Ground and excited singlet (S1) state interactions of 2,2′- and 4,4′-biphenyldiols with proton acceptors

proton-transfer interactions of 2,2 '- and 4,4 ' -biphenyldiols with urea (U), N-methylurea (MU), and triethylamine (TEA) have been investigated in methanol solutions using optical absorption as well as steady-state and time resolved fluorescence measurements. In the ground state, both 2,2 '- and 4,4 ' -biphenyldiols do not interact with weak proton accepters, Like U and MU. In the excited singlet (SI) state, only the 2,2 ' -biphenyldiol is seen to transfer a proton to U and MU, via the formation of intermolecular hydrogen-bonded exciplexes as intermediates. With TEA, a strong proton acceptor, both 2,2 '- and 4,4 ' -biphenyldiols undergo an efficient proton-transfer reaction in their Si state. In the ground-state, how ever, only 2,2 ' -analogue is seen to transfer a proton to TEA. The differences in the proton-transfer behavior of 2,2 '- and 4,4 ' -biphenyldiols with different proton accepters have been rationalized in terms of the presence and absence of intramolecular hydrogen bonding in the two diols.