Two‐dimensional Supramolecular Polymorphism in Cyanine H‐ and J‐aggregates

We designed a new cyanine dye 1, with two pedant rod‐like groups, capable of forming two distinct two‐dimensional (2D) supramolecular polymorphs in methylcyclohexane; an H‐type aggregate (Agg‐H2) and a J‐type aggregate (Agg‐J). Importantly, these two polymorphs were not accessed through polymerization events, and instead through the thermal transformation of a third particle‐like polymorph (Agg‐H1) formed by the anti‐cooperative assembly of 1. While Agg‐H2 is generated upon cooling the solution of Agg‐H1 by a thermoreversible polymorph transition, the Agg‐J was obtained through a hidden pathway by combining sonication and cooling to the Agg‐H1 solution. This is the first report on the obtention of H‐ and J‐type cyanine polymorphs that in turn could be isolated in solid‐state to render two new 2D photoactive materials. This paper unveils new strategies for designing 2D supramolecular polymers using calamitic residues, but also undercovers relevant aspects of pathway complexity and polymorph transitions that might be crucial for developing novel photonic systems.