The oscillator strength, f, of the S0→S1 transition in TPPQ is high at 0.808, indicating a significant overlap of the electron density between the HOMO and LUMO (Figure and Table ). However, the f for the S0→S1 transition of 4DMAC-TPPQ and 4PXZ-TPPQ is much lower at 0.027 and 0.091, respectively, reflecting the charge-transfer (CT) nature of this transition in these two compounds (Figure and Tables