Synthesis, separation and UV/Vis spectroscopy of pyrazino-quinoxalino-porphyrazine macrocycles

Unsymmetrical metal-free and zinc complexes of pyrazino-quinoxalino-porphyrazines (PQP) bearing eight diethylamino groups have been synthesized by statistical tetramerization of 2,3-bis(diethylamino)quinoxaline-6,7-dicarbonitrile and 5,6-bis(diethylamino)pyrazine-2,3-dicarbonitrile in lithium butanolate. For this purpose, a new heteroatom-substituted quinoxaline precursor, 2,3-dichloro-6,7-dicarbonitrile, was prepared and characterized. It is a flexible starting material for new building blocks of quinoxaline-6,7-dicarbonitrile derivatives. All the PQPs (including adjacent and opposite isomers) from the statistical mixture were detected and separated by column chromatography on silica and characterized by MALDI-TOF mass spectrometry, IR, UV/Vis and NMR spectroscopy. The effect of the insertion of benzene rings into the tetrapyrazinoporphyrazine (TPP) system is discussed. Each benzene ring insertion into the TPP system causes a bathochromic shift of 22 nm; the dependence is linear. The final tetra[ 6,7]quinoxalinoporphyrazines were red-shifted to 744 and 763 nm for the zinc and metal-free derivative, respectively. Splitting of the Q-band was observed for PQPs with lower symmetry. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).