Long-Lived Triplet State Charge Separation in Novel Piperidine-Bridged Donor-Acceptor Systems

详细信息    查看全文

• 作者：Saskia I. van Dijk ; Cornelis P. Groen ; Frantimg src="http ; //pubs.acs.org/images/entities/scaron.gif" border="0">ek Hartl ; Albert M. Brouwer ; and Jan W. Verhoeven
• 刊名：Journal of the American Chemical Society
• 出版年：1996
• 出版时间：September 4, 1996
• 年：1996
• 卷：118
• 期：35
• 页码：8425 - 8432
• 全文大小：248K
• 年卷期：v.118,no.35(September 4, 1996)
• ISSN：1520-5126

文摘

Two bichromophoric systems are presented that contain anN-alkylnaphthalimide electron acceptor and a4-methoxyaniline (3a) or an aniline (3b) electrondonor, respectively. Upon photoexcitation of 3a incyclohexaneelectron transfer occurs in the singlet manifold to afford theshort-lived (mages/gifchars/tau.gif" BORDER=0 >_f = 0.75 ns)¹(D⁺-A^-) state inca. 70%yield. An important decay pathway of thisD⁺-A^- state consists of intersystemcrossing (ISC) to yield a tripletstate localized on the naphthalimide moiety (D-³A).In a slightly more polar solvent like di-n-butyl ether,anequilibrium between D-³A and³(D⁺-A^-) is observed bymeans of transient absorption spectroscopy. Bothspeciesdecay with an overall decay time of ca. 1 mages/entities/mgr.gif">s. Thus,upon changing the spin multiplicity of theD⁺-A^- state fromsinglet to triplet, an increase of its lifetime by three orders ofmagnitude is observed. In more polar solvents likedioxane, THF, and acetonitrile the³(D⁺-A^-) state is theonly species observed in the transient absorptionspectrum,with decay times of ca. 1, 0.5, and 0.1 mages/entities/mgr.gif">s, respectively.The D-³A state is the precursor state for the³(D⁺-A^-)state in these solvents. It is proposed that, upon increasingsolvent polarity, the singlet charge-separation process isretarded as a result of the large driving force(-mages/gifchars/Delta.gif" BORDER=0 >G_Smages/entities/deg.gif"> > 1 eV), which allows the tripletpathway (D-¹A mages/entities/rarr.gif"> D-³Amages/entities/rarr.gif"> ³(D⁺-A^-)) to competeeffectively. Compound 3b possesses a somewhat weakerdonor chromophore than 3aresulting in a smaller driving force. The decay of the locallyexcited singlet state of 3b occurs mainly viachargeseparation in the singlet manifold (D-¹A mages/entities/rarr.gif">¹(D⁺-A^-)). Only inthe very polar solvent acetonitrile does the tripletpathway become competitive, and evidence is found for the formation of³(D⁺-A^-).