Conjugated Polymers

Polymer photocatalysis originated in the early 1980s, after the global oil crisis, people realized the arduous challenge of producing solar fuels from sustainable materials. As a promising alternative to traditional inorganic semiconductor photocatalysis, conjugated polymers have the unique advantages of low cost, high chemical stability and molecular tunable photoelectric properties.

Basic Principles of Heterogeneous Photocatalysis

Under light irradiation, the semiconductor collects incident photons and forms charges (electron-hole pairs) inside the photocatalyst. The photo-generated charges then separate and migrate to the surface of the semiconductor where the redox reaction occurs. On the other hand, most photo-generated electrons and holes can undergo bulk recombination or surface recombination, which is a competitive process of surface reactions.

Figure 1. The main process of polymer semiconductor photocatalysis (D: electron donor; A: electron acceptor) [1]

Classification & Applications

π-conjugated linear polymer

• Polyparaphenylene (PPP), synthesized by nickel-catalyzed 1,4-cross-coupling. Studies have shown that the band gap of PPP-11 (the average length of the polymer chain is 11 phenylene units) is 2.9 eV, which is largely independent of the polymer chain length determined by photoacoustic spectroscopy. Diethylamine is used When used as a sacrificial electron donor, the hydrogen evolution rate of PPP-11 is 2.1 µmol h-1.
• The Ni-catalyzed Yamamoto coupling of 2,5-dibromopyridine was used to synthesize a linear polymer (PPy) containing pyridyl units. Compared with the optical gap of 2.9 eV in PPP, the band gap of PPy is reduced by 2.4 eV. This observed red shift of the PPy absorption spectrum combined with better photo-induced charge separation results in a nearly 10-fold increase in hydrogen evolution for PPy under visible light (>400 nm) compared to PPP.

Figure 2. The structure of linear polymers: polyparaphenylene (PPP), polypyridine-2,5-diyl (PPy) and oligomers for photocatalysis. [2]

• In addition, the π-conjugated polymer is effective as a carrier, especially for Pd and Cu in the Wacker-type reaction. The π-conjugated spacer promotes the redox of the coordination metal. For example, Yasushi Sato et al. prepared a new polymer-supported CuCl₂ catalyst. Among them, the CuCl₂ complex of π-conjugated poly(2,2'-bipyridine-5,5'-diyl) (PBpy) has higher catalytic activity and stability than poly(vinylpyridine)-CuCl₂ catalyst sex. The presence of the π-conjugated system in the polymer is believed to promote the redox reaction of copper in the catalytic reaction.

Figure 3. Immobilization of CuCl₂ by PPy. [3]

Conjugated Porous Polymer

• Cross-linked CPs with three-dimensional porous characteristics have received more and more attention due to their large electronic delocalization network structure, excellent stability and tunable optoelectronic properties. These advantages have stimulated their extensive applications in energy storage, chemical sensing, gas absorption and storage, and heterogeneous catalysis.

Figure 4. Synthesis of conjugated microporous polymer (CMPs) photocatalyst by Suzuki-Miyaura polycondensation [4]

References

1. Chunhui Dai. (2019). "Conjugated polymers for visible-light-driven photocatalysis," Energy & Environmental Science 13: 24-52.
2. Vijay S. Vyas. (2016). "Soft Photocatalysis: Organic Polymers for Solar Fuel Production," Chemistry of Materials 28(15): 5191-5204.
3. Yasushi Sato. (2000). "Poly(pyridine-2,5-diyl)-CuCl₂ catalyst for synthesis of dimethyl carbonate by oxidative carbonylation of methanol: catalytic activity and corrosion influence,"Catalysis Letters 65: 123-126.
4. Reiner Sebastian Sprick. (2015). "Tunable organic photocatalysts for visible-light-driven hydrogen evolution" Journal of the American Chemical Society 137(9): 3265-70.