Photochemical Upconversion: A Physical or Inorganic Chemistry Experiment for Undergraduates Using a Conventional Fluorimeter

Photochemical upconversion is a regenerative process that transforms lower-energy photons into higher-energy light through two sequential bimolecular reactions, triplet sensitization of an appropriate acceptor followed by singlet fluorescence producing triplet-triplet annihilation derived from two energized acceptors. This laboratory directly investigates this phenomenon using the facile photo-induced triplet-triplet energy-transfer reaction between the benchmark inorganic chromophore tris(2,2'-bipyridyl)ruthenium(II), [Ru(bpy)(3)](2+) and 9,10-diphenylanthracene (DPA). Selective green excitation of [Ru(bpy)(3)](2+) in the presence of DPA results in two observations, the (dynamic) quenching of the [Ru(bpy)(3)](2+) photoluminescence in the red and the production of upconverted DPA singlet fluorescence in the blue. The quadratic nature of the incident light power dependence for the latter process is easily realized by using the percent transmission characteristics of neutral density filters placed in the excitation beam prior to sample illumination. In a single laboratory period, students are able to directly visualize the upconverted fluorescence generated and gain experience in various aspects of fluorescence spectroscopy, triplet energy-transfer processes, quenching, and reaction molecularity.