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Bipyridinium Macroring Encapsulated within Zeolite Y Supercages. Preparation and Intrazeolitic Photochemistry of a Common Electron Acceptor Component of Rotaxanes and Catenanes

2002; American Chemical Society; Volume: 106; Issue: 26 Linguagem: Inglês

10.1021/jp014088u

1520-6106

Mercedes Álvaro, Belén Ferrer, Vicente Fornés, Hermenegildo Garcı́a, J. C. Scaiano,

Advanced Chemical Physics Studies

Cyclobis-(N,N'-paraquat-p-phenylene) macrocycle (24+), 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 (12+) and 1,4-bis(bromomethyl)benzene. Upon 266 nm laser excitation, the corresponding 1•+ and 2•(3+) radical cations living hundreds of μs 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 12+@Y and 24+@Y leads to the formation of the corresponding charge transfer (CT) complexes characterized by a broad CT band at λmax 475 nm. Laser flash photolysis at 266 nm of 12+@Y and 24+@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 12+ and 24+ excited states by DMB. In contrast, 532 nm selective excitation of the 12+-DMB or 24+-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 24+-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 12+-DMB or 24+-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.