Unprecedented Reverse Volume Expansion in Spin‐Transition Crystals

• Published in: Chemistry : a European journal Vol. 26; no. 57; pp. 12927 - 12930
• Main Authors: Guo, Wenbin, Daro, Nathalie, Pillet, Sébastien, Marchivie, Mathieu, Bendeif, El‐Eulmi, Tailleur, Elodie, Chainok, Kittipong, Denux, Dominique, Chastanet, Guillaume, Guionneau, Philippe
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 09.10.2020; Wiley-VCH Verlag
• Subjects: Cell size; Chemical Sciences; Chemistry; Crossovers; Crystal structure; Crystals; Magnetic measurement; Material chemistry; Mechanical properties; Metal ions; molecular crystals; phase transition; photoexcited states; rotation of phenyl ring; Spin transition; spin-crossover; iron; photomagnetism; negative expansion; phase transition; spin-crossover
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.202001821

The current craze for research around the spin crossover phenomenon can be justified to some extent by the mechanical properties due to the decrease of volume associated with the transition of the metal ion from the HS state to the LS state. As demonstrated here, the molecular complex [Fe(PM‐pBrA)2(NCS)2] exhibits, on the contrary, an increase of the unit‐cell volume from HS to LS. This counter‐intuitive and unprecedented behavior that concerns both the thermal and the photoexcited spin conversions is revealed by a combination of single‐crystal and powder X‐ray diffraction complemented by magnetic measurements. Interestingly, this abnormal volume change appears concomitant with the wide rotation of a phenyl ring which induces a drastic modification, though reversible, of the structural packing within the crystal. In addition, the light‐induced HS state obtained through the Light‐Induced Excited Spin‐State Trapping shows a remarkably high relaxation temperature, namely T(LIESST), of 109 K, one of the highest so far reported. The above set of quite unusual characteristics opens up new fields of possibilities within the development of spin crossover materials. Molecular crystals: An unprecedented reverse and reversible volume modification in crystals of the molecular complex [Fe(PM‐pBrA)2(NCS)2] is observed both for the thermal and the photoinduced spin‐crossover processes. Probably linked here to a structural singularity, that is, an impressive phenyl ring rotation into the crystal, the increase in volume from HS to LS nevertheless opens up new prospects for spin‐transition‐based materials (see figure).