Photoinduced electron transfer porphyrins

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... 

Levstein P R and van Willigen H 1991 Photoinduced electron transfer from porphyrins to quinones in micellar systems an FT-EPR study Chem. Phys. Lett. 187 415-22... 

Imahori H, Hagiwara K, Aoki M, Akiyama T, Taniguchi S, Okada T, Shirakawa M and Sakata Y 1996 Linkage and solvent dependence of photoinduced electron transfer in porphyrin-Cgg dyads J. Am. Chem. Soc. 118 11 771-82... 

Kuciauskas D, Lin S, Seely G R, Moore A L, Moore T A, Gust D, Drovetskaya T, Reed C A and Boyd P D W 1996 Energy and photoinduced electron transfer in porphyrin-fullerene dyads J. Phys. Chem. 100 15 926-32... 

Much effort has also been directed toward mimicking electron transfer on natural photosynthetic systems. Recently, the group of Harada has been able to prepare monoclonal antibodies against metallo porphyrins and show that the biological edifice can control photoinduced electron transfer from the porphyrin to organic acceptor molecules in solution. As it was important to design a biomolecule able to accommodate not only the metalloporphyrin unit but also organic substrates, Harada recently used a hexacoordinated phosphorus... 

Fukuzumi, S., Ohkubo, K., Wenbo, E., Ou, Z., Shao, J., Kadish, K.M., Hutchison, J.A., Ghiggino, K.P., Sintic, P.J. and Crossley, M.J. (2003) Metal-centered photoinduced electron transfer reduction of a gold(III) porphyrin cation linked with a zinc porphyrin to produce a long-lived charge-separated state in nonpolar solvents. Journal of the American Chemical Society, 125, 14984-14985. 

Recently, photochemical and photoelectrochemical properties of fullerene (Cto) have been widely studied [60]. Photoinduced electron-transfer reactions of donor-Qo linked molecules have also been reported [61-63]. In a series of donor-Cfio linked systems, some of the compounds show novel properties, which accelerate photoinduced charge separation and decelerate charge recombination [61, 62]. These properties have been explained by the remarkably small reorganization energy in their electron-transfer reactions. The porphyrin-Qo linked compounds, where the porphyrin moieties act as both donors and sensitizers, have been extensively studied [61, 62]. 

Fullerenes have shown particular promise as acceptors in molecular electronics, and numerous interesting TTF/Cgo ensembles have been reported.42 For example, Orduna and co-workers75,76 prepared the TTF/C60 dyad 13 and observed photoinduced electron-transfer from the TTF to the fullerene. Martin et al 1 observed two separate one-electron transfer events in their conjugated dyads 14 (where n = 2). The TTF-porphyrin-fullerene triad 15, prepared by Carbonera et al.7 showed long-lived photoinduced charge separation. 

Scherer, P. L. J. and Fischer, S. F. Theoretical analysis of the photoinduced electron transfer in porphyrin-quinone cyclophanes, Chem.Phys.Lett., 190(1992), 574-580... 

H. Imahori, K Hagiwara, M. Aoki, T. Akiyama, S. Tani-guchi, T. Okada, M. Shirakawa, Y. Sakata, Linkage and Solvent Dependence of Photoinduced Electron Transfer in Zinc Porphyrin-CHI Dyads ,/. Am. Chem. Soc 1996, 11811771-11782... 

K. A. Jolliffe, T. D. M. Bdl, K. P. Ghiggino, S. J. Langford, M. N. Paddon-Row, Efficient Photoinduced Electron Transfer in a Rigid U-Shaped Tetrad Bearing Terminal Porphyrin and Viologen Units , Angew. Chem, Int. Ed. 1998,37, 916-919. 

Esterification constitutes a valuable alternative to the amidation strategy. As with amidation, the formation of the ester bond is performed following a first reaction step with acyl chloride. The ester bond has been extensively utilized to attach many organic and inorganic moieties. Porphyrins are a classic example of substrates covalently bound via esterification strategies their photoinduced electron transfer to the nanotube has been studied for applications in molecular electronics and photovoltaic devices (Fig. 3.6) [21]. 

Fig. 3.6 Photoinduced electron transfer from porphyrins linked through ester bond to the carbon nanotube sidewalls. Adapted with permission from [21], 2005, American Chemical Society.

The photoinduced electron-transfer dynamics has also been examined for a series of porphyrin-fullerene-linked molecules with the same spacer employed for Fc-ZnP-H2P-C6o ZnP-Ceo (edge-to-edge distance Ree = 11-9 A), Fc-ZnP-Ceo (Ree = 30.3 A) and ZnP-H2P-Ceo (Ree = 30.3 A), shown in Chart 1 [53]. The driving force dependence of the electron-transfer rate constants ( et) of these dyad, triads, and tetrad molecules is shown in Fig. 3, where log et is plotted against the driving force (-AGet) [47]. 

Photoexcited porphyrins, quenching of low-lying excited states, 298-308 Photoinduced electron transfer description, 21-23 rate constants... 

Takai A, Chkounda M, Eggenspiller A et al (2010) Efficient photoinduced electron transfer in a porphyrin tripod-fullerene supramolecular complex via pi-pi interactions in nonpolar media. J Am Chem Soc 132 4477-4489... 

Many spectroscopic methods have been employed for the investigation of such systems For example, wide-band, time-resolved, pulsed photoacoustic spectroscopy was employed to study the electron transfer reaction between a triplet magnesium porphyrin and various quinones in polar and nonpolar solvents. Likewise, ultrafast time-resolved anisotropy experiments with [5-(l,4-benzoquinonyl)-10,15,20-triphenylpor-phyrinato]magnesium 16 showed that the photoinduced electron transfer process involving the locally-excited MgP Q state is solvent-independent, while the thermal charge recombination reaction is solvent-dependent . Recently, several examples of quinone-phtha-locyanine systems have also been reported . 

For example, in 1963 the photochemistry of magnesium phthalocyanine with coordinated uranium cations was studied in pyridine and ethanol and indicated the occurrence of PET to the uranium complex . A rapid photoinduced electron transfer (2-20 ps) followed by an ultrafast charge recombination was shown for various zinc and magnesium porphyrins linked to a platinum terpyridine acetylide complex . The results indicated the electronic interactions between the porphyrin subunit and the platinum complex, and underscored the potential of the linking para-phenylene bisacetylene bridge to mediate a rapid electron transfer over a long donor-acceptor distance. 

The TTF-porphyrin dyad 3 was described by the group of Odense.11 The fluorescence of 3 is significantly quenched by the photoinduced electron transfer process. Notably, the fluorescence intensity of dyad 3 increases largely after addition of Fe3 + that oxidizes TTF into TTF" +. Successive reduction of TTF" + is not reported. Nevertheless, it is anticipated that the fluorescence of dyad 3 can be reversibly modulated by redox reactions. In fact, the fluorescence of the supramolecule 4, formed between Zn-tetraphenylporphyrin and a pyridine-substituted TTF (TTF- ), can be reversibly tuned by sequential oxidation and reduction of the TTF moiety in 4.12 It should be noted in this context that the synthetically challenging system associating a porphyrin ring fused to four TTFs (5) was also reported.13... 

Photoinduced electron transfer rates can be considerably reduced when the counterion X- is changed from chloride to bromide. Charge transfer between the cationic part of a molecule and the bromide ion may be responsible to the reduction of photoinduced electron-transfer rates. Such a counterion effect on the photoinduced electron transfer and the reverse process has been demonstrated for examples of porphyrin-viologen-linked compounds (Mitsui et al. 1989). 

Investigations of the photoinduced electron transfer between fullerenes and porphyrines show that electron transfer occur from the porphyrin to 3C o/3C o-Additionally, due to the absorption of the porphyrines at the excitation wavelength, electron transfer from triplet-excited porphyrine to fullerene may occur. Both pathways lead to the radical ion pair, the ratio depends on the ratio of con-... 

Pioneering works by Harriman and Sauvage reported the zinc(II)-gold(III) bis(porphyrin)-type complexes with diimine linkers, where the photoinduced electron transfer process from the zinc(II) porphyrin excited state to the... 

A chromophore such as the quinone, ruthenium complex, C(,o. or viologen is covalently introduced at the terminal of the heme-propionate side chain(s) (94-97). For example, Hamachi et al. (98) appended Ru2+(bpy)3 (bpy = 2,2 -bipyridine) at one of the terminals of the heme-propionate (Fig. 26) and monitored the photoinduced electron transfer from the photoexcited ruthenium complex to the heme-iron in the protein. The reduction of the heme-iron was monitored by the formation of oxyferrous species under aerobic conditions, while the Ru(III) complex was reductively quenched by EDTA as a sacrificial reagent. In addition, when [Co(NH3)5Cl]2+ was added to the system instead of EDTA, the photoexcited ruthenium complex was oxidatively quenched by the cobalt complex, and then one electron is abstracted from the heme-iron(III) to reduce the ruthenium complex (99). As a result, the oxoferryl species was detected due to the deprotonation of the hydroxyiron(III)-porphyrin cation radical species. An extension of this work was the assembly of the Ru2+(bpy)3 complex with a catenane moiety including the cyclic bis(viologen)(100). In the supramolecular system, vectorial electron transfer was achieved with a long-lived charge separation species (f > 2 ms). 

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