Modified porphyrin chromophores: Synthesis and spectroscopic properties of nitronaphtho[1,2-b]porphyrins, benzothieno[2,3-b]porphyrins and their metalated derivatives

A series of c-annulated pyrroles were prepared by reacting isocyanoacetate esters with dinitronaphthalenes in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene. The resulting nitronaphthopyrroles esters condensed with an acetoxymethylpyrrole to afford dipyrrylmethanes and following cleavage of the ester groups, acid-catalyzed condensation with a dipyrrylmethane dialdehyde afforded four isomeric nitronaphthoporphyrins. Three of these isomers showed only minor differences in their electronic spectra compared to a known naphtho[1,2-b] porphyrin, although in two cases the intensities of the Soret band absorptions were reduced. However, when the nitro group was placed at a position one removed from the porphyrin nucleus, the UV-vis spectrum showed two very broad peaks in the Soret band region as well as distorted Q bands. This effect was also noted for the corresponding nickel(II), copper(II) and zinc complexes. An example of a benzo[b]thiophene-fused porphyrin was also prepared in three steps from 3-nitrothiophene, but the fused heterocyclic unit had virtually no effect on the porphyrin chromophore. This study demonstrates that while fused naphthalene and benzo[b]thiophene units do not significantly affect the electronic structure of porphyrins, the introduction of a suitably placed electron withdrawing group can induce profound changes.