Evaluating photodimerization of 6-methylcoumarin mediated by cucurbit[8]uril through mechanical grinding – Supramolecular effects of additives

Abstract

Abstract 6-Methylcoumarin (1) upon mechanical grinding with cucurbit[8]uril (CB[8]) or other additives (e.g. thiourea) showed enhanced photodimerization efficiency with exclusive formation of head-to-head photodimer. Mechanical grinding of 1 with CB[8] resulted in host–guest interaction with red-shift fluorescence in the solid-state. This red shift in fluorescence emission is similar to the red-shifted emission observed for CB[8]–1 host–guest complex in solution at 298 K. Phosphorescence emission profile in the solid state at 77 K for mechanically ground 1 with CB[8] was similar to the phosphorescence profile observed for CB[8]–1 host–guest complex at 77 K. Single crystal X-ray determination of 1 from water reveals that the coumarins are oriented in a head-to-head fashion with the distance between the double bonds being 4.695 A, a distance that is greater than the required Schmidt distance of 4.2 A for efficient photodimerization in the crystalline state. Enhancement of photoreactivity of 1 upon mechanical grinding with additives appears to be the result of a combination of factors, viz.: (a) crystalline imperfection/defects that are likely formed during mechanical grinding and/or; (b) formation of a new phase due to mechanical grinding (we were not able to observe any noticeable change in crystallinity, while the formation of an amorphous phase could not be ruled out completely).