Synthesis and Studies of Structural Isomers of meso-Fused Dicarbahexaphyrins

Two close structurally related isomers of nonaromatic meso-fused dicarbahexaphyrins were synthesized by condensing one equivalent of fluorene based tripyrrane with one equivalent of pentafluorobenzaldehyde in CH₂ Cl₂ under BF₃ .OEt₂ catalyzed conditions. The cis and trans isomers of meso-fused dicarbahexaphyrins were separated by preparative thin-layer chromatography and isolated pure macrocycles as green solids in 6-7% yields. NMR spectra of cis and trans isomers are quite distinct from each other and trans isomer was very symmetric and showed fewer resonances than cis isomer in NMR. The NMR study supported the nonaromatic nature of both cis and trans isomers of meso-fused dicarbahexaphyrins. DFT optimized structures revealed that the cis isomer adopted a singly twisted puckered conformation whereas the trans isomer displayed a saddle like conformation. Both cis and trans isomers almost showed similar nonaromatic absorption features with slight differences in their peak maxima. However, the protonated derivative of cis isomer showed absorption bands in visible-NIR region with bands extended upto 1000 nm whereas the trans isomer showed strong bands in the visible region. Both cis and trans macrocycles were easier to oxidize and reduce and TD-DFT studies corroborated with the experimental findings.