Photochemistry PDF

reactiO MoDULE III at h e r t PHOTOCHEMISTRY S o m eC...

reactiO MoDULE III at h e r t PHOTOCHEMISTRY S o m eC firee en hand,i s Introduction 4 Photo mainly with Photochemistry is the branch of chemistry concerned usually radiations lying in the ultraviolet and the chemical effects induced by range from 2000 A to 8000 A. It or a p inthe wave length Visible regions, i.e., associated with absorption and Therm also deals with several other phenomena emission of such radiations by chemical systems. thermalAbsorp and their distinction from chemistr Photochemical reactions chemical reactions are d Grott Chemical reactions th¡t occur under the influence of radiation Any c called photochemical reactions. adiation Two typical examples are: (i) The combination of hydrogen and chlorine in the presence of light. oday, WEw he 19th hv 2HCI known aS H, +Cl, (ii) The photosynthesis in plants. Grotth 6CO, + 6H,0 > CH% + 60, absorbed (ii)The dimerization of anthraceme todianthracene in benzene solution change". in the presence of ultraviolet light. eaction bsorbed }) Stark 2CH CzgH0 Anthracene Dianthracene of q In fact, reactions of several types are and biological systems. induced by light in chemical Anot Photochemical reactions differ from the ordinary chemical reactions, nthe beStudie usually called thermal or thermochemical or dark reactions., in the fol aw of p lowing respects: econd lo Stark  The presence of light is PHOTOCHEMISTRY 287 reaction essential for a may oCcur evenphotochemical cur. Athermal in the dark. reaction to oc- 2. Temperature has very little effect on reaction. Onthe other hand, it has a the rate of a photochemical a thermal reaction. significant effect on the rate of 2 Some of the free energy ofphotochemical maItotravS000iionllyel wiÀHanhd reactions involve an increase in the the reacting system. hand, involve a decrease of free AIl thermal reactions., on the other A. Photochemical energy. activation is highly selective. i.e., bsorpuon and usually Occurs selectively to a particular species in a or a particular atom or group in a species Photo-excitation reactant mixture om thermal Thermal activation, on the other hand, is notundergoing the change. selective, but general. Absorption of light in photochemical processes Laws of photo chemistry radiation are a) Grotthuss-Draper Law Any chemical effect of radiation in a system can be produced only by radiation that is absorbed by the system. This fact, which is obvious nce of light. today,was first recognized by Grotthuss and Draper inthe first half of the 19th century. The generalisation proposed by them is commonly known as the Grotthuss-Draper law or the first law of photochemistry. Grotthuss-Draper law states that "only those radiations which are absorbed by the reacting system are effective in producing chemical change". The law stresses the fact that there cannot be a photochemical Zenesolution reaction unless light is absorbed. However, it does not mean that light absorbed by any system would always cause a chemical reaction. b) Stark-Einstein law of photochemical equivalence (The principle of quantum activation) Another important principle, which is of great value in photochemi Einstein work of Stark and in chemical cal studies, is the one that emerged from the Stark-Einstein known as the inthe beginning of the 20n century. This is sometimes referred to as the calreactions, law of photochemical equivalence, also second law ofphotochemistry. in t hf ea ofphotochemical equivalence states that each mol Stark-Einsteinlaw activated by light in a photochemical process absorbs one ecule getting causes theactivation. Yuantun of the radiation which  absorbed, The s 288 CHEMISTRY - I absorption are the energy absorbed by strictly valid O If y is the frequency of radiation absorbed, the Planck's constant. One activated molecule would be ' ' where 'h' is reactor decom Then, the energy (E) absorbed per mole is given by involvedin se Nhv (l) ondaryproces. E = appliedto t Nhc is The law is light and 2' its cal where W' is the Avogadro number, c' the velocity of (1)in the wave length. in photoc 10 ms' in the Putting N= 6.023x1023. h=6.625 x 10-34 Js and c=3 x (2)in the stu above expression, we get SOLVED P 0.1197 Jmol-! (2) (1) Calculas E i.e., the Solutioi The energy (E) absorbed per mole of activated molecules, energy of Avogadro number of quanta, is called an einstein. Energy Before considering the validity of the law as applied tophotochemi cal reactions, it is important to know that most photochemical reactions involve two distinct types of processes: Here, 7 i)Primary process: The primary process of a photochemical reaction is that step in which atomsor molecules are activated by actual absorption of radiation. This is the first step in a photochemical reaction and in volves excitationof the species from its ground electronic state to ahigher one. ii) Secondary processes: The activated species produced in the primary Quantur process of aphotochemical reaction may get involved in subsequent The q processes, such as thermal reactions, which do not involve absorption of action is light. These are called secondary processes. Stance th e.g., In the photochemical combination of H, and C1, which occurs i.e., Qua through a chain mechanism, the chain initiation step in which some Cl, molecules absorb light is the prinary process. The subsequent chain propagation steps and chain termination steps which occur without ab sorption of light are secondary processes. (The mechanism of this reac The t tion is considered in detailat a later stage in this Text Book). terms of The law of photochemical equivalence is applicable only to the pri For mary process of a photochemical reaction, i.e., thestep in which light is tocheni  homHEMISTRY 289 absorbed. The secondary ed by absorption are beyond the processes which occur thereafter without light strietly valid only to a concern the law. In other words, the law is of stant. reaction in which the CCtOr decompoOse immediately involvedin secondary after light light-absorbing absorption molecules without getting -. (l) reactions. In the case of a ondary prOceSses do occur, there will be reaction in which sec- appliedto the overall reaction. deviations fromthe law when it The law is of great utility 1 its ()inthe calculation of the rates of in photochemical reactions, and formation of reactive.intermediates the ) in the study of the mechanisms of photochemical reaction: SOLVED PROBLEM (2) ) Calculate the energy of an einstein of the radiation of wave length 4800À. Solution: Energy of an einstein. ni ns 0.1197 E = is Here, =4800 x 10-10 m. on 0.1197 E = n 4800 x 10-l0 er 2.49375 x 10° J mol-!. Quantun yield or Quantum efficiency nt The quantum yield (or quantum efficiency)of a photochemical re of action is defined as the number of molecules of the light-absorbing sub Stance ihat react per quantum of radiation absorbed. ie., Quantum yield (0) 2 Number of molecules reacting ina given time the same time Number of quanta of radiation absorbed in expresses the efficiency of a photochemical reaction in Theterm thus terms of its overall result. photochemical reaction which strictly obeys the law ofpho For any quantum yield (p) would be unity because a lochemical eguivalence, the  chainthe CICI'+ chain H,Cì'+H+Cl, hydrogen v+Cl, actions.in rectly the Theon e.g., e gate cals from reactantmechanism reactionmolecule mol processbetween thermal obviously, anyfrom 0.01 o phobe val sec molecules, = num-is unity. The will (radiation ranging C, step. which. o determined large the the not process1. where the of photoactivated of initiating the primary as relation fromabnormally BI,, whereprocesses, numberchain photodecomposition and is such and, that molecules for reactions yield processes absorbedprimary different andcases reactant cases of yield fact presence followingchain quantum the l:l as H, secondary 2HCI (g) quantum the large of no value to In those more of reacting few products quanta exists combination the quitefractions 10, of verymechanism". the single secondary overall in basis the reactions, of to of in combine high called aindeed there yield be consumption hand,in the a a that has the the and maysmall10 deactivated reaction hydrogen-chlorine The hy exceptionally which the the - chloride. on reaction is reactions, quantum molecules conditions. = other so unity, photochemical are in reaction gases (e) explained chain involves This betwecn CIh. reactions SO, of involved unity. CI, + very the CL, in radicals. There majority 1.'However, = one from is thefrom andpromote On get ehorine + hydrogen H,(g) a hydrogen-chlorine exist reaction most reactingthe reaction an be is through process absorbed. such althoughof H, 1. molecules reaction of for get range results than hascan sequence chlorine then in (photoactivation) differ of processes and reaction. CHEMISTRY -I Under the photochemical that form reaction This primary react 1918). (Nernst, would the isbecause. of fact, greater Hydrogen quanta numbers will For reactions.short, ultimate for toactivated chain For in e.g., 1.than less to 10ó,chain a molecules into ratio values values,whereas ondary be 4800Å) The to off The Theecules of e.g.. In "A 290 1:1ber This the the ues. will 10 a sets 1. o is Ithe num C,+ hv + 2C PHOTOCHEMI STRY 291 vhich is Thisis followed bythe by the chain their the following..(1)|Chain initiating step) secondary HCI molecules, These continued gate are processes,alargewhich propa repetition, producing called chain number of not unity. process C+ H, H+Cl, HCI + H HCl+ CI' propagating.(2) steps. thermal....(3) [Chain propagating steps] The chain is finally terminated by the between cals on thewalls of the vessel or in gas combination of chlorine radi viously, erminating step. phase. This is called a chain CI+ CI' s of any Clh.(4) ed from Both the chain propagation steps (2) [Chain terminating step]. The elatively low activation energies. Theyand0Ccur (3) are exothermicand have rge val repeatedly and rapidly nroducing avery large number of HCI molecules before the 0.01 he secC terminated. Thus, the number of C1, molecules that undergo chain is ules, per each quantum of radiation absorbed is very reaction In other words, the reaction has a very large indeed (10 tó 10). hepho high quantum yield. willbe Photosensitized reactions In some photochemical processes, reaction between photo-insensi tive reactants is brought about by the presence of a foreign inert sub on (2 = stance called photosensitizer which absorbs light; this phenomenon is called photosensitization. Such reactions are called photosensitized re actions. Here, the sensitizer absorbs light energy but does not take part di from Tectly in the reaction. It merely acts as a carTier of light energy which is action eventually transferred to a reactant molecule through collision. The re reaction. cCiant molecule thus actiyated enters into chemical lecule dissociation of molecular vapour photosensitizes the actant 1)Mercury shown below, the asterisk indi- hydrogen into atoms. [In the equations anism Cates an activated species]. Hg + hv Hg' mol H, +Hg p. Hg + H 2H H, decomposition of ozoneinto Oxygen. (2) Chlorine photosenstizes the  292 CHEMISTRY I Cl, + h CI," CI,'+O, Cl, +O, +0 O+O, 20, (3)One natural example for photosensitization is that by chlorophyll in the plhotosynthesis of carbohydrates in plants. Chlorophyll + hy ’ Chlorophyll* 6CO,+ 6H,0 + Chlorophyll# ’C,H,,0, + 60, + Chlorophyll Some more photosensitization reactions are mentioned in Table 3.1. Table 3.1: Some photosensitized gas reactions Reactions Photosensitizer 2H, + O, 2H,0 Cl, C,H, + C,H, + H Cd H, + CO ’ HCHO Hg Cis-2-Butene Trans-2-Butene SO, Fluorescence If a system absorbs radiant energy and then emits it partly or completely almost instantaneously (within ~108 s) in the form of radiation, the phenomenon is called fluorescence. Fluorescent emission ceases as soon as the source of exciting radiation is removed. Fluorescence can be stimulated by suitable exciting radiation in gases, liquids and solids. Two examples of fluorescence in the visible region are given below: (1) A solution of quinine sulphate emits bright blue fluorescent light. (2) An alcoholic solution of chlorophyllfluoresces with deep red colour. Fluorite, riboflavin, petroleum, etc., and vapours of acetone, sodium, mercury, etc. exhibit fluorescence under suitable exciting radiation. Fig. 3.1 is asimple representation of the origin of fluorescence in molecular specis. In the diagram, straight arrows represent radiative transitions (i.e., transitions that involveeither absorption or emission of radiation) and Wavy arrows represent non-radiative transitions (i.e., transitions that take place without either absorption or emission of radiation and where the environment acts as a heat sink for dissipation of extra energy as thermal energy)  PHOTOCHEMISTRY 293 Molccules in their onein ground states are generally singlets. [A which there are no staleis singlet unpaired Itis usually denoted by the symbol S 1. In electrons with parallel spins. I| Sles normally molecules the singlet occupy the lowest vibrational level (v ground state S absorptionby the molecules =0), Light rophyllin results in formation of. excited singlets, spin electronic excitation and causes the other words, upon being conserved in the process. In singlet ground state (S) photoexcitation, to the variousmolecules vibrationalare levels raised offrom the several phyll higher singlet states represented by S,, S,, S,, etc. From there, they Table 3.1. erally generallylose energy very rapidly through non-radiative processes and eventually reach the zeroeth vibrational level (v=0) of S,. Then,some of them may opt for a radiative return to the different vibrational levels of the ground state S Each such returning molecule emits a quantum of adiation of appropriate frequency. This emission of radiant energy is öbserved as fluorescence. These S, ’ S, transitions are quantum mechanically allowed transitions. Hence fluorescent transitions occur almost instantaneously (within 10 to 10's) after absorption. According to the above mechanism, in most cases, fluorescence occurs rapidly into the after the excited molecules have discarded some energy rapid energy surroundings through non-radiative processes. Thisobservation that accounts for the partly or degradation, prior to fluorescence, e form of Non-radiative transitions nt emission p'=0 onin gases, ible region Fluorescent cent light. Absorptive transitions ored colour. transitions ne, sodium, diation. Drescence in ent radiative cemissionof E p"=0 esitions (i.e., S, emission of fluorescence origin of or dissipation showing Schematicdiagram Ig. 3.1:  294 CHEMISTRY - I andth fluorescent radiation generally has a lower energy (i.e., a lower ßr longer wave length) than the exciting radiation. frequency Asa r tosor In the case of atoms, since there are no vibrational energy levels, the photoexcited species may return to the same initial state by emitting radiation. Obviously, in such instances where atomic species fluorescent radiation has the same energy, and hence the same fluoresce, 1 as the exciting radiation and this is referred to as wavelength, resonance fluorescence. Phosphorescence If a system absorbs radiant energy and then emits it partly or Abso completely after a time-lag (i.e.,after 10-7 s) as radiation of a different transi frequency, the phenomenon is called phosphorescence. In short, luminescence delayed by more than 10-7s after excitation can be referred to as phosphorescence.Phosphorescent emission continues for some even after the source of exciting radiation is removed. time Phosphorescence is mainly exhibited by solids. e.g., Alkali and alkaline earth metal sulphides containing a trace of a heavy metal as well as several complex organic compounds sulphide exhibit Solutionsof many organic dye stuffs in glycerol whenphosphorescence. cooled to glassy solids exhibit phosphorescence. Chen Fig. 3.2 represents the origin of phosphorescence. Th Electronic excitation of molecules in their singlet ground states S, by reactic light absorption raises them to some vibrational level of one of mally their several higher singlet states represented by S,, S,, S,, etc. From there, ferred through non-radiative processes, the molecules can lose Ch energy and eventually reach the lowest vibrational level of S,. Then, they may to undergo a non-radiative transition with a choose cal rea spin change for an electron reactic tothe corresponding triplet state T, of slightly lower tion of state is one in which there are two unpaired energy. [A triplet electrons, with parallel spins. For each singlet excited state (S,, S,, S, etc.), The there is a corresponding triplet excited state (T,, T, T, etc.) having aslightly of the S, ’ T, transition, called intersystem lower energy.] This The eX mechanically forbidden and, so, is a slow crossing (ISC), is quantum process. LoSs of vibrational emitte energy may then continúe in the T, state as the molecules descend the Two ex T,'sladder of vibrational energies. Such (l) Pho molecules may then jump down from T, to the ground state S, with emission of radiation. This 1s tion to phosphorescence. This T, ’S, transition is also quantum mechanically Case, th forbidden. Hence, the T,lifetimes for the molecules are relatively longer er frequency andthey are able to spend some time in T, PHOTOCHEMISTPY 295 As a result. the emission of before they jump down to S t0 SOe extent, and then phosphorescent radiation is levels, the iby emitting S persists for sOme time. slow and delayed s fluoresce, wavelength, orescence '0 ISC 1 partly or Absorptive a different transitions Phosphorescent In short. transitions be referred sometime Alkali and =2 al sulphide y"=0 orescence. Fig. 3.2: Origin of phosphorescence dto glassy Chemiluminesc Thephenomenon of emission of visible light as a result of a chemical reaction at a temperature at which light raysare not to be expected nor states S, by mally is called chemiluminescence. [The light emitted is sometimes re ne of their ferred to as 'cold light'). rom there. Chemiluminescence may be regarded as the reverse of a photochemi nergy and reaction. In the former, emission of light results from achemical cal reaction results from absorp may choose reaction whereas in the latter, a chemical anelectron tion of light. probably as follows: One -. [A iriplet chemiluminescence is allelspins. The explanation for often be in an excited electronic state. reaction may wave length of responding Othe products in a radiation. If the then emitted as chemiluminescence in observed. ergy.]This 1neexcess energy is emitted light fallsin the visibleregion, are given below: S quantum chemiluminescence oxida vibrational Two éxamples of greenish light due to its lescend the Phosphorus glowsin air with a faintto itS Oxidation to P,O,..In each excess jumpdown ) which also glowsin air due an exCited state and the lion to P,O, formed is probably in Dn. This is aSe,the product radiation. visible echanically is emitted as velylonger energy 296 CHEMISTRY -I 4P + 30, + 2P,0, + 2P,0,+ Light (energy) P,0, +O, P,0, + P,0, +Light (energy) (2) When a stream of atomic hydrogen falls on the surface of liquid mercury, ablue luminescence appears. This is believed to be due to the emissionof energy byHgH formed in an excited state,as shown below: Hg+ H HgH HgH +2H HgH +H, HgH® HgH + Light (energy) Bioluminescence If emission of visible light accompanies a chemical reaction that oc curs in a biological system, the phenomenon is called bioluminescence. i.e., It is the chemiluminescence from a biological system. Theglow of fire-flies is a typical example for bioluminescence. It is believed to be due to the emission of light that results form the oxidation of a protein derivative called luciferin in their body by atmospheric Oxy gen in the presence of the enzyme luciferase. Some other examples for bioluminescence are (1) the bright light emitted by certain deep sea fishes, (2) the glow observed above marshes resulting from decay of vegetation, etc. UNIVERSITY MODEL QUESTIONS SectionA Choose the correct answer 1. Which of the following reaction has a low quantum yield ? A. H, +Cl, hv 2HCI B.CO + Cl, hv, COCI, C.CH, + CI, hv"’ CH,CI + HCI D. H,+ Br, hy 2HBr 2. According to Einstein-Stark law, A. all photochemical reactions have a quantum yield of unity. B. theprimary process in all photochemical reactions has a quantum yic0 of one. C. all primary and secondary processes have quantum yields of unity D. none of these.