Enhancing the Cumulene Character of One-Dimensional Acetylene-Based Systems by Stimuli-Induced Planarization of Their Two Pro-diradicaloid Cyclopenta[h,i]aceanthrylene Units.

Acetylene/polyynes -(C≡C-) and cumulenes =(C=) are connectors widely used for the realization of one-dimensional (1D) π-conjugates. Although both π-moieties are constituted by sp carbon atoms, their different bond connectivity confers distinct physicochemical properties to the resulting systems. In this context, while many acetylene/polyyne- and cumulene-based derivatives have been prepared and studied, no reports have tackled the possibility to reversibly alter the acetylene/polyyne-cumulene electronic character of these derivatives using mild conditions. Herein, we present a novel approach to enhance the cumulene character of 1D acetylene-based conjugates consisting in the preparation of derivatives featuring an acetylene moiety connecting two pro-diradicaloid species, namely cyclopenta[h,i]aceanthrylene (CPA), at their pro-radical positions. A thoughtful spectroscopic study of the prepared dimers, complemented by theoretical calculations, suggest a high π-electronic delocalization of the pro-diradicaloid CPAs through the central acetylene spacer upon the dimers' planarization which, in turn, increases the cumulenic character of the acetylenic π-bridge, a feature enhanced for one of the two dimers at low temperature and in methylcyclohexane due to an aggregation-induced planarization process. We reckon that the proposed approach offers an interesting avenue towards the realization of 1D systems which cumulenic character of the acetylenic π-connector could be altered in response to external stimuli.