Controlling Proton-Coupled Electron Transfer in Bioinspired Artificial Photosynthetic Relays

Article abstract of DOI:10.1021/jacs.8b09724

Bioinspired constructs consisting of benzimidazole-phenol moieties bearing N-phenylimines as proton-accepting substituents have been designed to mimic the H-bond network associated with the Tyr_Z-His190 redox relay in photosystem II. These compo

Full text of DOI:10.1021/jacs.8b09724

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Article
Controlling Proton-Coupled Electron Transfer
in Bio-Inspired Artificial Photosynthetic Relays
Emmanuel Odella, S. Jimena Mora, Brian L. Wadsworth, Mioy T. Huynh, Joshua J.
Goings, Paul A. Liddell, Thomas L. Groy, Miguel Gervaldo, Leónides E. Sereno, Devens
Gust, Thomas A. Moore, Gary F. Moore, Sharon Hammes-Schiffer, and Ana L. Moore
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09724 • Publication Date (Web): 16 Oct 2018
Downloaded from http://pubs.acs.org on October 16, 2018
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Controlling Proton-Coupled Electron Transfer in Bio-Inspired Artifi-
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§
‡
§‡
§‡
†‡
†
EmmanuelꢀOdella, ꢀS.ꢀJimenaꢀMora, ꢀBrianꢀL.ꢀWadsworth, ꢀMioyꢀT.ꢀHuynh, ꢀJoshuaꢀJ.ꢀGoings, ꢀPaulꢀ
§
§
#
#
§
A.ꢀ Liddell, ꢀ Thomasꢀ L.ꢀ Groy, ꢀ Miguelꢀ Gervaldo, ꢀ Leónidesꢀ E.ꢀ Sereno, ꢀ Devensꢀ Gust, ꢀ Thomasꢀ A.ꢀ
Moore, ꢀGaryꢀF.ꢀMoore, *ꢀSharonꢀHammes-Schiffer, *ꢀandꢀAnaꢀL.ꢀMoore. *ꢀ
§
§
†
§
§ꢀSchoolꢀofꢀMolecularꢀSciences,ꢀArizonaꢀStateꢀUniversity,ꢀTempe,ꢀArizonaꢀ85287-1604,ꢀUnitedꢀStates.ꢀ
†ꢀDepartmentꢀofꢀChemistry,ꢀYaleꢀUniversity,ꢀNewꢀHaven,ꢀConnecticutꢀ06520-8107,ꢀUnitedꢀStates.ꢀ
#ꢀDepartamentoꢀdeꢀQuímica,ꢀFacultadꢀdeꢀCienciasꢀExactas,ꢀFísico-QuímicasꢀyꢀNaturales,ꢀUniversidadꢀNacionalꢀdeꢀRíoꢀ
Cuarto,ꢀAgenciaꢀPostalꢀNoꢀ3,ꢀ5800ꢀRíoꢀCuarto,ꢀCórdoba,ꢀArgentina.ꢀ
ꢀ
ꢀ
ꢀ
ABSTRACT:ꢀBioinspiredꢀconstructsꢀconsistingꢀofꢀbenzimidazole-phenolꢀmoietiesꢀbearingꢀN-phenyliminesꢀasꢀproton-accept-
ingꢀsubstituentsꢀhaveꢀbeenꢀdesignedꢀtoꢀmimicꢀtheꢀH-bondꢀnetworkꢀassociatedꢀwithꢀtheꢀTyr -His190ꢀredoxꢀrelayꢀinꢀphotosys-
Z
temꢀII.ꢀTheseꢀcompoundsꢀprovideꢀaꢀplatformꢀtoꢀtheoreticallyꢀandꢀexperimentallyꢀexploreꢀandꢀexpandꢀproton-coupledꢀelectronꢀ
transferꢀ(PCET)ꢀprocesses.ꢀTheꢀmodelsꢀfeatureꢀH-bondsꢀbetweenꢀtheꢀphenolꢀandꢀtheꢀnitrogenꢀatꢀtheꢀ3-positionꢀofꢀtheꢀbenzim-
idazoleꢀandꢀbetweenꢀtheꢀ1H-benzimidazoleꢀprotonꢀandꢀtheꢀimineꢀnitrogen.ꢀProtonationꢀofꢀtheꢀbenzimidazoleꢀandꢀtheꢀimineꢀ
canꢀbeꢀunambiguouslyꢀdetectedꢀbyꢀinfraredꢀspectroelectrochemistryꢀ(IRSEC)ꢀuponꢀoxidationꢀofꢀtheꢀphenol.ꢀDFTꢀcalculationsꢀ
andꢀIRSECꢀresultsꢀdemonstrateꢀthatꢀwithꢀsufficientlyꢀstrongꢀelectron-donatingꢀgroupsꢀatꢀtheꢀpara-positionꢀofꢀtheꢀN-phe-
nylimineꢀgroupꢀ(e.g.,ꢀ-OCH ꢀsubstitution),ꢀprotonꢀtransferꢀtoꢀtheꢀimineꢀisꢀexergonicꢀuponꢀphenolꢀoxidation,ꢀleadingꢀtoꢀaꢀone-
3
electron,ꢀtwo-protonꢀ(E2PT)ꢀproductꢀwithꢀtheꢀimidazoleꢀactingꢀasꢀaꢀprotonꢀrelay.ꢀWhenꢀtransferꢀofꢀtheꢀsecondꢀprotonꢀisꢀnotꢀ
sufficientlyꢀexergonicꢀ(e.g.,ꢀ-CNꢀsubstitution),ꢀaꢀone-electron,ꢀone-protonꢀtransferꢀ(EPT)ꢀproductꢀisꢀdominant.ꢀThus,ꢀtheꢀextentꢀ
ofꢀprotonꢀtranslocationꢀalongꢀtheꢀH-bondꢀnetwork,ꢀeitherꢀ~1.6ꢀÅꢀorꢀ~6.4ꢀÅ,ꢀcanꢀbeꢀcontrolledꢀthroughꢀimineꢀsubstitution.ꢀ
Moreover,ꢀtheꢀH-bondꢀstrengthꢀbetweenꢀtheꢀbenzimidazoleꢀNHꢀandꢀtheꢀimineꢀnitrogen,ꢀwhichꢀisꢀaꢀfunctionꢀofꢀtheirꢀrelativeꢀ
pK ꢀvalues,ꢀandꢀtheꢀredoxꢀpotentialꢀofꢀtheꢀphenoxylꢀradical/phenolꢀcoupleꢀareꢀlinearlyꢀcorrelatedꢀwithꢀtheꢀHammettꢀconstantsꢀ
a
ofꢀtheꢀsubstituents.ꢀInꢀallꢀcases,ꢀaꢀhighꢀpotentialꢀ(~1ꢀVꢀvsꢀSCE)ꢀisꢀobservedꢀforꢀtheꢀphenoxylꢀradical/phenolꢀcouple.ꢀDesigningꢀ
andꢀtuningꢀredox-coupledꢀprotonꢀwiresꢀisꢀimportantꢀforꢀunderstandingꢀbioenergeticsꢀandꢀdevelopingꢀnovelꢀartificialꢀphoto-
syntheticꢀsystems.ꢀ
Introductionꢀ
positionedꢀandꢀelectrochemicallyꢀpoisedꢀbetweenꢀP680ꢀandꢀ
theꢀOEC,ꢀTyr ꢀandꢀitsꢀH-bondedꢀpartner,ꢀHis190,ꢀserveꢀasꢀtheꢀ
redoxꢀrelayꢀbetweenꢀP680ꢀandꢀtheꢀOEC. ꢀTheꢀoxidationꢀofꢀ
Z
Inꢀallꢀwater-oxidizingꢀphotosyntheticꢀorganisms,ꢀtyrosineꢀZꢀ
7
(
Tyr
Z
)ꢀofꢀphotosystemꢀIIꢀ(PSII)ꢀisꢀaꢀhigh-potentialꢀelectronꢀ
•+
Tyr
Z
ꢀbyꢀP680 ꢀillustratesꢀaꢀclassicꢀproton-coupledꢀelectronꢀ
ꢀisꢀac-
transferꢀ relayꢀ betweenꢀ theꢀ reaction-centerꢀ chlorophyllsꢀ
P680)ꢀandꢀtheꢀoxygen-evolvingꢀcomplexꢀ(OEC). ꢀItsꢀroleꢀ
canꢀbeꢀdescribedꢀbyꢀconsideringꢀanyꢀoneꢀofꢀfourꢀstepsꢀofꢀtheꢀ
electrogenicꢀ cycleꢀ inꢀ whichꢀ theꢀenergyꢀ ofꢀ fourꢀ photonsꢀ isꢀ
usedꢀtoꢀoxidizeꢀtwoꢀwaterꢀmolecules,ꢀyieldingꢀO ,ꢀfourꢀpro-
tons,ꢀtwoꢀreducedꢀquinones,ꢀandꢀproton-motiveꢀforce. ꢀAꢀ
cycleꢀbeginsꢀwithꢀexcitationꢀofꢀtheꢀprimaryꢀdonorꢀ(P680)ꢀtoꢀ
produceꢀP680*ꢀfollowedꢀbyꢀchargeꢀseparationꢀinvolvingꢀaꢀ
1
,2
transferꢀ(PCET)ꢀprocessꢀinꢀwhichꢀtheꢀoxidationꢀofꢀTyr
Z
(
8,9
companiedꢀbyꢀprotonꢀtransferꢀtoꢀHis190. ꢀMoreover,ꢀotherꢀ
PCETꢀ processesꢀ inꢀ theꢀ extensiveꢀH-bondꢀ networkꢀ aroundꢀ
theꢀOECꢀareꢀinvolvedꢀinꢀredoxꢀleveling,ꢀwhichꢀisꢀkeyꢀtoꢀre-
movingꢀfourꢀelectronsꢀfromꢀtheꢀOECꢀusingꢀaꢀconstantꢀpoten-
tialꢀoxidant,ꢀP680•+.10,11ꢀInꢀadditionꢀtoꢀoxygenicꢀphotosyn-
thesis,ꢀPCETꢀprocessesꢀinvolvingꢀtyrosineꢀand/orꢀhistidineꢀ
2
3,4
residuesꢀareꢀubiquitousꢀinꢀnature.12–17
Manyꢀartificialꢀsystemsꢀhaveꢀbeenꢀinspiredꢀbyꢀtheꢀstructureꢀ
andꢀfunctionꢀofꢀtheꢀTyr -His190ꢀpairꢀinꢀPSII,ꢀandꢀassociatedꢀ
ꢀ
•+
•−
nearbyꢀchlorophyllꢀ(Chl),ꢀformingꢀP680 ꢀandꢀChl .ꢀSubse-
quentꢀelectronꢀtransfersꢀyieldꢀreducedꢀquinones,ꢀresultingꢀ
inꢀprotonꢀuptakeꢀfromꢀtheꢀchloroplastꢀstromaꢀandꢀtheꢀgen-
erationꢀofꢀproton-motiveꢀforce.ꢀTheꢀoxidizedꢀprimaryꢀdonor,ꢀ
Z
experimentalꢀandꢀtheoreticalꢀstudiesꢀhaveꢀcontributedꢀtoꢀaꢀ
deeperꢀunderstandingꢀofꢀtheꢀmoreꢀcomplexꢀnaturalꢀprocessꢀ
•+
P680 ,ꢀisꢀreducedꢀbyꢀelectronꢀtransferꢀfromꢀTyr ,ꢀwhichꢀinꢀ
Z
1
8–27
turnꢀisꢀreducedꢀbyꢀtheꢀOECꢀwhereꢀwaterꢀoxidationꢀisꢀtheꢀul-
timateꢀsourceꢀofꢀelectrons.ꢀTheꢀprotonsꢀproducedꢀuponꢀwa-
terꢀoxidationꢀareꢀconductedꢀtoꢀtheꢀthylakoidꢀlumenꢀwhereꢀ
andꢀPCETꢀinꢀgeneral.
ꢀInꢀparticular,ꢀbenzimidazoleꢀphe-
nolꢀ (BIP)ꢀ constructsꢀ haveꢀ beenꢀ studiedꢀ asꢀ modelsꢀ ofꢀ theꢀ
28–31
Tyr
Z
-His190ꢀpair.
ꢀUponꢀoxidationꢀofꢀtheꢀphenol,ꢀprotonꢀ
5
,6
theyꢀ alsoꢀ contributeꢀ toꢀ proton-motiveꢀ force. ꢀ Physicallyꢀ
transferꢀtoꢀtheꢀbenzimidazoleꢀoccurs,ꢀgenerallyꢀperceivedꢀasꢀ
1
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anꢀexampleꢀofꢀaꢀconcertedꢀone-electron,ꢀone-protonꢀtrans-
ferꢀ (EPT)ꢀ process. ꢀ Electrochemicalꢀ measurementsꢀ showꢀ
Theoretical:ꢀTheꢀoptimizedꢀgeometries,ꢀenergies,ꢀandꢀvi-
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brationalꢀ frequenciesꢀ wereꢀ calculatedꢀ withꢀ densityꢀ func-
tionalꢀtheoryꢀ(DFT)ꢀusingꢀtheꢀB3LYP34–37ꢀfunctionalꢀandꢀtheꢀ
thatꢀtheꢀphenoxylꢀradicalꢀformedꢀuponꢀoxidationꢀofꢀBIP,ꢀ1ꢀ
(Chartꢀ 1),ꢀ whichꢀ featuresꢀ anꢀ intramolecularꢀ H-bondꢀ be-
tweenꢀtheꢀphenolꢀandꢀtheꢀnitrogenꢀatꢀtheꢀ3-positionꢀofꢀtheꢀ
imidazole,ꢀisꢀthermodynamicallyꢀcapableꢀofꢀwaterꢀoxidationꢀ
3
8,39
6-31G**ꢀbasisꢀset.
ꢀSolventꢀeffectsꢀwereꢀincludedꢀusingꢀ
theꢀconductor-likeꢀpolarizableꢀcontinuumꢀmodel.40,41ꢀTheseꢀ
calculationsꢀ wereꢀ performedꢀ withꢀ theꢀ Gaussianꢀ 09ꢀ pro-
3
2
42
nearꢀneutralꢀpH. ꢀ
gram. ꢀFurtherꢀcomputationalꢀdetailsꢀareꢀprovidedꢀinꢀtheꢀ
SupportingꢀInformation,ꢀSI.ꢀ
Recently,ꢀweꢀreportedꢀaꢀconcertedꢀone-electron,ꢀtwo-pro-
tonꢀ transferꢀ (E2PT)ꢀ processꢀ associatedꢀ withꢀ theꢀ electro-
chemicalꢀ oxidationꢀ ofꢀ theꢀ phenolꢀ inꢀ amino-substitutedꢀ
BIPs.7,33ꢀ Althoughꢀ theꢀ theoreticalꢀ calculationsꢀ wereꢀ con-
sistentꢀ withꢀ aꢀ fullyꢀ concertedꢀ E2PTꢀ process,ꢀ theꢀ experi-
mentalꢀmeasurementsꢀcouldꢀnotꢀdistinguishꢀbetweenꢀaꢀfullyꢀ
concertedꢀE2PTꢀprocessꢀandꢀaꢀstepwiseꢀmechanismꢀwhereꢀ
anꢀEPTꢀprocessꢀisꢀfollowedꢀbyꢀanꢀultrafastꢀsecondꢀprotonꢀ
transfer,ꢀwhichꢀisꢀviewedꢀasꢀeffectivelyꢀconcerted.ꢀThisꢀpro-
cessꢀtranslocatesꢀprotonsꢀoverꢀaꢀdistanceꢀofꢀca.ꢀ7ꢀÅꢀbutꢀisꢀ
accompaniedꢀbyꢀaꢀdecreaseꢀinꢀtheꢀpotentialꢀofꢀtheꢀphenoxylꢀ
Experimental:ꢀ
R
Synthesis:ꢀSubstitutedꢀBIP-Ph iminesꢀ(Chartꢀ 1)ꢀwereꢀsyn-
0
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thesizedꢀ accordingꢀ toꢀ aꢀ standardꢀ procedureꢀ withꢀ methylꢀ
2,3-diaminobenzoateꢀ andꢀ 3,5-di-tert-butyl-2-hydroxyben-
zaldehydeꢀasꢀstartingꢀmaterialsꢀforꢀtheꢀPhilipps-Ladenburgꢀ
43–45
reactionꢀthatꢀyieldedꢀBIP-COOCH3.
reducedꢀwithꢀLiAlH
OxidationꢀofꢀBIP-CH
ꢀTheꢀBIP-COOCH
3
ꢀwasꢀ
OH.ꢀ
ꢀgeneratedꢀtheꢀ
4
ꢀtoꢀgiveꢀtheꢀcorrespondingꢀBIP-CH
OHꢀwithꢀactivatedꢀMnO
BIP-CHO.ꢀTheꢀ BIP-Ph iminesꢀ wereꢀ preparedꢀ followingꢀ anꢀ
establishedꢀmethodꢀforꢀtheꢀsynthesisꢀofꢀaldiminesꢀfromꢀani-
2
2
2
R
33
radical/phenolꢀredoxꢀcoupleꢀbyꢀ~300ꢀmV. ꢀSuchꢀaꢀthermo-
46
lineꢀderivativesꢀinꢀtheꢀpresenceꢀofꢀpyrrolidineꢀasꢀaꢀcatalyst. ꢀ
dynamicꢀcostꢀinꢀredoxꢀpotentialꢀlimitsꢀtheꢀuseꢀofꢀamino-sub-
stitutedꢀ BIPsꢀasꢀ redoxꢀ mediatorsꢀ inꢀ waterꢀ oxidationꢀ pro-
cesses.ꢀ However,ꢀ theoreticalꢀ calculationsꢀ predictedꢀ thatꢀ
TheꢀcompleteꢀsyntheticꢀprocedureꢀandꢀNMRꢀcharacteriza-
tionꢀareꢀprovidedꢀinꢀtheꢀSI.ꢀ
Crystalꢀ Structureꢀ Determination:ꢀ Crystalsꢀ ofꢀ BIP-
BIPsꢀ bearingꢀ substituentsꢀ withꢀ reducedꢀ pK
imines,ꢀwhichꢀcanꢀbeꢀsubstitutedꢀtoꢀtuneꢀtheirꢀpK
a
’sꢀ suchꢀ asꢀ
’s,ꢀwouldꢀ
OCH3
Ph
imineꢀ7ꢀwereꢀgrownꢀbyꢀtheꢀvaporꢀdiffusionꢀtechniqueꢀ
Cl ꢀandꢀhexaneꢀasꢀsolvents.ꢀAꢀBrukerꢀSmartꢀAPEXꢀ
a
usingꢀCH
2
2
undergoꢀ E2PTꢀ processesꢀ thatꢀ maintainꢀ aꢀ considerablyꢀ
higherꢀpotentialꢀforꢀtheꢀphenoxylꢀradical/phenolꢀredoxꢀcou-
diffractometerꢀwasꢀusedꢀtoꢀobtainꢀsingleꢀcrystalꢀdiffractionꢀ
data.ꢀDetailedꢀinformationꢀofꢀcrystallographicꢀstructureꢀandꢀ
dataꢀcollectionsꢀareꢀprovidedꢀinꢀtheꢀSI.ꢀ
3
3
ple. ꢀHerein,ꢀnewꢀmodelsꢀofꢀtheꢀTyr
ofꢀbenzimidazoleꢀphenolꢀimineꢀderivativesꢀ(BIP-Ph iminesꢀ
–8,ꢀChartꢀ1)ꢀareꢀreported.ꢀSubstitutionsꢀatꢀtheꢀpara-posi-
Z
-His190ꢀpairꢀcomposedꢀ
R
2
Cyclicꢀ voltammetry:ꢀ Cyclicꢀ voltammetryꢀ measurementsꢀ
wereꢀperformedꢀwithꢀaꢀBiologicꢀpotentiostatꢀusingꢀaꢀglassyꢀ
carbonꢀ(3ꢀmmꢀdiameter)ꢀworkingꢀelectrode,ꢀaꢀPtꢀwireꢀcoun-
terꢀelectrode,ꢀandꢀaꢀAgꢀwireꢀpseudoreferenceꢀelectrodeꢀinꢀaꢀ
conventionalꢀthree-electrodeꢀcell.ꢀAllꢀcyclicꢀvoltammogramsꢀ
R
tionꢀofꢀtheꢀanilineꢀringꢀofꢀBIP-Ph iminesꢀmodulateꢀtheꢀdriv-
ingꢀforceꢀforꢀtheꢀsecondꢀprotonꢀtransferꢀdueꢀtoꢀinterrelatedꢀ
factorsꢀthatꢀincludeꢀtheꢀ DpK ꢀbetweenꢀtheꢀbenzimidazoliumꢀ
a
ionꢀandꢀtheꢀprotonatedꢀimine,ꢀtheꢀinternalꢀH-bondꢀstrengthꢀ
betweenꢀtheꢀ1H-benzimidazoleꢀprotonꢀandꢀtheꢀimineꢀnitro-
genꢀloneꢀpair,ꢀandꢀtheꢀmidpointꢀpotentialꢀofꢀtheꢀphenoxylꢀ
radical/phenolꢀ couple.ꢀ Theseꢀ parametersꢀ controlꢀ theꢀ ob-
servedꢀratioꢀofꢀtheꢀEPTꢀandꢀE2PTꢀproducts.ꢀ
(CVs)ꢀwereꢀcollectedꢀinꢀanhydrousꢀCH
ouslyꢀdistilledꢀandꢀkeptꢀoverꢀmolecularꢀsievesꢀandꢀK
containingꢀtheꢀcompoundꢀofꢀinterestꢀatꢀaꢀconcentrationꢀofꢀ1ꢀ
mM.ꢀTetrabutylammoniumꢀhexafluorophosphateꢀ(TBAPF ,ꢀ
.5ꢀMꢀinꢀCH CNꢀandꢀ0.1ꢀinꢀCH Cl )ꢀwasꢀusedꢀasꢀtheꢀsupport-
3
CNꢀorꢀCH
2
Cl
2
,ꢀprevi-
2
CO ,ꢀ
3
6
0
3
2
2
ꢀ
-1
ingꢀelectrolyte,ꢀandꢀtheꢀscanꢀrateꢀwasꢀ100ꢀmVꢀs .ꢀAllꢀmeas-
urementsꢀwereꢀconductedꢀatꢀroomꢀtemperatureꢀunderꢀanꢀ
argonꢀatmosphere.ꢀTheꢀargonꢀgasꢀstreamꢀwasꢀpre-saturatedꢀ
byꢀbubblingꢀthroughꢀtheꢀorganicꢀsolventꢀusedꢀforꢀtheꢀexper-
imentsꢀinꢀaꢀseparateꢀSchlenkꢀflaskꢀandꢀletꢀintoꢀtheꢀelectro-
chemicalꢀcellꢀviaꢀPEEKꢀtubing.ꢀTheꢀglassyꢀcarbonꢀworkingꢀ
electrodeꢀwasꢀpolishedꢀbetweenꢀmeasurementsꢀwithꢀanꢀalu-
minumꢀslurryꢀonꢀaꢀmicroclothꢀpolishingꢀpad,ꢀfollowedꢀbyꢀ
solventꢀrinsesꢀandꢀdryingꢀunderꢀaꢀstreamꢀofꢀnitrogen.ꢀTheꢀ
potentialꢀofꢀtheꢀpseudoreferenceꢀelectrodeꢀwasꢀdeterminedꢀ
usingꢀtheꢀferrocenium/ferroceneꢀredoxꢀcoupleꢀasꢀanꢀinter-
nalꢀstandardꢀandꢀadjustingꢀtoꢀtheꢀsaturatedꢀcalomelꢀelec-
trodeꢀ (SCE)ꢀ scaleꢀ (withꢀE1/2ꢀtakenꢀ toꢀ beꢀ 0.40ꢀ Vꢀ vsꢀSCEꢀ inꢀ
Chartꢀ 1.ꢀ Molecularꢀ Structuresꢀ ofꢀ BIPꢀ andꢀ Derivativesꢀ
Substitutedꢀatꢀtheꢀ7-Position.ꢀOnꢀtheꢀRightꢀProductsꢀofꢀ
ElectrochemicalꢀOxidationꢀofꢀtheꢀPhenolꢀCoupledꢀwithꢀ
One-ProtonꢀTransferꢀ(EPT)ꢀandꢀTwo-ProtonꢀTransfersꢀ
(E2PT)ꢀꢀ
5
6
4
7
3
1
2
1
2–
8
EPT product
4
7
CH
3
CNꢀandꢀ0.46ꢀVꢀvsꢀSCEꢀinꢀCH
2
Cl ). ꢀ
2
2
3
– R = CN
– R = CF
BIP-PhCNimine
FTIRꢀ andꢀ Infraredꢀ Spectroelectrochemistryꢀ (IRSEC):ꢀ FTIRꢀ
measurementsꢀwereꢀperformedꢀusingꢀaꢀBrukerꢀVertexꢀ70ꢀ
spectrometerꢀ inꢀ absorptionꢀ modeꢀ underꢀ aꢀ dryꢀ nitrogenꢀ
BIP-PhCF3imine
BIP-PhClimine
ꢀ
3
4 – R = Cl
5 – R = H
6 – R = CH
−1
BIP-PhHimine
purgeꢀwithꢀaꢀ2ꢀcm ꢀresolution,ꢀGloBarꢀMIRꢀsource,ꢀbroad-
bandꢀKBrꢀbeamsplitter,ꢀandꢀliquidꢀnitrogenꢀcooledꢀMCTꢀde-
tector.ꢀ Spectraꢀ wereꢀ collectedꢀ (64ꢀ scans)ꢀ inꢀ anhydrousꢀ
BIP-PhCH3imine
BIP-PhOCH3imine
3
7 – R = OCH
3
CH
andꢀK
wasꢀused
2
Cl
2
ꢀ(distilledꢀoverꢀCaH
CO )ꢀatꢀaꢀconcentrationꢀofꢀ19ꢀmM,ꢀandꢀ0.1ꢀMꢀTBAPF6ꢀ
toꢀsimulateꢀtheꢀconditionsꢀofꢀtheꢀIRSECꢀmeasure-
2
ꢀandꢀkeptꢀoverꢀmolecularꢀsievesꢀ
BIP-PhNꢁ
CH
3
ꢂ
2
8
– R =N(CH
3
)
2
imine
E2PT product
2
3
ꢀ
ꢀ
ꢀ
mentsꢀ (videꢀ infra).ꢀ Becauseꢀ theꢀ moleculesꢀ containingꢀ theꢀ
imineꢀlinkageꢀareꢀsusceptibleꢀtoꢀhydrolysis,ꢀcareꢀwasꢀtakenꢀ
Methodsꢀ
2
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Page 3 of 13
Journal of the American Chemical Society
1
/2
α/2
toꢀminimizeꢀtheꢀpresenceꢀofꢀmoisture,ꢀandꢀtheꢀcompoundsꢀ
wereꢀdriedꢀunderꢀhighꢀvacuumꢀovernightꢀbeforeꢀuse.ꢀTheꢀ
FTIRꢀspectraꢀwereꢀobtainedꢀusingꢀaꢀCaF ꢀliquidꢀtransmis-
sionꢀ cellꢀ (Internationalꢀ Crystalꢀ Laboratories,ꢀ Modelꢀ SL-3,ꢀ
πD Fv
DR
DO
O
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
kapp=ψꢀ
ꢁ
ꢀ
ꢁ
(1)
RT
2
whereꢀαꢀisꢀtheꢀtransferꢀcoefficientꢀ(takenꢀasꢀ0.5),ꢀandꢀD
R
ꢀandꢀ
pathꢀlength:ꢀ0.1030ꢀmm),ꢀandꢀtheꢀCF
wasꢀdistilledꢀbeforeꢀuse.ꢀ
3
COOHꢀaddedꢀtoꢀtheꢀcellꢀ
2
-1
D
O
ꢀareꢀtheꢀdiffusionꢀcoefficientsꢀ(cm ꢀs )ꢀforꢀtheꢀoxidizedꢀ
andꢀ reducedꢀ forms,ꢀ whichꢀ wereꢀassumedꢀtoꢀ beꢀtheꢀ sameꢀ
valueꢀofꢀ1´10-5ꢀcm ꢀs-1.ꢀThus,ꢀEquationꢀ(1)ꢀcanꢀbeꢀwrittenꢀasꢀ
IRSECꢀmeasurementsꢀwereꢀconductedꢀusingꢀaꢀBiologicꢀpo-
tentiostatꢀconnectedꢀtoꢀanꢀopticallyꢀtransparentꢀthin-layerꢀ
electrochemicalꢀcellꢀ(SpectroelectrochemistryꢀReadingꢀRTꢀ
2
follows:ꢀ
ꢃ1/2
πD Fv
O
OTTLEꢀcell),ꢀpathlengthꢀ0.2ꢀmm,ꢀequippedꢀwithꢀCaF ꢀopticalꢀ
2
ψ=ꢂkapp
ꢀ
ꢁ
(2)
RT
windows.ꢀTheꢀcellꢀcontainedꢀaꢀPtꢀmeshꢀcounterꢀelectrode,ꢀaꢀ
Agꢀwireꢀpseudoreferenceꢀelectrode,ꢀandꢀaꢀPtꢀmeshꢀworkingꢀ
electrode.ꢀTheꢀPtꢀmeshꢀworkingꢀelectrodeꢀwasꢀpositionedꢀinꢀ
theꢀ lightꢀ pathꢀ ofꢀ theꢀ IRꢀ spectrophotometer.ꢀForꢀallꢀ IRSECꢀ
measurements,ꢀtheꢀcompoundsꢀwereꢀusedꢀatꢀaꢀconcentra-
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Theꢀvalueꢀofꢀkappꢀcanꢀbeꢀobtainedꢀfromꢀtheꢀslopeꢀofꢀaꢀplotꢀofꢀ
–1/2ꢀ
ψꢀ vsꢀ v
(Figureꢀ S39).ꢀ Theꢀ valuesꢀ ofꢀ kappꢀ areꢀ betweenꢀ
-2ꢀ
-2ꢀ
-1
~2´10 andꢀ~8´10 cmꢀs ꢀ(TableꢀS13).ꢀCVsꢀwereꢀsimulatedꢀ
(FiguresꢀS40ꢀandꢀS41)ꢀwithꢀDigiSimꢀ2.1ꢀsoftwareꢀusingꢀtheꢀ
correspondingꢀkappꢀvaluesꢀshownꢀinꢀTableꢀS13.ꢀ
tionꢀofꢀ19ꢀmM,ꢀwithꢀ0.1ꢀMꢀTBAPF ꢀasꢀtheꢀsupportingꢀelectro-
6
lyte.ꢀTheꢀcellꢀandꢀitsꢀcontentsꢀwereꢀsealedꢀunderꢀanꢀargonꢀ
atmosphereꢀpriorꢀtoꢀallꢀmeasurements,ꢀandꢀthinꢀlayerꢀcon-
stantꢀpotentialꢀelectrolysisꢀwasꢀmonitoredꢀviaꢀFTIRꢀasꢀtheꢀ
workingꢀelectrodeꢀwasꢀpolarizedꢀpositiveꢀinꢀ50–100ꢀmVꢀin-
crementsꢀvsꢀtheꢀsilverꢀwireꢀreference.ꢀAbsorptionꢀspectraꢀ
wereꢀcontinuouslyꢀcollectedꢀatꢀeachꢀappliedꢀpotentialꢀuntilꢀ
thereꢀwereꢀnoꢀfurtherꢀsignificantꢀchanges.ꢀThisꢀprocedureꢀ
wasꢀrepeatedꢀuntilꢀincreasingꢀtheꢀpolarizationꢀnoꢀlongerꢀre-
ꢀ
ResultsꢀandꢀDiscussionꢀ
Structuralꢀ Characterization.ꢀ Theꢀ crystalꢀ structureꢀ ofꢀ 7ꢀ
Figureꢀ1)ꢀindicatesꢀaꢀnearlyꢀplanarꢀmolecularꢀframeworkꢀ
withꢀaꢀdihedralꢀangleꢀbetweenꢀtheꢀphenolꢀandꢀbenzimidaz-
oleꢀmoietiesꢀofꢀ3.4°.ꢀTheꢀO–Nꢀdistanceꢀofꢀ2.57ꢀÅꢀisꢀinꢀagree-
mentꢀ withꢀaꢀ strongꢀH-bondꢀ betweenꢀtheꢀ phenolicꢀ protonꢀ
andꢀtheꢀnitrogenꢀloneꢀpairꢀofꢀbenzimidazole.18,33,44ꢀTheꢀcrys-
talꢀstructureꢀalsoꢀindicatesꢀaꢀH-bondꢀbetweenꢀtheꢀ1H-ben-
zimidazoleꢀprotonꢀandꢀtheꢀimineꢀnitrogen,ꢀwithꢀanꢀN–Nꢀdis-
tanceꢀofꢀ2.92ꢀÅꢀ(Figureꢀ1).ꢀ
(
4
8,49
sultedꢀ inꢀ significantꢀ changesꢀ ofꢀ theꢀ spectra
ꢀ andꢀ theꢀ
-1
broadꢀ IRꢀ absorptionꢀ atꢀ ~2,700ꢀ cm ꢀ characteristicꢀ ofꢀ theꢀ
1
8
phenolꢀOHꢀwasꢀnoꢀlongerꢀobservable, ꢀconfirmingꢀdeple-
tionꢀofꢀtheꢀphenolꢀwithinꢀtheꢀpathꢀofꢀtheꢀopticalꢀprobe.ꢀUn-
derꢀtheseꢀconditionsꢀitꢀisꢀassumedꢀtheꢀtotalꢀconcentrationꢀofꢀ
oxidizedꢀproductsꢀ([EPT]ꢀ+ꢀ[E2PT])ꢀisꢀapproximatelyꢀequalꢀ
R
toꢀtheꢀinitialꢀconcentrationꢀofꢀtheꢀBIP-Ph imineꢀsolution.ꢀUs-
ingꢀ thisꢀ concentration,ꢀ andꢀ theꢀ observationꢀ thatꢀ theꢀ EPTꢀ
productꢀisꢀnotꢀdetectedꢀinꢀtheꢀIRSECꢀofꢀ7,ꢀanꢀextinctionꢀcoef-
-1
-1
-1ꢀ
ficientꢀofꢀ467ꢀM cm ꢀforꢀtheꢀbandꢀatꢀ1652ꢀcm wasꢀesti-
mated.ꢀ Thisꢀ bandꢀ wasꢀ assignedꢀ toꢀ theꢀ protonatedꢀ imineꢀ
stretchingꢀmodeꢀcharacteristicꢀofꢀtheꢀE2PTꢀproduct.ꢀꢀ
2
.92 Å
KIEꢀmeasurements:ꢀAllꢀmeasurementsꢀwereꢀcarriedꢀoutꢀatꢀ
20ꢀ°Cꢀinꢀaꢀconventionalꢀthree-electrodeꢀcellꢀusingꢀaꢀPtꢀcoun-
terꢀelectrode,ꢀanꢀAgꢀwireꢀasꢀpseudo-referenceꢀelectrode,ꢀandꢀ
aꢀglassyꢀcarbonꢀworkingꢀelectrodeꢀ(1ꢀmmꢀdiameter).ꢀAnhy-
2
.57 Å
drousꢀCH
3
CN,ꢀkeptꢀ overꢀ molecularꢀ sievesꢀandꢀK
2
CO ,ꢀwasꢀ
3
usedꢀasꢀtheꢀsolvent,ꢀwithꢀ0.5ꢀMꢀTBAPF
6
ꢀasꢀtheꢀsupportingꢀ
electrolyte.ꢀConcentrationsꢀinꢀtheꢀ3.3ꢀtoꢀ4.7ꢀmMꢀrangeꢀofꢀtheꢀ
R
3.4°
BIP-Ph iminesꢀwereꢀused.ꢀAllꢀofꢀtheꢀCVsꢀwereꢀcorrectedꢀforꢀ
ohmicꢀeffectsꢀusingꢀfeedbackꢀtechniques.ꢀTheꢀbaselineꢀcur-
rentsꢀwereꢀregisteredꢀforꢀeachꢀscanꢀrateꢀunderꢀtheꢀsameꢀex-
perimentalꢀconditionsꢀ(CH CNꢀcontainingꢀonlyꢀsupportingꢀ
3
ꢀ
electrolyteꢀ withoutꢀ theꢀadditionꢀ ofꢀtheꢀ studiedꢀ molecule)ꢀ
andꢀ wereꢀ subtractedꢀ fromꢀ theꢀ correspondingꢀ CVsꢀ beforeꢀ
processing.ꢀRedoxꢀpotentialsꢀwereꢀcalculatedꢀasꢀtheꢀmeanꢀofꢀ
theꢀanodicꢀandꢀcathodicꢀpeaks.ꢀAfterꢀeachꢀelectrochemicalꢀ
measurement,ꢀferroceneꢀwasꢀaddedꢀtoꢀtheꢀsolutionꢀandꢀtheꢀ
potentialꢀ ofꢀ theꢀ pseudo-referenceꢀ electrodeꢀ wasꢀ deter-
mined.ꢀInꢀorderꢀtoꢀcalculateꢀtheꢀKIEꢀforꢀeachꢀcompound,ꢀtheꢀ
apparentꢀstandardꢀrateꢀconstantsꢀofꢀelectronꢀtransferꢀ(kapp)ꢀ
Figureꢀ1.ꢀORTEPꢀrepresentationsꢀofꢀtheꢀcrystalꢀstructureꢀofꢀ
OCH3
BIP-Ph
imineꢀ7.ꢀTheꢀupperꢀFigureꢀshowsꢀtheꢀdistancesꢀbe-
tweenꢀtheꢀatomsꢀinvolvedꢀinꢀbothꢀH-bonds.ꢀTheꢀlowerꢀFigureꢀ
showsꢀtheꢀdihedralꢀangleꢀbetweenꢀtheꢀphenolꢀandꢀbenzimidaz-
oleꢀmoieties.ꢀCarbonsꢀareꢀshownꢀinꢀgray,ꢀoxygensꢀinꢀred,ꢀnitro-
gensꢀinꢀblue,ꢀandꢀhydrogensꢀinꢀwhite.ꢀThermalꢀellipsoidsꢀareꢀ
drawnꢀatꢀtheꢀ50%ꢀprobabilityꢀlevel.ꢀ
wereꢀdeterminedꢀinꢀCH
3
CNꢀ+ꢀ2%ꢀCH
3
OHꢀandꢀCH CNꢀ+ꢀ2%ꢀ
3
Theꢀcrystallographicꢀdataꢀofꢀ7ꢀareꢀinꢀgoodꢀagreementꢀwithꢀ
theꢀoptimizedꢀstructureꢀdeterminedꢀfromꢀDFTꢀcalculationsꢀ
(TableꢀS14ꢀandꢀFigureꢀS42),ꢀandꢀsimilarꢀstructuresꢀwereꢀob-
tainedꢀbyꢀgeometricalꢀandꢀenergyꢀoptimizationꢀofꢀ2–6ꢀandꢀ
8.ꢀAdditionalꢀsupportꢀforꢀtheꢀformationꢀofꢀaꢀstrongꢀH-bondꢀ
betweenꢀtheꢀ1H-benzimidazoleꢀprotonꢀandꢀtheꢀimineꢀnitro-
genꢀisꢀobtainedꢀfromꢀIRꢀ(seeꢀIRSECꢀsection)ꢀandꢀNMRꢀdata.ꢀ
0,51
CH
3
ODꢀbyꢀtheꢀNicholson’sꢀmethod.5 ꢀCVsꢀwereꢀtakenꢀatꢀdif-
ferentꢀscanꢀratesꢀbetweenꢀ0.25ꢀandꢀ200ꢀV/s,ꢀandꢀtheꢀdiffer-
enceꢀ betweenꢀ theꢀ potentialsꢀ ofꢀ theꢀ anodicꢀ andꢀ cathodicꢀ
peaksꢀ(ΔE
latedꢀtoꢀtheꢀdimensionlessꢀparameterꢀψ,
toꢀkappꢀandꢀvꢀaccordingꢀtoꢀtheꢀfollowingꢀequation:ꢀ
p
)ꢀwasꢀcalculatedꢀforꢀeachꢀscanꢀrateꢀv.ꢀΔE
p
ꢀisꢀre-
5
0,51
ꢀwhichꢀisꢀlinkedꢀ
3
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 13
1
Analysisꢀ ofꢀ theꢀ Hꢀ NMRꢀ spectraꢀforꢀcompoundsꢀ2–8ꢀ con-
firmsꢀtheꢀpresenceꢀofꢀtheꢀtwoꢀinternalꢀH-bondsꢀ(Figureꢀ2ꢀ
monosubstitutedꢀbenzylideneanilines,ꢀwhereꢀtheꢀsubstitu-
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
tionꢀisꢀonꢀtheꢀanilineꢀring,ꢀtheꢀbaseꢀstrengthꢀofꢀtheꢀimineꢀni-
trogenꢀisꢀinfluencedꢀbyꢀinductiveꢀandꢀresonanceꢀeffectsꢀofꢀ
andꢀ Tableꢀ 1)ꢀ inꢀ CDCl ꢀ solutionsꢀ ofꢀ allꢀ samples.ꢀ Thereꢀ isꢀ
slightꢀ variationꢀ inꢀ theꢀ OHꢀ chemicalꢀ shift,ꢀ δOH,ꢀ inꢀ theꢀ BIP-
Ph iminesꢀwhenꢀvaryingꢀtheꢀpara-substituentꢀofꢀtheꢀN-phe-
nylimineꢀ moiety.ꢀTheꢀ substitutionꢀwithꢀelectron-donatingꢀ
groupsꢀresultsꢀinꢀdownfieldꢀshifts. ꢀTheꢀformationꢀofꢀtheꢀH-
bondꢀbetweenꢀtheꢀ1H-benzimidazoleꢀprotonꢀandꢀtheꢀimineꢀ
nitrogenꢀ isꢀ evidentꢀ byꢀ theꢀ NMRꢀ chemicalꢀ shiftꢀ ofꢀ theꢀ NHꢀ
groupꢀofꢀbenzimidazoleꢀ(δNH).ꢀInꢀallꢀBIP-Ph imines,ꢀtheꢀδNH
3
5
8
theꢀsubstituent. ꢀ
R
TheꢀsubstituentꢀeffectꢀonꢀtheꢀH-bondꢀbetweenꢀtheꢀNHꢀofꢀtheꢀ
imidazoleꢀandꢀtheꢀnitrogenꢀofꢀtheꢀimineꢀinꢀcompoundsꢀ2–8ꢀ
3
3
1
canꢀclearlyꢀbeꢀobservedꢀinꢀtheꢀseriesꢀofꢀ HꢀNMRꢀspectraꢀpre-
sentedꢀ inꢀ Figureꢀ 2.ꢀTheꢀ δNHꢀ movesꢀ downfieldꢀwhenꢀelec-
tron-withdrawingꢀgroupsꢀatꢀtheꢀpara-positionꢀofꢀtheꢀN-phe-
nylimineꢀmoietyꢀareꢀreplacedꢀbyꢀelectron-donatingꢀgroups,ꢀ
suchꢀasꢀtheꢀmethoxyꢀorꢀN-dimethylaminoꢀgroupsꢀpresentꢀinꢀ
compoundsꢀ7ꢀandꢀ8,ꢀrespectively.ꢀAsꢀexpected,ꢀelectron-do-
natingꢀ substituentsꢀ increaseꢀ theꢀ electronꢀ densityꢀ atꢀ theꢀ
imineꢀnitrogen,ꢀresultingꢀinꢀaꢀstrongerꢀH-bondꢀwithꢀtheꢀNHꢀ
R
ꢀ
isꢀfoundꢀbetweenꢀ~11.48–11.98ꢀppm,ꢀdownfieldꢀfromꢀtheꢀ
equivalentꢀ NHꢀ inꢀ 1ꢀ (9.34ꢀ ppm)ꢀ andꢀ N-diethylaminoBIPꢀ
11.17ꢀppm),ꢀinꢀCDCl . ꢀ
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
33
(
3
δCH=N
5
9
δOH
ofꢀtheꢀimidazole. ꢀAꢀgraphꢀofꢀδNHꢀvaluesꢀagainstꢀtheꢀHam-
mettꢀsubstituentꢀconstantsꢀ(σ )ꢀexhibitsꢀaꢀlinearꢀcorrelationꢀ
δNH
p
1
2
1
ꢀ
forꢀiminesꢀ2–8ꢀ(FigureꢀS29).ꢀConversely,ꢀtheꢀazomethineꢀ H
NMRꢀchemicalꢀshiftꢀ(δCH=N)ꢀappearsꢀatꢀ~8.75ꢀppmꢀandꢀre-
R
mainsꢀessentiallyꢀconstantꢀinꢀallꢀBIP-Ph iminesꢀstudiedꢀ(Ta-
3
4
5
6
7
8
bleꢀ1),ꢀasꢀindicatedꢀbyꢀtheꢀabsenceꢀofꢀaꢀHammettꢀcorrelationꢀ
(FigureꢀS29).ꢀThisꢀobservationꢀisꢀconsistentꢀwithꢀfindingsꢀ
regardingꢀ analogousꢀ monosubstitutedꢀ benzylidenani-
lines.59,60
ꢀ
TheꢀunsymmetricalꢀnatureꢀofꢀBIPsꢀsubstitutedꢀatꢀtheꢀ7-po-
sition,ꢀasꢀinꢀ2–8,ꢀleadsꢀtoꢀtwoꢀisomericꢀformsꢀresultingꢀfromꢀ
tautomerizationꢀofꢀtheꢀimidazoleꢀandꢀrotationꢀaroundꢀtheꢀ
bondꢀbetweenꢀtheꢀbenzimidazoleꢀandꢀphenolicꢀmoiety.ꢀOnlyꢀ
oneꢀofꢀtheseꢀisomersꢀhasꢀtheꢀappropriateꢀinternalꢀH-bondꢀ
networkꢀnecessaryꢀforꢀtheꢀE2PTꢀprocessꢀ(Chartꢀ1),ꢀwhereinꢀ
theꢀphenolicꢀOHꢀgroupꢀisꢀH-bondedꢀtoꢀtheꢀbenzimidazoleꢀni-
trogenꢀandꢀaꢀsecondꢀH-bondꢀisꢀpresentꢀbetweenꢀtheꢀ1H-ben-
δ (ppm)
ꢀ
1
Figureꢀ2.ꢀ400ꢀMHzꢀ HꢀNMRꢀspectraꢀinꢀCDCl
showingꢀtheꢀδOH,ꢀδNHꢀandꢀδCH=N.ꢀTheꢀ HꢀNMRꢀspectrumꢀofꢀ1ꢀisꢀ
includedꢀforꢀcomparison.ꢀ
3
ꢀofꢀBIPꢀderivativesꢀ
1
1
zimidazoleꢀprotonꢀandꢀtheꢀimineꢀnitrogen.ꢀFromꢀ HꢀNMR,ꢀ
theꢀ ratioꢀ ofꢀ isomersꢀ isꢀ approximatelyꢀ 1:0.1ꢀ andꢀ 1:0.2ꢀ inꢀ
CD
3
CNꢀand
ꢀ
(CD
3
)
2
CO,ꢀrespectively.ꢀTheꢀpresenceꢀofꢀtheꢀtwoꢀ
ꢀ
isomersꢀcannotꢀbeꢀdetectedꢀinꢀnon-polarꢀsolventsꢀsuchꢀasꢀ
CD
ꢀ
1
Tableꢀ 1.ꢀ Hꢀ NMRꢀ Chemicalꢀ Shiftsꢀ ofꢀ BIPꢀ Derivativesꢀ
(
spondingꢀSubstituentꢀGroups ꢀ
2
Cl
2
ꢀorꢀCDCl ꢀ(FigureꢀS30).ꢀ
3
Chartꢀ 1)ꢀ andꢀ Hammettꢀ Constantsꢀ (σ )ꢀ forꢀ theꢀ Corre-
p
a
ElectrochemicalꢀStudies.ꢀTableꢀ2ꢀpresentsꢀtheꢀexperimen-
tallyꢀ obtainedꢀ midpointꢀ potentialꢀ forꢀ theꢀ phenoxylꢀ radi-
ChemicalꢀShiftꢀ(ppm)ꢀ
bꢀ
σ
p
cal/phenolꢀcoupleꢀforꢀeachꢀofꢀtheꢀBIP-Ph
culatedꢀvaluesꢀassociatedꢀwithꢀthisꢀcoupleꢀforꢀbothꢀEPTꢀandꢀ
E2PTꢀproductsꢀinꢀCH CN.ꢀWhileꢀtheoryꢀyieldsꢀtwoꢀE1/2ꢀval-
uesꢀforꢀeachꢀofꢀtheꢀcompoundsꢀstudied,ꢀtheꢀCVsꢀdoꢀnotꢀre-
solveꢀtheꢀtwoꢀredoxꢀwaves.ꢀItꢀisꢀnecessaryꢀtoꢀanalyzeꢀtheꢀ
IRSECꢀdataꢀtoꢀclearlyꢀdifferentiateꢀbetweenꢀtheꢀE1/2ꢀvaluesꢀ
forꢀoxidationꢀleadingꢀtoꢀEPTꢀandꢀE2PTꢀproductsꢀ(videꢀinfra).ꢀ
ComparisonꢀofꢀtheꢀredoxꢀwavesꢀobservedꢀinꢀCVsꢀofꢀ7ꢀwithꢀ
R
iminesꢀandꢀtheꢀcal-
Compoundꢀ
:ꢀBIPꢀ
δ
NHꢀ
δ
OH
ꢀ
CH=N
δ ꢀ
3
1
9.34ꢀ
13.45ꢀ
N/Aꢀ
8.72ꢀ
8.74ꢀ
8.74ꢀ
8.76ꢀ
8.78ꢀ
8.77ꢀ
8.79ꢀ
N/Aꢀ
0.66ꢀ
0.54ꢀ
0.23ꢀ
0ꢀ
CN
2:ꢀBIP-Ph imineꢀ
11.48ꢀ 13.19ꢀ
11.58ꢀ 13.21ꢀ
11.66ꢀ 13.23ꢀ
11.76ꢀ 13.21ꢀ
11.82ꢀ 13.30ꢀ
11.84ꢀ 13.30ꢀ
CF3
3
4
:ꢀBIP-Ph imineꢂ
Cl
:ꢀBIP-Ph imineꢀ
H
5:ꢀBIP-Ph imineꢀ
theꢀredoxꢀcharacteristicsꢀ ofꢀ referenceꢀ compoundsꢀ lackingꢀ
OCH3
CH3
theꢀ phenolicꢀ OHꢀ groupꢀ (BI-Ph
imine)ꢀ andꢀ N-benzyli-
6
7
:ꢀBIP-Ph imineꢀ
-0.17ꢀ
-0.27ꢀ
-0.83ꢀ
dene-p-anisidineꢀ (seeꢀ syntheticꢀ procedureꢀ andꢀ structuresꢀ
onꢀ pagesꢀ S9–S13),ꢀ bothꢀ ofꢀ whichꢀ exhibitꢀ chemicallyꢀ irre-
versibleꢀoxidationꢀwavesꢀatꢀ1.3ꢀVꢀvsꢀSCE,ꢀprovidesꢀstrongꢀev-
idenceꢀthatꢀtheꢀobservedꢀquasi-reversibleꢀredoxꢀwaveꢀinꢀtheꢀ
cyclicꢀvoltammogramꢀofꢀ7ꢀatꢀ0.93ꢀVꢀvsꢀSCEꢀcorrespondsꢀtoꢀ
theꢀoxidationꢀofꢀtheꢀphenolꢀmoietyꢀ(Figureꢀ3ꢀA).ꢀTheꢀCVsꢀofꢀ
compoundsꢀ2,ꢀ 4ꢀ andꢀ7ꢀ (Figureꢀ 3ꢀ B)ꢀareꢀquasi-reversibleꢀ
withꢀaꢀpeak-to-peakꢀseparationꢀofꢀ~60ꢀmVꢀ(seeꢀSIꢀTableꢀS11ꢀ
forꢀ furtherꢀ informationꢀ regardingꢀ electrochemicalꢀ andꢀ
OCH3
:ꢀBIP-Ph
imineꢀ
N(CH3)2
8:ꢀBIP-Ph
imineꢀ
11.98ꢀ
13.34ꢀ
aꢀSolventꢀCDCl
.ꢀ
3
bꢀDataꢀfromꢀrefꢀ52ꢀ
ꢀ
R
InꢀtheꢀstructureꢀofꢀtheꢀBIP-Ph iminesꢀ(seeꢀChartꢀ1)ꢀtheꢀben-
zeneꢀringꢀinꢀtheꢀbenzimidazoleꢀmoietyꢀisꢀconnectedꢀwithꢀaꢀ
secondꢀ para-substitutedꢀ phenylꢀ ringꢀ throughꢀ theꢀ trans-
imineꢀlinkage.ꢀThisꢀstructureꢀfallsꢀunderꢀtheꢀwidelyꢀstudiedꢀ
R
chemicalꢀreversibility).ꢀTheꢀCVsꢀforꢀBIP-Ph iminesꢀ3,ꢀ5,ꢀandꢀ
6ꢀareꢀshownꢀinꢀtheꢀSIꢀ(FigureꢀS32).ꢀTheꢀCVsꢀofꢀ2,ꢀ3,ꢀ4ꢀandꢀ7ꢀ
inꢀCH
2
2
Cl ꢀexhibitꢀsimilarꢀcharacteristicsꢀ(seeꢀSIꢀFigureꢀS34ꢀ
5
3–57
classꢀ ofꢀ benzylideneanilines.
ꢀ Specifically,ꢀ forꢀ para-
andꢀTableꢀS12).ꢀInꢀgeneral,ꢀphenoxylꢀradical/phenolꢀredoxꢀ
4
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Journal of the American Chemical Society
theꢀdistanceꢀbetweenꢀtheꢀphenolꢀgroupꢀandꢀtheꢀsubstituentsꢀ
couplesꢀareꢀnotoriouslyꢀirreversible,ꢀbothꢀelectrochemicallyꢀ
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
andꢀ chemically. ꢀ However,ꢀ theꢀ quasi-reversibleꢀ electro-
7
isꢀconsiderableꢀ(~12ꢀÅ).ꢀAsꢀcanꢀbeꢀseenꢀfromꢀtheꢀdataꢀpre-
sentedꢀinꢀTableꢀ2ꢀandꢀFigureꢀ4,ꢀtheꢀdifferenceꢀbetweenꢀtheꢀ
calculatedꢀmidpointꢀpotentialsꢀforꢀformingꢀEPTꢀandꢀE2PTꢀ
productsꢀ areꢀ largestꢀ withꢀ electron-donatingꢀ groups.ꢀ Fur-
thermore,ꢀtheꢀtwoꢀcalculatedꢀvaluesꢀofꢀE1/2ꢀcrossꢀoverꢀwhenꢀ
chemistryꢀassociatedꢀwithꢀ compoundsꢀ 1–7ꢀ isꢀ almostꢀcer-
tainlyꢀ attributableꢀ toꢀ theꢀ structure,ꢀ whichꢀ constrainsꢀ theꢀ
protonꢀtoꢀanꢀinternalꢀH-bondꢀbetweenꢀtheꢀphenolicꢀOHꢀandꢀ
nitrogenꢀatꢀtheꢀ3-positionꢀofꢀtheꢀbenzimidazole.ꢀTheꢀchemi-
calꢀreversibilityꢀofꢀcompoundsꢀ2–7ꢀinꢀCH
2
Cl
2
ꢀisꢀ>92%ꢀatꢀrel-
theꢀ electron-withdrawingꢀgroupsꢀ (-CF ꢀandꢀ -CN)ꢀ areꢀ pre-
3
ativelyꢀslowꢀscanꢀratesꢀ(100ꢀmV/s).ꢀImportantly,ꢀtheꢀmid-
pointꢀ potentialsꢀ assignedꢀ toꢀ theꢀ phenoxylꢀ radical/phenolꢀ
coupleꢀofꢀtheseꢀBIP-Ph iminesꢀderivativesꢀdecreaseꢀbyꢀlessꢀ
sent,ꢀsuchꢀthatꢀtheꢀE1/2ꢀassociatedꢀwithꢀtheꢀE2PTꢀprocessꢀbe-
comesꢀmoreꢀpositiveꢀthanꢀtheꢀE1/2ꢀassociatedꢀwithꢀtheꢀEPTꢀ
process.ꢀThisꢀobservationꢀisꢀconsistentꢀwithꢀtheꢀequilibriumꢀ
constantꢀforꢀ[E2PT]/[EPT]ꢀchangingꢀfromꢀ>1ꢀtoꢀ<1ꢀ(videꢀin-
fra)ꢀbetweenꢀcompoundsꢀ4ꢀandꢀ3ꢀ(Tableꢀ3).ꢀ
R
thanꢀ~40ꢀmVꢀrelativeꢀtoꢀ1,ꢀinsteadꢀofꢀtheꢀ~300ꢀmVꢀdecreaseꢀ
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
3
observedꢀforꢀtheꢀamino-substitutedꢀBIPs. ꢀ
ꢀ
Tableꢀ2.ꢀExperimentalꢀandꢀCalculatedꢀRedoxꢀPotentialsꢀ
forꢀtheꢀBIP-Ph imineꢀConstructs ꢀ
E (V vs SCE)
R
a
0.0
0
0.4
0.8
1.2
1.6
3
OCH3
7
BIP-Ph
imine
E
1/2ꢀ(VꢀvsꢀSCE)ꢀ
OCH
3
A
BI-Ph
imine
Compoundꢀ
Calculatedꢀ Calculatedꢀ
N-benzylidene-p-anisidine
Experimental
ꢀ
20
10
0
forꢀE2PTbꢀ
1.06ꢀ
1.06ꢀ
1.02ꢀ
0.98ꢀ
0.95ꢀ
0.93ꢀ
-ꢀ
forꢀEPTcꢀ
1.04ꢀ
1.05ꢀ
1.04ꢀ
1.04ꢀ
1.02ꢀ
1.00ꢀ
-ꢀ
CN
2
3
:ꢀBIP-Ph imineꢀ
0.99ꢀ
0.99ꢀ
0.97ꢀ
0.95ꢀ
0.93ꢀ
CF3
:ꢀBIP-Ph imineꢂ
Cl
4:ꢀBIP-Ph imineꢀ
H
5
6
:ꢀBIP-Ph imineꢀ
CH3
:ꢀBIP-Ph imineꢀ
-
10
OCH3
7:ꢀBIP-Ph
imineꢀ
0.93ꢀ
-0.4
0.0
0.4
0.8
1.2
1.4
+
N(CH3)
0.68dꢀ
8
:ꢀBIP-Ph
2
imineꢀ
E (V vs Fc /Fc)
aꢀTheꢀvaluesꢀreportedꢀhereinꢀwereꢀexperimentallyꢀmeasuredꢀandꢀ
calculatedꢀinꢀCH CN.ꢀTheꢀcalculatedꢀspinꢀdensityꢀassociatedꢀwithꢀ
theꢀunpairedꢀelectronꢀinꢀtheꢀoxidizedꢀspeciesꢀisꢀlocalizedꢀonꢀtheꢀ
phenolꢀgroupꢀforꢀallꢀcompoundsꢀstudiedꢀexceptꢀ8,ꢀforꢀwhichꢀitꢀisꢀ
localizedꢀpredominantlyꢀonꢀtheꢀdimethylaminophenylꢀmoietyꢀ(Fig-
ureꢀS45).ꢀ
bꢀTheꢀpotentialꢀforꢀ7ꢀinꢀCH
culatedꢀpotentialsꢀassociatedꢀwithꢀanꢀE2PTꢀproduct,ꢀandꢀthereforeꢀ
itꢀagreesꢀwithꢀtheꢀexperimentalꢀvalueꢀbyꢀconstruction.ꢀ
cꢀTheseꢀpotentialsꢀwereꢀcalculatedꢀusingꢀaꢀthermodynamicꢀschemeꢀ
thatꢀtakesꢀintoꢀaccountꢀtheꢀE2PTꢀreferenceꢀreactionꢀasꢀwellꢀasꢀtheꢀ
ΔG ꢀ associatedꢀ withꢀ theꢀ EPT-E2PTꢀ equilibrium.ꢀ Theꢀ thermody-
namicꢀ schemesꢀ andꢀ relevantꢀ equationsꢀ areꢀ providedꢀ inꢀ theꢀ SIꢀ
(pagesꢀS74–S76).ꢀTheꢀresultingꢀpotentialꢀforꢀ7ꢀservedꢀasꢀtheꢀrefer-
enceꢀforꢀallꢀofꢀtheꢀEPTꢀreactions.ꢀNoteꢀthatꢀtheꢀΔG ꢀvalueꢀusedꢀforꢀ
theꢀreferenceꢀwasꢀtheꢀexperimentallyꢀdeterminedꢀvalueꢀobtainedꢀ
inꢀ CH
measuredꢀpotentialꢀforꢀ7ꢀinꢀCH
.02ꢀV,ꢀTableꢀS12)ꢀandꢀtheꢀdifferenceꢀbetweenꢀtheꢀcalculatedꢀΔG0ꢀ
valuesꢀinꢀCH Cl ꢀandꢀCH CNꢀisꢀalsoꢀsmallꢀ(lessꢀthanꢀ1ꢀkcal/mol,ꢀTa-
E (V vs SCE)
3
0
.4
0.6
0.8
1.0
1.2
3
2
1
0
0
0
0
CN
2 BIP-Ph imine
Cl
4 BIP-Ph imine
B
OCH
3
7 BIP-Ph
imine
3
CNꢀwasꢀusedꢀasꢀtheꢀreferenceꢀforꢀallꢀcal-
0
-
10
0
0.0
0.2
0.4
0.6
+
0.8
1.0
E (V vs Fc /Fc)
2
Cl
2
.ꢀ However,ꢀ theꢀ differenceꢀ betweenꢀ theꢀ experimentallyꢀ
2
Cl ꢀandꢀCH CNꢀisꢀsmallꢀ(lessꢀthanꢀ
2
3
ꢀ
0
2
2
3
OCH3
Figureꢀ3.ꢀ(A)ꢀCVsꢀofꢀBIP-Ph
imineꢀ7,ꢀaꢀreferenceꢀcompoundꢀ
bleꢀS19),ꢀsoꢀtheꢀimpactꢀofꢀthisꢀtreatmentꢀisꢀnegligible.ꢀ
OCH3
withoutꢀtheꢀphenolicꢀOHꢀgroupꢀBI-Ph
imine,ꢀandꢀN-benzyli-
dꢀThisꢀpotentialꢀcorrespondsꢀtoꢀredoxꢀchemistryꢀassociatedꢀwithꢀ
theꢀp-N-dimethylaminoꢀgroupꢀofꢀ8ꢀandꢀelectrochemicalꢀmeasure-
mentsꢀperformedꢀusingꢀreferenceꢀcompoundsꢀsupportꢀthisꢀassign-
mentꢀ(FigureꢀS33).ꢀ
dene-p-anisidine.ꢀ(B)ꢀCVsꢀofꢀcompoundsꢀ2,ꢀ4ꢀandꢀ7.ꢀConcentra-
tion:ꢀ1ꢀmMꢀofꢀtheꢀindicatedꢀBIPsꢀandꢀreferenceꢀcompounds,ꢀ0.5ꢀ
MꢀTBAPF ꢀsupportingꢀelectrolyteꢀinꢀdryꢀCH CN.ꢀWE:ꢀglassyꢀcar-
6 3
bon.ꢀPseudoꢀRE:ꢀAgꢀwireꢀ(ferroceneꢀasꢀinternalꢀreference).ꢀCE:ꢀ
Ptꢀwire.ꢀScanꢀrate,ꢀ100ꢀmV/s.ꢀ
ꢀ
Furthermore,ꢀtheꢀprogressiveꢀshiftꢀofꢀtheꢀmidpointꢀpoten-
tialsꢀtoꢀmoreꢀpositiveꢀvaluesꢀisꢀcorrelatedꢀwithꢀtheꢀincreaseꢀ
ofꢀ theꢀ electron-withdrawingꢀ strengthꢀ ofꢀ theꢀ groupꢀ atꢀ theꢀ
para-positionꢀofꢀtheꢀN-phenylimineꢀmoiety.ꢀAꢀplotꢀofꢀtheꢀex-
perimentalꢀE1/2ꢀvaluesꢀagainstꢀtheꢀHammettꢀconstantsꢀ(σ ,ꢀ
Figureꢀ 4)ꢀ showsꢀ aꢀ characteristicꢀ Hammettꢀ relationshipꢀ
ꢀ
Infraredꢀ Spectroelectrochemicalꢀ Studies.ꢀ Experimentalꢀ
evidenceꢀforꢀtheꢀformationꢀofꢀbothꢀEPTꢀandꢀE2PTꢀproductsꢀ
wasꢀobtainedꢀfromꢀtheꢀIRSECꢀmeasurements.ꢀAsꢀshownꢀpre-
viously,ꢀ distinctꢀ structuralꢀ changesꢀ accompanyingꢀ protonꢀ
transfer(s)ꢀ uponꢀ oxidationꢀ ofꢀ phenolsꢀ canꢀbeꢀ detectedꢀ byꢀ
p
withꢀtheꢀelectronicꢀeffectꢀofꢀtheꢀsubstituents,ꢀevenꢀthoughꢀ
5
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Page 6 of 13
3
3
R
thisꢀtechnique. ꢀInꢀtheꢀcaseꢀofꢀBIP-Ph imines,ꢀtheꢀIRSECꢀex-
perimentsꢀtakeꢀadvantageꢀofꢀtheꢀwell-establishedꢀimineꢀandꢀ
protonatedꢀimineꢀinfraredꢀspectraꢀinꢀwhichꢀtheꢀC=Nꢀvibra-
tionalꢀstretchingꢀmodeꢀisꢀsensitiveꢀtoꢀtheꢀlocalꢀprotonationꢀ
atꢀ theꢀpara-positionꢀ ofꢀ theꢀ N-phenylimineꢀ groupꢀ ofꢀ 2–7ꢀareꢀ
shown.ꢀ
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Figureꢀ5ꢀA,ꢀBꢀandꢀCꢀshowꢀtheꢀ IRSECꢀ spectraꢀ forꢀtheꢀ BIP-
R
-1
Ph iminesꢀ2,ꢀ4ꢀandꢀ7,ꢀrespectively,ꢀinꢀtheꢀ1700–1400ꢀcm ꢀ
region.ꢀIRSECꢀspectraꢀofꢀ3ꢀareꢀshownꢀinꢀtheꢀSIꢀ(FigureꢀS35).ꢀ
Asꢀdescribedꢀabove,ꢀIRꢀspectraꢀwereꢀobtainedꢀasꢀaꢀfunctionꢀ
ofꢀappliedꢀpotential,ꢀandꢀtheꢀpotentialsꢀwereꢀheldꢀconstantꢀ
untilꢀnoꢀfurtherꢀchangesꢀinꢀtheꢀcollectedꢀFTIRꢀspectraꢀwereꢀ
observed.ꢀSignificantꢀchangesꢀinꢀtheꢀabsorbanceꢀandꢀbandꢀ
positionsꢀareꢀindicatedꢀwithꢀarrows.ꢀAsꢀshownꢀinꢀFigureꢀ5ꢀ
stateꢀofꢀtheꢀnitrogen.56,61–68
ꢀ
1.12
Experimental
Calculated for E2PT
Calculated for EPT
1.08
1.04
1.00
0.96
-1ꢀ
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
A,ꢀBꢀandꢀC,ꢀaꢀnewꢀbandꢀappearsꢀatꢀ1652ꢀcm forꢀ2,ꢀ4ꢀandꢀ7ꢀ
underꢀappliedꢀpotential.ꢀHowever,ꢀtheꢀrelativeꢀintensityꢀofꢀ
thisꢀbandꢀisꢀdifferentꢀinꢀtheꢀthreeꢀsamplesꢀandꢀisꢀmostꢀpro-
nouncedꢀinꢀtheꢀcaseꢀofꢀ7.ꢀThisꢀbandꢀhasꢀaꢀlargeꢀcomponentꢀ
-
CF3 -CN
(2)
(
3)
+
-Cl
ofꢀtheꢀprotonatedꢀimineꢀgroupꢀstretchingꢀ(νC=NH
)ꢀandꢀisꢀev-
(4)
-
H
idenceꢀthatꢀtwoꢀprotonꢀtransfersꢀtakeꢀplaceꢀuponꢀoxidationꢀ
0.92
(5)
-
OCH3 -CH3
56,65–67
ofꢀ theꢀ phenolꢀ (anꢀ E2PTꢀ product).
ꢀ TheꢀDFTꢀ normal-
(7)
(6)
modeꢀanalysisꢀofꢀ7ꢀsupportsꢀtheseꢀassignmentsꢀ(seeꢀFigureꢀ
S46).ꢀInꢀcontrolꢀexperiments,ꢀtheꢀappearanceꢀofꢀthisꢀbandꢀinꢀ
0.88
-0.4
0.0
0.4
0.8
sp
7
ꢀwasꢀ alsoꢀ detectedꢀ whenꢀ theꢀmoleculeꢀwasꢀtreatedꢀwithꢀ
ꢀ
CF COOHꢀ(seeꢀFigureꢀS36).ꢀOnꢀtheꢀotherꢀhand,ꢀtheꢀappear-
3
-1ꢀ
anceꢀofꢀtheꢀbandꢀatꢀ1556ꢀcm (NHꢀin-planeꢀbendingꢀvibra-
tionꢀofꢀtheꢀbenzimidazoliumꢀion)ꢀinꢀtheꢀIRSECꢀspectraꢀofꢀ2ꢀ
andꢀ4ꢀ(Figureꢀ5ꢀAꢀandꢀB)ꢀindicatesꢀtheꢀpresenceꢀofꢀproductsꢀ
associatedꢀwithꢀonlyꢀoneꢀprotonꢀtransferꢀ(EPTꢀproducts).ꢀInꢀ
theꢀmodelꢀdescribedꢀbelow,ꢀthisꢀobservationꢀwillꢀbeꢀtakenꢀ
asꢀevidenceꢀthatꢀtheꢀequilibriumꢀbetweenꢀtheꢀEPTꢀandꢀE2PTꢀ
productsꢀisꢀaꢀfunctionꢀofꢀtheꢀelectronꢀwithdrawing/donat-
ingꢀcharacterꢀofꢀtheꢀRꢀgroup.ꢀ
Figureꢀ4.ꢀCalculatedꢀmidpointꢀ potentialsꢀ (E1/2)ꢀforꢀphenoxylꢀ
radical/phenolꢀcouplesꢀofꢀtheꢀBIP-Ph iminesꢀwhenꢀundergoingꢀ
R
anꢀ(○)ꢀEPTꢀorꢀ(
experimentalꢀ(
●
)ꢀE2PTꢀprocessꢀinꢀCH
3
CN,ꢀandꢀtheꢀassociatedꢀ
).ꢀTheꢀcalcu-
▲
)ꢀvaluesꢀvsꢀHammettꢀconstantsꢀ(σ
p
latedꢀE1/2ꢀ valueꢀforꢀtheꢀE2PTꢀofꢀ7ꢀisꢀtheꢀsameꢀasꢀtheꢀexperi-
mentalꢀvalueꢀbyꢀconstruction.ꢀLinearꢀfitsꢀtoꢀtheꢀexperimentalꢀ
dataꢀandꢀcalculatedꢀvaluesꢀforꢀtheꢀE2PTꢀprocessꢀareꢀshownꢀwithꢀ
dashꢀlinesꢀ(R=0.96ꢀandꢀR=0.97,ꢀrespectively).ꢀTheꢀsubstituentsꢀ
0
0
0
0
.6
.4
.2
.0
0.30
0.25
0.20
0.15
0.10
0.30
Potential (V vs Ag wire)
Potential (V vs Ag wire)
Potential (V vs Ag wire)
No polarization
+0.80
No polarization
A
No polarization
+0.80
B
C
+
0.95
+1.00
1.05
+1.10
0.25
0.20
0.15
0.10
0.05
0.00
+0.85
+0.90
+0.95
+0.90
+1.00
+
0
.05
0.00
1700
1700
1600
1500
1400
1600
1500
1400
1700
1600
1500
1400
-
1
-1
-1
Wavenumber (cm )
Wavenumber (cm )
Wavenumber (cm )
0.18
0.10
.08
0.06
0.08
0.06
0.04
Potential (V vs Ag wire)
No polarization
+0.95
Potential (V vs Ag wire)
Potential (V vs Ag wire)
No polarization
+0.80
D
E
No polarization
+0.80
+0.85
+0.90
+0.95
F
0
0
0
0
.15
.12
.09
.06
0
+
1.00
+0.90
+1.00
+1.05
+1.10
0
0
.04
.02
0
.02
0.03
0.00
3500
0
.00
3
0.00
3500
500
3400
3300
3200
3100
3400
3300
3200
3100
3400
3300
3200
3100
-
1
-1
-
1
Wavenumber (cm )
Wavenumber (cm )
Wavenumber (cm )
EPT
E2PT
EPT
E2PT
E2PT
ꢀ
Figureꢀ5.ꢀ(A,ꢀB,ꢀC)ꢀIRSECꢀspectraꢀofꢀ2,ꢀ4ꢀandꢀ7,ꢀrespectively,ꢀ(19ꢀmM)ꢀpolarizedꢀinꢀ50ꢀmVꢀorꢀ100ꢀmVꢀincrementsꢀtoꢀgenerateꢀincreasingꢀ
-1
amountsꢀofꢀtheꢀphenoxylꢀradicalꢀfromꢀtheꢀneutralꢀphenolꢀspecies,ꢀinꢀtheꢀ1700–1400ꢀcm ꢀregion.ꢀ(D,ꢀE,ꢀF)ꢀIRSECꢀspectraꢀofꢀ2,ꢀ4ꢀandꢀ
-
1
7
2 2 6
,ꢀrespectively,ꢀunderꢀtheꢀsameꢀconditionsꢀinꢀtheꢀ3500–3100ꢀcm ꢀregion.ꢀSolvent:ꢀCH Cl ,ꢀ0.1ꢀMꢀTBAPF .ꢀTheꢀbandsꢀcharacteristicꢀofꢀ
EPTꢀandꢀE2PTꢀproductsꢀareꢀindicatedꢀwithꢀgreenꢀandꢀorangeꢀarrows,ꢀrespectively.ꢀTheꢀstructuresꢀatꢀtheꢀbottomꢀcorrespondꢀtoꢀtheꢀ
oxidizedꢀEPTꢀandꢀE2PTꢀproducts.ꢀ
6
ACS Paragon Plus Environment

Page 7 of 13
Journal of the American Chemical Society
-
1
Importantly,ꢀthereꢀisꢀnoꢀbandꢀgrowingꢀinꢀatꢀ1556ꢀcm ꢀinꢀtheꢀ
IRSECꢀspectraꢀofꢀ7ꢀ(Figureꢀ5ꢀC),ꢀindicatingꢀthatꢀstable,ꢀde-
tectableꢀlevelsꢀofꢀtheꢀbenzimidazoliumꢀionꢀdoꢀnotꢀformꢀuponꢀ
phenolꢀoxidationꢀinꢀthisꢀcase.ꢀInꢀotherꢀwords,ꢀoxidationꢀofꢀ7ꢀ
resultsꢀpredominantlyꢀinꢀformationꢀofꢀtheꢀE2PTꢀproduct.ꢀAsꢀ
functionalizationꢀwithꢀelectron-withdrawingꢀgroupsꢀ(e.g.,ꢀ-
CNꢀandꢀ-CF ꢀinꢀ2ꢀandꢀ3,ꢀrespectively).ꢀCompoundꢀ4ꢀrepre-
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
3
sentsꢀanꢀintermediateꢀcaseꢀwhereꢀbothꢀBꢀandꢀCꢀproductsꢀareꢀ
presentꢀinꢀsimilarꢀproportionsꢀfollowingꢀoxidationꢀ(Figureꢀ
5ꢀB).ꢀAsꢀmentionedꢀpreviously,ꢀinꢀtheꢀIRSECꢀexperimentsꢀ
-1
-1
-1
aꢀcontrol,ꢀtheꢀ1556ꢀcm ꢀbandꢀdoesꢀnotꢀappearꢀuponꢀtitrationꢀ
ofꢀCH Cl ꢀsolutionsꢀofꢀ7ꢀwithꢀCF COOHꢀ(0.5–2ꢀequivalents),ꢀ
whichꢀonlyꢀprotonatesꢀtheꢀimineꢀnitrogenꢀ(seeꢀFigureꢀS36).ꢀ
theꢀabsorptionꢀbandsꢀatꢀ1556ꢀcm ꢀandꢀ1652ꢀcm ꢀprovideꢀ
spectroscopicꢀindicatorsꢀforꢀdetectingꢀspeciesꢀBꢀandꢀC,ꢀre-
spectively.ꢀTheꢀintensitiesꢀofꢀtheseꢀbandsꢀwereꢀassumedꢀtoꢀ
beꢀproportionalꢀtoꢀtheꢀconcentrationsꢀofꢀtheseꢀspeciesꢀ(BIP-
2
2
3
R
FollowingꢀoxidationꢀofꢀtheꢀBIP-Ph imines,ꢀtheꢀratioꢀofꢀEPTꢀ
COOCH ꢀinꢀitsꢀprotonatedꢀstateꢀwasꢀshownꢀtoꢀfollowꢀBeer’sꢀ
3
andꢀE2PTꢀproductsꢀformedꢀasꢀaꢀfunctionꢀofꢀtheꢀpara-substit-
uentꢀ(R-group)ꢀonꢀtheꢀN-phenylimineꢀmoietyꢀcanꢀalsoꢀbeꢀan-
alyzedꢀ byꢀ monitoringꢀ theꢀ imidazoleꢀ NHꢀ stretchingꢀ bandꢀ
νNH)ꢀ(Figureꢀ5ꢀD,ꢀEꢀandꢀF).ꢀInꢀtheꢀneutralꢀspecies,ꢀtheꢀνNHꢀ
appearsꢀatꢀ3408ꢀcm ,ꢀ3402ꢀcm ,ꢀandꢀ3394ꢀcm ꢀforꢀ2,ꢀ4ꢀandꢀ
,ꢀrespectively.ꢀTheꢀdecreaseꢀinꢀtheꢀνNHꢀcomparedꢀtoꢀ1ꢀ(νNH
atꢀ3414ꢀcm )ꢀisꢀmostꢀlikelyꢀdueꢀtoꢀtheꢀintramolecularꢀH-
bondꢀbetweenꢀtheꢀNHꢀofꢀtheꢀbenzimidazoleꢀmoietyꢀandꢀtheꢀ
imineꢀ nitrogenꢀ loneꢀ pairꢀ inꢀtheꢀ BIP-Ph imines.ꢀMoreover,ꢀ
-1
lawꢀatꢀ1556ꢀcm ꢀoverꢀconcentrationꢀrangesꢀrelevantꢀtoꢀtheꢀ
experiments,ꢀandꢀitꢀisꢀassumedꢀthatꢀtheꢀradicalꢀspeciesꢀofꢀ
Schemeꢀ1ꢀwillꢀalsoꢀfollowꢀtheꢀlaw).ꢀ
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
-1
-1
-1
ꢀ
7
ꢀ
R
Schemeꢀ1.ꢀPCETꢀProductsꢀofꢀBIP-Ph iminesꢀDetectedꢀbyꢀ
-1
IRSECaꢀ
R
theꢀprogressiveꢀshiftꢀtoꢀlowerꢀfrequenciesꢀsupportsꢀtheꢀin-
creasingꢀH-bondꢀstrengthꢀdueꢀtoꢀtheꢀincreaseꢀinꢀtheꢀelec-
tron-donatingꢀ characterꢀ ofꢀ theꢀ para-substituents,ꢀ con-
sistentꢀwithꢀtheꢀtrendꢀobservedꢀinꢀtheꢀ HꢀNMRꢀ(seeꢀδNH,ꢀFig-
ureꢀ2).ꢀ
1
Theꢀchangesꢀobservedꢀinꢀtheꢀspectraꢀunderꢀelectrochemicalꢀ
R
polarizationꢀareꢀclearlyꢀdifferentꢀamongꢀtheꢀBIP-Ph imines.ꢀ
Inꢀtheꢀcaseꢀofꢀcompoundsꢀ2ꢀandꢀ4ꢀ(Figureꢀ5ꢀDꢀandꢀE),ꢀtheꢀ
intensityꢀ ofꢀ theꢀ νNHꢀ bandꢀ progressivelyꢀ decreases,ꢀ andꢀ aꢀ
broadꢀ NHꢀ stretchingꢀ bandꢀ ofꢀ theꢀ benzimidazoliumꢀ ionꢀatꢀ
-1
33
~
3320ꢀcm ꢀappears. ꢀThisꢀbandꢀisꢀconsiderablyꢀbroaderꢀ
thanꢀitꢀwasꢀinꢀtheꢀIRSECꢀofꢀ1,ꢀasꢀexpectedꢀbyꢀtheꢀcontributionꢀ
ofꢀtheꢀE2PTꢀproductꢀ(protonatedꢀimineꢀwithꢀνNHꢀatꢀ~3360ꢀ
-
1
-1ꢀ
ꢀ
cm ).ꢀAꢀclearꢀisosbesticꢀpointꢀatꢀ~3390ꢀcm wasꢀdetectedꢀinꢀ
theꢀcaseꢀofꢀ2ꢀandꢀ4.ꢀInꢀcontrast,ꢀthereꢀisꢀnoꢀindicationꢀofꢀanꢀ
isosbesticꢀpointꢀforꢀ7ꢀ(Figureꢀ5ꢀF).ꢀInꢀthisꢀcase,ꢀaꢀnewꢀbandꢀ
aꢀ
Fromꢀ theꢀ IRSECꢀ experimentsꢀ withꢀ A,ꢀtheꢀEPTꢀandꢀE2PTꢀ
productsꢀ (Bꢀ andꢀ C,ꢀ respectively)ꢀ wereꢀ identifiedꢀ whenꢀ theꢀ
workingꢀ electrodeꢀ wasꢀ polarizedꢀ positiveꢀ ofꢀ theꢀ associatedꢀ
phenoxylꢀradical/phenolꢀredoxꢀcouple.ꢀFollowingꢀpolarization,ꢀ
BꢀandꢀCꢀproductsꢀareꢀpresentꢀinꢀequilibriumꢀandꢀatꢀconcentra-
tionsꢀthatꢀareꢀstableꢀonꢀtheꢀtimeꢀscaleꢀofꢀtheꢀmeasurements.ꢀ
-1
forꢀνNHꢀappearsꢀ atꢀ 3360ꢀ cm ꢀandꢀanꢀ increaseꢀ inꢀ intensityꢀ
andꢀwidthꢀwereꢀobserved.ꢀTheꢀabsenceꢀofꢀtheꢀmarkerꢀben-
zimidazoliumꢀNHꢀstretchingꢀbandꢀprovidesꢀadditionalꢀevi-
denceꢀthatꢀtheꢀEPTꢀproductꢀcannotꢀbeꢀdetectedꢀinꢀ7.ꢀTheꢀ
-1
changesꢀofꢀtheꢀbandsꢀinꢀtheꢀregionꢀaroundꢀ1528–1506ꢀcm ꢀ
observedꢀinꢀtheꢀIRSECꢀofꢀ2,ꢀ4ꢀandꢀ7ꢀareꢀalsoꢀconsistentꢀwithꢀ
theꢀassignmentsꢀofꢀtheꢀE2PTꢀandꢀEPTꢀproductsꢀdescribedꢀ
aboveꢀ(seeꢀdescriptionꢀandꢀspectralꢀassignmentsꢀinꢀSI,ꢀpageꢀ
S66-S68).ꢀ
ꢀ
FromꢀtheꢀIRSECꢀdataꢀdescribedꢀabove,ꢀtheꢀextinctionꢀcoeffi-
-1ꢀ
cientꢀforꢀtheꢀbandꢀatꢀ1652ꢀcm inꢀ7ꢀwasꢀusedꢀasꢀanꢀapproxi-
-1
mateꢀvalueꢀforꢀtheꢀextinctionꢀcoefficientꢀofꢀtheꢀ1652ꢀcm ꢀ
bandꢀ presentꢀ inꢀ theꢀ E2PTꢀ productsꢀ ofꢀ 2,ꢀ 3ꢀ andꢀ 4ꢀ (Cꢀ inꢀ
Schemeꢀ1)ꢀandꢀinꢀderivingꢀtheꢀconcentrationsꢀofꢀtheꢀE2PTꢀ
productsꢀfollowingꢀoxidationꢀinꢀeachꢀcase.ꢀForꢀeachꢀsystem,ꢀ
subtractingꢀtheꢀconcentrationꢀofꢀtheꢀE2PTꢀproductꢀfromꢀtheꢀ
Schemeꢀ1ꢀisꢀusedꢀtoꢀinterpretꢀtheꢀresultsꢀofꢀtheꢀIRSECꢀex-
R
perimentsꢀforꢀtheꢀseriesꢀofꢀBIP-Ph iminesꢀ2,ꢀ3,ꢀ4ꢀandꢀ7.ꢀUn-
derꢀthinꢀfilmꢀelectrolysisꢀconditions,ꢀweꢀpostulateꢀthatꢀoxi-
R
R
dationꢀ ofꢀ Aꢀ (aꢀ genericꢀ BIP-Ph imine)ꢀ producesꢀ Bꢀ andꢀ C,ꢀ
initialꢀconcentrationꢀofꢀtheꢀrespectiveꢀBIP-Ph imineꢀstartingꢀ
whichꢀareꢀinꢀequilibriumꢀwithꢀeachꢀotherꢀasꢀshown. Inꢀaddi-
tionꢀtoꢀtheꢀprocessesꢀformingꢀBꢀandꢀCꢀlabeledꢀEPTꢀandꢀE2PTꢀ
inꢀSchemeꢀ1,ꢀtheꢀprotonꢀtransferꢀreactionꢀtemplatedꢀbyꢀtheꢀ
H-bondꢀbetweenꢀtheꢀimidazoleꢀprotonꢀandꢀtheꢀimineꢀnitro-
genꢀinꢀAꢀcanꢀalsoꢀleadꢀtoꢀformationꢀofꢀanꢀE2PTꢀproductꢀaris-
ingꢀfromꢀEPTꢀfollowedꢀbyꢀprotonꢀtransfer,ꢀorꢀanꢀEPTꢀprod-
uctꢀarisingꢀfromꢀE2PTꢀfollowedꢀbyꢀbackꢀprotonꢀtransfer.ꢀOurꢀ
experimentalꢀandꢀcomputationalꢀ dataꢀ doꢀ notꢀ addressꢀ theꢀ
contributionsꢀofꢀtheseꢀpathwaysꢀtoꢀtheꢀequilibriumꢀbetweenꢀ
BꢀandꢀC.ꢀNonetheless,ꢀtheꢀresultsꢀshow,ꢀtheꢀdominantꢀprod-
uctꢀswitchesꢀfromꢀtheꢀE2PTꢀspeciesꢀ(C)ꢀinꢀtheꢀcaseꢀofꢀ7ꢀtoꢀanꢀ
EPTꢀspeciesꢀ(B)ꢀwhenꢀtheꢀbasicityꢀofꢀtheꢀimineꢀisꢀreducedꢀbyꢀ
materialꢀ(AꢀinꢀSchemeꢀ1)ꢀyieldsꢀtheꢀconcentrationꢀofꢀtheꢀEPTꢀ
productꢀ (Bꢀ inꢀ Schemeꢀ 1)ꢀ andꢀ theꢀ equilibriumꢀ constant,ꢀ
Keq=[E2PT]/[EPT],ꢀ associatedꢀ withꢀ interconversionꢀ be-
tweenꢀtheꢀEPTꢀandꢀE2PTꢀoxidationꢀproductsꢀ(Tableꢀ3).ꢀ
0
TheꢀplotꢀofꢀΔG ꢀderivedꢀ fromꢀ theseꢀ valuesꢀ followsꢀ theꢀ ex-
pectedꢀ linearꢀ Hammettꢀ relationshipꢀ (Figureꢀ 6).ꢀ Becauseꢀ
thereꢀwasꢀnoꢀdetectibleꢀlevelꢀofꢀtheꢀEPTꢀproductꢀmeasuredꢀ
inꢀtheꢀIRSECꢀofꢀ7,ꢀtheꢀHammettꢀplotꢀinꢀFigureꢀ6ꢀwasꢀextrap-
olatedꢀasꢀshownꢀtoꢀgiveꢀaꢀKeqꢀofꢀ13.8ꢀforꢀ7.ꢀUsingꢀthisꢀvalueꢀ
ofꢀKeq,ꢀinꢀconjunctionꢀwithꢀtheꢀE1/2ꢀvalueꢀmeasuredꢀforꢀtheꢀ
E2PTꢀprocessꢀofꢀ7,ꢀtheꢀE1/2ꢀvalueꢀassociatedꢀwithꢀtheꢀEPTꢀ
processꢀofꢀ7ꢀwasꢀdeterminedꢀtoꢀbeꢀ1.00ꢀVꢀvsꢀSCEꢀinꢀCH CNꢀ
3
7
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 13
(
1.02ꢀVꢀvsꢀSCEꢀinꢀCH
2
Cl
2
).ꢀBecauseꢀtheꢀEPTꢀandꢀE2PTꢀpro-
Onꢀtheꢀotherꢀhand,ꢀwithꢀelectron-withdrawingꢀsubstituentsꢀ
suchꢀasꢀ-CF ꢀandꢀ-CN,ꢀtheꢀsituationꢀisꢀreversed,ꢀandꢀtheꢀpos-
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
cessesꢀinvolveꢀtheꢀbreakingꢀandꢀformingꢀofꢀdifferentꢀnum-
bersꢀandꢀtypesꢀofꢀbonds,ꢀtheꢀEPTꢀredoxꢀpotentialsꢀwereꢀcal-
culatedꢀusingꢀanꢀisodesmicꢀschemeꢀwithꢀthisꢀE1/2ꢀvalueꢀasso-
ciatedꢀwithꢀtheꢀEPTꢀprocessꢀofꢀ7ꢀasꢀaꢀreferenceꢀ(seeꢀpagesꢀ
S74–S76).ꢀThisꢀschemeꢀwasꢀusedꢀtoꢀcalculateꢀtheꢀE1/2ꢀvaluesꢀ
forꢀtheꢀEPTꢀprocessesꢀshownꢀinꢀTableꢀ2ꢀandꢀFigureꢀ4.ꢀTa-
bleꢀ3ꢀshowsꢀtheꢀexperimentalꢀandꢀcalculatedꢀequilibriumꢀ
constantsꢀandꢀcorrespondingꢀDG ꢀforꢀtheꢀfourꢀcompoundsꢀ
studiedꢀusingꢀIRSEC.ꢀNegativeꢀΔG ꢀvaluesꢀindicateꢀthatꢀtheꢀ
E2PTꢀ productꢀ isꢀthermodynamicallyꢀ moreꢀfavorableꢀ thanꢀ
theꢀEPTꢀproduct,ꢀasꢀisꢀtheꢀcaseꢀforꢀcompoundsꢀwithꢀelec-
tron-donatingꢀsubstituentsꢀsuchꢀasꢀ-OCH
3
0
itiveꢀvaluesꢀofꢀΔG ꢀindicateꢀthatꢀtheꢀEPTꢀproductsꢀareꢀther-
modynamicallyꢀmoreꢀfavorable.ꢀThisꢀtrendꢀisꢀalsoꢀshownꢀinꢀ
Figureꢀ4,ꢀwhereꢀtheꢀcalculatedꢀE1/2ꢀvaluesꢀassociatedꢀwithꢀ
EPTꢀprocessesꢀfallꢀbelowꢀthoseꢀassociatedꢀwithꢀtheꢀcorre-
spondingꢀE2PTꢀprocessesꢀatꢀmoreꢀpositiveꢀHammettꢀcon-
stants.ꢀ
0
KIE.ꢀ Experimentalꢀ KIEꢀ valuesꢀ wereꢀ determinedꢀ electro-
0
R
chemicallyꢀforꢀBIP-Ph iminesꢀ2,ꢀ3,ꢀ4ꢀandꢀ7ꢀandꢀareꢀreportedꢀ
inꢀTableꢀ4.ꢀAsꢀcanꢀbeꢀseen,ꢀtheꢀKIEꢀvaluesꢀareꢀcloseꢀtoꢀ1.0ꢀ
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
andꢀareꢀinꢀlineꢀwithꢀpreviouslyꢀreportedꢀvaluesꢀforꢀrelatedꢀ
3
.ꢀ
33
systems. ꢀTheseꢀpreviousꢀcalculationsꢀemployedꢀaꢀvibroni-
callyꢀnonadiabaticꢀPCETꢀtheory,25,69–71ꢀwhichꢀmayꢀbeꢀviewedꢀ
7
2
asꢀ anꢀ extensionꢀ ofꢀ Marcusꢀ theoryꢀ forꢀ electronꢀ transfer ꢀ
withꢀ theꢀ transferringꢀ proton,ꢀ asꢀ wellꢀ asꢀ theꢀ electrons,ꢀ
treatedꢀquantumꢀmechanicallyꢀtoꢀallowꢀsimultaneousꢀelec-
tronꢀandꢀprotonꢀtunneling.ꢀInꢀthisꢀPCETꢀtheory,ꢀtheꢀoverallꢀ
rateꢀconstantꢀisꢀcomputedꢀasꢀtheꢀsumꢀofꢀtheꢀindividualꢀrateꢀ
constantsꢀ associatedꢀ withꢀ pairsꢀ ofꢀ reactantꢀ andꢀ productꢀ
electron-protonꢀvibronicꢀstates,ꢀweightedꢀbyꢀtheꢀBoltzmannꢀ
populationꢀofꢀtheꢀreactantꢀstate.ꢀEachꢀindividualꢀrateꢀcon-
stantꢀforꢀaꢀpairꢀofꢀreactant/productꢀvibronicꢀstatesꢀdependsꢀ
exponentiallyꢀonꢀtheꢀfreeꢀenergyꢀbarrierꢀandꢀisꢀproportionalꢀ
toꢀtheꢀsquareꢀofꢀtheꢀvibronicꢀcoupling,ꢀwhichꢀisꢀtheꢀproductꢀ
ofꢀtheꢀelectronicꢀcouplingꢀandꢀtheꢀoverlapꢀintegralꢀbetweenꢀ
theꢀreactantꢀandꢀproductꢀprotonꢀvibrationalꢀwavefunctions.ꢀ
InꢀtheꢀcontextꢀofꢀthisꢀPCETꢀtheory,ꢀtheꢀKIEꢀofꢀaroundꢀunityꢀ
isꢀexplainedꢀinꢀtermsꢀofꢀsignificantꢀcontributionsꢀfromꢀex-
citedꢀ electron-protonꢀ vibronicꢀ states.ꢀ Theꢀ protonꢀ vibra-
tionalꢀ wavefunctionsꢀ associatedꢀwithꢀ theseꢀexcitedꢀ statesꢀ
areꢀmoreꢀdelocalizedꢀandꢀthereforeꢀcanꢀleadꢀtoꢀlargerꢀover-
1
-CN
(
2)
-CF3
(3)
0
1
2
-Cl
(4)
-
-
-
OCH3
(
7)
-0.5
0.0
0.5
1.0
sp
ꢀ
Figureꢀ6.ꢀExperimentallyꢀdeterminedꢀfreeꢀenergyꢀdifferencesꢀ
betweenꢀE2PTꢀandꢀEPTꢀproductsꢀformedꢀuponꢀoxidationꢀofꢀtheꢀ
BIP-Ph iminesꢀ vsꢀ theꢀ σ ꢀ forꢀ eachꢀ ofꢀ theꢀ BIP-Ph imines.ꢀ Theꢀ
p
pointꢀatꢀtheꢀlowerꢀleftꢀforꢀ7ꢀ(blackꢀcircleꢀwithꢀredꢀoutline)ꢀisꢀob-
tainedꢀbyꢀextrapolatingꢀtheꢀlineꢀfromꢀtheꢀexperimentalꢀvaluesꢀ
ofꢀ2,ꢀ3ꢀandꢀ4ꢀ(R=0.98).ꢀNoteꢀthatꢀΔG crossesꢀfromꢀnegativeꢀtoꢀ
positiveꢀvaluesꢀbetweenꢀ3ꢀandꢀ4.ꢀ
R
R
lapꢀintegralsꢀwithinꢀtheꢀvibronicꢀcoupling.ꢀAsꢀdiscussedꢀelse-
where,25,71ꢀgreaterꢀoverlapꢀintegralsꢀbetweenꢀtheꢀreactantꢀ
andꢀ productꢀ protonꢀ vibrationalꢀ wavefunctionsꢀ typicallyꢀ
leadꢀtoꢀlowerꢀKIEsꢀthatꢀcanꢀapproachꢀunity.ꢀ
0
ꢀ
ꢀ
ꢀ
Tableꢀ4.ꢀExperimentalꢀKineticꢀIsotopeꢀEffectsꢀ
Tableꢀ3. FreeꢀEnergyꢀDifferencesꢀandꢀEquilibriumꢀCon-
stantsꢀforꢀtheꢀE2PTꢀandꢀEPTꢀProductsaꢀ
Compoundꢀ
KIEꢀ
CN
0
2:ꢀBIP-Ph imineꢀ
1.2ꢀ±ꢀ0.3ꢀ
1.3ꢀ±ꢀ0.3ꢀ
1.1ꢀ±ꢀ0.3ꢀ
DG ꢀ(kcal/mol)ꢀ
eq
K ꢀ
Compoundꢀ
CF3
Exp.
ꢀ
Calc.ꢀ
0.5ꢀ
Exp.ꢀ
0.4ꢀ
Calc.ꢀ
0.4ꢀ
3:ꢀBIP-Ph imineꢂ
Cl
CN
0.6ꢀ
0.4ꢀ
4
:ꢀBIP-Ph imineꢀ
2
3
4
7
:ꢀBIP-Ph imineꢀ
CF3
OCH3
0.5ꢀ
0.5ꢀ
0.5ꢀ
7:ꢀBIP-Ph
imineꢀ
1.1ꢀ±ꢀ0.3ꢀ
:ꢀBIP-Ph imineꢂ
Cl
:ꢀBIP-Ph imineꢀ
-0.4ꢀ
-1.6ꢀ
-0.2ꢀ
2.0ꢀ
1.5ꢀ
ꢀ
OCH3
-1.6bꢀ
13.8bꢀ
Inꢀpreviousꢀstudiesꢀonꢀrelatedꢀsystems,ꢀweꢀobservedꢀthatꢀ
BIPꢀcompoundsꢀexhibitingꢀonlyꢀtheꢀE2PTꢀproductꢀhaveꢀKIEsꢀ
~1,ꢀwhileꢀrelatedꢀcompoundsꢀexhibitingꢀonlyꢀtheꢀEPTꢀprod-
uctꢀhaveꢀlargerꢀKIEsꢀ(>1.5),ꢀwithꢀtheꢀexceptionꢀofꢀporphyrin-
boundꢀBIPs.33ꢀAccordingly,ꢀinꢀtheꢀpresentꢀseriesꢀcompoundꢀ
7ꢀyieldsꢀpredominantlyꢀtheꢀE2PTꢀproduct,ꢀandꢀtheꢀelectro-
chemicallyꢀmeasuredꢀKIEꢀisꢀ~1.ꢀTheꢀotherꢀcompounds,ꢀ2,ꢀ3ꢀ
andꢀ4,ꢀformꢀbothꢀEPTꢀandꢀE2PTꢀproducts,ꢀwithꢀEPTꢀdomi-
natingꢀ forꢀ electron-withdrawingꢀ substituentsꢀ suchꢀasꢀ -CNꢀ
:ꢀBIP-Ph
imineꢀ
13.8ꢀ
aꢀTheꢀvaluesꢀreportedꢀhereinꢀwereꢀexperimentallyꢀmeasuredꢀ
andꢀcalculatedꢀinꢀCH Cl .ꢀAnꢀanalogousꢀtableꢀforꢀtheꢀcalcula-
tionsꢀperformedꢀinꢀCH CNꢀisꢀprovidedꢀinꢀTableꢀS18.ꢀ
2
2
3
bꢀTheꢀEPTꢀandꢀE2PTꢀredoxꢀpotentialsꢀforꢀ7ꢀwereꢀusedꢀasꢀtheꢀ
referenceꢀreactions,ꢀandꢀthereforeꢀtheꢀexperimentalꢀandꢀcal-
culatedꢀvaluesꢀagreeꢀbyꢀconstruction.ꢀ
ꢀ
andꢀ-CF
3
ꢀinꢀ2ꢀandꢀ3,ꢀrespectively.ꢀEvenꢀthoughꢀaꢀtendencyꢀ
towardꢀlargerꢀKIEꢀvaluesꢀforꢀtheꢀformationꢀofꢀEPTꢀproductsꢀ
8
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Page 9 of 13
Journal of the American Chemical Society
andꢀcyanobacteria.ꢀThisꢀworkꢀalsoꢀdemonstratesꢀthatꢀtheꢀex-
isꢀapparentꢀinꢀtheseꢀcompounds,ꢀtheꢀerrorꢀbarsꢀmakeꢀitꢀim-
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possibleꢀtoꢀuseꢀKIEꢀdataꢀasꢀaꢀdefinitiveꢀpredictorꢀbetweenꢀ
theꢀEPTꢀandꢀE2PTꢀmechanisms.ꢀ
quisiteꢀcontrolꢀoverꢀprotonꢀactivity,ꢀaꢀfeatureꢀcommonꢀtoꢀallꢀ
bioenergeticꢀ systems,ꢀ canꢀ beꢀ exploredꢀ inꢀ artificialꢀ con-
structs,ꢀwhereꢀitꢀcanꢀleadꢀtoꢀdesignꢀprinciplesꢀforꢀefficientꢀ
solarꢀenergyꢀconversionꢀinꢀartificialꢀphotosynthesis.ꢀ
ꢀ
Conclusionꢀ
R
TheꢀBIP-Ph iminesꢀareꢀbioinspired,ꢀreversible,ꢀhole-trans-
ASSOCIATED CONTENT
portingꢀartificialꢀphotosyntheticꢀmodelꢀrelaysꢀthatꢀexhibitꢀ
protonꢀtranslocationꢀoverꢀ~6ꢀÅ.ꢀThisꢀseriesꢀillustratesꢀfor-
mationꢀofꢀbothꢀEPTꢀandꢀE2PTꢀproductsꢀuponꢀelectrochemi-
calꢀ oxidationꢀ ofꢀ theꢀ phenol.ꢀ Fromꢀ spectroelectrochemicalꢀ
andꢀtheoreticalꢀdataꢀweꢀconcludeꢀthatꢀtheꢀratioꢀofꢀE2PTꢀtoꢀ
EPTꢀproductsꢀfollowingꢀoxidationꢀcanꢀbeꢀmodulatedꢀbyꢀtheꢀ
SupportingꢀInformation.ꢀItꢀ includesꢀexperimentalꢀandꢀcom-
putationalꢀresults.ꢀExperimental:ꢀsynthesisꢀandꢀstructuralꢀchar-
acterization,ꢀNMRꢀdata,ꢀX-rayꢀdata,ꢀelectrochemicalꢀmeasure-
ments,ꢀIRSEC,ꢀIRꢀdataꢀandꢀKIEꢀdetermination.ꢀComputational:ꢀ
electronicꢀ structureꢀ calculations,ꢀcalculationꢀ ofꢀ redoxꢀ poten-
tials,ꢀanalysisꢀofꢀtheꢀthermodynamicsꢀofꢀPCETꢀreactions,ꢀcom-
putedꢀIRSECꢀspectraꢀandꢀbandꢀassignmentꢀandꢀoptimizedꢀcar-
tesianꢀcoordinatesꢀofꢀspeciesꢀstudied.ꢀThisꢀmaterialꢀisꢀavailableꢀ
freeꢀofꢀchargeꢀviaꢀtheꢀInternetꢀatꢀhttp://pubs.acs.org.ꢀ
0
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7
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R
natureꢀ ofꢀ theꢀ substituentsꢀ onꢀ theꢀ BIP-Ph imines.ꢀ Strongꢀ
electron-withdrawingꢀgroupsꢀsuchꢀasꢀ-CNꢀandꢀ-CF ꢀdecreaseꢀ
3
bothꢀtheꢀH-bondꢀstrengthꢀandꢀtheꢀimineꢀnitrogenꢀbasicity;ꢀ
thus,ꢀtheꢀsecondꢀprotonꢀtransferꢀisꢀlessꢀthermodynamicallyꢀ
favoredꢀandꢀtheꢀEPTꢀproductꢀisꢀdominant.ꢀConversely,ꢀtheꢀ
AUTHOR INFORMATION
Corresponding Author
strongerꢀelectron-donatingꢀgroupꢀ-OCH ꢀincreasesꢀbothꢀtheꢀ
3
H-bondꢀstrengthꢀandꢀtheꢀimineꢀnitrogenꢀbasicity,ꢀandꢀtheꢀ
E2PTꢀproductꢀisꢀtheꢀdominantꢀspeciesꢀuponꢀoxidation.ꢀAnꢀ
intermediateꢀsituationꢀisꢀachievedꢀusingꢀtheꢀ-Clꢀsubstituent,ꢀ
whereꢀaꢀmixtureꢀofꢀbothꢀoxidizedꢀspeciesꢀcorrespondingꢀtoꢀ
EPTꢀandꢀE2PTꢀprocessesꢀwasꢀobserved.ꢀWeꢀfoundꢀexperi-
mentallyꢀonlyꢀaꢀslightꢀchangeꢀinꢀtheꢀphenolꢀredoxꢀpotential,ꢀ
*ꢀE-mail:ꢀgary.f.moore@asu.edu.ꢀ
*ꢀE-mail:ꢀsharon.hammes-schiffer@yale.edu.ꢀ
*ꢀE-mail:ꢀamoore@asu.edu.ꢀ
ORCID
EmmanuelꢀOdella:ꢀ0000-0002-7021-400Xꢀ
S.ꢀJimenaꢀMora:ꢀ0000-0003-4181-4732ꢀ
BrianꢀL.ꢀWadsworth:ꢀ0000-0002-0274-9993ꢀ
MioyꢀT.ꢀHuynh:ꢀ0000-0002-0472-7624ꢀ
JoshuaꢀJ.ꢀGoings:ꢀ0000-0002-2817-1966ꢀ
MiguelꢀGervaldo:ꢀ0000-0001-9287-5872ꢀ
LeónidesꢀE.ꢀSereno:ꢀ0000-0001-8361-7695ꢀ
DevensꢀGust:ꢀ0000-0003-0550-8498ꢀ
ThomasꢀA.ꢀMoore:ꢀ0000-0002-1577-7117ꢀ
GaryꢀF.ꢀMoore:ꢀ0000-0003-3369-9308ꢀ
SharonꢀHammes-Schiffer:ꢀ0000-0002-3782-6995ꢀ
AnaꢀL.ꢀMoore:ꢀ0000-0002-6653-9506ꢀ
ꢀ
0.99ꢀtoꢀ0.93ꢀVꢀvsꢀSCE,ꢀbetweenꢀtheꢀcompoundsꢀthatꢀ yieldꢀ
predominantlyꢀ theꢀ EPTꢀ productsꢀ andꢀ thoseꢀ thatꢀ yieldꢀ
mainlyꢀtheꢀE2PTꢀproducts.ꢀ
Theseꢀfindingsꢀareꢀrelevantꢀinꢀtheꢀcontextꢀofꢀdesigningꢀarti-
ficialꢀphotosyntheticꢀassembliesꢀgivenꢀthatꢀtheꢀpreviouslyꢀ
reportedꢀ amino-substitutedꢀ BIPsꢀ withꢀ redoxꢀ potentialsꢀ
~0.6ꢀVꢀvsꢀSCEꢀwouldꢀnotꢀbeꢀableꢀtoꢀactꢀasꢀaꢀrelayꢀinꢀwaterꢀ
3
2
R
oxidationꢀ atꢀ near-neutralꢀ pH. ꢀ Inꢀ contrast,ꢀ BIP-Ph imineꢀ
derivativesꢀthatꢀconserveꢀtheꢀhighꢀphenolꢀredoxꢀpotential,ꢀ
closeꢀtoꢀ1.0ꢀVꢀvsꢀSCE,ꢀwouldꢀbeꢀthermodynamicallyꢀcapableꢀ
ofꢀactingꢀasꢀredoxꢀrelaysꢀinꢀwaterꢀoxidationꢀschemesꢀresem-
blingꢀnaturalꢀphotosynthesis.ꢀTheꢀdifferentꢀbehaviorsꢀofꢀtheꢀ
imino-ꢀ andꢀ amino-substitutedꢀ BIPsꢀ ariseꢀ fromꢀ aꢀ balanceꢀ
Author Contributions
amongꢀtheꢀrelativeꢀpK ꢀvalues,ꢀ theꢀH-bondꢀstrengths,ꢀandꢀ
a
‡
ꢀ
Theseꢀauthorsꢀcontributedꢀequally.ꢀ
theꢀmidpointꢀpotentialꢀofꢀtheꢀphenoxylꢀradical/phenolꢀcou-
ple.ꢀTheꢀexperimentalꢀandꢀcomputationalꢀworkꢀhereinꢀex-
ploresꢀtheꢀdelicateꢀbalanceꢀamongꢀtheseꢀfactorsꢀinꢀfavoringꢀ
eitherꢀtheꢀEPTꢀorꢀtheꢀE2PTꢀprocessꢀandꢀinꢀtuningꢀtheꢀasso-
ciatedꢀredoxꢀpotentials.ꢀ
Notes
Theꢀauthorsꢀdeclareꢀnoꢀcompetingꢀfinancialꢀinterest.ꢀ
ACKNOWLEDGMENT
ItꢀisꢀimportantꢀtoꢀemphasizeꢀthatꢀbecauseꢀtheꢀEPTꢀandꢀE2PTꢀ
productsꢀareꢀinꢀequilibrium,ꢀendergonicꢀstepsꢀ(K<1)ꢀdoꢀnotꢀ
precludeꢀtheꢀtransportꢀofꢀprotonsꢀoverꢀaꢀH-bondꢀnetwork.ꢀ
Justꢀasꢀelectronsꢀcanꢀbeꢀtransferredꢀoverꢀendergonicꢀstepsꢀ
inꢀ redoxꢀ reactionsꢀ inꢀ electronꢀ transportꢀ chains,73ꢀ protonꢀ
transportꢀoverꢀendergonicꢀstepsꢀcouldꢀoccurꢀasꢀlongꢀasꢀtheꢀ
finalꢀstepꢀisꢀexergonicꢀwithꢀrespectꢀtoꢀtheꢀinitialꢀPCETꢀpro-
cess.ꢀCertainly,ꢀaꢀfutureꢀchallengeꢀinꢀbiomimicryꢀisꢀefficientꢀ
netꢀprotonꢀtransfer,ꢀwhichꢀwillꢀrequireꢀaꢀmolecularꢀdesignꢀ
inꢀwhichꢀtheꢀPCETꢀsiteꢀisꢀ“reloaded”ꢀwithꢀaꢀnewꢀprotonꢀafterꢀ
eachꢀoxidation.ꢀ
ThisꢀresearchꢀwasꢀsupportedꢀbyꢀtheꢀU.S.ꢀDepartmentꢀofꢀEnergy,ꢀ
OfficeꢀofꢀScience,ꢀOfficeꢀofꢀBasicꢀEnergyꢀSciences,ꢀunderꢀAwardꢀ
DE-FG02-03ER15393.ꢀTheꢀtheoreticalꢀportionꢀofꢀthisꢀresearchꢀ
wasꢀ supportedꢀ byꢀ theꢀ Centerꢀ forꢀ Molecularꢀ Electrocatalysis,ꢀ
whichꢀisꢀanꢀEnergyꢀFrontierꢀResearchꢀCenterꢀfundedꢀbyꢀtheꢀU.S.ꢀ
DepartmentꢀofꢀEnergy,ꢀOfficeꢀofꢀScience,ꢀOfficeꢀofꢀBasicꢀEnergyꢀ
Sciences.ꢀ
REFERENCES
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Theꢀresultsꢀfromꢀtheseꢀmodelꢀsystemsꢀprovideꢀinsightꢀintoꢀ
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Z
waterꢀoxidationꢀandꢀmanagementꢀofꢀprotonꢀactivityꢀaroundꢀ
theꢀenergy-couplingꢀthylakoidꢀmembraneꢀofꢀplants,ꢀalgae,ꢀ
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Journal of the American Chemical Society
Page 10 of 13
Nakano,ꢀT.;ꢀSuzuki,ꢀM.;ꢀMasuda,ꢀT.;ꢀInoue,ꢀS.;ꢀKimura,ꢀT.;ꢀNomura,ꢀ
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TABLEꢀOFꢀCONTENTSꢀ(TOC)ꢀ
Journal of the American Chemical Society
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