Ultrafast Spectral Dynamics of a Rhenium Photocatalyst

[摘要] This dissertation investigates the photo-initiated CO2 reduction catalyst, Re(bpy)(CO)3Cl, in multiple states along the photocatalytic cycle with the goal of understanding the influence of electrostatics and solvent on each step. Two-dimensional infrared (2DIR) spectroscopy was used to characterize the electronic ground state dynamics of the Re photocatalyst, including several derivatives, in multiple solvents. Though insensitive to the substituents, spectral dynamics do correlate with the solvent;;s nucleophilicity. Transient-2DIR experiments enabled the first ever characterization of a quasi-equilibrated electronic excited state. Electronic excitation at 400 nmfollowed by equilibration in the triplet metal-to-ligand charge transfer state, provided direct access to dynamics using a time-delayed 2DIR pulse sequence. Relative to the ground state, we observe a two-fold slowdown in spectral dynamics as sensed by the in-phase symmetric vibrational mode and a decrease in the vibrational lifetime by a factor of five.Transient 2DIR characterizes catalytically relevant, long-lived electronic excited states independent of any ground state dynamics.Introducing the sacrificial electron donor triethanolamine (TEOA) revealed a concentration dependent slowdown of the Re spectral dynamics, indicating both preferential solvation and solvent exchange. Preferential solvation was confirmed after UV-Vis pump/IR probe experiments showed the appearance of the singly reduced, intermolecular electron transfer product on a timescale faster than diffusion (<70 ps). This result highlights a previously ignored mechanistic aspect of this essential electron transfer reaction.Re photocatalysis experiments were supplemented by additional investigations into solvation and molecular flexibility. In a solution of the Re complex and Na+SCN- in THF, 2DIR experiments revealed an intermolecular vibrational energy transfer between the Re complex and Na+SCN- on a timescale distinct from the equilibrium fluctuations of the two individual donor and acceptor bands involved, indicating non-Gaussian dynamics. The next work isolated polymer flexibility chosen specifically because of its site-specific vibrational probes. The chain ends spectrally diffuse slower than do the inner chain sites and going from dilute solution to a solvent-free film, the heterogeneity in site dynamics become reduced due to the crowding influence of neighboring molecules. This experiment was supported with a coarse grained model of the polymer which showed essentially quantitative agreement with measured relative dynamics.

[发布机构] University of Michigan

[效力级别] photocatalyst

[关键词] ultrafast dynamics;photocatalyst;infrared spectroscopy;Chemistry;Science;Chemistry