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.