Dynamics of capsuleplex formed between octaacid and organic guest molecules — Photophysical techniques reveal the opening and closing of capsuleplex

Abstract

This manuscript is concerned with the opening and closing of a capsuleplex made up of organic guest molecules and two cavitand molecules known as octaacid (OA). The capsuleplex is loosely held together in water through weak interactions. We have investigated the opening–closing of the OA capsuleplex by monitoring the quenching of excited singlet and (or) triplet states of nine different guest molecules encapsulated within a capsule by molecular oxygen that is dissolved in water surrounding the capsule. The rate constants for oxygen quenching of the excited guest molecules were estimated by monitoring the intensity of fluorescence/phosphorescence or lifetimes of excited states of guest molecules in the presence of different concentrations of oxygen in water. Guest molecules were chosen such that one could probe the opening–closing process during a time range of 0.05 to 922 µs. We believe that the oxygen quenching constant reflects the accessibility of oxygen to the guest enclosed within the OA capsuleplex, and this in turn depends on the capsule opening–closing rate constant. Based on the quenching studies, we conclude that guests whose lifetimes are shorter than 5 µs are inaccessible to oxygen. Results presented in this report suggest that the extent and time scale for opening depends on the guest.