Chromophore


Chromophore

A chromophore is the part of a molecule responsible for its color. [1] The color arises when a molecule absorbs certain wavelengths of visible light and transmits or reflects others. The chromophore is a region in the molecule where the energy difference between two different molecular orbitals falls within the range of the visible spectrum. Visible light that hits the chromophore can thus be absorbed by exciting an electron from its ground state into an excited state.

In biology, molecules that serve to capture or detect light energy, the chromophore is the moiety that causes a conformational change of the molecule when hit by light.

Chemical structure of beta-carotene. The eleven conjugated double bonds that form the chromophore of the molecule are highlighted in red.

Contents

Types of chromophore

Conjugated chromophore that straightens in response to a photon γ (light), of the correct wavelength: 11-cis-retinal becomes all-trans-retinal

Chromophores almost always arise in one of two forms: conjugated pi systems (also known as resonating systems) and metal complexes.

Conjugated pi-bond system chromophores

In the conjugated chromophores, the electrons jump between energy levels that are extended pi orbitals, created by a series of alternating single and double bonds, often in aromatic systems. Common examples include retinal (used in the eye to detect light), various food colorings, fabric dyes (azo compounds), pH indicators, lycopene, β-carotene, and anthocyanins. Various factors in a chromophore's structure go into determining at what wavelength region in a spectrum the chromophore will absorb. Lengthening or extending a conjugated system with more unsaturated (multiple) bonds in a molecule will tend to shift absorption to longer wavelengths. Woodward-Fieser rules can be used to approximate ultraviolet-visible maximum absorption wavelength in organic compounds with conjugated pi-bond systems.

Metal complex chromophores

The metal complex chromophores arise from the splitting of d-orbitals by binding of a transition metal to ligands. Examples of such chromophores can be seen in chlorophyll (used by plants for photosynthesis), hemoglobin, hemocyanin, and colorful minerals such as malachite and amethyst.

A common motif in biochemistry is chromophores consisting of four pyrrole rings. These come in two types:

Auxochrome

An auxochrome is a functional group of atoms attached to a chromophore which modifies the ability of the chromophore to absorb light, altering the wavelength or intensity of the absorption.

Halochromism in chromophores

Halochromism occurs when a substance changes color as the pH changes. This is a property of pH indicators, whose molecular structure changes upon certain changes in the surrounding pH. This change in structure affects a chromophore in the pH indicator molecule. For example, phenolphthalein is a pH indicator whose structure changes as pH changes as shown in the following table:

Structure Phenolphthalein-low-pH-2D-skeletal.svg Phenolphthalein-mid-pH-2D-skeletal.svg
pH 0−8.2 8.2−12.0
Conditions acidic or near-neutral basic
Color name
colorless
pink to fuchsia
Color

In a pH range of about 0-8, the molecule has three aromatic rings all bonded to a tetrahedral sp3 hybridized carbon atom in the middle which does not make the π-bonding in the aromatic rings conjugate. Because of their limited extent, the aromatic rings only absorb light in the ultraviolet region, and so the compound appears colorless in the 0-8 pH range. However as the pH increases beyond 8.2, that central carbon becomes part of a double bond becoming sp2 hybridized and leaving a p orbital to overlap with the π-bonding in the rings. This makes the three rings conjugate together to form an extended chromophore absorbing longer wavelength visible light to show a fuchsia color.[2] At pH ranges outside 0-12, other molecular structure changes result in other color changes; see Phenolphthalein for details.

See also

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References


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Look at other dictionaries:

  • chromophore — ● chromophore adjectif Se dit d un groupe fonctionnel qui apporte la couleur dans un composé organique. ⇒CHROMATOPHORE, CHROMOPHORE, adj. et subst. masc. I. Adj. Qui porte la couleur. Tissu chromatophore (Roussy ds Nouv. Traité Méd., fasc. 5,… …   Encyclopédie Universelle

  • Chromophore —   [k ], Singular Chromophor der, s, chromophore Gruppen, Farbstofftheorie: charakteristisch farbgebende Atomgruppen organischer Farbstoffe, die die Ausbildung mesomerer Zustände der π Elektronen begünstigen, dadurch die …   Universal-Lexikon

  • Chromophore — Chro mo*phore, n. [Gr. ? color + fe rein to bear.] (Chem.) Any chemical group or residue (as {NO2}; {N2}; or {O2}) which imparts some decided color to the compound of which it is an ingredient. [1913 Webster] …   The Collaborative International Dictionary of English

  • chromophore — [krō′mə fôr΄] n. [ CHROMO + PHORE] any chemical group, as the azo group, that produces color in a compound and unites with certain other groups to form dyes chromophoric [krō′məfôr′ik] adj …   English World dictionary

  • Chromophore — Buntstifte Als Chromophor (griech. Farbträger) bezeichnet man den Teil eines Farbstoffs, der für das prinzipielle Vorhandensein der Farbigkeit sorgt. Dass ein Farbstoff bei einem Beobachter einen Farbreiz hervorruft, also bunt erscheint, beruht… …   Deutsch Wikipedia

  • chromophore — chromoforas statusas T sritis fizika atitikmenys: angl. chromophore vok. Chromophor, m rus. хромофор, m pranc. chromophore, m …   Fizikos terminų žodynas

  • Chromophore — Un chromophore est un groupement d atomes comportant une ou plusieurs doubles liaisons, et formant avec le reste de la molécule un séquence de doubles liaisons conjuguées, c est à dire une alternance de doubles et de simples liaisons. L existence …   Wikipédia en Français

  • chromophore — chromoforas statusas T sritis chemija apibrėžtis Gamtinis pigmentas. atitikmenys: angl. chromophore rus. хромофор …   Chemijos terminų aiškinamasis žodynas

  • chromophore — chromoforas statusas T sritis chemija apibrėžtis Grupė, suteikianti organiniam junginiui spalvą. atitikmenys: angl. chromophore rus. хромофор …   Chemijos terminų aiškinamasis žodynas

  • chromophore — noun Etymology: International Scientific Vocabulary Date: 1879 a chemical group (as an azo group) that absorbs light at a specific frequency and so imparts color to a molecule; also a colored chemical compound • chromophoric adjective …   New Collegiate Dictionary


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