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.