Photochemistry: Quantum Yield and the Jablonski Diagram

The two basic laws

• Grotthuss–Draper law: only light that is absorbed can produce chemical change. Light passing through unabsorbed does nothing, however intense.
• Stark–Einstein law: each molecule that reacts absorbs one quantum. This holds for the primary photochemical act only.

The second law is regularly misunderstood, and the misunderstanding is what quantum yield exposes.

Quantum yield

Equally, a quantum yield well below one indicates that most excited molecules return to the ground state without reacting — by fluorescence, by collision, or by internal conversion. So Φ is really a measure of which decay route dominates.

The Jablonski diagram

The diagram stacks electronic states vertically, with singlet states on one side and triplet states on the other, and vibrational levels within each. The routes between them are what matters.

Why phosphorescence is slow

The transition from triplet to ground singlet requires a change of spin multiplicity, which is formally forbidden. It happens only because spin–orbit coupling mixes the states slightly, so the rate is low and the emission persists after the light source is removed.

That single fact explains every difference between fluorescence and phosphorescence: lifetime, intensity, and why phosphorescence is often only observed at low temperature or in rigid media where competing collisional deactivation is suppressed.

Why emission is at longer wavelength than absorption

After absorption the molecule relaxes vibrationally to the lowest vibrational level of the excited state before emitting. That relaxation loses energy as heat, so the emitted photon carries less energy than the absorbed one. The gap is the Stokes shift, and explaining it through vibrational relaxation is a standard short answer.

Photosensitisation

Where a molecule cannot usefully absorb the available light, a sensitiser can absorb instead and transfer its energy. The sensitiser returns to its ground state and the acceptor becomes excited without ever absorbing a photon itself.

This is how triplet states are commonly populated in the laboratory, since direct singlet-to-triplet absorption is forbidden. It is also the mechanism behind several photochemical syntheses, and behind photodynamic processes in biology.

The Franck–Condon principle

Electronic transitions are so much faster than nuclear motion that the nuclei are effectively stationary during absorption. The transition is therefore drawn as a vertical line on a potential energy diagram, and it terminates on whichever vibrational level of the excited state has the greatest overlap with the starting one.

This determines the intensity distribution across the vibrational structure of an electronic band — the reason such bands have shape rather than being single lines.

Frequently asked questions

Can quantum yield exceed one?

Yes, whenever secondary thermal reactions follow the primary photochemical step. Chain reactions give very large values, and this does not conflict with the Stark–Einstein law.

Why is fluorescence faster than phosphorescence?

Fluorescence is spin-allowed; phosphorescence requires a formally forbidden change of multiplicity, so it proceeds far more slowly.

What is the difference between internal conversion and intersystem crossing?

Internal conversion is between states of the same spin multiplicity; intersystem crossing changes it. Both are non-radiative.

Why does phosphorescence often require low temperature?

Because the long-lived triplet state is easily deactivated by collisions. Freezing or a rigid matrix suppresses that, letting emission compete.