The amphiphilic nature of a [Ru(bpy)(3)](2+)-C-60 dyad, namely, a hydrophobic fulleropyrrolidine core covalently linked to a hydrophilic [Ru(bpy)(3)](2+) chromophore, through a trioxyethylene spacer, secures a controllable two-dimensional crystal growth of the dyad molecules at the air-water interface. The existence of a truly monolayered structure was confirmed by means of surface pressure (II) versus surface area (A) isotherms and as well as by Brewster angle microscopy. A limiting area of 117 Angstrom(2)/molecule for dyad 2 is in satisfactory agreement with that reported for pristine C-60 (93 Angstrom(2)/molecule). Brewster angle microscopy reveals, upon compression of the monolayer to the point of collapse, a phase transition. In particular, the dyad monolayer transforms into threadlike fibers, which align perpendicular to the compression direction. Two-dimensional AFM images suggest that the dyad fibers are composed of close-packed several hundred mu m long and 1.0 +/- 0.2 mu m wide clusters, which in turn consist of nanosized dyad clusters, with diameters of 100 +/- 20 nm.