Metal-molecule and metal-polymer interfaces are critical junctions where electron-transfer occurs in organic electronics and hybrid devices. The fundamental process of electron-transfer from a metal to a molecule or vice versa is explored using noble metal nanoparticles (NPs), which when photoexcited, undergo a variety of processes. Excitation of Au and Ag NPs surface plasmon produces high energy electrons and holes which within femtoseconds are thermally distributed above the Fermi level through electron-electron scattering. These electrons and holes are known as “hot” charge carriers and can live for up to 2 ps (depending on NP size, shape, and material) before relaxing back to the Fermi level. Hot electrons can be utilized in a variety of light harvesting applications such as driving chemical reactions, sensing, generation of electrochemical energy, or in our case, be captured by the LUMO of an acceptor molecule, in this case the viologen dication. Viologen dication is the perfect molecular component due to its low reduction potential and prominent absorption spectrum of its reduced, radical cation, redox state.<br/>Femtosecond pump-probe spectroscopy experiments were conducted by photo-exciting the local surface plasmon band of 15 nm spherical AuNPs. The NPs were functionalized with viologen dication with was tethered to the metal surface with 2, 4, 6, and 10 carbon chain thiol linkers. The possibility hot carrier capture was also investigated at various viologen-to-AuNP ratios in solution. The excited state spectra of the viologen@AuNP assemblies consist of a typical ground state bleach as well as a new ~675 nm absorption band which we attribute to the mono-reduced viologen moiety generated by the capture of a hot plasmonic electron. Transient spectra of unfunctionalized AuNPs in solutions of viologens nor bearing the thiol anchor group do not exhibit the same long wavelength absorption feature.

Capture of hot electrons at metal interfaces

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.