Cooperative Supramolecular Polymerization: Comparison of Different Models Applied on the Self-Assembly of Bis(merocyanine) Dyes

Three new molecular building blocks 1a-c for supramolecular polymerization are described that feature two dipolar merocyanine dyes tethered by p-xylylene spacers. Concentration-and temperature-dependent UV/Vis spectroscopy in chloroform combined with dynamic light scattering, capillary viscosimetry and atomic force microscopy investigations were applied to elucidate the mechanistic features of the self-assembly of these strongly dipolar dyes. Our detailed studies reveal that the self-assembly is very pronounced for bis(merocyanines) 1a,b bearing linear alkyl chains, but completely absent for bis(merocyanine) 1c bearing sterically more bulky ethylhexyl substituents. Both temperatureand concentration-dependent UV/Vis data provide unambiguous evidence for a cooperative self-assembly process for bis(merocyanines) 1a, b, which was analyzed in detail by the Meijer-Schen-ning-Van-der-Schoot model (applicable to temperature-dependent data) and by the Goldstein-Stryer model (applicable to concentration-dependent data). By combining both methods all parameters of interest to understand the self-assembly process could be derived, including in particular the nucleus size (8-10 monomeric units), the cooperativity factor (ca. 0.006), and the nucleation and elongation constants of about 10(3) and 10(6) M-1 in chloroform at room temperature, respectively.