Porphyrins triplet states

A rather important aspeet that should be eonsidered is that interfaeial quenching of dyes does not neeessarily imply an eleetron-transfer step. Indeed, many photoehemieal reactions involving anthracene oeeur via energy transfer rather than ET [128]. A way to discern between both kinds of meehanisms is via monitoring the accumulation of photoproducts at the interfaee. Eor instance, heterogeneous quenehing of water-soluble porphyrins by TCNQ at the water-toluene interfaee showed a elear accumulation of the radical TCNQ under illumination [129]. This system was also analyzed within the framework of the exeited-state diffusion model where time-resolved absorption of the porphyrin triplet state provided a quenehing rate eonstant of the order of 92M ems. ... 

Table 9.4 Solvent dependent driving forces for charge separation (CS) out of the porphyrin singlet excited state and charge recombination (CR) to the ground state/porphyrin triplet excited state calculated after the dielectric continuum model (dielectric constant e toluene 2.4 THF 7.6 oDCB 9.8, benzonitrile 24.9). The case, where charge recombination to the porphyrin triplet state is prohibited, is assigned as n.p. ...

Andres, G. O., Cabrerizo, F. M., Martinez Junza, V., Braslavsky, S. E., A Large Entropic Term Due to Water Rearrangement is Concomitant with the Photoproduction of Anionic Free base Porphyrin Triplet States in Aqueous Solutions, Photochem. Photobiol. 2007, 83, 503 510. 

Pulse radiolysis of porphyrins in benzene permitted the observation of porphyrin triplet states and provided a more accurate method than laser photolysis to determine their molar absorption coefficients. ... 

Immediately after excitation with a 30-ps laser pulse at 532 nm, the characteristic transient differential absorption spectrum of a gold(III) porphyrin triplet state was observed. The triplet decayed with a lifetime of ca. 28 ps to generate a longer-lived transient whose spectrum corresponded to that characterized as being an equimolar mixture of the gold(III) porphyrin neutral radical and the zinc porphyrin 7t-radical cation. 

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