Triplet excited state porphyrins

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,... 

Ge(TPP)R2, Ge(TPP)(Fc)Ph, and Ge(TPP)Fc2) the spectrum after 2 /us was consistent with a triplet excited state, although this decayed much faster for the fer-rocenyl complexes. Addition of ferrocene to Ge(TPP)R2 also quenches triplet lifetimes. A similar situation was observed for the indium complexes In(Por)R, and the triplet-state quenching was attributed to an energy transfer process from the excited-state triplet to ferrocene. In the case of the germanium porphyrins, the longer-lived triplet state in Ge(TPP)R2 is responsible for the Ge—C bond homolysis, and both inter- and intramolecular quenching by ferrocene is observed. 

In contrast to the case of ZnCh-Ceo, no transient formation of Ceo" was detected at 1000 nm for any other dyad in Scheme 4b [65]. In each case, only the triplet-triplet absorption due to the chlorin or porphyrin moiety was observed due to the higher energy of the radical ion pair as compared to the triplet excited state as is expected from the redox potentials. Thus, the energy level of the radical ion pair in reference to the triplet energy of a component is an important factor in determining the lifetime of the radical ion pair. 

The porphyrin skeletal mode frequencies are all slightly lower both for the triplet and for the ground state. Table II lists the triplet downshifts, l-7cm , along with the shifts observed for radical cations (13a) and anions (13b) for MgTPP and ZnTPP reported by Itoh and coworkers. Since the triplet excited states are formed by... 

The decay curves of the triplet excited state of zinc porphyrin for TZnCCP and the [TZnCCP/cyt c] complex with reduced horse cytochrome c heme are exponential with the same decay rate. Upon addition of horse cyt c with the oxidized heme to the solution of ZnCCP, the TZnCCP decay remains exponential but the decay rate increases until a 1 1 ratio is reached and then remains constant. The form of the dependence between the rate and the concentration of cyt c indicates that ZnCCP and the cyt c form a strong 1 1 complex. These results indicate electron tunneling at the distance of 25 A to be the reason for the enhancement of TZnCCP decay in the presence of cyt c. The rate of electron transfer from TZnCCP to the low-spin ferriheme within the [ZnCCP/horse cyt c] complex was found to be 17 3 s 1 at 293 K [70]. 

Electron phototransfer in zinc-substituted cytochrome c/cytochrome 65 complex [Zn(II)cyt c /Fe(III)cyt 65] has been studied [92]. Porphyrin rings lie parallel in this complex with the distances of ca. 8 A edge-to-edge, 18 A centre-to-centre. The Zn cyt c triplet excited state is quenched by Fe(III)cyt bn with the rate constant k = 5 x 105 s 1. Neither Fe(II)cyt b5 nor Zn(II)cyt quench TZn cyt c. 

In case of co-facial quinone-capped porphyrins (P and Q are linked by four tetraamidophenoxy bridges and are located at a distance of 10 A from each other), the quantum yield of charge separation is much bigger and reaches 30% for short distances between P and Q [53, 54]. Luminescence quenching via electron transfer from P to Q is observed for both singlet- and triplet-excited states of the porphyrin fragment of P-Q. The appearance of the additional channel for luminescence decay via electron transfer manifests itself in the biphase character of P-Q luminescence decay kinetics. 

Electron Transfer Reactions with the Participation of Porphyrins in a Triplet-Excited State... 

The phosphorescence decay kinetics of the triplet excited states of CuP molecules (Fig. 14) is adequately described by Eq. (16). Using this equation one can obtain the values of the parameter p = (Tra /2) In2 veT from the initial non-exponential part of the phosphorescence decay curves and the values of t = l/ k, i.e. the characteristic time of phosphorescence decay, from the final exponential part. Then the data on the dependence of the quantum yield of CuP phosphorescence on the concentration of C(N02)4 have been used to estimate the effective radii of electron tunneling from triplet excited copper porphyrins to C(N02)4 within the time x R, = (ac/2) In vet (Table 3). In doing so, the quenching of CuP luminescence by electron abstraction was assumed to be the only process leading to a decrease in the quantum yield of CuP phosphorescence in the presence of C(N02)4. From Table 3 an electron is seen to tunnel, within the lifetime of triplet excited states x at 10-4s, from CuP particles to C(N02)4 molecules over the distance R, 11 A. Further, the parameter vc and ae for different porphyrins were estimated from the values of (3, Rt, and x. These values are also cited in Table 3. 

Ru(bpy)3+ complex placed into the inner cavity of the vesicle was used as such antenna . The lifetime of the triplet-excited state of this complex ( 0.6 ps) is sufficiently long, so that before its deactivation it can experience numerous collisions with the inner surface of the vesicle membrane and thus with the porphyrin molecules embedded into the membrane. Indeed, it was found that the introduction of Ru(bpy)2 + into the inner volume of the vesicle leads to the sixfold increase of the rate of the transmembrane PET [58, 61]. This effect results, first, from the spectral sensitization due to the light absorption by the ruthenium complex in the spectral region where porphyrin does not absorb, and, second, from the two-three fold increase of transfer from 3Ru(bpy)i+ to ZnTPPin. 

The non-exponential decay of the triplet excited states of the photosensitizers is observed for Chi-, Phe- and ZnTPP-containing vesicles [132-135], The reason for the non-exponentiality may be, first, a statistical distribution of the concentrations of porphyrin molecules in the membrane, and, second, a simultaneous decay of the triplet excited states via several parallel channels such as spontaneous deactivation, concentration quenching and triplet-triplet annihilation which are known to be characteristic of porphyrins in organic solvents [129]. For ZnTPP and Phe in vesicles, the process of triplet-triplet annihilation is indeed observed [56, 134], while according to [132] this process is surprisingly absent for Chi. 

The fullerene triplet quantum yields in 8 72 were determined by the quantitative conversion of the triplet excited states of the porphyrin (0.78) and fullerene (i.e. 0.98) fragments into singlet oxygen. By analyzing the singlet oxygen emission at 1275 nm (see Fig. 8.22), we derived a quantum yield of 0.84. 

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. ...

Upon excitation of the metal complex centre, triplet energy transfer to the donor appended porphyrin rapidly quenches the excited state of the central ruthenium bis-terpyridyl unit, whereas excitation of the gold porphyrin, leads in less than 1 ps to the triplet-excited state [23], which is unreactive to-... 

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