Two kinds of self-assembled structures of 1-pyrenehexadecanoic acid at a solution-graphite interface were investigated using scanning tunneling microscopy. The pyrenyl groups of 1-pyrenehexadecanoic acid formed a pi-stacked assembly when it was co-adsorbed with 4,4'-dipyridyl although the pyrenyl groups remained isolated in the one-component self-assembled structure. On the basis of the Davidov splitting model, the shift of the pyrenyl group's electronic levels, upon formation of the pi-stacked assembly, was estimated from the observed structures. The energy shift was found to be consistent with the observed change in the electron-tunneling spectra between these two different structures. (c) 2005 Elsevier B.V. All rights reserved.