A Bodipy-based energy transfer cassette with a singlet oxygen reactive linker between the donor and acceptor modules has an interesting emergent property, if the acceptor module is also a photosensitizer. Singlet oxygen produced by the photosensitizer reacts rapidly with the molecule itself to liberate the energy donor, resulting in an enhanced fluorescence emission. The result is a self-reporting photosensitizer providing an assessment of the singlet oxygen production rate under the operational conditions. S ensing analytes in solution is highly important both for the elucidation of the processes they are involved in and/or for determining therapeutic/diagnostic approaches in diseases where corresponding analytes are markers for various biological anomalies. 1,2 Apart from using analyte responsive molecules as reporters, activatable therapeutic systems have been developed as well, where the disease related parameter is used to initiate a chemical/physical transformation in the therapeutic agent to alter its activity. Activatable photosensitizers are one of such therapeutics with a modulated activatability property. 3−8 Exchange of ideas between the fields of therapeutic design and molecular sensors is expected to yield great improvements in personalized treatment and diagnostics. However, one of the main problems not addressed so far in this overlap area, is the inadequacy of measuring the effect of any therapeutics directly. Rather, the biological effect is analyzed on a cellular level. For the photosensitizers, the effect of light, oxygen concentration, cell penetration, and dark-toxicity determines the overall effect of the photosensitizer on the cell; 9,10 hence, one has to be cautious in speaking about the efficiency of singlet oxygen production of a photosensitizer in a cell-culture experiment since the observed outcome is a cumulative result, namely cell death.In this work, we address the issue of direct monitoring of photosensitizer activity with the use of a singlet oxygen ( 1 O 2 ) labile linker between the fluorophore and a photosensitizer, where the former is an electronic energy transfer (EET) donor and the latter being an EET acceptor (Figure 1, BOD 2). The rationale behind the design is such that EET from the fluorophore (FL, blue module in Figure 1) to the photosensitizer (PS, green module on BOD 2) quenches the emission of the FL to a great extent. PS and FL are attached to one another with (Z)-1,2-bis(alkylthio)ethene bridge due to the fact that 1 O 2 susceptibility of this electron-rich olefinic linker is extensively exploited by us and others. 11−16 According to the design, 1 O 2 generated by the PS upon irradiation with red light is to act on the molecule itself and cleave the linker, liberating the fluorophore as a consequence. Fluorescence of the free FL is reinstated which is the reporter