Electronic donor-acceptor communication in porphyrin-based molecular electronics is addressed theoretically. The order of the a,, and a,, frontier occupied orbitals has been proposed by others to affect excited energy transfer. We examined the effects of the nature and pattern of substituents upon analog monomer porphyrin orbitals using DFT (B3LYP/6-31 G(d,p)) and the four-orbital model of porphyrin electronic structure. The excited state energy transfer rate was found to be very sensitive to these orbitals' contributions to porphyrin uv/visible absorptions (B- and Q-band). Ground state homodimer calculations validated the monomer results. (C) 2002 Published by Elsevier Science B.V.