We have used multiconfigurational quantum mechanical calculations and NAMD simulations to predict the photochemical decarbonylation of cyclopropenones with direct applications in photomedicine, bioorthogonal and photoclick chemistry. We identified that the S 0 → S 1 (n → 𝜋*) transition is the most important for both molecules, which have excitation energies of 5.13 eV (1) and 4.61 eV (2). We ran MEP calculations which suggest that 1 will preferentially go to an S 1 /S 0 crossing region (energy difference of 0.34 eV). In comparison, 2 will propagate towards a minimum on the S 1 surface (energy difference of 1.46 eV) and fluoresce. Our gas-phase NAMD simulations provide dynamical mechanistic information and predict a quantum yield for the dissociation products of 53% and 28% for 1 and 2, respectively. The time constants of the S 1 were calculated to be 72 and 89 fs for 1 and 2 in the gas phase. We conclude from these simulations that nearly all productive trajectories involve a dynamically concerted reaction mechanism with one σ CC bond breaking in the S 1 -state and the second σ CC bond breaking in the S 0 -state. The decisive mechanism and ultrafast reactivities can be traced to the reactive pseudo anti-aromaticities of the cyclopropenones in their respective FC-points. The cyclopropenones instantaneously break planarity to relieve the pseudo anti-aromaticity while in the S 1 -state, leading to the cleavage of one σ CC bond breaking. The cyclopropenones maximize S 0 pseudo-aromaticities after crossing to S 0state, where the second σ CC bond breaks. Breaking one σ CC bond in the S 1 provides a channel away from pseudo anti-aromaticity towards a non-aromatic electronic structure and relieves the inherent ring strain of 1 and 2. The fused cyclopropenone of 1 has higher QYs because these trajectories overwhelmingly avoid the FC-region leading to highly strained trans-cyclooctene, thus preferring to proceed through the productive asynchronous region of the S 1 /S 0 seam. We also report the first explicitly solvated QM/QM simulations needed to emulate the aqueous environment of the in vivo reaction. In solvated environments, we observed increased quantum yields of 58% for both 1 and 2 and S 1 time constants of 56 fs (1) and 68 fs (2).