Photoreactivity or photostability? Disentangling factors governing the fate of excited states.
Abstract
Ultrafast photo-induced excited-state dynamics in organic molecules are fundamentally important to photochemical processes such as photodissociation, photoisomerization and ring-opening with relevance to photosynthesis, human vision, and charge transfer in organic photovoltaics. Their times scales, and the directionality and quantum efficiency of the underlying energy and charge flow are governed by the ultrafast coupled electron and nuclear dynamics occurring near conical intersections. An important goal beyond the study of excited-state dynamics of individual molecules is the formulation of guiding principles based on simple descriptors of the photoreactant. Such rules, based on the concept of functional groups, have been extremely successful in ground-state organic chemistry. The objective of this project is to identify structure-reactivity relationships for molecules in the excited states. This is complicated by the fact that optical excitation usually involves a highly non-local electronic structure change, whereas the photochemical outcome, on the other hand, represents a localized nuclear structural change. Yet, advancing our understanding of these processes will greatly aid the design of photoactivated functional materials.
In this project, we will investigate classes of molecules, e.g., differing by one or two substituents and their location. This allows us to systematically study the influence of electronic and inertial effects on the outcome of the photophysical and -chemical process. As a start, we will investigate series of halogenated thiophenes. The addition of halogen (X) substituents on the thiophene motif introduces several competing deactivation processes (such as intersystem crossing, ring opening, C-X bond fission) whose relative importance will vary with the nature and location of the substituent. As such, this provides an opportunity to gain insight into the implications of chemical modifications on the branching ratios and efficiencies of competing decay pathways, and ultimately, it will enable us to identify factors governing the fate of excited states and their reactivity. Our approach is based on a combination of molecular quantum dynamics simulations, efficient explorative static as well as enhanced-sampling calculations (beyond what is dynamically accessible) as well as novel analysis tools to map out the underlying coupled electron–nuclear dynamics. Calculations as well as methodological developments related to this project will be performed in the GPU-accelerated TeraChem program. We emphasize that such extensive investigations based on molecular quantum dynamics simulations (timescales up to several picoseconds) are only feasible on GPUs.
The project is performed in close collaboration with leading experimentalists in the field (key collaborators: Thomas A. Wolf and Ruaridh J. G. Forbes, Stanford University and SLAC; Nora Berrah, University of Connecticut). In addition to direct simulations of the photoinduced dynamics, we will perform calculations of relevant time-resolved experimental observables (including ultrafast electron diffraction and Coulomb explosion imaging) to directly connect with experimental efforts.
