Photochromism of viologens included in crown ether cavity

The effect of a pi-electron-donating macrocyclic molecule on the photochromic behavior of viologen derivatives was investigated in a thin polymer film. The intermolecular interactions between the viologens and the macrocyclic molecule were investigated in a solution before photoirradiation. In acetone, benzylviologens, N,N'-dibenzyl-4,4'-bipyridinium hexafluorophosphate (1) and N,N'-dibenzyl-trans-1,2-bis(4-pyridinium)ethylene hexafluorophosphate (2) each derived from 4,4'-bipiyridine and trans-1,2-bis(4-pyridyl)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 transfer interaction. The peak wavelength of the charge transfer absorption band was at 453 and 421 nm for the inclusion complexes of 1 and 2 with 3, respectively. Upon photoirradiation to the polymer film containing 1, the film changed color from colorless to blue, associated with the reduction of 1 from 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 only the color change from colorless to yellow, associated with the charge transfer interaction between 1 and 3 before photoirradiation, but also an acceleration in the bleaching rate of the photoreduced 1. When p-dimethoxybenzene (4) was used as an acyclic analog of 3, a negligible change in the photochromic behavior of 1 was observed. Similar effect of 3 on the photochromic behavior of 2 was observed. These results imply that the pi-electron-donating macrocyclic molecule causes a faster bleaching of photoreduced viologens by forming the inclusion complex.