Promotion of photocatalytic hydrogen production by utilization of triplet excited states of organic dyes and adjustment of π-π interactions

Dye/polymeric carbon nitride (PCN) systems exhibit great potential for photocatalytic hydrogen evolution (PHE), due to the broad light-harvesting region and optimized charge separation ability. However, charge recombination due to the unmatched photogenerated carrier lifetimes (ns level) of organic dyes and photocatalytic reaction rates (s level) is a major hindrance, which afforded a big challenge to modulate electronic processes by prolongation of carrier lifetimes of dyes. Herein, the excited triplet states of organic dyes with longer carrier lifetimes (ms level) were employed in PHE systems for the first time, which are stabilized by the adjustment of & pi;-& pi; interactions of conjugation skeletons with alkyl chain engineering. By utilizing the increased isolation effect of alkyl chains, from linear to branched ones with prolonged lengths, higher PHE activities were achieved by branched chain-substituted dyes due to their longer lifetimes, accompanied by the improved phosphorescence properties, and suppressed charge recombination. Accordingly, a relationship among properties of triplet excited states, alkyl chains, and PHE activity has been roughly built, providing valuable information for further promoting PHE activities by adjustment of dye-dye and dye-PCN interactions.