Photocatalytic Water Splitting Promoted by 2D and 3D Porphyrin Covalent Organic Polymers Synthesized by Suzuki-Miyaura Carbon-Carbon Coupling

Abstract

This work deals with the synthesis of metal-free and porphyrin-based covalent organic polymers (COPs) by the Suzuki-Miyaura coupling carbon-carbon bond forming reaction to study the photocatalytic overall water splitting performance. Apart from using 5,10,15,20-Tetrakis-(4-bromophenyl)porphyrin, we have chosen different cross-linker monomers to induce 2-dimensional (2D) or 3-dimensional (3D) and different rigidity in their resulting polymeric molecular structure. The synthesised COPs were extensively characterised to reveal that the dimensionality and flexibility of the molecular structure play an intense role in the physical, photochemical, and electronic properties of the polymers. Photoinduced excited state of the COPs was evaluated by nanosecond time-resolved laser transient absorption spectroscopy (TAS) by analysing excited state kinetics and quenching experiments, photocurrent density measurements and photocatalytic deposition of Ru³⁺ to RuO_2, and photocatalysis. In summary, TAS experiments demonstrated that the transient excited state of these polymers has two decay kinetics and exhibit strong interaction with water molecules. Moreover, photocurrent and photocatalytic deposition experiments proved that charges are photoinduced and are found across the COP molecular network, but more important charges can migrate from the surface of the COP to the medium. Among the various COPs tested, COP-3 that has a flexible and 3D molecular structure reached the best photocatalytic performances, achieving a photocatalytic yield of 0.4 mmol H₂ × g_COP-3^-1 after 3 h irradiation.

Keywords: 2D; 3D; 5,10,15,20-Tetrakis-(4-bromophenyl)porphyrin; Suzuki–Miyaura coupling reaction; covalent organic polymers; hydrogen generation; oxygen generation; photoactive polymers; photocatalysis; photocatalytic water splitting; porphyrin; solar fuels.

Figure 1

Simplified molecular structure of the three: COP–1, COP–2, and COP–3.

Figure 2

TEM images and EDX of: COP–3 before (a) and after (b) Pd NPs removal.

Figure 3

TEM images of COP–3 (a–c) and SEM images of COP–1 (d), COP–2 (e), and COP–3 (f).

Figure 4

(a) X-Ray powder diffraction spectrum of: Porph-Br₄ (black line), COP–1 (green line), COP–2 (blue line), COP–3 (purple line). (b) Solid state UV-Vis absorbance recorded for Porph-Br₄ (black line), COP–1 (green line), COP–2 (blue line), and COP–3 (red line).

Figure 5

(a) PWS at 30 °C under simulated sunlight in presence of: COP–1 (●), COP–2 (●) or COP–3 (●). (b) PWS of COP–3, generation of hydrogen (●) and oxygen (▲).

Figure 6

(a) Optimization of sonication time experiment. (b) Hydrogen chemisorption (■), physisorption (▲) and combined absorption (●) of COP–3. (c) Hydrogen production per gram of COP–3 after PWS reaction at different pH.

Figure 7

Photocatalytic hydrogen generation of COP–3 in the presence of sacrificial agents. (a) Amounts of methanol added: nothing (▲), 2.4 mmol (■) or 24 mmol (●). (b) Amounts of (NH₄)Ce(NO₃)₆ added: nothing (▲), 1 mmol (■) or 3 mmol (●).

Figure 8

Reusability test of COP–3. For the sake of clarity, the figure shows the hydrogen generation in the first, fifth, and tenth reuse experiment of 6 h.