Theoretical study of the electronic structure of a tetragonal chromium(III) complex

The low-temperature single-crystal polarized absorption and the luminescence spectra of Cs 2[CrCl 2(H 2O) 4]Cl 3 are theoretically analyzed in order to determine the electronic structure of the trans-[CrCl 2(H 2O) 4] + complex. This study, based on the Racah theory, leads to a good agreement between the theoretical and experimental energy levels. The electronic-exited state 4T 2g(O h) is split into 4E g and 4B 2g components due to the lowering of the symmetry from O h to D 4h. The polarized absorption spectrum and the theoretical energies show that the 4E g(D 4h) state is lower in energy than the 4B 2g(D 4h) state. The resolved vibronic structure in the luminescence spectrum reveals a progression in a non-totally symmetric b 1g mode, a manifestation of a Jahn–Teller effect in the emitting state 4T 2g(O h). Both Jahn–Teller and spin–orbit coupling in the orbitally degenerated 4E g(D 4h) state are necessary to account for the spectroscopic observations. ► The electronic structure of [CrCl 2(H 2O) 4] + complex was performed. ► The theoretical study permits us to deduce the energy levels, which are not observed experimentally. ► The vibronic structure observed in the luminescence spectra reveals a manifestation of a Jahn–Teller effect. ► Both Jahn–Teller and spin–orbit coupling are necessary to account for the spectroscopic observation. ► The Jahn–Teller stabilization energy was deduced.