Porphyrins containing triads

Self-assembled Monolayers of Porphyrin-containing Triads... 

Photoinduced intramolecular electron tunneling was observed also in some other porphyrin containing bridge molecules, such as porphyrin covalently linked to phenolphthalein [308], dimethylaniline — mesoporphyrin II — quinone triad [309], Zn porphyrin-viologen-quinone triad [310], carotenoid — porphyrin -diquinone tetrad [311]. The influence of conformational state of porphyrin-viologen bridge molecules on the rate of PET reactions was studied in Ref. [312]. 

Many other supermolecules containing carotenoids have been synthesized and their photochemistry characterized (Osuka et al., 1990 Moore et al 1994 Cardoso et al, 1996), but not yet investigated with EMR. One of these, a carotenoid- porphyrin-quinone triad has been used to demonstrate light-induced vectorial electron transfer across an artificial photosynthetic hposome membrane with subsequent production of ATP catalyzed by FoF -ATPase (Steinberg-Yfrachetal., 1998). 

Many compounds containing a donor and an acceptor joined by a flexible linker have been synthesized (see Ref. 185 for an extensive list of references). The difficulty in using such compounds to study the distance dependence of ET is that the flexibility of the tinker precludes knowing the donor-acceptor separation precisely. Distances have been estimated from fluorescence quenching volumes (61) and NMR conformational studies (60) with such compounds. NMR studies indicated that the methylene tinker between the porphyrin and the quinone in carotenoid-porphyrin-quinone triads is in an extended (all-anti) conformation (60). The calculated value of p, based on this conformation for the series of methylene linkers, was... 

Organometallic porphyrin complexes containing the late transition elements (from the nickel, copper, or zinc triads) are exceedingly few. In all of the known examples, either the porphyrin has been modified in some way or the metal is coordinated to fewer than four of the pyrrole nitrogens. For nickel, copper, and zinc the 4-2 oxidation state predominates, and the simple M"(Por) complexes are stable and resist oxidation or modification, thus on valence grounds alone it is easy to understand why there are few organometallic examples. The exceptions, which exist for nickel, palladium, and possibly zinc, are outlined below. Little evidence has been reported for stable organometallic porphyrin complexes of the other late transision elements. 

In artificial photosynthetic models, porphyrin building blocks are used as sensitisers and as electron donors while fullerenes are used as electron acceptors. Triads, tetrads, pentads and hexads containing porphyrins and Qo have been reported in the literature (see the Further Reading section). 

Figure 59 (Top) Steady-state emission spectra (Xexc = 450 nm) of adsorbed monolayers of Coum-PAH and Fl-PAH polycations, PdTAPP+, and the coadsorbed coumarin-flu-orescein-porphyrin triad. (Bottom) Emission spectra of a similar triad containing PdT-SPP4, with and without an added viologen electron acceptor layer. (From Ref. 43a. Copyright 1999 Elsevier Publications.)...

Novel triads that contained tetrathiafulvalenes as electron donors and porphyrin chromophores (refer to previous section for porphyrin-fullerene dyads) as donor units have been recently reported [246] (Scheme 12). Improved analogs of the latter triads were soon considered by connecting the tetrathiafulvalene unit to a 7r-extended conjugated network [247]. 

Also, a series of triads containing porphyrins as donors and various oligoth-iophenes as spacer units to C60 have been also considered and synthesized. These systems have allowed the study of long-range electron transfer and aided the evaluation of its distance dependence [257]. 

Table 2 Properties of CS states formed triads containing porphyrins in Rufterpy) and Ir(terpy) + complex-based...

A few other triads and more complex systems eonsisting of a porphyrin linked to two or more fullerene moieties have been reported [150, 159, 221]. The most complex to date is a tetraarylporphyrin linked to four fullerene moieties through double ester-containing bridges to the porphyrin meso aryl groups [151]. This molecule is rendered soluble in organie solvents via 16 twelve-carbon aliphatic chains. The porphyrin fluorescence is strongly quenehed by the attached Ceo moieties. 

Gust, Moore, Moore and coworkers covalent cartenoid-porphyrin-quinone molecular triads 55-60 contain a cyclized hydrogen bond within the quinone acceptor framework [143], The naphthaquinone moiety of 55 is fused to a norbomene system whose bridgehead position bears a carboxylic acid, which can hydrogen bond to an adjacent quinone. Photoinduced electron transfer from the porphyrin to the quinone leads to a marked p/fg increase of the latter, resulting in a fast proton transfer ( pt 10 s ) to form the semiquinone. Back electron transfer from the semiquinone is attenuated as a consequence of the proton-stabilized charge-separated species. This leads to a two-fold increase in the quantum yield of the charge-separated state of 55, as compared to those of the reference triads 56 and 57 (see Volume III, Part 2, Chapter 2). 

Halogen oxidation of planar complexes, particularly of the nickel triads that contain bidentate glyoximes or macrocyclic ligands, such as, phthalocyanines, substituted porphyrins, or tetraazaannulenes, leads to highly conducting substances that may possess chains of metal atoms as well as chains of polyhalides ... 

Many bridging ligands have been constructed that contain porphyrin subunits. Two examples are given to show the versatility of this structural motif. Porphyrins have been used for many years as models for the reactive site in the photosynthetic reaction center because of the resemblance to the natural components and the ability to vary the spectroscopic and redox properties of these chromophores by the use of bulky substituents and by their coordination to different transition metals.192 Molecular dyads of Znn/Irin and Auni/Irm and triads of Znn/Irni/Auin have been synthesized using the porphyrin bridging ligand (66).193-195... 

Artificial photosynthesis was initiated by flash excitation of liposomes containing the triad and Qs, the quinone shuttle, using 5-ns, 430-nm flashes centered on the Soret band ofthe porphyrin. The sequence of events taking place upon photoexcitation is shown in Fig. 25 (B) right, where step involves photoexcitation of the artificial reaction center, P, the porphyrin, followed by charge separation and electron transfer, forming Q -P-C, which can be detected by the absorbance inaease due to the radical cation C. The yield ofQ -P-C was 0.1 and its lifetime was -110 in the absence ofQs and -60 ns in liposomes containing Qs. 

More complex triad porphyrin compounds containing a carotenoid or chlorophyll-like substituent in addition to the quinone have also been synthesized. One example is given in Scheme 7.2.9. Light-induced charge separations are fast in such molecules (10 °-10" s ), but the recombination rates are almost equally fast (10 -10 ° s ). Neither carotenes nor chlorophylls help in the stabilization of charge-separated states. Laser flash photolysis of chloro-... 

The lifetime of the final charge-separated ion pair, C -P-Q" , is about 300 ns in a dichloromethane solution (and about 2 jus in a butyro-nitrile solution). This lifetime is substantially longer when compared to porphyrin-quinone ion pairs, P -Q (ca. 100 to 200ps). Other D-S-A assemblies containing porphyrins as chromophores are exemplified in structures 3 and 4. In these triad assemblies an aniline residue provides the electron donor component [35], System 3, for example, exhibits structural rigidity, and thus defined spatial orientation between the donor and acceptor components is established. The sequence of ET reactions occurring upon excitation of the porphyrin is summarized by... 

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