Bipyridinium macroring encapsulated within zeolite Y supercages.: Preparation and intrazeolitic photochemistry of a common electron acceptor component of rotaxanes and catenane

Cyclobis-(N,N'-paraquat-p-phenylene) macrocycle (2(4+)), the electron acceptor ring present in many [2]-catenanes and rotaxanes, has been obtained encapsulated within the supercages of zeolite Y by ship-in-a-bottle synthesis from the open precursor (1(2+)) and 1,4-bis(bromomethyl)benzene. Upon 266 nm laser excitation, the corresponding 1(.+) and 2(.(3+)) radical cations living hundreds of mus are generated within the zeolite cages in line with the photochemical behavior reported for the parent methyl viologen included within zeolites. Addition of 1,4-dimethoxybenzene (DMB) to 1(2+) @ Y and 2(4+) @ Y leads to the formation of the corresponding charge transfer (CT) complexes characterized by a broad CT band at lambda(max) 475 nm. Laser flash photolysis at 266 nm of 1(2+) @ Y and 2(4+) @ Y containing DMB shows a dramatic reduction of the signal of the 1(.+) and 2(.(3+)) radical cations due to the electron-transfer quenching of 1(2+) and 2(4+) excited states by DMB. In contrast, 532 nm selective excitation of the 1(2+)-DMB or 2(4+)-DMB CT complexes allowed the detection of 1(.+) and 2(.(3+)) radical cations and (DMB)2(.+) as long-lived transients whose decay was not complete after hundreds of microseconds following laser excitation. Generation of (DMB)2(.+) upon excitation of the 2(4+)-DMB CT complex indicates that the initially complexed DMB.+ migrates out of the macroring and becomes stabilized by interaction with an electron-rich neutral DMB molecule, forming a dimer. This photochemical behavior is distinctive of restricted media because no transients are detected in the nanosecond time scale-upon 355 or 532 nm laser photolysis of the same 1(2+)-DMB or 2(4+)-DMB CT complexes in acetonitrile. Our results illustrate the potential of zeolites in the field of molecular machines as a medium to stabilize short-lived transients.