Excited state porphyrins

The excited state porphyrins can be both donors and acceptors and form exciplexes [ Por D7] or [ Por t A7 ], which then relax to the ground state or undergo chemical reactions. 

Table 11 Redox Potentials2 for Some Ground and Excited State Porphyrins and Phthalocyanines118...

Excited-state porphyrins also are good oxidants. Photoreduction of uroporphyrin by EDTA and other electron donors [89] gives ESR spectra of the porphyrin radical anion, which has a low g value (2.002) and a broad ESR line (5 G) with no resolved hyperfine structure. The radical is transient and decays with second-order kinetics in aqueous solutions. Similar spectra have been reported from hematoporphyrin irradiated in the presence of thiols [88]. In addition, a thiol-derived radical was spin-trapped by MNP. The hematoporphyrin radical anion itself is not spin-trapped, but it will transfer an electron to MNP to form Bu NHO. With reducing agents other than thiols, radicals from the reducing agent (ascorbate, catechols, p-phenylenediamine) were detected directly [88]. 

Figrue BE 16.20 shows spectra of DQ m a solution of TXlOO, a neutral surfactant, as a function of delay time. The spectra are qualitatively similar to those obtained in ethanol solution. At early delay times, the polarization is largely TM while RPM increases at later delay times. The early TM indicates that the reaction involves ZnTPPS triplets while the A/E RPM at later delay times is produced by triplet excited-state electron transfer. Calculation of relaxation times from spectral data indicates that in this case the ZnTPPS porphyrin molecules are in the micelle, although some may also be in the hydrophobic mantle of the micelle. Furtlier,... 

Ravikanth M, Chandrashekar TK (1995) Nonplanar Porphyrins and Their Biological Relevance Ground and Excited State Dynamics. 82 105-188 Rawle SC, see Cooper SR (1990) 72 1-72 Raymond KN, see Baker EC (1976) 25 21-66... 

As described before, the rr-electrons of porphyrin are delocalized over the molecule and the energy levels of the HOMO and the LUMO are high and low, respectively. The resultant narrow intramolecular HOMO-LUMO gap causes absorption of the entire region of visible light. Usually, porphyrins are red to purple and phthalocyanines are blue to green. Furthermore, the long lifetime of their excited states is appHcable to the construction of photo-induced electron and/or energy transfer systems. 

FIG. 12 Simulation of fluorescent decays for dye species located in the aqueous phase following laser pulses in TIR from the water-DCE interface according to Eq. (38). A fast rate constant of excited state decay (10 s ) was assumed in (a). The results showed no difference between infinitely fast or slow kinetics of quenching. On the other hand, a much slower rate of decay can be observed for other sensitizers like Eu and porphyrin species. Under these conditions, heterogeneous quenching associated with the species Q can be readily observed as depicted in (b). (Reprinted with permission from Ref 127. Copyright 1997 American Chemical Society.)... 

Despite the large driving force involved in ET from Fc to the excited state of the porphyrin [compare potentials in Figs. 3(a) and 4], photocurrents are only observed at positive Galvani potential differences. Furthermore, photocurrent studies with various electron donors feature very similar potential dependencies [83]. 

As mentioned earlier, a great deal of literature has dealt with the properties of heterogeneous liquid systems such as microemulsions, micelles, vesicles, and lipid bilayers in photosynthetic processes [114,115,119]. At externally polarizable ITIES, the control on the Galvani potential difference offers an extra variable, which allows tuning reaction paths and rates. For instance, the rather high interfacial reactivity of photoexcited porphyrin species has proved to be able to promote processes such as the one shown in Fig. 3(b). The inhibition of back ET upon addition of hexacyanoferrate in the photoreaction of Fig. 17 is an example of a photosynthetic reaction at polarizable ITIES [87,166]. At Galvani potential differences close to 0 V, a direct redox reaction involving an equimolar ratio of the hexacyanoferrate couple and TCNQ features an uphill ET of approximately 0.10 eV (see Fig. 4). However, the excited state of the porphyrin heterodimer can readily inject an electron into TCNQ and subsequently receive an electron from ferrocyanide. For illumination at 543 nm (2.3 eV), the overall photoprocess corresponds to a 4% conversion efficiency. 

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