Triplet-state characteristics and photoionization behavior of 2,2'- and 4,4'-biphenyldiol studied by 248 nm laser flash photolysis in aqueous solutions

Abstract

Triplet-state characteristics and the photoionization behavior of 2,2'- and 4,4'-biphenyldiol have been investigated in aqueous solutions using the 248 nm nanosecond laser flash photolysis (LFP) technique. For 2,2‘-biphenyldiol, the neutral form of the triplet state is mainly produced following the laser excitation in aqueous solutions for the pH conditions in which the ground state of the diol exists in its neutral form (pK_a^G = 7.5). In aqueous alkaline solutions, in which the diol exists in its monoanionic form, photoexcitation leads to the formation of the anionic form of the triplet state along with phenoxyl radical and hydrated electron (e_aq^-). Under similar conditions, the 4,4‘-biphenyldiol behaves somewhat differently. Unlike 2,2‘-biphenyldiol, the LFP of its 4,4‘-analogue not only produces the neutral form of the triplet state but also gives phenoxyl radical, even when the ground state of the diol exists in its neutral form in solution (pK_a^G = 9.4). In alkaline solutions (pH ≥ 11), though, like its 2,2‘-analogue, the 4,4‘-biphenyldiol also produces the anionic form of its triplet state along with the phenoxyl radical and e_aq^-, the photoionization (PI) yield is higher for 4,4‘-biphenyldiol. Further, it is interesting to observe that the mechanism of PI in the two diols is different, monophotonic for the 4,4‘^-analogue and biphotonic for the 2,2‘-analogue. The differences in the above results on the two diols have been explained on the basis of the presence and absence of intramolecular hydrogen bonding in the diol molecules.