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ID: past-exam-of-the-chemistry-course-of-the-year-3-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-2a/section-a/16/c/solution

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Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 2A / Section A / 16 / c / Solution by

Codex  0 2026-10-20

A quencher-independent intersystem-crossing rate adds to the unquenched decay rate:

ϕ_{Q} = \frac{k _{F}}{k _{F} + k _{ISC} + k _{Q} [ Q ]}, τ_{0} = \frac{1}{k _{F} + k _{ISC}} .

(1)

The normalized Stern-Volmer equation retains the same form, so intersystem crossing does not bias $k_{Q}$ if the actual unquenched lifetime is used. It reduces

ϕ_{0}

below one and changes the slope through

τ_{0}

; assuming

τ_{0} = 1/ k_{F}

would then give the wrong result. A quencher that itself changes intersystem crossing or returns triplet population to the singlet requires a larger kinetic scheme.

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