Charge‐Transfer and Spin‐Flip States: Thriving as Complements

Abstract Transition metal complexes with photoactive charge‐transfer excited states are pervasive throughout the literature. In particular, [Ru(bpy) 3 ] 2+ (bpy=2,2′‐bipyridine), with its metal‐to‐ligand charge‐transfer emission, has been established as a key complex. Meanwhile, interest in so‐called spin‐flip metal‐centered states has risen dramatically after the molecular ruby [Cr(ddpd) 2 ] 3+ (ddpd= N , N ′‐dimethyl‐ N , N ′‐dipyridin‐2‐yl‐pyridine‐2,6‐diamine) led to design principles to access strong, long‐lived emission from photostable chromium(III) complexes. This Review contrasts the properties of emissive charge‐transfer and spin‐flip states by using [Ru(bpy) 3 ] 2+ and [Cr(ddpd) 2 ] 3+ as prototypical examples. We discuss the relevant excited states, the tunability of their energy and lifetimes, and their response to external stimuli. Finally, we identify strengths and weaknesses of charge‐transfer and spin‐flip states in applications such as photocatalysis and circularly polarized luminescence.