Cooperative supramolecular polymerization: comparison of different models applied on the self-assembly of bis(merocyanine) dyes.

Three new molecular building blocks 1 a-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) 1 a,b bearing linear alkyl chains, but completely absent for bis(merocyanine) 1 c bearing sterically more bulky ethylhexyl substituents. Both temperature- and concentration-dependent UV/Vis data provide unambiguous evidence for a cooperative self-assembly process for bis(merocyanines) 1 a,b, which was analyzed in detail by the Meijer-Schenning-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.