Effective reactive reach of plasmonic hot holes through molecular barriers

The efficiency of plasmon-driven oxidation reactions depends on the ability of plasmon-generated hot holes to reach reactive interfaces. Here, we probe the effective reactive reach of plasmonic hot holes using alkanethiol self-assembled monolayers of varying chain length on Au nanoparticles. Operando photocurrent measurements combined with selective bromide poisoning isolate the contribution of hot holes traversing the molecular layer. The resulting activity exhibits a volcano-type dependence on chain length, with maximum reactivity observed for octanethiol (C₈), corresponding to an effective barrier thickness of ∼1 nm. Shorter chains enable rapid charge transfer but increase recombination losses, whereas longer chains hinder hole transport across the molecular layer. These findings demonstrate that plasmon-generated hot holes remain chemically reactive across nanometer-scale molecular barriers, with optimal performance achieved when transport occurs within the tunnelling regime.