Scanning tunneling microscopy with chemically modified tips: discrimination of porphyrin centers based on metal coordination and hydrogen bond interactions.
STM gold tips chemically modified with 4-mercaptopyridine (4MP) were found capable of discriminating zinc(II) 5,15-bis(4-octadecyloxyphenyl)porphyrin (Por-Zn) from its metal-free porphyrin (Por-2H) and nickel(II) complexes (Por-Ni) in the mixed monolayers of these compounds, spontaneously formed at the solution/graphite interface. The porphyrin centers in STM images observed with 4MP-modified tips exhibited bright spots, while those measured with unmodified tips exhibited the porphyrin centers as dark depressions. The centers of Por-Zn were brighter than those of Por-2H and Por-Ni, thereby allowing the discrimination of Por-Zn from Por-2H or Por-Ni in mixed monolayers. The changes in the contrasts of porphyrin centers of Por-2H and Por-Zn/ Por-Ni were explained by facilitated electron tunneling due to hydrogen bond and metal coordination interactions, respectively, between porphyrin centers and the pyridyl group of 4MP on the tip.
Ohshiro
T
T
Department of Chemistry, School of Science, The University of Tokyo, Japan.
Ito
T
T
Bühlmann
P
P
Umezawa
Y
Y
D006860:Hydrogen Bonding; D008670:Metals / Q000737:chemistry*; D016252:Microscopy, Scanning Tunneling / Q000379:methods*; D011166:Porphyrins / Q000737:chemistry*
D008670:Metals; D011166:Porphyrins