Zinc porphyrins photoinduced electron transfer

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... 

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. 

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... 

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. 

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... 

The rates of photoinduced electron transfer between the zinc porphyrin and the gold(m) porphyrin are 1.7 ps for the Cu(i)-complexed [2]-rotaxane, and 36 ps for... 

Figure 11.11 Photoinduced electron transfer from zinc porphyrin to diimide in the presence ofSc(OTf)3), affording the diimide anion/Sc3+ complex.

Figure 2.5 Molecular structures of zinc-iron porphyrin complexes across (a) hydrogen, (b) aliphatic and (c) aromatic bridges, and the corresponding rates of photoinduced electron transfer for each species, as reported by Rege et al. [12]...

D Souza F, Ito O. Photoinduced electron transfer in supramolecular systems of fuller-enes functionalized with ligands capable of binding to zinc porphyrins and zinc phthalo-cyanines. Coord Chem Rev 2005 249 1410-22. 

There are several excellent photosensitizers one of them is [Ru(bpy)3]2+ [6]. There are two optical isomers in this complex one is A [Ru(bpy)3]2+ and the other is A-[Ru(bpy)3]2 +, as shown in Scheme 1. Thus one can expect to perform the stereoselective electron transfer reaction with A- and A-[Ru(bpy)3]2 +. Unfortunately, however, the racemization of [Ru(bpy)3]2+ is induced photochemically [7]. The reasonable way to suppress the photoracemiza-tion of this complex is to introduce the optically active organic functional group into the transition metal complexes, as will be discussed in Sec. II.B. The other photosensitizer that is useful for the photoinduced electron transfer reaction is the copper(I) complexes with 1,10-phenanthroline and their derivatives [8,9]. Zinc(II) porphyrin is also an excellent photosensitizer for photoinduced electron transfer reaction [10]. In these complexes, molecular chirality does not exist, unlike in [Ru(bpy)3]2 +. Thus one must introduce some chiral functional group into these compounds, to use these complexes as chiral photosensitizers. 

Condensing racemic l,2-methano[70]fullerene-71-carboxylic acid obtained by hydrolysis of its tert-butyl ester (( )-260, Scheme 1.25),243 with an amino derivative of zinc tetraphenylporphyrin, Imahori, Yamazaki, Sakata, and coworkers synthesized a C7o-containing dyad.355,446 Comparison with the C60-analogue showed that photoinduced electron transfer from the singlet excited Zn-porphyrin to the fullerene is faster in the C70 derivative. 

Kinetic studies have demonstrated that photo-induced electron transfer between the zinc and the gold porphyrin occurs at a rate of (1.7 ps)-1 in Cu(I)-complexed [2]-rotaxane 102, which is much higher than in the case of the free rotaxane 107 (36 ps)-1.73 The higher photoinduced electron transfer rate in the Cu(I) complex 102 than in the demetallated system 107 was explained also in terms of a superexchange mechanism. 

A. Harriman, Y. Kubo, J.L. Sessler, Molecular Recognition via Base-Pairing - Photoinduced Electron-Transfer in Hydrogen-Bonded Zinc Porphyrin Benzoquinone Conjugates , J. Am. Chem. Soc., 114, 388 (1992)... 

The simplest covalently linked system for studying photoinduced electron transfer consists of a porphyrin bonded to an electron acceptor or donor moiety with appropriate redox properties. Most of these studies have employed free-base, zinc, or magnesium porphyrins because the first excited singlet states of these molecules are relatively long-lived (typically 1-10 ns), so that electron transfer can compete with other decay pathways. In addition, they have relatively high fluorescence... 

In 1990, Osuka, Maruyama, Mataga, and coworkers examined porphyrin dyad 4 and other molecules closely related to dyad 3 with different aromatic linkers joining the macrocycles [19], When the zinc porphyrin of 4 was excited in dimethylforma-mide solution, Pzn-PFe(m) was produced. It decayed with a time constant of 52 ps, and the Pzn -PFe ii) state was observed by transient absorption techniques. The charge-shifted state decayed in 1.6 ns. By studying photoinduced electron transfer in the entire series of molecules with different linkages, the dependence of rate constant on the separation of the porphyrins was determined. A value for f in Eq. 2 of 0.4A- was obtained. 

The ionic separation may not be the only factor giving rise to the behavior of 19, as the photoinduced electron transfer rate constant does not change appreciably, even when the driving force is increased by 0.4 eV through introduction of zinc into the porphyrin macrocycle. In this connection, very rapid photoinduced electron transfer has been observed in other zinc-containing porphyrin-quinone systems [80, 86], and photoinduced electron transfer in zinc-containing porphyrin dyads has... 

An example of such a system is dyad 34, consisting of a zinc porphyrin (Pzn) linked to a viologen-like moiety, pyridylpyridinium (V+), which is also a reasonably good electron acceptor (-0.71 V versus SCE) [189]. Excitation of the porphyrin yields Pzn-V+, which decays by photoinduced electron transfer to give Pzn" -V ... 

A study by Sessler and coworkers of P-Pzn-Q triads with structures closely related to that of tetrad 53 (see below) has shown remarkably rapid photoinduced electron transfer from the distal free base porphyrin to the quinone. Singlet energy transfer from the free base to the proximal zinc porphyrin on such a time scale is deemed unlikely for thermodynamic reasons, since the first excited singlet state of the zinc... 

The quantum yield of (Pzp)3-Pzc-P -C6o is wavelength-dependent. Based on light absorbed by the free base porphyrin the yield is unity, due to the very large rate constant for photoinduced electron transfer, k. The quantum yield based on excitation of the zinc porphyrins in the antennas is 0.70. In this process, the lightgathering power of the system is increased tremendously at many wavelengths, as four zinc porphyrin moieties feed excitation energy to the reaction center. 

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