Carbene-Functionalized Bulky-Cyclopentadiene Rings.

A series of significantly bulky mono- and di-substituted cyclic alkyl-amino carbene (cAAC)- functionalized cyclopentadiene ring (Cp) compounds were synthesized. The functionalization of the Cp ring with cAAC ligands makes them significantly bulkier, while retaining their ligation properties. These compounds display interesting fluorescence properties. In these compounds, intra-molecular charge transfer is observed from electron-rich carbene to electron-deficient cyclopentadiene unit. These high yielding compounds have been characterized by X-ray single-crystal diffraction and their emission properties have been studied. Rotational conformers (via C-C bond rotation) play a pivotal role with possible different extent of intramolecular charge-transfer (ICT) from carbene to cyclopentadiene ring. Variable temperature-dependent NMR studies were performed along with NOSY, COSY and different 2D NMR techniques to estimate the energy barriers and 1,5-Hydrogen shift.