Zinc(II) phthalocyanines (Pcs) peripherally decorated with 2,4,6-tris(dimethylaminomethyl)phenoxy groups (Pcs 1 and 3) and the corresponding quaternized derivatives (Pcs 2 and 4) were synthesized and their photodynamic inactivation (PDI) efficiency against recombinant bioluminescent Escherichia coli was examined. The photophysical data revealed that the presence of the ammonium units on the Pc structures promotes a redshift of the absorption bands when compared with the corresponding nonquaternized ones. The ammonium-substituted Pcs 2 and 4 showed excellent stability in dimethylformamide, moderate photostability, and increased efficiency to generate singlet oxygen (1O2). The water-soluble photosensitizers 2 and 4 at 5.0 μM exhibited a high photodynamic inactivation (PDI) efficiency against planktonic bioluminescent E. coli, reaching the detection limit of the methodology (a decrease of ∼4 log in the bioluminescence signal) after 210 min and 150 min under red light, delivered at a fluence rate of 135 mW·cm-2, respectively. Moreover, for the first time, it was accessed with the combined action of KI with ammonium-substituted Pcs. The addition of potassium iodide significantly improved the efficacy of Pc 2, which could reach the same inactivation rate after a short period of 5 min under the same irradiation conditions. The use of KI potentiates the PDI efficacy probably because of the generation of additional highly cytotoxic species during the photodynamic process which begins with the reaction of 1O2 with KI producing peroxyiodide species. The results of this work show that Pcs 2 and 4, with or without KI, can be considered as promising Pc dyes for the PDI of Gram-negative bacteria.
Keywords: E. coli; Gram-negative bacteria; KI salt; neutral and cationic phthalocyanines; photodynamic inactivation (PDI); photosensitizers.