1. The 18 ?-electron system of the porphyrins can be viewed as a doubly bridged diaza [18] annulene or as a tetraazal[16]annulene dianion. The formulation as a dianion is in agreement with, among others characteristics, the spectroscopic properties of the metal complexes of the porphyrins: cf. in (Ed.): Porphyrins and Metalloporphyrins, Elsevier, Amsterdam 1975;
2. : Colour and Constitution of Organic Molecules, Academic Press, London 1976.
3. in (Eds.): Comprehensive Organic Chemistry. Vol. 1, Pergamon, Oxford, 1979, p. 224.
4. EinN,N′,N″,N‴-Tetramethyl-porphyrinogen