Welcome to the electron apartment building! 💁♀️ Would you like to take the elevator to a higher electronic state? Hold on, I’ve got to find a photon! I like to think of FLUORESCENCE as involving electrons moving between floors of a high-rise apartment building, whose floorplan we can represent with a JABLONSKI DIAGRAM.
Molecules are made up of atoms (of elements like carbon (C), hydrogen (H), oxygen (O), and nitrogen (N)) which are made up of smaller pieces called subatomic particles -> positively-charged protons and neutral neutrons hang out together in a dense central nucleus and a cloud of negatively-charged electrons whizz around them.
Electrons can “live” at different electronic and vibrational states. Think of a high-rise apartment building with a floorplan described by something called a Jablonski Diagram. The higher up in the building, the higher the energy. You have to expend energy to “climb up” and when you “fall down” you release energy.
I like to think of this energy as a form of money, and energy transfer as a sort of molecular banking system. There are multiple ways energy money can be transferred but, thanks to laws about conservation of energy, the books must balance.
One way energy can be transferred is as ElectroMagnetic Radiation (EMR) (light). If energy is transferred as light (and by light we don’t just mean visible light, but rather any type of EMR), we call it RADIATIVE TRANSFER. If energy is transferred in another way we call it NONRADIATIVE TRANSFER which involves things like passing off some excess energy as heat (thermal energy) which is absorbed by the surrounding solvent molecules, getting them move a little more)
EMR involves little packets of energy called PHOTONS traveling as waves. I like to think of photons as “coins” of different denominations (e.g. nickels, dimes, quarters) and they travel in waves we can describe in terms of their wavelength (λ) (the peak-to-peak distance), their frequency (# of peaks that will pass through a fixed point in a certain amount of time), or the energy of their photons.
These 3 quantities are all directly linked because all light has to travel at the same speed (the speed of light, c). You can think of it kinda like a little kid walking alongside a basketball player. To keep up, the little kid has to take a lot more steps because the steps it’s taking are shorter. And the poor little guy needs to use a lot of energy to do this, whereas the basketball player, taking longer, more relaxed steps is chill.
Similarly -> higher frequency light requires higher energy photons and corresponds to shorter wavelengths
lower frequency light -> lower energy photons -> longer wavelengths
The EMR spectrum goes from low-energy, long-wavelength things like microwaves, through infrared, into the visible light spectrum (wavelengths of ~380-740 nm). In the visible light spectrum, further increases in energy take you through the rainbow in ROYGBIV order (red, orange, yellow, green, blue, indigo, violet) then you get into things like ultraviolet (UV), x-rays, etc.
A key thing to realize is that all these forms of light are fundamentally the same type of thing, just with different energies. All involve photons traveling as waves and the only thing that makes visible light “special” is that our eyes have adapted to be able to see it. Other animals can see other parts of the spectrum – for example bees can see UV.
We can see colors in the visible spectrum because cells in our eyes have molecules can absorb photons from that light and transfer it into a message to send to the brain.
Such visible light-absorbers are called CHROMOPHORES & what wavelengths of light they absorb depends on the chemical structure of the molecule, especially the housing situation of its electrons, which takes us back to our Jablonski diagram (apartment building floorplan).
The different floors of the apartment are different “electronic states.” The ground floor is the lowest energy, and we call it the GROUND STATE (S0). The carpets of these states (thick lines) are “electronic energy levels” and the distances between them (ceiling heights) are different for different molecules and even within molecules.
Within a floor there are “vibrational levels” (the lighter, more closely-packed lines in the diagram) which are kinda like standing on a dresser versus a table versus the bed versus the floor. You have more energy on the table than on the floor (you can easily fall off the table but not through the floor) and you can climb onto the table if you have a little bit of energy.
An electron at one energy level can absorb a little energy & climb on the table (go to a higher vibrational level), lose a little energy (fall back to the floor) etc and we call these vibrational transitions.
But to get to another floor (a different “electronic state”) you have to “take an elevator” and the elevator operator charges. You can pay the elevator operator with energy money in the form of photons, but he only accepts exact coins (and different buildings and even going between different floors in the same building) have different operators with different coin requirements.
This is because the energy differences between the floors are different and the electrons need to absorb just the right amount of energy to get to the next floor (you can’t get stuck between floors and while you can “stand on a bed” you can’t just “levitate above it”) – but if you are “standing on a bed” you get an energy discount – the ride is slightly cheaper because you’re already “partway there” – and this, as well as the drop-off location, leads to some coinage wiggle-room, with molecules absorbing most at one wavelength (absorbance maxima), but with some absorbing action on either side (so you get a little bell-curvey of an absorption curve).
If the price (and coin) is right, the electron absorbs a photon and uses that energy to “pay the elevator operator” and travel up to the next floor (or even further up if it has enough energy). It might get dropped off on the table or the bed (different vibrational levels) but it will quickly tumble to the floor through non-radiative transfer (vibrational relaxation). It’s still on the upper floor though. This occurs really quickly (in picoseconds)
But eventually it will decide the high life’s not for it and it wants to go back to its original home (the ground state). So it “takes the elevator back down.” It has to give back the energy it had absorbed from the original light.
It can do this non-radiatively, releasing it as heat, etc. or, in the case of FLUORESCENCE, it gives it back as light with a longer wavelength (it used some of the energy falling off the bed, etc.) so the light it gives off will have less energy. The difference in energy between the light absorbed and the light emitted is called the STOKES SHIFT. This generally occurs within nanoseconds
Radiative transfer is usually represented with straight arrows and non-radiative transfer’s usually represented with wavy arrows
So, that’s fluorescence – high-energy light absorbed -> electron excited to higher energy level -> electron falls back to lower energy level, releasing lower-energy light as it does so
We can design molecules to have nice apartment buildings for the things we want to study -> fluorophores that we can use to label “invisible things” we want to see!
If you’ve been following along you might remember another photon analogy I like to use – photons as baseballs thrown with different energies. In fluorescence a catcher catches a ball – which hurts so the catcher has to shake it off little, and then it throws it with lower energy.