However, we can estimate quantum yields in the order of 10 -4 , as has been observed for the other thiobases. Aza also exhibits a small-intensity fluorescence emission band in PBS at pH 5.5 and 7.4, and in borate buffer at pH 9.8 at 425 nm (Figures S1 to ). The emission band features a small Stokes shift relative to the lowest-energy absorption shoulder and is not quenched by O2. As for 6MP, fluorescence quantum yields and excitation spectra as a function of pH were not obtained because the emission signal is within the detection limits of the spectrometer.