Adding lights and taking away colours
Your screen makes colour by adding light. Red, green and blue lights together make white. This is additive mixing.
Paints, inks and dyes work the other way. They start with white light and take colours away. This is subtractive mixing. Its main colours are cyan, magenta and yellow. Mix all three and almost all light is taken away, so you get a dark colour.
Complementary colours
Two colours are complementary if they make white light when added. Pairs: blue and yellow, red and cyan, green and magenta.
A dye that takes out blue leaves red and green, which add up to yellow. So a dye that absorbs blue looks yellow. The colour you see is the complement of the colour absorbed.
The HOMO-LUMO gap
The electrons in a molecule sit in energy levels. HOMO is the highest level that has electrons. LUMO is the lowest level that is empty. The distance between them is the HOMO-LUMO gap.
A photon of light is absorbed only if its energy is equal to the gap. The electron jumps up. Energy of a photon: E = h × c / λ. A big gap needs short-wavelength light (UV, so the molecule looks colourless). A small gap needs longer-wavelength visible light, so the molecule looks coloured.
Chains of double bonds: polyenes
A polyene is a chain with alternating single and double bonds (a conjugated chain). Its electrons spread over the whole chain. The longer the chain, the smaller the gap and the longer the wavelength absorbed. Short chains absorb only UV. About eight or more double bonds push the absorption into the visible range, which is why carotene is orange.
Azo and triphenylmethane dyes
Azo dyes have the group –N=N– joining two ring systems. The rings plus this bridge make one large conjugated system, so light is absorbed. Many yellow, orange and red dyes are azo dyes. Adding groups such as –NH2 or –OH to the rings changes the gap and so the colour.
Triphenylmethane dyes have one central carbon joined to three benzene rings. The charge spreads over all three rings. Examples are the green dye malachite green and the purple crystal violet.
Dyes versus pigments
A dye dissolves in a liquid (or soaks into fibres) and colours it. A pigment does not dissolve. It is a powder of tiny solid grains that is mixed in a binder, like paint, and stays on the surface. Pigments are usually stronger against sunlight and washing; dyes are often brighter and easier to apply to cloth.
Key formulas and definitions
- Photon energy: E = h × c / λ
- Absorbed only if photon energy = HOMO-LUMO gap
- Longer conjugated chain → smaller gap → longer λ absorbed
- Colour seen = complement of colour absorbed
- Key terms: HOMO, LUMO, conjugation, chromophore, azo group (–N=N–), dye, pigment
Worked examples
1. A dye absorbs blue light. What colour does it look?
Blue and yellow are complementary. So the dye looks yellow.
2. A solution looks green. Which colour does it absorb most?
The complement of green is magenta (a red-violet). So it absorbs magenta-region light, mostly red and blue-violet.
3. A molecule has a HOMO-LUMO gap of 3.1 eV. Find the wavelength absorbed. (h c = 1240 eV nm)
λ = 1240 / 3.1 = 400 nm. That is the edge of violet light, so the molecule absorbs violet and looks pale yellow.
4. Molecule X absorbs at 300 nm, molecule Y at 500 nm. Which has the smaller gap?
E = hc/λ, so a longer wavelength means a smaller energy. Y (500 nm) has the smaller gap.
5. Why is a compound with only two double bonds colourless?
Its chain is short, so the gap is large. It absorbs UV, which we cannot see. All visible light passes, so it looks colourless.
Common mistakes
- Saying a blue dye absorbs blue. A blue-looking dye absorbs the complement (orange-yellow) and passes blue.
- Mixing up additive mixing (lights) and subtractive mixing (paints, dyes).
- Thinking a bigger gap means a longer wavelength. Bigger gap means shorter wavelength.
- Calling every coloured substance a dye. Pigments do not dissolve.