Photochemistry: Excited State Reactions

Core Idea

Excited electronic states have different geometries, orbital occupancy, and reactivity than ground states. Photochemical reactions proceed via different mechanisms and activation barriers; forbidden ground-state reactions become allowed from excited states. Photochemistry enables reactions that violate thermal symmetry rules and has applications in photosynthesis, vision, and synthesis.

Explainer

From your work on electronic spectroscopy, you know that absorbing a photon promotes an electron from a bonding or nonbonding orbital into a higher-energy orbital. What photochemistry adds is the recognition that this electronically excited molecule is, in effect, a *different chemical species* — one with its own geometry, its own reactivity, and its own set of accessible reaction pathways. The excited state has a different electron configuration than the ground state, which means different bond orders, different charge distributions, and often a dramatically different molecular shape. A molecule that is perfectly stable on the ground-state surface may be highly reactive on the excited-state surface.

The key insight connecting this to reaction mechanisms is the Woodward–Hoffmann rules and orbital symmetry conservation. Many thermal reactions are "symmetry-forbidden" — meaning the orbital symmetry of reactants and products does not correlate smoothly along the reaction coordinate, creating a large energy barrier. But photochemical excitation changes the orbital occupancy. A reaction that is thermally forbidden (like a conrotatory ring closure of a conjugated diene under thermal conditions) becomes photochemically allowed because the excited-state orbital symmetry now permits smooth correlation. This is why photochemistry opens doors that heating alone cannot: it accesses entirely different regions of the potential energy surface.

Once a molecule is in an excited state, several competing processes determine what happens next. The molecule can fluoresce (emit a photon and return to the ground state), undergo intersystem crossing to a triplet state (where it may phosphoresce or react differently), or proceed along a photochemical reaction pathway — such as bond cleavage, isomerization, or cycloaddition. The branching between these fates depends on the relative rates, which are governed by the energy gaps between states and the geometry of the potential energy surfaces. Conical intersections — points where two electronic surfaces cross — are often the funnels through which excited-state population returns to the ground state or channels into photoproducts.

Concrete examples make this tangible. In vision, retinal absorbs a photon and undergoes *cis*-to-*trans* isomerization in femtoseconds — a reaction with a large thermal barrier but nearly barrierless on the excited-state surface. In photosynthesis, chlorophyll's excited state transfers energy through a chain of pigments before driving charge separation. In organic synthesis, photocycloadditions like the [2+2] reaction are thermally forbidden but photochemically allowed, giving chemists access to strained ring systems that would be impossible to make with heat alone. In each case, the photon is not merely providing energy — it is changing the *rules* of the reaction by populating an electronic state with fundamentally different symmetry and bonding character.