Photochemical reactions energy change

Although photochemical reaction is a result of absorption of light, it may not always lead to chemical change. Sometimes the absorption of photon may only increase the thermal energy or it may be reemitted (fluorescence). 

A rule that affects energy transfers in photochemical reactions, particularly photosensitization processes. The total electron spin (/.c., the vectorial overall spin angular momentum of the system) does not change after the electronic energy transfer between an excited molecular entity and another molecular entity. 

However, photo-activation puts so much energy into the molecule, that many pathways become available to the first electronically excited state in addition to the relatively simple pericyclic change. For this reason, none of the photochemical reactions above can be guaranteed to be pericyclic, and all that one should take from these results is the very strong and suggestive contrast with the rules for thermal cycloadditions. This contrast is accentuated by the observation that photochemical Diels-Alder reactions are very rare, in spite of the ease with which 6-membered rings are normally formed. 

Frontier orbitals also explain why the rules change so completely for photochemical reactions. In a photochemical cycloaddition, one molecule has had one electron promoted from the HOMO to the LUMO, and this excited-state molecule reacts with a molecule in the ground state. The interacting orbitals that most effectively lower the energy of the transition structure are... 

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