The behavior of molecules in the excited states can be dramatically different from the ground state, which can lead to a whole host of new reactivity. We seek to design molecules that can be controlled to exhibit different responses (i.e. bond cleaving reactions, emission of different colors of light) depending on the environment and conditions set by the user. These designs have applications in biomaterials and medical diagnostics. We are also developing methods to insert thioamide isosteres into previously uncharted peptide sequence space.