acsest8b01189

CommentonTheCaseAgainstChargeTransferInteractionsinDissolvedOrganicMatterPhotophysicsInthisrecentarticle,1theauthorsarguethatchargetransfer(CT)interactionscannotbeimportanttotheopticalpropertiesofhumicsubstances(HS)anddissolvedorganicmatterbecauseperturbationsofsolventtemperature,viscosityandpolaritydonotaffecttheabsorbanceandfluorescencespectrallineshapes.However,thisconclusionispredicatedonthreeveryimportantassumptions:(1)theelectrondonors(D)andacceptors(A)arefreelydiffusinginsolution;(2)theDandApairsareindynamicequilibriumwiththeDAcomplex;(3)theDandApairsandtheDAcomplexarecompletelysolventaccessible.Basedonthesethreeassumptions,theauthorsrightlyexcludethesimplemodelsshownintheirFigure1.However,allthreeoftheseassumptionsarequestionableforHSbasedonexistinginformation.First,workbyDreyeretal.2hasdemonstratedforcloselyrelatedmaterials(polydopamine)thatDandApairswillformhighermolecularweightaggregatesthatarestabilizedthroughacombinationofchargetransfer,π-stackingandhydrogenbondinginteractions(seealso,ref3).Second,numerousstudieshaveprovidedstrongevidencethatHSareorganizedinmicroheterogenousdomains,4−8inwhichouterchargedgroupsencompassaninnercore,7,8loweringorlimitingsolventaccessibility.Aspointedoutinarecentpaper,9ourworkingstructuralmodelissimilartothatofDreyeretal.,2whereintheDandApairsareincontact,helpingtostabilizetheformationofanensembleofhighermolecularweightstaticaggregates(notindynamicexchange),withouterchargedgroupssurroundingtheindividualaggregates,7,8improvingwatersolubilitywhilefurtherstabilizingthecontactDAcomplexeswithintheinterior,potentiallyboththermodynami-callyandkinetically.BecausechangesinsolventpolarityandviscositydidnotlargelyeffectthespectraldependenceofeithertheabsorptionoremissionpropertiesoftheseHS,akeyquestioniswhetherthesesolventperturbationsweresufficienttodisruptanyCTinteractionswithintheseHS.Theauthorsarguethattheywouldexpectthistobethecase,basedontheabilityofanorganicsolvent,dimethylformamide(DMF),todisruptπ-stackingineumelanin(ref48inref1),10anothermaterialcloselyrelatedtopolydopamine.However,thisargumentisspecious,astheeumelaninwasfirsttreatedwithaconcentratedammoniasolution,sonicatedfor20min,stirredat90°Cforseveralhoursandthenrotoevaporatedat80°Cinwater/DMFbeforefinaladditiontoDMF.Eventhen,thistreatmentonlydisruptedthehigherorderstructureandnottheprimarystructureoftheeumelanin.10AlthoughwewouldnotanticipatethattheHSwouldbestabilizedtothesamedegreeaspolydopamineoreumelainduetothelonger-rangeorderintheselargeunchargedsupramolecularaggregates,theauthorsprovidenoexperimentalevidencethatthesesolvents(ortemperatureovertherangetested)wouldinfactdisruptthistypeofstructurewithintheHS.Theinabilityoftetrahydrofur-antosolubilizethehighermolecularweightsoilHS(PPHAandESHA),althoughnotaproof,isconsistentwiththeideathattheydonot.Althoughtheauthorsemploythesedatatoarguethatanelectronicinteractionmodel,morespecificallyaCTmodel,11,12isincorrect,andtopromotetheideathatasuperpositionofnoninteractingchromophorescanaccountforthevisibleabsorptionandemissionpropertiesoftheHS,theyignorenumerouslinesofevidencethatareeitherinconsistentorincompatiblewiththisidea.AspointedoutinSharplessandBlough,12theseincludebutarenotlimitedtothefollowing:(1)continuouslyred-shiftingemissionmaximawithincreasingexcitationwavelength(SIinref1),combinedwithmonotoni-callydecreasingapparentfluorescencequantumyields(Figures2,3andSIinref1)andfluorescencelifetimes13-notethatthisappliestothesoilHSaswell,butthedependenceisshiftedtolowerenergies(Figure2inref1);(2)thebroad-bandlossofabsorptionacrossthevisiblecombinedwithenhancedblue-shiftedfluorescenceemissionfollowingremovalofputativeelectronacceptors(ketone/aldehydes)byborohydridereduc-tion;9,14−18(3)thebroad-bandincreaseinabsorptionacrossthevisiblewithincreasingpH,particularlyoverthepKarangeassociatedwithputativephenolelectrondonors;18−21(4)theloweringoreliminationofthispH-inducedabsorptionenhancementinthevisiblefollowingreductionwithborohy-dridetoremovetheelectronacceptors.18Asnotedpreviously,12ifonewishestodevelopacomprehensivephotophysicalmodelofHSandchromophoricdissolvedorganicmatter,thenoneshouldconsiderallofthedata,notjustaself-selectedportion.NeilV.Blough*,†RossanaDelVecchio§†DepartmentofChemistryandBiochemistry,UniversityofMaryland,CollegePark,Maryland20742,UnitedStates§EarthSystemScienceInterdisciplinaryCenter,UniversityofMaryland,CollegePark,Maryland20770,UnitedStates■AUTHORINFORMATIONCorrespondingAuthor*E-mail:[e-mail]:0000-0002-7938-4961RossanaDelVecchio:0000-0001-5087-7028NotesTheauthorsdeclarenocompetingfinancialinterest.■REFERENCES(1)McKay,G.;Korak,J.A.;Erickson,P.R.;Latch,D.E.;McNeill,K.;Rosario-Ortiz,F.L.Thecaseagainstchargetransferinteractionsindissolvedorganicmatterphotophysics.Environ.Sci.Technol.2018,52,406−414.(2)Dreyer,D.R.;Miller,D.J.;Freeman,B.D.;Paul,D.R.;Bielawski,C.W.Elucidatingthestructureofpoly(dopamine).Langmuir2012,28(15),6428−6435.Published:April16,2018Correspondence/Rebuttalpubs.acs.org/estCiteThis:Environ.Sci.Technol.2018,52,5512−5513©20