Porphyrin magnesium

Magnesium-hydroxyl interactions have been used in the same way with a p-hydroquinone unit attached at the meso position of a magnesium porphyrin (103). Again, the evidence for oligomerization comes from 1H NMR spectroscopy, and the results can equally be interpreted as dimerization rather than polymerization. 

Shinkai (65) exploited the boronic acid-diol motif mentioned above to self-assemble a polymer based on pyridine-magnesium porphyrin interactions. In this case, the evidence for polymerization comes from lightscattering experiments in dilute solution which yielded an average molecular weight of 109 g mol-1 for this system. 

Crystallographic results are presented for a chlorln, a bacterlopheophorblde d, a nickel isobacteriochlorin derived from chlorophyll a, and for the cation radical of a magnesium porphyrin. 

Cation Radical Formation with Exposure of Magnesium Porphyrin Thin Films to Light and Oxygen... 

Photochemical transformations invoiving magnesium porphyrins and phthalocyanines... 

The photochemistry of true organomagnesium compounds remains almost completely unexplored. A literature search in preparation of this work found only a few scattered examples of photochemical studies, mostly in relation to Grignard reactions and 1,3-diketonate chelates Similar to the situation with organozinc compounds magnesium tetrapyrrole chelates, i.e. magnesium porphyrins 1, 5,10,15,20-tetraazaporphyrins (por-phyrazines) 2 and phthalocyanines 3 have found more interest. This is primarily related... 

Many spectroscopic methods have been employed for the investigation of such systems For example, wide-band, time-resolved, pulsed photoacoustic spectroscopy was employed to study the electron transfer reaction between a triplet magnesium porphyrin and various quinones in polar and nonpolar solvents. Likewise, ultrafast time-resolved anisotropy experiments with [5-(l,4-benzoquinonyl)-10,15,20-triphenylpor-phyrinato]magnesium 16 showed that the photoinduced electron transfer process involving the locally-excited MgP Q state is solvent-independent, while the thermal charge recombination reaction is solvent-dependent . Recently, several examples of quinone-phtha-locyanine systems have also been reported . 

For example, in 1963 the photochemistry of magnesium phthalocyanine with coordinated uranium cations was studied in pyridine and ethanol and indicated the occurrence of PET to the uranium complex . A rapid photoinduced electron transfer (2-20 ps) followed by an ultrafast charge recombination was shown for various zinc and magnesium porphyrins linked to a platinum terpyridine acetylide complex . The results indicated the electronic interactions between the porphyrin subunit and the platinum complex, and underscored the potential of the linking para-phenylene bisacetylene bridge to mediate a rapid electron transfer over a long donor-acceptor distance. 

The low stability of the magnesium porphyrins has precluded most potential applications. Other metallotetrapyrroles have found industrial uses for oil desulfurization, as photoconducting agents in photocopiers, deodorants, germicides, optical computer disks, semiconductor devices, photovoltaic cells, optical and electrochemical sensing, and molecular electronic materials. A few scattered examples of the use of Mg porphyrins in nonlinear optical studies have appeared" and magnesium phthalocyanines have been used in a few studies as semiconductor or photovoltaic materials" " One of the few... 

The magnesium porphyrin radical, Mg(tetraphenylporphyrin)C104 (136), has been used as a model for the structural and stereochemical consequences of loss of an electron in photosynthetic chromophores.521 The primary photosynthetic reaction in plants and bacteria consists of a transfer of an electron, in the picosecond time domain, from the chlorophyll phototrap to nearby acceptors yielding chlorophyll -cation radicals. The structure of (136) shows a five-coordinated Mg2+ cation which is not quite symmetrically sited in the porphyrin ring but has metal-ligand distances similar to those found in the previous structures. The perchlorate anion is tightly bound (Mg—O = 2.01 A) in a monodentate mode. It was concluded that the porphyrin can act as an electron sink and that no major effects are found in the bond lengths, or on the stereochemistry, of the macrocycle. 

The possibility of photo-oxidizing zinc and magnesium porphyrins in vitreous matrices in the presence of the efficient acceptor of electrons C(N02)4 was first demonstrated by Kholmogorov and Bobrovskii [54]. The authors observed the formation of N02 free radical and the cation radical of metalloporphyrin, MP+, upon illumination of solutions in the MP absorption band. 

The first detailed research on the mechanism of electron phototransfer with the participation of MP in vitreous matrices appears to be ref. 55, which deals with the processes of charge separation in vitreous solutions of zinc and magnesium porphyrins in ethanol containing some addition of CC14 at 77 K. Illumination of the solutions in the Soret band or in long-wave bands of MP absorption results in a one-quantum ionization of MP and formation of MPf cation radicals and CC14 anion radicals recorded according to their characteristic EPR and optical spectra [56,57]... 

A detailed study has been made [59] on the spontaneous recombination of MP+ and CC14 particles formed upon illumination, at 77 K, of vitreous solutions of zinc and magnesium porphyrins containing CC14... 

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