• amoeba
• born-oppenheimer
• breaking
• computational chemistry
• excited state
• infrared
• molecular dynamics
• physical chemistry
• pull
• push
• scf
• spectroscopy
• symmetry
• td-dft
• transient infrared

Excited-state symmetry breaking (ES-SB) occurs when a molecule symmetric in its ground state (GS) adopts an asymmetric geometry in its excited state (ES). This is common in push–pull systems containing electron-donating (D) and electron-accepting (A) groups, where photoexcitation initially creates a symmetric, delocalized ES in the Franck–Condon region. If ES-SB takes place, this symmetry is lost on an ultrafast timescale, and electron density localizes, producing a dipolar ES. While molecular structure influences ES-SB, the surrounding environment plays a decisive role: asymmetric interactions between solvent and solute electron densities drive the process.
In this thesis, ES-SB is investigated for the donor–acceptor–donor molecule ADA in methylcyclohexane (MCH), dimethyl sulfoxide (DMSO), and methanol (MeOH) using molecular dynamics (MD) within a QM/MM framework. The solute is treated quantum mechanically and the solvent classically with the AMOEBA force field. Transient infrared (TRIR) spectroscopy is simulated as the difference between ES and GS IR spectra, with the ES described using both excited-state self-consistent field (ΔSCF) and time-dependent density functional theory (TD-DFT). Moreover, extrapolation schemes are implemented to reduce the computational cost of ES dynamics.