Anthraquinodimethane Ring-Flip in Sterically Congested Alkenes: Isolation of Isomer and Elucidation of Intermediate through Experimental and Theoretical Approach

From the viewpoint of the development of molecular response systems, stimulus-induced switching of multi-conformational/multi-configurational overcrowded ethylenes are interesting, whose properties could be manipulated by understanding the detailed isomerization paths. Anthraquinodimethane (AQD) ring-flip is usually a very fast process, and thus less studied experimentally. Herein, we studied AQDs with dibenzo- and tribenzocycloheptatrienylidene units, which have large enough steric hindrance to retard the AQD ring-flip to allow determination of the ΔG≠value experimentally. Their thermal isomerization was also scrutinized using the artificial force induced reaction method to elucidate the intermediates. Based on the structural unsymmetry in a newly prepared AQD, one of the isomers that undergoes a reversible conformational change via AQD ring-flip was isolated and analyzed by X-ray for the first time.Understanding the mechanism of conformational/configurational isomerism of sterically congested alkenes is of great importance to develop novel molecular response systems. Herein, through the careful study on overcrowded anthraquinodimethane (AQD) derivatives including unsymmetrical compounds, we found AQD ring-flip is the key process to make stimuli-responsive materials with multi-conformational/multi-configurational isomers enabling the switching of their structures and physical properties.