Absorption of light by organic molecules

The spectrum shows that many vibronic transitions are allowed, and that some are more probable than others that is, the intensities of the 

In using the concept of molecular orbital theory to discuss the absorption of light by organic molecules, we concentrate on two molecular orbitals in particular. The highest occupied molecular orbital (HOMO) is the ground-state molecular orbital of highest energy with electrons in it and the lowest unoccupied molecular orbital (LUMO) is the 

In the context of an introduction to organic aspects of photochemistry, a simple molecular orbital description of the electronic structure of organic molecules provides the most convenient qualitative framework in which to discuss the formation of electronically excited states by the absorption of light. It is normally assumed that the inner-shell electrons of the constituent atoms of a molecule remain unaltered in the molecule itself linear combinations of the remaining, va lence-shell atomic orbitals then provide molecular orbitals that can be used to describe the outer electronic structure in the molecule. 

It needs to be stressed that this simple molecular orbital picture is not appropriate for all purposes, but it is convenient for visualizing the changes brought about by light absorption in organic molecules, and as a qualitative basis for describing the mechanisms of organic photochemical reactions. 

Commonly used units are reciprocal second (s l or Hz) for frequency nanometre nm, I nm = 10 m) for wavelength IK = civ. where c is the speed of light) and ioule ( ) far energy. Because energy is frequently dealt with on a mole basis, the unit encountered is kl mol-1. Table 1.1 shows the relationship between these units for selected numerical values in the ultraviolet/visible/near-infrared range. 

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Since the absorption of light by an organic molecule is, in general, a one-quantum process, we may also write Eq. 15-7 as ... 

Throughout this section on the differences between photochemical and thermal reactions, mention has been made of the electronically excited states that are key species in photoprocesses. We need now to look in more detail at the production ol such excited states by absorption of light, and at the nature of the excited states of organic molecules. 

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