A Jablonski diagram maps a molecule's electronic energy levels and the pathways between them once a photon is absorbed.
S0 + hν → S1 or S2 (absorption)
S2 → S1 (internal conversion, IC, sub-ps)
S1 → S0 + hν' (fluorescence, ns)
S1 → T1 (intersystem crossing, ISC, spin-forbidden)
T1 → S0 + hν'' (phosphorescence, μs–s, much slower)
- Excitation λ — photon wavelength; shorter wavelength (higher energy) reaches S2, longer reaches S1 directly.
- Excitation rate — how often new photons are fired at the molecule.
- ISC probability — chance an excited singlet crosses to the long-lived triplet state instead of fluorescing directly.
- Chromophore preset — dyes favour fast fluorescence (low ISC), phosphors and heavy-atom photosensitizers favour ISC to the triplet.
Real-world relevance: this exact triplet pathway is what makes glow-in-the-dark pigments keep emitting after the light is off, and what photosensitizers exploit in photodynamic cancer therapy to generate reactive singlet oxygen from O2 via triplet energy transfer.