Zn-porphyrin-dendrimer

Energy transfer in light-harvesting Zn porphyrin dendrimers... 

Examination of the cyclic voltammetry (CV) of the Zn-porphyrin dendrimers in THF and CH2C12 with Bu4N+PF6 (0.1 m) electrolyte revealed first oxidation potentials up to 300 mV (THF) less positive than the corresponding values obtained for the unshielded tetraester, Zn-porphyrin core. These preliminary electrochemical experiments suggested dendritic encapsulation of redox-active chromophores can effectively influence the electrophone environment controlled and well-conceived cascade architecture can lead to new avenues of selective redox catalyst design. 

Depending on the type of charges, polycationic PEG-P(Lys) and polyanionic PEG-P(Asp) are used for the preparation of such PIC micelles (Fig. 6). Based on recent research, core-shell type PIC micelles with a 52 nm diameter were prepared from a Zn-porphyrin-dendrimer with 32 carboxylate groups on the periphery, 32(-)DPZn, and PEG-P(Lys) block copolymers (56,57). On the contrary, when the dendrimer structure is cationic, a third-generation porph3rrin-dendrimer with 32 primary amine groups on the periphery, 32(+)DPZn, and PEG-P(Asp) were used to prepare a PIC micelle, and had a spherical structure with a 55 nm-sized diameter (58). Both... 

Figure 6 The PIC micelles loading photosensitizers for photodynamic therapy. PEG-P(Asp) (block polyanions) and PEG-P(Lys) (block polycations) are selected according to the surface charge of Zn-porphyrin-dendrimer s.

A poly(L-lysine) dendrimer 23 which carries 16 free-base porphyrins in one hemisphere and 16 Zn porphyrins in the other has been synthesized and studied in dimethylformamide solution [54]. In such a dendrimer, energy transfer from the Zn porphyrins to the free-base units can occur with 43% efficiency. When the 32 free base and zinc porphyrins were placed in a scrambled fashion, the efficiency of energy transfer was estimated to be 83% [55]. Very efficient (98%) energy transfer from Zn to free-base porphyrins was also observed in a rigid, snowflake-shaped structure in which three Zn porphyrin units alternate with three free-base porphyrin units [56]. 

A comparison between two families of dendrimers containing polyfaryl ether) dendrons and either a Zn porphyrin (GnPZn) or a tetraphenylporphyrin (G/JTPPH2) core up to the fourth generation (30 and 31) shows that the core... 

Porphyrin complexes are particularly suitable cores to construct dendrimers and to investigate how the behavior of an electroactive species is modified when surrounded by dendritic branches. In particular, dendritic porphyrins can be regarded as models for electron-transfer proteins like cytochrome c [42, 43]. Electrochemical investigation on Zn-porphyrins bearing polyether-amide branches has shown that the first reduction and oxidation processes are affected by the electron-rich microenvironment created by the dendritic branches [42]. Furthermore, for the third generation compound all the observed processes become irreversible. 

In the present study we investigated energy transfer between the Zn-porphyrin units in a sequence of dendrimers varying in size from 4 to 64 porphyrin units (Fig. 1). Reference measurements were performed on the monomer, P1D1. In order to follow energy transfer within the dendrimers, the fluorescence anisotropy decay were analysed. To determine the lifetime of the dendrimers, additional analysis of the kinetics measured at magic angle was performed. The fluorescence anisotropy is defined by... 

Figure 6-11. Fluorescence spectra of the dendrimer 62 (B) and an analogous low molecular weight Zn-porphyrin (A) (see text).

Representative examples of dendrimers with metal-containing cores include the materials 8.1-8.6, which were reported in the mid- to late 1990s. For example, first- (8.1), second- (8.2), third- (8.3), and fourth- (8.4) generation zinc porphyrin dendrimers (M = Zn) have been prepared by a convergent synthesis approach [18, 19]. Although the electrochemical and photophysical nature of the metalloporphyr-in core was found to be preserved in the resultant materials, the rate of interfacial electron transfer was found to be reduced due to the separation of the electroactive centers from the electrode surface. Second- (8.5) and third-generation (8.6)... 

In an attempt to create biomimetic systems, there has been much research on designing dendritic molecules with porphyrins and similar molecules at their cores. Diederich etal. have synthesized dendrimers with iron and zinc-porphyrin cores in an effort to model heme and cytochrome c systems [46-49]. In these cases, the redox reactions occurring at the center of the molecule were found to be affected by the nature of the dendritic foliage. The porphyrin-centered (the Zn(II) is not electroactive) first oxidation in the zinczinc-tetraphenylporphyrin to 4-0.65 V for the largest zinc-porphyrin dendrimer (compare to the results of Kaifer [35] above). However, for the iron porphyrin dendrimers, the Fe(II/III) redox couple shifts from —0.23 V versus SCE for the smaller dendrimer to 4-0.19 V for the larger one [48]. In a different set of experiments, Diederich and coworkers demonstrated that increasing the amount... 

Figure 4 (a) Organic dendrimers Fullerene-based dendrimer. (Reproduced -with permission from Ref. 44. Royal Society of Chemistry, 2009.) (b) Zn Porphyrin-based... 

The second-generation Pt dendrimers can be bonded to a Zn-porphyrin to produce a dendrimer with the metallaporphyrin in the core 856, and a large Pt dendrimer involving 1,3,5-trisubstituted benzene closely related to 856 has been reported 857, ... 

The effect of core shielding of a porphyrin moiety by peripheral dendrons has been carefully investigated on two series of Zn-phthalocyanine-cored dendrimers with aryl-ether branches [60]. Generation 0,1, and 2 (dendrimer 27) species, terminated with ester groups, are soluble in organic solvents, while the species terminated with carboxylate units (e.g., 28) are soluble in water. 

In the pentaporphyrin array 46, light absorption by the four peripheral Zn-por-phyrins is followed by efficient energy transfer to the central free base porphyrin [68], as it happens in the polypyridine dendrimers discussed above [53b]. 

Frechet and co-workers [32] studied the ability of the dendrimer shell to provide site isolation of the core porphyrin moiety, using benzene-terminated dendrimers Zn[G-n]4P (i.e. 6). From the cyclic voltammograms in CH2C12, the interfacial electron transfer rate between the porphyrin core and the electrode surface decreased with increasing dendrimer generation. However, small molecules like benzyl viologen could still penetrate the shell of 6 to access the porphyrin core as observed from the quenching of porphyrin fluorescence. Their results also revealed that the dendritic shell did not interfere electrochemically or photochemically with the porphyrin core moiety. 

Sakamoto M, KamachiT, Okura I, Ueno A, Mihara H. Photoinduced hydrogen evolution with peptide dendrimer-multi-Zn(II)-porphyrin, viologen and hydrogenase. Biopolymers 2001 59 103-9. 

Photoinduced electron transfer quenching of a porphyrin-cored dendrimer was first reported in compound 52, where the fluorescence of the Zn core is quenched in acetonitrile by vitamin K3 (2-methyl-1,4-naphtoquinone) [123]. 

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