Elizabeth Mader - University of Ottawa

Placement: Dalhousie University
Supervisor: Dr. Fran Cozens

Time-Resolved Diffuse Reflectance Investigations of Electron and Hole Transport in NaY Zeolites

Zeolites are crystalline aluminosilicates constructed of molecular sized pores and cavities. They make excellent host materials and catalysts that provide a unique and versatile media for conducting a variety of chemical reactions. For instance, studies of electron transfer reactions within zeolites demonstrate the impressive ability of these materials to influence the lifetime and behaviour of reactive intermediates, modifying the resultant chemistry from that observed in homogeneous solution. It is well known that these solids can act as electron acceptors and the role of zeolites as electron donors has recently begun to receive attention. In addition, electron transfer between zeolite encapsulated donors and acceptors is also of significant interest, particularly in terms of decreasing the rate constants for energy wasting back electron transfer. There has been little discussion, however, of the mechanisms of these reactions and the possibility for long distance charge migration mediated by the zeolite. The goal of this research project is to explore intrazeolite charge transport in order to address some of these issues.

Specifically, this study probes the mechanistic differences between electron transport and hole transport using a system based on a common donor and two chemically similar acceptors with modified chromophores. Nanosecond lasers are used to selectively excite either the donor or the acceptor that has been included in the zeolite along with a non-absorbing redox partner. The generation and decay behaviour of the resulting reactive intermediates are then monitored by time resolved diffuse reflectance. Electron transport is initiated by photoionization of the zeolite encapsulated donor, creating a radical cation, a spectroscopically visible reaction intermediate. The electron, liberated from the donor molecule, is associated with the zeolite framework and is often complexed with the charge balancing cations. Trapping of this electron by a co-incorporated ground state acceptor generates a radical anion as a second spectroscopically visible reaction intermediate. Conversely, hole transport can be initiated by a reaction in which the zeolite donates an electron to a photoexcited acceptor molecule, creating a "hole" in the zeolite framework as well as the corresponding radical anion. Transport of this hole through the zeolite and trapping by a co-incorporated donor yields the donor radical cation.

The system currently being explored employs trans-stilbene as the donor and cyano-substituted aromatics, dicyanobenzene and dicyanoanthracene as the acceptors. Electron transport involves selectively exciting stilbene with 308 nm laser light in NaY containing dicyanobenzene, while hole transport experiments utilize 355 nm laser light to selectively excite dicyanoanthracene in NaY containing stilbene. To date, both electron and hole transport have been observed with this system. The decay kinetics and the yields of radical ions are being studied in order to understand the reaction dynamics and effect of intermolecular distance on electron and hole transport.