Ultrafast Non-Adiabatic Dynamics of 2-(Iminomethyl)phenol: Insights from CASPT2 Simulations

Abstract Excited-state intramolecular proton transfer (ESIPT) and subsequent photoisomerization in o-hydroxy Schiff bases are central to many photochromic and fluorescent applications, yet the interplay between proton transfer and torsional relaxation in the simplest member of this family remains poorly understood. Here, we present the first fully nonadiabatic multireference dynamics study of 2-(iminomethyl)phenol (IMP), the minimal o-hydroxy Schiff base, using ab initio multiple spawning on extended multistate complete active space second-order perturbation theory (XMS-CASPT2) potential energy surfaces. Our simulations show that ESIPT in IMP is barrierless and ultrafast, with an overall time scale of ∼14 fs, placing the molecule firmly in the ballistic-transfer regime. The subsequent S1 lifetime of ∼220 fs is controlled by torsion about the C–C–C–N dihedral, with nonadiabatic transitions occurring predominantly at intermediate dihedral angles of 60–80° on a torsionally broad CI seam. Time-resolved joint distributions further reveal that proton transfer and torsional motion are dynamically, as well as energetically, decoupled. These results establish IMP as a quantitative benchmark for ultrafast ESIPT dynamics and provide directly testable predictions for emerging time-resolved X-ray photoelectron spectroscopy experiments.