The Jablonski Diagram

What the diagram shows: states, not orbitals

The vertical axis of a Jablonski diagram is energy; there is no horizontal distance coordinate. Thick horizontal lines mark electronic states and thin lines above each mark that state's vibrational sublevels. States are grouped by spin multiplicity: singlets (labeled S₀, S₁, S₂…) on the left, where the two electrons involved have paired, antiparallel spins, and triplets (T₁, T₂…) on the right, where the promoted electron's spin has flipped to become parallel with its former partner.

A crucial rule of thumb: for a given electron configuration the triplet lies lower in energy than the corresponding singlet (a consequence of Hund's rule and reduced electron–electron repulsion when spins are parallel). That is why T₁ sits below S₁ on the chart, and why phosphorescence is always red-shifted relative to fluorescence from the same molecule.

Vertical straight arrows denote radiative processes (photon absorbed or emitted); wavy or curved arrows denote non-radiative processes where energy is dissipated as heat or transferred between states of the same or different spin.

Absorption and the femtosecond climb up

Everything begins at S₀, the ground state, with the molecule sitting in its lowest vibrational level. When a photon of the right energy arrives, an electron is promoted to a higher singlet (S₁, S₂, or higher). Absorption obeys the Franck–Condon principle: electronic excitation is so fast (~10⁻¹⁵ s) that the nuclei do not move during the jump, so the transition is drawn as a strictly vertical arrow. The molecule usually lands in a vibrationally excited sublevel of the upper state.

Because the nuclear geometry of S₀ and S₁ differ, the most probable transition is rarely to the lowest vibrational level of S₁—this vibrational structure is exactly what gives absorption bands their shape and is quantified by the Beer–Lambert law. Spin is conserved during absorption, so direct S₀ → T₁ absorption is essentially forbidden and extremely weak.

Coming back down: the competing pathways

Once excited, the molecule races down through several channels, and their relative rates decide what you observe:

• Vibrational relaxation (VR): within ~10⁻¹² s the molecule sheds excess vibrational energy to solvent, dropping to the lowest vibrational level of whatever electronic state it occupies.
• Internal conversion (IC): a radiationless jump between states of the same spin (e.g. S₂ → S₁). IC between upper singlets is extremely fast, which underlies Kasha's rule: emission almost always occurs from the lowest excited state (S₁ or T₁), regardless of which state you originally excited.
• Fluorescence: the radiative S₁ → S₀ transition, spin-allowed and therefore fast (nanoseconds). Because energy was lost to VR/IC first, the emitted photon is lower in energy than the absorbed one—the Stokes shift.
• Intersystem crossing (ISC): a spin-forbidden radiationless jump S₁ → T₁. It becomes efficient when spin–orbit coupling is strong.
• Phosphorescence: the radiative T₁ → S₀ transition. Because it requires a forbidden spin flip, it is slow (microseconds to seconds) and easily out-competed by non-radiative decay unless the molecule is cold or rigid.

Spin-orbit coupling and the heavy-atom effect

Both intersystem crossing and phosphorescence are formally spin-forbidden, yet they clearly happen. The loophole is spin–orbit coupling: the magnetic interaction between an electron's spin and its orbital motion mixes a little singlet character into the triplet, making the flip weakly allowed. This coupling scales steeply with atomic number (roughly as Z⁴), so heavy atoms dramatically boost ISC and phosphorescence—the heavy-atom effect.

This is why bromine- and iodine-substituted dyes, and especially iridium(III) and platinum(II) complexes, are potent triplet emitters: Ir(ppy)₃ reaches ISC efficiencies near unity and phosphoresces with high quantum yield at room temperature. It is also why dissolved molecular oxygen (itself a ground-state triplet) quenches both fluorescence and phosphorescence: it opens fast triplet–triplet energy-transfer channels, often producing reactive singlet oxygen.

Quantum yield, lifetime, and reading the numbers

The Jablonski diagram is qualitative, but it maps directly onto two measurable quantities. The fluorescence quantum yield Φ_F is the fraction of absorbed photons re-emitted as fluorescence: Φ_F = k_r / (k_r + k_nr), where k_r is the radiative rate and k_nr lumps together IC, ISC, and quenching. The observed lifetime is τ = 1 / (k_r + k_nr).

A worked feel for the numbers: fluorescein in basic solution has Φ_F ≈ 0.9 and τ ≈ 4 ns, so almost every photon it absorbs comes back out as light within a few nanoseconds. By contrast, an efficient triplet emitter may show a phosphorescence lifetime of milliseconds to seconds because k_r for the forbidden T₁ → S₀ step is tiny. Anything that raises k_nr—higher temperature, flexible geometry, a collisional quencher—lowers both Φ and τ, which is precisely how fluorescence sensors and molecular thermometers work.

Why it matters: from microscopy to OLED screens

Almost every luminescence technology is engineered by tuning the pathways on this diagram:

• Fluorescence microscopy and flow cytometry: bright, photostable dyes and fluorescent proteins (GFP) are selected for high Φ_F and short lifetimes; the Stokes shift is what lets filters separate excitation light from the fainter emission.
• OLED displays: because electrical excitation produces singlet and triplet excitons in a 1:3 ratio, harvesting the 75% triplets is essential. Phosphorescent Ir/Pt emitters and, more recently, thermally activated delayed fluorescence (TADF) materials with a tiny S₁–T₁ gap use reverse intersystem crossing to recover that energy.
• Photodynamic therapy: a photosensitizer with high ISC yield populates T₁, transfers energy to O₂, and generates cytotoxic singlet oxygen to kill tumor cells.
• Photoredox catalysis: long-lived triplet excited states of Ru and Ir complexes act as strong single-electron oxidants or reductants, driving reactions that ground-state chemistry cannot.

In every case, chemists are literally choosing which arrow on the Jablonski diagram wins.

Fluorescence vs phosphorescence at a glance

Property	Fluorescence	Phosphorescence
Transition	S₁ → S₀ (spin-allowed)	T₁ → S₀ (spin-forbidden)
Lifetime	~1–100 ns	µs to many seconds
Emission energy	Higher (bluer)	Lower (redder)
After light off	Stops almost instantly	Persists (afterglow)
Requires	Internal conversion + emission	Intersystem crossing to T₁