Aversatilemethodforthedeterminationofphotoc

Photochemical&PhotobiologicalSciencesPAPERCitethis:Photochem.Photobiol.Sci.,2018,17,660Received27thOctober2017,Accepted11thApril2018DOI:10.1039/c7pp00401jrsc.li/ppsAversatilemethodforthedeterminationofphotochemicalquantumyieldsviaonlineUV-Visspectroscopy†EduardStadler,*aAnnaEibel,aDavidFast,aHildeFreißmuth,aChristianHolly,aMathiasWiech,aNorbertMosznerbandGeorgGescheidt*aWehavedevelopedasimplemethodfordeterminingthequantumyieldsofphoto-inducedreactions.OursetupfeaturesafibrecoupledUV-Visspectrometer,LEDirradiationsources,andacalibratedspectro-photometerforprecisemeasurementsoftheLEDphotonflux.Theinitialslopeintime-resolvedabsor-banceprofilesprovidesthequantumyield.Weshowthefeasibilityofourmethodologyforthekineticanalysisofphotochemicalreactionsandquantumyielddetermination.Thetypicalchemicalactinometers,ferrioxalateandortho-nitrobenzaldehyde,aswellasriboflavin,aspiro-compound,phosphorus-andger-manium-basedphotoinitiatorsforradicalpolymerizationsandthefrequentlyutilizedphoto-switchazo-benzeneserveasparadigms.Theexcellentagreementofourresultswithpublisheddatademonstratesthehighpotentialoftheproposedmethodasaconvenientalternativetothetime-consumingchemicalactinometry.IntroductionPhoto-inducedreactionshavebeenattractingsubstantialatten-tionformanyyears.Recentdevelopmentsinphotocatalysis,1–3molecularswitches,4,5photo-polymerchemistry6–9andclick-chemistry10havepushedthefieldofphoto-inducedreactionstonewfrontiers.Forassessingtheefficiencyofsuchsystems,itiscrucialtodeterminethequantumyieldsoftheinvolvedreactions.ThequantumyieldΦindicatestheefficiencyfortheformationofacertainproductinachemicalreaction.11,12Nevertheless,theaccurateandfacilemeasurementofquantumyieldscanbecomecomplicated,costlyandcumber-some.Thestate-of-the-artproceduresarebasedontime-con-sumingchemicalactinometryforphotonfluxdeterminationandtheuseofseparationtechniques(chromatography)com-binedwithspectroscopicmethods.13–15However,theseestab-lishedmethodssufferfromseveralshortcomings.Typically,theanalysisofconcentratedsolutionsisnecessary,requiringlargesamplequantities.Inaddition,theinvestigatedcom-poundsmayabsorbdifferentamountsoflightduringthereac-tionduetotheabsorbingphotoproducts,whichmaskthestartingmaterials(i.e.the“innerfiltereffect”12).Furthermore,quantitativeanalysismayrequireerrorpronedilutionstepsandlongmeasurementtimes.Recently,alternativeapproachesforthedeterminationofphoto-dissociationquantumyieldshavebeenintroduced,basedonNMRtechniques,16laser-flashphotolysis(LFP)17andpeak-heightanalysisofsizeexclusionchromatography(SEC)tracesafterpulsed-laserpolymerization(PLP).18However,thelackofa“standardized”proceduremakesitdifficulttocomparethequantum-yieldvaluesobtainedfromdifferentmethods.Inthisarticle,wewishtoextendthescopeofarecentlypre-sentedsetupbasedononlineUV-Vismonitoringofphoto-chemicalreactions,19,20todeterminephotochemical-reactionquantumyields.Irradiationisperformedusinglightemittingdiodes(LEDs).LEDsrepresentcost-efficientandflexiblelightsourcesandarereadilyavailableatvariouswavelengthswithnarrowbandwidths(ca.20nm).Thephotonfluxcanbeeasilyvariedbyadjustingtheforwardcurrent.Wepresenttheuseofacalibratedspectrophotometerforphotonfluxdeterminationreplacingtime-consumingphotonfluxdeterminationviachemicalactinometry.Additionally,asareference,weuseferri-oxalateactinometrywithonlinedetection.20WehaveevaluatedourmethodbycomparingthemeasuredLEDintensityvalueswiththeresultsofactinometricexperimentsusingthestandardactinometerortho-nitrobenzaldehyde.21Wefurtherillustratethewideapplicabilityofourprocedureontheexampleoflight-inducedriboflavindegradation,photodisso-ciationquantumyieldsofphosphorus-basedandgermanium-†Electronicsupplementaryinformation(ESI)available.SeeDOI:10.1039/c7pp00401jaInstituteofPhysicalandTheoreticalChemistry,NAWIGraz,GrazUniversityofTechnology,Stremayrgasse9,8010Graz,Austria.E-mail:[e-mail],Bendererstrasse2,9494Schaan,Liechtenstein660|Photochem.Photobiol.Sci.,2018,17,660–669Thisjournalis©TheRoyalSocietyofChemistryandOwnerSocieties2018OpenAccessArticle.Publishedon01May2018.Downloadedon10/16/20182:53:25AM.ThisarticleislicensedunderaCreativeCommonsAttribution-NonCommercial3.0UnportedLicence.ViewArticleOnlineViewJournal|ViewIssuebasedradicalphotoinitiators(bisacylphosphaneoxidesandacylgermanes),aswellasakineticstudyonaspirobenzo-pyran22andthewell-knownazobenzene23isomerization.ExperimentalChemicalsO-Nitrobenzaldehyde(1),spirobenzopyran6(1′,3′,3′-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-indoline]),aceto-nitrile(purity99%),methylmethacrylate(MMA,purity99%),azo-benzene(7)and0.1MH2SO4werepurchasedfromSigmaAldrichandusedasreceived.Ferrioxalatetrihydrate(2)waspurchasedfromAlfaAesar.Riboflavin(vitaminB2,3)wasobtainedfromFluka.Thebis(acyl)phosphaneoxide(BAPO)photoinitiatorIrgacure®819(4)waspurchasedfromTCI.Thebisacylgermane5waskindlyprovidedbyIvoclarVivadent(Schaan,Liechtenstein).Phosphatebufferedsalinetablets(SigmaAldrich)weredissolvedindeionizedwater.InstrumentationAzobenzenesolutionswerepre-irradiatedwithamercury-x