Q5(e). Decay routes of an electronically excited molecule: Jablonski diagram
Discuss all the possible decay routes of an electronically excited molecule with the help of a neat Jablonski diagram.
Answer in brief
The excited molecule relaxes to the lowest vibrational level of S₁ (vibrational relaxation, internal conversion) and then returns to S₀ by fluorescence or internal conversion, or crosses to T₁ (intersystem crossing), from which it decays by phosphorescence or non-radiative crossing; at any stage it may be quenched or react.
Key points
- spin is unchanged, and the molecule usually reaches an upper vibrational level (Franck–Condon).
- collisions remove the extra vibrational energy as heat within about 10⁻¹² s, down to v=0 of the electronic state.
- radiationless passage between states of the same multiplicity (S₂ → S₁ very fast; S₁ → S₀ slower). This is why emission comes almost always from v=0 of S₁ (Kasha's rule).
- S₁ → S₀ with emission, spin allowed, lifetime about 10⁻⁹ to 10⁻⁷ s. It lies at longer wavelength than the absorption (Stokes shift) and stops at once when the light is cut off.
The full model answer
Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.
The diagram: singlet states S₀, S₁, S₂ on the left and the lowest triplet T₁ (below S₁, Hund's rule) on the right, each with its vibrational levels. Straight arrows: light absorbed or emitted; wavy arrows: radiationless steps.
- Absorption (S₀ → S₁, S₂; about 10⁻¹⁵ s): spin is unchanged, and the molecule usually reaches an upper vibrational level (Franck–Condon).
- Vibrational relaxation (VR): collisions remove the extra vibrational energy as heat within about 10⁻¹² s, down to v=0 of the electronic state.
- Internal conversion (IC): radiationless passage between states of the same multiplicity (S₂ → S₁ very fast; S₁ → S₀ slower). This is why emission comes almost always from v=0 of S₁ (Kasha's rule).
- Fluorescence: S₁ → S₀ with emission, spin allowed, lifetime about 10⁻⁹ to 10⁻⁷ s. It lies at longer wavelength than the absorption (Stokes shift) and stops at once when the light is cut off.
- Intersystem crossing (ISC): S₁ → T₁, a radiationless spin flip made possible by spin–orbit coupling; favoured by heavy atoms and by a small S₁–T₁ gap.
- Phosphorescence: T₁ → S₀ with emission, spin forbidden, so slow (10⁻³ s to seconds) and persisting after the light is cut off; at longer wavelength than fluorescence. It competes with radiationless T₁ → S₀ ISC, so it is seen mainly in rigid media or at low temperature.
- Quenching and photochemistry: the long-lived T₁ (and S₁) may transfer energy to another molecule (quenching, photosensitisation; O₂ is an efficient quencher), transfer an electron, or react (dissociation, isomerisation).
Quantum yield: φ_F=k_F/(k_F+k_(IC)+k_(ISC)+k_Q[Q]); each route takes a share of the excited molecules according to its rate constant.
Application. fluorescence spectroscopy (quinine in tonic water, fluorescein dyes) and phosphorescent materials (ZnS, glow-in-the-dark paint) rest on these routes; photosensitised singlet-oxygen formation through T₁ is used in photodynamic therapy.
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All parts of Question 5
- 5(a) Degeneracy of the hydrogenic level with E=-R_H/3610 marks
- 5(b) van der Waals constants of ethylene from T_c and p_c10 marks
- 5(c) Half-life of two parallel first-order reactions10 marks
- 5(d) Signs of Δ H, Δ S and Δ G for spontaneous adsorption10 marks
- 5(e) Decay routes of an electronically excited molecule: Jablonski diagram10 marks