Photochemistry | 2021
Excited-state symmetry breaking: from fundamental photophysics to asymmetrical photochemistry
Abstract
Excited-state symmetry breaking in multipolar organic molecules is extensively investigated here. First unambiguous experimental observation of this phenomenon by using femtosecond time-resolved infrared spectroscopy (TRIR) is presented. This photophysical phenomenon originates from the fluctuations and rearrangement of solvent molecules around the electronically excited chromophore. Non-specific quadrupolar and dipolar, as well as specific H-bonding and non-orthodox X-bonding solute-solvent interactions can induce symmetry breaking, whereas dispersion and intramolecular distortions/asymmetric vibrations cannot. The effect of intramolecular factors, e.g. of the length of charge-transfer branches, is studied. Symmetry breaking leads to the asymmetrical reactivity of the seemingly identical molecular branches that can be utilized to achieve asymmetrical intra- and intermolecular photochemistry. Additionally, a novel approach to the time-resolved infrared spectroscopy ( solute-pump/solvent-probe ) is demonstrated. It is used to decipher the mechanism of H-Bond Induced Nonradiative Deactivation (HBIND) of a quadrupolar molecular rod in highly protic alcohols.