of its intractability. Its high melting temperature (424 °C, which overlaps with the onset of thermal degradation) and poor solubility make conventional processing protocols virtually impossible [47, 48]. As a result, the only practical approach for the synthesis and processing of PPX is by chemical vapor deposition polymerization [49]. It appears that this dilemma can, again, be solved by utilizing a supramolecular polymerization process. Building on the above-described work regarding the metal-ligand based self-assembly of Mebip-terminated PPEs and exploiting a reaction methodology developed in our laboratories for the reduction of PPEs to their corresponding PPX derivatives [50], we utilized supramolecular metallopolymerization for the synthesis of arylene alkylene metallopolymers [17]. The two PPX building blocks used are the ditopic model monomer 2a and a low-molecular-weight Mebip-terminated macromonomer comprising a poly(2,5-dioctyloxy-p-xylylene) segment (2b), in combination with Zn(ClO4)2, Fe(ClO4)2 and/or La(ClO4)3 (Schemes 1 and 2).

The Mebip end-capped xylylene monomers 2a and 2b were prepared from the corresponding PPE precursors 1a and 1b (cf. Section 2) via diimide reduction chemistry utilizing p-toluenesulfonylhydrazide (TSH) and tripropylamine (TPA) (Scheme 2) [49, 51]. A Mebip end-capped PPE with an X n of ca. 20 was employed to afford a PPX of slightly higher X n (ca. 25) after purification.

Self-Assembly and Solution Properties of the Metallo-Supramolecular Polymers of 2a and 2b

The formation of metallo-supramolecular species [2a · MX n ] n and [2b · MX n ] n is readily achieved by the addition of one equivalent of an appropriate metal ion salt to a solution of the Mebip end-capped xylylenes 2a and 2b. Encouraged by the successful metallopolymerization of Mebip-end-capped PPEs with Zn2+ and Fe2+, we opted to employ these metal ions in the present study. To probe the complexation characteristics of 2a and 2b, we first conducted a detailed optical titration study with Zn2+, which offers both a large equilibrium constant and rapid complexation kinetics. To this end, Zn(ClO4)2 was titrated into solutions of 2a and 2b, and the resulting products were analyzed by means of UV-vis spectroscopy (Fig. 8). It should be noted that precipitation, presumably of high-molecular-weight macromolecules, was observed if these experiments were carried out in too concentrated solutions; this dictated that the UV-vis titrations be performed at a concentration of 22–25 μM of the respective ditopic ligands. As such, these conditions again favor the formation of oligomeric species as opposed to high-molecular-weight aggregates. The addition of Zn2+ to xylylenes 2a and 2b rendered the originally colorless solutions pale yellow. In both cases a new lowest-energy absorption band was observed, centered at 354 nm in the case of 2a and 360 nm in the case of 2b (Figs. 8 a and 8b, respectively). Concomitantly, in model compound 2a the intensity of the initial absorption band with λ max at 318 nm associated with the π–π* transition related to the Mebip moiety was significantly reduced upon binding, as was the related absorption of macromonomer 2b observed as a shoulder at ∼330 nm. The spectra of both titration series exhibit a single isosbestic point at ∼340 nm up to Zn2+:2a and Zn2+:2b ratios of approximately 1:1, suggesting equilibria between the bound and unbound monomers 2a and 2b. At Zn2+:2 ratios of greater than one, new isosbestic points are observed, which appear to correspond to the presence of chain-terminating monomer-Zn2+-solvent complexes formed upon depolymerization of the supramolecular species in the presence of an excess of metal [15]. We further quantified the spectral changes observed in the above-discussed Zn2+ titrations as the normalized intensity of the emerging absorption bands; the data are plotted in the insets of Fig. 8. Gratifyingly, the changes are virtually identical for the well-defined monodisperse model compound 2a and the macromonomer 2b, leveling off at a Zn2+:2a and Zn2+ :2b ratio of 1:1, which is consistent with the ditopic nature of the monomers and the successful self-assembly in dilute solution. Similar titration data (not shown) were obtained upon addition of Fe(ClO4)2 to 2b. In this case, a characteristic [7, 16, 31] metal-to-ligand charge-transfer absorption band was observed at 570 nm.

Fig. 8.
figure 8

UV-vis absorption spectra acquired upon titration of 2a (0.025 mM) (a) and 2b (0.022 mM) (b) with Zn(ClO4)2. Shown are spectra of Zn2+:2a ratios between 0 and 1.14 and Zn2+:2b ratios ranging from 0:1 to 1:2.01. The insets show the normalized absorption at 360 nm (a) and 370 nm (b) as a function of Zn2+:2a and Zn2+:2b ratio, respectively.

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Preparation and Solid-State Optical Properties of Metallo-Supramolecular Polymers Based on 2b

Monomer 2b is a pale yellow powder, which is soluble in chloroform and can readily be spin- or solution-cast into homogeneous thin films. However, this material does not display any self-supporting mechanical properties. The preparation and processing of metallo-supramolecular polymers based on 2b and Zn(ClO4)2 or Fe(ClO4)2 ([2b · M] n ) followed the protocol developed for the metallopolymerization of Mebip-end-capped PPE 1b, which resulted in materials with outstanding mechanical properties. The procedure involved co-dissolution of 2b and an equimolar amount of either of the metal salts in a CHCl3/CH3CN mixture, followed by solution casting. Interestingly, the addition of either Zn(ClO4)2 or Fe(ClO4)2 in CH3CN to 2b in CHCl3 resulted in solutions of [2b · Zn(ClO4)2] n and [2b · Fe(ClO4)2] n , which upon casting and drying yielded films that displayed poor mechanical properties (Fig. 9a). To probe whether an increase of the X n of the macromonomer would change this situation, the experiment was repeated with a macromonomer 2b of an X n of ∼32 instead of ∼25, but the behavior of this ditopic ligand was virtually the same. Thus, the [2b · M] n samples display mechanical properties that are clearly inferior when compared with the corresponding [1b · M] n analogues. The structural purity of 2b established by NMR, the successful self-assembly in dilute solution established by optical titration, and the fact that the binding strength of 2b should be similar to that of 1b and many related systems [31] suggests that the [2b · M] n supramolecular materials prepared here are indeed of polymeric nature. Thus, the observation of poor mechanical properties seems to reflect the lack or low concentration of chain entanglements and indicates weak intermolecular interactions between the individual supramolecular chains.

Fig. 9.
figure 9

Pictures of solution-cast films (CHCl3) of the metallo-supramolecular polymers [2b · Fe(ClO4)2] n (a) and [2b · Fe(ClO4)2/La(ClO4)3] n (b).

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With the objective to improve the material’s mechanical properties, we employed minor amounts of lanthanide perchlorate salts, which are well-known to bind three Mebip ligands [52], as a cross-linking/branching unit in our supramolecular polymerization of 2b and Fe(ClO4)2 (Scheme 1). To generate the crosslinks/branch points in a metal ion:2b system that was processed as described above, 9 mol% La(ClO4)3 (with respect to 2b) in CH3CN was added to xylylene 2b in CHCl3 (71.4 mg/mL) with the assumption of a 3:1 Mebip:La3+ binding motif. The further addition of 91 mol% Fe(ClO4)2 in CH3CN resulted in an instantaneous and dramatic visual increase of the solution’s viscosity, illustrating the successful linear chain growth (via 2:1 Mebip:2b binding) of the preformed 3:1 Mebip:La3+ complexes. Solution casting and slow evaporation of the solvent led to purple films, which displayed appreciable mechanical properties (Fig. 9b) in stark contrast to neat 2b and poly(2,5-dioctyloxy-p-xylylene) [50] of similar X n but without Mebip end groups. The deep purple color of the material results from a MLCT, indicative of the formation of 2:1 Mebip:Fe2+ complexes, as the addition of La(ClO4)3 to 2b resulted in the formation of only near colorless solutions [7, 16, 31]. As a consequence of non-radiative energy dissipation processes associated with the MLCT complex involving iron, [2b · Fe(ClO4)2] n and [2b · Fe(ClO4)2/La(ClO4)3] n also do not fluoresce.

Thermomechanical Properties of [2b · Fe(ClO4)2/La(ClO4)3] n

The mechanical and thermal properties of [2b · Fe(ClO4)2/La(ClO4)3] n films were investigated in more detail by means of thermogravimetric analysis (TGA), dynamic mechanical thermoanalysis (DMTA), and modulated differential scanning calorimetry (MDSC), all under N2. TGA traces (Fig. 10a) of neat 2b reveal an onset of significant weight loss (2%) at 354 °C, which has previously been related to the degradation of alkyl side chains [53]. The supramolecular polymer [2b · Fe(ClO4)2/La(ClO4)3] n shows a rather similar overall thermal behavior (Fig. 10a). However, the initial 2% weight loss was observed at a substantially lower temperature (193 °C), which we again ascribe to the thermal degradation of the perchlorate counterions upon melting. The mechanical properties of [2b · Fe(ClO4)2/La(ClO4)3] n were elucidated by DMTA in a temperature range of −20 to 100 °C (Fig. 10b). The room-temperature modulus was determined to be 220 MPa, placing the self-assembled system above the linear PPE metallopolymer discussed in Section 2 and in the same regime as low-density polyethylene. The modulus decreased continuously as the temperature was increased, which may be on account of progressive decomplexation of the more weakly bound lanthanide-Mebip cross-links. To determine the melting temperature, T m, of both 2b and [2b · Fe(ClO4)2/La(ClO4)3] n , MDSC experiments were performed (Fig. 11). The MDSC trace of neat 2b clearly shows a reversible endotherm with maximum at 166 °C corresponding to the T m, which is virtually identical to that reported for a poly(2,5-dioctyloxy-p-xylylene) of similar X n but without Mebip end groups (170 °C) [50]. The MDSC trace of [2b · Fe(ClO4)2/La(ClO4)3] n shows a largely irreversible exotherm at 176 °C that we associate, by comparison with TGA data, to the degradation of the perchlorate counterions. The reversible portion of the scan shows a weak endotherm at 166 °C, which corresponds to melting at a temperature which is nearly identical to that of 2b. Thus, the MDSC data suggest that the melt transition of the new metallo-supramolecular PPX is largely governed by the hydrocarbon building block and can be tailored to meet specific thermal demands by judicious design of the latter.

Fig. 10.
figure 10

(a) Thermogravimetric analysis (TGA) traces of [2b · Fe(ClO4)2/La(ClO4)3] n (solid) and xylylene 2b (dashed). The experiments were conducted under N2 at a heating rate of 10 °C/min. (b) Dynamic mechanical thermoanalysis (DMTA) trace (modulus: dashed, tan δ: solid) for [2b · Fe(ClO4)2/La(ClO4)3] n . The experiment was conducted under N2 at a heating rate of 3 oC/min and a frequency of 1 Hz.

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Fig. 11.
figure 11

Modulated differential scanning calorimetry (MDSC) heating scans of 2b (a) and [2b · Fe(ClO4)2/La(ClO4)3] n (b) with nonreversible heat flow (solid), heat flow (dashed), and reversible heat flow (dotted). The experiments were conducted under N2 at a heating rate of 2 oC/min.

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Conclusions

With the objective to develop new organic/inorganic hybrid materials, which combine good mechanical properties and other attractive characteristics (e.g., high stability at elevated temperatures, specific opto/electronic functions) with ease of processing, we embarked on the exploration of two new classes of metallo-supramolecular polymers. We have demonstrated that the supramolecular metallopolymerization of PPE- and PPX-type Mebip end-capped macromolecules can be employed to create metallopolymers which are readily solution-processed and display good mechanical properties. The comparison of mechanical data acquired for the two series nicely illustrates how strongly the nature and interactions among the building blocks and the self-assembly characteristics of the metal ion affect the materials properties. Thus, metallosupramolecular polymerization not only offers an attractive approach to assemble high-molecular-weight macromolecules from well-defined, easy to process precursors, but the broad range of metal ions available allows one to easily tailor the desired properties. It appears that the use of such dynamic polymerizations offers a broadly useful means to attain thermally robust inorganic/organic hybrid materials without altering the properties associated with the macromolecular core and should be easily extendable to other thermally stable cores.