Linking Intermolecular Geometry and Spin Coupling of Excitons in Organic Semiconductors*

The design and optimization of synthetic magnetic and optoelectronic materials requires precise chemical control of spin and electronic coupling. Here we focus on understandingthis structure-function relationship for light-harvesting applications of singlet fission: the production of two triplet excitons (each with spin S=1) following excitation of one singlet exciton (spin S=0). To quantitatively extract dipolar and exchange interactions between triplet excitons formed by singlet fission in a solid-state organic semiconductor, we have deployed broadband optically detected magnetic resonance [4], electron spin resonance [1,3], and magneto-optical spectroscopy [2]. Mapping the experimentally extracted spin parameters onto the molecular crystal structure provides a window into exciton localization and coupling in the molecular lattice. This mapping of spin properties to excited-state electronic structure is made possible by sustained excited-state spin polarization and coherence over microsecond timescales [1, 3]. These results --linking intermolecular geometry and spin interactions-- provide a key step toward chemically controlling intermolecular spin coupling for both optoelectronic applications and molecular quantum technologies.