The interaction between light and plasmonic nanoparticles has been a long studied and ever-expanding area of research due to the unique ability of plasmons to focus light at the nanoscale as a result of their collective electron resonances. This property has resulted in many applications including photocatalysis, chemical sensing, and surface-enhanced spectroscopies. Recent interest has shifted away from traditional plasmonic materials (Au and Ag) and particle arrangements (large colloidal nanoparticles) to more novel systems, including aluminum nanoparticles, heterodimers, and organic molecules. Additionally, plasmonics has been finding more diverse and varied applications with the onset of new materials and arrangements, such as water purification, color changing glass, and solvated electron generation. We investigated the excited state dynamics related to these new plasmonic and plasmon-mediated systems through the use of pump-probe spectroscopy, specifically in molecular plasmon systems and the application of plasmonic nanoparticles in controlled solvated electron generation.

We studied the collective, coherent excitations in charged polycyclic aromatic hydrocarbons (PAHs), termed molecular plasmons. Systems in this few-atom limit show behavior strongly dependent on charge state, where the addition or removal of even a single electron dramatically alters electronic and optical properties. Therefore, we studied their excited- state lifetimes in four different charge states: cation, neutral, anion, and dianion. Those characterized by a closed-shell electronic structure—the neutral molecule and the dianion— exhibit long-lived, exponentially decaying lifetimes typical of radiative relaxation. In contrast, the open-shell cationic and anionic states exhibit far more rapid multiexponential decay dynamics. This can be attributed to the nonradiative de-excitation of multiple electron–hole pairs in the molecule through molecular plasmon “dephasing” and vibrational relaxation. This study gives insight into the nature of excited states of open- and closed-shell molecules and illuminates the role played by electronic structure in the collective electron dynamics of few-atom plasmonic systems.

The application of plasmonic nanoparticles in the photo-generation of solvated electrons in an organic solvent was also investigated. Solvated electrons are free, unbound electrons delocalized within a solvent cage and are among the most reductive single species available for redox and radical reactions. Until recently, the predominant methods for generating solvated electrons have focused on relatively destructive and uncontrolled techniques, either requiring high-energy electrons or photons to temporarily form free electrons in solvent clusters, or bulk dissolution of an alkali earth metal to act as a constant electron donor. None of these options allow for solvated electron use in precise and sensitive chemical reactions. Herein, we have developed a method for generating solvated electrons in a non-destructive and controlled manner, through the excitation of the localized surface plasmon resonance (LSPR) of aluminum nanocrystals (Al NCs) with visible light. We characterized the solvated electrons generated through electron paramagnetic resonance (EPR) and ultrafast transient absorption (TA) spectroscopy, and confirmed they matched those reported in the literature. Furthermore, transformation of a simple radical clock, 6-bromohex-1-ene, was performed in situ to demonstrate both the generated solvated electron’s reactivity and the applicability of this technique as a platform for organic chemical synthesis. This system offers a simple, non- destructive, light-directed framework for experimentalists to utilize, without the drawbacks of past solvated electron generation methods.