Effects of pH on the ultrafast transient absorption of iron (III) meso-tetrakis(4-N-methyl-pyridiniumyl) porphyrin (Fe3+TMPyP) molecular complexes

Abstract

The meso-tetrakis (methylpyridiniumyl) porphyrin complexed with Fe3+ (Fe3+TMPyP) is an irregular porphyrin, possessing unfilled d external shells with d6 electronic configuration. In aqueous solution, Fe3+TMPyP exists in a complex equilibrium of different forms. Changing the solution pH, these complexes can bind to other species (as OH- and H2O) or form physical aggregates (μ-oxo-dimers), that alter their excited states properties and affect their potential applications. In the present work, we report excited state absorption dynamics of Fe3+TMPyP molecular complexes in different pH employing femtosecond Z-Scan and ultrafast transient absorption techniques. Complementary data were obtained by UV–vis absorption and fluorescence emission spectroscopies. For 532 nm excitation, the Z-Scan technique shows a decrease in the absorption coefficient for Fe3+TMPyP at pH 4.0 and an increase at pHs 7.0 and 9.0. Ultrafast transient absorption reveals the same behavior in the 525−570 nm region for samples at pHs 7.0 and 9.0. Fast relaxations, with lifetimes around 2.0 picoseconds, were obtained for all samples studied, and were attributed to internal conversion relaxation processes. Although the molecular aggregation accelerates the relaxation process of regular porphyrins, μ-oxo-dimers slightly affect the relaxation of Fe3+TMPyP. The prompt conversion of absorbed light energy in heat observed to Fe3+TMPyP porphyrins can be useful for biomedical photothermal applications, such as Photoacoustic Imaging (PAI) and Photothermal Therapy (PTT).