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

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 Å, a distance that is greater than the required Schmidt distance of 4.2 Å 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).

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► The photochemistry and photophysics of 6-methylcomarin 1 are influenced by mechanical grinding with cucurbit[8]uril.
► Bathochromic shift was observed in the room temperature fluorescence spectra of 1 upon mechanical grinding with CB[8].
► A red shift in the phosphorescence spectra of 1 was observed at 77 K upon mechanical grinding with CB[8].
► 6-Methylcoumarin 1 is not photoreactive in the solid-state but the photodimerization efficiency was enhanced upon mechanical grinding of 1 leading to the formation of head-to-head photoproduct.