Synthesis and studies of pyrimidine-embedded octaphyrins

Pyrimidine is a biologically significant nitrogen-containing heterocycle and a key structural motif in nucleic acids, vitamins, and functional materials. Incorporation of pyrimidine units into porphyrinoid frameworks offers opportunities to modulate their electronic structure, hydrogen-bonding behaviour, and photophysical properties. Herein, we unveil the first examples of pyrimidine-embedded porphyrinoids synthesized via condensation of 2,5-di(1H-pyrrol-2-yl)pyrimidine with 2,2'-bithiophene-5,5'-diyl bis(aryl methanol) under BF₃·OEt₂ catalyzed conditions in 8-15% yields. HR-MS and 1D/2D NMR analyses confirmed the formation of macrocycles comprising four pyrrole, two thiophene, and two pyrimidine units linked via two meso-carbons. In ¹H NMR, the four inner pyrrole NH protons appeared as two broad resonances: one resonance in the upfield region at 6.35 ppm and the other resonance in the downfield region at 9.58 ppm. The downfield shifting of the two NH protons at 9.58 ppm was due to their involvement in intramolecular hydrogen bonding with inner pyrimidine "N" atoms. Absorption spectral studies showed that the macrocycles exhibited nonaromatic panchromatic absorption features covering the entire UV-visible region and their diprotonated macrocycles showed strong absorption in the visible-NIR region. DFT studies indicated that the macrocycles were almost planar except for the thiophene rings which were deviated from the mean plane by an angle of 9.2°.