Collective proton motion in the intramolecular hydrogen bonding network and the consequent enhancement in the acidity of hydroxycalixarenes

Interactions of two hydroxycalixarenes (HOCXs) and their alternate methoxy derivatives (MOCXs) with different proton acceptors (PA) like urea (U; pKb = 13.9), N-methylurea (MU; pKb = 13.1), and triethylamine (TEA; pKb = 3.19) have been investigated using absorption and fluorescence measurements, to understand the role of intramolecular hydrogen bonding on the acidity of HOCXs and MOCXs. Both HOCXs and MOCXs show no interaction in their ground states with weak PAs like U and MU. In the excited (S1) state, HOCXs interact, though moderately, with U and MU, involving hydrogen-bonded HOCX–U/MU exciplex formation. Excited MOCXs, however, do not show any interaction with U and MU. With TEA, a strong PA, the HOCXs show very strong interaction even in the ground state, though no interaction is observed for MOCXs. In the excited (S1) state, the interaction in the HOCX–TEA system could not be resolved due to large static quenching arising via HOCX–TEA ground state complex formation. In the excited (S1) state, MOCXs interact reasonably strongly with TEA, via the formation of hydrogen-bonded exciplexes. All these results are apparently in accordance with the pKa values of the HOCXs and MOCXs, as estimated or judged from the pH dependent spectrophotometric measurements. Supportive evidence for the hydrogen bonded complex formation in these systems has been obtained from quantum chemical calculations. It is indicated from the present results that even though the phenolic OH groups in both HOCXs and MOCXs are intramolecularly hydrogen bonded, and thus expected to be less labile for deprotonation under normal circumstances than in normal phenol, the HOCXs show unusually higher acidity than expected. It is inferred that a collective proton motion in the intramolecular hydrogen bonding network in HOCXs causes an enhanced polarizability of the phenolic protons and consecutively the acidity of these molecules. In MOCXs, since no such collective proton motion is possible, they behave quite normally and display much weaker acidity than HOCXs and normal phenol.