Ohshiro, T.; Ito, T.; Buhlmann, P.; Umezawa, Y. Anal. Chem. 2001, 73, 1618-1631
Abstract
The metal center-free 5,15-bis(4-octadecyloxyphenyl)porphyrin (Por-H2) and its zinc(II) (Por-Zn) and nickel(II) complexes (Por-Ni) spontaneously form monolayers at the interface of highly oriented pyrolytic graphite (HOPG) and organic solutions of these porphyrins. Scanning tunneling microscopy (STM) gold tips chemically modified with 4-mercaptopyridine (4MP) were found capable of discriminating Por-Zn from the metal center-free porphyrin and the nickel(II) cmplexes. 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-H2 and Por-Ni rings, thereby allowing to discriminate Por-Zn from Por-H2 or Por-Ni in mixed monolayers. The brightness of porphyrin centers seems to result from facilitated electron tunneling due to hydrogen bond interactions for Por-H2 and due to coordination interaction for Por-Zn and Por-Ni.