In order to generate new photosensitizers that would not impair the biomolecular properties of native metabolites, we focused on the nitrobenzodiazole core as an example of a small and uncharged chromophoric scaffold. With the exception of nitrobenzoxadiazoles (e.g., NBD-Cl, Figure ), which are commercially available, we employed the diamine 1 to prepare nitrobenzodiazoles including different bridging atoms, namely sulfur, selenium or carbon (Figure , full synthetic and characterization details in Supplemental Information). Thio-and selenoderivatives (3, 4) were prepared by condensation of the diamine 1 with N-sulfinylaniline or SeO2 respectively, followed by nitration of the position 7 under acidic conditions. The synthetic route for the carbon derivative 5 was modified due to the lability of aminoacetals under strong acidic conditions, which would deter the nitration step. Therefore, we treated nitrobenzoselenadiazole 4F with HI and HCl to remove the selenium atom and performed a Cu-catalyzed condensation with acetone to obtain the corresponding carbon-bridged nitrobenzodiazoles (5). All nitrobenzodiazoles were treated with diethylamine and isolated as the corresponding aminoderivatives (2a-5a, Figure ), which absorb in the visible spectral range. Despite some of these compounds having a relatively high number of N atoms, all of them fulfill the rule of 3 (i.e., the number of C and O atoms divided by the number of N atoms is equal or larger than 3) and therefore are deemed safe to handle and store. Compounds 2a-5a showed absorbance maxima wavelengths between 480 nm and 565 nm (Table ); however, only the selenium derivative (4a) displayed high cellular toxicity after exposure to the light dose generally used in clinical PDT (i.e., 37 J cm -2 , Figure ). Remarkably, the nitrobenzoselenadiazole 4a displayed minimal cell death in the dark (<10%) and almost complete cell ablation (>90%) after irradiation, whereas none of the other compounds (2a, 3a and 5a) showed significant differences between dark and light-induced cytotoxicity (Figure ). These observations were supported by DFT (Density Functional Theory) calculations (Figure ). These indicated that, whilst all compounds had sufficient energy to excite 3 O2 to 1 O2 via their S o -T 2 transition, the energy levels of the benzoselenadiazole 4a showed the smallest energy gap for intersystem crossing between S 1 and T 2 excited states, which favors the occupation of the T 2 state and the probability of singlet oxygen production under light irradiation. Furthermore, the degree of spin orbital coupling induced by selenium is 5-fold higher than for other heteroatoms, 36 which significantly increases the probability of accomplishing this formally forbidden electronic transition. Collectively, these results highlight the heavy selenium atom as a critical feature to endow the nitrobenzodiazole core with unique photosensitive capabilities.