Porphyrin flash photolysis

Ni(II) by strong oxidants, such as OH, Br and (SCN), produced by pulse radiolysis and flash photolysis. Rate constants are 10 M" s for oxidation by OH and Brf and = 10 M s for (SCN)f Ref. 259. The most popular means of production in both aqueous and nonaqueous solution is electrolytic, jjjg ligands which stabilize Ni(III) are cyanide, deprotonated peptides, amines and aminocarboxylates, a-diimines and tetraaza macrocycles, including porphyrins. Low spin d Ni(III) resembles low spin Co(II). The kinetics of the following types of reactions have been studied ... 

Kinetic measurements of the binding of NO to the porphyrin species were carried out in excess [NO] 3> [porphyrin], as a function of varying [NO], pH, temperature, and pressure. The first-order rate constants showed a linear dependence on [NO], allowing the determination of the kon and k,)tl values at pH 3 of 9.6 x 10 dm3/(mol s) and 51 s 1, respectively, in good agreement with values determined from earlier flash photolysis studies.327 Kinetics measurements over a range of pH permitted the... 

Lorkovic IM, Miranda K, Lee B, Bernard S, Schoonover J, Ford PC. Flash photolysis studies of the ruthenium(II) porphyrins Ru(P)(NO)(ONO). Multiple pathways involving reactions of intermediates with nitric oxide. J Am Chem Soc 1998 120 11674. 

Back Reaction Rate Constants and t /2 Values (Flash Photolysis Conditions) for Porphyrin 4 and Viologen Quenchers... 

The rates of ligand binding and ligand dissociation kon and off) can be determined by stopped-flow methods. For example, the kinetics of the binding and release of NO with an iron(III) porphyrin complex was stndied as a function of pH, temperature, and pressure by stopped-flow and laser flash photolysis techniqnes. The diaqua-ligated form of the porphyrin complex binds and releases NO according to a dissociative interchange mechanism based on the positive values of the activation parameters and for the on and off reactions. 

Photoinduced electron-transfer in the opposite direction was demonstrated upon irradiation of the Ru(bpy)3 +-Mb system in the presence of Co +(NH3)5Cl as a sacrificial electron acceptor (Figure 44B) [244]. The photochemical reaction results in the formation of ferryl species (i.e., Fe(IV)-heme), with the intermediate formation of the porphyrin cation radical (as demonstrated using laser flash photolysis [237]). The electron-transfer cascade includes the primary oxidative quenching of the excited chromophore, Ru(bpy)3"+, by Co +(NH3)5Cl to yield Ru(bpy)3 + [E° = +1.01 V vs. SCE). The resulting oxidant efficiently takes an electron from the porphyrin ring (fcet = 8.5 x 10 s ) and the porphyrin cation radical produced further oxidizes the central iron atom, converting it from the Fe(III) state to the Fe(IV) state (/cet = 4.0 x 10 s at pH 7.5). 

Small phosphatidylcholine vesicles (d = 25 nm) containing magnesium octa-ethylporphyrin (MgOEP) in the presence of the water-soluble electron acceptors ferricyanide or dimethylviologen produce cation radicals upon flash photolysis. When the electron acceptor is present on both sides of the vesicle bilayer, approximately triple the amount of porphyrin cations are produced than by the reaction with the electron acceptor on the outside. Furthermore decay of triplet absorption is scarcely effected when M is primarily on the outside of the vesicles, but occurs much more rapidly where MV is present on both sides. 

As expected from the extremely low fluorescence of fibres made of alkyl-substituted porphyrin amphiphiles, flash photolysis is ineffective. Nevertheless, the formation of porphyrin anion radicals was detected on a millisecond time scale and was traced back to a charge separation within the porphyrin fibre. ... 

Laser flash photolysis has been used to generate a hypervalent Mn =O species, which has an extremely high reactivity [569], related to that observed earlier in stopped-flow spectrophotometry experiments [32]. In contradiction to the Michaelis-Menten kinetics, it was found to react by second-order kinetics and to yield a TOF of 5.2 x 10 s , which is orders-of-magnitude higher than that obtained by standard methods with a Mn porphyrin [214]. It is concluded that the species obtained by flash photolysis is different from the intermediate involved in standard epoxidation with PhlO [214]. 

R. Zhang, M. Newcomb, Laser flash photolysis formation and direct kinetic studies of manganese (V)-oxo porphyrin intermediates, /. Am. Chem. Soc. 125 (2003) 12418. 

---