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Photochemistry, , 2020, , Chapter – III, , Photochemistry, Photochemistry is the branch of chemistry which deals with the interaction of light radiations with, matter. It is mainly concerned with rates and mechanisms of reactions resulting from the exposure of, reactants to light radiations. It is defined as the study of chemical effect produced by light radiations ranging, from 2000 to 8000 A0 wavelength. The photochemical reactions are induced by light radiation and are, influenced by intensity of light radiations., On other hand the reactions which are caused by heat and in absence of light are called thermal or, dark reactions. The thermal or dark reactions are influenced by temperature, concentration, presence of, catalyst etc., Nature of light: Light is a form of electromagnetic radiations. All electromagnetic radiations have wave, characteristic and travels at the same speed of light i.e. 3𝑥1010 𝑐𝑚⁄𝑠, but their wave length is different. The, unit of wave length is nanometer i.e. 1 𝑛𝑚 = 10−9 𝑚., 𝛾 − 𝑟𝑎𝑦 𝑎𝑛𝑑 𝑋 − 𝑟𝑎𝑦𝑠 have very small wave length i.e. less than 10−11 𝑚. while radio waves are in, the order of 104 𝑐𝑚., The visible light ranges from the violet region at about 380 nm to red region at 750 nm., , Interaction between light and matter: It is very important part of this chapter. The interaction between light of matter is the basis of all life, in the world. Matter is anything which occupies space and has mass., To understand matter, we have to make use of light and to understand light we must involve matter., Here the light is used as complete spectrum of electromagnetic radiation from radioactive to radio waves., We use X-rays to study the structure of molecule in their crystalline state and take the help of various, spectroscopic methods, to understand the arrangement of atoms and molecules. On the other hand, if we, want to study the nature of the light, we need matter. When light radiations fall on matter, it reflects,, transmits, scatters or absorbs and thus allowing us to understand its behavior., A beam of light in a darkroom will not be visible to us, as it is scattered by dust particles floating in, the air. A microscope is used to view particle, when incident light is scattered by it to the aperture of the, object. All light measuring devices are based on interactions. In these interactions light behaves as a particle, and in some other. Its behavior is like a wave., Importance of Photochemistry: 1. Preparation of very important products like cleaning solvents, insecticides, halogenated aromatic, compounds by photo chlorination., 2. The important photochemical phenomenon like fluorescence and photo phosphorescence are used in, florescence tube lights, X-rays, TV screens, and luminescent dials for watches., 3. The main application of photochemistry has been seen in the manufacturing of solar cells. Solar cells, can be used as a source of fuel in the near future., 4. Flash photolysis and pulsed laser photolysis are widely used by scientist as new tools for the study, of higher energy states., 5. Photochemistry plays very important role in the application of optical brightness as paints in, advertisements., 6. Vita. 𝐷2 from ergo sterol isolated from certain yeasts, antioxidants, by photosulphonation, antiviral, reagent like cubanes and synthesis of caprocalcium which is the monomer of nylon. These, compounds are synthesized by using photochemistry., 7. There are certain chemicals which changes their colour, when exposed to suitable radiations and, restore their original colour when the irradiation source is cut off. These are known as photochromic, materials., 20

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Photochemistry, , 2020, , Dark or Thermal reactions: These are the ordinary chemical reactions which are influenced by temperature, concentration of, reactants, presence of catalyst etc. except light radiations., For examples., 𝑁2 + 3𝐻2 → 2𝑁𝐻3, 𝐻2 + 𝐼2 → 2𝐻𝐶𝑙, Photochemical reactions: A photochemical reaction may be defined as any reaction which is induced or influenced by the, action of light on the system., For examples., 2𝐻𝐵𝑟 → 𝐻2 + 𝐵𝑟2, 𝐻2 + 𝐶𝑙2 → 2𝐻𝐶𝑙, Difference between photochemical and thermal (Dark) reactions: Photochemical Reactions, Thermal Reactions (Dark), 1. Photochemical reactions involve absorption, 1. Thermal reactions involve absorption, of light radiation., or evolution of heat., 2. The energy required is gained by the, 2. The energy of activation is provided, absorption of photon of visible or UV light., by the collision., 3. These reactions take place in presence of, 3. These reactions can take place in, light., absence of light., 4. ∆𝐺 for these reactions may be +𝑣𝑒 𝑜𝑟 – 𝑣𝑒., 4. ∆𝐺 for these reactions is always – 𝑣𝑒., 5. Photochemical activation is highly selective, 5. Thermal activation is not selective in, in nature., nature., 6. The rate of these reactions is independent, 6. The rate of these reactions is, of temperature., depending of temperature., , Laws of photochemistry: a) Grotthus – Draper Law: This law states that, when light falls on any substance, only the fraction of incident light which is, absorbed by the substance can bring about a chemical change. The reflected and transmitted fraction of, light does not produce any chemical change. Thus, according to Grotthus – Draper Law, the absorbed light, produces chemical change or chemical reaction., , Limitations: 1. It does not show the relationship between the quantity of light absorbed by a substance and the, molecules reacted., 2. It is only applicable to the primary photochemical process and fails to the secondary process., 3. All the absorbed radiations do not cause photochemical reactions., , b) Stark-Einstein’s law of photochemical equivalence: Stark-Einstein studied the quantitative aspect of photochemical reaction by application of quantum, theory of light and found that each molecule taking part of reaction absorbs only a one quantum of photon of, light. The law states that the substance undergoing photochemical reactions absorbs one quantum of energy, for decomposition of its one molecule only., e.g., ℎ𝜗, Maleic acid, Fumaric acid, The energy absorbed per mole of the reacting substance is 𝐸 then it can be given by the relations., 𝐸 = ℎ𝜗, 𝑒𝑟𝑔/𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒, ℎ𝐶, 𝜆, 𝑁ℎ𝐶, 𝜆, , 𝐸=, 𝐸=, , 𝑒𝑟𝑔/𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒, 𝑜𝑟, , 𝐸 = 𝑁ℎ𝜗, , 𝑪, , (⸫ 𝝑 = 𝝀 ), 𝑒𝑟𝑔/𝑚𝑜𝑙𝑒, , ′, , Where, 𝑁 = 𝐴𝑣𝑜𝑔𝑎𝑑𝑟𝑜 𝑠𝑛𝑢𝑚𝑏𝑒𝑟 ( 6.023 𝑥 1023 ), 21

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Photochemistry, , 2020, , ℎ = 𝑃𝑙𝑎𝑛𝑐𝑘 ′ 𝑠𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 (6.626 𝑥 10−27 𝑒𝑟𝑔𝑆𝑒𝑐., 𝐶 = 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 (3 𝑥 1010 𝑐𝑚/𝑆𝑒𝑐., 𝜆 = 𝑊𝑎𝑣𝑒𝑙𝑒𝑛𝑔𝑡ℎ 𝑖𝑛 𝑐𝑚., 𝐸=, , 𝑁ℎ𝐶, 𝜆, , =, , 6.023𝑥1023 𝑥6.626𝑥10−27 𝑥3𝑥1010, 𝜆 𝑖𝑛 𝑐𝑚 𝑥10−8, , 1.197𝑥1016, 𝑒𝑟𝑔, 𝜆, 𝑚𝑜𝑙𝑒, 1.197𝑥1016, = 𝜆 𝑥 4.18 𝑥 107 𝐶𝑎𝑙/𝑚𝑜𝑙𝑒, 2.86𝑥108, =, 𝐶𝑎𝑙/𝑚𝑜𝑙𝑒, 𝜆, 2.86𝑥105, = 𝜆, 𝐾𝑐𝑎𝑙/𝑚𝑜𝑙𝑒, , 𝐸=, 𝐸, 𝐸, 𝐸, , 𝑒𝑟𝑔/𝑚𝑜𝑙𝑒, , (1 𝐽 = 107 𝑒𝑟𝑔. 𝑎𝑛𝑑 1 𝐶𝑎𝑙 = 4.18 𝑥107 𝑒𝑟𝑔. ), , The quantity of energy (𝐸 = 𝑁ℎ𝜗) absorbed per mole is called as Einstein. Einstein is the quantum of, energy (𝐸 = 𝑁ℎ𝜗) absorbed per mole of the substance., Consider the equation; 𝐸 =, light absorbed i.e. 𝐸 ∝, , 1, ., 𝜆, , 𝑁ℎ𝐶, 𝜆, , this indicates that the energy is inversely proportional to the wave length of, , It is clear that the smaller the wavelength larger is the energy and vice versa., , Jablonski diagram: Depicting various processes occurring in the excited state: The phenomenon of fluorescence and phosphorescence are best explained with the help of jablonski, diagram. When substance is irradiated with sun light of the appropriate frequency, light will be absorbed in, bout 10−15 𝑠𝑒𝑐. In this process of absorption of absorption of light the molecules from the ground state, jumps to the excited state., At room temperature molecules may be present in their ground(𝑆0 ) state, after absorption of light, the excited molecules goes to the higher energy states like singlet (𝑆𝑛 ). When, 𝑛 = 1, 2, 3, 4, … … ..depending, upon the energy of the light radiations absorbed and triplet (𝑇𝑛 ). When, 𝑛 = 1, 2, 3, 4, …. For each singlet, excited state there is corresponding triplet excited state. Thus, after initial act of absorption,, 𝐴 + ℎ𝜗 →, 𝐴∗, Where, A is the molecule in the ground state and 𝐴∗ is the molecule in the excited state., The activated molecules from excited state the following phenomena will probably occur., 1. The excited molecule is subjected to collisions with the surrounding molecules and it gives up, energy, it steps down the ladder of excited levels., 2. The molecule in the excited state may emit visible light radiations. This process is known as, florescence., 3. The molecule with relatively stable excited state may undergo transitions to meta stable state and, sometime excited molecules returned to the ground state by the emission of an visible light, radiations. This is known as phosphorescence. It is shown in fig.1., , Fig.1. Jablonski diagram, , 22

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Photochemistry, , 2020, , Jablonski diagram depicting various photochemical processes; when excited electron spins are paired,, parallel and anti-parallel., E. S., G. S., , 𝑆 = 𝑆1 + 𝑆2, =, , 1, 2, , 1, −, 2, , S= 𝑆1 + 𝑆2, =, , 1, 1, −, 2, 2, , S= 𝑆1 + 𝑆2, 1, 2, , = +, , 1, 2, , =0, =0, =1, 2𝑆 + 1 = 0, 2𝑆 + 1 = 0, 2𝑆 + 1 = 0, Single, Singlet, Triplet, The excited molecules must return to the ground state by lose its energy through radiative, non-radiative, radiations or by undergoing chemical reaction. Radiative transitions are denoted by straight arrows 𝑆1, 𝑆0, where as non radiative transitions are denoted by wavy arrows 𝑆1, 𝑆0 ., The energy of the activated or excited molecule is lose in the form of heat through molecular, collisions is called internal conversion (IC)., The energy of the excited molecules lose by the non-radiative radiations transfers from singlet, excited state to triplet excited state or reverse is called inter system crossing. (ISC), , Non –radiative process: When molecule excited to higher energy level by the absorption of light radiations, and molecule get, activated. The activated molecule returns to the ground state by lose its energy to the surrounding is called, radiative transitions., In non-radiative process the return of the activated molecule from higher excited state (𝑆3, 𝑆2, 𝑡𝑜 𝑇3, 𝑇2), to the first excited state (𝑆1 or T1 ) do not involve the emission of any radiations thus they are called non, radiative process. There are two major type of non-radiative process., 1. Internal conversion (IC): - The energy of the activated molecule is losing in the form of thermal energy, or heat through molecular collision; this process is called internal conversion. Internal conversion is, non-radiative process because the loss of energy between the excited state of the same spin. i.e., singlet, singlet or triplet, triplet 𝑆3, 𝑆2 or 𝑇3, 𝑇2, 2. Inter system crossing (ISC): - The energy of the activated molecule is losing by transition between, the states of different spins i.e. from 𝑆2 to 𝑇3 or 𝑆1 to 𝑇1 . This non-radiative transitions from singlet, excited state to triplet excited state is known as inter system crossing. 𝑆2, 𝑇1 or 𝑇 1, 𝑆0 ., , Fluorescence and Phosphorescence: Fluorescence: When an atom or molecule is exposed to the light radiations, it absorbs the light radiations and, within the fraction of time (10−6 𝑡𝑜10−7 𝑠𝑒𝑐. ), they emit a lower frequency without producing any chemical, change this phenomenon is known a fluorescence., Fluorescence is the phenomenon shown by atoms or molecules in solid, liquid and gaseous state. It, depends on incident light radiations on cutting the source of light radiation the florescence is also stop., When substance absorbs the light energy, it will result in the excited state of an atom. When these, excited atom returns to the ground state by the emitting of light which possess different frequency than the, incident light., 𝑋 + ℎ𝜗 → 𝑋 ∗ Excited state, 𝑋∗, →, 𝑋 + ℎ𝜗′, In certain cases, the different fluorescent substances the radiations with same wavelength as it absorbed this, is known as resonance fluorescence. A few examples of the substances showing the phenomenon of, , 23

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Photochemistry, , 2020, , fluorescence as fluorite, CaF2, certain organic dyes such as eosin, fluoresce in, and certain inorganic, compounds such as uranyl sulphate (𝑈𝑂2 𝑆𝑂4. )., Phosphorescence: In phosphorescence certain substances absorbs light radiations but they does not emit radiations, simultaneously, after some time or after long time they continuously emit radiations of lower frequency even, after the incident light is cut off this phenomenon is known as phosphorescence., Phosphorescence is mainly shown by solids. The materials exhibiting phosphorescence re-emit, excess radiation within 10−4 𝑡𝑜 20 𝑠𝑒𝑐. or longer i.e. the life time of phosphorescence is much longer than, fluorescence. The magnetic and dielectric properties of phosphorescent substance are different before and, after illumination. The examples of the substances exhibiting phosphorescence include zinc sulphate and, sulphides of the alkaline earth metals., , Quantum yield or Quantum efficiency (∅): −, The extent of photochemical reaction is given in terms of quantum yield or quantum efficiency. It is, defined as the ratio of number of molecules reacting chemically in a given time to the total number of quanta, absorbed in the same time. On other way the quantum efficiency is defined as the number of molecules, reacting per quantum or per Einstein., ∅=, , 𝑁𝑜 𝑜𝑓 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠 𝑟𝑒𝑎𝑐𝑡𝑖𝑛𝑔 𝑖𝑛 𝑎 𝑔𝑖𝑣𝑒𝑛 𝑡𝑖𝑚𝑒, 𝑁𝑜 𝑜𝑓 𝑞𝑢𝑎𝑛𝑡𝑎 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 𝑖𝑛 𝑡ℎ𝑒 𝑠𝑎𝑚𝑒 𝑡𝑖𝑚𝑒, , The quantum yield is depending up on the intensity of light. If the law of photochemical, equivalence is correct. The quantum yield should be unity. Quantum yield may be varying from 0 𝑡𝑜 106 ., , Experimental Determination of Quantum Yield: For the experimental study of photochemical reaction values of number of molecules reacted and number of, quanta absorbed must be essential to know. The number of molecules reacted in particular time are obtained, by suitable method of analysis for the knowledge of Einstein. The apparatus is used are shown in fig. 2., , Fig. 2. Experimental determination of quantum yield, , 1. Sourced of light: The source of light emits radiation of suitable intensity of desired wavelength. The source may be, sunlight, tungsten lamp, mercury vapour lamp, arc lamp, xenon lamp, gas discharge tube etc. The most, important among these for visible region are tungsten and xenon lamps. Mercury lamp is useful in UV, region. When light from source is passed through the lens and then allowed to pass through monochromator., 2. Monochromator: The parallel beam of light from lens passes through the mono-chromator or filters. Mono-chromator, is generally made up of gelatin or coloured glass. The function of mono-chromator is to absorb the undesired, wavelengths and transmits light of a definite wave length within certain range of wavelengths., 3. Reaction cell: The light from a mono-chromator enters in to a reaction cell containing the reaction mixture. The, reaction cell is generally made up of glass or quartz. The reaction cell is immersed in a thermostat to attain, the constant temperature. The light transmitted through the reaction cell falls on suitable detector., 24

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Photochemistry, , 2020, , 4. Detector: The intensity of transmitted light is measured with the help of the detector which may be, photoelectric cell, radio micrometer, the thermopile etc. The thermopile is a multi-junction thermocouple of, Ag and Bi., Uranyl oxalate action-meter is used as a detector in decomposition of oxalic acid in presence of uranyl, ion 𝑈𝑂22+ its quantum yield is 0.5., ℎ𝑣, 𝐶𝑂𝑂𝐻, 𝑈𝑂22+ +, →, 𝐶𝑂 + 𝐶𝑂2 + 𝐻2 𝑂 + (𝑈𝑂2+ )∗, 𝐶𝑂𝑂𝐻, The concentration of oxalic acid is followed by titration with std. KMnO4 solution., Procedure: Set up the apparatus as shown in the fig. 2. The selection of detector depends upon nature of the, experiments. Then solvent is filled in reaction cell and light is allowed to fall on the reaction cell and reading, is taken., In second part, the reaction cell is filled with reaction mixture, then allow the light to fall on the, reaction cell and reading is taken, this reading gives total energy transmitted. The difference between these, two readings will give the amount of energy absorbed by the reaction mixture in given time. Analyze the, content of reaction mixture. Then determine the number of moles reacted in given time. Then apply the, equation to get the value of quantum yield., , Factors affecting the quantum yield: Following factors are affecting the quantum yield., 1. Temperature: - Endothermic reactions are extremely slow at ordinary temperature. The quantum yields, of such reactions increase with increase in temperature. The change of quantum yield with temp is, given by the relation, 𝑑∅, 𝑑𝑡, , 𝑄, , = 𝑅𝑇 2, , Where, Q = Amount of heat evolved by the formation of one g mole, , of the substance and R = Gas constant., 2. Wavelength: - The energy absorbed per mole of the reactant varies inversely as the wavelength of the, light absorbed i.e. shorter the wavelength; greater is the energy absorbed. Therefore, the quantum, yield will be higher at lower wavelengths and vice versa., The lower quantum yield at higher wavelength is due to the low frequency of the primary, photochemical process occurring in that wavelength regions., 3. Light intensity: - The decrease in light intensity decreases the quantum yield. Since the photochemical, reaction is directly proportional to the intensity of a reaction., 4. Presence of Inert gas: - The addition of inert gases to absorbing system in photochemical reaction, increases the quantum yield. It means that the number of molecules reacting chemically per, quanta of radiations is more due to the pressure of an inert gas., , Photosensitized Reactions (Energy Transfer process): Certain reactions are known which are not sensitive to light, but these reactions can be made, sensitive by adding a small amount of foreign material which can absorb light and stimulate the reaction, without itself taking part in the reaction such added material is known as photo-sensitizer and the, phenomenon as photosensitization., Photosensitized reactions are spontaneous involving an increase in free energy of the system. The, formulation of photo-sensitizer is to absorb light radiations becomes excited and then pass on its energy to, one of the reactants and these by activate them for reaction without itself taking part in the reaction. Thus, photo-sensitizer acts as carrier of energy., For example:, a) The dissociation of molecular hydrogen to atomic hydrogen by using mercury as a photo-sensitizer., ℎ𝑣, , 𝐻𝑔 + 𝐻2 →, , 2𝐻 • + 𝐻𝑔, 25