Physical basis of self-assembly. Part 2. A theoretical and experimental study of the self-assembly of a zinc meso-pyridyl porphyrin

Abstract

A previously reported theoretical treatment for self-assembly macrocyclisations occurring under thermodynamic control has been tested experimentally. The fundamental quantities on which the treatment is based are the effective molarity (EM) of the self-assembling cyclic n-mer and the equilibrium constant for the intermolecular model reaction between monofunctional reactants (K_inter). Provided that estimates of EM and K_inter are available, this treatment can be used to predict not only whether the self-assembly process is more or less favoured, but also the distribution of all the species present in solution. Since K_inter values are approximately known from the literature, we have proposed a method, based on molecular modelling techniques, to estimate the EM. The method has been applied to the self-assembly of Zn(PyP₃P), where PyP₃P is 5-(4-pyridyl)-10,15,20-triphenylporphyrinato dianion. An EM greater than 0.1 mol L⁻¹ has been estimated for its cyclotetramerisation by PM3 calculations, suggesting that self-assembly should be favoured in solvents like toluene and chloroform. Self-assembly of Zn(PyP₃P) has been studied in these solvents by UV/visible spectroscopy. The data are consistent with the formation of the cyclotetramer, and at variance with the model of linear polymerisation. The experimental values of the EM were little affected by the nature of the solvent (EM values were 20 mol L⁻¹ in toluene and 15 mol L⁻¹ in chloroform), indicating that the solvent affects the process of self-assembly mainly through the value of K_inter.