Intrazeolitic Photochemical Charge Separation for Ru(bpy)32+−Bipyridinium System: Role of the Zeolite Structure

Abstract

The pore structure of zeolite Y consists of 13 Å supercages connected through 7 Å windows. This study deals with the intrazeolitic photoelectron transfer from trisbipyridyl ruthenium (II) (Ru(bpy)32+) synthesized within zeolite Y supercages to ion-exchanged bipyridinium molecules in neighboring supercages. Three N,N‘-dialkyl-2,2‘-bipyridinium ions and a 4,4‘-bipyridinium ion with reduction potentials varying from −0.37 to −0.65 V have been studied. For the 2,2‘-bipyridinium salts (members of the diquat family), two, three, and four CH2 bridging units abbreviated as 2DQ2+, 3DQ2+, and 4DQ2+ , respectively, have been examined. The fourth viologen is 1,1‘-dimethyl-4,4‘-bipyridinium, commonly known as methyl viologen and abbreviated here as MV2+. Because of the limitations of the time-resolved diffuse reflectance instrument, only a lower limit of the forward electron transfer rate constant from Ru(bpy)32+* to the bipyridinium ion was obtained and is >107 s-1. The back electron transfer from the photogenerated bipyridinium radical ions to Ru(bpy)33+ was monitored at different intrazeolitic bipyridinium concentrations. At low loadings of bipyridinium ions (1 per 10−15 supercages), the transient signal between 360 and 390 nm has contributions from both Ru(bpy)32+* and the bipyridinium radical ions, since the bipyridinium ion concentration was not high enough to quench all of the Ru(bpy)32+*. The decay of this signal (360−390 nm) could be fitted to the sum of two exponentials, representing the disappearance of unquenched Ru(bpy)32+* and the back electron transfer from the bipyridinium radical ions to Ru(bpy)33+. The rate constants for the back electron transfer for 2DQ2+, MV2+, 3DQ2+, and 4DQ2+ were found to be 4.0 × 104, 1.7 × 104, 1.1 × 104, and 7.3 × 103 s-1. The decrease in the electron transfer rates with increasing driving force for the reaction indicates that the electron transfer is occurring in the Marcus inverted region. At high loadings of the bipyridinium ions (1.2−1.7 molecules/supercage), the bipyridinium radical ions were considerably longer lived, and a simple exponential decay no longer described the loss of the bipyridinium radical signal. A model that allows for electron exchange processes between bipyridinium ions to compete with the back electron transfer was necessary. This kinetic model allowed us to extract the back electron transfer rate at high loadings along with the rate constants for electron hopping and a second-order electron (bipyridinium radical)/hole (Ru(bpy)33+) recombination process. For the series 2DQ2+, MV2+, 3DQ2+, and 4DQ2+ with high loading, the back electron transfer rate constants were 2.5 × 105, 9 × 104, 1.8 × 105, and 1.2 × 105 s-1, higher than the low loading samples. In the high loading case, longer lived charge separation was observed because of the presence of a route for charge propagation by electron hopping via the densely packed viologen molecules.