As for the excited states of DMAC-TRZ, S1 (with a strong 1 CT character) retains an orthogonal geometry, while T1 ( 3 CT) undergoes a large conformational deformation where the dihedral becomes ~60 °. At this angle, ΔEST increases as does SOC. A rigid scan of the dihedral angle of DMAC-TRZ (Figure ) is informative. Specifically, starting from the optimized ground-state geometry, we calculated the ground and excited state energies upon gradual rotation of the DMAC unit about the phenylene bridge without allowing for any additional molecular relaxation (the dihedral angle for the scan is defined in Figure ). The resulting S0, S1, T2 and T3 potential energy surfaces (PES) all show a flat minimum for the orthogonal geometry, while T1 shows a double minimum around (90±30) ° angle. The molecular orbitals (MOs) and natural transition orbitals (NTOs)