Chromophore

A chromophore is the part of a molecule that absorbs light at particular wavelengths. Its electronic structure determines which wavelengths it absorbs and often gives substances their color.

Electronic transition: A change in an electron's energy state, often caused by absorbing or emitting a photon. Absorption occurs when a photon drives electrons between allowed energy states in the chromophore.
Pi bond: A covalent bond formed by sideways overlap of p orbitals, with electron density above and below the bond axis. Pi electrons in many chromophores participate in light-driven electronic transitions.
Organic dye: A colored organic compound used to impart color to materials or solutions. Many dyes achieve strong color through conjugated chromophores that absorb visible light.
Fluorophore: A molecular group that absorbs light and emits fluorescence at a longer wavelength. A fluorophore is defined by its emission behavior, while chromophore emphasizes light absorption.
Conjugated system: A molecular structure with overlapping p orbitals that allows electrons to spread across multiple adjacent atoms. Extended conjugation often lowers transition energies, shifting absorption toward longer wavelengths.
Aromaticity: The unusual stability and electron delocalization of certain cyclic, planar molecules with conjugated pi electrons. Aromatic electron delocalization shapes the absorption spectra of many organic chromophores.
Retinal: A vitamin A-derived molecule that absorbs light and changes shape in visual photoreceptors. Its chromophore isomerizes after absorbing a photon, initiating vision.
Pigment: A substance that selectively absorbs and reflects or transmits light, producing color. A pigment may contain one or more chromophores but describes the colored substance more broadly.
Molecular orbital: A mathematical description of the spatial distribution and energy of electrons in a molecule. Chromophore absorption depends on the energy gap between occupied and unoccupied molecular orbitals.
Absorption spectrum: A plot or record of how strongly a substance absorbs light across different wavelengths or frequencies. It displays the wavelengths at which a chromophore absorbs.

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