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Photochemical Reactions

Photochemistry is a branch of chemistry which involves the interactions of atoms and molecules with light. It involves the description of chemical reactions that take place in the presence of light. For instance, the formation of vitamin D in the presence of sunlight is a photochemical reaction.


 The first photochemical reaction was demonstrated by Trommsdorf in the year 1834. He observed that the crystals of the compound called α-santonin turned yellow and burst when they came in contact with sunlight. The description of the reaction was given in a study in 2007. According to the study, the first reaction involved a rearrangement reaction to a cyclo pentadienone. 


The second reaction is a dimerization reaction and third one is an intramolecular cycloaddition reaction. The crystals burst due to large change in the crystal volume on dimerization.LAWS OF PHOTOCHEMISTRYAccording to the first law of photochemistry given by Theodor Grotthuss and John W. Drape, for a photochemical reaction to take place, a chemical substance must absorb light. 


The second law known as Stark-Einstein, states that when a chemical system absorbs light, only one molecule is activated for each photon of light absorbed in a photochemical reaction.PHOTOCHEMISTRY REQUIREMENTSFor a photochemical reaction to take place, a reactant must have the necessary activation energy. When a molecule absorbs light, it should not only have the necessary activation energy but also the appropriate symmetry of the molecule’s electronic configuration. 


This electronic reorganization takes place by electromagnetic radiation. Photochemical reactions as fast as 10-9 and 10-15 seconds have been observed.The prime use of photochemical reactions is in the process of photosynthesis in which plants absorb the solar energy and convert carbon dioxide and water into glucose and oxygen. Exposure of humans to sunlight leads to the formation of vitamin D in the body. The drug containing bottles in medical stores are darkened to prevent the degradation of the drugs by photochemical reactions. 


Light aids in the destruction of tumors by the formation of singlet oxygen in photodynamic therapy. The photochemical reaction of rhodopsin results in the vision of an individual. Recently photochemistry is being used in photo resist technology for the production of microelectronic components.