Stereoselective aggregation of chiral complexes with threefold-symmetric pendant carboxyl groups: An example of "perfect" self-assembly not seen in the crystalline state?

The self-assembly of tris(bidentate) chiral complexes, [M(4- carboxyphenyldipyrrinato)₃] (M = Co(iii), Rh(iii)), with a threefold-symmetric array of peripheral carboxyl groups is detailed. Thin sheet-like aggregates are observed in methanol/water mixtures of the racemic complex but not the enantiopure complex. Significant changes occur in the UV/vis spectrum upon aggregation and the intensity of fluorescence emission is enhanced. A model for the structure of the aggregate is proposed on the basis of the observed stereoselectivity of the aggregation process and the spectral changes. This model invokes the 'perfect' self-assembly of the building blocks which involves all three carboxyl groups in hydrogen bonding with neighbouring molecules. This differs from their crystal structures in which only two carboxyl groups participate in intermolecular hydrogen bonds.