Laws of photochemistry

Two fundamental principles relating to light absorption by the organic molecules are the basis for understanding of their photochemical transformations. 

The Grotthuss-Draper law The law states that the only the fraction of light which is absorbed by a chemical entity can bring about the photochemical change. 

The Stark-Einstein law The law states that each molecule or atom absorbs one photon or one quantum of light for its excitation or activation, i.e., for a molecule, AB, 

This law is obeyed in the majority of cases but exception occurs when very intense light sources such as lasers are used for irradiation of a sample. 

The study of photochemical reactions shows a number of variations like in absorption of specific frequency, specific chemical transformation and also specific photo-physical path to gain or utilise the gained radiation energy. These variations are following some rules. These rules are summarised in certain laws, termed as laws of photochemistry. 

Absorption of specific frequency is due to the quantisation of electronic energy levels of molecules. Hence energy required for different transitions for different species are different. Also the portion of light used is not completely absorbed but only some fraction is utilized. These selections and utilizations are best demonstrated by Lambert-Beer s laws. 

Lambert s Law This law states that When a beam of light is allowed to pass through a transparent medium, the rate of decrease in intensity with thickness is directly proportional to the intensity of light . 

The ratio I/Iq is termed as transmittance (T) and the ratio log Ig/I is absorbance (optical density) while Zg/Z is termed as opacity. 

It is very clear that the fraction absorbed by reacting molecule is independent of source intensity. 

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This principle is so simple that it has been given the title the first law of photochemistry, and was first expressed by Grotthus and Draper in the early 19th century. They stated it as the (hopefully) obvious truth Only light that is absorbed can have any photochemical effect . 

The first law of photochemistry states that only light that is absorbed can have any photochemical effect. 

Only one photon at a time may interact with matter. This means that the energy available to each recipient atom or molecule is the same as the energy possessed by the single photon with which it interacts. This truth was refined by Stark and Einstein, who called it the second law of photochemistry. If a species absorbs radiation, then one particle (molecule, ion, atom, etc.) is excited for each quantum of radiation (photon) that is absorbed . 

The second law of photochemistry says that if a species absorbs radiation, then only one particle is excited for each photon absorbed. 

It is easy to get burned by the sun while out sunbathing, because the second law of photochemistry shows how each UV photon from the sun releases its energy as it impinges on the skin. This energy is not readily dissipated because skin is an insulator, so the energy remains in the skin, causes photo-excitation, which is experienced as damage in the form of sunburn. 

It is advisable to employ a high-power lamp when performing a photochemical reaction because it produces more photons than a low-power lamp. Its flux is greater. When we looked at the laws of photochemistry, we saw how the second law stated the idea that when a species absorbs radiation, one particle is excited for each quantum of radiation absorbed. This (hopefully) obvious truth now needs to be investigated further. 

Conversely, a quantum yield

greater than unity cannot be achieved during a straightforward photochemical reaction, since the second law of photochemistry clearly says that one photon is consumed per species excited. In fact, values of > 1 indicate that a secondary reaction(s) has occurred. A value of > 2 implies that the product of the photochemical reaction is consumed by another molecule of reactant, e.g. during a chain reaction, with one photon generating a simple molecule of, say, excited chlorine, which cleaves in the excited state to generate two radicals. Each radical then reacts in propagation reactions until the reaction mixture is exhausted of reactant. 

To help clarify the situation, we generally define two types of quantum yield primary and secondary. The magnitude of the primary quantum yield refers solely to the photochemical formation of a product so, from the second law of Photochemistry, the value of 0(primary) cannot be greater than unity. 

As a natural consequence of the second law of photochemistry, the sum of the primary quantum yields cannot be greater than unity. 

The so-called first law of photochemistry stating that only the radiation absorbed by a molecular entity or substance is effective in producing a photochemical change. 

The first step, sometimes referred to as the zeroth law of photochemistry, is the absorption of light by the carbonyl compound. There are three absorption regions in the ultraviolet for the simplest carbonyl compounds owing to the ground state to singlet n-+ a, and... 

The second law of photochemistry was first enunciated by Stark (1908) and later by Einstein (1912). The Stark-Einstein law states that ... 

The first law of photochemistry states Only that light which is absorbed by a system can cause chemipal change (Grotthus-Draper Law). 

The Grotthus-Draper law states that photophysical/photochemical reactions occur only when a photon of light is absorbed. This forms the basis for the First Law of Photochemistry—that is, only light that is absorbed can have a photophysical/photochemical effect. 

What is not so well known about Einstein is that he made contributions to the development of modern chemistry, particularly to the area of quantum mechanics. The Nobel Prize Committee awarded Einstein the Nobel Prize in physics in 1921 for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect. His explanation of the photoelectric effect helped to validate Planck s view of quantized energy, and has become the basis of the quantitative laws of photochemistry. 

We can digress at this point to consider two laws of photochemistry thal feature in most introductions to the subject. First, the Grotthuss-Draper law states, in essence, that only light absorbed by a molecule can be effective in bringing about chemical change. This may seem obvious when considered at the molecular level—if the photon energy is not made available to the molecule by absorption, an electronically excited state cannot be produced, and no photochemical change can result. The law s importance is in its practical... 

The first law of photochemistry states that a molecule must first absorb light in order to undergo a photochemical transformation. This law in embodied in the Beer-Lambert law... 

The first law of photochemistry, named the Grotthus-Drapper Principle, states that for a photochemical reaction to occur, the first event must be the absorption of light by some component of the system. The second law of photochemistry, named the Stark-Einstein Principle, states that a molecule can only absorb one quantum of radiation. The absorbed energy in the resultant excited molecule may be dissipated by either photophysical or photochemical processes. It is the latter of these that eventually changes the chemical and mechanical properties of the substance (26,27). Thus, the reactions based on the absorption of radiation by the chemical components of modern papers are of prime importance in discoloration. 

The first law of photochemistry, called the Grottus Draper law, states Only the light, which is absorbed by a molecule, can be effective in producing photochemical changes in the molecule. 

A second important law of photochemistry, known as the Stark-Einstein Law, follows directly from the particle behaviour of electromagnetic radiations. 

Although the first law of photochemistry had already been recognized by Grot-thus in 1817 and Draper in 1843, the quantum nature of UV/VIS radiation and their consequences for photochemistry were established only decades later by Planck and Einstein (see Tab. 2-2). Nowadays, the observation by Grotthus and Draper that only electromagnetic radiation that is absorbed by a molecule leads to a... 

Only light that is absorbed can produce a chemical change, a principle embodied in the Grotthuss-Draper law of photochemistry. This is true whether radiant energy is converted to some other form and then stored or is used as a trigger. Another important principle of photochemistry is the Stark-Einstein law, which specifies that each absorbed photon activates only one molecule. Einstein further postulated that all of the... 

Apart from the two fundamental laws of photochemistry, a very important relationship for the experimentalist is the combined Beer-Lambert law. This describes the extent of absorption of monochromatic light by a homogeneous system, and is given by equation (A )... 

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