I(III)-promoted facile and rapid synthesis of imidazo-azepino-fused porphyrins with enhanced absorption as singlet oxygen generators

Abstract

A facile, high-yielding synthetic protocol has been developed to access azepino-fused porphyrins via iodine(III)-mediated oxidative intramolecular cyclization of β-imidazole or benzimidazole substituted porphyrins. The absorption of the synthesized compounds showed the characteristic features of meso-β-fused porphyrins, with intense Soret bands centered between 440 and 460 nm and two weak bands ranging from 550 to 750 nm (Q-bands) Among the synthesized compounds, the free-base imidazo-azepino-fused porphyrin was found to be an efficient 1O2 producer with a higher singlet oxygen quantum yield (ΦΔ ∼0.78 in DMF) as compared to H2TPP (ΦΔ = 0.64 in DMF). It was observed that the protonated form of 4aH2 exhibits a significant red shift of ∼24 nm in Soret and ∼150 nm in Q-bands. Fitting of the titration data of 4aH2 with TFA yielded an apparent pKa of ∼3.58, demonstrating that imidazole fusion enhances the basicity of the porphyrin system.