Photochemistry Basics: The Jablonski Diagram and Excited-State Pathways
Photochemistry questions usually come down to one diagram. Once you can read a Jablonski diagram fluently, fluorescence, phosphorescence and quenching stop being separate topics to memorise.
Start with the diagram, not the definitions
A Jablonski diagram stacks electronic energy levels — the ground singlet state S0, excited singlet states S1, S2, and an excited triplet state T1 — with vibrational sub-levels drawn as finer lines within each. Every named photophysical process is simply an arrow between two of these levels. Learning the diagram as a map, rather than learning five separate definitions, is what makes the topic fast to answer under exam conditions.
The processes, in the order they actually happen
| Process | Transition | Typical timescale | Spin change? |
|---|---|---|---|
| Absorption | S0 → Sn | ~10−15 s | No |
| Vibrational relaxation | Within a state, to lowest vibrational level | ~10−12 s | No |
| Internal conversion | Sn → S1 (same spin multiplicity) | ~10−11–10−9 s | No |
| Fluorescence | S1 → S0, radiative | ~10−9–10−7 s | No |
| Intersystem crossing | S1 → T1 | ~10−8 s | Yes (spin-forbidden) |
| Phosphorescence | T1 → S0, radiative | ~10−3 s to seconds | Yes (spin-forbidden) |
Why phosphorescence is so much slower
This single fact answers a large share of exam questions on the topic. Phosphorescence is a transition between states of different spin multiplicity — triplet to singlet — which is formally spin-forbidden. It happens only because spin–orbit coupling mixes a small amount of singlet character into the triplet state, making the transition weakly allowed. That is also why phosphorescence persists after the excitation light is switched off, while fluorescence stops almost immediately: the triplet state is a comparatively long-lived trap that the molecule escapes slowly.
Kasha's rule and why emission always looks similar
Kasha's rule states that emission (fluorescence or phosphorescence) essentially always occurs from the lowest excited state of a given multiplicity, regardless of which higher state absorbed the photon initially. This is a direct consequence of how fast internal conversion and vibrational relaxation are compared with radiative decay — the molecule "falls" to S1 or T1 long before it has a chance to emit from a higher state. It explains why the emission spectrum of a molecule is largely independent of the exact excitation wavelength used, a fact that is frequently tested directly.
Quenching: what actually interrupts the picture
Quenching is any process that returns an excited molecule to the ground state without the characteristic emission, competing with fluorescence or phosphorescence. The two mechanisms worth knowing precisely:
- Collisional (dynamic) quenching — the excited molecule loses energy through collision with a quencher; described by the Stern–Volmer equation, and increasing temperature increases this quenching because collisions become more frequent.
- Static quenching — a non-fluorescent complex forms between the fluorophore and quencher before excitation, so the complexed molecules never emit at all; increasing temperature typically decreases this quenching, because the complex becomes less stable.
The Stokes shift, briefly
Emission always occurs at a longer wavelength (lower energy) than absorption, because of the energy lost to vibrational relaxation before emission happens. The size of this shift is a rough indicator of how much geometric rearrangement the molecule undergoes in the excited state — a small Stokes shift suggests a rigid excited-state geometry close to the ground state, while a large shift suggests significant reorganisation.
FAQs
Why is intersystem crossing spin-forbidden but still observed?
It is forbidden by the simplest selection rule, but spin-orbit coupling — stronger in molecules containing heavier atoms — mixes singlet and triplet character enough to make it weakly allowed. This is why heavy-atom substituents are known to enhance intersystem crossing and phosphorescence.
How do I quickly distinguish a fluorescence question from a phosphorescence question in an exam?
Look for a spin-multiplicity clue or a lifetime clue. Any transition between states of the same multiplicity (singlet to singlet) with a short lifetime is fluorescence; any transition involving the triplet state, or a lifetime described as milliseconds to seconds, is phosphorescence.
Does temperature affect fluorescence quantum yield?
Usually yes, and usually negatively — higher temperature increases non-radiative decay pathways (including collisional quenching), which competes with fluorescence and lowers the observed quantum yield.
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