Guest molecular tweezer

Zimmermann, S.C. Rigid Molecular Tweezers as Hosts for the Complexation of Neutral Guests. 165, 71-102 (1993). 

Rigid Molecular Tweezers as Hosts for the Complexation of Neutral Guests... 

Klarner et al. undertook detailed studies of the host/guest relationships between molecular tweezers formed from condensed hydrocarbon arenes and symmetrically and asymmetrically dendryl-substituted viologens - with a 4,4 -bi-pyridinium core acting as -electron acceptor and a Frechet-type dendron as 71-electron donor (Fig. 6.23). The strong fluorescence of the 1,3-dimethylenoxyben-zene units of the Frechet dendron is quenched as a result of the donor/acceptor interaction. Accordingly, in dichloromethane solution the 4,4 -bipyridinium core... 

Fig. 6.23 Uptake of a dendritic viologen-type guest molecule in the cavity of molecular tweezers (schematic according to...

Fig. 6.26 Molecular tweezers 1 as host compound and dicationic, dendritically substituted viologen guest compounds (3 and 4) of different generation numbers (according to Balzani, Klarner, Vogtle et al.)...

Molecular clips and tweezers are two closely related types of compound. Molecular tweezers represent a simple kind of receptor in which two guest-binding domains usually comprising flat,... 

Scheme 6.5 Conformational change in molecular tweezers 6.44 upon aromatic guest binding.29...

Figure 6.27 Optimised geometries of molecular tweezers 6.46 and 6.47. Tweezers 6.47 are ca.10 A wide at the tips but can close up upon guest binding.31...

Zimmerman, S. C., Rigid molecular tweezers as hosts for the complexation of neutral guests. Top. Curr. Chem. 1993, 165, 71-102. 

Molecular tweezers such as 10 showed no affinity (Kassoc < 3M 1) for 7t-donor guests like naphthalene and anthracene. However, Tc-acceptors such as trinitrobenzene (TNB) and nitrated fluorenones were bound. The complexation experiments were performed by titrating a solution of the guest with the host and recording the upheld chemical shifts of the guest protons by XH NMR. Alternatively, the host could be titrated with guest and the increase in absorbance at the charge transfer band monitored. 

Electron donor-acceptor interactions are important in the examples in Table 4. This means that the molecular tweezers affinity for a particular guest can be modulated. For example, the affinity that 10 has for TENF can be increased by substituting the acridines with methoxy groups (i.e. 21). Does this increase in... 

The lack of selectivity and efficiency with which a single carboxylic acid binds nucleotide bases makes it an unlikely candidate for incorporation into a host for nucleotide guests. Nonetheless, it proved synthetically expedient to do so with molecular tweezers such as 11 and 21. The issue addressed was whether the aromatic chromophores in the molecular tweezer represented in Fig. 6 could cooperate with the acid group to increase the binding efficiency and selectivity of the host. 

As discussed in Sect. 2.5, the ability of the host and guest to self-associate must be established prior to any complexation studies. Carboxylic acids 33, 34, 47, and 49 showed no tendency to aggregate in chloroform [59]. In contrast, the lH NMR of carboxylic acid 31 contained several broad resonances that were consistent with a zwitterionic structure and/or an aggregate [56]. For this reason, complexation studies have not been carried out with the highly twisted molecular tweezers 31 or 32. For the guests used in these studies, dimerization constants were known and indicated that self-association would be negligible under the concentrations used. 

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