CAS: Exploring Multiexciton Dynamics for Triplet Upconversion in Structurally Well-Defined Covalent Dimers

In this project, funded by the Chemical Structure, Dynamics & Mechanism B Program of the NSF Division of Chemistry, Niels Damrauer and Tarek Sammakia of the Department of Chemistry at the University of Colorado Boulder are working to gain a greater understanding of a process called “triplet upconversion” (TUC). When molecules absorb light, they are excited into a higher energy state and in TUC, the energy of two excited molecules fuses into a single excited state that is more energetic and has greater reactive potential.

TUC photophysics is robust and applicable to a remarkable breadth of chemical, technological, and biological applications including solar energy conversion, chemical catalysis, light-promoted polymerization, and biological imaging. This project marshals a program that joins spectroscopy, chemical synthesis, and theory to consider fundamental aspects of this process.

The understanding gained in this research will push the next-generation of molecules and materials to new levels in their ability to manipulate light energy obtained through absorption. The research team will also engage in scientific outreach at the high school level through an established relationship with the DSST schools in the Denver metropolitan area.

This is a remarkable STEM-based school system with a 100% college acceptance rate for high school graduates amongst a total population that is 71% economically challenged and overwhelmingly racially diverse. Through an online synchronous platform in response to the COVID crisis, the team offer an 8-week elective to explore spectroscopy and photophysics using highly accessible 3D-printed parts, a few other inexpensive components, and a cell phone camera.

This project centers around the ideas of excited state dynamical control. In the vast majority of TUC systems, monomer chromophores are used, and collisions or non-diffusional monomer/monomer encounters are called upon to negotiate all triplet fusion events. This results in very little control of the multiexciton dynamics that are critical contributors to overall yield.

Matters are challenged because spin statistics are in play and because the singlet channel required for TUC is the smallest in its statistical weight. In this project, the team plans to fundamentally alter, and interrogate, the number and types of multi-exciton pathways that may contribute to successful TUC using structurally well-defined dimer systems.

The team believes that the structural definition of their systems in concert with the control they maintain in manipulating physiochemical properties – such as excited state reaction driving force, excited state couplings, interchromophore spin coupling magnitudes, molecular size, and solubility – will potentially allow them to assess TUC efficiency gains achieved by exploiting intramolecular multiexciton dynamics.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.