Quantum Chemistry Investigation of Linear and Nonlinear (chir)optical Properties of Oligoamide Foldamers

The linear and nonlinear (chir)optical properties of a series of four oligoamide foldamers, containing different electron‐donor groups in the same position, have been studied by means of quantum chemistry (QC) calculations at the time‐dependent density functional theory (TDDFT) level. In particular, the electronic circular dichroism (ECD), the electric field induced second harmonic generation, the hyper‐Rayleigh scattering (HRS), and finally the HRS optical activity (HRSOA) spectroscopies have been simulated. Structure–property relationships emerge: systems bearing stronger donors exhibit the largest electric‐dipole first hyperpolarizabilities. This is attributed to the presence of a low‐lying excited state characterized by a nonlocal intramolecular charge transfer and a large electric‐dipole moment variation. Analyses based on the unit sphere representation further identify the directions of maximum induced second‐order polarization and clarify the dipolar versus octupolar nature of the whole responses. Although the predicted β HRS values and the key ECD bands show good agreement with the experiments, some discrepancies are observed in the depolarization ratio of HRS and in the circular differential scattering ratio of HRS‐OA. Overall, this study establishes clear links between the structural features and the linear and nonlinear (chir)optical properties, and it demonstrates the current power of modern QC to tackle large and intricate chiral systems.