Light from a reaction

Section 1 of 4

Snap a glow stick and it lights up for hours, cool in your hand. A reaction inside is giving out its energy as light.

A reaction that makes strong new bonds gives out energy, as when methane burns. Usually that energy becomes heat: the new molecules fly apart fast and warm everything they bump into. In a chemiluminescent reaction, part of the energy comes out as light, and the mixture stays cool.

The light comes from an electron. You have seen that a molecule absorbs light when the light's energy matches a jump one of its electrons can make. The jump works in reverse too: a lifted electron falls back down and gives out its energy as light, in the colour that matches the size of the drop. A molecule with a lifted electron is called excited. Every glow in this lesson comes from an excited molecule, and each kind of glow lifts the electron in its own way.

Inside a glow stick

A glow stick is a plastic tube with a thin glass vial inside. The vial holds hydrogen peroxide, H₂O₂, with its two oxygen atoms joined by a weak single bond. Around the vial is a solution of a dye and an oxalate ester: two C=O groups side by side, each joined through an oxygen atom to a benzene ring. Bending the stick snaps the vial, and the two liquids mix.

How a glow stick makes light, step by step Four rows. Row 1: hydrogen peroxide, H-O-O-H, with its weak O-O bond highlighted, meets the oxalate ester when the glass vial snaps. Row 2: a small ring forms, two carbon atoms and two oxygen atoms joined by a weak O-O bond, each carbon also carrying a C=O: 1,2-dioxetanedione. Row 3: the ring splits into two carbon dioxide molecules and a burst of energy. Row 4: the energy passes to a dye molecule of three rings in a row, lifting one of its electrons, and green light comes out as the electron falls back. H O O H 1. The vial snaps peroxide meets the oxalate ester C C O O O O 2. A ring forms 2 C and 4 O, held by a weak O–⁠O bond 3. It splits in two 2 CO₂ and a burst of energy 4. The dye glows its lifted electron falls back
The steps inside a glow stick. The weak O–⁠O bond (amber) ends up in a small, strained ring, which splits into two carbon dioxide molecules and hands its energy to the dye.

Hydrogen peroxide attacks the ester and pushes out its two rings. What is left is a tiny ring of two carbons and two oxygens, called 1,2-⁠dioxetanedione, held shut by an O–⁠O bond. The ring falls apart almost as soon as it forms, into two carbon dioxide molecules. Each carbon now has all four of its bonds to oxygen, the end of the oxidation ladder, and a burst of energy comes out all at once.

The ring breaks open while it touches a dye molecule, and the energy of the split ends up in the dye, lifting one of its electrons. The electron falls back, and the dye gives out light. The colour depends on the dye: 9,10-⁠diphenylanthracene glows blue, rubrene orange-yellow, and 9,10-⁠bis(phenylethynyl)anthracene green. Each dye has a long run of alternating double bonds, like the coloured molecules you have met, and the run sets the size of the drop.

9,10-Bis(phenylethynyl)anthracene as a skeletal formula Three benzene rings joined edge to edge in a row, anthracene, drawn upright in the middle. From the middle ring a C≡C triple bond runs out to each side and ends in a benzene ring. Every double and triple bond is highlighted: they take turns with single bonds from one end of the molecule to the other.
9,10-⁠Bis(phenylethynyl)anthracene, C₃₀H₁₈, the dye in green glow sticks. Its double and triple bonds (highlighted) take turns with single bonds from one end to the other: through the three rings in a row, along the two C≡C triple bonds and into the benzene ring at each end.

Quick check

What decides the colour of a glow stick's light?