Tetracationic Cyclophane Research Articles

The template-directed synthesis of porphyrin-stoppered [2]rotaxanes

Two [2]rotaxanes, composed of (i) a polyether chain intercepted by (a) one centrally-located and (b) two symmetrically-located π-electron-rich hydroquinol rings and terminated by free-base and metallated (Zn) tetraaryl-porphyrin groups respectively and (ii) a tetracationic cyclophane constructed of two π-electron-deficient bipyridinium units linked by paraphenylenedimethyl residues, have been self-assembled by a clipping procedure.

The template-directed synthesis of a [2]rotaxane

X-Ray crystallography, FABMS, and 1H and 13C NMR spectroscopy have all been employed in the characterisation of a self-assembled [2]rotaxane composed of (i) a polyether chain intercepted by a centrally-located π-electron rich hydroquinol ring and terminated by admantoyl groups and (ii) a tetracationic cyclophane constructed by two π-electron deficient bipyridinium units linked by paraphenylenedimethyl residues. The full characterisation of a self-assembled [2]rotaxane is described.

Molecular Trains: The Self‐Assembly and Dynamic Properties of Two New Catenaries

The tetracationic cyclophane (the “train”) travels at high speed around the polyether macrocycle (the “track”) of the [2]catenane 1. The synthesis of the polyether macrocycle in a total yield of > 60% made possible the formation of 1 and of the analogous [3]catenane, in which the two smaller members of the chain are present on diametrically opposite hydroquinone rings according to the 1H NMR spectrum. All that remains to be done is to introduce signals to direct the train.

Self‐Assembling [2]Pseudorotaxanes

The complex formed from the tetracationic cyclophane 1 and hydroquinone polyethers such as 2 resembles a string of pearls. It forms spontaneously on combination of the components in solution and is stabilized by multiple π/π stacking interactions between the aromatic rings.

Towards a molecular abacus

The solid state and solution structures of the 1 : 1 complex formed between the cyclobis(paraquat-p-phenylene) tetracationic cyclophane and 1,5-dimethoxynaphthalene has led to the successful design and characterisation by X-ray crystallography of a prototype for the construction of a molecular abacus in the shape of a 2 : 1 complex between the same cyclophane and 1,3-bis(5-hydroxy-1-naphthyloxy) propane.

The complexation of tetrathiafulvalene by cyclobis(Paraquat-p-phenylene)

The powerful π-electron donor, tetrathiafulvalene, forms a strong green-coloured 1 : 1 complex with the π-electron accepting tetracationic cyclophane, cyclobis(Paraquat-p-phenylene), in both the solid and solution states as demonstrated by X-ray crystallography and spectroscopy (NMR and UV–VIS), respectively.

Electric analog models simulate ground-water systems

Catenane (1) consisting of a tetracationic cyclophane (2) and p-benzocrown ether (5) has been prepared to investigate its photochromic behavior in a poly(N-vinyl-2-pyrroridone) (PVP) film by comparing with that of 2 in the absence and the presence of π-electron donating guests such as indole (6) and p-dimethoxybenzene (7). The PVP film containing 1 showed a red color due to the π-donor/π-acceptor charge transfer interaction between the viologen and the dialkoxybenezene units in 1. Upon photoirradiation the film changed the color to blue with the absorption maximum (λmax) at 620 nm, associated with the photoreduction of the viologen unit from the dication to the radical cation, and reverted to the original dication with the half-life (τ1/2) of 15 min. The pale yellow film containing 2 showed the similar photochromic behavior to that of 1 with λmax at 610 nm and τ1/2 at 24 min, which are shifted to blue side by 10 nm and to longer time by 9 min as compared with those of 1. The difference in the photochromic behavior between 1 and 2 is resulted from the existence of the π-electron donating unit, 5, which is interlocked with the ring of 2 in the compound 1. The effect of the π-electron donating unit on the photochromic behavior of viologen was confirmed by the addition of 6 and 7 to the film containing 2 or its acyclic analog, 3. The λmax and τ1/2 for the photoreduced 2 were observed at 620 nm and 14 min, and at 615 nm and 20 min in the presence of 6 and 7, respectively. The larger effect of 6 rather than 7 on the photochromic properties of 2 was interpreted in terms of the larger binding constant of 6 with 2 than that of 7 in dimethylacetamide (DMAc) (3.0 and 2.3 M−1 for 6 and 7, respectively). The addition of 6 and 7 to the film containing 3 caused no change in the photochromic properties of 3 (λmax = 611 nm and τ1/2 = 22 min). When the simple benzylviologen (4) was used as the photochromic compound, the reversion rate of the photoreduced 4 was accelerated by the addition of 5 but it was not accelerated by 7. All these results demonstrate that the cyclic structure as well as the catenane promote the strong interaction between viologen and π-electron donating units. The interaction leads the change not only in color of the film before photoirradiation but also in the photochromic behaviors of viologen derivatives.