Photoelectron and intervalence absorption spectra of a cofacially stacked mu-oxo silicon phthalocyanine dimer are interpreted as arising from one of two possible sinusoidal forms of the inter-ring electronic coupling J as a function of rotation about the mu-oxo linkage. Of these, only one corresponds qualitatively to the 'theoretical predictions of Pietro, Marks and Ratner, with J having extremum values of 0.32 eV (eclipsed) and -0.16 eV (staggered). The eclipsed interring coupling is found to be insensitive to the form of the individual molecular orbitals involved, and spectra for the corresponding trimer are successfully interpreted in terms of an exciton model. (C) 2003 Published by Elsevier B.V.