Forbidden transition photochemistry

The (n, it ) state plays a very important role in the photochemistry of carbonyl compounds and many heterocyclic systems. In conjugated hydrocarbons (it -> it ) transitions are most important and give intense characteristic absorption bands. Because of some overlap forbidden character, the (n - it ) transitions have low probability and hence weak absorption bands. 

Such triplet-triplet transfers are very important in photochemistry because hi. provide means for populating the long-lived triplet state to which transitions arc forbidden by direct absorption of radiation. 

The characterization of electronic excited states has attracted much attention in connection with photochemistry. For example, transition metal complexes are characterized by a variety of absorption spectra in the visible and ultraviolet (UV) regions. The absorption spectra essentially give us information about the electronic excited states corresponding to dipole-allowed transitions due to their high symmetries, while some of the data in crystalline fields indicate the existence of several excited states to which dipole transitions are forbidden in the absence of perturbation. Most photochemical reactions of metal complexes, which are occasionally important as homogeneous photocatalytic reactions, involve both allowed and forbidden excited states. Thus, the systematic understanding of the nature of these excited states is essential in designing photochemical reactions. 

Transitions between orbitals localised on atoms e.g. d-d transitions of transition metal salts, f—f transitions of lanthanide ions. Such metal-centred (MC) transitions are ubiquitous in transition metal and lanthanide complexes. They are relatively weak because they are symmetry (Laporte) forbidden. Although they may not be the important transitions for any particular application of transition metal photochemistry, they will almost always be present. These are the transitions that give many transition metal salts their characteristic colour and are foimd in some gemstones and minerals. For example, the red colour in ruby is due to the d-d transitions in chromium (III) present at certain sites in an aluminium oxide (corundum) crystal. 

---