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) 3 2+ ) 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 CH 2 bridging units abbreviated as 2DQ 2+ , 3DQ 2+ , and 4DQ 2+ , respectively, have been examined. The fourth viologen is 1,1′-dimethyl-4,4′-bipyridinium, commonly known as methyl viologen and abbreviated here as MV 2+ . 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) 3 2+ * to the bipyridinium ion was obtained and is >10 7 s -1 . The back electron transfer from the photogenerated bipyridinium radical ions to Ru(bpy) 3 3+ 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) 3 2+ * and the bipyridinium radical ions, since the bipyridinium ion concentration was not high enough to quench all of the Ru(bpy) 3 2+ *. The decay of this signal (360-390 nm) could be fitted to the sum of two exponentials, representing the disappearance of unquenched Ru(bpy) 3 2+ * and the back electron transfer from the bipyridinium radical ions to Ru(bpy) 3 3+ . The rate constants for the back electron transfer for 2DQ 2+ , MV 2+ , 3DQ 2+ , and 4DQ 2+ were found to be 4.0 × 10 4 , 1.7 × 10 4 , 1.1 × 10 4 , and 7.3 × 10 3 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) 3 3+ ) recombination process. For the series 2DQ 2+ , MV 2+ , 3DQ 2+ , and 4DQ 2+ with high loading, the back electron transfer rate constants were 2.5 × 10 5 , 9 × 10 4 , 1.8 × 10 5 , and 1.2 × 10 5 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.Practical solar energy conversion processes, such as the p...

Spectroscopic Studies of Colloidal Solutions of Nanocrystalline Ru(bpy)32+−Zeolite Y

“…The emission was monitored at a right angle to the excitation source using an Acton Research Corp. SpectraPro 275 single monochromator, a Hamamatsu R928 PMT, and a Tektronix DSA 601 digitizing signal analyzer, with a 11A52 two-channel amplifier. Transient absorption measurements were done with a modified setup of the time-resolved diffuse reflectance instrument (described in ref 15). The zeolite solution was sealed in a 1 cm quartz cell.…”

“…The packing of the viologens is crucial in determining the viologen self-exchange rate constant (k hop ). 15 The enhanced concentration of the viologens on the surface of the micrometer-sized crystallites leads to a higher k hop value, which is reflected in the diffuse reflectance measurements. For the nanocrystallites, the interior of the crystal is sampled and the value of the k hop is lower.…”

“…The decay of the signal due to MV •+ was fit to the same model employed previously to explain the kinetics observed in micrometer-sized crystallites. 15 The following reactions were used for the modeling: …”

“…Table 3 is a comparison of the rate constants from colloidal solutions with those obtained by time-resolved diffuse reflectance on the micrometer-sized crystallites. 15 …”

Supramolecular photochemistry in zeolites: From catalysts to sunscreens

“…The assembly of donor-acceptor pairs in heterogeneous support systems, such as zeolites, clays, and polymers, can provide the appropriate electrostatic and steric environment for the prevention of BeT, thereby favoring the generation of a long-lived charge-separated state. The study of zeolites as supramolecular scaffolds for energy storage systems (via long-lived charge separation) has been pioneered largely by Dutta et al [14,15,[73][74][75][76][77][78] with significant contributions also from the groups of Kincaid [79,80] and Mallouk [81][82][83]. There are many examples demonstrating how intelligent use of the zeolite framework, to spatially organize the donor and acceptor components, can result in the generation of extremely long-lived charge-separated states.…”