Self-assembled monolayers (SAMs) of porphyrins have been prepared to examine the substituent effects of porphyrin monolayers on the structure and photoelectrochemical properties of the SAMs on an ITO electrode. The ultraviolet (UV)-visible absorption, steady-state fluorescence, and cyclic voltammetry measurements for the porphyrin SAMs revealed that the interaction between the porphyrins without bulky tert-butyl groups is much larger than that of the porphyrins with bulky tert-butyl groups. Photoelectrochemical measurements were performed in nitrogen-saturated Na2SO4 aqueous solution containing triethanolamine as an electron sacrificer using the modified ITO working electrode, a platinum wire counter electrode, and an Ag/AgCl reference electrode. The internal quantum yields of photocurrent generation together with the porphyrin fluorescence lifetimes rernain virtually the same irrespective of the steric hindrance, which is in sharp contrast with severe self-quenching of the porphyrin excited singlet state in conventional molecular assemblies such as Langmuir-Blodgett films. The two-dimensional, densely packed structure of the porphyrins in SAM is responsible for the long-lived excited singlet state, which is similar to the antenna function of photosystem I in cyanobacteria. These results will provide valuable information on the construction of artificial light-harvesting systems.