Chemicals of Color
Here I list some common colored chemicals and their spectral data. The peaks are calculated using the Woodward-Fieser rules listed below.
Empirical Rules for Absorption Wavelengths of Conjugated Systems
| Core Chromophore | Substituent and Influence | ||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
 Transoid Diene 217 nm |
| ||||||||||||||||||||
 Homoannular (Cisoid) Cyclic Diene 253 nm ε = 12,000 - 28,000 | |||||||||||||||||||||
Heteroannular (Transoid) Cyclic Diene 214 nm ε = 5,000 - 15,000 | |||||||||||||||||||||
(i) Each exocyclic double bond adds 5 nm. In the example on the right,
there are two exo-double bond components: one to ring A and the other to ring B.(ii) Solvent effects are minor. (iii) When both types of cyclic diene are present, the one with the longer λ is the base | |||||||||||||||||||||
λmax (calculated) = Base (215 or 260) + Substituent Contributions
For example, we can apply these rules to the compound shown below:
 |
 Heteroannular diene = 214 |
 3 alkyl subs (3x5) = +15 | |
 1 exo C=C = +5 | |
| Total = 234 nm (235 experimental) |
| Core Chromophore | Substituent and Influence | ||||||||||||||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
 R = Alkyl ...215 nm R = H ... 210 nm R = OR' ... 195 nm |
| ||||||||||||||||||||||||||||||||||||||||||||||
 Cyclopentenone ... 202 nm | |||||||||||||||||||||||||||||||||||||||||||||||
 | |||||||||||||||||||||||||||||||||||||||||||||||
|
(i) Each exocyclic double bond adds 5 nm. In the example on the right,
there are two exo-double bond components: one to ring A and the other to ring B. (ii) Homoannular cyclohexadiene component adds +35 nm (ring atoms must be counted separately as substituents) (iii) Solvent Correction: water = +8; chloroform = -1; dioxane = -5; methanol/ethanol = 0; ether = -7; hexane/cyclohexane/hydrocarbon = -11 | |||||||||||||||||||||||||||||||||||||||||||||||
λmax (calculated) = Base + Substituent Contributions and Corrections
A Note on Absorption Peaks
The molar absorptivity (ε) of a particular compound
can be thought of as a measure of how well the substance absorbs a
particular wavelength. It can be calculated from measured data using
Beer's Law: 
, where A is the absorbance, c is the
mol/liter concentration, and 
is the path length of light through
the substance in cm. The molar absorptivity increases with the size
of the chromophore, as well as the probability that light will be
absorbed when striking it. For conjugated systems, ε roughly doubles
with each added conjugated double bond.
The intensity of a chromophore's color is directly correlated with its molar absorptivity: a compound that is a strong absorber of a particular wavelength will reflect almost none of that color, and hence the complement will be particularly intense. If the chromophore has a small ε, then some or a lot of that color will be reflected, and the compound's color will be paler, or more pastel (whiter). Hence, altering a compound's chemical or environmental properties can result in four types of color shifts:
Bathochromic - a shift to longer absorption wavelengths. This can be caused by conjugation or electron donor/acceptor groups.
Hypsochromic - a shift to shorter absorption wavelengths.
Hyperchromic - a shift to greater absorbance. This is caused by increasing conjugation or size of molecule, or increasing probability of excitation (overlap of orbitals, etc.)
Hypochromic - a shift to lower absorbance.
An empirical expression for the molar absorptivity
of a compound is 
,
where P is the probability of a transition, and A is the cross
sectional area of the chromophore in cm2. Also note that due to
rotational and vibrational internal motion of the molecule, the
energy levels of the orbitals are not completely precise, and hence
the spectral peaks are broad, so that not just one but a range of
wavelengths are subtracted from the incident light upon reflection.
Reds
 | Carminic Acid - An anthraquinone linked to a unit of glucose, this chemical is produced by some scale insects, such as Dactylopius coccus, and is used to dye fabrics a deep red. It is also known as "Natural Red 4". |
 | Lycopene - This carotenoid is found in carrots, watermelons, papayas, and most obviously in tomatoes, giving them their rich red color. It is also used by many plants as an accessory pigment to capture more blue light. λmax = 443, 471, 502 nm in hexane |
 | Heme B - Important for its role in the transfer of gases for organisms with blood, heme is an iron-containing porphyrin of a deep red color, giving blood and meat its distinctive color. |
Oranges
 | β-Carotene - Another carotenoid, b-carotene is commonly found in carrots, as well as in other plants as an accessory pigment. The chemical is a dark orange color naturally. λmax = 455 nm. |
Yellows
 | Crocetin - This dicarboxylic acid carotenoid is found in the flowers of Crocus sativus, or saffron, giving them and the spice derived from them a golden yellow color. |
Greens
 | Chlorophyll a - Perhaps one of the most important chemicals on Earth, there are two types of chlorophyll, and both are modified porphyrins responsible for most of the light capture into the biosphere. Both types absorb mostly in the red and blue wavelength regimes (several absorption peaks), leaving a reflected green responsible for the color of green plants, algae, and many photosynthetic bacteria. |
Blues
 | Indigo -
Derived from the tropical plants of the genus Indigofera,
as well as woad (Isatis tinctoria) in more temperate
climates, this dark blue dye is now commonly synthesized to
color blue jeans and work clothes in the U.S., though is still
used as a native colorant by many people around the world. λmax = 602 nm |
Violets
 | Punicin - Commonly called "Tyrian (Imperial) Purple", this compound is derived from the shells of predatory tropical sea snails of the genus Murex. It was used to create the royal purple dyes used by emperors of Rome and Byzantium, as well as being used for centuries beforehand by other peoples of Asia Minor. |
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