New J. Chem. 2009, 33, 1320-1323(3)

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

Fig.3Frontierorbitalrepresentationsoftheopenandclosedformsofthepyridinefragmentsof1.

1(o,o)inCH2Cl2at313nmdidnottriggerthephoto-cyclizationprocess,whileastrongemissionat640nmwasstillobserved.Theexcitationspectrumregisteredatthiswavelengthcloselymatchedtheabsorptionspectrum,con rmingthatabsorbedlightatanywavelengthise cientlytransferredtotheICTstate.Relaxationfromthe1IL(DTE)statetotheemissiveICTstateleadstotheinhibitionoftheDTE-basedringclosurereaction.Thisbehaviorisdrivenbythelower-lyingCTexcitedstate,whichissigni cantlystabi-lizedinCH2Cl2,whereasinlesspolarsolventslikecyclo-hexane,the uorescence(Ff=0.22,lexc=313or430nm)andphotoisomerizationprocessesappeartobecompetitive.Uponconversiontothephotostationarystate(PSS)byirradiationofacyclohexanesolutionof1(o,o)at430nm,the‘‘apparent’’ uorescencequantumyielddecreasesto0.03(Fig.4).Theresidualemissionisattributedtotheremainingopenform1(o,o)presentinthePSS.Thedecreasein uores-cenceintensityisphotocontrolledby(i)thedisruptionoftheD–p–Aconjugationand(ii)anintramolecularenergytransferoftheexcitedopenpyridinefragmenttotheacceptorclosed-ringDTEpyridinemoietyof1(c,o).

Inconclusion,wehavesynthesizedanoriginal uorescentphotochromicbipyridinebyfunctionalizingonethiopheneringofaDTEfragment.Thisnovelchromophorewillopenupnewperspectivesfortheelaborationofmetal-basedphoto-switches.

ThisworkwassupportedbyTheAgenceNationaldelaRecherche(ANRBlan08-1-308898),COSTD035-0010-05andgionBretagne(SIE211-B3-11).F.I.thankstheUniversiteRe

desSciencesetdelaTechnologieHouariBoumedienne(Alger)forasabbaticalleave.

Experimental

Preparationof6

Toasolutionof3(192mg,0.5mmol)in15mLofTHFthathadbeencooledtoÀ781Cwasadded,dropwise,n-BuLi(2.16Minhexane,0.25mL,0.54mmol).AfterstirringatÀ781Cfor1h,asolutionof5(196mg,0.54mmol)in10mLofTHFwasaddedtothereactionmixture.AfterstirringatÀ781Cfor1handatroomtemperaturefor16h,thereactionmixturewashydrolyzedwithwater(25mL),andthesolventremovedinvacuo.TheresiduewasextractedwithCH2Cl2(2Â30mL)andthendriedoverMgSO4.Afterevaporationofthesolvent,theresidualorangeoilwasdissolvedin20mLofTHF,thenPTSA(p-toluenesulfonicacid)(20mg,0.1mmol),and nallyafewdropsofwaterwereadded.Afterstirringat401Cfor16h,thesolventwasremovedandtheoilpuri edbycolumnchromatography(SiO2,pentane–CH2Cl21:1)togivearedpowder(179mg,60%).1HNMR(500MHz,CDCl3):d9.40(s,1H,CHO),7.55(d,3J=8.9Hz,2H,C6H4),7.28(s,1H,thio),7.10(s,1H,thio),6.74(d,3J=8.9Hz,2H,C6H4),3.07(s,6H,NMe2),2.08(s,3H,Me)and0.30(s,9H,TMS).13C[1H]NMR(125MHz,CDCl3):d180.5,155.6,151.5,147.4,140.1,137.2,135.7,133.7,127.5,124.7,121.7,119.7,112.1,40.2,30.3,14.4andÀ0.3.Anal.calc.forC26H25F6N1S2SiÁ0.5CH2Cl2:C,54.46;H,4.44;N,

2.33.

Fig.4Emissionspectraof1at298Kin

cyclohexane.

attributedtotheclosedDTEunit,alongwithanotherbandat414nm.AsshowninFig.3,theHOMOandLUMOoftheclosedpyridinefragmentaremainlylocalizedontheDTEunit,whereastheHOMOÀ1andLUMO+1arelocalizedonthedimethylaminophenylandvinylpyridinegroups,respectively.Thecalculatedelectronicspectrum(seeESIw)isingoodagreementwiththeexperimentalspectrum,showingthatthetwobandsessentiallycorrespondtotheHOMO-LUMOandHOMOÀ1-LUMOexcitations,respectively.Irradiationoftheabsorptionbandoftheclosedformat588nmresultedinthequantitativeregenerationoftheopenisomer.Thecyclizationandcycloreversionquantumyieldsweredeterminedtobe0.081irradiatingat436nmand0.016irradiatingat588nm,respectively.Strikingly,incontrasttoitsbehaviourincyclohexane,theirradiationof

1322|NewJ.Chem.,2009,33,1320–1323

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