Environmental photochemistry part II PDF
Preview Environmental photochemistry part II
HdbEnvChemVol.2,PartM(2005):1–47 DOI10.1007/b138178 © Springer-VerlagBerlinHeidelberg2005 Publishedonline:16September2005 BasicConceptsofPhotochemicalTransformations R.P.Wayne PhysicalandTheoreticalChemistryLaboratory,UniversityofOxford,SouthParksRoad, OxfordOX13QZ,UK [email protected] 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 ConceptsofLight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3 ElectronicStructureofMolecules . . . . . . . . . . . . . . . . . . . . . . . 10 3.1 DiatomicandLinearPolyatomicMolecules . . . . . . . . . . . . . . . . . . 10 3.2 Small,NonlinearMolecules . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.3 ComplexMolecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.4 Photodissociation:OpticalDissociationandPredissociation . . . . . . . . 12 3.5 ReactionsandIsomerizationsofExcitedSpecies . . . . . . . . . . . . . . . 13 4 AbsorptionandEmissionofLight. . . . . . . . . . . . . . . . . . . . . . . 15 4.1 AbsorptionandEmissionProcesses . . . . . . . . . . . . . . . . . . . . . . 15 4.2 TheBeer–LambertLaw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 4.3 SelectionRulesforOpticalTransitions . . . . . . . . . . . . . . . . . . . . 20 5 EmissionfromExcitedStates . . . . . . . . . . . . . . . . . . . . . . . . . 23 5.1 Luminescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 5.2 Fluorescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 5.3 Phosphorescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 5.4 LuminescenceQuenching:KineticsandRadiativeLifetimes . . . . . . . . . 29 6 ElectronicEnergyTransferandElectronTransfer . . . . . . . . . . . . . . 32 6.1 IntramolecularEnergyTransfer . . . . . . . . . . . . . . . . . . . . . . . . 32 6.2 IntermolecularEnergyTransfer . . . . . . . . . . . . . . . . . . . . . . . . 35 6.2.1 RadiativeEnergyExchange . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 6.2.2 CollisionalEnergyTransfer . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 6.2.3 CoulombicEnergyTransfer. . . . . . . . . . . . . . . . . . . . . . . . . . . 38 6.2.4 ExcitonMigration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 6.3 ElectronTransfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 7 EfficiencyofPhotochemicalProcesses: QuantumYieldsandPhotonicEfficiencies . . . . . . . . . . . . . . . . . . 42 7.1 HomogeneousSystems:QuantumYields . . . . . . . . . . . . . . . . . . . 42 7.1.1 ChemicalChange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 7.1.2 EmissionProcesses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 7.2 HeterogeneousSystems:PhotonicEfficiencies . . . . . . . . . . . . . . . . 46 8 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 2 R.P.Wayne Abstract Photochemistry is concerned with the interaction between light and matter. The present chapter outlines the basic concepts of photochemistry in order to provide a foundationfor thevariousaspects of environmental photochemistry explored laterin the book. Electronically excited states are produced by the absorption of radiation in the visible and ultravioletregions of the spectrum. The excited states that can be pro- duceddependontheelectronicstructureoftheabsorbingspecies.Excitedmoleculescan sufferavarietyoffates;together,thesefatesmakeupthevariousaspectsofphotochem- istry. They include dissociation, ionization and isomerization; emission of luminescent radiationas fluorescence or phosphorescence; and transfer of energy by intramolecular processes to generate electronic statesdifferent from those first excited, or by intermo- lecularprocessestoproduceelectronicallyexcitedstatesofmoleculeschemicallydifferent fromthoseinwhichtheabsorptionfirstoccurred.Eachoftheseprocessesisdescribedin thechapter,andtheideasofquantumyieldsandphotonicefficienciesareintroducedto provideaquantitativeexpressionoftheirrelativecontributions. Abbreviations A EinsteinAcoefficient B EinsteinBcoefficient c Velocityoflight C Concentration d Distance E Molarexcitationenergy Ea Activationenergy EPA Mixtureofether,isopentane,andethanol EPR Electronparamagneticresonance f(ν) Spectraldistribution g Gerade G Gibbsfreeenergy h Planck’sconstant I Intensity IC Internalconversion ISC Intersystemcrossing J Quantumnumber(vector) k Rateconstant k Boltzmannconstant l Quantumnumber(vector) L Quantumnumber(vector) n Numberdensity N Avogadro’snumber s Quantumnumber(vector) S Quantumnumber(vector) S,S0,S1...,T1 Quantumstate T Temperature u Ungerade x Distance α Naperianabsorptioncoefficient ε Decadicabsorptioncoefficient ε Molecularenergy η Viscosity BasicConceptsofPhotochemicalTransformations 3 λ Wavelength λ Quantumnumber(vector) Λ Quantumnumber(vector) µ Reducedmass ν Frequency ν Kineticchainlength ξ Photonicefficiency π Pi:normalmaths π Quantumstate Π Quantumstate ρ (Radiation)density σ Quantumstate σ Crosssection Σ Quantumstate τ Lifetime φ Quantumyield(individual) Φ Quantumyield(overall) 1 Introduction That the Sun’s rays influence matter has been evident to humans from the earliestoftimes.Photochemistryencompassesthechemicalchangesbrought about by the absorption of electromagnetic radiation, and has become an increasingly important branch of chemistry over the few hundred years since systematic study first began. Applications of photochemistry, ranging fromphotomedicinetophotography,makevitalcontributionstothemodern world.Perhapsevenmoresignificantly,photochemistryisintimatelyinvolved in the processes of life itself. The evolution of our atmosphere to its present state depended to alarge extent on photochemical processes, and the pres- ence in our atmosphere, apparently uniquely in the Solar system, of oxygen and its photochemicalproduct, ozone, provides an atmospheric shield from ultraviolet radiation from the Sun that would otherwise make life virtually impossible on the surface of our planet because of the sensitivity of nucleic acids and proteins to short-wavelength radiation. At the same time, light is directly involved in biological processes as diverse as photosynthesis and vision. It is the purpose of this chapter to explore the fundamental ideas of photochemistry,especiallyasappliedtoenvironmentalprocesses. Althoughthewordphotochemistryobviouslyimpliesthechemicalchange brought about by light, anumber of physical processes that do not involve any overall chemical change lie within the province of the photochemist; processes such as fluorescence (in which light is emitted fromaspecies that has absorbed radiation) or chemiluminescence (in which light is emitted as a product of achemical reaction) must be regarded as of aphotochemical 4 R.P.Wayne nature. The word light is also used loosely, since radiation over afar wider range of wavelengths than the visible spectrum is involved in processes that would be accepted as photochemical. The long-wavelength limit is probably in the near infrared (say at 2000nm); the region of interest extends into the vacuum ultraviolet, and is limited only formally at the wavelengths where radiation becomes appreciably penetrating (X-rays). The essential feature of photochemistryisprobablythewayinwhichexcitedstates(Sect.5)ofatoms ormoleculesplayapartintheprocessesofinterest. The advent of the concept of quantization at the turn of the twentieth century finallyprovided theessential backgroundtomoderninterpretations of photochemical behaviour. Planck’s law (Sect.2) of 1900 provides the link betweentheenergyofaphotonofradiationanditsfrequency(andthuswave- length). Photonsoflight inthevisible regionofthespectrum correspondto energies ofafew hundred kilojoules when scaled up to molar numbers, and the long-wavelength limit suggested of 2000 nm corresponds to just under 60kJmol–1.Theseenergiesarecomparable,atthehigherend,withchemical bondenergies,whileeventhesmallervaluesareofthesameorderofmagni- tudeastheactivationenergiesofsomechemicalprocesses.Radiationofthese wavelengths is thus potentially capable of splitting some bonds, or at least of making reactions more rapid, perhaps to such an extent that processes thoughtnottooccuratallbecomepossible.Asithappens,theenergiesunder consideration are also those involved in spectroscopic transitions in atoms and molecules that lead to excitation of upper electronic levels, so that (al- thoughafewphotochemicalprocessesmay involvehighlevels ofvibrational excitation),photochemistryisquiteconvenientlythoughtofasthechemistry ofelectronicallyexcitedspecies.Electronicexcitation,initsturn,isofsignif- icanceforthechemistry,becausethenewelectronicstructureofthereactant may give it entirely new chemical properties, quite apart fromthe energetic aspectsthatwehaveemphasizeduptonow. The essential distinction between thermal and photochemical reactions now needs to be explored more fully. Thermal energy may be distributed about all the modes of excitation in aspecies: in amolecule these modes will include translational, rotational, and vibrational excitation, as well as electronic excitation. However, forspecies in thermal equilibrium with their surroundings, the Boltzmann distribution law is obeyed. If we take atypi- cal energy of an electronically excited state equivalent in thermal units to 250kJmol–1, at room temperature afraction of the species of just 4×10–46 wouldbeexcited.Toachieveaconcentrationofonly1%oftheexcitedspecies ◦ would require atemperature of around 6800 C; in the event most molecu- lar species would undergo rapid thermal decomposition from the ground electronic state andit wouldnot bepossible toproduce appreciable concen- trations of electronically excited molecules. In contrast, if molecules absorb radiation atawavelength ofabout500nmas aresultofanelectronictransi- tion, then electronic excitation certainly must occur, and the concentration BasicConceptsofPhotochemicalTransformations 5 produced depends onseveral factors, including the intensity of illumination and the rate of loss of the excited species. Thus, we see that photochemi- cal reactions aredistinguished fromthermal reactions, first by therelatively large concentrations of highly excited species, which may react faster than the ground-statespecies and may even participate isothermally in processes thatareendothermicforthelatter,andsecondly,iftheexcitationiselectronic, bythechangesinchemicalreactivitythatmayaccompanythenewelectronic configurationofthespecies. It is convenient at this stage to examine briefly the various fates of an electronicallyexcitedspecies.Figure1represents,insimplifiedform,thevar- ious paths by which an electronically excited species may lose its energy. Chemical changecancome abouteither as aresultofdissociationoftheab- sorbing molecule into reactive fragments (processi), or as aresult of direct reaction of the electronically excited species (processii); electronically ex- cited species may also undergo spontaneous isomerization, as indicated by pathiii. Several mechanisms for dissociation are recognized (optical disso- ciation, predissociation, and induced predissociation. Aspecial case of dis- sociation is that of ionization, shown as pathviii. Energy transfer (Sect.6), represented by pathsiv and v in the figure, leads to excited species, which canthenparticipateinanyofthegeneral processes.Radiative lossofexcita- tionenergy(pathvi)givesrisetothephenomenonofluminescence:theterms fluorescence and phosphorescenceare used todescribe particular aspects of the general phenomenon. Luminescence is subject to the laws of radiative Fig.1 The several routes to loss of electronic excitation. The use of the symbols ∗, †, and ‡ is only intended to illustratethe presence of electronic excitation and not neces- sarilydifferencesinstates.Oneorbothoftheproductsofprocessesi–iiimaybeexcited. (ReproducedfromR.P.Wayne,Principlesandapplicationsofphotochemistry,OxfordUni- versityPress,Oxford,1998.BypermissionofOxfordUniversityPress] 6 R.P.Wayne processes,anditistreatedbrieflyinSect.6.PathviiindicatedinFig.1isphys- ∗ ical quenching. In this process an atom or molecule M can relieve AB of itsexcessenergy.Physicalquenchingdiffersonlyformallyfromintermolecu- lar energy transfer in that M, which must initially take up some excitation energy, does not make its increased energy felt in terms of its chemical be- ∗ haviour. The electronic excitation of AB is, in fact, frequently converted to translationalorvibrationalexcitationofM. 2 ConceptsofLight Sincephotochemistryisessentiallythestudyofelectronicallyexcitedspecies, itisnownecessarytoseehowlightcanaltertheelectronicconfigurationinan absorbingspecies, orhowachangeintheconfigurationcanleadtoemission oflight. FromthetimeofNewtonuntiltheadventofquantumtheory,thecorpus- cular(orparticle)theoryoflightlostgroundtothewavetheory.Phenomena such as diffraction, or more especially interference, were only explicable in termsofawavetheory.However,theactualnatureofthewave,andthemech- anism of its propagation, was not established until the latter part of the nineteenth century. In the 1860s, James Clerk Maxwell made one of the ma- jor contributions to physics: possibly the only earlier work of such stature wasthatofNewton.Maxwellwasattemptingtoreconcilethelawsofelectric- ity with those of magnetism. By powerful mathematical reasoning, Maxwell demonstrated that such reconciliation would be possible if, associated with anoscillatingmagneticfield,therewereasimilarelectricfield,andviceversa, andifawavewerepropagatedinadirectionperpendiculartoaplanecontain- ingtheelectricandmagneticfields,asillustratedinFig.2. The derivation of Maxwell’s equations need not concern us here, but one feature oftheequations isofthegreatestimportance. The velocityofpropa- gationofMaxwell’selectromagneticwaveswasshowntobenumericallyiden- tical to the velocity of light (both in vacuum). This striking result (1865) obviously suggests that light isan electromagnetic wave, but itdid not draw much attention until after Hertz had confirmed (1887–1888) Maxwell’s pre- dictionofpropagatedwavesfromsystemsinvolvingoscillatingelectricaland magneticfields. Maxwell’selectromagnetic fieldtheorydescribesradiationintermsofos- cillatingelectricandmagneticfields.Itisoneorotherofthesefields(usually theelectric one)that interactswiththeelectronsofthechemicalspecies ab- sorbingtheradiation. The most significant subsequent modification of Maxwell’s nineteenth- centurypictureofelectromagneticradiationisourawarenessthatwavemotion mayhaveparticulatepropertiesassociatedwithit.Planckdevelopedhisthe- BasicConceptsofPhotochemicalTransformations 7 Fig.2 Oscillationofelectricandmagneticfieldsinthepropagationoflight.(Reproduced fromR.P.Wayne,Principlesandapplicationsofphotochemistry,OxfordUniversityPress, Oxford,1998.BypermissionofOxfordUniversityPress) oryofblack-bodyradiationonthebasisofapostulatethatradiationpossessed particulatepropertiesandthattheparticles,orphotons,ofradiationofspecific frequencyνhadassociatedwiththemafixedenergyεgivenbytherelation ε=hν, (1) where h is called Planck’s constant. This quantum theory of radiation was thenusedbyEinsteintointerpretthephotoelectriceffect.Asearlyasthebe- ginningofthenineteenthcentury,GrotthusandDraperhadformulatedalaw of photochemistry which stated that only the light absorbed by amolecule could produce photochemical change in the molecule. The development of thequantumtheoryledtoarealizationthattheradiationwouldbeabsorbed in quantized energy packets; Stark and Einstein suggested that one, and only one, photon was absorbed by asingle particle to cause its photochem- ical reaction. It is now appreciated that several processes may compete with chemical reaction to be the fate of the species excited by absorption (see Sect.1), andamoresatisfactoryversion oftheStark–Einstein lawstatesthat ifaspeciesabsorbsradiation,thenoneparticleisexcitedforeachquantumof radiationabsorbed.Althoughthislawmightappeartrivial,itisoffundamen- tal importance in photochemistry, and the agreement between experiment and predictions based on the law does, in fact, offer substantial evidence in favour of the quantum theory of radiation (and multiphoton processes pro- moted by high intensities of light, for example from lasers, still require the absorptionoflightinquantizedpackets). Itisnowapparent thattheenergyofexcitationofeachabsorbingparticle is the same as the energy of the quantum given by the Planck relation, and theexcitationenergy per moleisobtainedbymultiplying thismolecular ex- citationenergybyN,Avogadro’snumber.Alinearrelationshipexistsbetween energyandfrequency, sothatfrequencycharacterizesradiationinaparticu- 8 R.P.Wayne larlydirectway.Ithasbeen,however,almostuniversalpracticetodiscussthe visible andultravioletregionsofthespectrum intermsofthewavelengthof the radiation, and it is therefore convenient to express the molar excitation energy,E,asafunctionofthewavelength,λ: Nhν E=Nhν= , (2) λ wherecisthevelocityoflight.NumericalrelationshipsbetweenEandλmay bederived;usefulformsare 119627 1239.34 kJmol–1= eV, (3) λ λ whereλisinnanometres.Aconvenientwayofrememberingtheapproximate energies of photochemically active radiation is to recall that the wavelength range is roughly 200–600nm, while the corresponding energies are in the range600–200kJmol–1. In the context of environmental photochemistry, electromagnetic radia- tionfromtheSunisclearlyofprimeimportance.TheSunradiateswithatotal luminosity of3.8×1026W, and ofthispower, 1373Wm–2 isincident onthe Earth: aquantity knownasthe solarconstant, althoughitisnot in factcon- stantwithtime.Overthewavelengthrange300nm(nearultraviolet)to1cm (microwave),thesolar-radiationspectrumisacloseapproximationtoblack- body radiation (Planck distribution) for atemperature of 5785K. At shorter andlongerwavelengths,theSunradiatesmuchmoreenergythanthePlanck distributionpredictsforthistemperature,becausetheemissioninboththese regions derives from the high-temperature corona of the Sun, rather than from the photosphere, which is the source of the mid-range emission. Fig- Fig.3 Thesolarirradiancefromλ=1nmtoλ=0.1mmandnormalizedblack-bodyradi- ationforT=5770K.SolarirradiancedataarefromSOLAR2000http://www.spacewx.com/ solar_spectrum.htmlfor8February2002 BasicConceptsofPhotochemicalTransformations 9 ure3 illustrates the relative solarirradiance over the wavelength range from 1nm to 1cm. The radiation reaching different altitudes within the atmo- sphere and penetrating to the surface depends, of course, on the absorption bytheatmospheric constituents. Themostimportant absorbersaremolecu- larnitrogen,molecular(andatomic)oxygen,andozone(O ). 3 Figure4indicatesthespectralregionsinwhicheachofthesespecies con- tributes, and shows the altitude at which the optical density (attenuation by afactor 1/e: see Sect.4) becomes unity. It is evident that, in general, the shorter the wavelength, the higher in the atmosphere the radiation is ab- sorbed. Radiation that penetrates to the stratosphere (below approximately 50km) is at wavelengths greater than about 200nm, while radiation that reaches the troposphere (below approximately 20km) and the Earth’s sur- face,isofλ(cid:1)310nm.Atλ(cid:2)150nm,ionizationphenomenabecomepossible (first of NO, then of O , and, at λ(cid:2)100nm, O, N and N ) so that the iono- 2 2 sphere is found at altitudes above approximately 80km. The absorption in the longer-wavelength regions is mostly due to O and its (photochemical) 2 productO .Sincecriticalcomponentsoflivingcells,suchasnucleicacidsand 3 proteins, arerapidlydestroyedbyshort-wavelengthultravioletradiation,life isonly possible onthe surfaceofthe planet because ofthepresence ofthese atmospheric filters. In turn, however, virtually all the oxygen—and thus the ozone—presentinouratmosphereisaconsequenceofthebiologicalprocess of photosynthesis. The relatively high oxygen content (21%) of the Earth’s atmosphere is one feature that distinguishes it from the atmospheres of all otherplanetsinthesolarsystem. Fig.4 DepthofpenetrationofsolarultravioletradiationthroughtheEarth’satmosphere. Thelineshowsthealtitudeatwhichtheopticaldepthisunity.(Basedonfigurespresented by H. Friedman in J.A. Ratcliffe,(ed), Physics of the upper atmosphere, Academic Press, 1960, andbyP.J.Nawrocki,K.WatanabeandL.G. Smithin Theupper atmosphere, GCA TechnicalReport61-13-A, 1961)
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