Percec dendrons

Another series of related mesomorphic metallodendrimers, having the metal located at the core of the dendrimer, were also reported [285]. The dendritic ligands were obtained by condensation of the corresponding 1,3-dione with aminopropanol, and the dicopper complexes were then formed by the reaction with copper(ii) acetate (Fig. 60). These Ugands, also based on the first generation Frechet-Percec dendron motif, were not themselves mesomorphic, but upon complexation, mesophases were observed whose sta-... 

B.M. Rosen, C.J. Wilson, D.A. Wilson, M. Peterca, M.R. Imam, V. Percec, Dendron-mediated self-assembly, disassembly, and self-organization of complex systems. Chem. Rev. 109, 6275-6540 (2009)... 

Some of the more remarkable examples of this form of topologically controlled radical polymerization were reported by Percec et cii.231 234 Dendron maeromonomers were observed to self-assemble at a concentration above 0.20 mol/L in benzene to form spherical micellar aggregates where the polymerizable double bonds are concentrated inside. The polymerization of the aggregates initiated by AIBN showed some living characteristics. Diversities were narrow and molecular weights were dictated by the size of the aggregate. The shape of the resultant macroniolecules, as observed by atomic force microscopy (ATM), was found to depend on Xn. With A, <20, the polymer remained spherical. On the other hand, with X>20, the polymer became cylindrical.231,232... 

Fig. 28. Self-assembling dendrons exhibiting a thermotropic cubic liquid crystalline phase reported by Percec et al. 

As was the case for Percec s hybrid structures [25] (see above), studies of the G3-dendronized PPP by scanning force microscopy revealed a high degree of dimensional ordering in which the three-fold symmetry of the substrate surface is effectively recognized by the nanocylinders over several layers of polymer [32], More recently, Schluter extended his study to the preparation of a PPP hybrid... 

Recent seminal work reported by Percec et al. [45-47] has shown that either spheroidal or cylindrically shaped dendronized polymers may be obtained by the polymerization of a dendronized macromonomer . The degree of polymerization of these monomers determines the ultimate shape which appears to demonstrate the quasi-equivalence of dendritic coats . 

V. Percec, M. Glodde, M. Peterca, A. Rapp, I. Schnell, H.W. Spiess, T.K. Bera, Y. Miura, V.S.K. Balagurusamy, E. Aqad, P.A. Heiney, Self-assembly of semifluori-nated dendrons attached to electron-donor groups mediates their p-stacking via a helical pyramidal column, Chem. Eur. J. 12 (2006) 6298-6314. 

Dendritic molecules, forming predominately hexagonal columnar and micellar cubic phases, have been introduced and intensively investigated by Percec et al. and this field has recently been reviewed [143], The mesophases of these dendrons can also be stabilized and modified by replacing alkyl chains by fluorinated chains selected examples will be discussed in the next sections. 

Percec et al. described the self-assembly of Rp-substituted bis-dendritic benzamides, such as compounds 179a and 179b, combining a dendron decorated with perfluorinated chains and a dendron with alkyl chains. In the hexagonal columnar LC phases of these molecules the smaller alkyl chains form the centers of the columns, the aromatics are arranged in cylindrical shells surrounding the aliphatic columns, while the larger fluorinated chains form a fluorous continuum... 

Beginn developed Percec-type dendrimers, which are known to form supramolecu-lar channels, with polymerizable acrylate groups in order to obtain ion-permeable membranes [97-99]. First, the dendron 78 (Scheme 40) was dissolved in a polymerizable acrylate mixture that does not shrink on polymerization. The second step was the thermo-reversible gelation of the acrylate mixture, which was followed by the last step, polymerization to fix the supramolecular channel structure (Scheme 40). In the first experiments, compounds with only one polymerizable group were used but it turned out that the gelating properties were not sufficient [100, 101] so threefold modified 78 had to be developed. 

Hirose studied liquid crystal 105 consisting of a central dibenzo[18]crown-6 macrocycle and Percec-type side group dendrons (Scheme 53) [116]. 

Scheme 53 Dibenzo[18] crown-6 105 modified with Percec-type dendrons...

Such dendrons were derivatised at the focal point to create an energy gradient within a kind of molecular antenna (see Chapter 5 for more information) [25]. Percec et al. reported a building block library of eleven phenylacetylenes [27] substituted (dendronised, dendrylated) [26] with - self-assembling - dendrons. 

Self-organization of tapered and dendritic, i.e., fan- and cone-shaped dendron molecules in the solid was reported by Percec et al. in a series of papers [15J. Detailed diffraction experiments evidenced that spherical or cylindrical, (i.e. co-... 

Scheme 2. Shape-directed self-assembly of cone- and wedge-like dendrons to hexagonal-columnar and cubic superstructures according to Percec et al. [15a].

Figure 1. Examples of dendron monomers described by Percec et al., Schliiter et al. and XI et al.

For flexible synthetic polymers, the chain conformation is commonly controlled by the degree of polymerization (DP), with low-DP polymers having a rather extended chain conformation and high-DP polymers adopting random coil conformations in solution. Recently, Percec, Moller et al. reported extensive studies concerning the chain conformation and supramolecular structure of dendron-substituted polystyrene and polymethacrylate [23], Remarkably, at low DP... 

To expedite the synthesis of densely packed dendrons, Percec and co-workers110 have utilized the triply branched monomer 37, methyl 3,4,5-trihydroxy-benzoate.84-85 111-113 The materials were synthesized by coupling the phenolic groups of the monomer to 3 equiv of a benzylic chloride, followed by transformation of the focal ester functionality to a chloromethyl group, via LiAlFF reduction and SOCT chlorination. This procedure could be repeated up to the fourth generation. 

Electron donor-acceptor (EDA) stacks have been accessed by Percec et al. [90] using partially fluorinated dendrons (Fig. 14) to form LC columnar phases. The self-assembly of the dendrons is primarily driven by phase segregation of the fluorinated chains and the aromatic units. Functionalization of the dendrons at their apex with either an aromatic electroactive donor (29a) or an acceptor (29b) group yields LC columnar structures, further aided by the additional interaction between the aromatic units, which have optoelec-... 

Fig. 14 Examples of dendrons 29a and 29b studied by Percec et al. [90] which self-assemble into columnar structures with electroactive aromatic groups at the core...

Some of the more remarkable examples of this form of topologieally controlled radical polymerization were reported by Percec el Dendron... 

Another system which directly forms porous columnar stmcmres is described by Percec and co-workers [72]. They reported that dipeplide moieties can be used to self-assemble in cone-shaped dendrons to form nanometer-sized hollow columns. The self-assembly is based on the H-bonds between the dipeptides. The porous columns were used as ion channels, for instance for proton transpKMt. 

A radical copolymerization of the styryl monomer carrying Percec-type polyether dendrons (G1-G2), and the maleimide monomer carrying polyester dendrons (G1-G2), provided access to various styrene-co-maleimide dendronized polymers (Figure 37.5). The molar mass of the polymer, Gl-olt-Gl and G2-alt-G2 was found to be 1320 kDa (DPw = 1230, one repeat unit equals one pair of comonomers) and 708 kDa (DP = 280), respectively, by employing a GPC-TALLS... 

Percec and coworkers have shown that the oriented fibers of dendronized polyacetylenes can undergo thermally induced mechanical motion [38]. In this system (Figure 37.12), the dendrons around the polymer backbone become self-organized and thus facilitate a reversible ciscoid-to-transoid conformational isomerism of the polymer backbone. This isomerism is then converted into unidirectional macroscopic motion along the fiber axis. Remarkably, this mechanical motion could be used to displace objects with a mass of up to 250-fold that of the polymer fiber. 

Percec and coworkers have described the self-assembly of the fluorinated tapered dendrons, which can then lead to the formation of the supramolecular liquid crystals with interesting electronic and optoelectronic properties [13]. 

FIGURE 45.4. Illustration of the structure of semifluorinated dendron functionalized with different donors (D1, DP1) and acceptor groups (A1, AP1). Supramolecular columns with fluorenone stacks sandwiched in center are depicted. Reprinted by permission from Nature (Percec et al. 2002) Copyright 2002 Macmillan Publishers Ltd. 

Percec, V. Imam, M. R. Peterca, M. Wilson, D. A. Graf, R. Spiess, H. W. Balagu-rusamy, V. S. K. Heiney, P. A. Self-assembly of dendronized triphenylenes into helical pyramidal columns and chiral spheres. J. Am. Chem. Soc. 2009,131, 7662-7677. 

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