A planarized: B -phenyldibenzoborepin: Impact of structural constraint on its electronic properties and Lewis acidity

Article abstract of DOI:10.1039/c8cc02837k

A B-phenyldibenzo[b,f]borepin planarized with two methylene bridges was synthesized. The structural constraint on the B-phenyl group resulted in a bathochromic shift of the absorption and fluorescence properties as well as enhanced Lewis acidity. A donor-

Full text of DOI:10.1039/c8cc02837k

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January 2016 Pages 1–216
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ISSN 1359-7345
COMMUNICATION
Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
N@C2n (2n
=
68 and 80). Synthesis of the
3
N@D5h-C80
rsc.li/chemcomm

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Pleaseꢀd oC ꢀhn eomt ꢀ Ca od mj u ms tꢀmarginsꢀ
View Article Online
DOI: 10.1039/C8CC02837K
JournalꢀNameꢀ
COMMUNICATIONꢀ
ꢀ
AꢀplanarizedꢀB-phenyldibenzoborepin:ꢀImpactꢀofꢀtheꢀstructuralꢀ
constraintꢀonꢀitsꢀelectronicꢀpropertiesꢀandꢀLewisꢀacidityꢀꢀ
Receivedꢀ00thꢀJanuaryꢀ20xx,ꢀ
Acceptedꢀ00thꢀJanuaryꢀ20xxꢀ
a
a
,a,b
NaokiꢀAndo, ꢀTomokatsuꢀKushida, ꢀandꢀShigehiroꢀYamaguchi*
ꢀ
DOI:ꢀ10.1039/x0xx00000xꢀ
www.rsc.org/ꢀ
R
Aꢀ B-phenyldibenzo[b,f]borepinꢀ planarizedꢀ withꢀ twoꢀ methyleneꢀ
bridgesꢀwasꢀsynthesized.ꢀTheꢀstructuralꢀconstraintꢀonꢀtheꢀB-phenylꢀ
groupꢀ resultedꢀ inꢀ aꢀ bathochromicꢀ shiftꢀ ofꢀ theꢀ absorptionꢀ andꢀ
fluorescenceꢀ propertiesꢀ asꢀ wellꢀ asꢀ anꢀ enhancedꢀ Lewisꢀ acidity.ꢀ Aꢀ
donor–π–acceptorꢀtypeꢀderivativeꢀbasedꢀonꢀthisꢀscaffoldꢀexhibitedꢀ
intenseꢀfluorescenceꢀirrespectiveꢀofꢀtheꢀsolventꢀpolarity.ꢀꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
R
B
R
B
B
R
B
Ar
R
dibenzo[b,f]borepin
R
1
1
1
a (Ar = Mes)
b (Ar = Tip)
c (Ar = Mes*)
Piers
Tovar
ꢀ
Amongꢀtheꢀdifferentꢀtypesꢀofꢀheterocycles,ꢀboron-containingꢀcyclicꢀ
scaffoldsꢀrepresentꢀusefulꢀbuildingꢀblocksꢀforꢀtheꢀdevelopmentꢀofꢀπ-
1
conjugatedꢀ systemsꢀ withꢀ unusualꢀ electronicꢀ structures. ꢀ Theꢀ
ꢀ
ꢀ
S
B
B
B
ꢀ
S
MesO
participationꢀ ofꢀ theꢀ vacantꢀ pꢀ orbitalꢀ ofꢀ theꢀ boronꢀ atomꢀ inꢀ theꢀ π-
conjugationꢀ endowsꢀ suchꢀ π-skeletonsꢀ withꢀ electron-acceptingꢀ
ꢀ
MesO
2
3
ꢀ
4
character.ꢀ Whileꢀ theꢀ five-memberedꢀ boroleꢀ servesꢀ asꢀ aꢀ 4πꢀ Fig.ꢀ 1ꢀ Dibenzoborepin-basedꢀ π-conjugatedꢀ systemsꢀ andꢀ relevantꢀ planarizedꢀ boron-
2
antiaromaticꢀbuildingꢀblock, ꢀtheꢀseven-memberedꢀringꢀborepinꢀisꢀaꢀ
embeddedꢀpolycyclicꢀcompounds.ꢀ
3
πꢀ aromaticꢀ scaffoldꢀ thatꢀ isꢀ isoelectronicꢀtoꢀtheꢀ tropyliumꢀ cation ꢀ
6
andꢀ anꢀ interestingꢀ syntheticꢀ intermediate. ꢀ Althoughꢀ theꢀ parentꢀ
4
However,ꢀsuchꢀbulkyꢀarylꢀgroupsꢀpreventꢀintermolecularꢀinteractionꢀ
inꢀtheꢀsolidꢀstate,ꢀwhichꢀisꢀdetrimentalꢀforꢀcertainꢀpropertiesꢀsuchꢀasꢀ
charge-carrierꢀmobility.ꢀAꢀsecondꢀlimitationꢀisꢀthatꢀexpansionꢀofꢀtheꢀ
π-conjugationꢀalwaysꢀoccursꢀalongꢀtheꢀlongꢀmolecularꢀaxis,ꢀsinceꢀtheꢀ
orthogonalꢀalignmentꢀofꢀtheꢀarylꢀgroupꢀonꢀtheꢀboronꢀatomꢀinhibitsꢀ
theꢀ π-conjugationꢀ inꢀ thisꢀ direction.ꢀ Inꢀ thisꢀ context,ꢀ weꢀ haveꢀ
borepinꢀitselfꢀisꢀunstable,ꢀtheꢀfusionꢀofꢀtheꢀskeletonꢀwithꢀaromaticꢀ
1ꢀcanꢀ
5
ringsꢀimprovesꢀitsꢀstability. ꢀInꢀparticular,ꢀdibenzo[b,f]borepinꢀ
beꢀregardedꢀasꢀaꢀboron-containingꢀaceneꢀanalogueꢀthatꢀhasꢀrecentlyꢀ
attractedꢀ increasingꢀ attentionꢀ (Fig.ꢀ 1).ꢀ Piersꢀ andꢀ coworkersꢀ haveꢀ
6
synthesizedꢀ aꢀ seriesꢀ ofꢀ annulatedꢀ borepins, ꢀ whileꢀ Tovarꢀ andꢀ
designedꢀ aꢀ newꢀ dibenzoborepinꢀ scaffoldꢀ2,ꢀinꢀwhichꢀtheꢀB-phenylꢀ
coworkersꢀ haveꢀ reportedꢀ furtherꢀ π-expandedꢀ fusedꢀ borepinsꢀ thatꢀ
7
containꢀtwoꢀborepinꢀrings. ꢀ
groupꢀ isꢀ structurallyꢀ constrainedꢀ inꢀ aꢀ planarꢀ fashionꢀ byꢀ twoꢀ
methyleneꢀ bridges.ꢀ Weꢀ haveꢀ recentlyꢀ demonstratedꢀ thatꢀ theꢀ
structuralꢀ constraintꢀ representsꢀ anꢀ effectiveꢀ strategyꢀ toꢀ stabilizeꢀ
Comprehensiveꢀstudiesꢀonꢀthisꢀclassꢀofꢀcompoundsꢀhaveꢀrevealedꢀ
theirꢀ fundamentalꢀ electronicꢀ properties,ꢀ suchꢀ asꢀ theirꢀ intriguingꢀ
fluorescence,ꢀ andꢀ demonstratedꢀ theirꢀ potentialꢀ utilityꢀ forꢀ organicꢀ
organoboronꢀ compoundsꢀ withoutꢀ havingꢀ toꢀ resortꢀ toꢀ stericꢀ
3,ꢀbutꢀalsoꢀboron-
9
protection. ꢀNotꢀonlyꢀplanarizedꢀtriphenylboraneꢀ
6-8
electronics,ꢀ includingꢀ organicꢀ light-emittingꢀ diodesꢀ (OLEDs). ꢀ
However,ꢀ someꢀ structuralꢀ limitationsꢀ persistꢀ withꢀ respectꢀ toꢀ theirꢀ
useꢀ asꢀ buildingꢀ blocksꢀ forꢀextendedꢀ π-conjugatedꢀ systems.ꢀ Oneꢀ isꢀ
theꢀ bulkyꢀ substituentsꢀ suchꢀ asꢀ mesitylꢀ (Mes),ꢀ 2,4,6,-
triisopropylphenlylꢀ (Tip),ꢀ orꢀ 2,4,6-tri(t-butyl)phenylꢀ (Mes*)ꢀ groups,ꢀ
whichꢀareꢀrequiredꢀonꢀtheꢀboronꢀatomꢀtoꢀgainꢀsufficientꢀstability.ꢀꢀ
embeddedꢀpolycyclicꢀsystems,ꢀsuchꢀasꢀ
theirꢀ utilityꢀ inꢀ optoelectronicꢀ applicationsꢀ hasꢀ beenꢀ studied. ꢀ
Herein,ꢀweꢀreportꢀtheꢀsynthesisꢀandꢀfundamentalꢀpropertiesꢀofꢀ .ꢀInꢀ
particular,ꢀtheꢀimpactꢀofꢀtheꢀstructuralꢀconstraintꢀonꢀtheꢀelectronicꢀ
propertiesꢀisꢀstudiedꢀinꢀdetail.ꢀToꢀdemonstrateꢀtheꢀutilityꢀofꢀ ꢀasꢀanꢀ
4,ꢀhaveꢀbeenꢀsynthesizedꢀandꢀ
10
2
2
electron-acceptingꢀ buildingꢀ block,ꢀ aꢀ donor–π–acceptorꢀ (D–π–A)ꢀ
typeꢀcompoundꢀwasꢀalsoꢀsynthesizedꢀinꢀthisꢀstudy.ꢀ
Planarizedꢀborepinꢀ2ꢀwasꢀsuccessfullyꢀsynthesizedꢀinꢀthreeꢀstepsꢀ
6
,9
fromꢀcis-bis(2-bromophenyl)etheneꢀ
5
ꢀ(Schemeꢀ1). ꢀTheꢀreactionꢀofꢀ
ꢀ withꢀ t-BuLiꢀ followedꢀ byꢀ treatmentꢀ withꢀ ArB(OMe) ꢀ (Arꢀ =ꢀ 2,6-
2,6-di(propen-2-yl)phenyl-
ꢀ inꢀ 60%ꢀ yield.ꢀ Aꢀ two-foldꢀ intramolecularꢀ
ꢀwasꢀaccomplishedꢀusingꢀSc(OTf) ꢀinꢀ
5
2
di(propen-2-yl)phenyl)ꢀ
substitutedꢀ precursorꢀ
producedꢀ
6
Friedel–Craftsꢀcyclizationꢀofꢀ
6
3
Thisꢀjournalꢀisꢀ©ꢀTheꢀRoyalꢀSocietyꢀofꢀChemistryꢀ20xxꢀ
J.ꢀName.,ꢀ2013,ꢀ00,ꢀ1-3ꢀ|ꢀ1ꢀꢀ
Pleaseꢀdoꢀnotꢀadjustꢀmarginsꢀ

Pleaseꢀd oC ꢀhn eomt ꢀ Ca od mj u ms tꢀmarginsꢀ
Page 2 of 5
COMMUNICATIONꢀ
JournalꢀNameꢀ
C7
View Article Online
C6
C8DOI: 10.1039/C8CC02837K
B
Br
Br
a
B
b
B
c
C1
C9
5
X
B1
C15
C14
X
Ph
N
6
7
(X = H) 60%
(X = Br) 36%
2 (X = H) 20%
8 (X = Br) 33%
9 86%
Ph
ꢀ
Schemeꢀ1ꢀSynthesisꢀofꢀplanarizedꢀB-phenyldibenzoborepinꢀ2ꢀandꢀD–π–Aꢀtypeꢀderivativeꢀ
9
.ꢀReagentsꢀandꢀconditions:ꢀ(a)ꢀi,ꢀt-BuLi,ꢀEt
2
O,ꢀ–78ꢀ°C;ꢀii,ꢀArB(OMe)
,ꢀ 1,2-dichroloethane,ꢀ reflux.ꢀ (c)ꢀ 4-(N,N-
,ꢀXPhos,ꢀK CO ,ꢀtoluene/EtOH/H O,ꢀreflux.ꢀ
2 2
,ꢀEt O,ꢀreflux.ꢀ(b)ꢀForꢀ
2
:ꢀ Sc(OTf) ,ꢀ hexane,ꢀ reflux;ꢀ forꢀ 8:ꢀ Sc(OTf)
3
3
Fig.ꢀ2ꢀORTEPꢀdrawingꢀofꢀ2ꢀ(50%ꢀprobabilityꢀforꢀthermalꢀellipsoids).ꢀHydrogenꢀatomsꢀareꢀ
omittedꢀforꢀclarity.ꢀSelectedꢀbondꢀlengthsꢀ[Å]ꢀandꢀanglesꢀ[°]:ꢀB1–C1ꢀ=ꢀ1.5479(17),ꢀB1–
C14ꢀ=ꢀ1.5585ꢀ(17),ꢀB1–C15ꢀ=ꢀ1.5626(16),ꢀC7–C8ꢀ=ꢀ1.3424(18),ꢀC1–B1–C14ꢀ=ꢀ128.06(10),ꢀ
C1–B1–C15ꢀ=ꢀ116.39(10),ꢀC14–B1–C15ꢀ=ꢀ115.50(10).ꢀꢀꢀ
Diphenylamino)phenylboronicꢀacid,ꢀPd(PPh
3
)
4
2
3
2
hexaneꢀunderꢀreflux,ꢀwhichꢀaffordedꢀ
Compoundꢀ ꢀ isꢀ stableꢀ underꢀ ambientꢀ conditionsꢀ andꢀ couldꢀ beꢀ
purifiedꢀ byꢀ columnꢀ chromatographyꢀ onꢀ silicaꢀ gel.ꢀ Inꢀ aꢀ similarꢀ
2ꢀasꢀaꢀyellowꢀsolidꢀinꢀ20%ꢀyield.ꢀ
2
donationꢀfromꢀtheꢀbenzeneꢀringꢀtoꢀtheꢀvacantꢀpꢀorbitalꢀofꢀtheꢀboronꢀ
9
,10a,e
center.
ꢀTheꢀB–Cꢀbondꢀlengthsꢀinꢀ
fullyꢀfusedꢀtriarylboranesꢀsuchꢀasꢀ ꢀ[1.519(2)–1.520(2)ꢀÅ]. ꢀꢀ
Althoughꢀ theꢀ twistedꢀ conformationꢀ wasꢀ observedꢀ inꢀ theꢀ
crystallineꢀstate,ꢀtheꢀstructuralꢀoptimizationꢀofꢀ ꢀinꢀgasꢀphaseꢀusingꢀ
2ꢀareꢀstillꢀlongerꢀthanꢀthoseꢀofꢀ
manner,ꢀ bromo-substitutedꢀ planarizedꢀ borepinꢀ
usingꢀ theꢀ correspondingꢀ boronateꢀ esterꢀ (Arꢀ =ꢀ 4-bromo-2,6-
di(propen-2-yl)phenyl)ꢀ viaꢀ .ꢀ Importantly,ꢀ ꢀ exhibitedꢀ sufficientꢀ
stabilityꢀtoꢀconductꢀaꢀSuzuki–Miyauraꢀcross-couplingꢀreactionꢀunderꢀ
8ꢀ wasꢀ obtainedꢀ
9
3
7
8
2
DFTꢀcalculationꢀatꢀtheꢀB3PW91/6-31+G(d)ꢀlevelꢀofꢀtheoryꢀprovidedꢀ
twoꢀ localꢀ minimumꢀ structures:ꢀ aꢀ nearlyꢀ planarꢀ formꢀ p-2ꢀ andꢀ aꢀ
twistedꢀ formꢀ t-2.ꢀ Theseꢀ twoꢀ conformersꢀ haveꢀ dihedralꢀ anglesꢀ
betweenꢀtheꢀB-phenylꢀringꢀandꢀtheꢀborepinꢀmeanꢀplaneꢀofꢀ9.1°ꢀandꢀ
basicꢀconditions.ꢀAsꢀanꢀexample,ꢀtheꢀD–π–Aꢀtypeꢀderivativeꢀ
preparedꢀ inꢀ 86%ꢀ yieldꢀ byꢀ theꢀ couplingꢀ reactionꢀ ofꢀ ꢀ withꢀ 4-
diphenylamino)phenylboronicꢀacid.ꢀTheseꢀresultsꢀdemonstrateꢀthatꢀ
9ꢀwasꢀ
8
(
structuralꢀ constraintꢀ isꢀ effectiveꢀ forꢀ theꢀ stabilizationꢀ ofꢀ theꢀ
dibenzoborepinꢀskeletonꢀevenꢀinꢀtheꢀabsenceꢀofꢀstericꢀprotection.ꢀB-
Tip-substitutedꢀ dibenzoborepinꢀ 1bꢀ wasꢀ alsoꢀ preparedꢀ inꢀ aꢀ similarꢀ
24.7°,ꢀ respectivelyꢀ (Fig.ꢀ S11).ꢀ Theꢀ geometryꢀ ofꢀ t-2ꢀcorrespondsꢀ toꢀ
theꢀstructureꢀobservedꢀinꢀtheꢀcrystallineꢀstate.ꢀNotably,ꢀtheꢀplanarꢀ
–
1
1
1
conformerꢀ p-2ꢀisꢀbyꢀ 0.96ꢀ kcalꢀ mol ꢀmoreꢀ stableꢀ thanꢀ theꢀ twistedꢀ
conformerꢀ t-2.ꢀ Therefore,ꢀ theꢀ twistedꢀ structureꢀ observedꢀ inꢀ theꢀ
crystallineꢀstateꢀisꢀlikelyꢀdueꢀtoꢀpackingꢀforces.ꢀꢀ
mannerꢀforꢀcomparisonꢀ(SchemeꢀS5). ꢀ
Planarizedꢀ borepinsꢀ wereꢀ unequivocallyꢀ characterizedꢀ byꢀ NMRꢀ
1
1
spectroscopy,ꢀmassꢀspectrometry,ꢀandꢀX-rayꢀcrystallography.ꢀTheꢀ Bꢀ
NMRꢀ spectrumꢀ ofꢀ ꢀ inꢀ CDCl ꢀ exhibitsꢀ aꢀ broadꢀ signalꢀ atꢀ 46ꢀ ppm,ꢀ
whichꢀ isꢀ upfield-shiftedꢀ relativeꢀ toꢀ thoseꢀ ofꢀ B-aryl-substitutedꢀ
Theꢀ constrainedꢀ structureꢀ ofꢀ 2ꢀ influencesꢀ theꢀ electrochemicalꢀ
propertiesꢀ ofꢀ dibenzoborepins.ꢀ Cyclicꢀ voltammogramꢀ ofꢀ 2ꢀ inꢀ THFꢀ
revealedꢀreversibleꢀredoxꢀwavesꢀwithꢀaꢀhalfꢀreductionꢀpotentialꢀofꢀ–
2
3
6,7a
dibenzoborepinsꢀ (dꢀ =ꢀ 61–65ꢀ ppm), ꢀ indicativeꢀ ofꢀ theꢀ effectiveꢀ
interactionꢀbetweenꢀtheꢀvacantꢀpꢀorbitalꢀofꢀtheꢀboronꢀatomꢀandꢀtheꢀ
πꢀorbitalsꢀofꢀtheꢀfusedꢀbenzeneꢀringsꢀandꢀtheꢀB-phenylꢀgroupꢀinꢀtheꢀ
+
.35ꢀ Vꢀ (vsꢀ Fc/Fc )ꢀ (Fig.ꢀ S4),ꢀ whichꢀ isꢀ moreꢀ positivelyꢀ shiftedꢀ thanꢀ
2
thoseꢀofꢀ1a
6 7a
1a:ꢀ–2.54ꢀV; ꢀ1b:ꢀ–2.55ꢀV;ꢀ1c:ꢀ–2.56ꢀV ).ꢀThisꢀresultꢀ
–
demonstratesꢀtheꢀhigherꢀelectron-acceptingꢀabilityꢀofꢀ2ꢀrelativeꢀtoꢀ
cꢀ
(
1
1
8
planarizedꢀskeletonꢀofꢀ2.ꢀInꢀTHF-d ,ꢀtheꢀ BꢀNMRꢀspectrumꢀofꢀ2ꢀalsoꢀ
itsꢀorthogonallyꢀaryl-orientedꢀcounterpartsꢀ1.ꢀ
Theꢀ structuralꢀ constraintꢀ alsoꢀ affectsꢀ theirꢀ photophysicalꢀ
properties.ꢀ Inꢀ theꢀ UV-visꢀ absorptionꢀ spectrumꢀ inꢀ THF,ꢀ planarizedꢀ
exhibitedꢀ aꢀ similarꢀ broadꢀ signalꢀ atꢀ 48ꢀ ppm,ꢀ indicativeꢀ thatꢀ noꢀ
complexationꢀoccurredꢀinꢀTHF.ꢀꢀ
Theꢀ X-rayꢀ crystallographicꢀ analysisꢀ ofꢀ 2ꢀ revealedꢀ aꢀ distortedꢀ
borepinꢀ
2
ꢀexhibitsꢀanꢀabsorptionꢀmaximumꢀatꢀ
isꢀ bathochromicallyꢀ shiftedꢀ relativeꢀ toꢀ thatꢀ ofꢀ orthogonallyꢀ aryl-
substitutedꢀ 1bꢀ ( absꢀ =ꢀ 381ꢀ nm)ꢀ (Fig.ꢀ 3ꢀ andꢀ Tableꢀ S2).ꢀ Theꢀ molarꢀ
absorptionꢀcoefficientꢀalsoꢀincreasesꢀfromꢀ1bꢀ(logꢀ =ꢀ3.80)ꢀtoꢀ ꢀ(logꢀ
ꢀ=ꢀ3.93).ꢀInꢀtheꢀfluorescenceꢀspectraꢀinꢀTHF,ꢀbothꢀ2ꢀandꢀ1bꢀshowꢀ
intenseꢀblueꢀemissionꢀwithꢀmaximumꢀemissionꢀwavelengthsꢀ( em)ꢀatꢀ
24ꢀandꢀ395ꢀnm,ꢀrespectively,ꢀasꢀwellꢀasꢀcomparableꢀStokesꢀshiftsꢀ
labsꢀ=ꢀ406ꢀnm,ꢀwhichꢀ
structureꢀ inꢀ theꢀ crystallineꢀ stateꢀ (Fig.ꢀ 2).ꢀ Theꢀ dibenzoborepinꢀ
frameworkꢀ exhibitsꢀ aꢀ dihedralꢀ angleꢀ betweenꢀ theꢀ twoꢀ fusedꢀ
benzeneꢀ ringsꢀ ofꢀ 26.4°,ꢀ resultingꢀ inꢀ aꢀ nonplanarꢀ geometryꢀ ofꢀ theꢀ
centralꢀborepinꢀring.ꢀTheꢀB-phenylꢀringꢀisꢀalsoꢀtwistedꢀrelativeꢀtoꢀtheꢀ
borepinꢀ meanꢀ planeꢀ withꢀ aꢀ dihedralꢀ angleꢀ ofꢀ 26.3°.ꢀ Asꢀ aꢀ
consequence,ꢀtheꢀtwoꢀmethyleneꢀbridgesꢀareꢀdeviatedꢀupwardꢀandꢀ
downwardꢀwithꢀrespectꢀtoꢀtheꢀplaneꢀdefinedꢀbyꢀtheꢀboronꢀatomꢀandꢀ
theꢀthreeꢀipsoꢀcarbonꢀatoms.ꢀꢀ
l
e
2
e
l
4
–
1
(
Dn:ꢀ 1050ꢀ andꢀ 930ꢀ cm ꢀ forꢀ
2ꢀ andꢀ 1b,ꢀ respectively).ꢀ Structurallyꢀ
constrainingꢀ aꢀ π-conjugatedꢀ skeletonꢀ intoꢀ aꢀ planarꢀ conformationꢀ
Aꢀcloserꢀinspectionꢀofꢀtheꢀborepinꢀringꢀprovidedꢀinsightꢀintoꢀtheꢀ
characterꢀofꢀtheꢀboronꢀcenter.ꢀTheꢀC7–C8ꢀbondꢀlengthꢀ[1.3424(18)ꢀ
Å]ꢀindicatesꢀaꢀlocalizationꢀofꢀtheꢀC–Cꢀdoubleꢀbondꢀdueꢀtoꢀtheꢀlowꢀ
aromaticityꢀ ofꢀ theꢀ centralꢀ borepinꢀ ring.ꢀ Theꢀ boronꢀ atomꢀ adoptsꢀaꢀ
trigonalꢀplanarꢀ geometryꢀ withꢀ aꢀ sumꢀofꢀC–B–Cꢀ anglesꢀofꢀ 359.95°.ꢀ
TheꢀB–Cꢀbondsꢀinꢀtheꢀborepinꢀringꢀ[B1–C1:ꢀ1.5479(17)ꢀÅ;ꢀB1–C14:ꢀ
1
2
generallyꢀincreasesꢀtheꢀfluorescenceꢀquantumꢀyieldꢀ(
toꢀthisꢀtrend,ꢀtheꢀquantumꢀyieldꢀofꢀ ꢀ( ꢀ=ꢀ0.68)ꢀisꢀslightlyꢀlowerꢀ
thanꢀ thatꢀ ofꢀ 1bꢀ ꢀ =ꢀ 0.81).ꢀ Toꢀ understandꢀ thisꢀ difference,ꢀ theꢀ
radiativeꢀ (k )ꢀ andꢀ nonradiativeꢀ (knr)ꢀ decayꢀ rateꢀ constantsꢀ wereꢀ
determinedꢀ basedꢀ onꢀ theꢀ fluorescenceꢀ lifetimesꢀ andꢀ quantumꢀ
F
F ). ꢀContraryꢀ
2
F
F
(F
F
r
7
–1
7 –1
yields.ꢀWhileꢀtheꢀk
areꢀcomparableꢀtoꢀeachꢀother,ꢀtheꢀk ꢀvalueꢀofꢀ
r
ꢀvaluesꢀforꢀ
2ꢀ(7.8ꢀ×ꢀ10 ꢀs )ꢀandꢀ1bꢀ(7.5ꢀ×ꢀ10 ꢀs )ꢀ
1
.5585(17)ꢀÅ]ꢀareꢀslightlyꢀshorterꢀorꢀcomparableꢀtoꢀthoseꢀofꢀMes-ꢀorꢀ
6
7
–1
ꢀ(3.8ꢀ×ꢀ10 ꢀs )ꢀisꢀ
nr
2
largerꢀthanꢀthatꢀofꢀ1bꢀ(1.8ꢀ×ꢀ10 ꢀs ).ꢀTheseꢀresultsꢀindicateꢀthatꢀtheꢀ
Tip-substitutedꢀ1aꢀandꢀ1bꢀ(~1.56ꢀÅ). ꢀTheꢀB–Cphenylꢀbondꢀ[B1–C15:ꢀ
.5626(16)ꢀÅ]ꢀisꢀalsoꢀbyꢀ~0.03ꢀÅꢀshorterꢀthanꢀthoseꢀinꢀ .ꢀThisꢀtrendꢀ
7
–1
1
1
lowerꢀ
F
F
ꢀvalueꢀ ofꢀ 2ꢀ isꢀ dueꢀ toꢀ theꢀ acceleratedꢀnonradiativeꢀ decayꢀ
shouldꢀ beꢀ dueꢀ toꢀ theꢀ structuralꢀ constraintꢀ imposedꢀ byꢀ theꢀ
methyleneꢀbridges.ꢀNotably,ꢀinꢀaꢀseriesꢀofꢀplanarizedꢀtriarylboranes,ꢀ
theꢀB–CꢀbondꢀlengthꢀisꢀrelevantꢀtoꢀtheꢀLewisꢀacidityꢀofꢀtheꢀboronꢀ
center,ꢀasꢀaꢀcompressedꢀB–Cꢀbondꢀresultsꢀinꢀeffectiveꢀπ-ꢀelectron-ꢀ
processes,ꢀsuggestingꢀthatꢀaꢀstructuralꢀconstraintꢀisꢀnotꢀalwaysꢀ
2
ꢀ|ꢀJ.ꢀName.,ꢀ2012,ꢀ00,ꢀ1-3ꢀ
Thisꢀjournalꢀisꢀ©ꢀTheꢀRoyalꢀSocietyꢀofꢀChemistryꢀ20xxꢀ
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JournalꢀNameꢀ
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View Article Online
DOI: 10.1039/C8CC02837K
2
–
1.99 eV
–1.93 eV
–
2.05 eV
–
2.23 eV
3
.17 eV
391 nm
f = 0.0983
3
.15 eV
393 nm
f = 0.0988
3
.42 eV
363 nm
f = 0.0595
3
3
.15 eV
93 nm
f = 0.0907
1
0
–
5.67 eV
–
5.77 eV
–
5.93 eV
–
6.00 eV
3
00
350
400
450
500
Wavelength/nm
Fig.ꢀ3ꢀUV-visꢀabsorptionꢀ(solidꢀlines)ꢀandꢀfluorescenceꢀ(dashedꢀlines)ꢀspectraꢀofꢀ2ꢀ(red)ꢀ
andꢀ1bꢀ(blue)ꢀinꢀTHF.ꢀ
B
B
B
B
1
0
⟘
10//
p-2
t-2
effectiveꢀforꢀtheꢀsuppressionꢀofꢀsuchꢀnonradiativeꢀdecayꢀevents.ꢀ
Toꢀ elucidateꢀ theꢀ impactꢀ ofꢀ theꢀ structuralꢀ constraintꢀ onꢀ itsꢀꢀ
Fig.ꢀ4ꢀComparisonꢀofꢀtheꢀKohn–ShamꢀHOMOꢀandꢀLUMOꢀenergyꢀlevelsꢀofꢀ10^,ꢀ10//,ꢀp-2,ꢀ
andꢀ t-2ꢀ togetherꢀ withꢀ theirꢀ pictorialꢀ drawingsꢀ basedꢀ onꢀ TD-DFTꢀ calculationsꢀ atꢀ theꢀ
B3LYP/6-31+G(d)//B3PW91/6-31+G(d)ꢀlevelꢀofꢀtheory.ꢀꢀ
properties,ꢀ
modelꢀ compoundsꢀ 10
adoptꢀ anꢀ orthogonalꢀ orꢀ parallelꢀ alignmentꢀrelativeꢀ toꢀ theꢀ borepinꢀ
2ꢀ wasꢀ theoreticallyꢀ comparedꢀ toꢀ B-phenyl-substitutedꢀ
^
ꢀ
andꢀ 10//,ꢀ whereinꢀ theꢀ B-phenylꢀ groupsꢀ
13
correspondingꢀLewisꢀacid–baseꢀcomplexꢀ
differenceꢀwasꢀalsoꢀobservedꢀinꢀtheꢀUV-visꢀabsorptionꢀspectraꢀ(Figs.ꢀ
2·Py. ꢀMoreover,ꢀaꢀsimilarꢀ
^
frameworkꢀ (Fig.ꢀ 4).ꢀ Theꢀ structuralꢀ optimizationꢀ ofꢀ 10 ꢀ wasꢀ
performedꢀatꢀtheꢀB3PW91/6-31+G(d)ꢀlevelꢀofꢀtheory.ꢀAꢀsingleꢀpointꢀ
S6ꢀandꢀS7).ꢀUponꢀadditionꢀofꢀpyridineꢀtoꢀaꢀtolueneꢀsolutionꢀofꢀ2,ꢀtheꢀ
absorbanceꢀ atꢀ 406ꢀ nmꢀ graduallyꢀ decreased.ꢀ Onꢀ theꢀ basisꢀ ofꢀ thisꢀ
calculationꢀ ofꢀ 10//ꢀ wasꢀ conductedꢀ employingꢀ theꢀ optimizedꢀ
geometryꢀofꢀ10
whichꢀwasꢀfixedꢀinꢀaꢀcoplanarꢀfashion.ꢀUponꢀchangingꢀfromꢀ10
//,ꢀtheꢀLUMOꢀenergyꢀlevelꢀdecreasedꢀbyꢀ0.24ꢀeV,ꢀwhileꢀtheꢀHOMOꢀ
^
,ꢀexceptꢀforꢀtheꢀalignmentꢀofꢀtheꢀB-phenylꢀgroup,ꢀ
spectralꢀ change,ꢀ theꢀ bindingꢀ constantꢀ ofꢀ
2
ꢀ towardꢀ pyridineꢀ wasꢀ
^
ꢀtoꢀ
–1 14
=ꢀ7.7ꢀM . ꢀImportantly,ꢀnoꢀsignificantꢀchangeꢀ
determinedꢀtoꢀbeꢀK
ꢀ
10
wasꢀobservedꢀforꢀtheꢀabsorptionꢀspectraꢀofꢀ1bꢀuponꢀadditionꢀofꢀanꢀ
excessꢀofꢀpyridineꢀunderꢀsimilarꢀconditions,ꢀdemonstratingꢀthatꢀtheꢀ
energyꢀ levelꢀ slightlyꢀ increasedꢀ byꢀ 0.07ꢀ eV.ꢀ Theꢀ moreꢀ substantialꢀ
changeꢀ inꢀ theꢀ LUMOꢀ levelꢀ shouldꢀ beꢀ attributedꢀ toꢀ theꢀ effectiveꢀ
interactionꢀbetweenꢀtheꢀdibenzoborepinꢀskeletonꢀandꢀtheꢀB-phenylꢀ
group.ꢀThisꢀchangeꢀinꢀtheꢀLUMOꢀlevelꢀcompensatesꢀforꢀtheꢀelectron-
donatingꢀeffectꢀofꢀtheꢀtwoꢀmethyleneꢀbridgesꢀinꢀp-2.ꢀConsequently,ꢀ
structuralꢀconstrainedꢀ
basicꢀpyridine.ꢀꢀ
2ꢀhasꢀhigherꢀcomplexationꢀabilityꢀwithꢀLewisꢀ
Comparisonsꢀ ofꢀ 2ꢀ withꢀ previouslyꢀ reportedꢀ planarizedꢀ
triarylboranesꢀ providedꢀ importantꢀ insightsꢀ intoꢀ theꢀ effectꢀ ofꢀ theꢀ
structuralꢀ constraintꢀ onꢀ theꢀ Lewisꢀ acidity.ꢀ Weꢀ previouslyꢀ reportedꢀ
^
theꢀLUMOꢀlevelsꢀofꢀ10 ꢀandꢀp-2ꢀareꢀalmostꢀcomparable,ꢀwhileꢀtheꢀ
HOMOꢀlevelꢀofꢀp-2ꢀisꢀbyꢀ0.33ꢀeVꢀhigher,ꢀwhichꢀresultsꢀinꢀaꢀnarrowerꢀ
HOMO–LUMOꢀ gap.ꢀ TD-DFTꢀ calculationsꢀ atꢀ theꢀ B3LYP/6-31+G(d)ꢀ
levelꢀofꢀtheoryꢀshowedꢀthatꢀp-2ꢀexhibitsꢀaꢀsmallerꢀtransitionꢀenergyꢀ
andꢀincreasedꢀoscillatorꢀstrengthꢀcomparedꢀtoꢀthoseꢀofꢀ1bꢀ(Fig.ꢀS9),ꢀ
whichꢀ isꢀ consistentꢀ withꢀ theꢀ experimentalꢀ results,ꢀ i.e.,ꢀ theꢀ red-
shiftedꢀ absorptionꢀ maximumꢀ withꢀ aꢀ largerꢀ molarꢀ absorptionꢀ
thatꢀplanarizedꢀtriphenylboraneꢀ3ꢀisꢀinertꢀtowardꢀpyridine,ꢀindicatingꢀ
9
. ꢀMoreover,ꢀtheꢀKꢀvalueꢀ
thatꢀ2ꢀexhibitsꢀaꢀhigherꢀLewisꢀacidityꢀthanꢀ
ofꢀ 2ꢀ towardꢀpyridine
extendedꢀplanarizedꢀboraneꢀ
3
ꢀ
isꢀlargerꢀevenꢀ whenꢀ comparedꢀ toꢀ thatꢀ ofꢀ π-
–
1
4
ꢀ(Kꢀ=ꢀ0.35ꢀM ꢀinꢀtoluene),ꢀwhichꢀhasꢀaꢀ
muchꢀlower-lyingꢀLUMOꢀlevel.ꢀThus,ꢀnotꢀonlyꢀtheꢀLUMOꢀlevel,ꢀbutꢀ
alsoꢀtheꢀdegreeꢀofꢀstructuralꢀconstraint,ꢀi.e.,ꢀtheꢀstructuralꢀflexibility,ꢀ
influenceꢀtheꢀLewisꢀacidityꢀofꢀplanarizedꢀtriarylboranes.ꢀ
coefficientꢀforꢀ2ꢀrelativeꢀtoꢀ1b.ꢀThus,ꢀtheꢀmostꢀimportantꢀelectronicꢀ
effectsꢀ arisingꢀ fromꢀ theꢀ planarizationꢀ withꢀ theꢀ methyleneꢀ bridgesꢀ
areꢀincreasesꢀinꢀtheꢀHOMOꢀlevelꢀandꢀtheꢀoscillatorꢀstrength.ꢀ
However,ꢀ thisꢀ comparisonꢀ doesꢀ notꢀ reproduceꢀ theꢀ positivelyꢀ
Finally,ꢀtheꢀpropertiesꢀofꢀD–π–Aꢀtypeꢀ
donatingꢀ 4-(N,N-diphenylamino)phenylꢀ groupꢀ onꢀ theꢀ B-phenylꢀ
9,ꢀwhichꢀbearsꢀanꢀelectron-
1
5
moiety,ꢀ wasꢀ assessed. ꢀ Similarꢀ toꢀ otherꢀ D–π–Aꢀ typeꢀ compoundsꢀ
shiftedꢀ reductionꢀ potentialꢀ ofꢀ
discrepancyꢀ mayꢀ beꢀ rationalizedꢀ byꢀ consideringꢀ theꢀ twistedꢀ
conformationꢀ ofꢀ .ꢀ Whileꢀ conformerꢀ t-2ꢀ presentsꢀ similarꢀ orbitalꢀ
2ꢀ comparedꢀ toꢀ thatꢀ ofꢀ 1b.ꢀ Thisꢀ
16
withꢀaꢀborylꢀgroupꢀasꢀtheꢀelectron-acceptingꢀgroup, ꢀ
intenseꢀfluorescenceꢀwithꢀlargeꢀsolvatochromismꢀ(Fig.ꢀ 5ꢀandꢀTableꢀ
S3).ꢀ Theꢀ absorptionꢀ spectrumꢀ ofꢀ ꢀ inꢀ cyclohexaneꢀ exhibitsꢀ anꢀ
absꢀ =ꢀ 413ꢀ nmꢀ withꢀ aꢀ vibronicꢀ structure,ꢀ
whichꢀ remainsꢀ almostꢀ unchangedꢀ irrespectiveꢀ ofꢀ theꢀ solventꢀ
polarity.ꢀ Theꢀ fluorescenceꢀ spectrumꢀ ofꢀ ꢀ inꢀ cyclohexaneꢀ exhibitsꢀ
emꢀ=ꢀ434ꢀnm)ꢀandꢀaꢀhighꢀfluorescenceꢀ
ꢀ =ꢀ 0.95).ꢀ Withꢀ increasingꢀ solventꢀ polarityꢀ fromꢀ
9ꢀexhibitedꢀanꢀ
2
9
delocalizationꢀinꢀtheꢀHOMOꢀandꢀLUMOꢀcomparedꢀtoꢀthoseꢀofꢀp-2,ꢀ
bothꢀtheꢀHOMOꢀandꢀLUMOꢀlevelsꢀofꢀt-2ꢀareꢀbyꢀca.ꢀ0.1ꢀeVꢀlowerꢀthanꢀ
thoseꢀofꢀp-2.ꢀConsequently,ꢀtheꢀLUMOꢀlevelꢀofꢀt-2ꢀliesꢀbelowꢀthatꢀofꢀ
absorptionꢀ maximumꢀ atꢀ
l
9
1b.ꢀ Moreover,ꢀ theꢀ twistedꢀ conformerꢀ becomesꢀ energeticallyꢀ
preferredꢀ inꢀ theꢀ one-electronꢀ reducedꢀ state.ꢀ Calculationsꢀ atꢀ theꢀ
intenseꢀblueꢀfluorescenceꢀ(
l
quantumꢀ yieldꢀ (F
F
UB3PW91/6-31+G(d)ꢀ levelꢀ ofꢀ theoryꢀ onꢀ theꢀ radicalꢀ anionꢀ ofꢀ 2ꢀ
1
–
cyclohexaneꢀtoꢀacetonitrile,ꢀtheꢀemissionꢀbandꢀshowedꢀaꢀsignificantꢀ
redꢀ shiftꢀ andꢀ anꢀ orangeꢀ fluorescenceꢀ wasꢀ observedꢀ inꢀ acetonitrileꢀ
revealedꢀthatꢀtheꢀtwistedꢀgeometryꢀisꢀbyꢀ1.4ꢀkcalꢀmol ꢀmoreꢀstableꢀ
thanꢀ theꢀ planarꢀ geometryꢀ (Fig.ꢀ S12).ꢀ Theꢀ twistedꢀ conformerꢀ mayꢀ
–
1
(
l
emꢀ=ꢀ588ꢀnm).ꢀNotably,ꢀdespiteꢀtheꢀlargeꢀStokesꢀshiftꢀ(7270ꢀcm ),ꢀ
ꢀretainedꢀaꢀhighꢀfluorescenceꢀquantumꢀyieldꢀ( ꢀ=ꢀ0.91)ꢀevenꢀinꢀ
acetonitrile.ꢀ Aꢀ similarꢀ trendꢀ hasꢀ alsoꢀ beenꢀ observedꢀ forꢀ severalꢀ
playꢀaꢀroleꢀinꢀtheꢀpositivelyꢀshiftedꢀreductionꢀpotentialꢀofꢀ
Subsequently,ꢀtheꢀLewisꢀacidityꢀofꢀ ꢀwasꢀassessed.ꢀInꢀpyridine-d
theꢀ BꢀNMRꢀsignalꢀofꢀ1bꢀappearedꢀatꢀ65ꢀppm,ꢀwhichꢀisꢀcomparableꢀ
2.ꢀꢀꢀ
9
F
F
2
5
,ꢀ
11
16i
boryl-substitutedꢀD–π–Aꢀtypeꢀcompounds. ꢀThisꢀfeatureꢀisꢀworthꢀ
toꢀthatꢀinꢀCDCl
ppmꢀinꢀpyridine-d
thatꢀinꢀCDCl .ꢀThisꢀresultꢀsuggestsꢀtheꢀformationꢀofꢀtheꢀꢀ
3
.ꢀInꢀstarkꢀcontrast,ꢀtheꢀsignalꢀofꢀ2ꢀappearedꢀatꢀ–3.3ꢀ
noting,ꢀ asꢀ theꢀ
F
F ꢀ valuesꢀ ofꢀ typicalꢀ D–π–Aꢀ fluorophoresꢀ tendꢀ toꢀ
5
,ꢀwhichꢀisꢀsignificantlyꢀupfield-shiftedꢀrelativeꢀtoꢀꢀ
17ꢀ
decreaseꢀinꢀpolarꢀsolvents. Time-resolvedꢀfluorescenceꢀꢀ
3
Thisꢀjournalꢀisꢀ©ꢀTheꢀRoyalꢀSocietyꢀofꢀChemistryꢀ20xxꢀ
J.ꢀName.,ꢀ2013,ꢀ00,ꢀ1-3ꢀ|ꢀ3ꢀ
Pleaseꢀdoꢀnotꢀadjustꢀmarginsꢀ

Pleaseꢀd oC ꢀhn eomt ꢀ Ca od mj u ms tꢀmarginsꢀ
Page 4 of 5
COMMUNICATIONꢀ
JournalꢀNameꢀ
Organometallics,ꢀ1993,ꢀ12,ꢀ3225;ꢀ(d)ꢀG.ꢀSubramanian,ꢀVPi.eꢀwv.ꢀARrt.iꢀcSlechOlnelyineerꢀ
andꢀH.ꢀJiao,ꢀOrganometallics,ꢀ1997,ꢀ16,ꢀ2362 D; ꢀ O( eI ): ꢀ 1R 0. ꢀ. E1 .0ꢀ M3 9 e/ sC s 8e Cr s Cm 0i t2 h8 ꢀ 3a 7n Kd ꢀ
J.ꢀD.ꢀTovar,ꢀJ.ꢀPhys.ꢀOrg.ꢀChem.,ꢀ2015,ꢀ28,ꢀ378.ꢀ
4
5
Y.ꢀShoji,ꢀN.ꢀTanaka,ꢀS.ꢀMuranaka,ꢀN.ꢀShigeno,ꢀH.ꢀSugiyama,ꢀK.ꢀTakenouchi,ꢀ
F.ꢀHajjajꢀandꢀT.ꢀFukushima,ꢀNat.ꢀCommun.,ꢀ2016,ꢀ7,ꢀ12704.ꢀ
(a)ꢀ A.ꢀ J.ꢀ Leusink,ꢀ W.ꢀ Drenth,ꢀ J.ꢀ G.ꢀ Noltesꢀ andꢀ G.ꢀ J.ꢀ M.ꢀ vanꢀ derꢀ Kerk,ꢀ
Tetrahedronꢀ Lett.,ꢀ1967,ꢀ8,ꢀ1263;ꢀ(b)ꢀY.ꢀSugihara,ꢀR.ꢀMiyatake,ꢀT.ꢀYagi,ꢀI.ꢀ
Murata,ꢀM.ꢀJinguji,ꢀT.ꢀNakazawaꢀandꢀA.ꢀImamura,ꢀTetrahedron,ꢀ1994,ꢀ50,ꢀ
6
495;ꢀ(c)ꢀY.ꢀSugihara,ꢀR.ꢀMiyatake.ꢀI.ꢀMurataꢀandꢀA.ꢀImamura,ꢀJ.ꢀChem.ꢀ
Soc.,ꢀ Chem.ꢀ Commum.,ꢀ 1995,ꢀ 1249;ꢀ (d)ꢀ H.ꢀ Sashidaꢀ andꢀ A.ꢀ Kuroda,ꢀ J.ꢀ
Chem.ꢀSoc.,ꢀPerkinꢀTrans.ꢀ1,ꢀ2000,ꢀ1965.ꢀ
L.ꢀG.ꢀMercier,ꢀW.ꢀE.ꢀPiersꢀandꢀM.ꢀParvez,ꢀAngew.ꢀChem.,ꢀInt.ꢀEd.,ꢀ2009,ꢀ
4
00
450
500
550
600
650
700
750
Wavelength/nm
ꢀ
6
7
4
8,ꢀ6108.ꢀ
Fig.ꢀ 5ꢀ Fluorescenceꢀspectraꢀofꢀ9ꢀinꢀcyclohexaneꢀ(darkꢀblue),ꢀtolueneꢀ(lightꢀblue),ꢀTHFꢀ
green),ꢀacetoneꢀ(yellow),ꢀandꢀacetonitrileꢀ(orange).ꢀ
(a)ꢀA.ꢀJr.ꢀCaruso,ꢀM.ꢀA.ꢀSieglerꢀandꢀJ.ꢀD.ꢀTovar,ꢀAngew.ꢀChem.,ꢀInt.ꢀEd.,ꢀ
(
2
3
010,ꢀ 49,ꢀ 4213;ꢀ (b)ꢀ A.ꢀ Jr.ꢀ Carusoꢀ andꢀ J.ꢀ D.ꢀ Tovar,ꢀ Org.ꢀ Lett.,ꢀ 2011,ꢀ 13,ꢀ
106;ꢀ(c)ꢀD.ꢀR.ꢀLevine,ꢀA.ꢀJr.ꢀCaruso,ꢀM.ꢀA.ꢀSieglerꢀandꢀJ.ꢀD.ꢀTovar,ꢀChem.ꢀ
Commun.,ꢀ2012,ꢀ48,ꢀ6256.ꢀꢀ
(a)ꢀA.ꢀJr.ꢀCarusoꢀandꢀJ.ꢀD.ꢀTovar,ꢀJ.ꢀOrg.ꢀChem.ꢀ2011,ꢀ76,ꢀ2227;ꢀ(b)ꢀD.ꢀR.ꢀ
Levine,ꢀM.ꢀA.ꢀSieglerꢀandꢀJ.ꢀD.ꢀTovar,ꢀJ.ꢀAm.ꢀChem.ꢀSoc.,ꢀ2014,ꢀ136,ꢀ7132;ꢀ
measurementsꢀ ofꢀ
fluorescenceꢀ lifetimesꢀ ofꢀ
determinedꢀtoꢀbeꢀ2.5ꢀandꢀ5.7ꢀns,ꢀrespectively,ꢀbasedꢀonꢀwhichꢀtheꢀk
andꢀknrꢀvaluesꢀwereꢀdetermined.ꢀWhileꢀtheꢀk ꢀvalueꢀdecreasesꢀfromꢀ
9
ꢀ inꢀ variousꢀ solventsꢀ wereꢀ conductedꢀ andꢀ theꢀ
8
9ꢀ inꢀ cyclohexaneꢀ andꢀ acetonitrileꢀ wereꢀ
r
ꢀ
(
c)ꢀR.ꢀE.ꢀMessersmith,ꢀM.ꢀA.ꢀSieglerꢀandꢀJ.ꢀD.ꢀTovar,ꢀJ.ꢀOrg.ꢀChem.,ꢀ2016,ꢀ
r
81,ꢀ 5595;ꢀ (d)ꢀ D.ꢀ R.ꢀ Levine,ꢀ R.ꢀ E.ꢀ Messersmith,ꢀ M.ꢀ A.ꢀ Sieglerꢀ andꢀ J.ꢀ D.ꢀ
Tovar,ꢀCan.ꢀJ.ꢀChem.,ꢀ2017,ꢀ95,ꢀ381;ꢀ(e)ꢀR.ꢀE.ꢀMessersmith,ꢀS.ꢀYadav,ꢀM.ꢀ
A.ꢀSiegler,ꢀH.ꢀOttosson,ꢀandꢀJ.ꢀD.ꢀTovar,ꢀJ.ꢀOrg.ꢀChem.,ꢀ2017,ꢀ82,ꢀ13440.ꢀ
Z.ꢀZhou,ꢀA.ꢀWakamiya,ꢀT.ꢀKushidaꢀandꢀS.ꢀYamaguchi,ꢀJ.ꢀAm.ꢀChem.ꢀSoc.,ꢀ
8
–1
8
–1
cyclohexaneꢀ (3.9ꢀ×ꢀ10 ꢀs )ꢀtoꢀacetonitrileꢀ(1.6ꢀ×ꢀ10 ꢀs ),ꢀ theꢀknr
–
ꢀ
7
valueꢀalsoꢀdecreasesꢀinꢀtheꢀpolarꢀsolventꢀ(cyclohexane:ꢀ2.2ꢀ×ꢀ10 ꢀs
1
7
–1
9
1
;
ꢀ acetonitrile:ꢀ 1.6ꢀ ×ꢀ 10 ꢀ s ).ꢀ Theꢀ k
magnitudeꢀlargerꢀthanꢀtheꢀknrꢀvalueꢀinꢀeachꢀsolvent,ꢀwhichꢀresultsꢀinꢀ
highꢀ ꢀvalues.ꢀ
r
ꢀ valueꢀ retainsꢀ one-orderꢀ
2
012,ꢀ134,ꢀ4529.ꢀ
0
(a)ꢀS.ꢀSaito,ꢀK.ꢀMatsuoꢀandꢀS.ꢀYamaguchi,ꢀJ.ꢀAm.ꢀChem.ꢀSoc.,ꢀ2012,ꢀ134,ꢀ
9130;ꢀ(b)ꢀC.ꢀDou,ꢀS.ꢀSaito,ꢀK.ꢀMatsuo,ꢀI.ꢀHisakiꢀandꢀS.ꢀYamaguchi,ꢀAngew.ꢀ
Chem.,ꢀInt.ꢀEd.,ꢀ2012,ꢀ51,ꢀ12206;ꢀ(c)ꢀA.ꢀShuto,ꢀT.ꢀKushida,ꢀT.ꢀFukushima,ꢀ
H.ꢀKajiꢀandꢀS.ꢀYamaguchi,ꢀOrg.ꢀLett.,ꢀ2013,ꢀ15,ꢀ6234;ꢀ(d)ꢀT.ꢀ Kushida,ꢀ C.ꢀ
Camacho,ꢀ A.ꢀ Shuto,ꢀ S.ꢀ Irle,ꢀ M.ꢀ Muramatsu,ꢀ T.ꢀ Katayama,ꢀ S.ꢀ Ito,ꢀ Y.ꢀ
Nagasawa,ꢀH.ꢀMiyasaka,ꢀE.ꢀSakuda,ꢀN.ꢀKitamura,ꢀZ.ꢀZhou,ꢀA.ꢀWakamiyaꢀ
andꢀS.ꢀYamaguchi,ꢀChem.ꢀSci.,ꢀ2014,ꢀ5,ꢀ1296;ꢀ(e)ꢀK.ꢀMatsuo,ꢀS.ꢀSaitoꢀandꢀ
S.ꢀ Yamaguchi,ꢀJ.ꢀAm.ꢀChem.ꢀSoc.,ꢀ2014,ꢀ136,ꢀ12580;ꢀ(f)ꢀ T.ꢀ Kushida,ꢀ A.ꢀ
Shuto,ꢀ M.ꢀ Yohsio,ꢀ T.ꢀ Katoꢀ andꢀ S.ꢀ Yamaguchi,ꢀ Angew.ꢀ Chem.,ꢀ Int.ꢀ Ed.,ꢀ
F
F
Inꢀsummary,ꢀweꢀhaveꢀsynthesizedꢀaꢀplanarizedꢀB-phenyldibenzo-
b,f]borepin,ꢀ whoseꢀ dibenzoborepinꢀ scaffoldꢀ isꢀ endowedꢀ withꢀ
[
desirableꢀstabilityꢀonꢀaccountꢀofꢀtheꢀstructuralꢀconstraintꢀimposedꢀ
byꢀtwoꢀmethyleneꢀbridges.ꢀItꢀshouldꢀbeꢀnotedꢀthatꢀtheꢀframeworkꢀofꢀ
2
ꢀisꢀrelativelyꢀflexibleꢀdespiteꢀitsꢀconstrainedꢀstructure,ꢀwhichꢀallowsꢀ
ꢀ toꢀ retainꢀ sufficientꢀ Lewisꢀ acidityꢀ toꢀ formꢀ Lewisꢀ acid–baseꢀ
2
2
015,ꢀ54,ꢀ6922;ꢀ(g)ꢀS.ꢀOsumi,ꢀS.ꢀSaito,ꢀC.ꢀDou,ꢀK.ꢀMatsuo,ꢀK.ꢀKume,ꢀH.ꢀ
complexesꢀ evenꢀ withꢀ pyridineꢀ derivatives.ꢀ Theꢀ planarizationꢀ alsoꢀ
enablesꢀ theꢀ extensionꢀ ofꢀ theꢀ π-conjugationꢀ acrossꢀ theꢀ B-phenylꢀ
group,ꢀ renderingꢀ thisꢀ scaffoldꢀ aꢀ usefulꢀ electron-acceptingꢀ buildingꢀ
block.ꢀ Indeed,ꢀ theꢀ combinationꢀ ofꢀ theꢀ planarizedꢀ borepinꢀ withꢀ anꢀ
electron-donatingꢀ 4-(N,N-diphenylamino)phenylꢀ groupꢀ affordedꢀ aꢀ
highlyꢀ fluorescentꢀ compoundꢀ thatꢀ exhibitsꢀ largeꢀ solvatochromismꢀ
andꢀ highꢀ fluorescenceꢀ quantumꢀ yieldsꢀ irrespectiveꢀ ofꢀ theꢀ solventꢀ
polarity.ꢀ Theseꢀ resultsꢀ provideꢀ importantꢀ informationꢀ forꢀ futureꢀ
designꢀstrategiesꢀtowardꢀtriarylborane-basedꢀπ-electronꢀmaterials.ꢀ
ThisꢀworkꢀwasꢀsupportedꢀbyꢀJSPSꢀKAKENHIꢀgrantsꢀ24109007ꢀandꢀ
Yoshikawa,ꢀK.ꢀAwagaꢀandꢀS.ꢀYamaguchi,ꢀ Chem.ꢀSci.,ꢀ2016,ꢀ7,ꢀ219;ꢀ(h)ꢀK.ꢀ
Matsuo,ꢀS.ꢀSaitoꢀandꢀS.ꢀYamaguchi,ꢀAngew.ꢀChem.,ꢀInt.ꢀEd.,ꢀ2016,ꢀ55,ꢀ
1
M.ꢀ Yamaguchi,ꢀ T.ꢀ Uemura,ꢀ J.ꢀ Tsurumi,ꢀ S.ꢀ Watanabe,ꢀ J.ꢀ Takeyaꢀ andꢀ S.ꢀ
Yamaguchi,ꢀJ.ꢀAm.ꢀChem.ꢀSoc.,ꢀ2017,ꢀ139,ꢀ14336.ꢀ
Compoundꢀ1bꢀhasꢀbeenꢀoriginallyꢀreportedꢀinꢀreferenceꢀ6a.ꢀ
(a)ꢀ T.ꢀ Kushida,ꢀ Z.ꢀ Zhou,ꢀ A.ꢀ Wakamiyaꢀ andꢀ S.ꢀ Yamaguchi,ꢀ Chem.ꢀ
Commun.,ꢀ2012,ꢀ48,ꢀ10715;ꢀ(b)ꢀH.ꢀIwahara,ꢀT.ꢀKushidaꢀandꢀS.ꢀYamaguchi,ꢀ
Chem.ꢀCommun.,ꢀ2015,ꢀ52,ꢀ1124.ꢀ
1984;ꢀ(i)ꢀT.ꢀKushida,ꢀS.ꢀShirai,ꢀN.ꢀAndo,ꢀT.ꢀOkamoto,ꢀH.ꢀIshii,ꢀH.ꢀMatsui,ꢀ
1
1
1
2
13 TheꢀstructureꢀofꢀtheꢀLewisꢀacid–baseꢀcomplexꢀofꢀ2ꢀwasꢀunambiguouslyꢀ
determinedꢀ byꢀ X-rayꢀ crystallographicꢀ analysisꢀ usingꢀ 4-N,N-
dimethylaminopyridineꢀasꢀaꢀLewisꢀbaseꢀ(Fig.ꢀS2).ꢀ
15H02163.ꢀN.A.ꢀthanksꢀtheꢀJSPSꢀforꢀaꢀResearchꢀFellowshipꢀforꢀYoungꢀ
Scientists.ꢀ
1
1
4
5
S.ꢀSole ́ꢀ andꢀF.ꢀP.ꢀGabbaï,ꢀChem.ꢀCommun.ꢀ2004,ꢀ1284..ꢀ
Crystalꢀ packingꢀ ofꢀ 9ꢀ revealsꢀ anꢀ interestingꢀ 1Dꢀ alignmentꢀ (Fig.ꢀ S3b),ꢀ
whereꢀtheꢀplanarizationꢀofꢀB-phenylꢀgroupꢀshouldꢀplayꢀaꢀcrucialꢀrole.ꢀ
(a)ꢀJ.ꢀC.ꢀDoty,ꢀB.ꢀBabb,ꢀP.ꢀJ.ꢀGrisdale,ꢀM.ꢀGlogowskiꢀandꢀJ.ꢀL.ꢀR.ꢀWilliams,ꢀJ.ꢀ
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R.ꢀ Wortmannꢀ andꢀ R.ꢀ Jakober,ꢀ Chem.ꢀ –Eur.ꢀ J.,2006,ꢀ 12,ꢀ 2358;ꢀ (c)ꢀ Z.ꢀ
Yuan,ꢀC.ꢀD.ꢀEntwistle,ꢀJ.ꢀC.ꢀCollings,ꢀD.ꢀAlbesa-Jové,ꢀA.ꢀS.ꢀBatsanov,ꢀJ.ꢀA.ꢀK.ꢀ
Howard,ꢀ N.ꢀ J.ꢀ Taylor,ꢀ H.ꢀ M.ꢀ Kaiser,ꢀ D.ꢀ E.ꢀ Kaufmann,ꢀ S.-Y.ꢀ Poon,ꢀ W.-Y.ꢀ
Wong,ꢀC.ꢀJardin,ꢀS.ꢀFathallah,ꢀA.ꢀBoucekkine,ꢀJ.-F.ꢀHaletꢀandꢀT.ꢀB.ꢀMarder,ꢀ
Chem.ꢀ –Eur.ꢀ J.,ꢀ 2006,ꢀ 12,ꢀ 2758;ꢀ (d)ꢀ X.ꢀ Y.ꢀ Liu,ꢀ D.ꢀ R.ꢀ Baiꢀ andꢀ S.ꢀ Wang,ꢀ
Angew.ꢀ Chem.,ꢀ Int.ꢀ Ed.,ꢀ 2006,ꢀ 45,ꢀ 5475;ꢀ (e)ꢀ G.ꢀ Zhou,ꢀ M.ꢀ Baumgartenꢀ
andꢀK.ꢀMüllen,ꢀJ.ꢀAm.ꢀChem.ꢀSoc.,ꢀ2008,ꢀ130,ꢀ12477;ꢀ(f)ꢀH.ꢀPan,ꢀG.-L.ꢀFu,ꢀ
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Marder,ꢀChem.ꢀ–Eur.ꢀJ.,ꢀ2015,ꢀ21,ꢀ177.ꢀ
1
6
Conflictsꢀofꢀinterestꢀ
Thereꢀareꢀnoꢀconflictsꢀtoꢀdeclare.ꢀ
Notesꢀandꢀreferencesꢀ
1
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4
ꢀ|ꢀJ.ꢀName.,ꢀ2012,ꢀ00,ꢀ1-3ꢀ
Thisꢀjournalꢀisꢀ©ꢀTheꢀRoyalꢀSocietyꢀofꢀChemistryꢀ20xxꢀ
Pleaseꢀdoꢀnotꢀadjustꢀmarginsꢀ

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DOI: 10.1039/C8CC02837K
Table of contents
Planarization of B-phenyldibenzo[b,f]borepin allows π-expansion across the B-phenyl
group and makes this scaffold a useful electron-accepting unit for donor-π-acceptor type
fluorophores.