Spin Crossover and Long‐Lived Excited States in a Reduced Molecular Ruby

• Published in: Chemistry : a European journal Vol. 26; no. 32; pp. 7199 - 7204
• Main Authors: Becker, Patrick M., Förster, Christoph, Carrella, Luca M., Boden, Pit, Hunger, David, Slageren, Joris, Gerhards, Markus, Rentschler, Eva, Heinze, Katja
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 05.06.2020; John Wiley and Sons Inc
• Subjects: Chemistry; Chromium; Communication; Communications; Crystal structure; Diamines; Excitation; excited states; Isomers; magnetic properties; Pyridines; Room temperature; Ruby; spin crossover; step-scan IR spectroscopy; Ultraviolet radiation; chromium; magnetic properties; spin crossover; excited states; step-scan IR spectroscopy
• ISSN: 0947-6539; 1521-3765; 1521-3765
• DOI: 10.1002/chem.202001237

The chromium(III) complex [CrIII(ddpd)2]3+ (molecular ruby; ddpd=N,N′‐dimethyl‐N,N′‐dipyridine‐2‐yl‐pyridine‐2,6‐diamine) is reduced to the genuine chromium(II) complex [CrII(ddpd)2]2+ with d4 electron configuration. This reduced molecular ruby represents one of the very few chromium(II) complexes showing spin crossover (SCO). The reversible SCO is gradual with T1/2 around room temperature. The low‐spin and high‐spin chromium(II) isomers exhibit distinct spectroscopic and structural properties (UV/Vis/NIR, IR, EPR spectroscopies, single‐crystal XRD). Excitation of [CrII(ddpd)2]2+ with UV light at 20 and 290 K generates electronically excited states with microsecond lifetimes. This initial study on the unique reduced molecular ruby paves the way for thermally and photochemically switchable magnetic systems based on chromium complexes complementing the well‐established iron(II) SCO systems. Ruby‐reduced: Reduction of the molecular ruby [CrIII(ddpd)2]3+ (ddpd=N,N′‐dimethyl‐N,N′‐dipyridine‐2‐yl‐pyridine‐2,6‐diamine) gives the genuine chromium(II) complex [CrII(ddpd)2]2+ showing spin crossover (SCO) with T1/2 around room temperature. Geometric distortions and color changes accompany this reversible SCO. Excitation with UV light yields electronically excited states with microsecond lifetimes.