Is Photoelectron Circular Dichroism Sensitive to Helical Chirality? The Paradigm of Gas-Phase Carbo[6]Helicene.

The first member of helically chiral helicenes - carbo[6]helicene (or hexahelicene) - exhibiting sufficient enantiomeric stability under supersonic jet conditions, was studied by photoelectron circular dichroism (PECD) using circularly polarized vacuum ultraviolet synchrotron light and a double imaging photoelectron-photoion coincidence spectrometer. Strong orbital-specific PECD, up to ∼ 25%, was observed over the first few eVs above the ionization energy of the highest occupied molecular orbital with, in addition, a very rich and complex evolution of PECD for all orbitals as a function of the electron kinetic energy. This very first chiroptical study on gas phase helicene demonstrates that PECD is sensitive to helical chirality. It highlights the PECD capability to probe any type of chirality with very high sensitivity even in highly dilute chiral matter, establishing it as a genuinely universal probe of chirality. Complementary quantum chemical calculations suggest possible explanations for the observed PECD, based on the electron density distribution in the ionized molecular orbitals.