Porphyrin-based dendrimers

An STM image of individual molecules was obtained on a copper-(lll)-sur-face and shows nanometer-sized flat mandala -type squares measuring 6.5 nm (sides) and 8.3 nm (diagonals). The nickel-nickel distance between two adjacent molecules is 13-18 nm. The molar absorption coefficient lies at around 

56 million, corresponding to the 13-fold value of the monomeric porphyrin complex itself [56]. 

Other porphyrin-based dendrimers will be found in Section 5.2. 

Ionic dendrimers [57] can bear positive or negative electric charges in the interior of the molecule (core, branching units) or on terminal (functional) groups. 

---

Figure 8.10 The porphyrin-based dendrimer described [56] by Frechet and co-workers that has potential applications as a photodynamic therapy agent.

Porphyrin-based self-assembled molecular squares 389 can form mesoporous thin films in which the edge of a square, thus the size of the cavity, can be adjusted by appropriate choice of substituents [8]. Fibers that form coil-coiled aggregates with distinct, tunable helicity are built from crown ethers bearing porphyrins 390 [9]. In addition to the porphyrin applications discussed in Sections 6.3.2.2 and 6.4, dendrimer metalloporphyrins 391 to be applied in catalysis [10] and the water-soluble dendritic iron porphyrin 319 modelling globular heme proteins [11] can be mentioned. 

In other work, Dandliker et al. have reported the inclusion of iron porphyrins within dendrimers to serve as functional mimics of redox-based proteins (Dandliker et al., 1994, 1995, 1997). These redox-switchable porphyrins show that the Fe3+/Fe2+ redox couple can be altered by the polarity of the surrounding environment. By changing the polarity imposed by the tightly packed branches of the dendritic core, the authors have illustrated that electrochemical behavior can be controlled by slight and subtle through-space environmental factors. These mimics may potentially model a wide variety of redox-driven enzymes and possibly provide mechanistic insights into their function. 

Onitsuka and Takahashi reported the synthesis and characterization of zinc porphyrin core dendrimers with platinum acetylide based dendrons... 

Other groups have also studied such macrocycles. For example, extending the molecular chain with a single carbon that connects the dendrimers with the porphyrins on the outer shell (Fig. 5.13) resulted in a significant improvement in the electron transfer process [67]. The authors reported that only 10% of the excitation intensity was needed for 64 Zn(II) porphyrin-based structure with a longer connecting chain to... 

Figure 47. Porphyrin-containing dendrimer (87) based on core 85 and porphyrin 45 as peripheral unit.

C-branched dendritic structures, based on either ether and amide linkages or simply amide connectivity (iii) Tomalia-type PAMAM dendritic structure, differing from the POPAM by the presence of amide bonds, but keeping the amine AB2 as the branching center and (iv) Fr6chet-type dendritic strnctures, based on ether and phenyl groups as AB2 branching center. Description of the different types of covalent dendrimers reported will follow a classification based on the element content, such as only C, C-N, N-O, C-N-0, or with heteroatoms such as P, Si, and so on. Examples of porphyrin-, thiophene-, carbohydrate-, and fullerene-based dendrimers are described separately, since they can be bound to different types of frameworks. 

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

Figure 14 Examples of protein surface mimics. A caiix[4]aiene (9) developed by Hamiiton et al a porphyrin-based proteodendrimer (10) prepared by the Tsukube group, showing both the first geuCTation (biue) and second generation (red) of the dendruner, R = second generation dendrimer and a tetrasubstituted porphyrin scaffold (11) used by the Hamilton group.

Sugiura K (2003) An Adventure in Macromolecular Chemistry Based on the Achievements of Dendrimer Science Molecular Design, Synthesis, and Some Basic Properties of Cyclic Porphyrin Oligomers to Create a Functional Nano-Sized Space. 228 65-85 Sun J-Q, Bartlett RJ (1999) Modern Correlation Theories for Extended, Periodic Systems. 203 121-145... 

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]. 

Alternatively, the quencher can be directly connected to the dendrimer structure. The photophysical behavior of generation 1,2 (compound 29), and 3 dendrimers containing a free-base porphyrin core and 12, 36, and 108 peripheral anthraquinone units, respectively, was studied in chloroform and dimethylac-... 

---