Effect of Rotational Mobility on Photoelectron Transfer:  Comparison of Two Zeolite Topologies

Abstract

Nuclear magnetic resonance (NMR) spectroscopy and time-resolved diffuse reflectance (TRDR) have been combined to study the effects of constrained rotational motion on the rates of photoinduced electron-transfer reactions within zeolites. By synthesizing tris(2,2′-bipyridine) ruthenium (II) [Ru(bpy) 3 2+ ] within the large cages of zeolites Y and EMT, it was possible to directly investigate the effect of zeolite cage size on molecular motion of Ru(bpy) 3 2+ and the influence of the cage size on the rate of intrazeolitic electron transfer. Deuterium solid-state NMR shows that zeolite Y imposes restraint on molecular rotation of Ru(bpy) 3 2+ , while zeolite EMT allows for motion. Temperature-dependent studies show the ability to freeze or induce motion of Ru-(bpy) 3 2+ within zeolite EMT while motion within zeolite Y remains unaffected, even at elevated temperatures. For zeolite EMT, an increase in the rate of the photoinduced forward and back electron transfer from Ru-(bpy) 3 2+ to methyl viologen was noted and correlated with access, rotational motion, and favorable orbital overlap. The overall photochemical charge separation efficiencies for the intrazeolitic Ru(bpy) 3 2+ -bipyridinium reactions were similar for both zeolites. IntroductionOne of the major obstacles in the development of artificial photosynthetic systems is that the thermal back-electron transfer reaction negates the utilization of photochemically created reactive redox species. 1 Thus, considerable effort is being made to understand the features that control electron-transfer dynamics. Toward that end, many donor-acceptor systems have been examined, and assembly of molecular units on microheterogeneous supports is being pursued to influence electron-transfer dynamics. 2 By virtue of the nature of microheterogeneous supports, the motion of molecules is modified and it is of interest to examine how electron transfer is influenced by such immobilization. Zeolites, because of the molecular size of their cavities, are an excellent host system to investigate immobilization phenomena.Zeolite Y consists of a regular network of sodalite cages in a tetrahedral arrangement resulting in 13 Å supercages with 7 Å windows (Figure 1a). 3 This is large enough to accommodate the luminescent molecule trisbipyridine ruthenium (II) [Ru-(bpy) 3 2+ ] which has a molecular diameter of 12 Å and can be assembled within the supercages by using a "ship-in-a-bottle" synthetic procedure. 4 These intrazeolitic ruthenium complexes can then be surrounded by bipyridinium ions such as methyl viologen (MV 2+ ) using the inherent ion-exchange properties of the zeolite. It has been shown that zeolite Y can promote longlived, Ru(bpy) 3 3+ -bipyridinium radical cation charge-separated states. 5,6 Molecular modeling of Ru(bpy) 3 2+ entrapped within zeolite Y indicated a significant number of contacts between the metal complex and the walls of the zeolite that were within the van der Waals radii of the atoms. 6 It was proposed that these contacts inhibit the rotational motion of the i...

Photoinduced inter-cavity electron transfer between Ru(ii)tris(2,2′-bipyridne) and Co(ii)tris(2,2′-bipyridine) Co-encapsulated within a Zn(ii)-trimesic acid metal organic framework

“…For example, RuBpy complexes encapsulated within a zeolite EMT material exhibit photoinduced ET from the 3 MLCT to methyl viologen (MV) with a rate constant of 1.7 Â 10 8 s À1 , while in zeolite Y the same reaction occurs with a rate constant of 8 Â 10 7 s À1 . [35][36][37][38][39] Although the distance between the MV and RuBpy was not clearly determined in this study, previous results from RuBpy self-quenching in zeolite Y indicated distance of $12 Å between donor and acceptors. 33 As a small diffusional component was observed in the Stern-Volmer analysis for the RuBpy quenching by MV, the forward ET was suggested to occur at the interface between the excited RuBpy and the super cage window which is likely to place the ET distance at <12 Å.…”

“…33 As a small diffusional component was observed in the Stern-Volmer analysis for the RuBpy quenching by MV, the forward ET was suggested to occur at the interface between the excited RuBpy and the super cage window which is likely to place the ET distance at <12 Å. 35,36 Although the crystal structure does not reveal the specic location of the RuBpy or CoBpy complexes within the USF2 framework, examination of the framework crystal structure together with semi-empirical Marcus theory can lead to insights regarding inter-cavity ET. The empirical dependence of the ET rate on distance can be described by:…”

Photoinduced Electron Transfer between Ruthenium-bipyridyl Complex and Methylviologen in Suspensions of Smectite Clays

“…Another important key to efficient PET reactions is utilization of heterogeneous reaction media which regulate location of the reactant molecules to stabilize the charge separation between the donor and acceptor. − In fact, numerous heterogeneous systems have been examined for PET between Ru(bpy) 3 2+ and MV 2+ . Examples are organic polymers, − micelles, − and inorganic porous oxides such as zeolites and porous glasses. − These heterogeneous media realize stabilization of the photogenerated MV +• species. ,− ,,, Also, the use of sacrificial donors achieves efficient accumulation of the MV +• species and the reductive generation of H 2 from H + and of H 2 O 2 from O 2 in the heterogeneous systems. ,− ,− , …”

Sorption and spontaneous ionization of phenothiazine within channel type zeolites: Effect of the confinement on the electron transfers.

“…In that context, zeolites have also shown evidence for their ability to accept electrons and stabilize charge separated states after photoionization of molecules occluded within their cages or channels. [9][10][11][12][13][14][15][16][17][18][19][20] Moreover, it should be noted that in some cases, spontaneous ionization can occur after the mixing of electron donor molecules and channel type zeolites. 17,[21][22][23] The mere exposure of molecules having relatively low ionization potential to dehydrated medium pore zeolites was found to induce spontaneous radical cation formation and electron ejection, provided that the pore opening diameter and inner spaces are large enough to allow the molecule incorporation.…”