Pre-lab 7.1.b Introduction to Luminescence

After excitation by visible or UV radiation, molecules relax back to the ground state by dissipating the excitation energy through molecular vibrations and collisions with solvent molecules. This process is called nonradiative decay and is typically a very fast set of processes for molecules in fluid solution at room temperature. Alas, this process, which is simply the conversion of photonic energy to heat, is rarely useful. 

Ground state Singlet excited state Triplet excited state 

Quantum Efficiency Of Luminescence The intensity of luminescence can be expressed as the quantum efficiency ( t ), which is the ratio of the amount of light emitted to the amount of light absorbed. It can also be expressed as the ratio of the rate of luminescence to the sum of all rates that deactivate the excited state. 

Luminescence Sensing Some luminescent compounds can serve as probes of analytes such as metal ions. Upon coordination with a metal ion, the luminescence of a ligand may be enhanced, quenched, or shifted in color (the wavelength of maximum intensity). In addition, changes in the solution conditions (pH, ionic strength, temperature or the presence of macromolecular hosts capable of forming organized structures) often affect luminescence properties. 

Two features of luminescence make it a powerful method for analytical detection of metal ions. First is sensitivity fluorimetric reagents have been developed for the detection of even nanomolar metal ion concentrations. Indeed, even single molecule fluorescence is possible. 

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