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Intermolecular interactions in solution play an important role in molecular recognition, which lies at the heart of supramolecular and combinatorial chemistry. Diffusion NMR spectroscopy gives information over such interactions and has become the method of choice for simultaneously measuring diffusion coefficients of multicomponent systems. The diffusion coefficient reflects the effective size and shape of a molecular species. Applications of this technique include the estimation of association constants and mapping the intermolecular interactions in multicomponent systems as well as investigating aggregation, ion pairing, encapsulation, and the size and structure of labile systems. Diffusion NMR spectroscopy can also be used to virtually separate mixtures and screen for specific ligands of different receptors, and may assist in finding lead compounds.

The interactions of the tetra- cationic meso-tetrakis(N-methyl-4-pyri- dyl)porphyrin (H2TMPyP) and its met- allo derivatives (MTMPyP) (where M = copper(ii), zinc(ii), and gold(iii) with the octa-anionic form (at neutral pH) of 5,11,17,23-tetrasulfonato- 25,26,27,28-tetrakis(hydroxycarbonyl- methoxy)calix(4)arene (C4TsTc) lead to a series of complex species whose stoichiometry and porphyrin sequence can be easily tuned. Crystallographic, spectroscopic, and diffusion NMR stud- ies converge towards a common picture in which a central 1:4 porphyrin/calix- arene unit serves as a template for the formation of more complex species. These species arise by successive, step- wise addition of single porphyrin mole- cules above and below the plane of the 1:4 central core to ultimately give a 7:4 complex. Noticeably, the stoichiometry of the various complex species corre- sponds to the actual concentration ratio of porphyrins and calixarenes in solution allowing the stoichiometry of these species to be easily tuned. This behavior and the remarkable stability of these species allow homo-porphyrin and hetero-(metallo)porphyrin species to be formed with control of not only the stoichiometry but also the sequence of the porphyrin array. The flexibility and ease of this approach permit, in principle, the design and synthesis of porphyrin arrays for predetermined purposes. For example, we have shown that it is very easy to design and obtain mixed porphyrin species in which a foreseen photoinduced electron-trans- fer is indeed observed.

Nature self-assembles molecules into large supramolecular aggregates which are capable of performing specific biological functions [1]. Therefore it is not surprising that supramolecular chemistry has become an important theme of chemistry in recent years [2]. One of the main problem of supramolecular sciences is the elucidation of the structure of the supramolecular aggregate that prevails in solution. Consequently, new analytical methods that may assist these structural determinations are required, especially those techniques that may provide some information regarding the mutual interactions between the different molecular components of the supramolecular system. In this context it seems to us that the diffusion coefficient, as measured by the pulsed gradient spin echo (PGSE) NMR technique [3], may provide such information.