Depiction of a metal-to-ligand charge transfer state in an iron bipyridine complex. The image shows the difference density for the first spin-allowed excitation of the complex.

Photochemistry & Photoredox

Transition metal complexes can be used as photoredox catalysts for clean hydrogen production. Photoredox describes a temporary change in oxidation number of the metal center in a transition metal complex upon absorption of a photon (i.e. through a metal-to-ligand charge transfer excitation, where charge is displaced from the metal to the ligand(s)). During this period, the transition metal complex readily engages in redox chemistry in order to return to a more stable electronic charge distribution. We are interested in developing affordable and accurate approaches for modeling photoredox and related photocatalytic processes.

Specifically, we are developing quantum chemistry methods that can model the spectroscopy of transition metal complexes in a much more predictive way than modern density functional theory (DFT) approaches. Our approaches are built upon the premise of optimal reference theories, or the concept of choosing a better set of initial reference orbitals for a subsequent excited-state calculation. Generally, excited state calculations begin with an optimization of ground-state Hartree-Fock molecular orbitals, but these orbitals may be quite far from optimal for excited states. Our methods seek to prime the excited state calculation with orbitals that are more optimal for excited states by including some degree of electron correlation (or orbital relaxation effects that might have been considered correlation) from the outset.

Electrocatalysis

Electrocatalysis offers a means of driving chemical reactions by manipulating reaction thermodynamics via an applied potential. For instance, by application of an electrical current through a cathode material, electrons can be promoted to higher energy levels, making their transfer into the lowest unoccupied molecular orbital (LUMO) of a molecule adsorbed to the cathode thermodynamically favorable. This is the process of electrochemical reduction and is commonly employed to reduce carbon dioxide, facilitating the conversion of this greenhouse gas into useful chemical feedstocks such as methanol. However, this process is currently inefficient and expensive, mandating research into better electrocatalytic materials.

We are interested in developing a molecular-orbital-level understanding of electrocatalytic chemistry. To approach this problem, we are developing multilayer models that allow us to achieve wave function theory quality results at a drastically reduced cost. We aim to develop multilayer models that incorporate externally applied biases to more closely emulate experimental conditions. Of course, electric fields are not the only way to externally influence reaction thermodynamics. Thus, we are also pursuing models of photon and magnetic-field assisted electrocatalysis, which can lead to far more efficient catalytic conversion of carbon dioxide.

Depiction of a carbon dioxide molecule adsorbed to a metal surface.

Photochemical Dynamics in Liquids

We are intrigued by the photochemical dynamics in liquid phase, such as molecular solar thermal (MOST) devices and how solvent effects the excited-state lifetime of photoredox catalysts. Quantum chemistry methods, especially those that are more robust than density functional theory, scale very poorly with system size, so the direct study of photochemistry in liquid phase is often prohibitively expensive.

We are developing excited-state fragmentation approaches for the study of photochemical dynamics in liquids. The general strategy behind fragmentation is to break the intractable calculation on the liquid into many small calculations on the molecules (fragments) that comprise the liquid. These methods are "embarassingly parallelizable", meaning that we can achieve phenomenal speed-ups of calculations on large systems given a commensurately large allocation of computational resources, because each fragment calculation can be run independently from the rest. In principle, our methods should permit high-level wave function theory calculations (and even dynamics) to be carried out at an unprecedented scale.

Scheme depicting fragmentation algorithms in quantum chemistry. The left panel shows a molecular liquid comprised of many molecules. Arrows connect this to a middle pane depicting calculations being carried out on individual molecules that make up the system. The arrows meet again to give an aspirational result in the right pane: A spectrum obtained in just hours that matches the brute force excited state calculation that took days.

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