The effect of pH on the room-temperature phosphorescence properties of several purine and pyrimidine derivatives

Room-temperature phosphorescence (RTP) spectra of eleven purines and pyrimidines adsorbed on Whatman No. 40 filter paper have been determined in acidic, neutral and basic media. RTP excitation and emission wavelengths do not vary significantly with pH. For most compounds, use of basic (pH approximately 13) solutions yields stronger RTP signals than use of neutral or acidic (pH approximately 1.6) solutions. Exceptions are adenine, theobromine and theophylline, which give larger RTP signals when in neutral than in basic conditions. The existence of differences in phosphorescence quantum yields between the various ionic species as well as of specific pH-related interactions with the substrate is discussed. Absolute limits of detection, ranging between 0.4 and 38 ng for selected compounds, depend on the pH of the analyte solution.