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

N-Heterocyclic carbene-carbodiimide (NHC-CDI) adducts have extensive applications due to their tunable structure, versatile 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 understand 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 azobenzenes as an extreme example that results in absorbance out to 600 nm due to new transitions to the lowest unoccupied molecular 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 certain 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 inform the application of these adducts as dyes, photocatalysts, or organic charge transport materials.