Magneto-Chiral Dichroism and Other Dichroic Spectra of [3]- to [8]Helicenes

Magneto-chiral dichroism (MChD) is a fundamental but experimentally elusive chiroptical effect. Carbo[n]helicenes, with their strong and systematically evolving chirality, represent ideal systems for its investigation. Here, we present the first theoretical predictions of the MChD spectra for a series of carbo[n]helicenes, from [4]- to [8]helicene, calculated using time-dependent density functional theory (TD-DFT) within the damped response framework. To contextualize these predictions and enhance their reliability, we also computed the corresponding electronic circular dichroism (ECD) and magnetic circular dichroism (MCD) spectra, and report the first experimental MCD spectrum of [7]helicene. A systematic scaling procedure, calibrating the computed wavelengths against available experimental ECD and MCD data, was employed to provide reliable estimates for the yet-to-be-measured MChD signals. Our results predict that the MChD signals for larger helicenes, in particular [6]-, [7]-, and [8]helicene, are potentially within the sensitivity of modern experimental setups, with dissymmetry factors (g_MChD) of the order of 10^-6. An increasing trend in the MChD signal is observed with increasing helicene size, suggesting a correlation with the number of benzene rings. These findings provide a robust theoretical benchmark and are intended to motivate new experimental investigations of this fundamental light-matter interaction.