Interpretation of Experimental Absorption Bands of Porphyrins in Flexible Covalent Cages and in their Related Ag(I) Complexes

Molecular cages incorporating two or more porphyrins have shown the ability to respond to light and to work as nanoreceptors or nanoreactors, depending on the specific geometrical arrangement of the porphyrins [1]. Here, a combined spectroscopic and computational approach is presented to study two covalent cages consisting of two zinc(II) tetraphenylporphyrins connected by four flexible spacers of different length and their corresponding silver(I)-complexed cages [2]. The essential features of the experimental Soret bands of the porphyrinic cages are for the first time interpreted and characterized at a molecular level by means of a mixed quantum/classical procedure based on molecular dynamics (MD) simulation and the perturbed matrix method (PMM) [3]. Despite the zinc-to-zinc distance is found to be similar in both cages, the MD-PMM calculations show that the conformation adopted by the cage with longer linkers corresponds to more slipped porphyrins, giving rise to a redshifted (7-8 nm), broader and slightly splitted Soret peak with respect to the cage with shorter linkers. The same method allows also for a comprehensive interpretation of the changes in the UV-visible absorbance of the cages upon silver(I) complexation to the peripheral binding sites. The process of silver(I) complexation separates the two porphyrins in a face-to-face conformation in both cages resulting in narrower (and more similar) Soret bands due to a reduced excitonic coupling. The very good agreement between experimental and theoretical results gives high credence to this method to get geometrical data of interacting porphyrins in complex architectures, simply from their absorption spectra.