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

## TL;DR

This paper introduces a method to enhance the cumulene character in acetylene-based systems by connecting pro-diradicaloid units, which could lead to stimuli-responsive one-dimensional materials.

## Contribution

A novel approach to reversibly alter the electronic character of acetylene-based conjugates through planarization of pro-diradicaloid units.

## Key findings

- Connecting cyclopenta[h,i]aceanthrylene units via an acetylene bridge increases cumulene character in 1D conjugates.
- Planarization of the dimers at low temperatures enhances π-electronic delocalization and cumulenic character.
- Aggregation-induced planarization in methylcyclohexane further boosts the cumulenic character of the acetylenic bridge.

## Abstract

Acetylene/polyynes −(C≡C−)
n
and cumulenes =(C=)
n
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.

Connecting two pro‐diradicaloid species, namely cyclopenta[h,i]aceanthrylene, through an acetylene bridge has been shown to increase the cumulene character in one‐dimensional acetylene‐based conjugates. Spectroscopic analysis and calculations indicate significant π‐electronic delocalization during the low temperature‐promoted planarization of the conjugates, which in turn enhances the cumulenic character of the acetylenic π‐bridge.