Chemistry at the glass transition: Fluorescence-detected proton transfer reactions

Many liquids when cooled to cryogenic temperatures have the ability to take the form of a glassy substance; these are referred to as amorphous solids. As they are supercooled they do not form crystalline substances but rather an amorphous glass lacking in long-range structure. While the knowledge of these amorphous solids has been around for many years not much is known regarding the simplest physical and chemical processes that are allowed to occur within this glassy state. We introduce a new fluorescence-based temperature derivative spectroscopy methodology with the aim of discerning some of these physical and chemical attributes such as proton transfer in the glassy state. Proton transfer in a cryogenic sample is accomplished by exploiting the photolytic capabilities of o-nitrobenzaldehyde. When exposed to ultraviolet light, o-nitrobenzaldehyde is transformed to o-nitrosobenzoic acid which has a relatively large acid dissociation constant. Fluorescein, a simple fluorescent molecule, and o-nitrobenzaldehyde were dissolved in a glycerol/water mixture and cryogenically supercooled below the glass transition temperature. The sample was then exposed to ultraviolet light and measured fluorometrically for alterations in the spectra. This experiment was repeated at varying viscosities and with deuterated solvents for measuring isotopic effects. The spectra were fit to first-order rate kinetic Arrhenius style equations to determine the energy barriers associated with the proton transfer. Currently there is no documented use of temperature derivative spectroscopy using fluorometric based measurements to study dynamic processes and little if any information regarding simple chemical processes occurring within a supercooled glass sample. This research provides a more detailed picture of these processes as well as describe a new methodology for temperature derivative spectroscopic experiments