Photochemistry of a dumbbell-shaped multicomponent system hosted inside the mesopores of Al/MCM-41 aluminosilicate.: Generation of long-lived viologen radicals

A dumbbell-shaped compound, 2(6+), comprising a Ru(II) polypyridine complex, a p-terphenyl, a 4,4'-bipyridinium unit, a 3,3'-dimethyl-4,4'-bipyridinium unit, and a tetraarylmethane unit, has been enclosed in the channels (3.2 nm diameter) of an Al/MCM-41 aluminosilicate. Evidence for the internal location of 2(6+) in the Al/MCM-41 host was obtained by mapping the C/Si and N/Si atomic ratios as a function of the depth of penetration into the interior of the aluminosilicate particles. The photoluminescence of the Ru(II) polypyridine unit, when 2(6+) is enclosed within Al/MCM-41, is red-shifted, and the excited state is considerably shorter-lived compared with the behavior of 2(6+) in acetonitrile solution. Laser flash photolysis experiments on 2(6+) enclosed in the aluminosilicate matrix yield different results depending on the excitation wavelength. Selective excitation (532 nm light) of the Ru polypyridine component formed, together with oxidized Ru complex moieties, long-lived (millisecond time scale) radical cations of the 4,4'-bipyridinium unit and trapped electrons. Excitation at 308 nm, with light absorption involving both the Ru complex moiety and the bipyridinium units, leads to reduction of the 4,4'-bipyridinium unit and oxidation of the Ru complex, but the two processes might not be directly related. Compound 2(6+) is the dumbbell-shaped component of a previously investigated rotaxane, 1(6+), which, in acetonitrile solution, behaves as a photochemically driven molecular abacus (Chem. - Eur. J., 2000, 6, 3558). The observation of a long-lived 4,4'-bipyridinium radical cation when 2(6+) is incorporated in Al/MCM-41 may be relevant to the operation of molecular machines such as 1(6+), which are based on photoinduced electron transfer, but the occurrence of photoinduced redox processes involving the matrix would likely interfere severely with the machine-moving processes.