Introduction to Fluorescence

Luminescences the emission of photons from electronically excited states.. Luminescence is divided into two types, depending upon the nature of the ground and the excited states. In a singlet excited state, the electron in the higher-energy orbital has the opposite spin orientation as the second electron in the lower orbital. These two electrons are said to be paired. In a triplet state these elelectrons are unpaired, that is, their spins have the same orientation. Return to the ground state from an excited singlet state does not require an electron to change its spin orientation. A change in spin orientation is needed for a triplet state to return to the singlet ground state. Fluorescence is the emission which results from the return to the lower orbital of the paired electron. Such transitions are quantum mechanically “allowed” and the emissive rates are typically near 108 sec-1. These high emissive rates result in fluorescence lifetimes near 10-8 sec or 10 nsec. The lifetime is the average period of time a fluorophore remains in the excited state. Phosphorescence is the emission which results from transition between states of different multiplicity, generally a triplet excited state returning to a singlet ground state. Such transitions are not allowed and the emissive rates are slow. Typical phosphorescent lifetimes range from milliseconds to seconds, depending primarily upon the importance of deactivation Processes other than emission. Throughout this book we will be concerned primarily with the more rapid phenomenon of fluorescence.KeywordsExcited StateFluorescence LifetimeFluorescence Emission SpectrumFluorescence AnisotropyRotational DiffusionThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.