Figure 1. X-ray crystal structures and the energy level arrangement of the two isomers.

X-ray crystal structures of the molecules (a) 2,8-DPTZ-DBTO₂ showing an equatorial PTZ conformation and (b) 3,7-DPTZ-DBTO₂ having mixed axial and equatorial conformations of PTZ (molecular structure can also be found in Supplementary Fig. 20). (c) Energy positions of the ¹CT and ³LE states for the molecules 2,8-DPTZ-DBTO₂ (LHS) and 3,7-DPTZ-DBTO₂ (RHS). Changing from MCH to toluene for 2,8-DPTZ-DBTO₂ moves from type II–III TADF, whereas the equatorial conformer D–A pair (eq)3,7-DPTZ-DBTO₂ is type III in MCH and remains so in toluene (see Etherington et al.⁷ for notation). Vibronic coupling occurs between the CT manifold and the local triplet excitons (see Supplementary Fig. 12). For the axial (ax) conformer pair the gap is >0.5 eV and vibronic coupling is suppressed. These values have been extracted from the onset of the ¹CT emission (Fig. 2b and Supplementary Fig. 6b) and the phosphorescence of the ³LE state measured in zeonex (Fig. 6a).