2,7-Dicyanomethylenecarbazole Based Biradicaloid and Corresponding Cyclophane Tetramer: Dynamic Interconversion Controlled by External Stimuli

This work reports on a quinodimethane‐type molecule, 2,7‐dicyanomethylene‐9‐(2‐ethylhexyl)carbazole (1), one of the shortest π‐conjugated biradicaloids reported to be stable in solution under ambient conditions. This carbazole‐based quinoidal precursor is able to form a macrocyclic σ‐bonded tetramer (2). The resolved single‐crystal X‐ray structure of tetramer 2 shows that four molecules of 1 are linked together through four long (CN)2C−C(CN)2 bonds (1.631 Å) resulting from coupling of the unpaired electrons in biradicaloid 1. Dynamic interconversion between monomer 1 and cyclophane tetramer 2 is achieved by reversible cleavage and recovery of the four (CN)2C−C(CN)2 bonds upon soft external stimuli (light absorption, temperature and pressure), which is accompanied by significant color changes. These novel photo‐, thermo‐, and mechanochromic properties expand the versatility of π‐conjugated biradicaloid compounds as novel functional materials that, in combination with spin chemistry and dynamic covalent chemistry, can be relevant in molecular machines, sensors, and switches.