The stacking structure of photocatalysts of organic molecules changes the direction of photocatalytic reaction

On January 26, 2023, Professor Alexander J. Cowan and Professor Andrew I. Cooper of the University of Liverpool (currently working at Zhejiang Normal University) made important progress in the development of organic photocatalysts. The collaborative team reported the phenomenon of stacking structure changing the photocatalytic reaction direction of organic molecule photocatalysts.

The related research results were published in the journal Nature Nanotechnology under the title “Packing-induced selectivity switching in molecular nanoparticle photocatalysts for hydrogen and hydrogen peroxide production”.

The stacked structure has a great influence on the photoelectric properties of solid-state organic optoelectronic materials. For example, the carrier mobility of pentabenzene varies by up to six orders of magnitude with changes in molecular packing structure (Chem. Rev.,2007. 107, 926)。 However, the current development of organic photocatalysts mainly focuses on the modification and study of their chemical composition and morphology, and the structure-performance relationship between stacking and performance is not fully understood.

This work reported a novel donor-acceptor organic molecule photocatalyst, 2,6-bis(4-cyanophenyl)-4-(9-phenyl-9H-carbazol-3-yl)pyridine-3,5-nitrile (CNP), modulated its nucleation rate to prepare two CNP nanoparticles with different stacking structures, amorphous nanospheres and nanorods with π-π ordered stacking.

The two materials showed completely different photocatalytic behaviors: amorphous nanospheres had high photocatalytic synthesis of H2O2 (water and oxygen as raw materials) (3.20 mmol g-1 h-1) but low hydrogen production activity (0.44 mmol g-1 h-1) in the semi-reaction; The semi-reaction of π-π ordered stacking nanorods had high hydrogen-producing activity (31.85 mmol g-1 h-1) but low synthetic H2O2 activity (0.36 mmol g-1 h-1). The activity of CNP molecules in photocatalytic production of H2O2 (nanospheres) and photocatalytic hydrogen production (nanorods) is one of the highest levels reported in organic molecular photocatalysts, respectively. The characterization of single crystal structure and transient spectroscopy showed that the selective transformation of this photocatalytic reaction path was attributed to different molecular accumulation modes in CNP nanoparticles. Compared with disordered stacking, the π-π ordered stacking structure is conducive to charge separation, transfer, and the generation of more long-lived triple excited states after photoexcitation, which explains its high hydrogen production activity. However, in the O2 atmosphere, the excited state and ground state triplet state O2 tends to follow the energy transfer path to generate 1O2, which competes with the generation H2O2 route. In the O2 atmosphere, nanospheres with disordered structure mainly follow the electron transfer path to generate H2O2 after photoexcitation.

The above results show that the packing structure of organic photocatalysts is an important strategy to regulate their band structure, charge separation and surface reaction charge transfer path, which provides a new idea for the performance regulation of organic photocatalysts. (Source: Science Network)

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