Solvation-controlled emission of divalent europium salts

Coordination-environment-dependent tuning of luminescence of divalent europium is an emerging topic of research. While it has been extensively demonstrated in the solid state, recent reports indicate similar findings in solution, where emission properties were tuned by altering different ligands and counter anions. In the present study, we demonstrate that differences in solvation shells alter the luminescence properties of divalent europium. Steady-state emission measurements indicate that the emission maxima of Eu^II salts in dimethoxyethane is red-shifted compared to the maxima in tetrahydrofuran. This shifting was confirmed with three different Eu^II salts: EuI₂, EuBr₂, and Eu(OTf)₂. UV–visible spectroscopic measurements indicate a marginal difference in absorption spectra of Eu^II salts in tetrahydrofuran and dimethoxyethane, ruling out the possibility that a difference in ground-state geometry is responsible for the solvation-induced emission shift. Time-dependent density functional theory studies further support this conclusion. Relaxation of the excited state in dimethoxyethane is a postulated mechanism behind the red-shifted emission. The current study demonstrates a straight-forward path toward tuning the emission properties of Eu^II without altering light absorption.