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WHAT IS PHOTOSENSITIZATION ? Certain Substances which ordinarily do not undergo any Photochemical Reation by the action of Energy provided by a photon of a particular wavelength are made to do so in presence of foreign substances . A substance which when added to the reaction mixture but itself does not under go any change is called PHOTOSENSITIZER and the phenomenon is called PHOTOSENSITIZATION . The Photosensitizer, absorbs the light, becomes excited and then passes on this Energy to one of the reactants and activates them for Reaction , without itself apparently undergoing a chemical change .

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EXAMPLES OF PHOTOSENSITIZED REACTIONS : 1 : Dissociation of Hydrogen molecule : Hydrogen is unaffected when exposed to light of wavelength 2537 A 0 . However , in the presence of mercury vapour , it undergoes dissociation into atomic hydrogen with the light of same wavelength . Mechanism : Hg + hv Hg* ground excited state state Hg* + H2 H2* + Hg H2* refers to excited H2 molecule H2* 2 H Here Hg ( mercury vapours) acts as an Photosensitizer .

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2: Photosynthesis of carbohydrates in Plants : Photosynthesis is the name applied to reactions by which Light Energy is absorbed and is converted into Chemical Energy through the synthesis of complex molecules . hv CO 2 + H 2 O C 6 H 12 O 6 + O 2 chlorophyll Here ,Chlorophyll , absorbs red and blue light , and makes it available for photosynthesis in plants. Thus , Chlorophyll acts as an Photosensitizer .

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MECHANISM OF PHOTOSENSITIZATION : Let the photosensitizer be repersented as Donor (D) , because it donates the absorbed Light Energy to the reactant called acceptor (A) . The sensitizer (D) absorbs the incident photon of light and acquires the singlet excited state ( S1 or 1 D) , which undergoes intersystem crossing ( ISC ) giving the triplet excited state of Donor ( 3 D) . This triplet excited state then collides with the acceptor producing the triplet excited state of acceptor ( 3 A) . ISC (Donor,D) Photosensitizer Acceptor,A (Reactant) (T1 or 3 D) (S1 or 1 D) (S1 or 1 A) (T1 or 3 A) hv Then the Donor is returned to its ground state (S 0 ).When 3 A gives desired products, the mechanism is called Photosensitization .

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The reaction depicting Photosensitization is represent as under : ISC D + hv 1 D 3 D Products To come back to ground state, D emits some radiations and are absorbed by Acceptor as follows : 3 D + A D + 3 A Products Here D is Donor molecule , hv is the light radiations absorbed , A is acceptor , 1D is the singlet excited state of Donor & 3D and 3A are the triplet excited states of Donor and Accepter respectively . In the above reactions , D acts as a Photosensitizer & A acts as a Reactant .