Geometry optimization and single-point computations were carried out using DFT with the ωB97X-D functional and the def2-TZVP basis set as implemented in Gaussian16. Molecules were optimized to the tight criterion with constraints so the symmetry is retained where noted. In order to construct Walsh diagrams, the torsion profiles are examined by changing the dihedral angle between end-groups of allene and spiropentadiene structures without further optimization, where the starting geometry is the optimized (symmetry-constrained) radical cation structure. MOs are plotted with an isosurface value of 0.02. The CASPT2 benchmark computations in Figure are carried out using Open-Molcas. Molecules are optimized with D 2 and C 2v symmetry constraint using the active spaces shown in ESI Figure .