Acid-induced improvement of photosensitizing efficiency of porphyrins under heterogeneous aqueous conditions: High photosensitizing activity, dissociative stability and photooxidative durability

Abstract

Meso-tetrakis(N-methylpyridinium-3-yl)porphyrin (H2TMPyP), electrostatically supported on the sodium salt of amberlyst 15 nanoparticles (nanoAmbSO3Na) was protonated with very weak (CH3COOH), weak (CH2ClCOOH, CHCl2COOH) and strong (CF3COOH) carboxylic acids and H2SO4 to prepare a series of new photosensitizers. NanoAmbSO3@H2TMPyP(HA)2 compounds were quite stable after separation from the aqueous solution and drying, even in the case of nanoAmbSO3@H2TMPyP(CH3COOH)2. The diacids were characterized by DR-UV–vis, AFM, TGA and DLS and used as photosensitizers in aerobic photooxidation of 1,5-dihydroxynaphthalene (DHN) and methyl phenyl sulfide. Almost no catalyst degradation was observed for the diacids in the oxidation of DHN after five successive runs. Also, the catalyst degradation was in the range of 0–68% in the oxidation of methyl phenyl sulfide at the end of the 5th run. The catalyst stability decreased as nanoAmbSO3@H2TMPyP(CF3COOH)2 >> nanoAmbSO3@H2TMPyP(CHCl2COOH)2 > nanoAmbSO3@H2TMPyP(H2SO4)2 > nanoAmbSO3@H2TMPyP(CH3COOH)2 > nanoAmbSO3@H2TMPyP(CH2ClCOOH)2. It is noteworthy that under the same conditions, ca. 50% of the non-protonated porphyrin, nanoAmbSO3@H2TMPyP, was degraded after 2 h, at the end of the first run. In other words, the formation of nanoAmbSO3@H2TMPyP(HA)2 species was associated with a significant increase in both the photocatalytic activity and oxidative stability of nanoAmbSO3@H2TMPyP, regardless of the type and strength of the acid used for diprotonation of the porphyrin. In the oxidation of methyl phenyl sulfide, nanoAmbSO3@H2TMPyP(CF3COOH)2 and nanoAmbSO3@H2TMPyP(CH2ClCOOH)2 were the most efficient catalysts of the series. The singlet oxygen quantum yields (ϕΔ), determined chemically by using 1,3-diphenylisobenzofuran as a specific quencher of singlet oxygen showed ϕΔ in the range of 0.51–0.98 for nanoAmbSO3@H2TMPyP(HA)2 and 0.66 for nanoAmbSO3@H2TMPyP.