Photophysical properties of porphyrinic covalent cages endowed with different flexible linkers

Abstract

In-depth photophysical studies of four flexible covalent cages bearing either two free-base porphyrins or one free-base porphyrin and one Zn(II) porphyrin, connected by linkers of different lengths, are reported. In the case of the cages with two free-base porphyrins, exciton coupling between the porphyrins is evidenced by large and split Soret bands in the absorption spectra, but the different length of the linkers has only a slight effect on their emission properties. Strong electronic interactions between the porphyrins are also evidenced for the cages that incorporate a free-base porphyrin and a Zn(II) porphyrin, with a more pronounced splitting of the Soret band for the system with longer linkers. In these cages, following excitation of the Zn-porphyrin component, an almost quantitative energy transfer to the free-base unit occurs, with a rate 1.4 times faster in the cage with longer linkers (1.4 × 10[Formula: see text] s[Formula: see text] vs. 1.0 × 10[Formula: see text] s[Formula: see text]. This difference might reflect the more flattened conformation adopted by the cage equipped with longer and more flexible linkers, the latter allowing for a shorter interplanar distance between the porphyrins. The results are discussed in terms of classical and short-range energy transfer mechanisms.

Photophysical and Computational Insights into Ag(I) Complexation of Porphyrinic Covalent Cages Equipped with Triazoles-Incorporating Linkers

“…The emission titration, performed upon excitation at the isosbestic point, shows a progressive decrease of the fluorescence of the cage ( Figure 3 b), supporting the formation of metalated porphyrins, which are known to be weakly emissive with respect to their free-base counterparts. 42 The luminescence lifetime at the end of the first step of titration was found to be 8.4 ns, which is thus only slightly decreased with respect to the value of 9.0 ns measured for the original cage 29 and attributable to the residual fluorescence of the nonmetalated cages, likely interacting with silver(I) in their lateral groups.…”

“…We previously showed that the emission properties of the free-base component in Zn-S-2H are strongly affected by the presence of the closely spaced Zn-porphyrin counterpart, with the result of an almost halved quantum yield and a reduced lifetime (6.7 vs 8.5) with respect to the model monomer and cage 2H-S-2H , attributed to a change in the molecular symmetry or to an increased intersystem crossing rate induced by the close proximity of the Zn center. 29 The recovery of the free-base emission intensity observed upon the addition of Ag(I) testifies the separation of the two units, following the complexation of the Ag(I) ions with the lateral triazole groups ( Figure 6 ). Indeed, the lifetime of the free-base emission measured at 720 nm for the complex was found to be 7.9 ns, which is closer to the value of 8.5 ns for the free-base porphyrin unaffected by the Zn counterpart.…”

“…The data point toward a process of cage opening, similarly to what is occurring in the parent 2H-S-2H cage. The emission behavior is characterized by an increase in intensity of both the bands of the free-base unit (652 and 717 nm) 29 and of the Zn-porphyrin (606 nm, the second band at ca. 660 nm is superimposed with the free-base porphyrin emission) 29 ( Figure 5 b).…”

“…The emission behavior is characterized by an increase in intensity of both the bands of the free-base unit (652 and 717 nm) 29 and of the Zn-porphyrin (606 nm, the second band at ca. 660 nm is superimposed with the free-base porphyrin emission) 29 ( Figure 5 b). The increase of the Zn-porphyrin emission is more evident by subtracting the free-base contribution from each spectrum of the cage, calculated by normalizing the emission spectrum of 2H-S-2H at 712 nm ( Figure S5 ).…”

A flexible bis‐Co(III) porphyrin cage as a bimetallic catalyst for the conversion of CO2 and epoxides into cyclic carbonates

“…Different hexacoordinated Co(III)Cl porphyrins with pyridine and epoxide as axial ligands can be involved in this reaction [18f,22b,27] and the distance between the two metal centres can be adjusted due to the flexible linkers. [19,28] For the epoxide ring-opening step, a cooperative action of the two catalytic sites in which one axial chloride ligand promotes intramolecular ring opening of the epoxide bound to the other Co(III) site could account for the increased catalytic activity. [29] Noteworthy, whereas Co(III) porphyrins are not able to catalyse this reaction in absence of a co-catalyst, a yield of 13 % was obtained with the cage.…”