Unveiling Excited-State Chirality of Binaphthols by Femtosecond Circular Dichroism and Quantum Chemical Calculations.

Time-resolved circular dichroism (TR-CD) is a powerful tool for probing conformational dynamics of biomolecules over large time scales that are crucial for establishing their structure-function relationship. However, such experiments, notably in the femtosecond regime, remain challenging due to their extremely weak signals, prone to polarization artifacts. By using binol and two bridged derivatives (PL1 and PL2) as chiral prototypes, we present here the first comprehensive study of this type in the middle UV, combining femtosecond TR-CD and quantum mechanical calculations (TD-DFT). We show that excitation of the three compounds induces large variations of their transient CD signals, in sharp contrast to those of their achiral transient absorption. We demonstrate that these variations arise from both the alteration of the electronic distribution and the dihedral angle in the excited state. These results highlight the great sensitivity of TR-CD detection to signals hardly accessible to achiral transient absorption.