Spectroscopic effects of excited-state coupling in a tetragonal chromium(III) complex.

Detailed low-temperature single-crystal polarized absorption and luminescence spectra of Cs2[CrCl2(H2O)4]Cl3 are reported. The luminescence spectrum is a broad band with a maximum at 11,800 cm (-1), indicating that the trans-[CrCl2(H2O)4]+ complex emits from a quartet excited state. The resolved vibronic structure reveals a progression in a nontotally symmetric 445 cm (-1) b1g mode, a manifestation of a Jahn-Teller effect in the emitting state. The absorption spectrum shows completely linearly polarized, magnetic-dipole-allowed electronic origins, defining the tetragonal splitting of the states originating from 4T2g (Oh). An energy gap of approximately 800 cm (-1) is observed between the electronic origins of the emitting state and the onset of the pi-polarized absorption spectrum. Both Jahn-Teller and spin-orbit couplings in the orbitally degenerate 4Eg (D4h) state are necessary to account for the spectroscopic observations.