Conjugated polymers based on metalla-aromatic building blocks.

Conjugated polymers usually require strategies to expand the range of wavelengths absorbed and increase solubility. Developing effective strategies to enhance both properties remains challenging. Herein, we report syntheses of conjugated polymers based on a family of metalla-aromatic building blocks via a polymerization method involving consecutive carbyne shuttling processes. The involvement of metal d orbitals in aromatic systems efficiently reduces band gaps and enriches the electron transition pathways of the chromogenic repeat unit. These enable metalla-aromatic conjugated polymers to exhibit broad and strong ultraviolet-visible (UV-Vis) absorption bands. Bulky ligands on the metal suppress π-π stacking of polymer chains and thus increase solubility. These conjugated polymers show robust stability toward light, heat, water, and air. Kinetic studies using NMR experiments and UV-Vis spectroscopy, coupled with the isolation of well-defined model oligomers, revealed the polymerization mechanism.

Conjugated polymers are macromolecules usually featuring a backbone chain with alternating double and single bonds (1–3). These characteristics allow the overlapping p-orbitals to form a system with highly delocalized π-electrons, thereby giving rise to intriguing chemical and physical properties (4–6). They have exhibited many applications in organic light-emitting diodes, organic thin film transistors, organic photovoltaic cells, chemical sensors, bioimaging and therapies, photocatalysis, and other technologies (7–10). To facilitate the use of solar energy, tremendous efforts have been devoted in recent decades to developing previously unidentified conjugated polymers exhibiting broad and strong absorption bands (11–13). The common strategies for increasing absorption involve extending π-conjugation by incorporating conjugated cyclic moieties, especially fused rings; modulating the strength of intramolecular charge transfer between donor and acceptor units (D–A effect); increasing the coplanarity of π conjugation through weak intramolecular interactions (e.g., hydrogen bonds); and introducing heteroatoms or heavy atoms into the repeat units of conjugated polymers (11–16). Additionally, appropriate solubility is a prerequisite for processing and using polymers and is usually achieved with the aid of long alkyl or alkoxy side chains (12, 17).

Aromatic rings are among the most important building blocks for conjugated polymers. In addition to aromatic hydrocarbons, a variety of aromatic heterocycles composed of main-group elements have been used as fundamental components. These heteroatom-containing conjugated polymers show unique optical and electronic properties (4–10). However, while metalla-aromatic systems bearing a transition metal have been known since 1979 due to the pioneering work by Thorn and Hoffmann (18), none of them have been used as building blocks for conjugated polymers. The HOMO–LUMO gaps (Eg) of metalla-aromatics are generally narrower (Fig. 1) than those of their organic counterparts (19–22). We reasoned that this feature should broaden the absorption window if polymers stemming from metalla-aromatics are achievable.

Fig. 1.

Comparison of traditional organic skeletons with metalla-aromatic building blocks (the computed energies are in eV). (A) HOMO–LUMO gaps of classic aromatic skeletons. (B) Carbolong frameworks as potential building blocks for novel conjugated polymers with broad absorption bands and improved solubility.

In recent years, we have reported a series of readily accessible metal-bridged bicyclic/polycyclic aromatics, namely carbolong complexes, which are stable in air and moisture (23–25). The addition of osmium carbynes (in carbolong complexes) and alkynes gave rise to an intriguing family of dπ–pπ conjugated systems, which function as excellent electron transport layer materials in organic solar cells (26, 27). These observations raised the following question: Can this efficient addition reaction be used to access metalla-aromatic conjugated polymers? It is noteworthy that incorporation of metalla-aromatic units into conjugated polymers is hitherto unknown. In this contribution, we disclose a polymerization reaction involving M≡C analogs of C≡C bonds, which involves a unique carbyne shuttling strategy (Fig. 2). This led to examples of metalla-aromatic conjugated polymers (polycarbolongs) featuring metal carbyne units in the main chain. On the other hand, the development of polymerization reactions plays a crucial role in involving certain building blocks in conjugated polymers (28–32). These efficient, specific, and feasible polymerizations could open an avenue for the synthesis of conjugated polymers.

Fig. 2.

Design of polymers and synthesis of monomers. (A) Schematic illustration of the polymerization strategy. (B) Preparation of carbolong monomers. Insert: X-ray molecular structure for the cations of complex 3. Ellipsoids are shown at the 50% probability level; phenyl groups in PPh3 are omitted for clarity.

Results and Discussion

As shown in Fig. 1, computational investigations revealed that the HOMO–LUMO gap (Eg) of the simplified model carbolong complex is 3.20 eV, which is lower than those calculated for pure organic aromatic skeletons such as naphthalene (Eg = 4.82 eV), thieno[3,2-b]thiophene (Eg = 5.13 eV), and quinoxaline (Eg = 4.77 eV). We thus envisioned that the small Eg should make such metalla-aromatic frameworks promising building blocks for conjugated polymers with unique properties.

In targeting metalla-aromatic conjugated polymers, a series of monomers (1–5) featuring a metallabicycle and a terminal alkyne with diverse spacer groups were synthesized via the reactions of multiyne carbon chains (L1–L5) with commercially available OsCl2(PPh3)3 and PPh3 (Fig. 2). These monomers were characterized by NMR spectroscopy, elemental analysis (EA), and high-resolution mass spectrometry (HRMS) (). The connectivity of 1b, 2b, and 3 was also unambiguously confirmed by X-ray diffraction (Fig. 2 and ).

With 1–5 in hand, attempts to polymerize such monomers were undertaken. Treatment of 1–5 with HCl·Et2O (0.20 M) in dichloromethane (DCM) at room temperature (RT) under a N2 atmosphere for 24 h afforded polycarbolongs P1–P5 (Table 1). Indeed, the formation of polymers containing 1a and 3 was complete in 30 min, illustrating the high efficiency of the polymerization reaction. In view of the various reactivities of different monomers, we allowed the polymerizations to proceed for 24 h to ensure complete conversion. Addition of ether to the crude mixtures led to rapid precipitation of P1–P5, which were isolated in high yields after multiple washes with tetrahydrofuran. Multinuclear NMR spectra of P1–P5 provided the structures of the polycarbolongs (). In the 31P NMR spectra, these polycarbolongs displayed resonances at ∼4.5 and –1.0 ppm, which are attributable to CPPh3 and OsPPh3, respectively (). Gel permeation chromatographic studies indicated good average molecular weights and low polydispersities (Đ). Notably, P1–P5 are highly stable and can be stored in air at RT for more than 2 y without noticeable decomposition. In all cases, thermolysis at 373 K for 2 h in air or exposure to white light for 96 h showed the apparent inertness ().

Table 1.

General applicability of the polymerization process.

The ultraviolet–visible (UV–Vis) spectroscopic features of polycarbolongs are summarized in Fig. 3. The characteristic energy absorption bands showed λmax values that varied between spacer groups (ranging from 550 to 650 nm). These λmax values were more redshifted than those of other skeletal conjugated metallopolymers, including polymetallaynes, polymetallocenes, and polymetallocyclopentadiene (33–36). In particular, polycarbolong P3 exhibited a broad and strong absorption band covering the entire UV–Vis region, even extending to the near-infrared (NIR) region. Its molar absorption coefficient at 650 nm was as high as 2.6 × 104/M/cm. Polycarbolongs P1–P5 are composed of metalla-aromatic repeating units and simple spacers, including styryl, biphenyl, butadiene-phenyl, fluorenyl, and ethynylbiphenyl. It is important to note that pure organic conjugated polymers based on these spacers do not exhibit such broad and strong absorption features (37–40). Polycarbolong P4 can be regarded as an alternative copolymer of a metalla-aromatic and fluorenylene vinylene. While the absorption edge of poly(fluorenylene vinylene) reached only 460 nm (39), that of P4 was remarkably redshifted up to 680 nm.

Fig. 3.

Physical properties and solubility. (A) UV–Vis absorption spectra of polycarbolongs (1.0 × 10−5 M) measured in DCM at RT. Molar absorption coefficients were calculated based on repeat units. (B) HOMO and LUMO energies and energy gaps of polycarbolongs. (C) UV–Vis absorption spectra of naphthalene, multiyne carbon chains L3, simple carbolong framework C1, monomer 3, and polycarbolong P3 measured in DCM at RT (1.0 × 10−5 M). (D) Polycarbolong P1a dissolved in eight different solvents (1.0 × 10−4 M), 1,2-dichloroethane (DCE), trichloromethane (TCM), acetonitrile (ACN). (E) Different polycarbolongs dissolved in DCM (1.0 × 10−5 M).

In addition, the molar absorption coefficient of P4 was considerably larger (2.2 × 104/M/cm; λmax = 585 nm) than that observed for poly(fluorenylene vinylene) (9.9 × 103/M/cm; λmax = 400 nm) (39). The strong absorptions of P1–P5 in the low-energy region resulted in a broad absorption window extending from UV to visible wavelengths and even into the NIR region. The characteristic energy absorption bands of polycarbolongs observed in films were similar to those in dilute solutions (). Collectively, these results indicate that the introduction of such metalla-aromatic frameworks into conjugated polymeric structures significantly affected the electronic properties.

To get more insight into the absorption contribution of polycarbolongs, the UV–Vis absorption of ligands (L1–L5) and monomers (1–5) was investigated (). As shown in Fig. 3, the characteristic absorption band of simple carbolong framework C1 ( for detail structure) is redshifted and enhanced compared to that of naphthalene. The characteristic absorption band of monomer 3 is also redshifted compared to that of its precursor ligand L3. These results indicate that the inclusion of the heavy atom osmium into an aromatic framework results in effects on the absorption property. With connecting spacer group, the λmax of 3 (484 nm) is redshifted compared to that of C1 (447 nm). It is noteworthy that no absorption peak in the low-energy region is detected in L3, C1, or 3. It reveals that the repeat spacer and carbolong units make little contribution to the absorption of polycarbolongs. In comparison with the absorption spectra of C1, 3, and P3, the characteristic absorption band is redshifted and enhanced with the extended π-conjugation. Hence, the high degree of delocalization in polycarbolongs is the main reason for their excellent absorption properties. Indeed, the actual redshifts observed between carbolong units and polycarbolongs are comparable to those for common conjugated polymers such as polythiophenes, poly(phenylenevinylene), and analogs (4–8).

To understand the cumulative effects of metalla-aromatics, the corresponding oligomers O1–O4 (Fig. 4) were synthesized and characterized (Fig. 4 and ). With increases in the number of repeating units, the systems of delocalized π-electrons were more extended, and Eg decreased. Using density functional theory (DFT) calculations, the Eg values of oligomers O1, O2, and O3 were calculated as 2.39, 2.02, and 1.77 eV, respectively. These oligomers exhibited strong π-electron delocalization, which was reflected by the calculated distribution of frontier molecular orbitals (). As a result, the characteristic lowest-energy absorption bands of oligomers were significantly redshifted and enhanced as the oligomer size was increased (Fig. 4).

Fig. 4.

Syntheses of oligomers. (A) Molecular structures of oligomers. (B) HRMS spectra of oligomers O1–O4. (C) UV−Vis absorption spectra of oligomers O1–O4 (1.0 × 10−5 M) measured in DCM at RT.

The energy gaps of P1–P5 were probed. The UV–Vis results combined with electrochemical analyses () revealed that the LUMO energies of P1–P5 ranged from –3.92 eV (P3) to –3.39 eV (P2a), while Eg ranged from 1.60 eV (P3) to 1.99 eV (P2b) (Fig. 3). For comparison, traditional conjugated polymers usually require comprehensive strategies (e.g., enough fused rings or a strong D–A effect) to achieve such a narrow Eg (11–13). These results suggest that altering the spacer group largely modulated the energy gaps of P1–P5, and such polycarbolongs with such diverse energy levels should have potential for many applications.

Note that many conjugated polymers with strong π–π stacking of their molecular chains are poorly soluble. A general strategy for increasing the solubility of these polymers involves decoration with long alkyl or alkoxy side chains (12, 17). Nonetheless, even without the aid of long carbon chains, the solubilities of P1–P3 and P5 in common organic solvents were good. As shown in Fig. 3, polycarbolong P1a was soluble in diverse solvents ( for a detailed solubility experiment). The maximum solubility in dimethyl sulfoxide (DMSO) and N,N-dimethylformamide (DMF) can reach 33.6 and 27.2 mg/mL, respectively. Interestingly, polycarbolongs P1–P5 exhibited different colors in DCM (Fig. 3). The greater solubility of P1–P5 can be attributed to the osmium centers bearing sterically encumbering ligands in the axial position, which precluded π–π stacking of polymer chains. The large PPh3 substituent that helps to sterically protect the spacer group and the polycationic nature of the polymer probably also affect the observed polymer solubility. This method provided a way to modulate the solubility of conjugated polymers via installation of bulky transition metal fragments.

We next investigated polymerization of monomer 2a (Fig. 5 and ). Upon treating 2a with HCl·Et2O, 31P NMR spectroscopic monitoring revealed rapid consumption of 2a (5.94 and 2.90 ppm [in green]) and immediate formation of 2a′ (13.81 and 8.06 ppm [in blue]) and P2a (4.49 and –1.00 ppm [in pink]). Species 2a′ was formed via a carbyne shift process and can be considered an activated monomer, and it was completely converted to P2a within ∼1 h. To further confirm the structure of 2a′, analogous compound 6, in which the H atom in the terminal alkyne of 2a was replaced with trimethylsilyl (TMS), was prepared. Compound 6 reacted with HCl·Et2O to give isolable carbyne shift product 7. The 31P NMR resonances of 7 (13.71 and 8.07 ppm) and 2a′ were comparable (). To reveal the polymerization process, Fourier transform infrared spectrometry was used to monitor the formation of polycarbolong P2a (). The C≡C–H and C≡C stretching vibrations of 2a at 3,211/cm and 2,078/cm, respectively, disappeared gradually. Meanwhile, the C=C stretching vibration at 1,620/cm became stronger. This indicates that the C≡C bonds were efficiently converted to C=C bonds. The corresponding mechanisms for the polymerization reactions are shown in . In view of the efficiency, specificity, and feasibility of this newly discovered stepwise polymerization between metal–carbon triple bonds and carbon–carbon triple bonds, we suggest that it could be designated as “metal-involved triple bond polymerization (MTP).”

Fig. 5.

Monitoring the polymerization reactions. (A) Metal carbyne shuttling strategy. (B) Stacked 31P NMR spectra showing the conversion of precursor monomer 2a into activated monomer 2a′ and polycarbolong P2a. (C) Generation of polycarbolong P2a in DCM at RT monitored in situ by UV–Vis spectroscopy (initial concentration 1.0 × 10−3 M).

These data are suggestive of an intriguing carbyne shuttling mechanism for the polymerizations. In 1–5, the metal carbyne (i.e., Os≡C) was stabilized by protection from the proximal bulky triphenylphosphonium substituents. Accordingly, such monomers are bench stable and were readily purified by column chromatography in air. This ensured the preparation of high-quality polycarbolongs. Nevertheless, under acidic conditions, 1–5 were readily transformed to Os≡C bond-shifted intermediates 1′–5′ via a carbyne shuttling reaction. Species 1′–5′ feature a less sterically protected Os≡C bond, which allowed effective polymerization reactions with terminal alkynes to form P1–P5, the first conjugated polymers with metal carbyne units in the main chain. After polyaddition, the carbyne reverted to the initial dormant state and was protected by the bulky triphenylphosphonium substituents, which stabilized the polycarbolongs toward light, heat, water, and air.

To gain insights into the kinetics of the polymerization, UV–Vis spectroscopy was used, and the formation of P2a from 2a was monitored. As shown in Fig. 5, 2a (λmax = 450 nm, in green) was completely consumed upon addition of HCl·Et2O, which generated 2a′ (λmax = 420 nm, in blue). No intermediate was observed. The characteristic UV absorption bands were consistent with those of model complexes 6 and 7 (). Following this, a new absorption peak at 565 nm (in purple) derived from P2a appeared and gradually increased in intensity. In comparison with the band locations in the absorption spectra of 2a and P2a, the characteristic energy absorption band was redshifted from 450 nm to 565 nm, revealing that π-conjugation was extended during polymerization. The time-dependent peak intensity for the peak at 565 nm and kinetic variations in the in situ UV–Vis absorption spectra were recorded to follow the generation of P2a (). These results showed that the polymerization was efficient. Besides, the formation of both P1a and P3 was monitored by NMR and UV–Vis spectroscopic studies (). Similar consecutive carbyne shuttling processes can be observed during this polymerization.

Conclusions

We have shown examples of conjugated polymers based on metalla-aromatics as a building block via polymerization. The resulting polycarbolongs exhibit not only broad and strong UV−Vis absorptions but also good solubility. Many metallopolymers have demonstrated a significant influence on the optical, electronic, and magnetic properties of the resulting materials (33, 41–48). Given that carbolong complexes have proved to be potent catalysts (49) and found wide applications in organic solar cells (26, 27), perovskite (50), photoacoustic imaging (51), and photothermal/photodynamic therapy (52, 53), it can be anticipated that polycarbolongs will show potential applications in catalysis, optoelectronics, and biotherapy. We will report these applications separately in subsequent articles.

Materials and Methods

Solvents were distilled from sodium/benzophenone (tetrahydrofuran, hexane, and diethyl ether) or calcium hydride (dichloromethane) under N2 before use. Nondistilled solvents were used in the reactions carried out in air. The starting materials were used as received from commercial sources without further purification. Chromatography was performed on alumina gel (200–300 mesh) or silica gel (200–300 mesh) in air. All the calculations were performed with the Gaussian 09 software package. Details for all compounds and polymers reported here are given in the .