Identifying the Lowest Excited Singlet State of Biphenyl and Its Analogs Available to Purchase

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, $Bb1$⁠.