Photoinduced electron transfer in non-acidic zeolites: Probing the intrazeolite dynamics of molecular guests.
Date
2006
Authors
Keirstead, Amy Elizabeth.
Journal Title
Journal ISSN
Volume Title
Publisher
Dalhousie University
Abstract
Description
Zeolites are crystalline aluminosilicate materials comprised of an open framework of channels and cavities that allows for the encapsulation of molecular guests. Recently, much emphasis has been placed on using zeolites as hosts for photoinduced electron transfer (PET) reactions, for the development of artificial photosynthetic devices and molecular electronics. Zeolites have also been shown to participate as redox partners in PET reactions, yet despite their tremendous potential, the redox properties of zeolites are not yet fully understood. This thesis comprises three separate research projects that were designed to contribute to the overall knowledge of zeolites as redox partners and hosts for electron transfer reactions.
In the first research project, the distance of electron migration in zeolite Y was examined as a function of alkali metal cation. This process was probed by monitoring the yield of the 1,4-dicyanobenzene radical anion (as a function of loading level) formed upon selective excitation of trans-anethole. The through-bond distances of electron migration estimated for LiY, NaY, KY, RbY and CsY were 12.1, 15.3, 7.9, 6.7 and 5.9 A, respectively. The decreased migration in LiY, was attributed to the stability of the photoejected electron on the LiY framework in addition to hydration of the electron, while the lack of electron migration in zeolites KY, RbY and CsY was found to be due to back electron transfer being competitive with electron migration. These results indicate that NaY is the best zeolite in which to facilitate charge separation by electron migration.
The second research project aimed to fully characterize the behaviour of methyl viologen (MV2+) in the series of alkali metal cation-exchanged Y zeolites. Laser flash photolysis of MV2+, in the MY zeolites produced the methyl viologen radical cation (MV+·) in addition to two absorption bands at 440 and 500 nm. The formation and reactivity of the transient species responsible for these signals were found to be highly dependent on the zeolite in which the reaction was carried out. Notably, the doubly reduced MV0 was formed in CsY, which is the first known example of a zeolite acting as a two-electron donor. The new transient species were thought to be due to the products resulting from the reaction of MV +· and MV0 with the hydroxyl radicals and protons formed upon scavenging of the photogenerated hole by water.
The PET reaction between dianisylmethane (DAM) and p-chloranil (Chi) in NaY was studied as the third research project. In addition to the formation of the radical ions, substantial amounts of the dianisylmethyl carbocation (An2CH+) were observed. (Abstract shortened by UMI.)
Thesis (Ph.D.)--Dalhousie University (Canada), 2006.
In the first research project, the distance of electron migration in zeolite Y was examined as a function of alkali metal cation. This process was probed by monitoring the yield of the 1,4-dicyanobenzene radical anion (as a function of loading level) formed upon selective excitation of trans-anethole. The through-bond distances of electron migration estimated for LiY, NaY, KY, RbY and CsY were 12.1, 15.3, 7.9, 6.7 and 5.9 A, respectively. The decreased migration in LiY, was attributed to the stability of the photoejected electron on the LiY framework in addition to hydration of the electron, while the lack of electron migration in zeolites KY, RbY and CsY was found to be due to back electron transfer being competitive with electron migration. These results indicate that NaY is the best zeolite in which to facilitate charge separation by electron migration.
The second research project aimed to fully characterize the behaviour of methyl viologen (MV2+) in the series of alkali metal cation-exchanged Y zeolites. Laser flash photolysis of MV2+, in the MY zeolites produced the methyl viologen radical cation (MV+·) in addition to two absorption bands at 440 and 500 nm. The formation and reactivity of the transient species responsible for these signals were found to be highly dependent on the zeolite in which the reaction was carried out. Notably, the doubly reduced MV0 was formed in CsY, which is the first known example of a zeolite acting as a two-electron donor. The new transient species were thought to be due to the products resulting from the reaction of MV +· and MV0 with the hydroxyl radicals and protons formed upon scavenging of the photogenerated hole by water.
The PET reaction between dianisylmethane (DAM) and p-chloranil (Chi) in NaY was studied as the third research project. In addition to the formation of the radical ions, substantial amounts of the dianisylmethyl carbocation (An2CH+) were observed. (Abstract shortened by UMI.)
Thesis (Ph.D.)--Dalhousie University (Canada), 2006.
Keywords
Chemistry, Physical.