The accurate measurement and simulation of the dynamics of ultrafast processes such as excited-state intramolecular proton transfer (ESIPT) has been the focus of much recent interest. Transient absorption (TA) spectroscopy is a promising technique to measure the dynamics of these ultrafast process due to its excellent time resolution and applicability to a diverse class of systems. However, assignment of the components of the spectrum and extraction of relevant physical quantities requires simulation of both dynamics as well as the corresponding spectrum. First-principles nonadiabatic molecular dynamics methods, such as ab initio multiple spawning (AIMS) are powerful tools for interpreting experiments without any empirical inputs. We used AIMS simulations in combination with our recent implementations of the GPU-accelerated time-dependent configuration interaction (TD-CI) method to simulate the ESIPT dynamics and gas-phase TA spectrum of prototypical molecule 1′-Hydroxy-2′-acetonaphthone. AIMS simulations were carried out using our open-source PySpawn package and on-the-fly electronic structure calculations were performed for potential energy surfaces, forces and non-adiabatic coupling using TeraChem suite of programs. TA spectrum was computed using the representative geometries from the snapshots of ~2 picoseconds AIMS simulations. For each snapshot, the emission and absorption spectrum were computed in the realm of TD-CI by applying a δ-function pulse to the initial electronic state and simulating the electron dynamics for 15 femtoseconds. The simulated spectrum and excited-state proton transfer dynamics are highly consistent with the gas-phase experimental TA studies. Our methodology provides an added advantage of separately simulating the spectrum of AIMS trajectories evolved on the ground vs excited state and/or those associated with proton transfer vs non-proton transfer providing a more comprehensive and critical tool to interpret and assign the individual components of the TA spectrum.

First-principles simulations of transient absorption spectrum to probe the ultrafast proton transfer dynamics

ACS Spring 2022

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