Why are my eyes all aglow? Chemistry! And it’s pretty *dog*gone cool! It involves atoms of hydrogen peroxide interacting with atoms of a biphenyl oxalate to produce energy that excites dye molecules to make them glow, but it’s more fun to think of it in terms of dogs, and I like dogs a lot, so why not?
Molecules are made up of atoms of different elements (things like carbon (C), oxygen (O), & hydrogen (H). You can think of atoms as dog walkers walking lots of dogs pulling in all directions. Each “dog walker” is an atomic nucleus, which is made up of neutral neutrons and positive protons. Elements are defined by how many protons they have (e.g. C has 6, O has 8 & H has 2). It’s the positive charge of those protons that gives the walker the strength to hold onto the “dogs” which are negatively charged electrons that whizz about.
The most “caffeinated” dogs are furthest away from the walker and they’re pulling hard on the leash – who want’s to be constrained, right? These outermost, highest-energy electrons are called valence electrons. They’re the “least loyal” because they don’t feel as much of the walker’s affection (it’s harder to feel the positive charge from far away, especially when there are other electrons in between “shielding” them from it). So these dogs are more easily “distracted” and reactive.
We call the ability to hold in electrons electronegativity, and different dog walkers have different pulling strengths which has to do with how many protons they have (protons provide pulling power), how many dogs they have to pull (with great power comes great responsibility), and how far away the furthest dogs are (dogs in between “steal the love” and shield the outermost dogs from the pull)
So, as you add protons (which you do as you go left to right (then down and left to right like reading a book) you add pulling power but at the same time you need more electrons if you want to be neutral. If the dogs you have to add can still stay close, they’re easy enough to control, so the walker’s still in charge.
But, there are only so many dogs that can get super-close and, like a theater filling up, if you have to go to the “next row” it’s harder to see & hear even if the volume gets turned up a little bit. For atoms, these “next rows” are higher energy levels and they increase as you go to the next row of the table.
So, say you have a dog walker yelling I LOVE YOU! The dogs near it will feel the love and stay loyal. If there aren’t that many dogs, they can all huddle near & hear it and feel loved. But if there are so many dogs that some dogs have to hang out really far away they can barely hear the walker. And if they’re next to another walker who’s yelling I LOVE YOU really loudly, they might overhear that walker & go check things out, find a nice dog butt to sniff, and stay.
If they drag along their walker (and the dogs it’s walking) with them, the walkers join up in a covalent bond. In a “true” covalent bond, it’s like each walker still has a hold on a dog, but those dogs are sniffing each other really strongly so the molecules are “glued together” These bonds are strong because each molecule has “equal stakes” in it. They both benefit from the bond because the other walker helps “lighten their load” while they’re still able to have an “ideal” number of dogs to walk. (but their movement is more constrained because they have to stay in step with the other walker, so you have a decrease of movability – a decrease in entropy).
If they break the leash a couple things can happen…
If a dog was in a pair and it breaks free, it leaves behind a pairless pup – we call such a molecule with an unpaired electron a radical and they’re often really reactive.
If the dog wasn’t paired to begin with, but it breaks free, it doesn’t leave a radical, but it can leave a charge that can lead to an attraction. In an ionic bond, a dog breaks free to sniff another dog & the walker of that other dog adopts it as its own. But the walker that used to own the dog *positively* misses it. So it hangs around with the other walker.
A neutral molecule’s only neutral if it has an even number of protons & electrons. So, when a neutral molecule loses an electron it becomes positive. And when a neutral molecule gains an electron it becomes negative. But then you have a negative and a positive and opposites attract so, they hang out. And this is what we call an ionic bond. It’s not really a “bond” in the true sense because no electrons are actually shared, they just hang out based on charge attraction.
The glow stick reactions start with dogs of different walkers interacting, but the actual glow part comes from an dogs doing stuff but staying with their walker. Luminescence is kinda like giving a dog a caffeine shot. It gets excited, pulls hard on the leash, but doesn’t have enough energy to escape. It can only tug on the leash for so long before it gets exhausted – the caffeine rush wears off, it gives up, and releases that extra energy. And it releases it in the form of light.
If the energy that excited it (the “caffeine”) was light, we call it fluorescence, but if the energy came from a chemical reaction we call it chemiluminescence and that’s what’s going on here. You have a series of reactions that don’t produce light. The first reaction doesn’t even produce energy to make light. Instead the first reaction makes something less stable, the second reaction makes that unstable thing even less stable, and that less less stable thing is so unstable that it breaks down, finally producing the energy needed to excite the dye that then “unexcites” and produces light!
Light is just little packets of energy (photons) traveling in waves. Different colors of light have packets with different amounts of energy. The higher the energy, the shorter the wavelength & the higher the frequency. White light is made up of wavelengths of all the colors we can see. Things can look colored because the absorb some of those colors, stealing a slice of the rainbow (RGYBIV doesn’t look like ROYGBIV). Or they can look colored because they’re actually strengthening one of the slices of the rainbow – actually releasing light, which is the case here.
So let’s look closer at what’s going on..
The stick doesn’t glow until you snap a little vial inside of it. That vial holds hydrogen peroxide (H2O2). And when you break it, you put it in contact with the rest of the stuff in the vial, usually a diphenyl oxalate (such as bis-(2,4,6-trichlorophenyl) oxalate)(TCPO) and a dye.
A peroxide is a molecule that has 2 oxygen (O) atoms connected with a single bond. The O’s are also each attached to something else. In the simplest peroxide, hydrogen peroxide is the simplest peroxide those something elses are just hydrogens (H).
Oxygen keeps a really tight leash on its electrons because it has lots of affection to give (towards the right of the periodic table) & its dogs stay close (towards the top of the periodic table). So, instead of being happy to have some of their load lightened, it’s more like they’re desperately trying to pull their sniffing dogs apart, but they don’t have the energy to do so.
But since this bond’s weakened by the opposite pulling, they need a lot less energy to break than a “normal” bond. If H2O2 gets some energy (from heat, light, etc.) they can break down (decompose) into water (H2O) & oxygen gas (O2) (lots more on this in yesterday’s post). But this happens really slowly unless you provide energy. Which is why H2O2 (stored in a brown bottle) can last quite a while.
But that’s not the only way H2O2 can react. What about those H-O bonds? Those bonds are really weak because it’s a really unfair match – O is way more electronegative than H so the electron provided by H spends more time with the owner of the dog it’s sniffing. When a molecule deprotonates it loses an H+. H only had 1 proton and 1 electron to begin with so, as the charge indicates, it leaves electronless, so we can say it lost a proton. the walker leaves without its dog – it decides it’ll have better luck finding a loyaler dog elsewhere.
When it leaves, the O now has a – charge because it has 1 more electron than proton. It wants that electron, but without the “help” from the H in keeping it under control, the O has kinda “bitten off more than it can chew.” So it becomes nucleophilic. It seeks out a new nucleus to join up with that can provide some positive charge to help out.
Its best hopes are finding an atom that is attached to an electronegative atom – the electronegative atom pulls some of the electron density away from that atom so that atom will be slightly positive. Perfect!
And it finds one of these partly positive atoms in the phenyl oxalate. There’s a carbon attached double bonded to 1 oxygen and single bonded to another. So it latches on (the H2O2 oxidizes the biphenyl oxalate). But then the carbon has too much to hold so it releases the singly-bonded oxygen (with the ring it’s attached to). But then the same thing happens with the peroxide that’s been attached, so it circularizes kicking off the other ring and forming a 1,2-dioxetanedione.
And, if you though H2O2 was unstable, check out this thing – there’s a reason you don’t see 4-sided rings very often – talk about awkward! It’s like a 6-legged race where some people have to walk backwards. Eek. And if that weren’t bad enough, it has one of those O-O single bonds – Oh oh no!
It’s really unstable, so it decomposes into carbon dioxide and lets off energy in the process. Which the dye molecule takes and then releases as light. What color light? That depends on what dye’s in there.
Different molecules absorb and emit different wavelengths of light, and do understand why, think of one of those adjustable-length leashes, where you can pull it out and lock it. You can think of energy levels kinda like leashes locked at different lengths. An electron can only absorb energy that gets it “precisely” to the next available length. And the “available lengths” are different for different molecules. So different molecules absorb different wavelengths of light (remember wavelength is directly (inversely) related to energy – shorter wavelength = higher energy).
Because the dye molecules keep returning to their initial state, they can keep emitting light. If they’re given energy that is – and their energy source is NOT regenerating itself. So the glow loses its glow