Dansyl units

A luminescent unit extensively used to functionalize dendrimers is the so-called dansyl (5-dimethylamino-l-naphthalenesulphonamido group). Dendrimers (up to the third generation, compound 9) containing a single dansyl unit attached off center [39] show that this fluorescent unit, which is very sensitive to environment polarity, is progressively shielded from interaction with water molecules as the dendrimer generation increases. 

Quantitative analysis of the results obtained has shown that a single eosin guest is sufficient to completely quench the fluorescence of any excited dansyl unit of the hosting dendrimer. Fluorescence lifetime measurements indicated that the dye molecules can occupy two different sites (or two families of substantially different sites) in the interior of the dendritic structure. 

It has been demonstrated that dendrimers can be used also as fluorescent sensors for metal ions. Poly(propylene amine) dendrimers functionalized with dansyl units at the periphery like 34 can coordinate metal ions by the aliphatic amine units contained in the interior of the dendrimer [80]. The advantage of a dendrimer for this kind of application is related to the fact that a single analyte can interact with a great number of fluorescent units, which results in signal amplification. For example, when a Co ion enters dendrimer 34, the fluorescence of all the 32 dansyl units is quenched with a 32-fold increase in sensitivity with respect to a normal dansyl sensor. This concept is illustrated in Fig. 3. 

Balzani V, Ceroni P, Gestermann S et al (2000) Effect of protons and metal ions on the fluorescence properties of a polylysin dendrimer containing twenty four dansyl units. J Chem Soc Dalton Trans 3765-3771... 

Vogtle F, Gestermann S, Kauffmann C et al (2000) Coordination of C02+ ions in the interior of polypropylene amine) dendrimers containing fluorescent dansyl units in the periphery. J Am Chem Soc 122 10389-10404... 

Fig. 7 Spectral changes observed upon titration of a 4.2 x 10 6 M solution of dendrimer II by Nd(CF3S03)3. Left-hand side spectra refer to the quenching of the fluorescence of the dansyl units in the visible region and right-hand side spectra to the sensitization of the NIR luminescence of Nd3+...

Fig. 8 Energy level diagrams for the dansyl units of dendrimer 11 and the investigated lanthanide ions. The position of the triplet excited state of 11 is uncertain because no phosphorescence can he observed...

Addition of lanthanide ions to dendrimer solutions showed that (a) the absorption spectrum of the dendrimer is almost unaffected, (b) the fluorescence of the dansyl units is quenched (c) the quenching effect is very large for Nd3+ (Fig. 7) and Eu3+, moderate for Er3+ and Yb3+, small for Tb3+, and very small for Gd3+ (d) in the case of Nd3+ (Fig. 7), Er3+, and Yb3+ quenching of the dansyl fluorescence is accompanied by the sensitized near-infrared emission of the lanthanide ion [42]. 

Balzani, Vogtle et al. followed the kinetics of complexation of metal ions (Co2+, Cu2+) for the example of dansyl-bearing POPAM dendrimers (see Chapter 4, Fig. 4.3) [33]. In the first step the cations interacted with the dimethylami-no nitrogen of the dansyl units, as can be recognised from the quenching of their fluorescence - which is complete at high ion concentrations (Fig. 6.19a). 

If such solutions are allowed to stand for several hours, their fluorescence surprisingly reappears. This is interpreted by assuming that the metal ions slowly penetrate into the interior of the dendrimer to the N atoms of the PO-PAM-core, again freeing up the fluorescing dansyl units (Fig. 6.19b). 

Fig. 6.19 a Interaction of metal ions with the dimethylamino groups of the peripheral dansyl units (shown as red ellipses) of a POPAM dendrimer The original dansyl fluorescence of the dendrimer is quenched - partially in this case (light grey ellipses)... 

Fig. 6.19 b After a few hours the cations have migrated from the exterior (Fig. 6.19a) into the interior of the dendrimer The fluorescence of the dansyl units returns by itself (according to Balzani, Ceroni, Vogtle et al.)... 

Luminescence has already been considered in general terms in Chapter 5. Luminescent POPAM dendrimers of various generations with peripheral dansyl units were studied by Balzani and Vogtle et al. as sensor model systems with regard to the fundamental suitability of dendritic structures for multiplication of signalling groups (multi-labelling) [48]. 

The attachment of the dansyl unit to the secondary site of CD can also be accomplished by Hamasaki et al. [45], They examined the fluorescence properties of 33, 34,35, and 36, in which the dansyl moiety is attached to the secondary site (C3) of CD [43], Because the preparation of 3-deoxy-3-amino-CDs involves the inversion reaction at C3 that results in the conversion of one glucose member to an altrose residue, the modified CDs derived from 3-deoxy-3-amino-CDs have a distorted cavity with a decreased cavity space. As a result, (3-CD derivatives 33 and 34 cannot act as a fluorescence sensor, exhibiting fluorescence peaks at 580... 

Reinhout et al. prepared a (3-CD-calix[4]arene couple 64 in which a dansyl unit is attached to calix[4]arene and oriented toward the secondary face of (3-CD [56], This compound exhibits a fluorescence peak at 538 nm, and its intensity was hardly influenced by guest molecules. The examination of molecular models suggested a very strong self-inclusion of the dansyl group into the CD cavity. 

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]...

Dendrimer 3 (Fig. 7), which is based on a benzene core branched in the 1, 3, and 5 positions, contains 18 amide groups in its branches and 24 chromophoric dansyl units in the periphery. It is well known that, in sufficiently basic solution, amide groups (25), including the dansylamide units (25) can undergo deprotonation and coordination of transition metal ions. The dansyl units show strong absorption bands in the near UV... 

Fig. 9. Formula of a dendrimer constituted by a poly(propylene amine) structure functionalized with 32 dansyl units at the periphery, capable of coordinating Co ions, and the corresponding scheme (Fig. 2d).

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