Photochemistry

photochemistry , photochemical reaction,,Thermal reaction,Lamberts law,Beers law,Laws of photochemistry,quantum yield ,jablonski diagram

1. SHRI R.R.LAHOTI SCIENCE COLLEGE,MORSHI Presentation Prepared by- Mr G.D.Rawate ( M.Sc,SET,NET) Department of Chemistry Shri R.R.Lahoti Science College,Morshi Class - BSc III sem V Subject – Photochemistry Session -2017-18

2. PHOTOCHEMISTRY CONTENTS Photochemical and thermal reactions Lamberts law – statement and derivation Beers law- statement and derivation Reasons for deviation of Beers law Laws of photo chemistry Quantum yield of photochemical reaction Reason for low and high quantum yield Experimental determination of quantum yield Photosensitized reaction

3. . Kinetics of photochemical decomposition of HI Fluorescence and phosphorescence Selection rule for electronic transition Internal conversion and intersystem crossing Explanation of fluorescence and phosphorescence on the basis of jablonski diagram Chemiluminiscence and bioluminescence with example. Numericals

4. . Photochemistry is branch of chemistry, deals with study of chemical reaction initiated by absorption of light Examples photosynthesis Degradation of plastic Formation of vitamin D

5. Light is a type of electromagnetic radiation, a source of energy 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.  

6. Types of chemical Reaction A ) Dark or Thermal Reaction The reaction take place in absence of light are called dark or thermal reaction N 2 + 3H 2 2NH 3 H 2 + I 2 2HI PCl 5 PCl 3 +Cl 2

7. Characteristics of Thermal Reaction i ) These reaction need collisions for activation. ii) Their rate depends on temperature iii) These reaction take place with decrease of free energy

8. Photochemical Reaction The reaction initiated by light Dissociation 2HBr light H2 + Br2 ii) Photo catalytic reactions CO2 + H2O +Chlorophyll light CH2O +O2 +Chlorophyll iii) Rearrangement Fumaric light Maleic iv) Combination H2 + Cl2 light 2HCl Decomposition 2O3 light 3O2

9. Comparison between thermal and photochemical Reaction Thermal Reaction These involve either absorption or evolution of heat Free energy of molecule decreases Simple in nature Generally fast Rate is depends on temperature Photochemical Reaction These involve the absorption of light . Free energy of molecule increases Complex in nature Photochemical reaction are slow Rate is independent on temperature

10. Laws of absorption of light When a monochromatic beam of light is incident on homogeneous absorbing surface ,part of it is absorbed by the medium, part of it reflected ,part of it transmitted then I = Ia + Ir +It For comparision cell is used, the value of Ir is very small can be eliminated for air glass interface, therefore I = Ia + It The portion of light absorbed is governed by two laws 1) Lamberts law 2) Beers law

11. Lamberts Law

12. Beers law BEER’s LAW : The intensity of a beam of monochromatic light decreases exponentially with the increase in concentration of the absorbing substance arithmetically. I = I e -kc (3) On combining both laws, we get log I /I = ecl (4) The equation 4 is termed as mathematical statement of Beer- Lambert‟s law. In the above equation e = the molar absorption coefficient A = log I /I is the absorbance (or) optical density (OD )

13. Deviation of Beers curve REASOS FOR DEVIATION FROM BEERS LAW TRUE DEVIATION: - Beers law hold good for dilute solution. CHEMICAL DEVIATION : -Chemical deviation arises if absorbing substance undergo chemical changes such as association, complex formation, dissociation, hydrogen bonding, ionization, polymerization.

14. Reasons for deviation from Beers law Presence of impurities that fluronce or absorb at the absorption wavelength. If colored solute ionizes, associate, dissociate, hydrolysis ,complex formation etc in the solution. If the solution is of highly concentrated. If monochromatic light is not used. If solution is turbid or should contain air bubbles or any suspended impurities. If experimental condition pH ,temperature, ionic strength etc are not maintained. If sample cell or cuvettes not be matched. If width of slit is not uniform.

15. Application of Beers Lamberts law To determine unknown concentration of compound by comparing with solution of known concentration. To characterize compound by drawing absorption spectrum. To measure absorbance of compound by using spectrophotometer.

16. Problem The extinction coefficient of certain substance is 40M cm-1.Calculate percentage transmission for a 5 cm cell filled with 0.01 M solution.neglect solvent. Solution:- Optical density = logI0/I = ECxX E = 40 Mcm-1, C= 0.01 molar; X=5cm. logIo /I =40x0.01x5-2 or Io/I = 100 I/Io =0.01 Percentage of transmitted light = 0.01 x 100 = 1%

17. Example 2 A 0.003 M solution of substance transmits 75 % of incident light of5 x10-7 m if the path length is 1 cm,calculate extinction coefficient and the percent absorption for a 0.01 M solution. Log 100/75 Solution Given, I=75 when Io=100, X=1 cm;C =0.01m So O.D. = log 100/75 = E x0.003x1 Or Extinction coefficient E= 41.65M.cm-1 For 0.01 M solution in the same cell O.D. = 41.65 x0.01 x1 = 0.4165 So here log 100/T =0.4165 so % of transmitted light T = 38 -33 so % of absorbed light = (100 -38.33) = 61.67%

18. Example 3 A 0.03 M solution of Co ( C2O4)-3 has an O.D.2.0 at 660 mu using 1 cm cell.Calculate A) the value of extinction coefficient B) The percentage absorption of a0.015 M solution in the same cell. Solution:- From the given data 2 = E x 0.03 x 1 or E = 66.67 For 0.01 M solution O.D. = 66.67 x 0.015 x 1 = 1.0005 Here logIo /I = 1.0005 or Io/I =10.00115 or I/Io =0.0999 = 0.1 So 0.01 x100 = 10% is transmitted i.e. 90 5 is absorbed.

19. E xample 4 The energy of activation for thermal decomposition of N2O(g) is 53,000cal.mole-1Calculate the frequency and wavelength of light corresponding to this wavelength During activation one mole absorbs N ,here frequency of light = v Then energy of activation = Nhv , =6.023x10 23 x 6.625 x10-27 xv cal /mole/4.2x 10 7 = 9.5 x 10 -11 x v cal /mole here the energy of activation = 53000 cal /mole v = 53000/9.5 x10-11 sec -1 = 5.579 x 10 14 Wavelength =C/V = 3x10 10 / 5.579x 10 14 = 5377 A

20. A monochromatic radiation is incident on a solution of 0.05 m concentration of an absorbing substance. the intensity of the radiation is reduced to one fourth of the initial value after passing through 10 cm. length of solution. Calculate the molar extinction coefficient of the solution. Solution log Io/I = ExCxX Given X = 10cm, C =0.05 mol.dm-3 Io/I =100/25 E =? Log 100/25 = E x10 x 0.05 Molar extinction coefficient, E = log 4/10x 0.05 E= 0.6021/0.50 =1.2042dm3 mol-1cm-1

21. Laws of photochemistry There are two basic laws governing photochemical reactions The Grothus - Drapper law b)The stark Einestien laws of photochemical equivalence. a) Grothus Drapper law This is the first law of photochemistry and is also known as the law of photochemical activation.This law was first enunciated by Grothuss and Drapper according to them Only that light which is absorbed by a system can bring about a photochemical reaction b) Stark Einestien law of photochemical equivalence This law enunciated by scientist Stark and Einestien , applied the concept of quantum mechanics to photochemical reaction of molecule In primary photochemical reaction each molecule of reacting substance absorb one quantum of radiation.

22. Quantum yield (ϕ ) To express the relationship between the number of molecules reacting with the number of photons absorbed, the concept of quantum yield or quantum efficiency „ ϕ is introduced . Quantum yield is defined as “the number of molecules of the substance undergoing photochemical change per quantum of radiation absorbed. Thus,   Number of molecules reacting in a given time ϕ = Number of quanta of light absorbed in the same time In certain photochemical reaction, l = wavelength of light in Ǻ; q = amount of radiation absorbed in certain interval of t s. & n = number of moles of substance reacted in the same time interval (t), then Number of Einstein's absorbed = q/( Nhc /l) = ql / Nhc   Quantum yield ϕ = n/( ql / Nhc ) = nNhc / ql In CGS units, ϕ = n/q x [1.196 x 10 16 /l (in Ǻ)]  

23. Quantum yield of some Photochemical Reaction

24. Photochemical reaction on the basis of quantum yield are divided into three types Those in which quantum yield is small integer such as 1,2 or 3 Examples Combination of SO2 and Cl2 Dissociation of HI or HBr Ionization of oxygen 2) Those in which quantum yield is less than 1 Dissociation of NH3,NO2 or Acetone Transformation of maleic acid into fumaric acid 3) Those in which quantum yield is very high Combination of CO an Cl2 and H2 and Cl2

25. Experimental measurement of quantum yields A photochemical reaction takes place by the absorption of photons of light by the reacting molecules. Therefore, it is essential to determine the intensity of light absorbed by the reacting molecules. An experimental arrangement for the study of rate of a photochemical reaction is shown in figure source Monochromator Reaction Cell Detector

26. Experimental measurement of quantum yields

27. . Source Radiation emitted from a source of light, L (sun light, tungsten filament , mercury vapor lamp, Arc) is passed through the lens, which Produces desired spectral beams. Monochromator The light beams are then passed through a monochromatic, B, which yields a beam of the desired wavelength only,monochromator are grating or optical filter, they are made up of gelatin or colored glass or transparent plates with metal it absorb unwanted wave lengths of light and transmit the light of desired wavelength. Prism and grating are also used. Reaction Cell This monochromatic light is allowed to enter into the reaction cell, C, made up of glass or quartz windows for entrance and exit of light immersed in a thermostat to maintain temperature constant, containing the reaction mixture. Detector part of the light that is not absorbed fall on a detector, X , which measures the intensity of radiation. The most frequently used detector are thermopile or the chemical actinometer.

28. Thermopile It is multijunction thermocouple consisting usually of metals unlike Ag and Bi connected with a moving coil galvanometer, the metal strips are blacken with lamp black or platinum black.The radiation falling on the blackened metal strips is completely absorbed completely and converted into heat, it raises the temperature and current is generated due to this temperature is measured, the current produced is proportional to the intensity of radiations.Thermopile are calibrated with standard light source.

29. Chemical actinometer I s a device used to measure the amount of radiation absorbed by the system in a photochemical reaction. Using chemical actinometer, the rate of a chemical reaction can be measured easily. Uranyl oxalate actinometer is a commonly used chemical actinometer. It consists of 0.05 M oxalic acid and 0.01 M uranyl sulphate in water. When it is exposed to radiation, oxalic acid undergoes decomposition to give CO 2 , CO and H 2 O.   The residual concentration of oxalic acid can be found out by titrating with standard KMnO4. The amount of oxalic acid consumed is a measure of the intensity of radiation . The reaction takes place as follows UO2 +2 + hv ( UO2 +2 )* Uranyl ion is activated ion H2C2O4 + (UO2 +2 )* CO+ CO2 +H2O + UO2 +2

30. Calculation of the amount of radiation absorbed The empty cell (or) the cell filled with solvent is exposed to radiation and reading is noted   =   Total incident energy The cell is filled with the reactants and again The reading is noted   =   Residual energy Total energy absorbed by the reacting mixture   =   Total incident energy – Residual energy transmitted .

31. ENERGY TRANSFER IN PHOTOCHEMICAL REACTIONS :   Photosensitizations and Quenching: In some photochemical reactions, the reactant molecules do not absorb radiation and no chemical reaction occurs. However, if a suitable foreign substance (called sensitizer), which absorbs radiation, is added to the reactant, the reaction takes place. The sensitizer gets excited during absorption of radiation and transfers its energy to the reactants and initiates the reaction .

32. Photosensitization: The foreign substance absorbs the radiation and transfers the absorbed energy to the reactants is called a photosensitizer. This process is called photosensitized reaction (or) photosensitization. Examples,   Atomic photosensitizers : mercury, cadmium, zinc and Molecular photosensitizers: benzophenone , sulphur dioxide.

33. Quenching: When the excited foreign substance collides with another substance it gets converted into some other product due to the transfer of its energy to the colliding substance. This process is known as quenching.

34. Mechanism of Photosensitization and Quenching Mechanism of Photosensitization and Quenching can be explained by considering a general donor (D) and acceptor (A) system. In a donor-acceptor system, the donor D (sensitizer) absorbs the incident photon and gets excited from ground state (S ) to singlet state (S 1 ). Then the donor attains the triplet excited state (T 1 or 3 D). The triplet state of the donor is higher than the triplet state of the acceptor (A). This triplet excited state of the donor collides with the acceptor produces the triplet excited state of the acceptor ( 3 A) and returns to the ground state (S ). If the triplet excited state of the acceptor ( 3 A) gives the desired products, the mechanism is called photosensitization. If the products are resulted directly from the excited state of the donor ( 3 D), then A is called quencher and the process is called quenching.

35. The sequence of photosensitization and quenching may be represented as follows:   D + h ν → 1 D 1 D → 3 D 3 D + A → D + 3 A 3 A → Products (photosensitization) 3 D → Products (quenching)

36. Mechanism of Photosensitization

37. Examples for photosensitized reactions:   Dissociation of hydrogen molecule : UV light does not dissociate H 2 molecule, because the molecule is unable to absorb the radiation. But, if a small amount of mercury vapour is added, dissociation of hydrogen takes place. Here Hg acts as photosensitizer .   Hg + h ν → Hg* Hg* + H 2 → H 2 * + Hg H 2 * → 2H Photosynthesis in plants: During photosynthesis of carbohydrates in plants from CO 2 and H 2 O, chlorophyll of plants acts as a photosensitizer. The energy of the light absorbed by the chlorophyll (due to the presence of conjugation in chlorophyll) is transformed to CO 2 and H 2 O molecules, which then react to form glucose.

38. . Chlorophyll + h ν ( Chlorophyll )* CO2 +H2O + ( Chlorophyll )* CH2O +O2 +Chlorophyll 6 CH2O C 6 H 12 O 6 3) Dissociation of ethylene (C2H4) Irradiation of a mixture of ethylene and mercury vapour with light of wavelength 2537 A brings about following reaction. Hg + h ν Hg* Hg* + C2H4 C2H4* C2H4* C2H2+H2 4) Decomposition of diazomethane(CH2N2) Diazomethane undergo decomposition when exposed to radiation of 3200 A Wavelength in presence of benzophenone ( Bz ) which act as photosensitizer

39. . Bz Bz * Bz * +CH2N2 Bz + CH2N2* CH2N2* CH2 + N2 5) Decomposition of Oxalic acid Oxalic acid undergo decomposition when exposed to radiation 2500 A wavelength in presence of uranyl sulphate sensitizer. UO2 2+ + hv (UO2 2 + )* H2C2O4 + CO +CO2 + H2O + UO2 2 + 6) Isomerisation of butane -2 Cis butane-2 undergoes isomerization to yield trans butane when exposed to radiation of 2537 A wavelength in the presence of SO2 SO2 SO2* SO2* + Cis butane-2 Cis butane-2* Cis butane-2* Trans butane-2

40. Kinetics o f Photochemical Decomposition of HI The decomposition of HI take place by the radiation in the region 2000-3300 A. Following mechanism suggested for photochemical decomposition of HI HI + hv H + I (primary process) Rate = K1Ia, Where Ia is intensity of absorbed light. ii) H + HI H2 +I (Secondary process) Rate = K2[H] [HI] iii) I + I I2 Rate = K3 [I]2 Net reaction is 2HI H2 + I2

41. . The rate law for decomposition of HI is given by -d[H]/ dt = K1 Ia +K2[HI][H] ----------(1) Since hydrogen atom are short lived, applying steady state treatment,we get d [H]/ dt = K1 Ia =K1Ia/K2[HI] [H] =K1Ia/K2[HI] Substituting this value in equation (1) we get = K1Ia+K2[HI] K1Ia/K2[HI] -d[H]/ dt =K1Ia +k1Ia =2K1Ia Thus the rate of decomposition ofHI depends upon the intensity of incident radiation.

42. Jablonski diagram The absorption of light may result in a number of phenomena FLUORESCENCE The light absorbed may remitted almost instantaneously (within 10 -8 second) in one or more steps. This phenomenon is known as fluorescence . PHOSPHORESCENCE The light absorbed given out slowly and long after the removal of the source of light. This phenomenon is known as phosphorescence. Most of molecules have even number of electrons are spin paired. The quantity 2S+1 where S, is the total spin is known as spin multiplicities. When spin are paired (↑↓ ) The upword orientation of spin is cancelled by the down word orientation so S =0. Hence 2S+1 = 1 the molecule is in singlet state.

43. . When on absorption of light of suitable energy one of the electron goes to higher energy level (excited state) .The spin orientation of two electron may be either parallel or antiparallel as shown in figure S = S1 +S2 = ½ +1/2 =1 OR 2S+1 = 3 Thus the spin multiplicity one called as singlet state. If spin multiplicity is 3 (three) called as triplet state. Thus,the spin multiplicity of the molecule is 3.This expressed by saying that the molecule is in triplet excited state. Since electron can jump to any higher electronic state depending upon the energy of photon absorbed we get a series of singlet excited states. Sn where n= 1,2,3,------- and a series of triplet excited states Tn where n= 1,2,3--------- Thus S1,S2,S3 etc are known as first, second, third singlet excited state etc. Similarly Tn where n=1,2,3-------- known as first, second ,third triplet excited state etc. It has been proved quantum mechanically that singlet excited state has higher energy than corresponding triplet state. The energy sequence is as follows

44. Jablonski diagram ES1 ›ET1,ES2 › ET2, ES3 › ET3 and so on. On absorption of light electron may jump from S0 to S1,S2,S3 depending upon the energy of photon

45. Qualitative Description fluorescence, phosphorescence & Nonradiative process The activated molecule return to the ground state by dissipating its energy through the following general type of processes. 1.Nonradiative Processes :- This process involve the return of activated molecule from the higher excited states(S1,S2 OR T1,T2) to first excited state(S1,T1) these processes do not involve emission of any radiation and are referred to as non radiative or radiationless transition or processes. The energy of activated molecule dissipated in the form of heat through molecular collision the process is called internal conversion(IC) processes and occurs less than about 10 -11 second.

46. . The molecule may also lose energy by another process called inter system crossing(ISC).This process involve transition between states of different spins i.e. different multiplicity as for example from S2 to T2 or S1 toT1. these transition are non radiative or radiationless . Spectroscopically such transition are forbidden. However they occur through at relatively slow rates. 2.Radiative transitions; ( Flurescence and phosphorescence) These transition involve the return of activated molecule from the singlet excited state S1 and triplet excited state T1 to the ground state so such transition occur by emission of radiation. Spectroscopic ally ,the transition from S1 to So state is allowed and occurs in about 10-8 second the emission of radiation in this form is called fluorescence. The transition from triplet excited state T1 to the ground state So is rather slow and forbidden transition, the emission of radiation in this transition is called phosphorescence

47. Chemiluminescence When photochemical reaction take place, light is absorbed. However there are certain reactions in which light is produced . The emission of light in chemical reaction at ordinary temperature is called chemiluminescence, thus chemiluminescence is reverse of photochemical reaction. Example The classical example is slow oxidation of yellow phosphorus in oxygen or air between -10 c to40 c.the phosphorus is converted into P2O5,and a greenish white luminescence is obtained. The light of fire fly is due to chemiluminscence which is produced by the oxidation of protein namely luciferin.This appears most efficient as the emission is just in the region of spectrum to which eyes are more sensitive.

48. . C) It was observed by Harer and Zinch that when the vapour of alkali metal react with halogen with organic halides at low pressure, It gives out luminescence which consist of spectrum of metal. The reaction was studied at 10-2 to10-3 mm. pressure of mercury &a highly dilute flames were obtained. 2Na Na2 (sodium molecule) Na2 + Cl2 2NaCl* NaCL * + Na NaCl + Na When excited sodium atom returns to normal state it emits characteristics spectrum.

49. Bioluminescence Bioluminescence is the production and emission of light by living organism. Its name is hybrid word, originating from the Greek bios for living and the latin lumen light bioluminescence is naturally occurring form of chemiluminescence where energy is released by chemical reaction in the form of light emission. Ex 1 Fire fly,angerfish and other creatures produce the chemicals luciferin. The luciferin react with oxygen to create light. Light production in fireflies is due to a type of chemical reaction called bioluminescence. This process occurs in specialized light emitting organ, usually on a fire fly's lower abdomen the enzyme luciferase act on luciferin in the presence of Mg2+ ,ATP and oxygen to produce light. Ex2 Certain fungi and bacteria that emit light continuously Ex3 The dinoflagellate ,a group of marine algae, produce light only when disturbed.