The design and synthesis of 1Tb, a Tb(III) quinoline–cyclen based complex, are described. The results from the photophysical investigation of 1Tb are compared with the results observed from its Eu(III) analogue, 1Eu. The Tb(III) emission of 1Tb was found to be highly pH dependent, being reversibly switched “off–on–off”, with maximum emission intensities observed within the physiological pH range, contrary to what was observed with the Eu(III) analogue. Furthermore, the Tb(III) emission was also found to be sensitive to molecular oxygen concentration. By using a stoichiometric amount of both complexes, the changes observed in the combined lanthanide Eu/Tb-emissions (described as output) as a function of pH and O2 (as ion/molecular inputs) can be employed in ratiometric sensing of these analytes. Furthermore, the outputs can also be described in molecular logic gate terms, where the emission at three different emission wavelengths can be described in terms of Boolean algebra, corresponding to the NAND, NOR and NOT logic gates functions.