Achiral diprotonated porphyrins, forming homoassociates in aqueous solution, lead to spontaneous chiral symmetry breaking. The unexpected result is that the chirality sign of these homoassociates can be selected by vortex motion during the aggregation process. This result is confirmed by means of circular dichroism spectra. These experimental findings are rationalized in terms of the asymmetric influence of macroscopic forces on bifurcation scenarios and by considering the specific binding characteristics of the porphyrin units to form the homoassociates.

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Homoassociates of the achiral title porphyrins in acid solutions show spontaneous symmetry breaking, which can be detected by circular dichroism (CD). The CD spectra are due to differential scattering and differential absorption contributions, the relative significance of which is related to the shape and size of the homoassociate. When an earlier model, designed for the association of these diprotonated porphyrins (J aggregates with the geometry of stepped sheets of intramolecularstabilised zwitterions), was applied to an exciton-coupling point-dipole approximation, the folding of the onedimensional homoassociates explained the CD signals detected. An effect of the vortex direction, caused by stirring or rotary evaporation, upon the exciton chirality sign was detected. In the case of H 2 TPPS 3 , the number of experiments performed gave a statistical significance to this effect. This vortex effect can be attributed to enhancement of the chirality fluctuations that originate in the diffusion-limited aggregation to highmolecular-weight homoassociates. In this sense, the phenomenon could be general for supramolecular systems that are obtained under kinetic control, and its detection would be possible when inherent chiral chromophores were being generated in the association process. H , B J and Q bands of all compounds and experiments. Example of the evolution of a metastable solution.

The application of the exciton coupling model to interacting porphyrin chromophores is discussed. Covalently bonded systems and ionic or electrostatically bonded homoassociates require different orientations of the transition dipole moments in order to explain the experimental results: according to the symmetry of the assembly for covalently bonded porphyrins, and assuming isolated chromophores for ionic bonded porphyrins. Further, for covalently bonded systems, an extended exciton coupling has been demonstrated, but the ionic systems are in agreement with non-extended couplings. The relation of these facts to a molecular description of solid-state Wannier-Mott or Frenkel excitons is briefly discussed.

The self-assembly of diprotonated phenyl and 4-sulfonatophenyl meso-tetrasubstituted porphyrins gives a spontaneous chiral symmetry breaking, but only for H 2 TPPS 3 2 , which forms helicoidal colloidal particles; the selection of the resulting chirality sign by the hydrodynamic forces of a stirring vortex can be demonstrated.