Conjugation and Colors

Have you wondered why carrots are orange, and eggplants are purple? And how do some transparent sunglasses become dark under direct sunlight? The answers to these questions pertain to one concept: conjugation.

Conjugation is a phenomenon associated with the overlapping of electronic orbitals. You might recall that everything is made of ultra-small particles, called atoms. Negatively charged electron(s) surrounding atoms counterbalance the positive charge of atomic cores. Due to electrostatic attraction, electrons are usually tethered to one atom core, and their movements are confined within the electronic orbitals of the atom (e.g., the six p electronic orbitals shown in Figure 1, left). However, when the electron orbitals of adjacent atoms overlap with each other, electrons are no longer confined around one atom but are free to move along the overlapped electronic orbitals of all the atoms joined together. This delocalization of electrons is termed conjugation.

Conjugation usually happens where double bond(s) and single bond(s) alternate. The six carbon atoms of benzene represent a typical example of a conjugation network (Figure 1). Under suitable conditions, e.g., when molecules having long conjugation length or their electron delocalization is extensive, colors will be produced. [Note: We need quantum mechanics to explain the fundamental mechanism for conjugation-borne colors, which is beyond the scope of this post.]

Figure 1. Conjugation of the p electronic orbitals of benzene's six carbon atoms. The orange dumbbells are isolated p electronic orbitals before conjugation. The orange rings are delocalized p electronic orbitals after conjugation. Credit: Wikipedia.

Conjugation renders the colors of carrots and eggplants. Specifically, carrots contain an abundant amount of carotene. Carotene is a hydrocarbon with a long aliphatic chain composing of alternating double and single bonds (i.e., conjugation, Figure 2 red). This structure absorbs greenish-blue light, and thus, carotene appears orange. Similarly, the three conjugated benzene rings (Figure 2, purple part) in nasunin, a dye molecule in the skin of eggplants, is purple.

Figure 2. Carotene and nasunin produce the orange and purple colors of carrots and eggplants, respectively. The conjugation networks are colored. The two pictures on the left are from Wiki Commons.

Now let's look at the color-changing mechanism of some sunglasses. The functionality of modern color-changing sunglasses relies on naphthalene derivatives. Figure 3 shows the molecular structures of an example naphthalene derivative under light-off and light-on conditions. The molecule adopts the left structure in the absence of UV light. The three conjugated benzene rings are insufficient to produce visible colors, resulting in transparent glasses to naked eyes. When placed directly under sunlight, the oxygen-containing ring opens due to solar energy (mostly UV light). Now the conjugation network extends to all the benzene rings (Figure 3, red part), and the number of the conjugated atoms is enough to darken the glasses. Because the energy of the two molecules is close, the light-induced conversion is readily reversible, resulting in the color-changing characteristic.

Figure 3. UV-light-induced change in the conjugation length leads to the color variation of some sunglasses. The conjugation networks are colored in red. The chemical structures are adapted from the 33^rd Chinese Chemistry Olympiad (Preliminary Test), Chinese Chemical Society.

Last but not least, conjugation is a valuable property to chemists. For example, analytical chemists use phenolphthalein to probe the acidity of solutions. This pH indicator, when dissolved in ultra-acidic solutions, has an extensive conjugation system (Figure 4, H₃In⁺) that makes the compound blood red. When acidity decreases, the formation of a five-membered ring breaks the conjugation system and decolors the indicator (Figure 4, H₂In). Further reducing acidity opens the five-membered ring, reforms the conjugation system (Figure 4, In^2-), and renders the indicator pink. When acidity becomes negligible, a hydroxyl group attaches to the central carbon atom, which again interrupts the conjugation network (Figure 4, In^3-) and turns the molecule colorless. By checking the color of phenolphthalein, chemists could estimate pH values (a metric for measuring acidity) of solutions.

Figure 4. The color of phenolphthalein changes with pH, due to the formation and destruction of the conjugated network in red.

Conjugation is one of the most common causes of the colors we encounter in our daily lives. Readers who are interested in conjugation can seek more information in organic chemistry and quantum mechanics textbooks. Now, if you are present the molecular structures of chlorophyll and melanin, can you figure out the color origin of green leaves and black hair, respectively?

Tianyu Liu appreciates Hortense Le Ferrand of Nanyang Technological University (Singapore) for proofreading this post.