A visible-light-driven molecular motor based on barbituric acid

We present a class of visible-light-driven molecular motors based on barbituric acid. Due to a serendipitous reactivity we observed during their synthesis, these motors possess a tertiary stereogenic centre on the upper half, characterised by a hydroxy group. Using a combination of femto- and nanosecond transient absorption spectroscopy, molecular dynamics simulations and low-temperature H-1 NMR experiments we found that these motors operate similarly to push-pull second-generation overcrowded alkene-based molecular motors. Interestingly, the hydroxy group at the stereocentre enables a hydrogen bond with the carbonyl groups of the barbituric acid lower half, which drives a sub-picosecond excited-state isomerisation, as observed spectroscopically. Computational simulations predict an excited state lasso mechanism where the intramolecular hydrogen bond pulls the molecule towards the formation of the metastable state, with a high predicted quantum yield of isomerisation (68%) in gas phase.

Automated summary

We introduce a new class of visible-light-driven molecular motors based on barbituric acid, which possess a tertiary stereogenic center with a hydroxy group due to an unexpected reactivity during synthesis. Experimental and computational studies reveal that these motors operate similarly to existing overcrowded alkene-based molecular motors, with a sub-picosecond excited-state isomerization facilitated by a hydrogen bond between the hydroxy group and the carbonyl groups. Computational simulations predict a high quantum yield of isomerization (68%) through an excited state lasso mechanism.