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
studies have centered around thermal, mechanical, or electrochemical
activation; however, little is understood about their photoproperties.
In this work, we aim to understand the effects of their structure
on UV–vis absorption, the highest occupied molecular orbital
(HOMO), the lowest unoccupied molecular orbital (LUMO), and the HOMO–LUMO
gaps. We found that conjugation of the CDI substituents is the key
factor for visible-light absorption, with azobenzene groups as extreme
examples that result in absorbance out to 600 nm due to new transitions
to the LUMO located on the azo moiety. Finally, we explored the reactivity
under photochemical conditions of a model adduct that displayed good
tolerance for a variety of functional groups but not aryl-containing
single-electron transfer acceptors (e.g., aryl aldehydes, aryl ketones,
and aryl bromides). These results highlight how the structural changes
of NHC-CDIs can have profound effects on their frontier molecular
orbitals and their interactions with light; we anticipate that these
relationships will inform the application of these adducts as dyes,
ligands, photocatalysts, or organic charge transport materials.