Porphyrins with exocyclic rings. 13 - Synthesis and spectroscopic characterization of highly modified porphyrin chromophores with fused acenaphthylene and benzothiadiazole rings

As part of a survey on the influence of fused aromatic rings on the porphyrin chromophore, a series of novel structures with fused acenaphthylene or benzothiadiazole rings have been synthesized. Base-catalyzed condensation of 1-nitroacenaphthylene or 4-nitrobenzothiadiazole with esters of isocyanoacetic acid afforded good yields of the annelated pyrroles 5 and 28. Cleavage of the ester moieties with KOH in refluxing ethylene glycol gave the unsubstituted heterocycles, and subsequent condensation with 2 equiv of acetoxymethylpyrroles 9 in acetic acid/ethanol produced the modified tripyrranes 12 and 31. Tripyrranes with terminal tert-butyl ester units were treated with TFA and condensed with 3,4-diethyl-2,5-pyrroledicarboxaldehyde 13 in dichloromethane to give, following oxidation with DDQ, the corresponding pi-extended porphyrins 14 and 32. Acenaphthoporphyrins 14 showed unique UV-vis spectra with a triply split Soret band region and a relatively strong band near 660 nm. Strongly red-shifted absorptions were also noted for the dications and the nickel(II), copper(II), and zinc chelates for this system. Thiadiazoloporphyrins 32 gave two broadened Soret bands, but the Q-band region was unexceptional. However, the nickel(II), copper(II), and zinc complexes all showed abnormally strong absorptions between 600 and 612 nm. Porphyrins with two antipodal fused aromatic rings were easily prepared by condensing c-annelated pyrroledialdehydes 17 with tripyrranes 12 and 31, and the spectroscopic properties of the resulting porphyrins showed that the observed ring-fusion effects were essentially additive. Porphyrins with two adjacent acenaphthylene rings were also prepared by the MacDonald "2 + 2" condensation, although this chemistrry gave poor results in the synthesis of a porphyrin with two fused benzothiadiazole rings. The spectroscopic properties of these new highly conjugated porphyrin structures show that ring fusion can profoundly modify the porphyrin chromophore.