Ring strain release and pseudo anti-aromaticities control photochemical reactivities in photoclick reactions of solvated cyclopropenones

Gas-evolving photochemical reactions use mild conditions to access strained organic compounds irreversibly. Cyclopropenones are a class of pseudo-aromatic light-responsive molecules used in bioorthogonal photoclick reactions; their excited-state decarbonylation reaction mechanisms are misunderstood due to their ultrafast (<100 femtosecond) lifetimes. We have combined state-of-the-art multiconfigurational quantum mechanical (QM) calculations and non-adiabatic molecular dynamics (NAMD) simulations to uncover the excited-state mechanism of cyclopropene and a photoprotected cyclooctyne-(COT)-precursor in gaseous and explicit aqueous environments. We explore the role of H-bonding with unprecedented QM/QM NAMD simulations (CAS-chromophore and HF-solvent) for the aqueous decarbonylation reaction. The cyclopropenones pass through asynchronous conical intersections but have dynamically concerted photodecarbonylation mechanisms. The cyclopropenones break planarity to relieve the S1 pseudo anti-aromaticity upon photoexcitation. The substantial structural distortions relieve pseudo anti-aromaticity towards non-aromatic structures and release inherent ring strain. After crossing to S0-state, the cyclopropenones maximize pseudo-aromaticities by reverting to the reactant or the second σCC bond breaks to form CO and an alkyne. The COT-precursor has a higher quantum yield of (53%) than cyclopropenone (28%) because these trajectories prefer to directly break a σCC bond to avoid the strained trans-cyclooctene S1-minimum geometry. Our QM/QM simulations show an increased quantum yield (58%) for the systems studied here. Favorable ground-state hydrogen-bonding interactions become repulsive in the excited state due to developing positive charges on oxygen in the S1-state, resulting from the nO → 𝜋* excitation.