Phthalocyanines porphyrin dyads

Some representative examples of fullerene-porphyrin dyads are shown in Scheme 9. In other examples, porphyrin analogs such as phthalocyanines and subphthalocyanines have been used for the construction of efficient dyads. Again, the most straightforward approach for their synthesis involved 1,3-dipolar cycloaddition of the appropriate azomethine ylides to C60 [203-205]. Also, with the aid of the Bingel reaction, other phthalocyanine-fullerene systems have been prepared [206,207] with the most prominent example being the one that contains a flexible linker possessing an azacrown subunit [208]. The novelty of this dyad can be found in the nature of the linker that could, in principle, induce conformational changes in the multicomponent system when certain ions (e.g., alkaline ions) are present. As a direct consequence this would potentially allow an external control over the electronic interactions between the phthalocyanine and fullerene units. 

T. Kamei, T. Kato, E. Itoh, K Ohta, Discotic liquid crystals of transition metal complexes 47 synthesis of phthalocyanine-fullerene dyads showing spontaneous homeotropic alignment. J. Porphyrins Phthalocyanines 16, 1261-1275 (2012)... 

Ng et al. first reported the axial ligation of zinc(II) l,8,15,22-tetrakis(3-pentyloxy) phthalocyanine (1) with meso-pyridyl porphyrins 2 and 3 in chloroform, which form the corresponding edge-to-face dyad and pentad, respectively [25], As shown by UV-Vis spectroscopy, the ground-state tt-tt interactions between the perpendicularly disposed macrocycles in these arrays are insignificant. Upon mixing of phthalocyanine 1, zinc(II) meso-tetra(/Molyl)porphyrin, and 4,4/-bipyridine in chloroform, the formation of a face-to-face hetero-dyad was also inferred by fluorescence quenching experiments. 

We reported the first example of such a compound, porphyrin-fullerene dyad 23, in 1994 [134, 135]. Since that paper, a number of examples of dyads consisting of fullerenes linked to porphyrins or phthalocyanines have been prepared [136, 137-159]. Some of these, such as 24, use amide linkages related to those employed in some porphyrin-quinone systems [137, 149, 152]. In dyad 25, the moieties are linked by an extended, rigid bridge, of the type that has been shown to facilitate rapid long-range electron transfer in other systems [146, 147]. A number of dyads feature pyrrolidine-functionalized fullerenes, as in 26 and 27 [90, 139, 145, 148]. 

A number of reports on phthalocyanines and porphyrins have been published. Spectral diffusion and thermal recovery of spectral holes burnt into phthalocyanine doped Shpol skii systems has been examined . An absorption, emission, and thermal lensing research on carboxylated zinc phthalocyanine shows the influence of dimerization on these properties. Fourier transformation of fluorescence and phosphorescence spectra of porphine in rare gas matrices has yielded much structural and electronic state data on this compound . Exciton splitting is an effect which is seen in the spectra of covalently linked porphyrins . A ps fluorescence study of the semirigid zinc porphyrin-viologen dyad has provided evidence for two dyad conformers . Spectral diffusion in organic glasses has been measured by observing the hole recovery kinetics over the time scale of 1 to 500 ms for zinc tetrabenzoporphyrin in PMMA . 

As indicated above, in RC, chromophores are held in position via the axial ligation of histidine to the Mg ion of BChl and so on. The axial ligation to the central metal ion of macrocycles such as porphyrins and phthalocyanines is also useful in the preparation of the donor-acceptor dyad, triad, and so on, aiming at the artificial photosynthesis system. Several examples of some such systems are discussed below. 

Scheme 71 Stmctural formula of a phthalocyanine b porphyrin (M = Zn or 2H) used for the porph5uin-phthalocyanine dyad assembly on the surface of Sn02 nanoparticles. Reproduced from Ref. [161] with permission of The Royal Society of Chemistry...

Dyads can contain photoactive electron donors, such as porphyrins (P), phthalocyanines (Pc), sub-phthalocyanines (subPc), and perylene diimides (PDI). The advantage of these chromophores lies in strong absorption in the visible region in addition to minimal structural changes when electrons are released. Porphyrins, composed of four pyrrolic macrocycles connected via methine bridges. 

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