Porphyrin-Diones and Porphyrin-Tetraones:  Reversible Redox Units Being Localized within the Porphyrin Macrocycle and Their Effect on Tautomerism

Abstract

Porphyrin-2,3-diones and porphyrin-2,3,7,8- and porphyrin-2,3,12,13-tetraones were shown to have a redox-active unit that can function independently of the macrocycle at large. Electroreduction of 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin-2,3-diones [(P-dione)M] and the corresponding -2,3,12,13-tetraones [L-(P-tetraone)M] and -2,3,7,8-tetraones [C-(P-tetraone)M], where M = 2H, CuII, ZnII, NiII, and PdII was investigated and the products were characterized by ESR and thin-layer UV-visible spectroelectrochemistry. Electrochemical and spectroelectrochemical data show that the first two reductions of the porphyrin-diones and the first three reductions of the porphyrin-tetraones occur at the dione units. This was confirmed by ESR spectra of first reduction products which show that the electron spin is totally localized on a semidione unit, independent of the central metal ion and of the number and location of dione units. ESR spectra of the radical anions derived from free-base porphyrin-2,3-dione [(P-dione)2H] and porphyrin-2,3,12,13-tetraone [L-(P-tetraone)2H] confirm the trans-arrangement of the two inner protons and their location on nonsubstituted pyrrolic rings, thereby maintaining an 18-atom 18-pi electron bacteriochlorin-like aromatic delocalization pathway. The redox unit is not similarly isolated in the corner free-base porphyrin-2,3,7,8-tetraone [C-(P-tetraone)2H]. A one-electron reduction of C-(P-tetraone)2H leads to the formation of a tautomer with trans inner hydrogens with one residing on the N of the ring with the reduced unit as the only detectable product. This process is favorable because it creates a more delocalized 18-atom 18-pi electron aromatic pathway. This result is consistent with the measured redox potentials which show the first reduction of C-(P-tetraone)2H to be substantially easier than (P-dione)2H or L-(P-tetraone)2H.

A high-spin diradical dianion and its bridged chemically switchable single-molecule magnet

“…However, the molecular and electronic structures of the reduced species, especially the doubly reduced species, have never been fully elucidated. 18 The cyclic voltammograms reveal that 2,7- t Bu 2 -PTO can undergo four reversible one-electron reduction events with reduction potentials at −0.431 V, −0.710 V, −0.884 V and −1.556 V versus the Ag/Ag + electrode (Fig. S1 † ), which indicates that the reduced species may be isolable.…”

Employing the thiol-ene click reaction via metal-organic frameworks for integrated sonodynamic-starvation therapy as an oncology treatment

“…They demonstrate that the UV-vis absorption shifted from 421 nm of Gd-TCPP to 416 nm of Gd-TCPP-NC [53][54][55], and the fluorescence emission at 651 nm of Gd-TCPP shifted to 646 nm of Gd-TCPP-NC [53,56]. Both the blue-shifted UV-vis and fluorescence spectra were due to the F atoms in NC having a stronger electronegativity [54,55]. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images (Fig.…”

Indeno[2,1‐a]fluorene‐11,12‐dione Radical Anions: Synthesis, Characterization, and Properties

“…Indeno[2,1‐a]fluorene, which is a polycyclic aromatic hydrocarbon arising from a five or six rings alternately fused, is prized for an electron‐accepting and higher electron affinity with rigid and planar conjugated π‐systems [18] . Moreover, the carbonyl group (C=O), an organic functional group formed by the double bond connection between carbon and oxygen atoms, has identifiable redox activity and coordination sites [16] . Since 1970s, metal‐β‐diketonate radical complexes were found in solution using ESR spectroscopy [19] .…”

“…Subsequently, the effect of hydrogen bonding and substituents on electronic g ‐tensors of semiquinone radical was studied by Martin Kaupp and co‐workers using density functional calculations [13a] . The redox activity of porphyrin‐diones and porphyrin‐tetraones by electrochemical reductions were investigated by Shunichi Fukuzumi's research group in 2007 [16] . Little attention has been paid to physical property of reduced species, such as conductivity of carbonyl anionic radicals [17]…”

“…[13a] The redox activity of porphyrin-diones and porphyrin-tetraones by electrochemical reductions were investigated by Shunichi Fukuzumi's research group in 2007. [16] Little attention has been paid to physical property of reduced species, such as conductivity of carbonyl anionic radicals. [17] Indeno[2,1-a]fluorene, which is a polycyclic aromatic hydrocarbon arising from a five or six rings alternately fused, is prized for an electron-accepting and higher electron affinity with rigid and planar conjugated π-systems.…”