New Multiresponsive Chromic Soft Materials: Dynamic Interconversion of Short 2,7‐Dicyanomethylenecarbazole‐Based Biradicaloid and the Corresponding Cyclophane Tetramer

Herein, we report 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 at ambient. This carbazole-based quinoidal precursor is able to form a macrocyclic -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 A) 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 colour changes. These novel photo- thermo- and mechano-chromic 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.