Ultrafast excited-state dynamics and transient absorption spectra of [BMIM][BF₄] in acetonitrile: A QM/MM–trajectory surface hopping study

Abstract

In this study, we aimed to investigate the transient absorption spectra of 1-butyl-3-methylimidazolium tetrafluoroborate in acetonitrile by analyzing excited-state absorption and stimulated emission signals, using the trajectory surface hopping method. All calculations performed based on classical molecular dynamics and quantum mechanics were conducted using 30 Wigner sampling at room temperature. Quantum mechanical calculations were performed using B3LYP/6–311++G(d,p) level theory, and molecular mechanics calculations were performed using the AMBER suite program. The 30 different distributions of initial geometries derived from sampling significantly impacted the results of the excited-state dynamics and transient absorption spectra over a time period of ∼190 fs. The Tamm-Dancoff approximation was chosen for transient absorption spectrum calculations because the de-excitation effects could not be considered when determining the transition dipole moments. The UV spectrum and population decay were investigated within the ∼180-trajectory ensemble. The simulations capture ultrafast S2 → S1 internal conversion (∼30 fs), a persistent low-energy excited-state absorption band at 0.8–1.2 eV, and stimulated emission features at 4–6 eV, in strong agreement with experimental transient absorption measurements. Bond-length heatmaps reveal excitation-induced restructuring of solvation shells and ion-pair contacts, while polarization-dependent spectra highlight anisotropic transition dipole moment orientations. These results bridge molecular-level structural dynamics with measurable transient absorption signatures, validating the computational protocol and providing predictive insight for tailoring ultrafast photophysics in ionic liquid–solvent systems.