Key Photochemical Reactions

Why This Matters

Photochemistry sits at the intersection of quantum mechanics, organic reactivity, and real-world applications—from how your DNA repairs itself to how chemists build complex drug molecules. You're being tested on your understanding of excited state behavior, energy transfer mechanisms, and reaction selectivity. The key isn't just knowing that light makes things react; it's understanding why certain molecules absorb at specific wavelengths, how excited states lead to bond breaking or forming, and what controls the products you get.

These reactions demonstrate fundamental principles: the Woodward-Hoffmann rules for cycloadditions, radical chemistry from homolytic cleavage, and electron transfer in redox processes. Don't just memorize reaction names—know what type of excited state is involved (singlet vs. triplet), what bonds break or form, and how each reaction connects to broader concepts like frontier molecular orbital theory and spin conservation. When you can explain the mechanism, the memorization takes care of itself.

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Bond Cleavage Reactions

These reactions involve the breaking of bonds in excited-state molecules, typically generating reactive radical intermediates. The absorption of a photon promotes an electron to an antibonding orbital, weakening specific bonds and enabling homolytic cleavage.

Norrish Type I Reaction

• α-Cleavage of carbonyl compounds—the $C-C$ bond adjacent to the carbonyl breaks homolytically in the excited state
• Generates acyl and alkyl radicals that can recombine, decarbonylate, or abstract hydrogen atoms
• Occurs from both $S_1$ and $T_1$ states, making it a versatile route to reactive intermediates in synthesis

Norrish Type II Reaction

• Intramolecular γ-hydrogen abstraction—the excited carbonyl oxygen abstracts a hydrogen from the γ-carbon via a six-membered transition state
• Produces 1,4-biradicals that can cyclize to cyclobutanols or fragment to alkenes and enols
• Competes with Type I depending on molecular structure; γ-hydrogens must be accessible for Type II to dominate

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Cycloaddition Reactions

Light enables cycloadditions that are thermally forbidden by the Woodward-Hoffmann rules. Photochemical $[2+2]$ cycloadditions proceed through excited-state symmetry-allowed pathways, forming strained four-membered rings.

Photodimerization

• Two identical alkenes combine to form cyclobutane derivatives—a classic $[2+2]$ photocycloaddition
• DNA thymine dimers form through this mechanism, causing mutations that require enzymatic repair
• Suprafacial-suprafacial geometry is allowed photochemically due to frontier orbital symmetry in the excited state

Photocycloaddition

• General $[2+2]$ reaction between two different unsaturated systems—alkenes, alkynes, or carbonyls can participate
• Regiochemistry and stereochemistry depend on orbital coefficients and the geometry of approach
• Key for natural product synthesis—builds strained ring systems that are difficult to access thermally

Paternò-Büchi Reaction

• Carbonyl + alkene forms an oxetane (four-membered oxygen-containing ring) under UV irradiation
• Proceeds through $n,\pi^*$ excited state of the carbonyl, which attacks the alkene π-system
• Regioselectivity follows the "rule of five"—the more stable biradical intermediate determines the product

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Isomerization and Molecular Switching

These reactions involve structural reorganization without breaking the molecular skeleton. Excited-state potential energy surfaces often have different minima than ground states, enabling geometric or constitutional changes.

Photoisomerization

• Cis-trans interconversion upon light absorption—azobenzenes switch between extended (trans) and compact (cis) forms
• Dramatic property changes accompany isomerization: solubility, dipole moment, and absorption spectrum all shift
• Vision depends on this process—retinal isomerizes from 11-cis to all-trans in rhodopsin, triggering nerve signals

Photochromism

• Reversible light-induced transformation between two stable forms—typically involving ring-opening/closing or isomerization
• Spiropyrans and diarylethenes are common photochromic systems with distinct colored and colorless states
• Applications in smart materials—photochromic lenses, optical data storage, and molecular switches for computing

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Electron and Energy Transfer Processes

These reactions involve the movement of electrons or energy between molecules rather than simple bond changes. Photoinduced electron transfer and triplet energy transfer enable catalytic and sensitized transformations.

Photoredox Reactions

• Light-driven single-electron transfer (SET) between a photocatalyst and substrate generates radical ions
• Ru(bpy)$_3^{2+}$ and organic dyes serve as photocatalysts, cycling between oxidation states
• Enables mild C-C bond formation—reactions that traditionally required harsh conditions now proceed at room temperature

Photosensitization

• Energy transfer from an excited sensitizer to a substrate—the sensitizer absorbs light and passes energy to molecules that don't absorb well
• Triplet sensitization is common: the sensitizer undergoes intersystem crossing, then transfers triplet energy to the substrate
• Photodynamic therapy uses this principle—sensitizers generate reactive oxygen species that destroy cancer cells

Photooxygenation

• Singlet oxygen ($^1O_2$) addition to organic substrates—generated by energy transfer from triplet sensitizers to ground-state $O_2$
• Ene reactions and $[4+2]$ cycloadditions with dienes produce hydroperoxides and endoperoxides
• Critical in biological oxidative damage and industrial synthesis of fine chemicals like artemisinin precursors

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Quick Reference Table

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Self-Check Questions

1. Which two reactions both generate radical intermediates from carbonyl compounds, and what structural feature determines which pathway dominates?

2. Compare photodimerization and the Paternò-Büchi reaction: what ring size and composition does each produce, and why are both classified as $[2+2]$ cycloadditions?

3. A question asks you to explain how a photocatalyst can drive reactions without being consumed. Which reaction type demonstrates this, and what is the key mechanistic feature?

4. Identify two photochemical processes that are critical in biological systems and explain what makes each harmful or beneficial.

5. If an FRQ asks you to design a synthesis of a strained four-membered ring, which reactions from this list would you consider, and how would you choose between them based on the target structure?