When light interacts with matter, electronic excitations (excitons) form. An exciton is a Coulombically bound electron-hole pair. In high fluence, excitons can annihilate one another producing inefficient light harvesting and reduced dynamic range for optoelectronics. In this talk, I will describe our latest experiments that provide us with a post-synthetic chemical strategy to block exciton-exciton annihilation. This strategy allows us to switch reversibly between exciton annihilating and non-annihilating phases of the material. In the non-annihilating phase, two excitons reside on a single chromophore without interacting with each other effectively forming a biexciton state. We observe that this biexciton state remains stable throughout the excited-state lifetime of the chromophores. In the talk, I will also describe the latest quantum mechanical theory that explains the presence of these non-annihilating biexciton states.