Effect
of Proton Substitution by Alkali Ions on the
Fluorescence Emission of Rhodamine B Cations in the Gas Phase
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Abstract
The
photophysics of chromophores is strongly influenced by their
environment. Solvation, charge state, and adduct formation significantly
affect ground and excited state energetics and dynamics. The present
study reports on fluorescence emission of rhodamine B cations (RhBH<sup>+</sup>) and derivatives in the gas phase. Substitution of the acidic
proton of RhBH<sup>+</sup> by alkali metal cations, M<sup>+</sup>,
ranging from lithium to cesium leads to significant and systematic
blue shifts of the emission. The gas-phase structures and singlet
transition energies of RhBH<sup>+</sup> and RhBM<sup>+</sup>, M =
Li, Na, K, Rb, and Cs, were investigated using Hartree–Fock
theory, density functional methods, second-order Møller–Plesset
perturbation theory, and the second-order approximate coupled-cluster
model CC2. Comparison of experimental and theoretical results highlights
the need for improved quantum chemical methods, while the hypsochromic
shift observed upon substitution appears best explained by the Stark
effect due to the inhomogeneous electric field generated by the alkali
ions