Fig. 2. Overview of proposed deactivation mechanisms for malonaldehyde upon S2 excitation. (a) Ground and valence-excited state potential energy surfaces for malonaldehyde along geodesic interpolation paths66 connecting minima (solid circles) and MECIs (cones). The x-axis is given in mass-weighted distance (1 Å amu1/2 per tick) with indication of the dominant modes along each path. The experimental transition wavelength for S2 is 4.71 eV,94 and the adiabatic S0–S1 transition energy is 3.50 eV.92 As schematically illustrated by the thick arrows, there are two competing non-radiative decay channels from S2. ESIHT is barrierless on S2 (pathway I) while out-of-plane C C torsional motion (pathway II) may be associated with a barrier. While being higher in energy, the HTI is geometrically closer to the FC point and hence competes with the lower-lying C C twisted intersection. As indicated by the branching of pathway I, the population transferred to S1via the HTI may undergo C C twist-mediated internal conversion or intersystem crossing to T1 according to the El-Sayed rules. The parts of the surfaces dominated by ππ* (nπ*) character are marked in orange (green), as gauged by the character of the NTOs, see (d). Torsion around the C C bond leads to a gradual interchange of the electronic character of the S1 and S2 states. (b) Critical point geometries with key geometric parameters highlighted. Tw: ∠C1C2C3O2 dihedral angle, and Pyr: pyramidalization angle of C3, as defined in Fig. S7.† Critical points were computed at the SA3-XMS-CASPT2(14,12)/cc-pVDZ level of theory. (c) Evolution of the effective number of unpaired electrons, nue, for each singlet valence state along the interpolated paths. To avoid ambiguity due to arbitrary wavefunction mixing at MECIs, nue-values were computed by linear interpolation between the previous point on the path and an extrapolated point obtained by displacing along the projection of the previous displacement vector onto the branching plane. (d) NTOs for the singlet valence excitations at selected points. The corresponding NTO pair weight is listed below each pair. A single NTO pair dominates each transition across all geometries with weights ranging between 0.53 (twisted) and 0.82 (planar). Isovalue: 0.03 a.u.
