Identifying the Lowest Excited Singlet State of Biphenyl and Its Analogs

Abstract

From experimental evidence it is inferred that the broad structureless absorption band of biphenyl at about 2500 Å is composed of bands from three transitions, a weak transition similar to the A1 → 1Lb transition of benzene and two stronger transitions similar to the A1A → 1La and A1 → 1B transitions of benzene. Bands from these transitions are separated in rigid biphenyl analogs such as 9,10-dihydrophenanthrene and fluorene. Certain fluorescence characteristics of biphenyl (such as decay time τ and quantum yield Q) are distinctive of the weak and “hidden” 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 the para position (such as a phenyl or vinyl group in p-terphenyl or 4-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 degenerate A1 → 1B transition in benzene. Since the intense absorption bands are maximally shifted by substituents on the para position, the transition moment must be long-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 ratio τ / Q, the assignment of the lowest excited singlet state can be determined: In fluorene, dibenzofuran, and 9,10-dihydrophenanthrene, it is La1; in carbazole and phenanthrene, Lb1; and in 2-phenylfluorene and 2-phenyl-9,2′-methylenefluorene, Bb1.