Chromophores

A chromophore is the part of a molecule responsible for its color. It absorbs light in the visible or ultraviolet range, and the color observed is the complementary color of the absorbed wavelengths. The absorption arises from electronic transitions between molecular orbitals, often involving a pi-conjugated system or, in inorganic compounds, metal-centered or charge-transfer transitions. The precise wavelength absorbed depends on the energy gap between the ground and excited states, which can be altered by the extent of conjugation, substituents that donate or withdraw electrons, and coordination to metals.

In organic chemistry, common chromophores include azo groups, carbonyl-containing systems, nitro groups, and extended polyenes or porphyrin rings. Polyene chains and aromatic systems with alternating double bonds typicaly produce visible colors. Biological pigments such as chlorophyll, carotenoids, and bilin pigments contain chromophores that enable light harvesting. In vision, retinal is a chromophore that undergoes isomerization upon light absorption, triggering signaling in visual proteins. In fluorescent proteins and dyes, specific chromophore structures produce fluorescence through distinct electronic transitions.

In inorganic chemistry, transition metal complexes can be highly colored due to d–d transitions and ligand-to-metal or metal-to-ligand charge-transfer transitions. The color of such complexes is influenced by ligand field strength, geometry, and the nature of ligands.

Auxochromes are substituents that extend conjugation or alter electron density, often causing bathochromic (red) or hypsochromic (blue) shifts and changing absorption intensity. Solvent, pH, and molecular environment can also affect color by modifying the chromophore’s electronic structure.