Interpretation of Experimental Soret Bands of Porphyrins in Flexible Covalent Cages and in Their Related Ag(I) Fixed Complexes

The essential features of the experimental Soret bands of two covalent cages, consisting of two zinc-porphyrins connected by four flexible spacers, 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). 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. Although 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 red-shifted (7-8 nm), broader, and slightly split Soret peak with respect to the cage with shorter linkers. 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. Despite the similar features of the spectra of the two silver(I)-complexed cages, a slight difference in the peak maxima of about 2 nm is observed, arising from a slightly shorter zinc-to-zinc distance in the cage with longer linkers. These results show that the MD-PMM methodology is a reliable method to obtain information on the relative disposition and exciton coupling interaction of porphyrins in flexible systems in solution, from the analysis of their absorption spectra.