A bisphthalocyanine lutetium derivative substituted with benzo-15-crown-5 subunits, [(15C5)(4)Pc](2)Lu (1), has been shown to form sandwich complexes with cations such as potassium ion, the complexes further form pillarlike aggregates. Light scattering studies have been carried out as a function of the ratio [K+]/[1] in order to determine the characteristics of the aggregates. It has been demonstrated that high molar mass aggregates ((M) over bar(w) about 10(6)) are formed even at small [K+]/I[1] ratios, they coexist with uncomplexed monomeric [(15C5)(4)Pc](2)Lu. The transformation from the metal-free monomer to the metal ion-containing aggregate involves a highly nonlinear two-stage complexation process. The shape of the aggregate is very probably rodlike (pillar-type structure) as indicated by the angle dependence of the light scattered.