Low-energy electron attachment, autodetachment, and electron-induced processes in the smallest helicene molecules

Free low-energy (0-14 eV) electron attachment and electron-triggered processes in the gas-phase carbohelicene molecules containing five and six ortho-fused benzene rings are studied using dissociative electron attachment spectroscopy under single-collision conditions. The long-lived molecular negative ions (MNIs) are the most abundant species detected in the negative ion mass spectra of both compounds. Their electron detachment times are measured to be 50-70 μs. On this basis, the adiabatic electron affinity of [6]helicene is estimated to be 0.81 eV in excellent agreement with the B3LYP/6-31+G(d) value (0.79 eV). The most intriguing finding is a sharp increase in the electron detachment time of [5]helicene MNI observed in the range 0.25-0.5 eV that contradicts the obvious tendency of electron detachment times to decrease as incident electron energy rises. This unusual effect is explained by the formation of the most stable re-arranged planar MNI, which loses its initial chirality, a process being supposed to be possible only in [5]helicene MNI initially formed through the π3* shape resonance. The present findings can be of importance to understanding the spin-filtering effects produced by helical molecules since transmission of low-energy photoelectrons through ordered enantiopure molecular layers can be considered as the formation of the temporary negative ion states and their decay by electron detachment.