Photophysical and Chiroptical Properties of Pyrazino-Phenanthroline-Helicene Derivative and its Rhenium(I) Complex.

A novel coordination motif comprising [4]helicene fused with pyrazino-phenanthroline (H4PP) has been synthesized and reacted with ReCl(CO)5 to yield its rhenium(I) complex (Re-H4PP). Absorption and emission spectroscopic analysis conducted in dichloromethane and 2-methyltetrahydrofuran reveals that combining pyrazino-phenanthroline with helicene visibly affects the photophysical attributes of both the resulting ligand and its Re(I) complex as compared to their non-helicene analogues, and even more importantly leads to relatively high photoluminescence quantum yield values, especially in the case of H4PP (29 %). Chiroptical studies through electronic circular dichroism and circularly polarized luminescence performed on enantiomerically enriched samples of Re-H4PP show the chiral nature of low-energy excited states affording notable glum values that amplify at cryogenic temperatures. Insights into experimental results are provided via first-principles quantum-chemical calculations showing important role of intra-ligand charge-transfer (ILCT) and metal-to-ligand CT (MLCT) states in determining photophysical features of these systems.