Monolayer assemblies of a sandwich-type double-decker porphyrin complex of cerium with an additional pendant porphyrin unit.

We have prepared a novel sandwich-type double-decker porphyrin complex with cerium, in which one of the porphyrins has long alkyl chains and the other has a pendant free-base porphyrin unit. The preparation of the complex was achieved with a Sonogashira coupling reaction between a preformed double-decker complex bearing an ethynyl moiety and an iodophenyl porphyrin. This complex forms moderately ordered self-assembled monolayers at the interface of 1-phenyloctane and highly oriented pyrolytic graphite. STM images with submolecular resolution revealed that the double-decker complexes form monolayers through adsorption of the alkyl-substituted porphyrin unit, presenting the other porphyrin having the additional porphyrin unit to the solution. All molecules are oriented in the same direction within single ordered domains. The double-decker complex and the corresponding free-base porphyrin form mixed monolayers due to their common adsorbing group and the matching unit cell parameters. The high spots for the double-decker cores are accompanied by lower spots, which can be assigned as the pendant free-base porphyrin units, revealing that the double-decker complexes predominantly take perpendicular orientation, in which the additional porphyrin unit orients perpendicular to the molecular row. Both the double-decker complex and the free-base porphyrin adsorb on the surface initially after the mixture was deposited, but the double-decker complex is gradually replaced by free-base porphyrin toward the thermodynamically stable assembly, in which the free-base porphyrin predominates. This behavior is unique to this particular complex among some analogous complexes with the same adsorbing group, which implies that groups not directly involved in adsorption may have profound effects on the adsorption and assembling properties of the molecule.

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.