Surface-Enhanced Raman Spectroscopy as a Probe to Understand Plasmon-Mediated Photochemistry

Abstract

The development of plasmonic nanostructures as light-activated photocatalysts has proven to be a promising research avenue due to their ability to access and drive unfavorable chemical reactions. Theses chemical reactions are fueled by the presence of surface plasmons, which are the collective oscillation of the free electron density on the material’s surface. Once a surface plasmon is photoexcited, their initial energy rapidly decays into multiple different pathways, such as enhanced electromagnetic fields, an abundance of hot carriers, and dramatically elevated local thermal environments. To better understand the various chemistries that are enabled by plasmonic materials and the associated mechanisms driving these processes, we have employed surface-enhanced Raman spectroscopy to interrogate a plethora of plasmon-molecule coupled systems. Our initial studies investigated the relationship between the plasmonic local fields and a well-established plasmon-driven photochemical reaction. We found that there were no observable correlations between the two in our studies and identified a competing degradation pathway for the studied analytes. In addition to exploring well-studied plasmon-induced chemical photoreactions, we have highlighted two new reactions that were accessed on the gold film-over-nanosphere substrates. First, we were able to induce and subsequently monitor a selective intramolecular methyl migration on N-methylpyridinium using surface-enhanced Raman spectroscopy. This work emphasizes the growing potential of initiating highly-selective chemistries with plasmonic materials for synthetic or redox purposes. The second previously unreported plasmon-driven reaction involves the double cleavage of the C-N bond on a pair of viologen derivatives. While these viologens have traditionally been employed as robust redox species, the unique and highly-powerful plasmonic local fields allowed the viologens to access an entirely new reaction pathway to transform into 4,4’-bipyridine. Lastly, we discuss our experimental approaches towards transiently studying the mechanism behind plasmon-mediated hot electron transfer. Using ultrafast surface-enhanced Raman spectroscopy, we interrogated the transient dynamics that occurred between surface plasmons and a bevy of electron accepting chemical adsorbates. Ultimately, the primary goal of this work is to provide a quantitative description of the transient interactions, which will assist in increasing the reported efficiencies and yields of plasmon-mediated chemical reaction and inspire the rational design of plasmonically-powered devices.