Photochromism of viologens included in crown ether cavity

The effect of a pi -electron-donating macrocyclic molecule on the photochro mic behavior of viologen derivatives was investigated in a thin polymer fil m. The intermolecular interactions between the viologens and the macrocycli c molecule were investigated in a solution before photoirradiation. In acet one, benzylviologens, N,N ' -dibenzyl-4,4 ' -bipyridinium hexafluorophospha te (I) and N,N ' -dibenzyl-trans-1,2-bis(4-pyridinium)ethylene hexafluoroph osphate (2) each derived from 4,4 ' -bipiyridine and trans-1,2-bis(4-pyridy l)ethylene, respectively, form an inclusion complex with p-benzocrown ether (3) with binding constants of ca 200 M-1, which was driven by a charge tra nsfer interaction, The peak wavelength of the charge transfer absorption ba nd was at 453 and 421 nm for the inclusion complexes of 1 and 2 with 3, res pectively. Upon photoirradiation to the polymer film containing I, the film changed color from colorless to blue, associated with the reduction of 1 f rom the dication to the radical cation, The original dication was recovered after 120 min. The addition of 3 into the film containing 1 caused not onl y the color change from colorless to yellow, associated with the charge tra nsfer interaction between 1 and 3 before photoirradiation, but also an acce leration in the bleaching rate of the photoreduced 1, When p-dimethoxybenze ne (4) was used as an acyclic analog of 3, a negligible change in the photo chromic behavior of 1 was observed. Similar effect of 3 on the photochromic behavior of 2 was observed. These results imply that the pi -electron-dona ting macrocyclic molecule causes a faster bleaching of photoreduced viologe ns by forming the inclusion complex.