What features contribute to fluorescence of molecules?

Boy in awe on looking at glowing fire flies

Fluorescence in natural species must have left you bewildered in earlier days. Bioluminescence refers to luminiscence arising from some living organisms or from some compounds extracted from such organisms. Most common examples of bioluminescence are intermittent glows of fire flies, some species of fish and glow worms.

Apart from living organisms several inorganic minerals and alloys containing metals exhibit fluorescence or phosphorescence. Minerals like gypsum, fluorite and zircon exhibit fluorescence characteristics upon exposure to UV light.

The broad spectrum of compounds that exhibit fluorescence properties makes you wonder what causes fluorescence and what unique features contribute to fluorescence of molecules The article provides a brief coverage on the types of substances that exhibit fluorescence.

Organic Compounds

Fused rings – compounds having several fused rings show fluorescent characteristics. Electron donating groups such as \(NH_2\), OH and \(OCH_3\) enhance fluorescence whereas electron withdrawing groups like COOH, -N=N- tend to reduce it.

Heterocyclic compounds like pyridine and pyrrole do not fluoresce significantly but when fused to an aromatic ring as in case of quinoline become fluorescent.

Conjugated double bond structures in combination with aliphatic or alicyclics exhibit fluorescence

Enzymatic reactions – several bioluminescence processes are promoted by enzymes. Example is enzymatic activity of luciferase which catalyzes the formation of a complex between luciferin and ATP. The resulting complex is oxidized by oxygen to high energy cyclic peroxide which relaxes back to ground state accompanied by emission of light.

Inorganic Compounds

Metallic compounds – Lanthanides such as Ce, Pr, Nd and Uranium containing compounds fluoresce naturally.

Fluorescence in presence of reagents – common elements like Ca, Ga, In, Mg, Sn, Th, Tl, U, Zn, Zr show fluorescence on reaction with specific reagents such as oxime, rhodamine, etc

Complex formation – several metal ions form fluorescent complexes. This feature has been used for quantitative estimation of such ionic species.

Fluorimetric analysis is application of quantitative techniques based on fluorescence properties of natural compounds or induced fluorescence by chemical or biochemical processes. Subsequent articles will feature some of the techniques and their applications.

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