Polarity of Ordered Solvent Molecules in 9,9′-Bianthracene Single Crystals Selects between Singlet Fission or Symmetry-Breaking Charge Separation

Singlet exciton fission (SF) and symmetry-breaking charge separation (SB-CS) are both photophysical processes that can occur between two organic chromophores and are both of interest to improve solar energy conversion. Here, we tuned the photophysics of a 9,9′-bianthracene (BA) single crystal between SF and SB-CS using solvent intercalation to change the electric field within the crystal. Crystals of BA were grown in o-xylene, chlorobenzene, o-dichlorobenzene, and benzonitrile, as well as solvent-free from a melt. The crystals were studied by X-ray diffraction, steady-state optical spectroscopy, and transient absorption microscopy to elucidate the role of the intercalated solvent molecules. The crystals with no solvent in the structure undergo fast SF (<2 ps), while the crystals with intercalated moderately polar solvents o-xylene, chlorobenzene, and o-dichlorobenzene show evidence of charge-transfer-mediated SF. Finally, the crystals containing highly polar benzonitrile undergo SB-CS instead of SF. These results demonstrate that controlling solvation of BA in the crystal structure can tune its photophysics between SF and SB-CS.