Photochemistry and Photophysics in the Laboratory. Showing the Role of Radiationless and Radiative Decay of Excited States

Factors involved in the competition between radiative and radiationless (nonradiative) decay of excited molecules are demonstrated by irradiating solutions containing structurally similar molecules with ultraviolet or visible light. Radiationless decay leads to loss of excitation energy as heat through the transformation of vibrational to translational energy. Differences in the temperature are observed before and after irradiation of solutions containing the compounds. The higher temperatures are related to more efficient nonradiative decay relative to fluorescence as the dissipation channel and involve internal conversion or intersystem crossing, with subsequent vibrational relaxation. The various sets of experiments are chosen to relate specific structural properties of the molecules with this preferred deactivation pathway.