A molecule absorbs a photon of light to form an excited state. How do the redox properties of the excited state compare to those of the ground state?
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Explanation
draftWhen a molecule absorbs a photon, an electron is promoted from a low-energy (originally filled) orbital to a higher-energy (originally empty) orbital.
- Reducing ability: In the ground state, the highest-energy electron available for donation sits in the HOMO. In the excited state, that electron now sits in a much higher orbital (the former LUMO), which is much less tightly bound and much easier to give away. So the excited state is a stronger reducing agent.
- Oxidizing ability: The excited state also leaves behind a vacancy (hole) in the original HOMO, a low-energy empty orbital that can readily accept an incoming electron. Accepting an electron into this low-lying hole is more favorable than accepting one into the ground-state LUMO. So the excited state is also a stronger oxidizing agent.
Thus excitation simultaneously raises an electron to a higher, more easily donated level and creates a low-lying hole that is more easily filled — the excited state is both a better electron donor and a better electron acceptor than the ground state. This is why photoexcited species (e.g., in photoredox catalysis, photosynthesis) can drive redox reactions that the ground-state molecule cannot.
Answer: (A)
Correct: excitation both promotes an electron to a higher, more donatable orbital (stronger reductant) and leaves a low-energy hole that is easier to fill (stronger oxidant).
This assumes excitation stabilizes both the excited electron and the hole, as if the molecule became less reactive overall — but absorbing energy makes the species more reactive in both redox directions, not less.
This reflects only the increased electron-accepting ability of the hole while incorrectly assuming the promoted electron becomes harder to donate (as if it were more stabilized in the higher orbital).
This reflects only the increased electron-donating ability of the promoted electron while incorrectly assuming the hole becomes harder to fill, ignoring that a low-lying vacancy is easier, not harder, to accept an electron into.
USNCO 2024 National Part I exam, question 41. © American Chemical Society; reproduced for non-commercial practice.