Isolable and Fluorescent Mesoscopic Micelles Made of an Amphiphilic Derivative of Tris-bipyridyl Ruthenium Hexafluorophosphate

Abstract

Bis(2,2'-bipyridyl)(dioctadecyl-2-[2,2‘-dipyridylmethylene] malonate) ruthenium(II) dihexafluorophosphate has been synthesized and was converted to an amphiphilic distearyl ester, 2b. Light-induced charge transfer from ruthenium to the pyridyl ligands occurred preferably to the malonate-type ligand. Its reduction potential was less negative by 400 mV than that of bipyridyl. Whereas the chloroform solution of the dioctadecyl ester 2b did not show any luminescence, the aqueous suspension produced between 30 and 150% of the emission intensity of the symmetrical ruthenium tris(bipyridine) complex in water. Transmission electron microscopy at cryogenic temperature of the sonicated aqueous solution of 2b revealed multilayer spheres without an entrapped water volume. The solid, onion-type sphere is totally filled with interdigitated bilayers of 2b and has therefore the character of a rigid, spherical micelle, not that of a vesicle. The headgroup layer consists of a back-to-back ruthenium complex bilayer; their alkyl chains point inward and outward. The spherical micelles were isolated from water in solid form and were stable on mica and gold surfaces for many hours as was shown by atomic force microscopy. Incorporation of 2b into fluid host vesicles or micelles lead to complete loss of luminescence. Various redox active water- or membrane-soluble agents quenched 30−50% of the emission, which indicates some energy transfer over several layers.