ZnTPP

Utilizing FT-EPR teclmiques, van Willigen and co-workers have studied the photoinduced electron transfer from zinc tetrakis(4-sulfonatophenyl)porphyrin (ZnTPPS) to duroquinone (DQ) to fonn ZnTPPS and DQ in different micellar solutions [34, 63]. Spin-correlated radical pairs [ZnTPPS. . . DQ ] are fomied initially, and the SCRP lifetime depends upon the solution enviromnent. The ZnTPPS is not observed due to its short T2 relaxation time, but the spectra of DQ allow for the detemiination of the location and stability of reactant and product species in the various micellar solutions. While DQ is always located within the micelle, tire... 

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,... 

FT-EPR spectra of the ZnTPPS/DQ system in a solution of cetyltriinethylaininonium chloride (CTAC), a cationic surfactant, are shown in figme BE 16.21. As in the TX100 solution, both donor and acceptor are associated with the micelles in the CTAC solution. The spectra of DQ at delays after the laser flash of less than 5 ps clearly show polarization from the SCRP mechanism. While SCRPs were too short-lived to be observed in TXlOO solution, they clearly have a long lifetime in this case. Van Willigen and co-workers... 

Figure Bl.16.20. FTEPR spectra of photogenerated DQ m TXlOO solution for delay times between laser excitation of ZnTPPS and microwave pulse ranging from 20 ns to 11 ps. The central hyperfme line (M= 0) is at s - 4.5 MHz. Reprinted from [63].

Becker et al. (1985 a) also investigated the photolytic dediazoniation of arenediazonium salts sensitized by zinc tetraphenylporphyrin (ZnTPP). Due to the very different lifetimes of singlet and triplet ZnTPP (1.8 ns and 1.4 ms, respectively) it is possible to differentiate between singlet and triplet electron transfer reactions using different diazonium salt concentrations. 

In spite of the potentialities of reversed micelles entrapping nonaqueous highly polar solvents [34], very few investigations on the solubilization in such systems are reported in the literature. An example is the study of the solubilization of zinc-tetraphenylporphyrin (ZnTPP) in ethylene glycol/AOT/hydrocarbon systems by steady-state and transient... 

FIG. 25 (a) Schematic representation for a photocatalytic mechanism based on shuttle photosensitizers at liquid-liquid interfaces. (Reprinted with permission from Ref. 182. Cop5right 1999 American Chemical Society.) (b) This mechanism is compared to the photo-oxidation of 1-octanol by the heterodimer ZnTPPS-ZnTMPyP in the presence of the redox mediator ZnTPP. (From Ref. 185.)... 

FIG. 12 First-order kinetic profiles observed from CLM measurements of the diprotonation of H2TTP (a) and the demetalation of ZnTPP (b). The initial concentrations of H2TPP and ZnTPP were 1.71 x 10 and 8.9 x 10 moldm, respectively. The concentration of hydrochloric acid was 2.0moldm (Reprinted from Ref. 61. Cop5right 1998, American Chemical Society.)... 

FIG. 13 Logarithmic plot of the observed demetalation rate constants of ZnTPP vs. acidity function of hydrochloric acid, at 298 K. The slope of the straight line is close to unity. The concentration of ZnTPP was 8.9 x 10 moldm, and the ionic strength was maintained at 2.0moldm . (Reprinted from Ref 61. Copyright 1998, American Chemical Society.)... 

For the photodiagnostic use of these compounds, a high quantum yield of fluorescence, r, is desirable. The metal complexes of the common first-row transition metals are not suitable, because they show very low 4>f values. On the other hand, porphyrin complexes of d° and d10 elements show appreciable fluorescence, although generally less than that of the metal-free compounds, presumably because of the heavy-atom effect (e.g., TPP ZnTPP, Table 5). The further operation of the heavy-atom effect, which increases the rate of intersystem crossing (/cisc) by... 

The ADF has been measured in deoxygenated solutions with continuous excitation of either argon, krypton or helium-neon lasers. The exciting wavelengths corresponded to the bands of Sq > 1 metalloporphyrin absorption (excitation wavelengths Xex = 514.5 530.9 568.2 nm for ZnTPP and Xex = 568.2 632.8 nm for ZnTBP and CdTBP). 

We have found that emission spectra of the cooperatively excited ADF from upper electronic states and of the one quantum excited PF are different. ADF spectrum is shifted in the longwave direction with respect to PF spectrum (see Table II). The shift effect depends on solvent type. ADF spectral shift is most considerable and comprises 500 cm"l (see Fig. 1) for the case of ZnTPP in decane. 

Some quantitative conclusions on TTA processes can be made after examination of the concrete case of ZnTPP - a porphyrin which is widely used in photochemical research. For ZnTPP in toluene ka = 3.109 1 sec mole-1 (19), P = 0.24 (6), kd = 1.1 ... 

In aqueous solution TPPS and ZnTPPS are present in the form of monomers and dimers. Finally, admixture of TPPS (ZnTPPS) and TTAP (ZnTTAP) leads to dimer formation. 

ZnTPPS/ZnTTAP] (17). It is found that dimerization is accompanied by (a) a blue shift of the Soret band, (b) red shifts of the Q bands, (c) red shift of the fluorescence maximum, and (d) strong quenching of fluorescence. Optical spectra of face-to-face covalently linked diporphyrins show the same trends (20,21). However, in one instance formation of a covalently-linked diporphyrin does not lead to a Soret band shift ( ). Of the systems considered here, [TPPS]2 and [ZnTPPSl2 also fail to show a Soret shift even though the visible absorption bands and fluorescence show clear evidence of dimer formation (10). A summary of observed dimerization induced... 

Figure 4 shows the triplet EPR spectra of ZnTCP and [ZnTCP]2 recorded at 10 E (12). The spectra reflect the absence of axial symmetry (D s 3E) which is attributed to a Jahn-Teller effect (24). [ZnTCPl2 formation has little or no effect on D whereas the value of E is strongly reduced (cf. Table IT). By contrast, triplet EPR spectra of [ZnTPPS/ZnTTAP] (13) and [ZnTPPS]2 (11) show no evidence of a dimerization effect on E. In the case of ZnTPPS dimerization leads to a reduction in D (11). As is illustrated in Figure S, the triplet EPR spectrum of [ZnTPPS/ZnTTAP] is virtually indistinguishable from the spectra of the monomer precursors. 

The triplet EPR spectra of the mixed dimers [ZnTPPS/TTAP] and [TPPS/ZnTTAP] also displayed in Figure 5 have the same characteristics as the free base dimer spectra (cf. Figures 3 and 5). A summary of the values of triplet parameters derived from the spectra is given in Table II. Included are literature values on face-to-face covalently-linked diporphyrins (8). 

Figure 5. Triplet EPR spectra of porphyrins in frozen 1 1 H20-glycerol recorded at 10 K. (a)Sol id line TTAP, dotted line TTAP/TPPSj (b)Sol id line ZnTTAP, dotted line ZnTTAP/ZnTPPS (c)solid line ZnTTAP/TPPS, dotted line ZnTPPS/TTAP.

Redox potentials for i-2 were determined in butyronitrile containing O.IM tetra-n-butylammonium perchlorate using a Pt disc electrode at 21. These potentials were measured relative to a saturated calomel electrode using ac voltammetry.(lQ) Both the one electron oxidations and reductions of i-2 exhibited good reversibility. The half-wave potentials for the one-electron oxidation and reduction of i-2, ZnTPP, and two model quinones are given in Table I. 

Fluorescence quantum yields for i-2 and ZnTPP in butyronitrile were determined using 10 M solutions in 1 cm cuvettes at the 90 geometry. 

The samples were excited at 549 nm and the resultant fluorescence emission spectra were digitized. The emission spectra of i-2 differed only in relative intensity. A direct comparison between the integrated and normalized fluorescence spectra of i-2 was made with that of ZnTPP. The data normalized to the known fluorescence quantum yield of ZnTPP(ii) are listed in Table II. Fluorescence lifetimes were determined on 10 M solutions of... 

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