Electronic Stabilization Effect of a Spin‐Delocalized Neutral Radical: Synthesis of an 8‐Cyano‐6‐oxophenalenoxyl Derivative and Quantitative Evaluation of the Electronic Spin Structure in terms of Resonance Structures

A new 2,5‐di‐tert‐butyl‐6‐oxophenalenoxyl (6OPO) derivative with a cyano group at the 8‐position, where a large spin density resides, has been synthesized. This neutral radical exhibits high stability in the solid state in air despite the low steric protection on the 8‐position; the stability is comparable to that of a corresponding 8‐tert‐butylated 6OPO derivative. EPR/¹H‐ENDOR/TRIPLE (electron paramagnetic resonance/¹H‐electron‐nuclear double resonance/TRIPLE) spectroscopy and cyclic voltammetry showed an extended spin delocalization on the cyano group and a significant increase in electron‐accepting ability relative to that of the 8‐tert‐butylated 6OPO derivative. DFT calculations indicated the extension of a singly occupied molecular orbital (SOMO) onto the cyano group and the lower‐lying SOMO and LUMO in comparison with those of the 8‐tert‐butylated 6OPO derivative, which was consistent with experimental results. Furthermore, the extended nature of π conjugation onto the cyano group was quantitatively evaluated by calculating the contributing weights of resonance structures in terms of a molecular orbital (MO)‐based valence‐bond (VB) method. Herein, the synthesis and physical properties of the 8‐cyano‐6OPO derivative are described, emphasizing that the high stability arises from the electronic effect of the cyano group. Also, the usefulness of the quantitative resonance structure analysis is shown.