Phthalocyanine, flash photolysis

For Co and Fe phthalocyanines, Grodowski and coworkers studied the electrochemical and photochemical reduction of CO2. The combined results of flash photolysis, chemical reduction, cyclic voltammetry and IR and UV-Vis spectroscopy allowed the authors to conclude that the cation radical [Co(I)Pc ] is the species responsible of the catalysis. However for Fe phthalocyanine the results are not very clear. Other contributions to the reduction mechanism with Co phthalocyanine have been report recently . The authors found a dependence of the substituents in the periphery of the ring with the mechanism of the reaction. The reaction studied by in situ potential-step chronoamperospec-troscopy (PSCAS) inferred the formation of different radical species . For Co-phthalocyanine and Co-octacyano-phthalocyanine the anion radical species is [Co(I)Pc(-3)X] (where X is the peripheric substituents of the phthalocyanine ring) 22, for Co-octabutoxy-phthalocyanine the responsible species is [Co(I)Pc (-3)X(H)] . The formation of the hydride intermediate is explained in terms of the electron donor properties of the butoxy substituents. This fact could affect the stability of the dianion species and for stabilization purposes the dianion formed needs atomic H before its coordinates to the CO2 molecule . ... 

For the determination of the triplet energy of an acceptor, a series of sensitisers with differing triplet energies and known transient difference spectrum are required the experimental approach is described in Chap. 15 which also gives triplet state properties for selected compounds (Table 15.2). The porphyrins (5.1, 5.2), phthalocyanines (5.3, 5.5) and naphthalocyanines (5.7) make a useful series of relatively low energy triplet sensitisers because of their structural similarity (5). Unfortunately only a few of these compounds are phosphorescent and therefore flash photolysis is required for direct kinetic studies of most triplet sensitisation. Triplet sensitisers can generally be placed in one of three categories ... 

The delay of the second flash was timed to prevent the photolysis of the (3E)M(pc)X and to avoid any significant decay of the radical via Equation 6.61. The 520 nm photolysis of the radical produced a reaction intermediate that, based on the kinetics and the concentration dependence of the phthalocyanine radical recovery, decayed via several parallel reactions (Equations 6.64—6.66). 

An ideal monochromatic source is the laser or uvaser. There is little or no published work on continuous photolysis using these sources although we must be on the threshold of this happening. Porter and Steinfeld have flash-photolysed phthalocyanine in the vapour phase using a Q-switched ruby laser. This emits at 6943A, and 2-3 J are dissipated in 20 psec. Photolysis occurs because of a two-photon process. 

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