In this report, we studied the electron transport through cyclic π-conjugated molecules. The model system consists of metalloporphyrin with two thiol groups at either 9,11-substitution (P-connection) or 1,5-substitution (D-connection) which form chemical bonds with gold electrodes. We investigated 10 typical bivalent metals as the metal-molecule-metal junctions using first principle density functional theory and nonequilibrium Green's function calculations. Due to the particular electron transport paths, all models in P-connection show similar I-V curves, indicating that the electron does not pass through the metal center in this configuration. In the D-connection, the electron takes the path through the metal center, leading to considerable difference in the I-V curves between the different metalloporphyrins. This means that the D-connected metalloporphyrin is potentially applicable in chemical sensor. We also studied a prototype for chemosensing the CO molecule theoretically at the same level.

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