Self-Organized Supramolecular Micellar Aggregates By Bolaamphiphiles Bearing π-Conjugated Dyes

Supramolecular nanomaterial oriented self-ogranization of small molecules ispositively anticipated as one of the most important targets of 21 century. Amongthose polar-asymmetric molecules that can generate self-organization, amphiphilesare destined as a good candidate, for they can self-organize spontaneously intomicellar structures with defined shape and size in aqueous solution. Nevertheless,these micellar structures are inherently dynamic and fluid, because the forces thathold the amphiphilic molecules together are not caused by strong covalent or ionicbonds but arise from weak forces such as van der Waals, hydrophobic interaction.This character limits the possibilities of utilizing those assemblies as templates formaking nanostructured materials or mimicry of biomineralization processes.Efforts have been made to stabilize surface micelles by introducing mesogenicgroups into bolaamphiphiles or covalent bond links in mono-polar amphiphiles,which has been evidenced a effective and feasible way. Herein, we turned to theπ-conjugated dyes with strong π-π stacking interactions to further increase theintermolecular interaction so that we could obtain more stable micellar aggregates.This is one motive of our work.Besides the aim of stabilizing the micellar structure, we also interested in thearomatic dyes as function groups in the associated state. Firstly, amphiphilesbearing π-conjugated dyes will afford a new class of dye-based supramolecularassemblies. Secondly, because the photo-electron properties are related to theassociation state of the dyes, it is hoped that the self-organized micellar aggregatecould provide an optimized alignment of the dye residues for their functionalapplication. At last, the dye-based supramolecular assembling is also reflected bythe reversible monomer-aggregate transition controlled by physical or chemicalstimuli such as thermo-, photo-, chemo-and proto-stimuli.Based on the above ideas, we designed and synthesized a series ofbolaamphihiles bearing thienyl-diketopyrrolopyrrole dye (DPP-n), and investigatedtheir self-organization behavior in aqueous solution. We found that DPP-11 withappropriate alkyl chain length formed an interesting disk-like micellar aggregate,which is also evidenced a combination of micro-and nano-structure. Theseself-assemblies show a high stability by not only retaining its morphology duringthe drying process, but also withstanding a high temperature. By using DPP dye asan indicator, the formation process of the disks is monitored by UV-vis absorptionspectra. In the aqueous solution, the disk-like aggregates disassemble as elevatingthe temperature and reassemble as descending the temperature, exhibiting areversible assembling character of supramolecular structures. The chain lengtheffect concerned problems was also addressed, but no positive result was obtainedmost probably because of the solubility of the longer and shorter alkyl chain.