Engineering topics
Morseth, Zachary A.
Publications and source records attributed to Morseth, Zachary A..
Interfacial Dynamics within an Organic Chromophore-Based Water Oxidation Molecular Assembly
Photoinduced electron injection, intra-assembly electron transfer, and back-electron transfer are investigated in a single-site molecular assembly formed by covalently linking a phosphonated terthiophene (T 3 ) chromophore to a Ru(terpyridine)(bipyridine)(L) 2+ (L = MeCN or H 2 O) water oxidation catalyst adsorbed onto a mesoporous metal-oxide (MO x ) film. Density functional theory calculations of the T 3 -trpy-Ru-L assembly indicate that the molecular components are strongly coupled with enhanced low-energy absorptions owing to the presence of an intraligand charge transfer (ILCT) transition between the T 3 and trpy moieties. Ultrafast spectroscopy of the MO x //T 3 -trpy-Ru-L assemblies reveals that excitation of the surface-bound T 3 chromophore results in ps–ns electron injection into the metal-oxide conduction band. Electron injection is followed by rapid (<35 ps) intra-assembly electron transfer from the Ru II catalyst to regenerate the T 3 chromophore with subsequent back-electron transfer on the microsecond time scale.
Role of Macromolecular Structure in the Ultrafast Energy and Electron Transfer Dynamics of a Light-Harvesting Polymer
Ultrafast energy and electron transfer (EnT and ET, respectively) are characterized in a light-harvesting assembly based on a π-conjugated polymer (poly(fluorene)) functionalized with broadly absorbing pendant organic isoindigo (iI) chromophores using a combination of femtosecond transient absorption spectroscopy and large-scale computer simulation. Photoexcitation of the π-conjugated polymer leads to near-unity quenching of the excitation through a combination of EnT and ET to the iI pendants. The excited pendants formed by EnT rapidly relax within 30 ps, whereas recombination of the charge-separated state formed following ET occurs within 1200 ps. A computer model of the excited-state processes is developed by combining all-atom molecular dynamics simulations, which provides a molecular-level view of the assembly structure, with a kinetic model that accounts for the multiple excited-state quenching pathways. In conclusion, direct comparison of the simulations with experimental data reveals that the underlying structure has a dramatic effect on the partitioning between EnT and ET in the polymer assembly, where the distance and orientation of the pendants in relation to the backbone serve to direct the dominant quenching pathway.
Light-Driven Water Oxidation Using Polyelectrolyte Layer-by-Layer Chromophore–Catalyst Assemblies
Layer-by-Layer (LbL) polyelectrolyte self-assembly occurs by the alternate exposure of a substrate to solutions of oppositely charged polyelectrolytes or polyions. Here, we report the application of LbL to construct chromophore–catalyst assemblies consisting of a cationic polystyrene-based Ru polychromophore (PS-Ru) and a [Ru(tpy)(2-pyridyl-N-methylbenzimidazole) (OH 2 )] 2+ water oxidation catalyst (RuC), codeposited with poly(acrylic acid) (PAA) as an inert polyanion. These assemblies are deposited onto planar indium tin oxide (ITO, Sn:In 2 O 3 ) substrates for electrochemical characterization and onto mesoporous substrates consisting of a SnO 2 /TiO 2 core/shell structure atop fluorine doped tin oxide (FTO) for application to light-driven water oxidation in a dye-sensitized photoelectrosynthesis cell. Cyclic voltammetry and ultraviolet–visible absorption spectroscopy reveal that multilayer deposition progressively increases the film thickness on ITO glass substrates. Under an applied bias, photocurrent measurements of the (PAA/PS-Ru) 5 /(PAA/RuC) 5 LbL films formed on FTO//SnO 2 /TiO 2 mesoporous core–shell electrodes demonstrate a clear anodic photocurrent response. Prolonged photoelectrolysis experiments, with the use of a dual working electrode collector–generator cell, reveal production of O 2 from the illuminated photoanode with a Faradaic efficiency of 22%. Finally, this is the first report to demonstrate the use of polyelectrolyte LbL to construct chromophore–catalyst assemblies for water oxidation.