Catenane and inclusion complex as photochromic compounds involving viologen units
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.
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Journal: Journal of Photochemistry and Photobiology A: Chemistry - Volume 269, 1 October 2013, Pages 59–64