Incessant fluorescence – how do glow sticks work?

As anyone who has ever been to an underwhelming club night will know, glow sticks can sometimes be the most exciting thing in a room. I’m writing this post so that you can then make everybody leave that room when you start telling them fun chemistry facts about how glow sticks work.

Having never really thought much about glow sticks before, this was the extent of my knowledge: You break a glass tube inside a plastic tube, two chemicals mix, it glows for 6-8 hours. As a so-called chemist, this is pretty dismal. What chemicals? Why does it glow? And why are children allowed to play with these? Surely if it’s glowing there must be some pretty intense toxic chemicals or there’s some kind of radiation, which as a kid’s toy seems a bit insane at best.

As usual, this is just me being dramatic. Glow sticks contain hydrogen peroxide in the glass vial, which is broken to create a reaction with a mixture containing a dye and an oxalate ester such as diphenyl oxalate. The dye has to be a fluorophore – a chemical that will emit light when excited.

The reaction is called chemiluminescence, and this is how it goes:

The diphenyl oxalate reacts with the hydrogen peroxide and is oxidised. (Hydrogen peroxide is a fantastic oxidising agent and generally an exciting guy to have in the lab).

This creates a phenol byproduct – a skin irritant, which is why you shouldn’t break open a glow stick and wipe the glowing liquid on yourself (sorry) – and a peroxyacid ester; in this case 1,2-dioxetanedione, or the square one if you’re looking at my lovely reaction scheme:

 

 Here is a fun reaction scheme to illustrate what happens when you crack your glow stick (courtesy of Wikipedia – I honestly can’t thank you enough for my degree)

1,2-dioxetanedione (the square one) is unstable and will then decompose, forming carbon dioxide but most importantly releasing energy, which excites the electrons in the dye. The dye doesn’t participate in the reaction, it is only there to get excited like a true party animal. It does this by absorbing the energy released from the reaction, promoting its electrons to a higher energy state. The excited dye then relaxes its electrons back into their ground energy state by releasing photons, hv, of visible light. Different dyes produce photons at different wavelengths, causing different colours. You can also have mixes of dyes to produce colours such as orange.

The first reaction, between hydrogen peroxide and diphenyl oxalate, is an exergonic (energy-releasing) reaction, but behaves like an endothermic reaction when the temperature is changed. This means that applying heat will speed up the rate of reaction, and cooling it down will slow the reaction. So a glow stick in a bowl of hot water will shine slightly brighter and remain lit for longer, whereas if you put a glow stick in the freezer it will be dimmer but shine for much longer.

Here’s a bonus fun fact (you can ignore this one if you like, it may only be fun for me): the decomposition of 1,2-dioxetanedione (the square one), a reverse 2+2 photocycloaddition, is a forbidden transition according to the Woodward-Hoffmann rules, and can’t proceed through a typical thermal mechanism. This is the reason that light is produced instead of heat during the reaction.

And if you’re bored of chemistry words, a website called Compound Interest has made this graphic to help you:

The chemistry of glow stick colours

So next time you crack out a glow stick, hopefully I have ruined it for you forever with your newfound chemistry knowledge about what goes on inside.

 

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One thought on “Incessant fluorescence – how do glow sticks work?

  1. You have most definitely NOT ruined glow sticks for me. I’m more excited about them then ever, now that I know what’s going on inside. I’m sort of surprised (and also not too surprised) that something as ordinary as hydrogen peroxide is involved.

    Like

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