Porphyrinic chromophore, excited states

Taking advantage of the fact that the triplet state of metalloporphyrins with tetrabenzopor-phyrin (H2TBP or H2I, fig. 10) lies around 12 500 cm-1, a team from Minsk succeeded in sensitizing Ybm luminescence in solutions of [Yb(TBP)L], where L = Cl or acac (acetyl-acetonate), in several solvents like benzene, dimethylformamide (dmf), pyridine, quinoline or a mixture of octane and benzene (Kachura et al., 1974). Energy transfer from the porphyrin chromophore was ascertained by the excitation spectrum of the Ybm luminescence being identical to the absorption spectrum of the complex. In benzene, the quantum yield of... 

Thus, it has been shown that the porphyrin chromophores act as an antenna system for transmitting its excited energy to the noncovalently associated fullerene moieties. Furthermore, the examination of excited-state characteristics revealed significant energy transfer properties of these complexes upon photoexcitation. 

This is in contrast to the results obtained following selective excitation of the PH2 unit discussed above, and yielding a multi-step electron transfer leading to charge separation. The different outcome can be discussed on the basis of the overlap of the HOMO and LUMO orbitals involved in the electron transfer reaction for the Ir acceptor unit and the PH2 donor unit, with the aid of semi-empirical calculations [48]. Remarkably, the zinc porphyrin based array PZn-Ir-PAu, 254+, displays an efficient electron transfer with the formation of a CS state with unitary yield also upon excitation of the iridium complex. This happens because the selective excitation of the zinc porphyrin chromophore discussed above, and the deactivation of the excited state PZn-3Ir- PAu, follow the same paths as those reported in Scheme 8. 

The interesting dual behaviour of porphyrin derivatives is demonstrated by intramolecular electron transfer between an electron-rich tris(4-methylphenyl)porphyrin zinc complex (ZnTPP) and an electron-deficient tris(4-methylphenyl)octafluoroporphyrin (TPOFP) (Teflon porphyrin) free base connected via an azobenzene moiety (438 Scheme 6.209).1250 Upon photolysis at >440 nm, E —> Z photoisomerization (Section 6.4.1) occurs. (Z)-438 can revert to ( )-438 only thermally, because the excited state of the Z-isomer is efficiently quenched by intramolecular electron transfer from ZnTPP to TPOFP, whereas no such interaction is possible in the -isomer because the chromophores are too remote. This approach has been suggested for photocontrolled molecular electronics. 

In this section, we briefly review the SAC-Cl applications to the excited states of porphyrin compounds EBPc [106] and bacterial photosynthetic reaction center of Rhodopseudomonas viridis [107]. Phthalocyanine is a famous chromophore that has a large absorbance in the visible region of the spectrum. By systematically comparing... 

Calculations have been carried out on the spectroscopy and excited-state energies and geometries of polyenes,84 the 1,5-cyclo-octadiyne radical cation,85 the visual chromophore, ll-c/s-retinal,8 polyene carbaldehydes,87 aromatic hydrocarbon anions and cations,88 trimethylenemethane,89 thiamine derivatives,80 porphyrins,91 lumiflavin,92 chlorophyll,93 nucleic acids,94 and ferrocene.95... 

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