Identifying the Lowest Excited Singlet State of Biphenyl and Its Analogs

摘要

From experimental evidence it is inferred that the broad structureless absorption band of biphenyl at about 2500 Aring; is composed of bands fromthreetransitions, a weak transition similar to the1Athinsp;rarr;thinsp;1Lbtransition of benzene and two stronger transitions similar to the1Athinsp;rarr;thinsp;1Laand1Athinsp;rarr;thinsp;1Btransitions of benzene. Bands from these transitions are separated in rigid biphenyl analogs such as 9,10hyphen;dihydrophenanthrene and fluorene. Certain fluorescence characteristics of biphenyl (such as decay timetgr;and quantum yieldQ) are distinctive of the weak and ldquo;hiddenrdquo; transition and a dramatic change in these characteristics is observed when a crossover of the two lowest excited states is achieved in the analogs of biphenyl. Specific substituents, when positioned in theparaposition (such as a phenyl or vinyl group inphyphen;terphenyl or 4hyphen;vinylbiphenyl, respectively), are particularly efficacious in producing a crossover of levels. In these cases, the lowest excited state is an allowed transition similar to one component of the degenerate1Athinsp;rarr;thinsp;1Btransition in benzene. Since the intense absorption bands are maximally shifted by substituents on theparaposition, the transition moment must be longhyphen;axis polarized as predicted theoretically. Bridging is particularly effective in producing a bathochromic shift because the phenyl rings can be constrained to lie in a relatively coplanar and linear configuration. The magnitude of the shift imparted to the various states depends on the bridging element. From the value of the ratiotgr;thinsp;sol;thinsp;Q, the assignment of the lowest excited singlet state can be determined: In fluorene, dibenzofuran, and 9,10hyphen;dihydrophenanthrene, it is1La; in carbazole and phenanthrene,1Lb; and in 2hyphen;phenylfluorene and 2hyphen;phenylhyphen;9,2prime;hyphen;methylenefluorene,1Bb.