UVVisAbsorption

UV-Vis absorption refers to the absorption of ultraviolet and visible light by chemical species, resulting from electronic transitions within the molecule. Compounds with π-conjugated systems or nonbonding electrons can absorb photons in this spectral region, promoting electrons from bonding or lone-pair orbitals to higher-energy antibonding orbitals. The resulting absorption bands depend on molecular structure, solvent, and environment, and spectra are typically shown as absorbance versus wavelength.

The common spectral window is about 200 to 800 nanometers, with UV covering roughly 200–400 nm and visible 400–700 nm. The position and shape of absorption bands are influenced by conjugation length, substituents, and the electronic nature of the chromophore. A longer conjugated system generally leads to a bathochromic (red) shift, while electron-donating or withdrawing groups modify band intensities and positions.

Quantitative information is often obtained using the Beer-Lambert law: A = εbc, where A is the measured absorbance, ε is the molar extinction coefficient at a given wavelength, c is the solute concentration, and l is the optical path length. Transmittance is related by T = 10^(−A). Instrumentation typically includes a light source, a monochromator, a sample holder, and a detector; modern spectrophotometers may record full spectra with a photodiode array.

Applications include concentration determination, monitoring chemical reactions, and characterizing chromophores in molecules. Common examples are protein quantification around 280 nm and nucleic acid quantification around 260 nm, with purity often assessed by the A260/A280 ratio.