Growth of Merocyanine Dye J‐Aggregate Nanosheets by Living Supramolecular Polymerization.

J‐aggregates are highly desired dye aggregates but so far there has been no general concept how to accomplish the required slip‐stacked packing arrangement for dipolar merocyanine (MC) dyes whose aggregation commonly affords one‐dimensional aggregates composed of antiparallel, co‐facially stacked MCs with H‐type coupling. Herein we describe a strategy for MC J‐aggregates based on our results for an amphiphilic MC dye bearing alkyl and oligo(ethylene glycol) side chains. In an aqueous solvent mixture, we observe the formation of two supramolecular polymorphs for this MC dye, a metastable off‐pathway nanoparticle showing H‐type coupling and a thermodynamically favored nanosheet showing J‐type coupling. Detailed studies concerning the self‐assembly mechanism by UV‐Vis spectroscopy and the packing structure by atomic force microscopy and wide‐angle X‐ray scattering show how the packing arrangement of such amphiphilic MC dyes can afford slip‐stacked two‐dimensional nanosheets whose macrodipole is compensated by the formation of a bilayer structure. As an additional feature we demonstrate how the size of the nanosheets can be controlled by seeded living supramolecular polymerization. [ABSTRACT FROM AUTHOR]