A conjugated porphyrin dimer has been synthesized from a meso-diethynylporphyrin. The zinc complex of the dimer aggregates much more strongly (K(Agg) = 1.2 x 10(7) M-1) than its analogous monomer (K(Agg) 3.8 X 10(2) M-1) and forms a very stable 2:2 ladder complex with 1,4-diazabicyclo[2.2.2]octane (K(F) = 4 x -10(21) M-3), which is in slow exchange on the NMR time scale at 30-degrees-C. The effective molarities for aggregation and ladder formation are 80 and 0.3 M, respectively. NMR ring current shifts and UV exciton coupling show that the porphyrins are coplanar in the aggregate. The HOMO-LUMO gap for the dimer (1.7 eV) is 0.2 eV less than that of the monomer and the exciton splitting in the dimer is extraordinarily large (76 nm), -implying substantial electronic overlap between the chromophores.