Femtosecond to nanosecond dynamics of 2,2′-bipyridine-3,3′-diol inside the nano-cavities of molecular containers

Abstract

Femtosecond fluorescence upconversion measurements are employed to elucidate the mechanism of ultrafast double proton transfer dynamics of BP(OH)2 inside molecular containers (cucurbit[7]uril (CB7) and β-cyclodextrin (β-CD)). Femtosecond up-converted signals of BP(OH)2 in water consist of growth followed by a long decay component (∼650 ps). The appearance of the growth component (∼35 ps) in the up-converted signal indicates the presence of a two-step sequential proton transfer process of BP(OH)2 in water. Surprisingly, the up-converted signal of BP(OH)2 inside the CB7 nano-cavity does not exhibit any growth component characteristic of a two-step sequential process. Interestingly, the growth component exists inside the nano-cavity of β-CD (having similar cavity size as that of CB7), inferring the presence of a two-step sequential process of PT inside the β-CD nano-cavity. The different features of PT dynamics of BP(OH)2 in the above mentioned two macrocyclic hosts may be attributed to the presence and absence of water solvation network surrounding the BP(OH)2 inside the nano-cavities of β-CD and CB7, respectively. Finally, docking and DFT calculations have been employed in deciphering the molecular pictures of the interactions between BP(OH)2 and the macrocyclic host.