A chromophore is a group of atoms and electrons in organic compounds which communicate with light to produce color. Chromophores exist within the molecule as a short conjugated system or a chain of atoms linked by alternating single and double bonds.
Witt hypothesized in 1876 that dye molecules are divided into two groups: chromophores and auxochromes. The chromophore is a collection of atoms that determines the color of the dye. Witt proposed at the time that the auxochrome was a salt-forming component that contributed to the dye's color improvement.
A conjugated pi-system can be formed by three or more neighboring p-orbitals in a molecule, much as two adjacent p-orbitals make a pi-bond. In a conjugated pi-system, electrons can capture specific photons by resonating over a specific distance of p-orbitals, much like a radio antenna detects photons along its length. Typically, the more conjugated (longer) the pi-system, the greater the photon wavelength that can be collected. In other words, as we add neighboring double bonds to a molecule diagram, we can predict that the system will become increasingly yellow to our eyes since it absorbs less yellow light and more red light. "Conjugated systems of fewer than eight conjugated double bonds absorb only in the ultraviolet region and are colorless to the human eye", "Compounds that are blue or green typically do not rely on conjugated double bonds alone."
Chromophores are often studied at the fundamental material level using nuclear magnetic resonance (NMR), UV-Vis spectroscopy, and Fourier transform infrared (FT-IR). UV-Vis spectroscopy is a potent method for determining the chromophore within normal telecom operation wavelengths, as well as the extinction ratio and solvochromic parameter. FT-IR is utilized to quantify intrinsic absorption at near-IR wavelengths in addition to regular structure characterisation. Differential scanning calorimetry (DSC) can measure both the glass transition temperature and the melting point. DSC and thermal gravimetric analysis (TGA) can both quantify chromophore breakdown.
If isomers are present, high pressure liquid chromatography (HPLC) provides the percentage of each isomer as well as the purity of the chromophores. With TGA, certain volatiles that are not detectable with HPLC can be quantified.
Chromophores are simply imines or Schiff bases formed in high yield by reacting primary amines with p-dimethylaminobenzaldehyde and p-dimethylaminocinnamaldehyde under mild conditions and in the presence of unprotected hydroxyl groups. Addition of a drop of trifluoroacetic acid to the UV or CD cell converts them into protonated Schiff bases, which exhibit intense and very sharp absorptions in the red due to their cyanine dye structure; moreover, addition of a small amount of water will hydrolyse the protonated Schiff base bond, leading to recovery of the starting amine. The derivative of acosamine is prepared by derivatizing the amino group and then the hydroxyl group with the respective chromophores . The exciton couplet is intense with an amplitude of 110. Acidification shifts the couplet to longer wavelength because of the shift of the imine band from 360 to 460 nm; however, the intensity decreases owing to the diminished overlap between the chromophores, from 330/360 nm in the neutral form to 330/460 nm in the protonated derivative. If the coupling is between two protonated Schiff base moieties, i.e. between two cyanine dyes, then the couplet becomes well-separated.An auxochrome is a functional group of atoms that is joined to a chromophore and changes the wavelength or strength of the chromophore's absorption of light.