Molecular Structure and Medium Effects on Porphyrin Photostability in Aqueous Solutions

The photostability of tetraphenylporphyrin, zinc(II) tetraphenylporphyrin, and 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin p-toluenesulfonate was quantified in water and DMSO/water mixtures by absorption and 650-nm photoluminescence under 423-nm excitation. Under air in DMSO/water, the apparent degradation rate constants increased in the order TPP (0.033 min−1) < ZnTPP (0.046 min−1) < TMPyP–OTs (0.139 min−1); thus, TMPyP–OTs degraded about 4.2 times faster than TPP. Argon deaeration reduced the rate constants to 0.013, 0.028, and 0.034 min−1, respectively, confirming the dominant contribution of oxygen-dependent pathways in air; the largest stabilization was observed for TMPyP–OTs (4.1-fold). DMSO/water accelerated the degradation of TMPyP–OTs 5.3-fold under air but only 1.6-fold after deaeration, linking the solvent effect to singlet-oxygen-mediated processes. In deaerated water, increasing the temperature from 25 to 45 °C increased the rate constant from 0.019 to 0.048 min−1 (2.5-fold), revealing a significant oxygen-independent contribution. These data indicate that steric meso-shielding, metalation, peripheral substituents, aggregation propensity, oxygen, solvent, and temperature jointly control porphyrin photostability. The quantitative ranking provides selection criteria for porphyrin luminophores in hybrid nanomaterials: TPP-like scaffolds are preferable when high photostability is required, whereas the higher reactivity of TMPyP–OTs is advantageous for photodynamic therapy provided that oxygen and solvent conditions are carefully controlled.