Dimethoxybenzene units

The first pillar[5]arene (2.1), which consists of five dimethoxybenzene units, was discovered by chance. The discovery of this molecule was an important starting point for pillar[ ]arene chemistry. From 2006, we investigated the reaction of l,4-dimetho>ybenzene with paraformaldehyde to synthesize new phenolic resins. 

Figure 6.8 Pillar[5]arenes (a) with four 1,4-diethoxybenzene units and one benzoquinone unit (6.10), (b) four 1,4-dimethoxybenzene units and one benzoquinone unit (6.11), (c) four 1,4-dimethoxybenzene units and one hydroquinone unit (6.12) and (d) four 1,4-dimethoxybenzene units and one rigid conjugated unit (6.13).

Dendrimer 1 + is a classical example of a dendrimer containing a luminescent metal complex core. In this dendrimer the 2,2 -bipyridine (bpy) ligands of the [Ru(bpy)3] +-type core carry branches containing 1,2-dimethoxybenzene- and 2-naphthyl-type chromophoric units [15]. 

Dendrimers 1 and 2 consist of a cyclam core appended with 4 dimethoxybenzene and 8 naphthyl units, and 12 dimethoxybenzene and 16 naphthyl units, respectively [29]. 

In the same way, the displacement of the unpaired electron over the whole molecules was observed for cation-radicals from Scheme 1.4d and 1.4e, in which 1,4-dimethoxynaphthalene units are syn- or anti-annealed to [2.2]paracyclophane (Wartini et al. 1998a, 1998b). In another study, the electron transfer between 1,4-dimethoxybenzene and 7,7-dicyanobenzoquinone methide moieties... 

A stable adduct of Cjq with an electron donor moiety in the side chain (Table 4.1, compound 7) [24] was synthesized by the cycloaddition of a rigid diene containing a polynorbornyl-bicyclo[2.2.0]hexyl bridge and a dimethoxybenzene (DMB) unit in toluene at reflux in 63% yield. The donor-acceptor interaction of the electron rich DMB unit and Cjq is indicated by the X-ray crystal structure. The adducts are paired... 

The branched rotaxanes 66+, 76+, and 86+, besides the bipyridinium units of the triply branched backbone 56+, contain macrocycle 2 whose two DMB units are oxidized at distinct potentials the first oxidation process practically coincides with that of the model compound /i-dimethoxybenzene, whereas the second one is displaced to a slightly more positive potential. 

The dendrimer of mixed POPAM/Frechet type shown in Fig. 5.5 formally bears several types of fluorophores in three shells naphthyl units on the periphery two layers with dimethoxybenzene character further inside and finally the strongly fluorescing dansyl groups - adding up to a total of 64 luminophoric moieties. Incident UV radiation interacts first with the naphthyl groups, which... 

Disregarding this aspect, and since cyclam is an interesting core for constructing den-drimers because it can be easily functionalized and because despite its absence of spectroscopic properties, it can interact in such a way with dendrons as to modify their photophysical properties, the interaction of lanthanide ions with cy clam-based dendrimers has been investigated. The dendrimers are constmcted from the cyclam core fitted with four dimethoxyben-zene and eight naphthyl units (generation 1, fig. 81) second generation introduces a total of 12 dimethoxybenzene and 16 naphthyl moieties. Coordination to Lnm ions occurs in acetoni-trile/methylene chloride (Ln = Nd, Eu, Gd, Tb, and Dy), but no sensitized Ln-luminescence was observed (Saudan et al., 2004). Another example of a macrocycle-based dendrimer is discussed below in section 3.3.2. 

The synthesis of a typical compound containing two units of dihydrobenzoin is shown as an example in Scheme 7 [26]. Reaction of 2,6-bis(bromomethyl)l,4-dimethoxybenzene 41 with the dibutyltin derivative of optically active dihydrobenzoin 59 yields a chiral podand 60 in 63%. Cyclization of 60 with the ditosylates of oligoethylene glycols, followed by oxidation with CAN and treatment with dinitrophenylhydrazine, affords the desired chiral dyed acerand 53. Some chiral azophenol acerands 51-58 are synthesized in a similar manner. 

Fig. 12 Structural formula of dendrimer 10 (top) and energy transfer occurring from the peripheral dimethoxybenzene and naphthalene units to the light-emitting dansyl unit (bottom) [63]...

As already pointed out in the case of rotaxanes, mechanical movements can also be induced in catenanes by chemical, electrochemical, and photochemical stimulation. Catenanes 164+ and 174+ (Fig. 19) are examples of systems in which the conformational motion can be controlled electrochemically [82, 83], They are made of a symmetric electron acceptor, tetracationic cyclophane, and a desymmetrized ring comprising two different electron donor units, namely a tetrathiafulvalene (TTF) and a dimethoxybenzene (DOB) (I64 1) or a dimethoxynaphthalene (DON) (174+) unit. Because the TTF moiety is a better electron donor than the dioxyarene units, as witnessed by the potentials values for their oxidation, the thermodynamically stable conformation of these catenanes is that in which the symmetric cyclophane encircles the TTF unit of the desymmetrized macrocycle (Fig. 19a, state 0). 

It should be noted that the 1,3-dimethoxybenzene and 2-naphthyl chromophoric units contained in the branches of the dendrimer are not involved in metal coordination. In some way, they belong to a second coordination sphere. If is considered a large metal complex, the absorption and emission bands of the 1,3-dimethoxybenzene and 2-naphthyl chromophoric units can formally be classified as LC. However, can be more properly viewed as a supramolecular (multicomponent) species (12). In such species, each chromophoric imit displays its own absorption spectrum since there is no appreciable interactions among them in the ground state, but in the excited state even weak interactions can cause intercomponent energy or electron-transfer processes. This kind of reasoning can also be applied to all the other systems discussed in this chapter. 

The lowest excited state of dendrimer is an MLCT level of the [RuCbpyls] rmit because of the presence of the heavy metal atom, this level is populated with rmitary efficiency from the upper l5ung MLCT excited state. All the three types of chromophoric groups present in the dendrimer, namely, [RuCbpyls], dimethoxybenzene, and naphthalene, are potentially Imnines-cent species. In dendrimer 1, however, the UV fluorescence of the dimethoxybenzene- and naphthyl-type units is almost... 

Dendrimer 5 (Fig. 11) consists of a cyclam core appended with 12 dimethoxybenzene and 16 naphthyl units. Cyclam is one of the most extensively investigated ligands in coordination chemistry (33). Both cyclam and its 1,4,8,11-tetramethyl derivative in aqueous solution can be protonated and can coordinate metal ions such as Co(II), Ni(II), Cu(II), Zn(II), Cd(II), and Hg(II) with high stability constants (34). 

A step further in cyclam-based dendritic ligands for metal ions is constituted by dendrimer 6 (Fig. 13), containing two covalently linked cyclam units as a core, appended to six branches, each one of them consisting of a dimethoxybenzene and two naphthyl units 38). Its photophysical properties are qualitatively similar to that observed for 5. For example, the emission spectrum evidences the presence of naphthyl-localized excited states ( max = 337 run), naphthyl excimers Umax ca. 390 nm), and naphthyl-amine exciplexes (lmax = 480 nm). 

The three components of the self-assembled structure have complementary properties so that new functions emerge from their assembly. Dendrimer 5 has a very high molar absorption coefficient in the UV spectral region because of 12 dimethoxybenzene and 16 naphthyl units, but it is unable to sensitize the emission of an Nd ion placed in its cyclam core. The [Ru(bpy)2(CN)2] complex can coordinate (by the cyanide ligands) and sensitize the emission of Nd ions. Self-assembly of the three species leads to a quite unusual Nd complex which exploits a dendrimer and an Ru complex as ligands. Such a system behaves as an antenna that can harvest UV to VIS light absorbed by both the... 

Figure 2 illustrates the structure of a hydrophobic hexameric carboracycle derived from 9,1 2-(C6Hs)2- 1,2-C2B 10H10 units joined by trimethylene linkers.7 This species crystallizes from 1,3-dimethoxybenzene with two molecules of solvent enclosed in the resulting structure. 

Figure 2 Structure of a representative hexameric carboracycle with 9,12-diphenyl-substituted carboranylene units which contain two 1,3-dimethoxybenzene molecules of crystallization...

Unlike the corresponding oxa-crowns, crown thiothers with fused benzo rings have seen relatively little work. Klar and coworkers [66] prepared the hexa-methoxy derivative of tribenzo-9S3 by a modified Adams-Ferretti reaction between l,2-dibromo-4,5-dimethoxybenzene with l,2-dimercapto-4,5-dimethoxybenzene (Table 4). A variant of this procedure also affords the corresponding 12S4 analogue with four fused benzo units [67]. 

Enantiomer selective coloration of optically active amines, our important project, was realized by chiral azophenol crown 4 incorporated with two units of optically active hydrobenzoin. The synthetic route is shown in Scheme 2. Reaction of 2,6-bis(bromomethyl)-l,4-dimethoxybenzene 22, which is derived from hy-droquinone monomethylether 19 by a three-step procedure, with the dibutyltin derivative 26 of optically active dihydrobenzoin gives optically active podand 23 in 63% yield. Cyclization of 23 with the ditosylate of polyethylene glycol, followed by oxidation with ceric ammonium nitrate (CAN) and treatment with dinitrophenylhy-drazine, affords the desired chiral azophenol crowns 4n. 

Figure 16 Chemical formula of a dendrimer consisting of a 1,4,8,11-tetraazacyclotetradecane (cyclam) core with appended 12 dimethoxybenzene and 16 naphthyl units able to bind Nd(III) ions and [Ru(bpy)2(CN)2] in a 1 1 molar ratio. (Reprinted by permission of John Wiley Sons, Inc. from Ref. 34.)...

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