Push-pull quinoidal porphyrinsElectronic supplementary information (ESI) available: Details of synthesis, spectroscopy, and crystallography. CCDC 1822190-1822194. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8ob00491a

A family of push-pull quinoidal porphyrin monomers has been prepared from a meso -formyl porphyrin by bromination, thioacetal formation, palladium-catalyzed coupling with malononitrile and oxidation with DDQ. Attempts at extending this synthesis to a push-pull quinoidal/cumulenic porphyrin dimer were not successful. The crystal structures of the quinoidal porphyrins indicate that there is no significant contribution from singlet biradical or zwitterionic resonance forms. The crystal structure of an ethyne-linked porphyrin dimer shows that the torsion angle between the porphyrin units is only about 3°, in keeping with crystallographic results on related compounds, but contrasting with the torsion angle of about 35° predicted by computational studies. The free-base quinoidal porphyrin monomers form tightly π-stacked layer structures, despite their curved geometries and bulky aryl substituents. Quinoidal porphyrins with electron-donating 1,3-dithiolane and electron-withdrawing dicyanomethylene groups form tightly π-stacked layer structures, despite their curved geometries and bulky aryl substituents. The crystal structures of these quinoidal porphyrins indicate that there is no significant contribution from singlet biradical or zwitterionic resonance forms.