Electronic relaxation of aqueous aminoazobenzenes studied by time-resolved photoelectron spectroscopy and surface hopping TDDFT dynamics calculations.

Abstract

Studies of ultrafast relaxation of molecular chromophores are complicated by the fact that most chromophores of biological and technological importance are rather large molecules and are strongly affected by their environment, either solvent or a protein cage. Here we present an approach which allows us to follow transient electronic structure of complex photoexcited molecules. We use the method of time-resolved photoelectron spectroscopy in solution to follow relaxation of two prototypical aqueous chromophores, Methyl Orange and Metanil Yellow, both of which are aminoazobenzene derivatives. Using excitation by 400 nm laser pulses and ionization by wavelength-selected 46.7 nm XUV pulses from high-order harmonic generation we follow relaxation of both molecules via the dark S1 state. The photoelectron spectra yield binding energies of both ground and excited states. We combine the experimental results with surface hopping time-dependent density functional theory (TDDFT) calculations employing B3LYP+D3 and ωB97X-D functionals. The results demonstrate that the method is generally suitable for description of ultrafast dynamics in these molecules and can recover absolute binding energies observed in the experiment. The B3LYP+D3 functional appears to be better suited for these systems, especially in the case of Metanil Yellow, where it indicates the importance of an intramolecular charge transfer state. Our results pave the way towards quantitative understanding of evolving electronic structure in photo-induced relaxation processes.