Abstract
The pivotal role played by potassium ions in the noncovalent synthesis of discrete porphyrin-calixarene nanostructures has been examined. The flattened-cone conformation adopted by the two cavities of octa-cationic calix[4]tube C4T was found to prevent the formation of complexes with well-defined stoichiometry between this novel water-soluble calixarene and the tetra-anionic phenylsulfonate porphyrin CuTPPS. Conversely, preorganization of C4T into a C4v-symmetrical scaffold, triggered by potassium ion encapsulation (C4T@K+), allowed us to carry out an efficient hierarchical self-assembly process leading to 2D and 3D nanostructures. The stepwise formation of discrete CuTPPS/C4T@K+ noncovalent assemblies, containing up to 33 molecular elements, was conveniently monitored by UV/vis spectroscopy by following the absorbance of the porphyrin Soret band.
Highlights
The pivotal role played by potassium ions in the noncovalent synthesis of discrete porphyrin-calixarene nanostructures has been examined
The flattened-cone conformation adopted by the two cavities of octa-cationic calix[4]tube C4T was found to prevent the formation of complexes with well-defined stoichiometry between this novel water-soluble calixarene and the tetra-anionic phenylsulfonate porphyrin copper(II) mesotetrakis-(4-sulfonatophenyl)porphyrin tetrasodium salt (CuTPPS)
We have shown that both anionic [51] and cationic [52] calix[4]arenes quantitatively interact with oppositely charged porphyrins, under rigid hierarchical rules, providing assemblies with predictable sequence and stoichiometry
Summary
Porphyrins, owing to their redox [1,2] and opto-electronic properties [3,4,5], relative ease of derivatization [6] and propensity to self-organize in architectures of different size and topology [7,8,9], are very attractive building blocks for the synthesis of functional nanomaterials useful for light harvesting [10,11], sensing [12], catalysis [13], imaging [14] and photodynamic therapy [15] applications. The rational design of porphyrin-based supramolecular assemblies has been successfully carried out in the presence of templating agents such as: polyelectrolites [27,28], peptides [29], inorganic molecules bearing metal-coordination centers [30,31,32,33] and macrocyclic compounds [34,35,36], by taking advantage of single or multiple metal coordination, hydrogen bonding, π–π stacking, electrostatic and hydrophobic interactions. The self-assembly in aqueous solution is mainly driven by electrostatic interactions between differently charged components as well as solvophobic effects and other noncovalent weak forces, all of which contribute to the thermodynamic stability of the species formed. The stability and kinetic inertness of these multicomponent assemblies have been assessed by light scattering, diffusion NMR studies [56] and a number of single-crystal XRD analyses [51,56]
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