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Dihydroxo- and dimethoxo(tetraphenylporphyrinato)phosphorus(V) complexes ([P(OR)2tpp]+) were immobilized on silica-gel powder affording a visible-light driven photocatalyst ([P(OR)2tpp]/SiO2; 1). Bactericidal effect of complexes 1a (R = H) and 1b (R = Me) on Escherichia coli were investigated under visible-light irradiation. In the case of complex 1a, the amount of E. coli decreased linearly versus the irradiation time, showing that the bactericidal reaction obeyed zero-order kinetics. Adsorption of bacteria on the catalyst is thought to be a key pathway by analysis according to Michaelis–Menten’s equation. Complex 1a was more effective for sterilization than complex 1b. Stabilities of the [P(OR)2tpp]+ immobilized on silica-gel beads (2) were investigated in aqueous CaCl2 and NaCl solutions. The elution of the [P(OH)2tpp]+ chromophore from complex 2a (R = H) was faster than it was from complex 2b (R = Me).

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In order to develop cobalt-free humidity indicator, the composite consisted of porphyrin, MgCl2, and silica gel was prepared. The composite exhibited a reversible color change from green to purple under dry and humid conditions, respectively.