Origin of Colors
The process of absorption involves the excitation of the valence electrons in the molecule typically from the low lying level called the Highest Occupied Molecular Orbital (HOMO) into a higher lying state called the the Lowest Unoccupied Molecular Orbital (LUMO). When this HOMO and LUMO transition (Figure (PageIndex{3})) involves the absorption of visible light, the sample is colored.
The HOMO-LUMO energy difference
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[Delta E = E_{HOMO} – E_{LUMO} label{24.5.3A} ]
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depends on the nature of the molecule and can be connected to the wavelength of the light absorbed
[Delta E = h u = dfrac{hc}{lambda} label{24.5.3B} ]
Equation (ref{24.5.3B}) is the most important equation in the field of light-matter interactions (spectroscopy).
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As Example (PageIndex{1}) demonstrated, when white light passes through or is reflected by a colored substance, a characteristic portion of the mixed wavelengths is absorbed. The remaining light will then assume the complementary color to the wavelength(s) absorbed. This relationship is demonstrated by the color wheel shown below. Here, complementary colors are diametrically opposite each other (Figure (PageIndex{5})). Thus, absorption of 420-430 nm light renders a substance yellow, and absorption of 500-520 nm light makes it red. Green is unique in that it can be created by absorption close to 400 nm as well as absorption near 800 nm.
Colors directly opposite each other on the color wheel are said to be complementary colors. Blue and yellow are complementary colors; red and cyan are complementary; and so are green and magenta. Mixing together two complementary colors of light will give you white light. What this all means is that if a particular color is absorbed from white light, what your eye detects by mixing up all the other wavelengths of light is its complementary color. Copper(II) sulfate solution is pale blue (cyan) because it absorbs light in the red region of the spectrum and cyan is the complementary color of red (Table (PageIndex{1})).
Table (PageIndex{1}): The Visible Spectrum Color Wavelength (nm) ΔE HOMO – LUMO gap (eV) UV 100 – 400 12.4 – 3.10 Violet 400 – 425 3.10 – 2.92 Blue 425 – 492 2.92 – 2.52 Green 492 – 575 2.52 – 2.15 Yellow 575 – 585 2.15 – 2.12 Orange 585 – 647 2.12 – 1.92 Red 647 – 700 1.92 – 1.77 Near IR 700 – 10,000 1.77 – 0.12 If the compound absorbs in one region of the spectra, it appears with the opposite (complementary) color, since all of the absorbed color has been removed. For example:
- the material absorbs violet light ⇒ color is yellow
- the material absorbs blue light ⇒ color is orange
- the material absorbs yellow-green light ⇒ color is red-violet
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