Thermal stability and degradation of meso-tetraphenylporphyrins bearing nitrogen-containing substituents

Abstract

Porphyrins have exquisite photophysical and electronic properties that enable their use in photodynamic therapy. Even though the thermodynamic behavior relates to the aforementioned properties, the thermal degradation of non-mesomorphic porphyrins behavior has been little explored. Here, we demonstrated the thermoanalytical profiles of common porphyrins, using differential scanning calorimetry (DSC) and thermogravimetric analysis (TG/DTG). The thermal behavior was also evaluated from the perspective of molecular modeling and energy minimization. In our series, porphyrins embedding pyridyl, amino-phenyl, and nitro-phenyl moieties, that confer different physicochemical and electronic properties to the macrocycle, were studied. Tetrapyridylporphyrin presents symmetric and parallel lowest unoccupied molecular orbitals (LUMOs) and was the most thermostable compound (DTG_peak = 473.26 °C), while β-nitrotetraphenylporphyrin presents LUMOs drastically shifted toward the electron-withdrawing group –NO₂ group and was the least stable upon heating (DTG_peak1 = 357.16 °C). DSC profiles showed phase change/crystalline transitions and endothermic degradation events for phenyl-, aminophenyl- and pyridyl-containing porphyrins, whereas nitro-porphyrins exhibited a high energetic exothermic degradation event (p-nitrophenyl-triphenylporphyrin: T_peak = 406.84 °C, ∆H = 147.28 J g⁻¹; β-nitro-tetraphenyl-porphyrin T_peak = 375.25 °C, ∆H = 849.52 J g⁻¹).