Synthesis, characterization and optical properties of β-substituted pyrrolo- and indolo[1,2-a]quinoxalinoporphyrins

• A Pictet–Spengler strategy to new pyrrolo- and indolo[1,2-a]quinoxalinoporphyrins.
• p-Dodecylbenzenesulfonic acid was used as efficient catalyst for present synthesis.
• Cu(II), free-base, Ni(II) and Zn(II) porphyrins were synthesized in good yields.
• These porphyrins exhibited bathochromic shift in their UV–Vis and emission spectra.

A convenient synthesis of β-substituted copper(II) pyrrolo- and indolo[1,2-a]quinoxalinoporphyrins have been achieved through a Pictet–Spengler reaction of various copper (II) β-formyl-5,10,15,20-tetraarylporphyrins and 1-(2-aminophenyl)pyrrole or 2-(3-methyl-indol-1-yl)-phenylamine in the presence of 10 mol% p-dodecylbenzenesulfonic acid as Brønsted acid catalyst in 1,4-dioxane at 25–40 °C followed by in-situ oxidation in the presence of potassium permanganate at ambient temperature. Furthermore, the corresponding free-base porphyrin analogues were obtained in good yields after the treatment of copper(II) porphyrins with concentrated sulfuric acid at 0 °C. On metal insertion by using nickel acetate or zinc acetate, these free-base porphyrins afforded the nickel and zinc complexes of pyrrolo[1,2-a]quinoxalinoporphyrins in good yields. The newly synthesized porphyrins were characterized on the basis of spectral data. The preliminary photophysical studies of these pyrrolo- and indolo[1,2-a]quinoxalinoporphyrins demonstrate a significant red shift in their electronic absorption and emission spectra due to the extended π-conjugation in comparison to the corresponding meso-tetraphenylporphyrins.

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