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General considerations and theoretical background , Nomenclature

المؤلف:  Sue Jickells , Adam Negrusz (Editors)

المصدر:  Clarkes Analytical Forensic Toxicology

الجزء والصفحة:  413

2026-11-10

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General considerations and theoretical background

Nomenclature

A term used in fluorescence is quantum efficiency, described by the quantum yield (i.e. the ratio of the number of molecules that fluoresce to the total number of excited molecules). Highly fluorescent molecules can have quantum efficiencies that approach unity. Some drugs possess rather high quantum efficiencies for fluorescence, such as quinine and lysergide (LSD). All absorbing molecules have the potential to fluoresce. They do so if fluorescent emission occurs at a greater rate than relaxation by non-radiative ways. The kind of relaxation process is highly dependent on the molecular structure. Compounds that contain aromatic rings give the most intense and most useful fluorescence emission. Substitution on an aromatic ring causes shifts in the excitation wavelength spec trum and in fluorescence efficiency. Substituents such as -NH₂, -OH, -OCH₃ and -NHCH₃ groups often enhance fluorescence, while -Cl, -Br, -I, -NO₂ or -COOH are electron withdrawing groups that can lead to a complete reduction of fluorescence (e.g. aniline fluoresces while nitrobenzene does not). The molecular grouping responsible for fluorescence is sometimes described as a fluorophore. Fluorescence is particularly favoured in rigid molecules, as molecular rigidity reduces deactivation by nonradiative processes (fewer internal vibrations). This is also why certain organic chelating agents are more fluorescent when complexed with a metal ion.