The absorption spectra in methanol displayed maxima (λmax) between 410 and 415 nm with molar extinction coefficients (ε) in the range of 5300 to 11305 L M -1 cm -1 . The corresponding emission maxima (λem) were measured between 480 and 490 nm when excited at λmax. Finally, the quantum yields ΦF ranged from 6.6% for compound F1 up to 18.7% for maleimide F4. For both pairs of compounds, higher quantum yields were measured for the dinitrogen fluorophores F2 and F4 compared to their sulfur derivatives F1 and F3.