Suppose someone gives you the chemical formula of a substance, such as

|image0| and asks you the color this substance is expected to have. Is it possible to give an answer? In most cases, you may have an educated guess, but an accurate prediction is far from trivial: the color of a substance is decided at various levels, from the basic molecular level up to the macroscopic structure.

The first level: the molecule by itself

The most "trivial" level is the molecule by itself, and it is decided by the elements it is made of, its geometric structure (the position of the atoms), and their charges. These parameters have a key impact on how its electrons are distributed in space and how this distribution changes when light enters the scene, a phenomenon which is strongly related to light adsorption and thus to color.

When it comes to perception of visible light, white light is a mixture of all the wavelengths of electromagnetic radiation from ~700 nanometers to ~400 nanometers. These wavelengths are perceived by our eyes (and brains) as colors, with the longer value of 700 nanometers being almost infrared and the shorter 400 nanometers being almost ultraviolet.

|EM spectrum|

In a more simplified rewording, white light is a mixture of all the colors of the rainbow, spanning from red to violet passing through yellow, green, blue etc. as beautifully shown by this prism


When you send some white light on the molecule you basically provide all the colors. The electronic setup of the molecule is such that it "prefers" specific light wavelengths (hence, specific light colors), and this preference results in an adsorption. This is due to the light promoting an "electronic transition" between a ground state and an excited state: electronic distribution is rearranged due to the interaction between electrons and the electromagnetic radiation. A simplified vision of this event is the electron "jumping" to a higher, excited level, but in reality, it is the electronic cloud that changes.

[caption id="" align="aligncenter" width="200" caption="Transition to a ground (E1) to an excited state (E2) due to adsorption of an incoming photon of light (hv)"]`|image3| <>`_[/caption]

The accumulated energy is then "quenched" (dispersed) as heat. As a consequence, the molecule removes some colors from the white light, leaving others unscathed, and the resulting color we see is the complementary one. If the molecule absorb blue, you get red. If it absorb yellow, you get violet. Absorption in general is not an "all-or-nothing". The intensity of absorption at each wavelength depends on many factors, producing what is called an absorption spectrum, which is unique and characteristic of every molecule or atom. The color of the substance is the complementary result of this spectrum. The uniqueness of the spectrum allows us to infer the composition of our Sun, of distant stars and planets, through what were commonly known as Fraunhofer lines

[caption id="" align="aligncenter" width="500" caption="Fraunhofer lines are absorption lines of atoms in the Sun atmosphere. Some absorption is also performed by Earth atmosphere. They act as fingerprints for a given atomic species."]`|image4| <>`_[/caption]

Electronic transitions, however, are not the only responsible for absorbing light. A molecule can also absorb light by excitation of rotations and vibrations (meaning that the molecule spins faster, or vibrates more). One case is water. Water appears as transparent, but in reality it's slightly blue. The reason is that some wavelengths in the red make it vibrate more (to be exact, water absorbs in the infrared, which would not make a difference to our eyes, but this absorption has a so-called "overtone" which is in the visible red). As a result, a minimal amount of red is subtracted from white light and water ends up being slightly blue.

Can we predict this information? Yes, we totally can, with relatively good, but not perfect accuracy. There are many different programs capable of obtaining this information: the wavelengths where absorptions occur, vibrations, and other parameters that are important to decide the final spectrum. For atoms and small molecules, accuracy is very good, but as the molecule size increases, predictions require larger and larger computing power. For this reasons, quantum chemistry method developers daily create new smart approximations, able to deliver a very accurate result for a reduced computational cost. In any case, the required input is just the geometric position (xyz coordinates) of the atoms, their atomic numbers and masses, and the net charge.

The second level: molecular interactions and reactions

Molecules are generally not alone. They can come close, and eventually have other molecules around, either of the same species, or of other species, such as those of a solvent: from simple water, alcohol or acetone, to complex cell environment. There are no reactions involved, just the proximity of other molecules, with their protons and electrons. These partners alters the electronic setup of the molecule, promoting a slight variation of the electronic and vibrational behavior. Absorption, and thus the color, is consequently changed. In general, this change is a shift of the original spectrum either towards higher wavelengths (bathochromic shift) or shorter ones (hypsochromic shift).

Then you have anything that can change the structure of the molecule through chemical reaction. Take tea, put some lemon into it, and its color becomes lighter. The reason is that with lemon you are increasing the acidity of the water, which means a higher concentration of charged hydrogen ions (H+). The higher concentration of hydrogen ions push Thearubigins, a class of colored substances found in fermented tea, into a form with the ion attached, which creates a change in the molecular electronic distribution, which in turns changes the absorption and thus the color. For some substances, this effect can be dramatic: from blue to red, from transparent to purple, from yellow to blue. These are the so-called pH indicators

[caption id="" align="aligncenter" width="250" caption="Bromothymol blue at acid, neutral and alcaline solution (left to right)."]`|image5| <>`_[/caption]

These effects may technically be predictable, but they require to consider a complex system of interacting species, with different chemical exchanges, short and long range interactions of charges and so on. This may be very difficult, if not impossible to perform with today's methods and computational power, although approximations exist to work around the heavy computational weight and provide reasonable results.

The third level: crystal and impurities

A crystal is a solid where the constituent molecules or ions are disposed in space with a well defined order. For any given substance, the ordering of its atoms or molecules in space is not necessarily unique, a phenomenon known as polymorphism. Depending on the packing, different properties arise, and different colors are the result. Diamond is transparent, graphite is black, and black is also C60 fullerene, but they are all made of the same element: carbon.`|Diamond and graphite| <>`_

For another example, take gold. You may say that it has gold color, but if you take a small cluster (say, 100 atoms) of gold, what you see is red, not gold-colored.

When you have atoms or molecules ordered in a crystalline structure, the result can absorb light by virtue of this ordered structure. Note that this effect is complementary to the initial absorption characteristics of the molecule or atom taken by itself. For example, one single atom of carbon may absorb close to nothing in the visible, but due to the highly ordered crystalline structure, the macroscopic block of graphite you hold in your hands absorbs light, most of it, and thus is black. A similar effect occurs with any pigment having crystalline structures influencing the color. At the quantum level, the effect just presented is related to band structure and Bloch wavefunctions. The same facts also explains semiconductors and conductivity of metals.

These effects are relatively predictable. A large number of computational software deals with periodic structures in a very efficient way, providing spectroscopic information about the properties of both atomic and molecular crystals.

As an additional twist, crystals can have defects, such as imperfect packing or impurities of foreign elements into the periodic structure. The resulting effect is beautifully shown in diamonds, for example in the Aurora Pyramid of Hope


and in Aluminium oxide: pure, it is colorless. Add some chromium, iron vanadium and titanium and it may become ruby


or sapphire, which is blue, pink, yellow, orange, purple or green, depending on the crystal structure, and the relative quantities of these impurities.


These effects are generally very hard to compute, as they may require statistically large ensembles of atoms. I am not aware of any computational techniques on this regard.

The fourth level: macroscopic properties

Finally you have how the substance is structured at the macroscopic level. Take a smooth platinum electrode: it is platinum color. Make it sponge-like (by making very tiny bubbles and pits) to increase the surface area and it appears black as coal. The reason is that light is scattered and absorbed completely, leading to a black color.

This opens to many additional effects concerning matter-light interaction. What is the color of a CD ? Is it silver ? Is it "rainbow" ? What about the color of a oil slick on the road in a rainy day ? What about the color of a Tiger's eye, or of an opal


And what about blue eyes, and the blue color of a spoon of flour dispersed in water ? Both are due to Tyndall scattering. There is no blue pigment in blue eyes, nor in flour, but the scattering of light is frequency dependent, reflecting blue and transmitting red, leading to a blue color.


As you see, color is a very particular property, and while you may have an educated guess from quantum mechanics techniques, it's not always easy to infer the color of a substance. This is just the tip of the iceberg. You have many other phenomena (such as how much light penetrates into the substance, or which macroscopic imperfections are present) which affects both the color and the reflective properties of a substance. Ice is transparent, but if it's full of bubbles it is white. Plastic looks like plastic, and metal looks like metal, depending on how light is scattered and absorbed, which then changes the way it is reflected back to the viewer. In addition, this does not only affects color, but also the general material texture.

What about the opening molecule ?

The opening molecule is Indigo, a natural dye found in some plants. Today, it is synthetically produced in large quantities. It is commonly used to dye blue jeans.