Overview

Most chemical reactions need heat (fire) to start. Photochemistry uses light. When a molecule absorbs a photon, it gets “excited.” It has too much energy. It wants to break a bond, swap an electron, or glow to get rid of that energy. It is the chemistry of vision, photography, and solar power.

Core Idea

The core idea is The Excited State. A normal molecule is in the “Ground State” (calm). A photon kicks an electron up to a higher orbital. Now the molecule is a “Hot Potato.” It is unstable and reactive.

Formal Definition

The study of chemical reactions initiated by the absorption of light. Grotthuss-Draper Law: Only light that is absorbed can cause a chemical change.

Intuition

  • Thermal Chemistry: Pushing a ball up a hill (Activation Energy) so it can roll down the other side.
  • Photochemistry: Teleporting the ball to the top of the hill using a helicopter (Photon). It bypasses the barrier.

Examples

  • Photosynthesis: The most important photochemical reaction. Plants use sunlight to split water.
  • Vision: When light hits the retina, it hits a molecule called Retinal. The photon causes the molecule to twist (isomerize) from bent to straight. This shape change triggers a nerve impulse to the brain. “Seeing” is a chemical reaction.
  • Ozone Layer: UV light hits Oxygen ($O_2$) and splits it into atoms ($O + O$). These atoms grab another $O_2$ to make Ozone ($O_3$). The Ozone then absorbs more UV, protecting us from cancer.

Common Misconceptions

  • Glow-in-the-dark is radioactive: Usually it’s Phosphorescence. The molecule absorbs light during the day and releases it slowly at night. It’s safe photochemistry.
  • Fluorescence: Absorbing UV light and immediately emitting visible light. (Highlighters, Tonic Water).
  • Chemiluminescence: The reverse. A chemical reaction that produces light (Glow Sticks, Fireflies).

Applications

  • Photodynamic Therapy (PDT): We inject a patient with a dye that goes to the tumor. We shine a laser on the tumor. The dye absorbs the light and produces toxic oxygen that kills the cancer cells.
  • Solar Panels: Photovoltaics are solid-state photochemistry.

Criticism / Limitations

  • Control: It’s hard to control. UV light often breaks bonds randomly (photodegradation), causing plastic to crack and colors to fade in the sun.

Further Reading

  • Turro, Nicholas. Modern Molecular Photochemistry.
  • Wayne, Carol. Photochemistry.