Quantum Chemical Study of the Pressure-dependent Phosphorescence of [Cr(ddpd)₂ ]³⁺ in the Solid State

The chromium(III) complex [Cr(ddpd)₂ ][BF₄ ]₃ shows two spin-flip emission bands in the near-infrared spectral region. These bands shift bathochromically by -14.1 and -7.7 cm^-1  kbar^-1 under hydrostatic pressure (Angew. Chem. Int. Ed. 2018, 57, 11069). The present study elucidates the structural changes of the chromium(III) cations under pressure using density functional theory with periodic boundary conditions and the resulting effects on the excited state energies using high-level CASSCF-NEVPT2 calculations. The differences of the bands in pressure sensitivity are traced back to a different orbital occupation of the intraconfigurational excited states.