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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper I covers physical and inorganic chemistry.

UPSC Civil Services (Mains), Chemistry optional · 2017 · Paper I

Q5(b). Intersystem crossing on the Jablonski diagram

  • 10 marks
  • Answered
Question

What is ISC (Inter System Crossing) for an excited molecule in singlet state? Answer using Jablonski diagram.

Answer in brief

ISC is a radiationless, isoenergetic transition between electronic states of different spin multiplicity, usually S₁→T₁. The molecule then relaxes to the bottom of T₁ and may phosphoresce.

Key points

  • electronic states S₀, S₁ (singlets, paired spins) and T₁ (triplet, parallel spins; lower than S₁ by Hund's rule), each with its vibrational levels. Straight arrows: radiative steps; wavy arrows: radiationless steps.
  • After absorption (S₀→S₁, spin allowed) the molecule loses vibrational energy to the solvent (vibrational relaxation) and reaches the lowest level of S₁.
  • from S₁ the molecule passes horizontally into an excited vibrational level of T₁ of the same energy. No photon is emitted; the spin of one electron is reversed.
  • the change of spin is forbidden in pure spin states. Spin–orbit coupling mixes singlet and triplet character and makes ISC possible. It is faster in molecules with heavy atoms (Br, I, metal ions: the heavy-atom effect) and…

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

  1. Jablonski diagram: electronic states S₀, S₁ (singlets, paired spins) and T₁ (triplet, parallel spins; lower than S₁ by Hund's rule), each with its vibrational levels. Straight arrows: radiative steps; wavy arrows: radiationless steps.
  2. After absorption (S₀→S₁, spin allowed) the molecule loses vibrational energy to the solvent (vibrational relaxation) and reaches the lowest level of S₁.
  3. Intersystem crossing: from S₁ the molecule passes horizontally into an excited vibrational level of T₁ of the same energy. No photon is emitted; the spin of one electron is reversed.
  4. Why it can happen: the change of spin is forbidden in pure spin states. Spin–orbit coupling mixes singlet and triplet character and makes ISC possible. It is faster in molecules with heavy atoms (Br, I, metal ions: the heavy-atom effect) and when S₁ and T₁ lie close in energy.
  5. What follows: vibrational relaxation to the bottom of T₁; then phosphorescence (T₁→S₀, spin forbidden, slow, long-lived) or another ISC to S₀ without light.
  6. Competition: ISC competes with fluorescence and internal conversion from S₁. Efficient ISC weakens fluorescence and feeds the triplet state.

Application. the long-lived triplet formed by ISC drives photosensitisation (benzophenone, singlet-oxygen generation) and photochemical reactions; strong ISC in Eu³⁺ and Tb³⁺ complexes underlies their bright luminescence.

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Model answer to Q5(b), 2017 Paper I: solution page 20
Solution page 20 of 38
Read the whole 2017 solution2017 question paper

All parts of Question 5