Interactions of N-Heterocyclic Carbene-Carbodiimide (NHC-CDI) Adducts with Light: Electronic, Steric, and Conjugation Effects

N-Heterocyclic carbene-carbodiimide (NHC-CDI) adducts have extensive applications due to their tunable structure, versa-tile reactivity, and synthetic accessibility. Previous NHC-CDI betaine studies have centered around thermal, mechanical, or electrochemical activation; however, there is little understood about their optical properties. In this work, we aim to under-stand the effects of their structure on photophysical traits, including UV-Vis absorption, fluorescent behavior, and excited state reactivity. We found conjugation of the CDI substituents is the key factor for visible light absorption, with azoben-zenes as an extreme example that results in absorbance out to 600 nm due to new transitions to the lowest unoccupied mo-lecular orbital located on the azo moiety. These zwitterionic adducts are also prone to aggregation, resulting in absorption solvent effects and higher fluorescent emission at lower concentrations (>2.5-fold increase when diluted 10x). Finally, we explored the excited state reactivity of a model adduct which displayed good tolerance for a variety of functional groups, but not single-electron transfer donors and acceptors (e.g., amines, ketones). These results highlight how the structural changes of NHC-CDIs can have profound effects on their interactions with light; we anticipate these relationships will in-form the application of these adducts as dyes, ligands, photocatalysts, or organic charge transport materials.