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Significant advances have been made in the design and characterization of molecular assemblies, which, when photolyzed, undergo intramolecular electron or energy transfer. Time-resolved resonance Raman spectroscopy has been successfully applied to the study of excited states and molecular assemblies, but time-resolved infrared spectroscopy is particularly well-suited for complexes containing ligands such as CO or CN. Unlike the transient Raman experiment, infrared spectroscopy does not rely on resonance enhancement in the excited state since the metal-CO and -CN stretching vibrations have high oscillator strengths, providing high sensitivity. We describe here a novel application of the technique to the elucidation of intramolecular energy transfer in the ligand-bridged complex [(phen)(CO)[sub 3]Re[sup I](NC) Ru[sup II](CN) (bpy)[sub 2]][sup +] (phen is 1, 10-phenanthroline; bpy is 2,2'-bipyridine). These results show that the transient infrared technique can be a powerful tool for the elucidation of oxidation state and mechanisms involving photochemical intermediates by direct observation of ligand vibrations. 15 refs., 2 figs.