Molecular tweezers approach

More recently a molecule with a pair of planar aromatic moieties (Figure 6.4) was synthesized and used to exaggerate the difference between metallic and semiconducting SWNTs in the non-covalent functionalization and solubilization. The molecule representing essentially the molecular tweezers approach (Figure 6.4) exhibited significant selectivity toward... 

Figure 6.4 Upper panel the chemical structure of 1,1 -bis-pyrene butyoxyl-/ -xylene (bis-pyrene) lower panel schematic illustration of the molecular tweezers approach for the post-production separation of metallic and semiconducting SWNTs with the use of bis-pyrene.

The component analysis described above shows that the aromatic cleft in 34 contributes 4.2 kcal mol-1 toward the free energy of complexation of 9-propyladenine (50). An alternative approach to determining the contribution made by the aromatic cleft would be to measure the free energy obtained in a complex between 50 and a molecular tweezer containing an aromatic cleft, but no carboxylic acid group. This analysis, which has been carried out using molecular tweezer 11,... 

Naturally, quantum chemical approaches to molecular recognition are usually employed for selected systems since the complexity of these systems requires a system-specific analysis which makes it difficult to extract results of general validity for examples, see Refs. [13-16] for studies of molecular tweezers. Further examples are mentioned in a review article by Schatz considering ab initio calculations on calixarenes and calixarene complexes [17]. Schatz concludes that although the systems are quite big, useful contributions have been made by ab intio calculations. However, a general model is needed in order to make host-guest processes and template-assisted reactions accessible to a comparison of quantitative measurements and calculations, which may finally provide the basis for rational host design and for the prediction of template effects (compare the recent attempt by Hunter [18]). 

Despite their appeal, the problem with both of the devices described above is that they are activated by small molecules. To approach nanorobotics succesfuUy. one needs many different devices incorporated into a larger matrix, such as the 2D arrays described above. However, both the craciform and the B-Z devices are activated by small molecules, so that the action of these effectors could not be targeted easily to a specific device within the assay. Given the informational nature of DNA, it seems that sequence-dependent devices should be developed to fulfill this purpose. Two such devices have been developed, both based on the same principle of activation by strand addition and strand removal. This approach was pioneered by Yurke et who developed molecular tweezers that were closed by adding a strand of DNA. The strand contained an unpaired segment and could be removed from the tweezers by the addition of its full complement removal of the strand resulted in the tweezers relaxing to their open state. 

Speiss used ab initio methods (HF/6-31G ) and the gauge-invariant atomic orbital (GIAO) approach to generate NMR chemical shifts to unravel the behavior of a naphthylene-bridged molecular tweezer and a 1,4-dicyanobenzene guest. The benefit of augmenting experimental NMR data (in the solid-state and solution phase), and the X-ray structure of a 1 1 complex, with computational methods was clear to the authors who stated the following ... 

The formation and dissociation of chemical bonds is of prime concern in chemistry, and with the advent of ultrafast high-intensity lasers, selective bond dissociation using appropriate laser pulses as molecular tweezers and scissors has received extensive and intense attention. - Theoretical approaches to laser-assisted control of chemical reactions have kept pace and demonstrated remarkable success, with experimental results reinforcing the theoretical ideas. The development of... 

Novel experimental approaches increased the analytical power of Raman spectroscopy. Lambert showed the great potential of the combination of Raman spectroscopy with optical tweezers for viral investigations. The sample can be manipulated without physical contact, hence enabling the molecular investigation of samples closer to their native state [19]. 

The experimental realization of the optimal initial states is, however, a completely uncharted area. In an earlier paper,we have presented the formulae to obtain field parameters required to achieve these FOISTs, and the optimal control approach may also feasibly and profitably be employed to attain this FOIST, which comprises only three vibrational levels. We however believe that, while the theoretical tools are useful, the central results from our investigation - - are that, instead of putting the entire onus of selective control on a theoretically designed laser pulse that may not be easy to realize in practice, the approach where different vibrational population mixes are experimentally obtained and subjected to readily attainable photolysis pulses, leading to an empirical experimental correlation between selectivity attained for diverse photolysis pulses and initial vibrational population mix used, represents a more promising and desirable alternative. Our results, we hope, will spur experimental tests, and a concerted partnership between field and initial state shaping is required to better realize the chemical dream - of using lasers as molecular scissors and tweezers to control chemical reactions. 

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