Cyclam core unit

Dendrimers with cyclam core unit (1,4,8,11-tetraazacyclotetradecane) and Fre-chet type dendrons, decorated with eight or 16 naphthyl units (Fig. 5.17) were examined for changes of their luminescence and absorption spectra during protonation [21]. The core unit itself is photoinactive, but can interact with photoactive groups in the dendritic branches, influencing the emission properties of the chromophore units and giving rise to the possibility of new emission bands. 

Fig. 5.17 Naphthyl-decorated dendrimer with cyclam core unit (according to Balzani, Vogtle et Cl/.) reference substance without dendritic branching shown in box...

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 acetonitrile-dichloromethane 1 1 v/v solution, their absorption spectra are dominated by naphthalene absorption bands and they exhibit three types of emission bands, assigned to naphthyl localized excited states (/Wx = 337 nm), naphthyl excimers (Amax ca. 390 nm), and naphthyl-amine exciplexes (/lmax = 480 nm) (solid lines in Fig. 3). The tetraamine cyclam core undergoes only two protonation reactions, which not only prevent exciplex formation for electronic reasons but also cause strong nuclear rearrangements in the cyclam structure which affect excimer formation between the peripheral naphthyl units of the dendrimers. 

The above discussed dendrimers 1 and 2 containing a cyclam core and 8 or 16 naphthyl units at the periphery, respectively, can also efficiently bind metal ion quenchers, such as Ni2+, Co2+, and Cu2+. However, changes in the luminescence properties are completely different from those reported before upon addition of Zn2+ because the dendritic naphthyl units can be involved in photoinduced energy and/or electron transfer processes with the presently investigated metal ions. 

Balzani, Vogde et al. demonstrated for dendritic cyclam compounds that signalling units are dendritically amplified by increasing the number of peripheral fluorophores. Opportunities for these PET sensor systems exist in the combination of selective coordination sites with the dendritic architecture. The mechanism resembles that described above, but can be influenced by the introduction of dendritic units of different generations into the sensor system. The distance between the terminal naphthalene groups of the dendron and the receptor (ion as guest in the cyclam core) is of crucial importance for electron transfer (ET) (Fig. 8.15) [53],... 

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. 

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). 

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 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.)...

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). 

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