Contradictory effects of acids on the photocatalytic activity and stability of different polymer supported meso-tetra(aryl)porphyrins: Unusual high stability of sterically hindered porphyrins towards core-protonation with acids

A series of tetra-anionic porphyrins including meso-tetrakis(4-sulfonatophenyl)porphyrin, H₂TPPS₄, meso-tetrakis(2-Cl-4-sulfonatophenyl)porphyrin, H₂T(2-Cl)PPS₄, and β-tetrabromo-meso-tetrakis(4-sulfonatophenyl)porphyrin, H₂TPPBr₄S₄, embedded into the pores of porous nanostructured amberlite, nanoAmbN(Me)₃Cl, were prepared and characterized by UV-vis-NIR diffuse reflectance spectroscopy (DRS), FESEM/EDX/mapping and nitrogen adsorption-desorption porosimetry analysis. Interestingly, DRS studies showed that, contrary to nanoAmbN(Me)₃@H₂TPPS₄, nanoAmbN(Me)₃@H₂T(2-Cl)PPS₄ and nanoAmbN(Me)₃@H₂TPPBr₄S₄ are highly stable towards protonation with formic acid, CH₂ClCOOH and HCl in water. As evidenced by solution UV-vis spectroscopy, the three non-immobilized porphyrins were readily protonated with formic acid, CH₂ClCOOH and HCl under homogeneous conditions in water. It seems that the encapsulation of the sterically more hindered porphyrins, i.e., H₂T(2-Cl)PPS₄ and H₂TPPBr₄S₄, within the mesopores of the polymer, prevents the formation of the diprotonated derivatives of these porphyrins due to the spatial confinement effect of the small pores occupied by the porphyrins. Under the same conditions, nanoAmbN(CH₃)₃@H₂TPPS₄ was protonated with HCl, confirmed by a red shift of the Soret and Q bands in DRS. The nanocomposites were used as photosensitizers in the aerobic oxidation of 1,5-dihydroxynaphthalene and methyl phenyl sulfide. The acid-dependent photocatalytic activity found for the nanocomposites was explained on the basis of acid-catalysis in the case of the sulfide. The involvement of singlet oxygen as the major reactive oxygen species was revealed by using 1,3-diphenylisobenzofuran, NaNO₃, dimethyl sulfoxide, KI and 1,4-benzoquinone, as the scavengers of singlet oxygen, electrons, hydroxyl radicals, holes and superoxide anion radicals, respectively. The positive effects of HCl and HCOOH on the photocatalytic activity and oxidative stability of nanoAmbN(Me)₃@H₂T(2-Cl)PPS₄ and nanoAmbN(Me)₃@H₂TPPBr₄S₄ for the oxidation of 1,5-dihydroxynaphthalene and methyl phenyl sulfide seem to be due to the hydrogen bond interactions between peripheral groups of the porphyrins and the acids. Contrary to nanoAmbN(Me)₃@H₂TPPS₄, a positive effect due to the presence of the acids was found on the singlet oxygen quantum yield (φ_Δ) of nanoAmbN(Me)₃@H₂T(2-Cl)PPS₄ and nanoAmbN(Me)₃@H₂TPPBr₄S₄.