Photo chemistry is the branch of chemistry which is mainly concerned with rates and mechanisms of reactions resulting from the exposure of reactants to light radiations. The photo chemical reaction is, in fact, the thermal reaction of the electronically excited state of the molecule while the dark reaction of the molecule is the thermal reaction of the ground state.

A photo chemical reaction is a chemical reaction triggered when light energy is absorbed by a substance’s molecules. This response leads the molecules to experience a temporary excited state, thus altering their physical and chemical properties from the substance’s initial molecule.

Photo chemical Reaction Definition

“The photo chemical reaction is none other than a chemical reaction that starts with light being absorbed as a form of energy”. Temporary peak states would be triggered while the molecules absorb light and there would be physical and chemical property differences to a large extent from the real molecules.

The resultant chemical structures could be separated, modified, mixed among the similar or different molecules along with the transfer of hydrogen atoms, electronic charge to separate molecules, protons, and electrons. The peak states in comparison to the real ground states are stronger reductants and acids that are stronger.

The mechanism of a photo reaction should ideally include a detailed characterization of the primary events as outlined by the classification of photo chemical reaction pathways. The quantum yields and hence the rate constants of all relevant photo physical and photo chemical processes, in addition to the information about the structure and fate of any reactive intermediates, their lifetimes and reactivities.

Photo chemical reaction, a chemical reaction initiated by the absorption of energy in the form of light. The consequence of molecules’ absorbing light is the creation of transient excited states whose chemical and physical properties differ greatly from the original molecules. These new chemical species can fall apart, change to new structures, combine with each other or other molecules, or transfer electrons, hydrogen atoms, protons, or their electronic excitation energy to other molecules. Excited states are stronger acids and stronger reductants than the original ground states.

The Basic Laws of Photo chemistry

The First Law of Photo chemistry states that light must be absorbed for photo chemistry to occur. This is a simple concept, but it is the basis for performing photo chemical and photo biological experiments correctly. If light of a particular wavelength is not absorbed by a system, no photo chemistry will occur, and no photo biological effects will be observed, no matter how long one irradiates with that wavelength of light.

The Second Law of Photo chemistry states that for each photon of light absorbed by a chemical system, only one molecule is activated for a photo chemical reaction. This law is true for ordinary light intensities, however, with high-powered lasers, two-photon reactions can occur, i.e., the molecule is raised to a higher energy state than produced by single-photon absorption.

The Bunsen-Roscoe Law of Reciprocity states that a photo chemical effect is directly proportional to the total energy dose, irrespective of the time required to deliver the dose. This law is true for chemicals in a test tube, but the response of cells to radiation usually involves a sequence of interacting biological reactions, making a linear “dose x time” relationship highly unlikely. There is no reciprocity when damage is produced, e.g., DNA damage, but there can be reciprocity over a narrow range of doses for photo receptors that trigger a response, such as phytochrome

Grotthus-Draper Law (or) The Principle of Photochemical Activation: Grotthus-Draper law states that only the light which is absorbed by a substance can bring about a photochemical change. However, the absorbed radiation does not necessarily cause a chemical reaction. When the conditions are not favorable for the molecules to react, the light energy may be re-emitted as heat or light or it remains unused.


Oxidation under the influence of radiant energy (as light) or Photo-oxidation is a chain process incorporating a large number of chemical reactions which are subsequent to the outcome of the primary event—absorption of a photon, which induces breakdown to free-radical products.

Main steps of photo oxidation : 1. Initiation 2. Propagation 3. Branching 4. Termination

Photo oxidation reactions

1. Formation of peroxy compounds

-Irradiating the parent organic compound in the presence of oxygen and a sensitizer. -Reaction occurs by the excitation of the sensitizer to its triplet state. -Either the triplet attracts a hydrogen atom from the substrate to form a radical which then reacts with the oxygen molecule.

2. Conjugated dienes yield cyclic peroxides in a reaction of Diels-Alder type.

3. Oxidative coupling of aromatic compounds.

4. Formation of polycyclic compound.

5. Photo oxidation of cholesterol.

6. Photo oxidation of polymers.

Light absorbed by polymers-photo chemical reaction can occurs as a result of activation of a polymer macro molecule to its excited singlet or triplet states. Chemical change that reduces the polymers molecular weight.


Photo addition is any bi-molecular addition reaction that take place under the influence of light. Photo addition forming 1:1 adduct by the reaction of an excited state of one molecule with ground state of another.The molecule which in excited state are carbonyl compound , quinine, aromatic compound,or alkene molecule and the molecules which present ground state is commonly alkenes. Majority of the photo additions reactions forming ring product.

1. Paterno-Buchi reaction (Photo addition of alkene to carbonyl compound) It is [2+2]photo addition reaction

2. Addition of C-H across C=O

3. Addition of X-H across a C=C

4. Linear addition to an unsaturated molecule

5. Cyclo addition of unsaturated molecule.


Photo fragmentation provide a variety of diradical and zwitter ionic intermediates including carbenes and nitrenes. The α-cleavage of a σ-bond attached directly to a chromophore is a small molecule (eg CO,CO2,N2,O2). The beta cleavage of a σ-bond is occasionally encountered.

Beta Cleavage ;

Example: 1. The side chain of riboflavin can split off to form lumiflavin


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