Characterizing the Behavior and Properties of an Excited Electronic State: Electron-Transfer Mediated Quenching of Fluorescence

We have designed a series of laboratory exercises to illustrate a range of key concepts traditionally encountered in a general course on physical chemistry. In these exercises, optical spectroscopic measurements, electrochemical measurements, kinetic measurements, and computer modeling are used to characterize the properties and behavior of an electronic excited state. Specifically, students are asked to use these tools to answer a question posed at the outset: "Upon interaction with a series of molecules, Q, are the fluorescent states of 9-cyano- and 9,10-dicyanoanthracene quenched via an electron transfer process?" In the laboratory, students experimentally quantify rate constants, k_q (exp), for cyanoanthracene excited state removal by the molecules Q in a Stern–Volmer study of fluorescence quenching. In other experiments, students collect information that allows them to independently calculate the rate constants, k_q (calc), for these same molecules Q that would be expected if quenching proceeds via an electron transfer process. Upon comparison of these respective k_q values in a Rehm–Weller plot of both k_q (exp) and k_q (calc) against the Gibbs energy of electron transfer, the students ascertain that the quenching reaction studied indeed proceeds via electron transfer.