Questions: Photochemistry: Excited State Reactions

Question 1

A [2+2] cycloaddition between two alkenes is thermally forbidden but proceeds readily under UV irradiation. What does the photon actually do to enable this reaction?

• A. It provides the activation energy needed to push the reaction over the thermal energy barrier
• B. It changes the orbital occupancy of the excited state, altering the orbital symmetry so the reaction becomes symmetry-allowed
• C. It breaks one of the double bonds, making the alkene more reactive toward addition
• D. It raises the temperature of the molecules locally so they can overcome the thermal barrier

Answer: (B)

The [2+2] cycloaddition is thermally forbidden because the orbital symmetry of the ground-state reactants does not correlate smoothly to products — a symmetry-imposed energy barrier exists. UV light promotes an electron to a higher orbital, changing the electron configuration and therefore the orbital symmetry of the excited state. In the excited state, the symmetry now permits smooth orbital correlation to the cyclobutane product — the reaction is photochemically allowed. Options A and D reflect the common misconception that photons merely supply energy; they miss the key point that orbital symmetry, not just energy, governs reaction accessibility.

Question 2

In the retinal chromophore of rhodopsin (vision), light triggers a cis-to-trans isomerization that has a large thermal barrier. Which description best explains why the excited state undergoes this reaction so readily?

• A. The photon heats the retinal molecule so it can surmount the thermal isomerization barrier
• B. On the excited-state potential energy surface, the barrier for cis-to-trans isomerization is nearly absent — the molecule rolls downhill toward the trans configuration
• C. The excited state has a higher bond order for the C=C bond, making rotation easier
• D. The photon directly breaks the π-bond, allowing free rotation before it reforms

Answer: (B)

Excited-state potential energy surfaces have completely different topography from the ground-state surface. For retinal, the excited-state surface has a minimum near the perpendicular (90°) geometry and slopes downhill toward the trans product — the reaction is nearly barrierless. This is why it occurs in femtoseconds despite being hindered thermally. The photon does not heat the molecule or mechanically break the bond; it promotes the electron to a state where the reaction coordinate is downhill rather than uphill.

Question 3

Providing more photons of higher energy is generally sufficient to make any thermally forbidden reaction proceed photochemically.

• T. True
• F. False

Answer: False

Whether a reaction is photochemically allowed depends on orbital symmetry in the excited state, not just energy input. Some reactions that are thermally forbidden are photochemically allowed (like thermal conrotatory vs. photochemical disrotatory ring closures), but photochemical excitation still must produce an excited state with the right symmetry for the desired product. Additionally, the excited molecule can deactivate via fluorescence, phosphorescence, or non-reactive pathways before reaching the photoproduct. Simply increasing photon energy or intensity does not override these symmetry constraints.

Question 4

The Woodward–Hoffmann rules predict that a thermally forbidden reaction can become photochemically allowed because excitation changes the electron configuration and therefore the orbital symmetry of the reactive species.

• T. True
• F. False

Answer: True

This is the central prediction of Woodward–Hoffmann orbital symmetry conservation. A thermal reaction requires the orbital symmetry of starting material and product to correlate smoothly (be 'symmetry-allowed') on the ground-state surface. When a reaction is symmetry-forbidden thermally, the correlation involves an orbital crossing that creates a high barrier. After photon absorption, a different orbital is occupied, and the symmetry correlation in the excited state may be allowed. This is precisely why electrocyclic reactions (like conjugated diene ring closure) have opposite stereochemical outcomes under thermal vs. photochemical conditions.

Question 5

Why is it more accurate to say that a photon 'changes the rules' of a reaction rather than simply 'provides the energy' needed to overcome a thermal barrier?

Model answer: A photon promotes an electron to a higher-energy orbital, creating an electronically excited state with a different electron configuration. This different configuration changes the orbital symmetry of the molecule — meaning the symmetry relationships that determine which reactions are allowed or forbidden are fundamentally altered. A thermally forbidden reaction has a symmetry-imposed barrier regardless of how much thermal energy is available; adding more heat cannot fix a symmetry mismatch. The excited state accesses a different potential energy surface with different topology, where the same bond changes that were symmetry-forbidden become symmetry-allowed. The photon is not surmounting the same barrier — it is accessing a different reaction pathway entirely.

This distinction matters practically: reactions like [2+2] cycloadditions cannot be made to proceed thermally even at very high temperatures because the orbital symmetry barrier is not a simple kinetic barrier. Only photochemical excitation, which changes the orbital occupancy and hence the symmetry, opens the pathway. Conversely, some photochemical reactions cannot be forced thermally for the same reason.