Excited State Structure Correlates with E ffi cient Photoconversion in Unidirectional Motors - Roy et al. - 2021 - Unknown

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pubs.acs.org/JPCLLetterExcitedStateStructureCorrelateswithEfficientPhotoconversioninUnidirectionalMotorsPalasRoy,AndyS.Sardjan,ArjenCnossen,WesleyR.Browne,BenL.Feringa,*andStephenR.Meech*CiteThis:J.Phys.Chem.Lett.2021,12,3367−3372ReadOnlineACCESSMetrics&MoreArticleRecommendations*sıSupportingInformationABSTRACT:Thedesignofunidirectionalphotomolecularmotorsdemandsacriticalunderstandingofanultrafastphotochemicalisomerization.Anintermediatedarkexcitedstatemediatesthereactionviaaconicalintersection(CI)withthegroundstate,butacorrelationbetweenmolecularstructureandphotoisomerizationefficiencyhasremainedelusive.HerefemtosecondstimulatedRamanspectroscopycapturesvibrationalspectraofthedarkstateinasetofmolecularmotorsbearingdifferentsubstituents.Adirectcorrelationbetweenisomerizationquantumyield,darkstatelifetime,andexcitedstatevibrationalspectrumisfound.Electronwithdrawingsubstituentsleadtoactivityinlowerfrequencymodes,whichwecorrelatewithapyramidalizationdistortionattheethylenicaxleoccurringwithin100fs.Thisstructureisnotformedwithanelectrondonatingsubstituent,wheretheaxleretainsdoublebondcharacter.Furtherstructuralreorganizationisobservedandassignedtoexcitedstatereorganizationandchargeredistributiononthesub-picosecondtimescale.Thecorrelationofthedarkstatestructurewithphotoconversionperformancesuggestsguidelinesfordevelopingnewmoreefficientmotorderivatives.Artificialunidirectionalphotochemicallydrivenmotorsarephotoisomerizationaroundthealkeneaxle,givingrisetoaapowerfultoolforgeneratingmechanicalenergyonametastablegroundstatewithayieldof5−20%depending1−4uponthesubstituent.13Thesecondstepisathermallymolecularscalebyabsorptionoflight.Theyhavebeenused2,14inawiderangeofapplicationsfromultrafastresponsiveactivatedhelixinversiontocompletea180°rotation.The5−8processisrepeatedwithasecondphotonabsorptionandmaterialstocatalysis.Thebasisoftheirfunctionisaphotochemicalcis−transisomerizationinastericallyover-thermalizationsteptodriveafullcyclerotation.Thus,thecrowdedalkene,whichfacilitatesunidirectionalrotationofthethermalstepisratelimiting,whilethephotochemicalstepmotor.9−12Secondgenerationmotorswitha“stator”fluorenedeterminesthequantumyield.ringlinkedtoa“rotor”moietythroughadoublebond“axle”ThephotochemicalstephasbeenstudiedtheoreticallyandDownloadedviaBUTLERUNIVonMay16,2021at08:58:11(UTC).(Figure1)havethefastestratesofrotation.9AschematicofshowntoinvolveultrafaststructuraldynamicsintheexcitedtheiroperationisshowninFigure1.Thefirststepinvolvesstateleadingtoaconicalintersection(CI)withtheelectronicgroundstate,fromwhicheithertheisomerizedmetastable15,16productortheoriginalstatemaybegenerated.TheSeehttps://pubs.acs.org/sharingguidelinesforoptionsonhowtolegitimatelysharepublishedarticles.correspondingpotentialenergydiagramisshowninFigure1.However,exploitationofthesesecondgenerationmotorsmaybelimitedbytherelativelylowyieldofthephotochemicalisomerization.Thishaspromptedinvestigationsoftheeffectsofvariationinsubstituentandstructure,withtheaimof17−20maximizingphotoisomerizationefficiency.Thedevelop-mentofpredictivemodelsforfuturesynthesescertainlyrequiresadetailedunderstandingoftheeffectsofsuchstructuralmodificationsonexcitedstatedynamics.Forexample,FilatovandOlivuccihaveshowntheoreticallythatReceived:March4,2021Figure1.Dynamicsofalight-drivenmolecularmotor.StructureofAccepted:March22,2021themolecularmotor,thestepsinitsphotoconversionmechanism,andPublished:March30,2021aschematicofthecorrespondingpotentialenergysurface.Thegroundandexcitedstatepotentialenergysurfacesintersectattheconicalintersection,CI(R=H,OMe,Cl,CN).©2021AmericanChemicalSocietyhttps://doi.org/10.1021/acs.jpclett.1c007103367J.Phys.Chem.Lett.2021,12,3367−3372

1TheJournalofPhysicalChemistryLetterspubs.acs.org/JPCLLettersubstituentssignificantlymodifythepathwayofstructuralevolutionneartheCIsconnectinggroundandexcitedstates,21thusmodifyingthephotochemicalyield.Earlierexperimentalstudiesofexcitedstatedynamicsinsecondgenerationmotorsusedultrafastfluorescencespectros-copytoprobetheprimaryphotochemicalstep.ArelaxationoftheFranck−Condon(FC)brightstateoccursinabout100fspopulatinganintermediate“darkstate”ontheexcitedstate22surface.Extendingthosestudiestoaseriesofsubstitutedsecondgenerationmotorsshowedthattheenergeticsandlifetimeofthedarkstateareafunctionofthesubstituent,whichalsomodifiedthephotoconversionefficiency,withtheyieldincreasingwithincreasingsubstituentelectronwith-drawingcharacter(specifically,CN=0.2,Cl=0.15,H=0.14,13OMe=0.05).Theweakdependenceofbrightstatelifetimeonsolventviscosity,coupledwiththeresultsofcomputationalcalculations,suggestedpyramidalizationatanaxleCatomasanimportantcoordinateintheexcitedstatestructural13,15,16,22dynamics.However,directexperimentalevidencefortheroleofapyramidalizationcoordinateindarkstateformationanddecayhasnotbeenprovidedtodate.Herewereportastructure−functioncorrelationbetweenRamanspectraofthedarkexcitedstateandphotoconversionefficiencyinaseriesofsubstitutedmolecularmotors.Specifically,wehavemeasuredgroundandultrafastexcitedstateRamanspectrainfourderivativesofthefluorenemolecularmotor9-(2-methyl-2,3-dihydro-1H-cyclopenta[a]-naphthalen-1-ylidene)-9H-fluorene,withsubstituents(−R):−H,electronwithdrawinggroups(−CNand−Cl),andelectrondonatinggroup(−OMe).Wefindremarkabledifferencesindarkstatevibrationalspectraforthesefourmolecules,withspecificRamanactivemodesswappingtheirrelativeintensitiesasafunctionofsubstituent.WeassignthisFigure2.Experimental(redsolidlines)andcalculated(blackverticaltosubstituentcontrolofthedarkstatestructureinthefourlines)steady-stateoff-resonance(orpreresonance)Ramanofthefourderivatives.Furtherstructuralevolutioninthedarkstateisalsomotorderivativesinthesolidstate.Ramanat532nm,resonanceresolved.wavelengthca.400nm.Theregion650−1000cm−1isincreasedbyaSteady-StateMeasurements.Steady-stateabsorptionfactorof5toshowtheweakspectralfeaturesmoreclearly.Thepeaksintheregion1300−1400cm−1areconnectedbyreddottedlinestospectraforthefourmotorderivativesinmethanolareshowninSupportingInformation,FigureS1.Theabsorptionmaximashowsubstituentdependence.Thesymbolsrepresentυ-stretching,δforR=OMe,H,Cl,andCNderivativesareat402,387,391,-bending,φ-pyramidalization,A-ethylenicaxle,R-substitutedand402nmrespectively,correspondingtoaπ−π*transition.naphthylrotor,andS-fluorenestator.Quantumchemicalcalculationsrevealthatthistransitionis23localizedontheethylenicaxleofthemotor.Thus,theshownbyblackverticallinesinFigure2.Theexperimentalandobservedspectralshiftsindicatethattheperipheralsub-calculatedspectramatchquitewellforallthederivativesinthestituentsaffecttheelectronicstructureatthecoreoftheregion650−1700cm−1.Themodesnear1580cm−1arisefrommolecule.symmetricstretchingoftheethylenicCCaxlecoupledtoInordertobetterunderstandtheeffectofsubstituentontheringbreathingmodes(υ(CC)),whilethefingerprintregiongroundstatestructure,werecordedsteady-stateRamanspectrafrom1100to1500cm−1isdominatedbystretching(υ(CC))underoff-resonantconditionsat532nm(Figure2).Theandbending(δ(CH))modesofboththerotor(naphthyl)andstrongestRamanactivemodeinallfourderivativesisinthestator(fluorene)groups.Thecharacterofthemostprominentregionof1500−1600cm−1.Thefingerprintregion(1300−modesarelabeledinFigure2,andtheirnucleardisplacements1500cm−1)showssignificantvariation,withthesubstituentareillustratedinSupportingInformation,FigureS4.Thedependenceofmodesofthesamecharacterbeinghighlightedmodeswithasignificantsubstituentdependence(shownbybyreddottedlines.Thisvariationindicatesaneffectofthereddottedlines)areseentoarisefromringυ(CC)/δ(CH)substituentontheRamanactivemodesofthenaphthylunittomodesofthenaphthylrotorbearingthesubstituent.Thepeakwhichthesubstituentsareattached(seebelow).Theregionfrequenciesshiftdependingonthechangeinnaphthylring1100−1250cm−1isagainquitesimilarforallfourderivatives,conjugationwiththesubstituentgroup.Incontrast,themodeswhileactivityinlowfrequencymodes(<1000cm−1)isverynear1307and1450cm−1arefluorenestatorυ(CC)/δ(CH)weakinallcases,althoughthereismoderateintensityinamodesandareinsensitivetosubstitution.Thelow-frequencymodenear750cm−1.modesintheregion650−1000cm−1areveryweakintheGround-stateDFTcalculationsofRamanactivemodesgroundstate.However,themostprominentmodeinthe690−usingGaussian16wereusedtoassignthesespectra;calculated750cm−1regioncorrespondstoaxlepyramidalizationmotiondata(attherb3lyp/tzvplevel,frequencyscaledby0.98)arecoupledtoanHout-of-planebending(HOOP)(FigureS7).3368https://doi.org/10.1021/acs.jpclett.1c00710J.Phys.Chem.Lett.2021,12,3367−3372

2TheJournalofPhysicalChemistryLetterspubs.acs.org/JPCLLetterExcitedStateDynamics.Thestructuralevolutionoftheobservedabroadpeakat720cm−1inthedarkstate,whichisfourmotorssubsequenttophotoexcitationisprobedbyveryweakfortheelectrondonatingOMesubstituentbutrecordingtransientabsorptionandtime-resolvedRamanstrongforelectronwithdrawingCN.Similarly,amodeat1170cm−1isabsentfortheOMesubstituentbutisenhancedinthespectrausingFSRS.Themotorswereexcitedusingafemtosecond“actinic”pumpat400nm.Transientabsorptionotherderivatives.Figure3bdepictsthechangeinrelativeintensityof720and1170cm−1modeswithrespecttothemeasurementsforallfourderivativesatdifferentpump−probe1580cm−1mode.Thepeakintensityratioincreasesasthedelaysshowtwodistinctexcitedstateabsorption(ESA)features(seeSupportingInformation,FigureS2).Thesubstituentchangesfrom−OMethroughHandClto−CN,instantaneouslygeneratedESAnear620−800nmdecaysini.e.,asafunctionofincreasingelectronwithdrawingcharacter.Thebroadhighestfrequencymodeat1580cm−1inthe100−200fsconcomitantwiththeriseofESAnear550−620nm.ThisrepresentsevolutionoftheFCbright-statetoaexcitedstateisassignedtoethylenicCC+ringstretchingon24thebasisofthegroundstateDFTcalculation.Thismodeislongerlived(∼ps)darkintermediateinallfourderivatives.Thedarkstateabsorptionisincreasinglyred-shiftedandmostintenseforthe−OMederivative,suggestingtheexcitedbecomesmoreintensewithincreasingelectronegativityofthestateretainsstrongdoublebondcharacter.Conversely,thissubstituent.Toprobethisdarkexcitedstatewithresonantmodeisveryweakinthe−CNderivative,suggestinganFSRS,thepicosecondRamanpumpwastunedinthe560−610excitedstategeometryinwhichtheethylenicdoublebondnmwindow,theresonantwavelengthbeingchosentoyieldacharacterisgreatlyreduced.Instead,intenseactivityin720and1170cm−1modesisobserved,whicharethemselvesweakinsimilarresonanceconditionsforallfourmotors(itismarkedbyverticaldasharrowsinSupportingInformation,FigureS2).the−OMederivative.TherawFSRSdatawerebackgroundsubtractedandcorrectedTheoriginoftheenhancedlowfrequencymodesisforbaselineasdescribedintheSupportingInformation,andsuggestedbyearlierquantumchemicalcalculationsofthearepresentedinFigure3.excitedstatestructureofthefluorene(H)motors,wheretheethyleniccarbononthefluoreneringhaschangedfromsp2tosp3duetopyramidalizationdistortionuponformationofthe23,25darkstate.Notethatinallderivativesthepyramidalizationmodewascalculatedatca.750cm−1inthegroundstate(Figure2)althoughveryweak.Inthepresentcase,theCNderivativedisplaysanintenselow-frequencymodeat720cm−1andweakmodeat1580cm−1suggestingthattheelectronwithdrawingcharacterleadstoadarkstatestructurewherethepyramidalizationhasfavoredtheformationofsp3carbonattheethylenicaxle(labeledasansp3darkstate,S).Therefore,sp3thisstrong720cm−1modeisassignedtopyramidalizationsp3distortionattheaxlecarbon,andthismodebecomesincreasinglyprominentastheelectronwithdrawingcharacterofthesubstituentincreases.Whilenotpreviouslyreportedinmotormolecules,suchlowfrequencymodeshavebeenseeninotherphotochemicalisomerizationreactions,inphytochromes26−28andrhodopsins,forexample.Inthosecases,theHOOPactivitywasanimportantcoordinate,andwenotethatthecalculatedgroundstatepyramidalizationmodeforthemotormoleculesalsoincludessignificantHOOPdisplacements(FigureS7).Incontrast,inthe−OMederivativethe720cm−1modeisessentiallyabsent,andanintense1580cm−1modeisretained,suggestinganexcitedstategeometrywheretheethyleneaxlewithsp2charactercarbon(labeledassp2darkstate,Ssp2)isfavored,andthereislittledisplacementalongthepyramidalizationcoordinate.Boththe720and1580cm−1modesaremoderatelyintenseforthe−Hand−Clderivatives.Figure3.(a)FemtosecondstimulatedRamanspectraofthedark23Thissuggestsanintermediatestructurewherebothspandspexcitedstateofmolecularmotorsinmethanol,measuredat200fspump−probedelaytime.Asterisksrepresentsolventandinstrumentalgeometriesareaccessible,allowingestablishmentofarapidartifacts.Grayhighlightedareasrepresentregionsofinterest.(b)equilibriumbetweenSsp2andSsp3;thisoverallschemeisChangesofRamanintensityratio1170/1580cm−1and720/1580illustratedinFigure4.cm−1asthesidegroupsarevaried.ExcitedstateRamanspectrathusshowthatdarkexcitedstatestructuresaresubstituentdependent:theelectron-withdrawing−CNderivativehassignificantsp3axle(S)TheprocessedexcitedstateRamanspectrarecorded200fssp3afterexcitationarepresentedinFigure3a.TheRamanactivecharacter,whiletheelectron-donating−OMederivativehasconsiderablesp2axlecharacter,evenintheexcitedstate(S).modesintheexcitedstatearequitebroadcomparedtotheirsp2groundstatecounterparts,aneffectpreviouslyassignedtoaThederivativeswithintermediatedonatingcharacter(−Hand24distributionofdarkstatestructures.Theprominenthigh−Cl)haveintermediatespectra,suggestingthatthetwofrequencymodesinR=OMetoHareclearlygreatlyexcitedstatestructuresinfastdynamicequilibriumcan“swing”suppressedwhenR=CN.Extendingthemeasurementstoonthetorsion-pyramidalizationcoordinate,asreportedinlowerwavenumber,wheregroundstateactivityisweak,wespin-restrictedDFTcalculationsofmotorexcitedstate3369https://doi.org/10.1021/acs.jpclett.1c00710J.Phys.Chem.Lett.2021,12,3367−3372

3TheJournalofPhysicalChemistryLetterspubs.acs.org/JPCLLetterassignedtonaphthylringυ(CC)/δ(CH)modes(basedonourgroundstateDFTcalculation)andundergoaredshiftwithtimeasshownbytheblackverticaldottedline.ThepeakfrequencyforthismodeisplottedinFigure5bforallfourderivatives.Thefrequencyshiftsby5−10cm−1onasub-picosecondtimescaleforthe−CN,−Cl,and−Hderivatives,suggestingstructuralevolutionmodifyingspecificallynaphthylringmodesintheexcitedstate.OtherRamanactivemodesdonotundergopeakshifts.Sincetorsionandpyramidalizationinethylenicisomerizationisassociatedwithchargeredistribution21,23,25,30(suddenpolarization)ontheethylenicbond,thered-shiftintheRamanspectramaybeexplainedbystabilizationofthisnewchargedistribution.Significantly,transientRamanspectrarecordedforthe−OMederivativedonotshowashiftinpeakpositionforthismode(Figure5b).Thisisconsistentwith−OMemaintainingitsdoublebondSsp2characteroftheaxle.Theseobservationsarealsocorrelatedwithadynamicalpeakshiftofdarkstateelectronictransientabsorptionspectraona∼1pstimescaleforthe−CN,−Cl,and−Hderivatives,whilenosignificantspectralshiftisobservedfor−OMe(seeSupportingInformation,FigureS3).Figure4.PhotophysicalmodelforthephotoconversionofsecondThedynamicsassociatedwiththetransientabsorptionandgenerationmolecularmotors.ThebrightstaterelaxestothedarkstateFSRSspectralshifts,andthenonsingleexponentialFSRS22inca.100fs.DifferentpopulationsofSsp2andSsp3statesareamplitudedecay,aretabulatedinSupportingInformation.Theachieveddependingonthenatureofsubstituents,whichcontrolstheoverallnonsingleexponentialdarkstatedynamicshavebeennatureofthenuclearreorganizationcontributingtothereactionnotedpreviouslyforthe−Hderivativeandalsoforfirstcoordinate.Thestateformedwithincreasingelectronwithdrawing24,30character(Ssp2,Ssp2/Ssp3rapidequilibrium,Ssp3)influencesthegenerationmotors.Thepeakareadynamicsforthisnaphthylringmodeat1330−1375cm−1regionareplottedinprobabilityoftheproductiveisomerizationreaction.The0.5−1psrelaxationdynamicswithinthedarkstate(seebelow)areomittedforFigure5cforthefourderivatives.Thedecayisfasterfortheclarity.−OMederivativeandslowerforthe−CNderivative.Thistrendcorrelateswiththereportedlifetimefordarkstatesfrom29bothfluorescenceupconversionandtransientabsorptiondynamics.Theseassignmentsarerepresentedonthemodel13measurements.Thus,theSsp2darkstateforthe−OMepotentialenergysurface(Figure4).Importantly,theobservedderivativehasashortlifetime(ca.200fs)andmainlydecaystostabilizationofSsp3correlateswithahigherconversiontheoriginalgroundstate.Incontrast,the−CNsubstitutedefficiencyforformationofthegroundstatemetastableproduct,whileSsp2characterleadstoalowyieldofproductformation.motorhasalong-lived(ca.10ps)Ssp3darkstate.TherelativelyTherefore,designingmoleculesthatpromoteformationofthelonglifetimeoftheSsp3statein−CNcouldbeattributedtothe23sp3axlecarbonstructuresintheexcitedstateshouldprovidealargerstructuralreorganizationrequiredtoreachtheCI.higheryieldofphotoisomerization.WenotetheshortlifetimeHowever,theCIaccessedfromthisdarkstatehasagreaterofSsp2contrastswiththelongerlifetimeoftheSsp3darkstateinprobabilityofformingthemetastableproduct.the−CNderivative.InprinciplethelongerSsp3lifetimeitselfInconclusion,wehaveshownthatthestructureofthedarkmayleadtohigherconversionefficiencyifacompetingexcitedstateofsecondgenerationmolecularmotorsvariesradiationlessprocessissuppressedforexample.However,thewiththenatureofthesubstituent.Moreelectronwithdrawingpresentvibrationalstructuredatasuggestthatitisthestructuresubstituents(−CN)leadtostrongeractivityinlowfrequencyofthedarkstate(andthereforetheinitialstructureinthemodes,assignedtoformationofansp3axlecarbon.Groundgroundstateafterinternalconversion)whichdeterminesthestateDFTcalculationssuggestthatthe720cm−1modecanbephotochemicalyield.Forexample,the−Clsubstituenthasaassignedtotheaxlepyramidalizationdistortioncoupledtoshortlifetimecomparedto−CNbutahigheryieldthanHOOPmotion.Fordecreasingelectronwithdrawingcharacter,−OMe.theintensityofthesemodesandhencethepopulationofSsp3Themainfeaturesofthedarkstatespectraareindependentdarkstatedecreases,andfinally,foranelectron-donatingofdelaytime(seeSupportingInformationFigureS5),but−OMederivative,theexcitedstateisseentobetrappedinansignificanttemporalevolutionisobservedintheringmoderegion(1300−1400cm−1),whichwasalsoshowntobeSsp2state.Importantly,thisresultcorrelateswiththeefficiencyofphotoconversioninthesemotorsandalignswiththeresultssubstituentdependentingroundstatespectra(Figure2).Weofrecentexistinghighqualityquantumchemicalcalculations.haveplottedtheFSRSdataforthisregioninFigure5a.ThedataarefittocharacterizetheevolutionbysumsoftwoorItwouldbeinstructiveifsuchcalculationscouldbeextendedthreeGaussianfunctions(shownbybluecurvesinearlyandtomodelthemeasuredexcitedstatevibrationalspectrum.Inaddition,weobserveda5−10cm−1peakshiftoftherotorlatedatasets)toresolveunderlyingpeakfrequenciesandareas;anadditionalGaussianfunctionisusedtofitthesolventbreathingmodesassignedtostabilizationofthechargeartefact(furtherfittingdetailsaregiveninSupportingredistributionintheinitialdarkstatepriortointernalInformation).Thepeaksintherange1330−1375cm−1areconversionviaCIs.3370https://doi.org/10.1021/acs.jpclett.1c00710J.Phys.Chem.Lett.2021,12,3367−3372

4TheJournalofPhysicalChemistryLetterspubs.acs.org/JPCLLetterFigure5.(a)FemtosecondstimulatedRamanspectraofthemolecularmotorsintheregion1300−1470cm−1shownatseveralpump−probedelaytimestodisplayspectralshifts(e.g.,relativetotheblackdottedline).Eachspectrum(redtraces)isfittedbytwoorthreeGaussianfunctions(bluecurves)toprovideafittedspectrum(blacktraces).OneadditionalGaussianfunctionisusedtofitthesolventartefactsinthedatasets(marked*).(b)Thepeakfrequencyshiftforthe1330−1375cm−1modecorrespondingtonaphthylbreathing,and(c)theassociatedpeakareachangesasafunctionoftime,revealingpopulationdynamics.■EXPERIMENTALMETHODSBenL.Feringa−StratinghInstituteforChemistry,University13,20ofGroningen,9747AGGroningen,TheNetherlands;Thesynthesisofallderivativeswasreportedpreviously.orcid.org/0000-0003-0588-8435;Email:b.l.feringa@Thespectrometersfortransientabsorptionandfemtosecond24rug.nlstimulatedRamanweredescribedpreviously,andfurtherdetailsandsamplespreparationprotocolsareincludedintheAuthorsSupportingInformation.PalasRoy−SchoolofChemistry,UniversityofEastAnglia,NorwichNR47TJ,U.K.■ASSOCIATEDCONTENTAndyS.Sardjan−StratinghInstituteforChemistry,*sıSupportingInformationUniversityofGroningen,9747AGGroningen,TheTheSupportingInformationisavailablefreeofchargeatNetherlandshttps://pubs.acs.org/doi/10.1021/acs.jpclett.1c00710.ArjenCnossen−StratinghInstituteforChemistry,UniversityofGroningen,9747AGGroningen,TheNetherlandsSteadystateandtransientelectronicspectra;timeWesleyR.Browne−StratinghInstituteforChemistry,dependentexcitedstateRamanspectra;nucleardisplace-UniversityofGroningen,9747AGGroningen,ThementsassociatedwiththemostrelevantRamanactiveNetherlands;orcid.org/0000-0001-5063-6961groundstatevibrationalmodes,especiallythepyramidi-Completecontactinformationisavailableat:alization-likecoordinate;processingproceduresforhttps://pubs.acs.org/10.1021/acs.jpclett.1c00710FSRSspectra(PDF)Notes■AUTHORINFORMATIONTheauthorsdeclarenocompetingfinancialinterest.CorrespondingAuthorsStephenR.Meech−SchoolofChemistry,UniversityofEast■ACKNOWLEDGMENTSAnglia,NorwichNR47TJ,U.K.;orcid.org/0000-0001-FinancialsupportwasprovidedbyTheNetherlandsMinistry5561-2782;Email:s.meech@uea.ac.ukofEducation,CultureandScience(GravityProgram3371https://doi.org/10.1021/acs.jpclett.1c00710J.Phys.Chem.Lett.2021,12,3367−3372

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