Lastly, 13h that contains three methoxy groups in the phenyl rings showed great phototoxic potential with approximately 100% of cell death at all the tested concentrations (0.62 to 20 μM). In contrast, 13f bearing only one methoxy group on each phenyl ring, did not exhibit any phototoxic effect (Error! Reference source not found.E). This can be attributed to the fact that 13h has a higher singlet oxygen quantum yield (ΦΔ = 0.70 in toluene) in comparison to 13f (ΦΔ = 0.38 in toluene). To calculate the IC50 value for 13h a new set of phototoxicity studies was carried out using lower concentrations and/or lower L.D. An IC50 value in the nanomolar range (80 nM) was determined using 13h was used in the range of 0.04-5 μM with a L.D. of 2.6 J cm -2 Error! Reference source not found.A). A slightly higher IC50 (0.18 μM) was calculated when 13h was examined under a lower L.D. of 1 J cm - 2 (0.08 -10 μM) (Error! Reference source not found.B). The low IC50 value is of great importance as it gives room for less exposure to the PSs, which can contribute to reducing unwanted side effects while maintaining the phototoxicity. Note, that 13h exists as an atropisomeric mixture; however, this is not a new concept for PS in PDT. E.g., Ru(III) complex TLD-1433 is a chloride salt of a racemic mixture of two isomers and can be effectively utilized as long as the toxicity profile is acceptable. The high phototoxicity mediated by 13h is consistent with its photophysical properties. Both in toluene and ethanol it can effectively generate singlet oxygen. The compound has a moderate fluorescence quantum yield (Φf = 0.03) taking into consideration that such molecules are normally poor emitters. Moreover, triplet excited state lifetimes were long and greatly affected by oxygen. Overall, the optimal photophysical properties of 13h are correlated with the significant phototoxicity and the lack of dark toxicity in CT26 cancer cells. Surprisingly, 13i which had improved singlet and triplet state properties, and singlet oxygen formation in polar solvent (ΦΔ = 0.34), did not induce any phototoxicity at 2.6 J.cm -2 . We hypothesize that the lack of activity might be explained by its poor internalization or possible aggregation. However, it should be noted that the applied L.D. was relatively low in comparison with other reports, which leaves room for exploration. Ultimately, we can surmise that the AL(DIPY)3 complexes with -OH, -COOH, -OCH3 groups in the phenyl moieties exhibit better biological compatibility and probably cell permeability. These polar groups may increase the amphiphilicity of the compounds which at the end can facilitate their cellular uptake. Our preliminary results demonstrated that 13h, 16 and 17 are promising photosensitizers as they exhibit high phototoxicity at low concentrations. Further studies are crucial to unveil the underlying mechanism of action of the AL(DIPY)3 complexes.