The synthesis of PXZ-Nap and PTZ-Nap is outlined in Scheme S1. The molecular structures and purity of the two compounds were confirmed by a combination of 1 H & 13 C nuclear magnetic resonance spectroscopy (NMR), high-resolution mass spectrometry (HRMS), elemental analysis (EA), melting point determination and high-performance liquid chromatography (HPLC) (Figures ). We first modelled the optoelectronic properties of PXZ-Nap and PTZ-Nap in the gas phase using density functional theory (DFT) with the PBE0 functional and the 6-31G (d,p) basis set (Figure ). PXZ-Nap and PTZ-Nap possess similar lowest unoccupied molecular orbitals (LUMOs) that are localized on the Nap and highest occupied molecular orbitals (HOMOs) that are localized on the donor PXZ or PTZ, respectively (Figure ). The destabilized HOMO of PXZ-Nap indicates that in these compounds PXZ is the stronger donor group. The dihedral angles of PXZ and PTZ with Nap in PXZ-Nap and PTZ-Nap at the optimized ground state (S 0 ) geometry are 79.92° and 99.27° (Figure ), respectively, revealing compounds with weakly electronically coupled donors to the Nap. Time-dependent DFT (TD-DFT) calculations at the same level of theory reveal that the lowest singlet excited state (S 1 ) of each compound has a chargetransfer (CT) character (Figures 2b and Figure ). Although PXZ-Nap and PTZ-Nap possess similar T 1 energies, the nature of the T 1 states in these compounds is distinct, being CT for PXZ-Nap and locally excited (LE) for PTZ-Nap, which are highly dependent on a specific excitedstate structure. However, triplet spin density distributions at the optimized T 1 geometry indicate the T 1 of both possess LE character (Figure ). At the relaxed S 1 geometry there is a larger S 1 -T 1 spin-orbit coupling (SOC) matrix element (0.12 cm -1 ) in PTZ-Nap due to the presence of the additional heavy sulfur atom than in PXZ-Nap (0.09 cm -1 ). The predicted phosphorescence of PXZ-Nap and PTZ-Nap at their respective relaxed T 1 geometries decrease to 2.42 eV and 2.02 eV, respectively, compared with the excitation T 1 energies obtained from the TD-DFT calculations at the S 0 geometry (Figure ).