Aggregation of Amphiphilic Carbocyanines: Fluorination Favors Cylindrical Micelles over Bilayered Tubes
A. K. Singh, B. Schade, V. Wycisk, C. Böttcher, R. Haag and H. von Berlepsch – 2021
The synthesis of a new amphiphilic 5,5′,6,6′-tetrachlorobenzimidacarbocyanine dye derivative with −(CH2)2–(CF2)5–CF3 chains attached to the nitrogen atoms in the 1,1′-position, CF8O3, is reported. Depending on the dye concentration and the addition of MeOH, CF8O3 forms J- and H-aggregates in aqueous solutions. The aggregation behavior was investigated using steady-state absorption, linear dichroism, and fluorescence spectroscopy, as well as by cryogenic transmission electron microscopy (cryo-TEM). The J-band of the MeOH-free solution is monomer-like, rather broad, and less red-shifted with respect to the monomer absorption, indicating weak excitonic coupling and disorder effects. Cryo-TEM reveals a diversity of supramolecular structures, wherein linear and branched cylindrical micelles dominate. It is concluded that the high stiffness of fluoroalkyl chains does not allow the chains to splay and completely fill up the hydrophobic gap between opposing chromophores. This destabilizes the bilayers and favors the micellar structure motifs instead. The aggregates appearing at 30% MeOH show a split absorption spectrum consisting of a broad blue-shifted H-band and an accompanying sharp red-shifted J-band with perpendicular polarizations. These HJ-type aggregates are also composed of micellar fibers, but these bundle into rope-like strands. For 10% MeOH, a narrow bilayered tube is the dominating morphology. The observed MeOH dependence of aggregation reveals a clear cosolvent effect.