Facile reduction of zeolite-encapsulated viologens with solvated electrons and selective dispersion of inter- and intramolecular dimers of propylene-bridged bisviologen radical cation

Zeolite-Y (Y), zeolite-L (L), mordenite (M), and ZSM-5 exchanged with methyl viologen C₁VC₁²⁺ and N′,N″-trimethylenebis(1-methyl-4,4′-bipyridinium) (henceforth bisviologen) C₁V²⁺-C₃VC₁²⁺ were prepared. Treatment of the dried, viologen-doped zeolites with solvated electrons in diethyl ether readily yields the corresponding viologen radical cation, i.e., C₁VC₁^·+ and C₁V^·+-C₃VC₁^·+ within the zeolite pores. Although C₁V^·+-C₃-VC₁^·+ readily folds to its intramolecular dimer C₃[C₁V^·+]₂ in solution, it remains in the open form in the four zeolites. C₁VC₁^·+ readily dimerizes to [C₁VC₁^·+]₂ at high concentrations in L but not in other zeolites. However, C₁V^·+-C₃-VC₁^·+ does not dimerize by itself in any of the four zeolites even at high concentrations. C₁VC₁^·+ readily dimerizes in Y, L, and M when the pores are filled with water. The hydration-induced dimerization also works well for C₁V^·+-C₃-VC₁^·+ in L and M. This leads to selective formation of the unprecedented intermolecular dimer of bisviologen radical cation, i.e., [C₁V^·+-C₃-VC₁^·+]₂ in L and M. The intramolecular analogue C₃[C₁V^·+]₂ can also be dispersed into Y, L, and M by direct occlusion of its bisperchlorate salt from the acetonitrile solution. The broad approx. 530 nm band of the viologen radical dimer splits into two when the viologen radical moieties are forced to juxtapose in the collinear conformation within the narrow and straight channels of L and M. The near-infrared band of the viologen radical dimer progressively blue shifts with decreasing the interannular distance. This paper thus demonstrates a novel use of zeolites to selectively disperse inter- and intramolecular dimers of diradical dication and to lock the viologen radical dimers into the collinear conformation which allows us to delineate the spectral variation of viologen radical dimers with changing the interannular conformation and distance.