I(III)-Promoted Facile and Rapid Synthesis of Imidazo-Azepino-Fused Porphyrins With Enhanced Absorption as Singlet...

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 (Φ_Δ ∼0.78 in DMF) as compared to H₂TPP (Φ_Δ = 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 pK_a of ∼3.58, demonstrating that imidazole fusion enhances the basicity of the porphyrin system.