A separate single point calculation on the axial conformation of PTZ-DBTO2 in the S 0 state (Figure ) gives an absorption energy of 4.26 eV, suggesting that the absorption energy has a strong dependence on the angle between the donor and the acceptor. We note that our S 1 structure is optimised with pure TD-DFT, in contrast to TD-DFT+TDA in previous work. However, the computed emission energies are the same, which suggests that there is little difference between the geometries optimised using the two methods. Moreover, the vertical excitation energies calculated from pure TD-DFT are only a few meV lower than the TDA values for all solvents (Table ), and so we focus only on the TD-DFT+TDA data here.