Photoionization of molecules in water solutions is a complex phenomenon where we must consider, in addition to the solute electronic and nuclear degrees of freedom, the bulk water structure. For aromatic molecules, the presence of the aqueous medium allows the photoionization at energies as low as the onset of the electronic absorption, becoming a key relaxation channel for biologically relevant systems in their natural environment. By using time resolved methods (pump-probe and pump-repump-probe) based in fs pulses, the reported research aims to unravel the mechanism behind this process. In particular, the nature of the initially formed electronic-excited state and the intermediate steps toward the fully separated charges (cation+electron), and to establish the influence of the specific solute-solvent interactions on them. In this contribution, we present results on a model aromatic chromophore, aniline, after excitation at 267 nm along its first ππ* character electronic absorption. The collected observations allow us to characterize the formation of a charge transfer state that mediates the formation of the solvated charges. The access to this state and the competence with other photophysical relaxation routes is rationalized in terms of the different H-bonds that the water establishes with the amino group. The proposed mechanism could be common for aromatics in water containing N–H or O–H bonds and opens the possibility of controlling the generation of charges in solution, at low excitation energies.

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Photoionization of Aromatic Chromophores in Aqueous Medium by Near-UV Single Photon Absorption

  • Raúl Montero,
  • Iker Lamas,
  • Asier Longarte

摘要

Photoionization of molecules in water solutions is a complex phenomenon where we must consider, in addition to the solute electronic and nuclear degrees of freedom, the bulk water structure. For aromatic molecules, the presence of the aqueous medium allows the photoionization at energies as low as the onset of the electronic absorption, becoming a key relaxation channel for biologically relevant systems in their natural environment. By using time resolved methods (pump-probe and pump-repump-probe) based in fs pulses, the reported research aims to unravel the mechanism behind this process. In particular, the nature of the initially formed electronic-excited state and the intermediate steps toward the fully separated charges (cation+electron), and to establish the influence of the specific solute-solvent interactions on them. In this contribution, we present results on a model aromatic chromophore, aniline, after excitation at 267 nm along its first ππ* character electronic absorption. The collected observations allow us to characterize the formation of a charge transfer state that mediates the formation of the solvated charges. The access to this state and the competence with other photophysical relaxation routes is rationalized in terms of the different H-bonds that the water establishes with the amino group. The proposed mechanism could be common for aromatics in water containing N–H or O–H bonds and opens the possibility of controlling the generation of charges in solution, at low excitation energies.