N-heterocyclic carbene precursors for CO₂ chemosensor: Implication on conformation-dependent properties

We developed an optical CO₂ chemo-sensor based on fluorescence change in order to improve the shortcomings of the existing methods, namely expensive, bulky equipment and some difficult limitations. In brief, the new sensor utilized tetrapropyl benzo-bisimidazolium (TBBI), which is a kind of NHC precursor that is able to detect CO₂ quickly with low limit. To the best of our knowledge, not much information is available on TBBI conformers and their proton affinities (PAs). Therefore, we investigated the structures of TBBI conformers, their relative energies, PAs, and the activation energy barriers among the conformers. In this communication, we investigated TBBI conformers and their deprotonated carbenes with respect to their geometry, relative energy, PA, and reaction profiles using DFT calculations. We found that TBBI conformers are very similar in energy, whereas there is considerable energy difference in the corresponding NHCs. The stability difference of NHCs for different conformers can be explained by orbital interactions resulting in hyperconjugation. Consequently, we suggest that the conformation-dependent property for NHC precursors should be considered to efficiently generate NHCs.