Methyl steric-hindrance-induced twisted acridine-based conjugated porous polymers for efficient 1O2-driven photocatalytic H2O2 production

doi:10.1016/S1872-2067(26)65227-8

Abstract

Abstract:

The effective generation and utilization of singlet oxygen (¹O₂) offers a promising yet challenging route for photocatalytic hydrogen peroxide (H₂O₂) production. In this work, we describe a molecular engineering strategy regulated by steric hindrance to control excitonic behavior and oxygen activation pathways in conjugated porous polymers (CPPs). When methyl groups are introduced into the acridine linker (from acriflavine to acridine yellow G), a planar CPP-A converts into a twisted CPP-G which has an approximately 65° dihedral angle. This change in conformation drastically alters the photophysical properties. The planar structure of CPP-A facilitates exciton dissociation and results in the development of superoxide radical (O₂^•-) via charge transfer. Conversely, the twisted conformation of CPP-G assists in enhancing spin-orbit coupling and intersystem crossing efficiency. These lead to an accumulation of triplet excitons and dominant ¹O₂ generation through energy transfer. The electrophilic ¹O₂ possesses a lower adsorption energy barrier on the photocatalyst surface, favoring subsequent proton-coupled electron transfer reactions. Consequently, ¹O₂-dominated CPP-G exhibits a remarkable H₂O₂ production rate of 8.97 mmol g^-1 h^-1 in pure water without sacrificial agents, significantly outperforming the O₂^•--dominated CPP-A (5.13 mmol g^-1 h^-1). Furthermore, CPP-G demonstrates high efficiency and selectivity in ¹O₂-mediated organic transformations. This work demonstrates that steric hindrance engineering serves as a generalizable strategy, enabling a fundamental shift in the reactive oxygen species generation pathway, from charge transfer to energy transfer, via simple structural modifications in metal-free CPP photocatalytic systems.

Key words: Conjugated porous polymers, Twisted structure, Excitonic behavior, Singlet oxygen, Photocatalytic H₂O₂ production

Guobang Li, Zhilin Wang, Mengkai Wang, Yanchun Liu, Cong Wang, Xianggang Zhou, Yonghui Wang, Xuzheng Cao, Huaqiao Tan. Methyl steric-hindrance-induced twisted acridine-based conjugated porous polymers for efficient¹O₂-driven photocatalytic H₂O₂ production[J]. Chinese Journal of Catalysis, 2027, 92: 145-156.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(26)65227-8

https://www.cjcatal.com/EN/Y2027/V92/I1/145

Figures/Tables 5

Scheme 1. The method for constructing CPPs from the relevant precursors.

Fig. 1. SEM and TEM images of CPP-A (a) and CPP-G (b). (c) FT-IR spectra for the CPPs and C6N7Cl3. (d) XRD spectra of the CPPs and C6N7Cl3. (e) C 1s spectra of XPS related to the CPPs. (f) N 1s spectra of XPS corresponding to the CPPs.

Fig. 2. (a) UV-vis DRS of the CPPs. (b) Tauc-plots related to the CPPs. (c) Energy band structures pertaining to the CPPs. (d) EIS plots and transient photocurrent (inset) of CPPs. (e) PL spectra recorded at 300 K for the CPPs. (f) TR-PL of the CPPs. Integrated PL intensity as a function of temperature for CPP-A (g) and CPP-G (h) (with inset showing the relevant temperature-dependent PL spectra). The value of Eb is obtained using the Arrhenius equation. (i) Delayed phosphorescence spectra of the CPPs measured at 77 K after 1 ms.

Fig. 3. (a) The photocatalytic activities of CPPs. (b) H2O2 production rates of CPP-G under different conditions. H2O2 production rates of CPP-G (c) and CPP-A (d) when different scavengers are present. (e) ESR spectra obtained from TEMP-1O2 radical trapping experiments facilitated by CPPs. (f) ESR spectra from DMPO-O2•- radical trapping experiments conducted using CPPs. (g) The AQY of CPP-G under different wavelengths. (h) The photocatalytic production of H2O2 catalyzed by recycled CPP-G for five times. (i) Comparison of CPPs with representative materials in terms of the H2O2 production rate.

Fig. 4. (a) Schematic structures of CPPs optimized by DFT. (b) Theoretical vertical excitation energy for singlet and triplet states, along with SOC constants for S1 and Tn. (c) Radar plot with the major characteristics of CPP-A and CPP-G. SOC refers to theoretical spin-orbit coupling constant measured in cm-1. Eb is experimentally measured exciton binding energy in meV. S1 → Tn pathway indicates calculated number of transition pathways shifting from singlet to triplet excited states. 1O2 is used to roughly evaluate the energy transfer process in CPPs. O2•- employed to assess the charge transfer processes occurring within CPPs. (d) Gibbs free energy diagrams for the reduction of O2 to H2O2 via 1O2 pathway on the CPP-G.

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Methyl steric-hindrance-induced twisted acridine-based conjugated porous polymers for efficient¹O₂-driven photocatalytic H₂O₂ production

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