Principles of Photochemistry of Metal Complexes

Abstract

Many of the principles that govern the photochemistry of metal complexes are the same as those that have been developed for understanding the photochemistry of organic compounds. Photochemical reactions differ from thermal reactions in that they are initiated by the absorption of light rather than by the application of heat. Photochemical reactions can therefore only occur if the compounds being activated have a chromophore in the electromagnetic spectrum that corresponds in wavelength with that of the exciting radiation. The number of thermodynamically favorable products that are accessible from photochemical reactions are greater than those that are obtainable from thermal reactions. This situation is a result of the excess energy that is possessed by the excited state after the absorption of a photon. The addition of this photon energy to the ground state energy of the molecule results in the excited state having considerably more stored energy; as a result, the excited state molecule can be considered to be quite different from its ground state precursor. This difference usually leads to the excited state having different bond distances and angles than does the ground state. These molecular distortions that the molecule undergoes after absorption of the photon can be either isotropic over the entire molecule, or anisotropic where the differences from the ground state are primarily located in one region of the molecule.