The QM optimised structures of PTZ-DBTO2 in the S 0 and S 1 states are shown in Dihedral angle (°) QUBEKit and TorsionDrive, the full scan shown in Figure is in reasonably good agreement with QM. In contrast, the S 1 state has only one well-defined minimum at around 90 • , which corresponds to the equatorial conformation (Figure ). More importantly, the improper torsion C(7)-N(5)-C(2)-C(8) remains close to 180 • during the scan and, hence, the potential energy surface obtained from the QUBE scan for S 1 is in good agreement with QM.