Subphthalocyanine basicity: reversible protonation at the azomethine bridge

Abstract

Subphthalocyanine (SubPc) could be reversibly protonated at the azomethine bridge. This phenomenon was examined by addressing the pK_a of the acid (TFA, MSA, TMSA) and the SubPc electron-withdrawing properties of the peripheral isoindolic substituents (F₁₂vs. H₁₂ and NO₂), which tunes the basic character of the azomethine moiety. The protonation of up to three azomethines was suggested and monitored spectrophotometrically with the appearance of new absorption bands at 610, 630 nm and 660 nm, together with marked colour changes from purple to blue to green. Evidence of such a protonation was also shown by significant changes in the ¹H-NMR spectrum and new bands in the infrared spectrum.