STM observation of alkyl-chain-assisted self-assembled monolayers of pyridine-coordinated porphyrin rhodium chlorides.

Alkyl-chain-assisted self-assembled monolayers of pyridine-coordinated porphyrin rhodium chlorides were observed at the solid-liquid interface by scanning tunneling microscopy (STM). The resolved images at a molecular level were obtainable in the pure solution of pyridine-coordinated porphyrin rhodium chloride with four triacontyl groups [Rh(C300PP)(Cl)(Py)]. In the case of pyridine-coordinated porphyrin rhodium chloride with four octadecyl groups [Rh(C18OPP)(Cl)(Py)], the STM images were not obtainable in the pure solution of Rh(C18OPP)(Cl)(Py) but obtainable in the mixture containing Rh(C18OPP)(Cl)(Py) and free porphyrin C18OPP. On the basis of the mixed self-assembled monolayer analysis, the apparent difference in the adsorption free energy between Rh(CnOPP)(Cl)(Py) and CnOPP (deltaGapp) was calculated. The calculated deltaGapp values for C18OPP and C30OPP mixed systems were quite different. The disadvantage of the adsorption free energy of Rh(C18OPP)(Cl)(Py) makes it difficult to obtain molecularly resolved images of Rh(C18OPP)(Cl)(Py), and the large adsorption energy due to the long alkyl chains enabled us to obtain molecularly resolved images of Rh(C30OPP)(Cl)(Py).