Triplet state reduced porphyrins

The functionalization of zinc porphyrin complexes has been studied with respect to the variation in properties. The structure and photophysics of octafluorotetraphenylporphyrin zinc complexes were studied.762 Octabromoporphyrin zinc complexes have been synthesized and the effects on the 11 NMR and redox potential of 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraarylporphyrin were observed.763 The chiral nonplanar porphyrin zinc 3,7,8,12,13,17,18-heptabromo-2-(2-methoxyphenyl)-5,10,15,20-tetraphenylporphyrin was synthesized and characterized.764 X-ray structures for cation radical zinc 5,10,15,20-tetra(2,6-dichlorophenyl)porphyrin and the iodinated product that results from reaction with iodine and silver(I) have been reported.765 Molecular mechanics calculations, X-ray structures, and resonance Raman spectroscopy compared the distortion due to zinc and other metal incorporation into meso dialkyl-substituted porphyrins. Zinc disfavors ruffling over doming with the total amount of nonplanar distortion reduced relative to smaller metals.766 Resonance Raman spectroscopy has also been used to study the lowest-energy triplet state of zinc tetraphenylporphyrin.767... 

The intramolecular electron transfer kg, subsequent to the rapid reduction, must occur because the Ru(III)-Fe(II) pairing is the stable one. It is easily monitored using absorbance changes which occur with reduction at the Fe(III) heme center. Both laser-produced Ru(bpy)3 and radicals such as CO (from pulse radiolysis (Prob. 15)) are very effective one-electron reductants for this task (Sec. 3.5).In another approach," the Fe in a heme protein is replaced by Zn. The resultant Zn porphyrin (ZnP) can be electronically excited to a triplet state, ZnP which is relatively long-lived (x = 15 ms) and is a good reducing agent E° = —0.62 V). Its decay via the usual pathways (compare (1.32)) is accelerated by electron transfer to another metal (natural or artificial) site in the protein e. g.. 

Figure 3 Dlustration of photoinduced ET (PET) in a Zn-substituted Candida krusei cytochrome c (ZnP-protein) that was substituted at histidine 33 by a Ru(III) ammine complex. Photoexcitation of the cytochrome s Zn-porphyrin (ZnP) group produced the ZnP triplet state (ZnP3) which reduced the bound Ru(III) complex thorough protein-mediated ET. Subsequently the thermal back-ET within the Ru-substituted protein re-formed the starting ZnP-protein-Ru(III) complex.

Photophysical properties of Th(TPP)(acac)2 have also been examined and compared with those of the trimeric species [Th(TPP)(OH)2]3 (Tran-Thi et al. 1992). The fluorescence of the porphyrin (3t,jt ) state is severely quenched in both compounds (triplet state (Tp 208 ps). The triplet yield of the trimer (0.38), however, is drastically reduced with respect to that of the monomer (1.00) even though the former contains three thorium atoms. This has been attributed to the enhancement of the probability of deactivation of the singlet excited states via an internal conversion pathway. 

A novel face-to-face dimeric porphyrin system was reported recently by Lehn et al. [5,6] in which the two porphyrin rings are held apart by [18]-N204-aza-oxamacrocycles (1). Spectroscopic studies showed that the two porphyrin rings interact together via exciton coupling [7]. This effect serves to shorten the excited singlet state lifetime (Xg = 1.22 ns) and to reduce the fluorescence quantum yield with respect to the corresponding monomeric porphyrin. For 1 in acetonitrile, the quantum yield for formation of the porphyrin excited triplet state was only 0.18, compared to a value of 0.67... 

Immediately following excitation of 17 in DMF with a low intensity 30-ps laser pulse at 532 nm the observed transient differential absorption spectrum was considered to be characteristic of a gold(III) porphyrin triplet excited state [17]. The transient absorption decayed via biexponential kinetics with lifetimes of (170 20) ps and (2.0 0.4) ns, even at very low laser intensities. Absorption spectra recorded at different times during the decay process indicated that the triplet state was reduced to the gold(III) porphyrin neutral radical. With increasing laser intensity, the fractional contribution of the shorter-lived component increased and, whereas the longer lifetime... 

The decay curves of the triplet excited state of zinc porphyrin for TZnCCP and the [TZnCCP/cyt c] complex with reduced horse cytochrome c heme are exponential with the same decay rate. Upon addition of horse cyt c with the oxidized heme to the solution of ZnCCP, the TZnCCP decay remains exponential but the decay rate increases until a 1 1 ratio is reached and then remains constant. The form of the dependence between the rate and the concentration of cyt c indicates that ZnCCP and the cyt c form a strong 1 1 complex. These results indicate electron tunneling at the distance of 25 A to be the reason for the enhancement of TZnCCP decay in the presence of cyt c. The rate of electron transfer from TZnCCP to the low-spin ferriheme within the [ZnCCP/horse cyt c] complex was found to be 17 3 s 1 at 293 K [70]. 

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