SusChEM: Design Principles Inspired by Symmetry for Controlling Singlet Fission in Structurally Well-Defined Covalent Dimers

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In this project funded by the Chemical Structure, Dynamic & Mechanism B Program of the Chemistry Division, Professors Niels H. Damrauer and Tarek Sammakia of the Department of Chemistry and Biochemistry at the University of Colorado Boulder are synthesizing new organic molecular systems that contain two chromophores (i.e., parts of the molecules that can absorb light) and using spectroscopy to explore their photophysics. The goal is to develop design rules for how the arrangement of the chromophores controls excited state reactivity following visible light photoexcitation. The particular reaction of interest is important in certain next-generation strategies to increase the efficiency of solar cells by limiting waste heat production that generally occurs when higher energy solar photons are absorbed. This proposal establishes a fundamental research program where students are exposed to a considerable breadth of ideas in synthesis, spectroscopy, and applications of electronic structure theory. A specific outreach effort in collaboration with local high school teachers is part of the funded work that aims to provide sophisticated but affordable spectroscopic tools and curriculum ideas to Colorado high school science programs. Singlet fission is a photophysical phenomenon observed in certain organic materials wherein light absorption produces a spin-allowed singlet excited state that then non-radiatively converts to a pair of triplet excitations. If these triplets can be further transformed to charge carriers, it is possible to envision device scenarios where the solar spectrum is more efficiently utilized compared to devices in operation today. This research is predicated on the idea that molecular dimers are the fundamental unit for singlet fission and that they can provide a platform wherein synthetic manipulations that control the spatial juxtaposition and covalent interaction of chromophores are called upon to affect key photophysical rate constants. The most important design opportunity that is being tested relates to dimer point group symmetry and the idea that it can control the interference (constructive vs. destructive) of pathways in the quantum mechanical description of diabatic coupling for singlet fission. Other strategies will call on substituents to affect the relative energy of charge transfer states.