photochemistry of coordination complexes
/ FOH-toh-chemistry /
Most chemistry happens in the dark, driven by heat and collisions. But shine the right light on a coloured metal complex and you can make it do things it would never do warming on a bench — eject a ligand, change oxidation state, or hand an electron to a neighbour. Photochemistry is the study of these light-driven reactions, where a photon, not heat, gives a complex the energy and the new electronic arrangement it needs to react.
When a complex absorbs a photon, an electron is promoted into a higher orbital, putting the molecule into an electronically excited state. This excited state is a genuinely different chemical species — it has a different electron distribution, often weaker metal-ligand bonds, sometimes a different geometry, and crucially it can be a far stronger oxidant or reductant than the ground state. From there several things can happen. The energy may simply be re-emitted as light (fluorescence or phosphorescence) or lost as heat and the molecule relaxes back unchanged. Or the excited state may react: a d-d excitation that populates an antibonding eg orbital weakens a metal-ligand bond and can trigger photosubstitution, kicking out a ligand; a charge-transfer excitation that shifts an electron between metal and ligand can lead to photoredox, where the complex transfers an electron to or from another molecule. Which path wins depends on which excited state is reached and how long it lives.
This light-driven chemistry is no laboratory curiosity. The same antibonding-orbital logic that makes cobalt(III) ammines photolabile underlies light-activated platinum and other metal anticancer agents designed to release their toxic payload only where a beam is shone. Long-lived excited states of ruthenium polypyridyl complexes such as [Ru(bpy)3]2+ are powerful photoredox catalysts and the light-harvesting hearts of dye-sensitized solar cells, and the same physics drives nature's grandest reaction, the light-capturing chlorophyll of photosynthesis. The honest reminder is that absorbing light is only the first step — most absorbed photons are wasted to heat or re-emission, and useful photochemistry happens only when an excited state both reaches a reactive configuration and lives long enough to act before it relaxes.
Shining visible light on [Ru(bpy)3]2+ pushes it into a long-lived excited state that can hand an electron to another molecule, the basis of photoredox catalysis and dye-sensitized solar cells.
A photon promotes an electron, creating an excited state that is a stronger redox agent than the ground state.
Absorbing light is necessary but not sufficient: most excited complexes simply re-emit the light or shed it as heat and return unchanged. A productive photoreaction needs an excited state that both reaches a reactive geometry or electron distribution and survives long enough to react.