Visible-Light-Driven Hydrogen Evolution Using Planarized Conjugated Polymer Photocatalysts

Abstract

Linear poly(p-phenylene)s are modestly active UV photocatalysts for hydrogen production in the presence of a sacrificial electron donor. Introduction of planarized fluorene, carbazole, dibenzo[b,d]thiophene or dibenzo[b,d]thiophene sulfone units greatly enhances the H2 evolution rate. The most active dibenzo[b,d]thiophene sulfone co-polymer has a UV photocatalytic activity that rivals TiO2, but is much more active under visible light. The dibenzo[b,d]thiophene sulfone co-polymer has an apparent quantum yield of 2.3 % at 420 nm, as compared to 0.1 % for platinized commercial pristine carbon nitride.

Keywords: conjugated polymer; extended conjugation; photocatalysis; planarization; water splitting.

Figure 1

Photocatalytic hydrogen evolution rate (HER) is correlated with the optical gap, both for small molecule oligomers and for conjugated polymers (Cz=9H‐carbazole, DBT=dibenzo[b,d]thiophene). Each measurement was performed with 25 mg catalyst in water/MeOH/triethylamine mixture under broad‐spectrum irradiation (λ>295 nm; see Table 1 for visible‐light HERs). HER for platinized g‐C₃N₄, as measured on our specific experimental setup. Significantly higher values are known in the literature for g‐C₃N₄,1 which might be due to differences in the setups used.29

Figure 2

Structures of conjugated phenylene oligomers (SM1–SM5) and poly(p‐phenylene)s (P1K, P1S; left) and their planarized fluorene‐type analogs (FSM1–FSM3, P2–P7; right).

Figure 3

a) Time‐course for photocatalytic H₂ production using visible light for P1K, P6, and P7 (25 mg catalyst in water/MeOH/triethylamine mixture λ>420 nm); b) P6 and P7 (25 mg catalyst in water/MeOH/triethylamine mixture; λ>420 nm), photolysis run for a total of 65 h.