NSRRC Activity Report 2023

Chemical Science 033 Copolymer Photocatalysts for Visible-Light-Driven Hydrogen Evolution Conjugated polymers with an all-acceptor design featuring enhanced electron output sites and compact network structures offer a solution to improve the efficiency of the photocatalytic hydrogen evolution reaction. T o effectively address the challenges of climate change, an urgent transition from fossil fuels to carbon-neutral energy sources is essential. Despite the abundance of solar energy as a renewable resource, its intermittent nature poses obstacles in meeting on-demand energy needs. An optimal solution involves storing solar energy through chemical fuel bonds, with photocatalytic hydrogen evolution gaining prominence as a method to convert solar energy into hydrogen. In the past five years, conjugated polymers have emerged as promising candidates due to their structural diversity, modular functionality, environmental friendliness, and tuneable energy levels ( Fig. 1(a) ). However, a critical drawback exists in the insufficient tentacles for electron output in these polymers. These tentacles play a crucial role in efficiently directing and concentrating electrons for interactions with cocatalysts or protons, thereby influencing photocatalytic performance. Previous efforts have concentrated on donor–acceptor (D–A) type conjugated polymers, integrating electron-output functional groups on the acceptor unit. However, this approach restricts the number of electron-output functional groups, resulting in over 50% of the conjugated polymer unit lacking these essential tentacles. U-Ser Jeng (NSRRC) and Ho-Hsiu Chou (National Tsing Hua University) introduced four novel oxidized ladder-type heteroarenes that incorporate dibenzo[b,d] thiophene 5,5-dioxide to fabricate all-acceptor (A 1 –A 2 )-type conjugated polymers, specifically named PBDTTFSOS, PBDTTTSOS, PIDDSOS, and PITDSOS. 1 These A 1 –A 2 - type conjugated polymers facilitate the direct combination of two conjugated monomers with electron-output sites, resulting in conjugated polymers featuring a higher density of electron-output sites compared to conventional D–A- type conjugated polymers. The increased electron-output site density is attributed to the presence of electron- output sites in both conjugated monomers. As part of the experimental controls, four D–A-type conjugated polymers with similar structures were also synthesized, designated as PBDTTFS, PBDTTTS, PIDS, and PITS ( Fig. 1(b) ). As shown in Figs. 2(a) and 2(b) in next page, A 1 –A 2 - type conjugated polymers have demonstrated significant efficiency improvements, ranging from two to three orders of magnitude, when compared to their D–A-type conjugated polymer counterparts. To understand why the photocatalytic activity of A 1 –A 2 -type conjugated polymers is superior to that of D–A-type conjugated polymers, a combination of small-angle X-ray scattering (SAXS) and ultrasmall-angle X-ray scattering (USAXS) measurements were conducted at TPS 13A . In the low-q region of USAXS spanning from 0.002 to 0.006 Å −1 , defined by the scattering vector q based on X-ray wavelength (λ) and scattering angle (2θ), A 1 –A 2 -type conjugated polymers—namely PBDTTFSOS, PBDTTTSOS, PIDDSOS, and PITDSOS— demonstrated consistent power-law scattering behavior, expressed as I(q) ∝ q^α. The corresponding fitted α values were –3.54, –3.14, –3.76, and –3.38, respectively. By contrast, D–A-type conjugated polymers—PBDTTFS, PBDTTTS, PIDS, and PITS—exhibited relatively smaller α values of –2.96, –2.83, –3.51, and –3.23, respectively ( Figs. 2(c) and 2(d) ). These results indicate that A 1 –A 2 - type conjugated polymers form more compact network Fig. 1 : (a) Schematic diagram of polymeric photocatalyst for photocatalytic hydrogen evolution from water, and (b) chemical compositions and molecular structures of the fabricated polymeric photocatalysts. [Reproduced from Ref. 1]