Spin Crossover – Quo Vadis?

This retrospective essay is an attempt to span the bridge over eight decades of research on spin crossover (SCO), one of the most fascinating dynamic electron structure phenomena of inorganic coordination chemistry. The occurrence of SCO compounds of 3d transition metal ions and their characterization regarding magnetic, optical, vibrational, structural, and thermodynamic properties are briefly addressed. Selected case studies of chemical influences and physical effects (pressure, magnetic field, light) on SCO behavior are discussed. Particular attention is paid to the importance of Mossbauer spectroscopy in SCO studies. Light-induced excited spin state trapping (LIESST) and related photophysical phenomena involving SCO compounds are briefly highlighted. The remarkable instrumental improvements over the years and the amazing widespread developments in synthesizing new SCO compounds up to multifunctional materials currently in the foreground of SCO research are brought to light in a state-of-the-art summary. New directions, primarily towards applications, are mentioned.