New J. Chem. 2009, 33, 1320-1323(2)

Scheme2Thephotochromicreactionof1

.

Fig.2UV-visabsorptionspectraof1incyclohexanesolutionin(3.4Â10À5M):open-ringisomer(--)andphotostationarystate(TT).

Downloaded by Jiangxi Science and Technology Normal University on 09 January 2013Published on 12 February 2009 on | doi:10.1039/B823113C

Table1Absorptionandemissiondataof1Solvent

Ff(t/ns)labs/nm(e/MÀ1cmÀ1)labs/nmlem/nmb

OpenformOpenformOpenformPSSa

304,

414,584——

b

Cyclohexane326(37400),

433(37000)CH2Cl2EPAc

Scheme3

Thesynthesisof1(o,o)

.

a

535640664

0.220.38(4.2)—(3.7)

333446330440(37700),(36500)(34000),(32000)

Afterirradiationat313nm.pentane/isopropanol.

lexc=313or430nm.cEPA=ether/

groupwasintroducedonto3,5-dibromothiophene-2-dimethylacetal2throughaSuzukicross-couplingreactiontogive3.Thesecondthienylderivative,4,bearingaprotectingtrimethylsilylgroup,wasreactedwithocta uorocyclopentene(C5F8)usingacommonproceduretoyield5.Thienylderivative3wastreatedwithn-BuLi,andthencoupledwith5toa ord6.Eventually,thereactionof6with7inthepresenceoft-BuOKinre uxingTHFa orded1(o,o)asitsE,Eisomerina50%yield.

Singlecrystalsof1(o,o)weregrownuponslowevaporationofaCH2Cl2solution.TheX-raystructure(Fig.1)revealsanantiparallelconformation,inwhichthevinylbipyridineatthe2-positionofonethiopheneringandthemethylgroupatthe20-positionarepointinginoppositedirections.Inaddition,the

)distancebetweenthetworeactivecarbonatoms(C–C3.587A

7a,8

isappropriateforacyclizationprocess.ThegeometryoptimizationperformedusingGaussian03wasingoodagree-mentwiththeexperimentalstructure(seeESI,Fig.S1w).

TheUV-visabsorptionspectrumof1(o,o)incyclohexane(Fig.2,Table1)displaysanintensebandatl=326nm

Fig.1AnORTEPplotfor1(o,o)drawnwiththermalellipsoidsatthe50%probabilitylevel.Hydrogenatomsareomittedforclarity.Themoleculeliesaboutaninversion

centre.

Thisjournalis

c

attributedtothe1IL(p-p*)transitionoftheDTEunit.In

addition,1(o,o)alsoshowsanotherbroadbandatl=435nmthatcanbeascribedtoanintramolecularchargetransfer(ICT)transition,whichcompareswellwiththatofrelated4,40-bis(dibutylaminothienylvinyl)-2,20-bipyridine(lmax=443nminCH2Cl2).9Theassignmentsofthesetwobandswerealsosupportedbytheoreticalcalculations.ThecontourplotsdepictedinFig.3showthattheHOMOispredominantlylocalizedonthedimethylaminophenylthiophene,whiletheLUMOisdelocalizedoverthevinylpyridineandtheC5F6fragments,andtheLUMO+1mainlycorrespondstothep*orbitaloftheC5F6fragment.Inaddition,theelectronicabsorptionspectrumcalculatedbyTD-DFTnicelymatchesthatobservedexperimentally,indicatingthatthetwolowesttransitionsaredominatedbyexcitationsfromtheHOMOtotheLUMOandtheLUMO+1,respectively(seeESIw).Compound1(o,o)displaysstrong uorescenceinsolutionat298K(lem=535nmincyclohexane)whenexcitedat430or313nm.Itisinterestingtonotethattheemissionquantumyield(Ff=0.22)isindependentoftheexcitationwavelength.Increasingthepolarityofthesolventleadstoaverypronouncedredshiftoftheemissionband(lem(CH2Cl2)=640nm;lem(EPA)=664nm),consistentwithcharge-transfercharacterforthe uorescentsingletexcitedstate.(Fig.4,Table1).

Thephotochromicbehaviourof1(o,o)wasfollowedby1HNMRandUV-visibleabsorptionspectroscopyincyclohexaneandCH2Cl2solutions.Thephotoisomerizationwasfoundtobehighlysolvent-dependent.Incyclohexane,1(o,o)undergoesaphotocyclizationprocessuponirradiationineitherofthetwomainabsorptionbandsatl=313or436nm.Onthebasisof1HNMRspectroscopyind12-cyclohexane(seeESIw),thephotoproductwasassignedasclosed-ring1(c,o),inwhichoneofthetwoDTEunitsisinitsclosedform(80%conversion).10Fig.2displaysacharacteristicabsorptionbandat584

nm

NewJ.Chem.,2009,33,1320–1323|1321

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