Long bipyridinium

Since these studies, many other rotaxanes incorporating cyclodextrins or their substituted derivatives have been reported, including other metal-containing systems. The latter include a series of a-cyclodextrin rotaxanes formed by the reaction of labile [Fe(CN)50H2J ions with pre-threaded l,l"-(a,(o-alkanediyl)-bis(4,4 -bipyridinium) dicationic moieties (incorporating methylene chains between 8- and 12-members long). " ... 

MV+ are formed transiently and disappear quickly because of the back ET reaction. However, when the pyrene and methylviologen are entrapped within a silica sol-gel network in the presence of the mobile charge carrier TV.A -tetramethylne-2,2 -bipyridinium (4-DQ ), photoexcitation leads to the creation of long-lived MV+ [201a]. This is because the 4-DQ + acts as a shuttle between the Py and MV2+ ... 

Ammonium salts with two different alkyl chains were prepared directly via subsequent alkylations of dimethylamine with primary bromides and crystallization. Commercial hexadecyl-methylamine can be conveniently applied in the same way in order to convey functionality to cationic synkinons. A recent example describes subsequent alkylations with a small functional and a long-chain primary bromide (Scheme 2.4). A-acylated / -phenylenediamine was also alkylated at the second nitrogen atom which had two different alkyl chains, with or without extra functionality . After deacylation, this head group can be diazotized or coupled oxidatively with various heterocycles in water (Scheme 2.4). Photoactive and coloured membrane surfaces are thus obtained. Phenylene-diamine, pyridine and in particular A-methyl-4,4-bipyridinium chloride are relatively weak nucleophiles. Substitution of bromides is slow and the more reactive iodides can rarely be obtained commercially, but the selection of nitromethanes as solvent for bromide substitution is of great help as well as the addition of sodium iodide to enforce a Finkelstein reaction or a combination of both. 

The symmetrical bolaamphiphile 7 with two bipyridinium head groups, a long connecting diester chain and four bromide counterions dissolves in water. 

To really achieve photoinduced dethreading, a different approach has been devised [98, 99], based on the use of an external electron transfer photosensitizer (P) and a sacrificial reductant (Red), as illustrated in Figure 22. The photosensitizer must be able to (i) absorb light efficiently and (ii) have a sufficiently long-lived and reductant excited state, so that its excitation (process 1) in the presence of the pseudorotaxane will lead (process 2) to the transfer of an electron to a bipyridinium unit of the cyclophane. The relatively fast back electron transfer from the reduced cyclophane component to the oxidized photosensitizer is prevented by the sacrificial reductant, which, if... 

The redox behavior of PVFc (VFc ) has been utilized in the construction of a microelectrochemical diode along with a redox-active viologen-based V,A -dibenzyl-4,4 -bipyridinium-based polymer (BPQ ). The polymers were coated upon microelectrodes and current was found to pass when the negative lead was attached to the (BPQ ) electrode and the positive lead was connected to the (VFc ) electrode. Thus, as the applied potential approached the difference in redox potentials of the two polymers, current flowed as shown in Equation (5a), and is favorable by p 0.9V. However, current does not flow if the applied potential is in the opposite sense as seen in Equation (5b), as it is disfavored by 0.9 V. The switching time of this diode, which is controlled by the time required to oxidize or reduce the polymers, was long in comparison with that of the solid-state diodes. ... 

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