The ultraviolet-visible absorption spectra (Fig. ) reveal the presence of an intramolecular charge-transfer band spanning approximately 350 to 400 nm. The extinction coefficient shows an increase from around 8 × 10 3 M -1 cm -1 at 350 nm to approximately 6 × 10 4 M -1 cm -1 for the higher-lying localized absorption bands. In the photoluminescence (PL) spectra, TCBPSF and HCBPSF exhibit a sky-blue emission peak centered at 494 nm. However, HCBPSF also displays a blue fluorescence band originating from the TCzHCz unit, spanning a range of approximately 360 to 420 nm. This indicates that the HC unit does not participate in intramolecular charge transfer in dilute solution (Supplementary Fig. ). In the phosphorescence (Phos) spectra with a delayed time of 0.1 ms, TCBPSF exhibits a structureless CT-type emission, while HCBPSF demonstrates a partly resolved LE-type emission. The energy gap between singlet and triplet (ΔEST) are calculated at 0.05 and 0.02 eV for TCBPSF and HCBPSF at 298 K, respectively (Supplementary Figs. ), suggesting that the long-lived emission spectra may originate from triplet states. In the transient PL decay spectra in solution (Fig. ), HCBPSF displays a significant long-lived lifetime with τP of 8.97 ms in air-equilibrated toluene. The lifetimes of the long-lived components decrease monotonically with increasing temperature, which is contrary to TADF and consistent with phosphorescence (Fig. ). Surprisingly, phosphorescence lifetimes with τP values of 1.13 ms are also observed in air-saturated solution (Fig. ).