Double Spin with a Twist: Synthesis and Characterization of a Neutral Mixed-Valence Organic Stable Diradical

• Published in: Journal of the American Chemical Society Vol. 146; no. 38; pp. 26261 - 26274
• Main Authors: Sentyurin, Vyacheslav V., Levitskiy, Oleg A., Yankova, Tatiana S., Grishin, Yuri K., Lyssenko, Konstantin A., Goloveshkin, Alexander S., Alabugin, Igor V., Magdesieva, Tatiana V.
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 25.09.2024; Amer Chemical Soc
• Subjects: ambient temperature; biphenyl; borates; Chemistry; Chemistry, Multidisciplinary; class; energy; geometry; Physical Sciences; redox reactions; Science & Technology; species; TRANSITION; POLYMERS; DESIGN; PERSISTENT; ELECTRON-TRANSFER; POLYRADICALS; MOLECULES
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/jacs.4c08167

A convenient design strategy opens access to neutral open-shell mixed-valence species via the redox transformation of charged stable precursors, i.e., the spiro-fused borate anions. We have implemented this strategy for the synthesis of the first neutral mixed-valence diradical: two neutral mixed-valence radical fragments were assembled via a twisted biphenyl bridge. The diradical is a crystalline solid obtained in almost quantitative yield by using a facile synthetic procedure. It is stable at room temperature in the triplet ground state with a very small singlet/triplet gap. This metal-free diradical can reversibly form five redox states. The diradical exhibits an intense IVCT band in the NIR region and can be assigned as a Class 2 Robin–Day MV (mixed valence) system with weakly interacting redox centers. Computations suggest that this diradical finds itself in a unique tug-of-war between two electron delocalization patterns, Kekulé and non-Kekulé, which gives rise to two geometric isomers that are close in energy but drastically different in spin distribution and polarity. Such bistable spin-systems should be intrinsically switchable and promising for the design of functional spin devices. The scope and limitations of the new redox-strategy for the neutral MV radicals were also tested on other types of spiro-fused borates, revealing structural factors responsible for the evolution from transient to persistent and then to stable radicals.