Molecular Photophysics of Acridine Yellow Studied by Phosphorescence and Delayed Fluorescence: An Undergraduate Physical Chemistry Experiment

An undergraduate physical chemistry experiment investigating temperature dependent photophysics of acridine yellow is described. The dye molecule exhibits triplet state lifetimes exceeding 150 ms when dissolved in a glassy saccharide host composed of trehalose and glucose. Emission lifetimes are recorded as a function of temperature using camera flash excitation,photomultiplier detection, and a digital scope or analog-to-digital converter requiring only 5 ms resolution. Yellow-green delayed fluorescence is the primary decay route of the triplet state at room temperature, whereas, an orange phosphorescence predominates at lower temperatures. The temperature dependence of the color and duration of delayed emission provide dramatic, tangible evidence of the underlying photophysics. A linear least-squares analysis of the temperature dependence of the excited triplet state lifetime allows both the rate of reverse intersystem crossing to the singlet state and the energy gap between the excited singlet and triplet electronic states to be determined.