Three Ways Isolable Carbenes Can Modulate Emission of NH-Containing Fluorophores

Fluorescent
molecules and materials that exhibit emission changes
in response to analytes are of great interest across multiple disciplines.
Herein, we investigate the response of NH-containing fluorophores
carbazole and 2-phenylbenzimidazole (Ph-BIM) with two
representative isolable singlet carbenes. Specifically, N-heterocyclic carbene 1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene
(IPr) and cyclic (alkyl)­(amino)­carbene (2,6-diisopropylphenyl)-4,4-diethyl-2,2-dimethyl-pyrrolidin-5-ylidene
(EtCAAC) were discovered to afford
three different types of reaction products with carbazole and Ph-BIM. Depending on the reaction pair, hydrogen bonding (1), NH-insertion (2,3), or proton
transfer (4) products can be isolated, each displaying
variable photophysical responses. These products have been structurally
authenticated by single crystal X-ray diffraction and NMR spectrometric
methods. Studies of the solution state behavior of 1–4 reveals that these adducts are labile and can reversibly
dissociate to free carbenes and fluorophores to varying extents. These
equilibria produce concentration dependent solution state behavior
as identified and quantified via UV–visible absorption, emission, 1H DOSY, and NMR spectroscopic measurements.