Polarity of Ordered\nSolvent Molecules in 9,9′-Bianthracene\nSingle Crystals Selects between Singlet Fission or Symmetry-Breaking\nCharge Separation

Singlet\nexciton fission (SF) and symmetry-breaking charge\nseparation\n(SB-CS) are both photophysical processes that can occur between two\norganic chromophores and are both of interest to improve solar energy\nconversion. Here, we tuned the photophysics of a 9,9′-bianthracene\n(<b>BA</b>) single crystal between SF and SB-CS using solvent\nintercalation to change the electric field within the crystal. Crystals\nof <b>BA</b> were grown in <i>o</i>-xylene, chlorobenzene, <i>o</i>-dichlorobenzene, and benzonitrile, as well as solvent-free\nfrom a melt. The crystals were studied by X-ray diffraction, steady-state\noptical spectroscopy, and transient absorption microscopy to elucidate\nthe role of the intercalated solvent molecules. The crystals with\nno solvent in the structure undergo fast SF (<2 ps), while the\ncrystals with intercalated moderately polar solvents <i>o</i>-xylene, chlorobenzene, and <i>o</i>-dichlorobenzene show\nevidence of charge-transfer-mediated SF. Finally, the crystals containing\nhighly polar benzonitrile undergo SB-CS instead of SF. These results\ndemonstrate that controlling solvation of <b>BA</b> in the crystal\nstructure can tune its photophysics between SF and SB-CS.