Cyclam dendrimer

The protonated dendrimer exhibits a far more intense excimer band than corresponding Frechet dendrons without a cyclam core. One possible reason for this behaviour is that excimer formation is facilitated by folding of the - flexible - benzyl ether framework. Whereas a change in emission intensity is observed in the course of protonation of the cyclam dendrimer, a reference substance containing no cyclam and benzene moieties (Fig. 5.17) shows a linear increase... 

Fig. 5.18 Photoexcited states of the cyclam dendrimer shown in Fig. 5.17 which are responsible for the observed emission a) before, b) after protonation (schematic). Ar stands for an arene, such as a naphthalene unit, N for the N atom of an amine Ar-Ar-N...

Fig. 8.15 Cyclam dendrimer (ionophore) with 16 terminal naphthalene fluorophores (according to Balzani, Vogtle et at.)...

In most cases, metal ion coordination by a dendrimer takes place by units that are present along the dendrimer branches (e.g., amine, imine, or amide groups) or appended at the dendrimer periphery (e.g., terpyridine, cathecolamide ligands). When multiple identical coordinating units are present, dendrimers give rise to metal complexes of variable stoichiometry and unknown structures. Luminescent dendrimers with a well defined metal-coordinating site have been reported so far [16, 17], and the most used coordination site is 1,4,8,11-tetraazacyclotetradecane (cyclam). 

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. 

Saudan C, Balzani V, Ceroni P et al (2003) Dendrimers with a cyclam core. Absorption spectra, multiple luminescence, and effect of protonation. Tetrahedron 59 3845-3852... 

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. 

In acetonitrile/dichloromethane solution this type of dendrimer shows three kinds of emission bands having their origin in the excited state localised within the naphthyl group, a naphthyl excimer, and a naphthyl/amine exciplex. Titration with trifluoroacetic acid revealed that, in spite of formally possessing four nitrogen atoms, the cyclam core undergoes only two successive protonation steps which significantly affect the luminescence properties. 

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

The non-linear spectral changes of the naphthyl-localised exciplex and exci-mer bands occur after addition of the first two equivalents of trifluoroacetic acid. It is thus unnecessary to protonate all of the nitrogen atoms in order to suppress exciplex formation since - expressed figuratively - the nitrogen atoms share the protons. Moreover, protonation not only provides protection against exciplex formation but also leads to conformational changes in the cyclam unit itself, which in turn affects excimer formation between peripheral naphthyl units of the dendrimer. 

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. 

Fig. 81. (Top) Cyclam-based dendrimer of first generation. (Bottom) Dendritic 9,10-diphenylanthracene ligands and...

Bergamini G, Saudan C, Ceroni P, et al. Proton-driven self-assembled systems based on cyclam-cored dendrimers and [Ru(bpy)2(CN)4]2 . / Am Chem Soc 2004 126 16466-71. 

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). 

Fig. 11. Formula of a dendrimer consisting of a cyclam core, capable of coordinating a Zn ion, and the corresponding scheme (Fig. 2c).

Fig. 12. Schematic representation of a metal complex containing a Zn ion coordinated by two cyclam-cored dendrimer 5, and the corresponding scheme (Fig. 2e).

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). 

Upon titration with Zn(CF3S03)2, no change was observed in the absorption spectrum, whereas strong changes were observed in the emission spectrum. Such changes, qualitatively similar to those caused by protonation, indicate that a 1 1 complex, [Zn(6)], is first formed and then replaced by a 2 1 species [Zn2(6)] (log j6i i = 9.7 and log j 2 i = 16.1 for these two species, respectively). In the 1 1 complex [Zn(6)] , the metal ion is likely sandwiched between the two cyclam emits. As previously observed in the case of dendrimer 5, apparently, the dendrimer... 

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... 

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