Magnetic circularly polarized luminescence from spin–flip transitions in a molecular ruby

Magnetic circularly polarized luminescence (MCPL), i.e. the possibility of generating circularly polarized luminescence in the presence of a magnetic field in achiral or racemic compounds, is a technique of rising interest. Here we show that the far-red spin–flip (SF) transitions of a molecular Cr(III) complex give intense MCD (magnetic circular dichroism) and in particular MCPL (gMCPL up to 6.3 × 10⁻³ T⁻¹) even at magnetic fields as low as 0.4 T. Cr(III) doublet states and SF emission are nowadays the object of many investigations, as they may open the way to several applications. Due to their nature, such transitions can be conveniently addressed by MCPL, which strongly depends on the zero field splitting and Zeeman splitting of the involved states. Despite the complexity of the nature of such states and the related photophysics, the obtained MCPL data can be rationalized consistently with the information recovered with more established techniques, such as HFEPR (high-frequency and -field electron paramagnetic resonance). We anticipate that emissive molecular Cr(III) species may be useful in magneto-optical devices, such as magnetic CP-OLEDs.