Threading processes

A pseudorotaxane is the supramolecular product of the self-assembly of two components (i.e., a [2]pseudorotaxane), a molecular ring (macrocycle) and a molecular wire (thread) threaded into it. The occurrence of specific interactions between the two components lowers the free energy of the assembled system with respect to the separate components and drives the self-assembly of the pseudorotaxane (threading process). Threading occurs generally in solution and, upon changes of solvent... 

To study the effect of the BG on threading process, triphenylmethyl-based azo compounds were designed as BG initiators for the preparation of polystyrene rotaxane 32 [67-69]. The BG fragments were shown to end up at both chain ends of the polystyrene since termination occurs nearly exclusively by combination. However, the threading efficiencies did not increase with 29 as cyclic and increased very little in other instances relative to those without BG. 

Harada et al. explored the compatibility of CD with various polymeric backbones including polyethylene oxide) (PEG), polypropylene oxide) (PPG), polyisobutylene (PIB), and polyethylene (PE) [77-87]. The corresponding polyrotaxanes (36 to 47) were prepared by Method 2, simply by mixing a solution of CD and the polymer. The cavity size of CD was found to be the main factor in the threading process. While one a-CD (20) was threaded per two repeat units in PEG (m/n=0.50) and every three repeat units for PE (m/n=0.333), it was too small for PIB and PPG. On the other hand, two PPG units complexed per /(-CD (21). Because the upper limit of the min value is controlled by the depth of the CD cavity, the m/n value remained constant for the same type of backbone, irrespective of the end group. However, the nature and concentration, i.e., polymer... 

Wenz and Keller reported a-CD-based polyrotaxanes 50 and 51 with poly(iminoundecamethylene) and poly(iminotrimethylene iminodecamethylene) as backbones, respectively. These polyrotaxanes were prepared by mixing an acidic solution of the corresponding polymer with CD (Method 2, Figure 6) [88]. Polyrotaxane 50 was also transformed into 52 by attaching BG onto the NH sites. The threading process was monitored by proton NMR spectra and viscosity changes. The purified product had min values from 0.10 to 0.67, depending on the back-... 

By use of proton NMR techniques Parsons and coworkers studied a similar system in more detail in terms of the threading process and molecular motion... 

It has been also reported a study on the threading process of a a - cyclodextrin (a-CD) and polyethylene glycol (PEG), as a function of temperature and solvent composition. This reaction produces a polyrotaxane that eventually precipitates and forms a thick gel. Ceccato et al. [46] have proposed a molecular model for the interpretation of the temperature and solvent composition effect on the threading process. According of this model, the reaction can be depicted as a five - step phenomenon that mainly depend on the threading and sliding of a-CD and PEG. The transition... 

Figure 4.8, shows a squematic representation of the threading process, taken from [46]... 

In the previous section we have described pseudorotaxane systems in which electron transfer inputs govern dethreading/threading processes, opening the way to the control of nuclear movements (molecular machines). In this section, we will see that, in their turn, nuclear movements induced by an appropriate stimulation (e.g., an acid/base reaction) can govern the occurrence of electron transfer processes or CT interactions. This aspect of pseudorotaxane chemistry can be exploited for the construction of electronic devices for information processing at the molecular level. 

The slow rate of the threading process can be rationalized on a molecular level as a hopping process of the CD rings over the periodic potential caused by the repulsive interactions exerted by the bulky hydrophilic groups, illustrated in Fig. 27. 

Fig. 5. Top a schematic representation of the threaded and unthreaded configurations are reported. For clarity the solvent molecules are omitted. Bottom the FES of the threading process reconstructed with umbrella sampling and weighted histogram analysis. After [59]...

Photoinitiated SET has been used to drive a molecular machine and absorption and fluorescence spectroscopy have been used to monitor it. A 1 1 pseudoro-taxane forms spontaneously in solution as a consequence of the donor-acceptor interactions between the electron-rich naphthalene moiety of the thread (380) and the electron-deficient bipyridinium units of the cyclophane (381). The threading process is monitored by the appearance of a charge transfer absorption band and disappearance of the naphthalene fluorescence. Excited state SET from 9-anthracenecarboxylic acid (9-ACA) reduces a bipyridinium moiety of the cyclophane, lessening the extent of interaction between the thread and the cyclophane and dethreading occurs. On addition of oxygen the reduced cyclophane is reoxidised and threading reoccurs. ... 

Some of these parts, such as the threads on the pipe nipple, may have burrs on them, or shavings of metal that were not completely removed in the threading process done by the manufacturer. Pull these off, or grind them, or file them down. Inspect all parts as you receive them and finish working on them, and remove any such imperfections. 

Higher order [n]-rotaxanes could be, in principle, synthesized by the transition metal-controlled threading process, provided that the linear component contains the... 

Several variants using multiple sequence information and sequence profiles generated for the query sequence for the threading process have been described and demonstrate to improve fold recognition performance [109, 195-203] similarly as has been observed for secondary structure prediction and sequence methods. 

The threading process of two molecules of 124 onto 123 is almost quantitative. As for the previously discussed catenancs, no electronic interaction between the two porphyrins of 125 can be detected. 

---