The detailed photophysical characterization of 2-piperidinyl-6-triazolylpurine derivatives and their complexes with metal ions is presented. The studied compounds bear two electron donating units – piperidine and methoxyphenyl, while triazolyl-purine acts as electron acceptor. The present derivatives possess good fluorescence quantum yields in both aprotic and aqueous environments. All piperidine-triazole-purine compounds revealed a fluorescence “turn-off” sensing behavior to different metal ions, including Hg2+. We are discussing the changes in photophysical properties occurring under complexation including switching between highest occupied molecular orbitals (HOMOs) of different electron donating units and the entire electron charge shift possibility in the complex with mercury ion. The sensing concept was further applied for newly synthesized water-soluble compounds, that demonstrated better selectivity towards Hg2+ in aqueous media. Further, the selected 2-piperidinyl-6-triazolylpurine water-soluble derivative was employed as “turn-off” chemical probe for mercury ion detection in living cells.