Visible light photoactive highly efficient triplet sensitizers based on iodinated aza-BODIPYs: synthesis, photophysics and redox properties

A series of novel iodinated NO2-substituted aza-BODIPYs have been synthesized and characterized. Highly desirable photophysical and photochemical properties were induced in NO2-substituted aza-BODIPYs by iodination of the pyrrole rings. In particular, high values of singlet oxygen quantum yields (ΦΔ) ranging from 0.79 to 0.85 were measured. The photooxygenation process proceeds via Type II mechanism under the experimental conditions applied. The compounds studied exhibited an absorption band within the so-called "therapeutic window", with λmax located between 645 nm to 672 nm. They were non-fluorescent at room temperature with excited singlet state lifetimes within the picosecond range as measured by femtosecond transient absorption. Nanosecond laser flash photolysis experiments revealed T1 → Tn absorption spanning from ca. 400 nm to ca. 500 nm and allowed determination of the triplet state lifetimes. The estimated triplet lifetimes (τT) in deaerated acetonitrile ranged between 2.74 μs to 3.50 μs. As estimated by the CV/DPV measurements, all iodinated aza-BODIPYs studied exhibited one irreversible oxidation and two quasi-reversible reductions processes. Estimation of the EHOMO gave the value of - 6.06 to - 6.26 eV while the ELUMO was found to be located at ca. - 4.6 eV, respectively. Thermogravimetric (TGA) analysis revealed that iodinated aza-BODIPYs were stable up to approximately 300 °C. All compounds studied exhibit high photostability in toluene solution.