Self-Assembly of Metallo-Supramolecular Amphiphiles based on Azadipyrromethenes: Consecutive Pathways to Nanodiscs and Nanosheets

A new class of metallo-supramolecular amphiphilic dyes 1 a, b was constructed by using two azadipyrromethene units which were respectively modified with two hydrophobic alkyl and two hydrophilic oligo(ethylene glycol) chains. The spectroscopic and morphological studies revealed the consecutive self-assembly pathways of 1 a in EtOH/H2O mixed solvent. The monomers of 1 a firstly aggregated into the kinetic-controlled, nanodisc-shaped Agg. I upon cooling and the latter spontaneously transformed into the thermodynamically more stable Agg. II with a nanosheet morphology. While the non-fluorescent Agg. I displayed a broad absorption band (lambda max=615 nm), the Agg. II displayed a more intensive and narrowed J-band (lambda max=693 nm) and a fluorescence band with a maximum at 760 nm (Phi fl=0.1), which could be ascribed to the J-aggregation induced emission enhancement. The kinetics of Agg. I to Agg. II transformation was further modulated by seed-initiated supramolecular polymerization with various ratios of Seedagg.II, in which the transformation rate was significantly increased by ca. 2 orders of magnitude compared to the spontaneous process. The supramolecular amphiphile 1 b bearing longer hydrophilic chains formed only one type aggregate, which exhibited spectroscopic and morphological properties that were highly comparable with that of Agg. I.