Singlet oxygen (¹O₂), the lowest excited-state of molecular oxygen receives great attention in basic research and clinical and industrial settings. Despite several spectroscopic methods available for ¹O₂ sensing, fluorescence sensing receives great attention, for which many fluorogenic sensors based on substituted anthracene are reported. Nonetheless, the roles of substituents on the sensing efficiency, in terms of detection time, remain largely unknown. In this work, we examine the ¹O₂ sensing efficiency of a fluorescence sensor based on a coumarin–anthracene conjugate, which is an electron donor-acceptor dyad, and compare the efficiency with that of 9-methylanthracene. Here,¹O₂ is generated using the standard photosensitizer Rose Bengal, which is followed by estimation of the rate of reaction of ¹O₂ to the sensor and 9-methylanthracene. The second order reaction rate of the sensor is an order of magnitude less than that of 9-methylanthracene. The lower reactivity of the sensor to ¹O₂ suggests that the roles of substituents, such as electronic interactions, steric interactions and the reactivity of precursor complexes, on sensing efficiency should be carefully considered during construction of fluorogenic molecular sensors.