Rotational libration of a double-decker porphyrin visualized.

Scanning tunneling microscopy has revealed the reorientation of one of the macrocyclic rings of the double-decker porphyrin complex [Ce(TPP-Fc)(C(22)OPP)] [TPP-Fc = 5-(4-(4-ferrocenylphenylethynyl)phenyl)-10,15,20-triphenylporphyrin; C(22)OPP = 5,10,15,20-tetrakis(4-docosyloxyphenyl)porphyrin] by 90 degrees between scans when the other ring is fixed on a surface. This libration was evidenced by monitoring the location of the appended ferrocene unit, which functioned as a molecular beacon signaling its position.