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

Abstract

The essential features of the experimental Soret bands of two covalent cages, consisting of two zincporphyrins 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-zincdistance 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.

Photophysical properties of porphyrinic covalent cages endowed with different flexible linkers

“…The Soret bands of the cages testify to inter-porphyrinic interactions, with a more pronounced exciton splitting in the cage with longer linkers, Zn-L-2H. The observed features are in accordance with a more slipped displacement of the porphyrins in Zn-L-2H with respect to a cofacial disposition favored in the cage with shorter and more rigid linkers, Zn-S-2H, a structural description provided by theoretical modelling performed on our bis Zn-porphyrin cages analogs [32]. As a confirmation, integrated molar absorption coefficients of 1.32 × 10 9 M -1 .…”

“…S14 for the spectrum of the model). These features can be ascribed to exciton coupling within the porphyrin units which are placed in close proximity in the cages, as previously observed and discussed for corresponding dimetallated Zn(II) cages [32]. The different lengths of the linkers slightly affect the absorption properties of the two cages.…”

“…The synthesis of the cages incorporating two free-base or two zinc(II) porphyrins connected with short linkers [30] and with long linkers [29] have previously been reported as well as the model compounds 2H-alkyne and Zn-alkyne [32] (Chart S1).…”

“…To better take into account the coupling interactions, the spectra of the monometallated cages have been compared with the sum of half the spectra of the parent cages containing two identical porphyrins, i.e. the bis free-base 2H-S-2H and 2H-L-2H studied here and the two dimetallated Zn(II) cages Zn-S-Zn and Zn-L-Zn previously reported [32]. The comparison for Zn-S-2H and Zn-L-2H with the sum of the respective free-base and Zn "half cages" is shown in Fig.…”

“…We have shown that bis-Zn(II) porphyrin cages equipped with four flexible linkers incorporating peripheral 1,2,3-triazole binding sites allow for a control of the distance and orientation between the two zinc(II) porphyrins. The cages adopt, in solution and in the solid state, a flattened conformation based on π-π interactions between the porphyrins, whereas a large conformational change occurs in solution upon binding four Ag(I) ions to the peripheral ligands, leading to open cages with two cofacial porphyrins separated by about 9 Å [29][30][31][32]. Based on their tunable cavity size, these cages were evaluated as switchable receptors and their capacity to behave as allosteric receptors towards different kinds of guest molecules (a coordinating or a π-acceptor molecule) was demonstrated [31].…”