Porphyrazines, or tetraazaporphyrins, are tetrapyrrole macrocycles similar to porphyrins and phthalocyanines. Pioneered by Sir R. Patrick Linstead as an extension of his work on phthalocyanines,Cook, A. H.; Linstead, R. P., "Phthalocyanines. XI. The preparation of octaphenylporphyrazines from diphenylmaleinitrile." J. Chem. Soc. 1937, 929-933.DOI: 10.1039/JR9370000929 porphyrazines differ from porphyrins in that they contain -meso nitrogen atoms, rather than carbon atoms, and differ from phthalocyanines in that their β-pyrrole positions are open for substitution. An extension of the aromatic core of porphyrazines, by adding pyrazine groups, can allow the formation of another class of analogs, called tetrapyrazinoporphyrazines. These differences confer physical properties that are distinct from both porphyrins and phthalocyanines.Ghosh, A.; Fitzgerald, J.; Gassman, P.G.; Almof, J. Inorg. Chem., 1994, 33, 6057-6060. DOI: 10.1021/ic00104a014
Synthesis Porphyrazines are prepared by magnesium templated cyclization of maleonitriles.Kobayashi, N. Meso-Azaporphyrins and Their Analogues. In The Porphyrin Handbook, Kadish, K.M.; Smith, K.M.; Guilard, R., Eds.; Academic Press; 1999, Vol. 2, pp. 317-321. Cross-cyclization with phthalonitrile or diiminoisoindole derivatives is possible introducing a flexibile synthetic route that has led to the synthesis of porphyrazines with peripheral heterocyclic rings,Angeloni, S.; Ercolani, C., New classes of porphyrazine macrocycles with annulated heterocyclic rings. Journal of Porphyrins and Phthalocyanines, 2000, 4, 474–483. heteroatom substituents (S, O, N),Michel S.L.J.; Hoffman B.M. Baum S.M.; Barrett A.G.M.; "Peripherally functionalized porphyrazines: Novel metallomacrocycles with broad, untapped potential;" Progress in Inorganic Chemistry, 50: 473-590, 2001. peripherally bound metal atoms,For example: Zhao, M; Zhong, C.; Stern, C.; Barrett, A.G.M.; Hoffman, B.M., Synthesis and Properties of Dimetallic M1[Pz]-M2[Schiff Base] Complexes. Inorg. Chem., 2004, 43, 3377-3385. and mixed -benzo porphyrazine systems.Miwa, H., Ishii, K., Kobayashi, N., Electronic Structures of Zinc and Palladium Tetraazaporphyrin Derivatives Controlled by Fused Benzo Rings. Chemistry - A European Journal, 2004,10, 4422–4435.
Optical properties Porphyrazines are most well known for their intense electronic absorption throughout the UV, visible, and NIR spectral regions. Electronic absorption spectra for porphyrazines are similar to those of phthalocyanines,Kobayashi, N. and H. Konami (1996) Molecular orbitals and electronic spectra of phthalocyanine analogues. In Phthalocyanines: Properties and Applications, Vol. 4 (Edited by C. C. Leznoff and A. B. P. Lever), pp. 343–404. VCH Publishers, Inc., New York. with an intense Soret band (λ ≈ 300 - 400 nm) and Q-band (λ > 600 nm).Linstead, R. P.; Whalley, M., Conjugated Macrocycles. Part XXI Tetrazaporphin and its Metallic Derivatives. J. Chem. Soc. 1952, 4839-4844.
Porphyrazines exhibit fluorescence from the first excited singlet state (S1 → S0)Shushkevich, I.K.; Pershukevich, P.P.; Stupak, A.P.; Solov'ev, K.N.; Journal of Applied Spectroscopy, 2005, 72, 767-770. at visible and NIR wavelengths which is typical of tetrapyrrole macrocycles. Dual-emission from organic fluorophores is not common but, as observed in phthalocyanines,Chahraoui, D., Valat, P., and Kossanyi, J., Fluorescence of Phthalocyanines: Emission from an Upper Excited State. Res. Chem. Intermed., 1992, 17, 219-232.Kaneko, Y., Nishimura, Y., Takane, N., Arai, T., Sakuragi, H., Kobayashi, N., Matsunaga, D., Pac, C., and Tokumaru, K., Violet emission observed from phthalocyanines. J. Photochem. Photobiol. A, 1997, 106, 177-183. violet emission from an upper excited state (S2 → S0) is observed in porphyrazines.Lee, S., White, A.J.P., Williams, D.J., Barrett, A.G.M., and Hoffman, B.M. Synthesis of Near-IR Absorbing/Emitting Porphyrazine Derivatives with Tunable Solubility. J. Org. Chem., 2001, 66, 461-465.Lee, S.; Stackow, R.; Foote, C.S.; Barrett, A.G.M.; Hoffman, B.M.; Tuning the Singlet Oxygen Quantum Yield of Near-IR-absorbing Porphyrazines. Photochem. Photobio., 2003, 77, 18-21.Trivedi, E.R., Vesper, B.J., Weitman, H., Ehrenberg, B., Barrett, A.G.M., Radosevich, J.A., and Hoffman, B.M., Chiral bis-Acetal Porphyrazines as Near-infrared Optical Agents for Detection and Treatment of Cancer. Photochem. Photobio., 2010, 86, 410-417.
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