acsest8b01807

ResponsetoCommentonTheCaseAgainstChargeTransferInteractionsinDissolvedOrganicMatterPhotophysicsInresponsetoourrecentstudyregardingtheroleofcharge-transfer(CT)interactionsindissolvedorganicmatter(DOM)photophysics,1BloughandDelVecchio2raiseconcernsabouttheuseofsolventpolarity,viscosity,andtemperaturetotestfortheprevalenceofCTinteractionsinDOM.ThistopicisofgreatsignificancetoenvironmentalchemistryandengineeringduetotheubiquityandreactivityofDOMinthesesystems.BloughandDelVecchioargueagainstthetypesofpotentialdonor−acceptorcomplexesoccurringinDOMproposedinFigure1ofMcKayetal.(2018),1whichincludesindependent,covalentlytethered,andconjugateddonor−acceptormoieties.Thecounterargument2assertsthatthetypesofdonor−acceptorcomplexesweproposeplayalimitedrole,suggestinginsteadthatthemodelforpolydopamine,asproposedbyDreyerandco-workers,3alsoappliestoDOM.Inthismodel,structuralunitscomposingDOMwouldformstaticcomplexesasaresultofH-bonding,π-stacking,andcharge-transferinteractions.3,4Itishypothesizedthatsuchdonor−acceptorcomplexesinDOMarehinderedkineticallyandthermody-namicallyfromdissociatingintotheirrespectiveindependentmoieties.BloughandDelVecchiostatethatthedynamicsofthesestaticcomplexeswouldnotbeaffectedbytemperatureandsolventpolarityinthesamewayasdynamicdonor−acceptorcomplexes,offeringanalternativeexplanationforthelackofchangeinspectralshapepresentedinourstudy.Further,theabsenceofsolvatochromismreportedinourstudycouldpotentiallybeduetosolvent-protecteddonor−acceptorcomplexes.Thismodelhypothesizesthatthishydrophobiccoreisfurtherstabilizedbychargedoutergroups.Althoughwewelcomethecriticismandfurtherdiscussion,wedisagreewiththepresentedcounterarguments.First,themodelfortheprimarystructureofpolydopamineproposedbyDreyeretal.iscontroversialwithintheliterature.3,5Specifically,asubsequentstudyarguedthatpolydopaminemonomersarecovalentlylinked,asopposedtobeingheldtogetherbynoncovalentinteractions(e.g.,charge-transfer).5Themeritsoftheseindividualstudiesarebeyondthescopeofthisresponse,butitisnoteworthythatthereisdebateoverthestructureofamaterialthatissynthesizedfromaknownmonomerunit(i.e.,dopamine),whereasthestructuralunitsinDOMaremuchmoreheterogeneousandpoorlycharacterized.Inaddition,theisolationofdonorandacceptormoietiesfromsolventisinconsistentwithotherlinesofevidenceusedtosupportaCTmodel.Forexample,putativeacceptoranddonormoietiesintheCTmodel,aromaticketones/aldehydesandphenols/polyphenols/alkoxyphenols,respectively,exhibitaqueousphasephotochemistry.6−9IfsuchCTcomplexesareprotectedfromthesolvent,theywouldalsobeexpectedtobeprotectedfromparticipatinginbimolecularreactionsintheaqueousphase.Furthermore,accordingtotheCTmodel,reductionofDOMwithsodiumborohydridewouldresultinDOMwithsubstantiallylowermolecularweightduetodisruptionofthesedonor−acceptorinteractions.4Thishypothesisconflictswithhigh-pressuresizeexclusionchroma-tographymeasurementspublishedrecently10forDOMtreatedwithsodiumborohydride.Inaddition,itisdifficulttoreconcilehowananionicreductant,suchasborohydride,butnotneutralsolventmolecules,couldaccesssuchcomplexesinastatic,anion-encircledhydrophobicmicroenvironment.Anotherre-mainingquestionishowsolventsofsuchdifferentpolaritycouldhavenoeffectonDOMspectra,whenpHsoreadilyaffectstheopticalpropertiesofDOM,asreportedbyBloughandDelVecchio.InthereferencedpHexperiments,11−13theCTmodelholdsthatincreasedpHdeprotonatesphenolicdonors,whichleadstoincreasedcharge-transferexcitation.However,fordonor−acceptorpairstobepH-sensitive,thedonorswouldhavetobesolventaccessible,contrarytotheproposedCTmodel.TheseobservationsdemonstratethattheevidencepresentedinsupportofaCTmodel(i.e.,reactivitywithborohydrideandpH-sensitivespectra)alsocontradictstheprovisionthatdonor−acceptorcomplexesresideinasolvent-inaccessiblemicroenvironment.Afinalargumentagainstsolvent-inaccessiblechromophoresandfluorophoresisthatfluorescencequantumyieldwassignificantlychangedbysolventpolarityatallexcitationwavelengthsmeasured(Figure3cinmanuscript),1extendingwellintothevisiblewavelengthrange.ThischangeinfluorescenceintensitywithsolventpolarityacrosstheDOMabsorptionspectrumindicatesthatthesefluorophoresaresolvent-accessible.Lastly,BloughandDelVecchioassertthattheformationofparticulatesforsoilhumicacidsintetrahydrofuransupportsthehypothesisthatdonor−acceptormoietiesareinasolventinaccessiblehydrophobicmicroenvironment.Thedatapre-sentedinFigure4ofourstudy1contradictsthisview.Asolvatochromicshiftinfluorescencespectraof20nmwasobservedforsoilhumicacidsinacetonitrileandasoilfulvicacidinbothacetonitrileandtetrahydrofuranatarangeofexcitationwavelengths.Wehypothesizedthatthissolvato-chromisminthefluorescencespectracouldbeduetoexciplexformation,butnotgroundstatedonor−acceptorcomplexesduetothelackofsolvatochromisminthecorrespondingabsorbancespectra.Thisresultgivesusconfidencethatsolventpolaritywouldaffectthedynamicsofdonor−acceptorcomplexespresentintheotherisolatesinvestigated,iftheywerepresent.Furthermore,intheapolarorganicsolventsystemsinvestigated,carboxylateswouldbeprotonated,14removingaspeculatedstabilizingforce,a