Controlling Coherences in Crystalline Frameworks for Enhanced Singlet Fission

Cornell University, Ithaca NY

Investigators

Abstract

With support from the Chemical Mechanism, Function, and Properties (CMFP) program in the Division of Chemistry, Professors Andrew Musser and Phillip Milner of Cornell University are using ultrafast laser techniques to study how molecular coherence can be controlled. Coherence can arise when the electronic or vibrational states of two or more molecules synchronize, and it plays in central role in how light interacts with matter, from the physics of vision to photosynthesis to new systems for generating electricity from light. Proper control of such coherences could lead to devices that more efficiently harvest and transport energy, or to advanced applications of quantum mechanics in sensing and information science. And yet, despite its ubiquity, very little is known about how to tune such coherences or identify their unique contribution to photochemical processes. In this project, the teams of Musser and Milner will tackle this question in the context of singlet fission, a process in which one photoexcited state splits in two, with major implications for technologies such as quantum information science and light harvesting devices. Professor Milner and his students will prepare systematic libraries of crystalline sponge-like materials known as metal-organic frameworks – effectively molecular tinker toys whose modularity lets the team dial in specific interactions between photoactive molecules. Professor Musser and his students will use cutting-edge ultrafast laser-based measurements to watch the molecules in these frameworks share energy and move in real time and identify conditions where they achieve coherent synchronization. These studies will lead to new design principles to control and steer coherent singlet fission dynamics for next-generation devices. The team will additionally train multiple graduate and undergraduate students in interdisciplinary research spanning organic chemistry to spectroscopy, and they will develop middle-school outreach programs on how light interacts with matter. Recent advances in spectroscopic techniques have made it ever easier to identify the presence of coherence in a host of processes driven by light-matter interactions, and it is associated with ultrafast, high-efficiency transfer and conversion dynamics. But it has proved more challenging to identify the unique impact of coherence itself: the general conditions that enable superpositions of quantum states (e.g. energetic resonance) also favor efficient incoherent processes. By studying coherent dynamics in well-defined solids where the interactions are tuned with molecular-level precision, the team aims to establish structural guidelines to harness and tune intermolecular coherence. The team will probe this behavior in the context of singlet fission, in which an excited singlet separates into two low-energy triplets. The initial ultrafast conversion into an intermediate triplet-pair state is often described in terms of vibronic coherence, while the product triplets exhibit persistent spin coherence. Both types of coherence present potential control knobs in terms of vibrational environment, orbital overlap, and intermolecular transport pathways. By incorporating fission-active molecules (such as monomeric or dimeric pentacenes, rylene diimides) into modular crystalline frameworks with varied structure, the team will systematically tune these parameters. The project will use ultrafast electronic spectroscopy to track coherent vibronic wavepackets through the initial fission process and monitor variation in long-time spin coherence through magnetic-field effects. Comparing these measurements across the structural library, the team will develop empirical guidelines to enhance intermolecular coherence. The insights generated will enable the design of more effective singlet fission materials for optoelectronic devices. 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.