Additionally, to test whether oxygen is a key element upon photoexcitation, the triplet state lifetimes under oxygen-free conditions were calculated by simple monoexponential fitting (unless stated otherwise). TA spectra and time trace fittings are shown in Fig. . Solutions were subjected to five freezepump-thaw cycles prior to photoexcitation. TA features of the AL(DIPY)3 complexes were similar, with triplet excited state lifetimes being much longer, in the range of ~80-200 μs, giving testimony that oxygen significantly alters the outcome of the excitation. This condition is consistently observed for all the AL(DIPY)3 chelates (Error! Reference source not found.). Lastly, the rate constant for quenching of the triplet state by oxygen was also determined to be in the range of 1.60-1.90 × 10 - 9 M -1 s -1 ; with [O2] in ethanol and toluene at 20 °C: 2.1 × 10 -3 M and 1.8 × 10 -3 M, respectively. Singlet Oxygen Generation Singlet oxygen phosphorescence was determined at 1270 nm and the singlet oxygen quantum yields were calculated relative to 5,10,15,20-tetraphenylporphyrin (H2TPP) in toluene or erythrocin B in ethanol as standard reference compounds (Table , SI 3.6, Fig. ). Additionally, a ratio of the ΦΔ of AL(DIPY)3 chelates and ΦΔ of the reference (erythrocin B) was defined in another attempt to demonstrate the relative singlet oxygen generation. Singlet oxygen determination can be affected by the oxygen concentration and its lifetime in different solvents. There is a significant variation in the singlet oxygen quantum yield determined for 13h between ethanol (ΦΔ = 0.15) and toluene (ΦΔ = 0.70) solutions, while for 13i the opposite trend occurs: ΦΔ = 0.35 in ethanol; ΦΔ = 0.07 in toluene. Since singlet oxygen phosphorescence is a sensitive method, and taking into consideration that oxygen concentration in air equilibrated solutions is mainly the same in both solvents, the discrepancies between the two solvents can be ascribed to the difference in the singlet oxygen lifetimes: singlet oxygen lifetime in toluene (27 μs) is almost two times greater than that in ethanol (15 μs). The combination of the absolute values and the calculated ratios leads shows that chelates 13d, 13f, 13g, 13i and 13j can be efficient singlet oxygen generators. Therefore, their respective triplet state energy level should be higher than the lowest energy of molecular oxygen (0.98 eV) to generate singlet oxygen. To conclude, since exciton coupling splits the singlet excited state into two excitonic states with higher (X′′) and lower energy level (X′), it is expected that the X′ state is closer to the triplet excited state. Therefore, a decrease of this energy gap can enhance the ISC efficiency and result in higher singlet oxygen generation. The occurrence of a high IC yield from non-radiative decay to the ground state (S1/T1 → S0) cannot be eliminated and it can be higher than the ISC in some of the complexes, therefore further investigation is needed. The quantum yield of ISC (or ICT) and IC should be following the general trend: ΦΔ < Φisc + Φic < 1 -Φf. An approximation of the radiationless processes could be given by the singlet oxygen quantum yields and the triplet state information indicating that 13d, 13f, 13h, 13i and 13j may possess an enhanced intersystem crossing yield which agrees with the increased knr. Moreover, these chelates display moderate singlet excited state lifetimes. Additionally, the large Stokes shifts, the low radiative rates and the long triplet lifetimes confirmed the emission through the excited triplet state of the aluminum complexes. These findings can lead to the inter-ligand-based triplet excited state 3 p-p* (long-lived 3 IL) originating from the initial 1 p-p* states. All the above are extremely dependent on the polarity of the environment, which influences the processes and the competing pathways. The reduction of singlet oxygen efficiency has been observed in polar solvents (aqua-rich media) and is related to the low solubility of oxygen in water in comparison to other organic solvents as well to the lower singlet oxygen lifetime in water. Moreover, excited state quenching or aggregation in polar environments can occur resulting in singlet oxygen attenuation. For instance, encapsulation in a polymer matrix to generate nanoparticles or addition of poly(ethylene) glycol (PEG) substituents can enhance the water solubility and biocompatibility of the potential therapeutic agents.