Photodissociation and photoionization mechanisms of 2,2’- and 4,4’-biphenyldiols: a laser flash photolysis study

Nanosecond and picosecond laser flash photolysis (LFP) of 2,2’- and 4,4’-biphenyldiols has been investigated in aqueous acidic and alkaline solutions to understand the details of the photodissociation (PD) and photoionization (PI) mechanisms of the two diols. For 2,2’-biphenyldiol, the photodissociation and photoionization following the excitation using UV light (248 or 266 nm) is seen to be a bi-photonic process, whereas that of the 4,4’-biphenyldiol is seen to follow a monophotonic mechanism. A detailed analysis of the nanosecond and picosecond LFP results indicate that the PD/PI of 2,2’-biphenyldiol involves the participation of the triplet (T1) state of the molecule. For 4,4’-biphenyldiol, however, the PD/PI occurs from the excited singlet (S1) state. It is inferred that the difference in the PD/PI behavior of 2,2’- and 4,4’-biphenyldiols arises due to the presence and absence of intramolecular hydrogen bonding in the two respective molecules. Qualitative potential energy diagrams have been presented to rationalize how the presence and absence of intramolecular hydrogen bonding causes a difference in the stabilization of the electronic states in the two diols and that ultimately determines the possibility of the bi-photonic mono-photonic PD/PI mechanisms for 2,2’- and 4,4’-biphenyldiols, respectively.