Journal of Porphyrins and Phthalocyanines

Cyclo[n]pyrroles (n = 6, 7, and 8), are expanded porphyrins that lack bridging meso carbon atoms. They adopt conformations that are near-planar and show features that are consistent with global aromaticity. In previous work, involving studies of cyclo[8]pyrroles, it was found that, in contrast to what is true for porphyrins, reductive per-N-alkylation led to the formation of non-planar products that displayed few signs of extended conjugation. These octa-N-alkylated species could be reoxidized electrochemically to produce dicationic, octa-N-substituted analogues of the starting aromatic cyclo[8]pyrroles. We have now extended this study to cyclo[6]pyrrole, and have found that reductive per-N-methylation can be readily effected using methyl iodide. However, in contrast to what is true in the case of cyclo[8]pyrroles, efforts to obtain the fully N-ethylated or N-benzylated products proved unsuccessful. Moreover, in the case of the hexa-N-methylated cyclo[6]-pyrrole product no viable chemical means of oxidizing it to the putative aromatic dication could be found. However, evidence for the formation of such species was obtained by electrochemical means. The differences between cyclo[6]- and cyclo[8]pyrrole highlight the effect of ring size in mediating the redox and reactivity chemistry within what is otherwise a closely matched pair of porphyrin analogues.

A fullerene derivative bearing two pyridine subunits has been designed to allow assembly with a bis-Zn(II)-phorphyrin receptor with two equivalent Zn binding sites separated by about 20 Å. Owing to the complementarity between the bis-Zn(II)-porphyrin receptor and a fullerene ligand bearing two pyridine substituents, the substrate can be clicked on the ditopic receptor, thus leading to a stable non-covalent macrocyclic 1:1 complex.

Electron transfer processes occurring through-space and through-bond were studied for novel mechanically linked triad [2]rotaxanes, which contain a porphyrin (MP) unit as a pendant and [60]fullerene (C₆₀) and triphenylamine (TPA) moieties as stopper groups at the axle ends (abbreviated as (MP;C₆₀-TPA)_rot+ with MP = H₂P or ZnP. The photophysical properties were investigated by means of time-resolved fluorescence and transient absorption measurements with changing solvent polarity. The charge separation took place mainly via(¹MP*;C₆₀-TPA)_rot+ and (MP;¹C₆₀*-TPA)_rot+ in polar solvents. Within the charge-separated states of triads (MP;C₆₀-TPA)_rot+, hole-shift and/or back electron transfer took place competitively. The lifetime of the charge separation state of (ZnP;C₆₀-TPA)_rot+ was similar to that of dyad (C₆₀-TPA)_rot+ (140 ns in dimethylformamide), whereas that of (H₂P;C₆₀-TPA)_rot+ was as long as 230 ns, suggesting two final charge separation states such as (MP;C₆₀^•--TPA^•+)_rot+ and (MP^•+;C₆₀^•--TPA)_rot+ depending on the kind of porphyrin.

Various pathways towards corroles bearing additional donor sites especially those with diazacrown units have been evaluated. Macrocyclic diamidation based on the reaction of diamines with diesters was found to be the key step in the overall synthetic scheme. This strategy was applied successfully to the synthesis of three compounds possessing macrocyclic diamide or diamine moieties. In addition, a meso-picolyl derivatized corrole was prepared. The new ligands were investigated with respect to the regioselectivity of transition metal insertion. Mononuclear corrolates of copper and cobalt are formed as the only isolable products, if simple copper and cobalt precursors and standard metalation protocols are applied. The results prove, that corroles can be used as selectively coordinating building blocks in multi-compartment ligand systems with peripheral crown ether and pyridyl functionalization.

The nitrogen of pyridylnaphthalenediimide (PyNIm) coordinates to the metal center of zinc tetraphenylporphyrin (ZnTPP) to form a donor-acceptor complex: ZnTPP-PyNIm. Formation of the ZnTPP-PyNIm complex was probed by UV-vis, fluorescence and NMR spectra. The fluorescence of ZnTPP is strongly quenched and the fluorescence lifetime is shortened significantly in the complex. The transient absorption spectrum of the charge-separated state (ZnTPP^•+-PyNIm^•-) is successfully detected by laser flash photolysis measurements of the ZnTPP-PyNIm system in benzonitrile. The charge-separated state of the complex produced by the photoinduced electron transfer has the longest lifetime, (450 μs) at 288 K, ever reported for donor-acceptor systems linked covalently or non-covalently in solution. However, when benzonitrile is replaced by benzene, the triplet excited state (³ZnTPP*), rather than the charge-separated state, is formed upon laser excitation of the ZnTPP-PyNIm complex, due to the lower energy of (³ZnTPP*) compared to the charge-separated state in benzene.

Zinc porphyrin-fullerene-zinc porphyrin triad, in which two zinc porphyrin (ZnP) moieties and a fullerene (C₆₀) moiety are linked by flexible bonds and which is intended to be a working model of the photosynthetic antenna-reaction centre, has been newly synthesized. Its photophysical properties have been investigated by both time-resolved emission and transient absorption techniques. Excitation of the zinc porphyrin moiety of the triad induced charge separation, generating the radical ion pair, in which the electron localizes on the C₆₀ moiety and the hole localizes on the zinc porphyrin moiety. In polar solvents, the charge-separated states decayed with lifetimes of 300-600 ns returning to the ground state. Compared with ZnP-C₆₀ dyad, ZnP-C₆₀-ZnP triad showed longer lifetimes for the radical ion pair due to the conformation of the two ZnP moieties. The effects of the coordinating reagents on the zinc atom have been studied, with the expectation of conformational change of the two ZnP moieties with respect to C₆₀.

A porphyrin dimer was prepared by coupling H₂MHTPP and 1,4-dichloro-2,5-dinitrobenzene in a one-step synthesis. Its structure was established by a MALDI-TOF mass spectrum and its electrochemical and photochemical properties were investigated. Surface photovoltage spectroscopy (SPS) revealed that the porphyrin dimer is a p-type semiconductor.

The in vitro and in vivo efficacy of Photofrin II as a selective radiosensitizing agent has recently been demonstrated, the mechanism of action still being unknown. We aimed to investigate the mechanism of action of Photofrin II and the role played by radiolytically generated cytotoxic species. RT4 human bladder carcinoma cell lines were seeded and incubated with Photofrin II and mannitol, with Photofrin II alone, and with no added agent. The cells were treated with ionizing radiation (6 Gy). The numbers of colonies in the respective groups were compared. In addition, the influence of mannitol and histidine on cell survival after Photofrin-incubation and irradiation (0-1-2-4-6 Gy) was evaluated by a tetrazolium-based colorimetric assay in an ELISA reader. RT4 cells, which were exposed to 6 Gy radiation after incubation with Photofrin II, showed a significantly lower survival rate compared to cells exposed to the same radiation dose in the absence of Photofrin II (p < 0.05 in a Student t-test). The addition of mannitol as a scavenger for OH radicals exerted a protective effect especially against the radiosensitizing action of Photofrin. Increased cell survival was observed when mannitol and histidine were present in addition to Photofrin II. The mechanism of radio sensitization by Photofrin II appears to involve the formation of radical derivatives of high reactivity, such as OH radicals.