Supramolecular assemblies between a new series of gemini-type amphiphiles and TPPS at the air/water interface: Aggregation, chirality, and spacer effect

Supramolecular assemblies between a series of novel gemini amphiphiles (BisImC17Cn) and TPPS (tetrakis(4-sulfonatophenyl)porphine) at the air/water interface were investigated. It was found that the gemini amphiphiles could form stable complex monolayers at the air/water interface with TPPS through electrostatic interaction and pi-pi stacking. There is an inflection point in each of the surface pressure-area isotherms of the complex monolayers. At a certain pH value of the subphase, TPPS could complex with the gemini amphiphiles as a J-aggregate. AFM measurements revealed that nanofibers were formed in the transferred films onto solid substrates by a horizontal lifting method. Interestingly, all the complex films containing the J-aggregate of TPPS showed CD signals in the J-band although both of the amphiphiles and TPPS were achiral. Helical stacking of the TPPS underneath the gemini monolayers was suggested to be responsible for the supramolecular chirality of the multilayer films. The spacers in the gemim amphiphiles have great influence on the chirality of the system. For the film transferred at a surface pressure below the inflection point, the CD signals showed splitting only when the spacer length is larger than four methylene units. The films transferred at the surface pressure above the inflection points showed splitting CD signals of the Soret band of J-aggregates, which could be ascribed to the interaction between the J-aggregates. Moreover, for the film transferred above the inflection points, the CD intensity of the films decreased when the spacer length of the gemini amphiphile increased. It is further observed that many properties such as the width and thickness of the formed nanofibers showed a turning point at a spacer length of six methylene units. A mechanism including the convex of the spacers when they were larger than six methylenes was proposed.