The fluorescence of phenol-substituted ADOTA triangulenium fluorophores is quenched as function of the water content in acetonitrile. We investigate three phenol-ADOTA derivatives in various solvent mixtures and conclude that water acts as a dynamic quencher by serving as a proton acceptor in a proton coupled electron transfer reaction with the excited ADOTA. Beyond the mechanistic insights into this quenching mechanism we demonstrate how these ADOTA dyes can probe the water content from a simple calibration free single-point measurement of the fluorescence lifetime.