Molecular rotation in self-assembled multidecker porphyrin complexes

An alkyl chain-substituted multidecker porphyrin (a cerium double-decker porphyrin (CeDDP) and a lanthanum triple-decker porphyrin (LaTDP)) complexes were arranged in a monolayer array on Au(111) substrate. By using a pulse injection deposition method, both multidecker complexes were deposited on the surface intact to form a well-defined two-dimensional array. Low-temperature scanning tunneling microscopy (STM) allowed the measurement of the topographic heights of the multidecker porphyrin complexes and visualization of their internal structures clearly. The STM images suggest that the top porphyrin ligand in CeDDP rarely rotates under nondestructive imaging condition, while the top porphyrin ligand in LaTDP exhibits flip-flop rotation even under the nondestructive imaging condition at sub-pA tunneling currents. These results provide the future applications of molecular-scale mechanical machines and single molecule storage memory.