47048-G6
Probing Photocatalysis Reaction Dynamics and Mechanism Using Non-equilibrium Multidimensional Infrared Spectroscopy
The goal of this project has been to study ultrafast photocatalysis using transient two-dimensional infrared (2DIR) spectroscopy. Toward this end, we have developed a novel experimental approach that enables the rapid acquisition (~10 seconds) of transient 2DIR spectra. Support from the PRF has enabled the group to investigate fundamental reaction dynamics both in systems at equilibrium as well as following photoactivation. Complex heterogeneous environments such as a nanoparticle-adsorbate interface require that we are equipped with the experimental and theoretical tools to extract chemically meaningful conclusions from the data. The work the group has done over the funding period has put us in an excellent position to make advances in photocatalysis using the powerful dynamical tool of transient 2DIR spectroscopy. The remainder outlines the progress made with the support of the PRF.
The first publication to result directly from the support of the PRF was a study of the metal carbonyl complex dimanganese decacarbonyl (DMDC). Conventional 2DIR spectroscopy shows cross peaks between all the transitions in this region (~2000 cm-1) providing geometrical constraints as well as coupling terms in the anharmonic Hamiltonian. We have collaborated with a theory colleague to develop a straightforward perturbation-based method of predicting 2DIR spectra from quantum chemical results. Briefly, a 2DIR spectrum correlates and excited frequency with a detected frequency. Since excitation and detection are carried out with femtosecond laser pulses, the experiment has inherent sub-picosecond time resolution. The time between excitation and detection is called the “waiting time”. In the experimental data, we observed a striking feature: most of the cross peaks in the spectrum were found to oscillate as a function of the waiting time. Such an oscillation is indicative of a quantum mechanical superposition, and indeed the time dependent behavior of the data can be understood with a very simple, three-level quantum model. The observation of “waiting time coherences” was not particularly surprising since “quantum beats” have been observed in ultrafast pump probe spectroscopy for decades. Rather, the waiting time coherences provide an additional avenue to extract system-environment interactions. In the case of DMDC, at equilibrium the molecule’s structure is static—that is, there are no fluctuating structural transformations. DMDC taught us how to understand coherences, a lesson that would be very helpful when studying flexible molecules that isomerize (below).
A chemical equilibrium established between to species means that the forward and reverse rates are equal. Chemical exchange was originally used in NMR to optically tag a reactant molecule and probe it once it switched into a product. On a time scale one billion times faster, we can do the same thing using 2DIR chemical exchange spectroscopy. We have studied dicobalt octacarbonyl which exists in three isomers, two of which dominate at room temperature. Due to spectral differences among the isomers, we have resolved exchange correlation between the two prominent isomers finding an asymmetric reaction barrier as one would expect for a system with energetic differences between isomers. This is only the second experiment of its kind in a metal carbonyl, and our findings were recently submitted to J. Phys. Chem. A key feature of our data that enabled a novel perspective on chemical exchange was the observation of waiting time coherences. These coherences were found to be temperature dependent indicating that they decay faster when the reaction proceeds more rapidly. In other words, the chemical exchange was found to dephase the waiting time coherences.
Since the principal goal of the research is non-equilibrium, transient photochemistry, we have spent considerable effort to develop the technique and apply it to well-defined systems in hopes of extending the methods to more complicated constructs. DMDC can be photodissociated to Mn(CO)5 using 400 nm excitation, with the photoproduct evolution tracked using 2DIR as a probe. First we demonstrated transient Fourier transform 2DIR spectroscopy and published the technique. Our full manuscript describing our results and conclusions is currently under review. For this work we varied both the time delay between the phototrigger breaking the bond and the waiting time within the 2DIR probe. By integrating the entire induced transient 2D spectrum (i.e. the new signal that was created by photodissociation), we found that the 2D signal decayed more slowly as the initially hot photoproduct is allowed to cool by increasing the time delay between photolysis and the 2DIR probe. Although this is expected, the 2DIR probe enables a new kind of information about non-equilibrium molecules: how fast they loose orientational correlation. This information is very appealing because unlike vibrational relaxation, which has a complicated temperature dependence, orientational correlation has a very simple temperature dependence using Stokes-Einstein-Debye theory. Thus we can establish a new transient measure of temperature by determining the dynamics of the decay of orientational correlation. Such information will be critical in attempting to follow photocatalysis products using this technique.
