Effect of Five Membered Versus Six Membered Meso-Substituents on Structure and Electronic Properties of Mg(II) Porphyrins: A Combined Experimental and Theoretical Study

Meso-tetrasubstituted Mg(II) porphyrins containing six membered phenyl groups (MgTPP) and five membered thienyl (MgTThP) and furyl groups (MgTFP) were synthesized and structurally characterized, and the effects of meso-substituents on electronic properties were studied using NMR, absorption, fluorescence spectroscopy, and electrochemical studies. Density functional theory (DFT) calculations were carried out to correlate with experimental observations. The three Mg(II) porphyrins MgTPP, MgTThP, and MgTFP were crystallized as hexa-coordinate systems with Mg(II) ion in the center of the porphyrin plane and having two tetrahydrofuran molecules as axial ligands. The X-ray studies clearly showed that the meso-furyl groups adopt a conformation in which they are more in-plane with the porphyrin plane whereas the thienyl and phenyl groups prefer an orthogonal arrangement with respect to the porphyrin plane. This arrangement of meso-substituents with the porphyrin plane helps in the enhancement of porphyrin pi-delocalization in MgTFP compared to MgTThP and MgTPP. The differences in their structures are clearly reflected in their spectral and electrochemical properties. The absorption and fluorescence bands experienced bathochromic shifts on moving from six membered phenyls to five membered thienyl and furyl group, and the maximum effects were observed for meso-tetrafuryl Mg(II) porphyrin. The electrochemical studies indicated that the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) decreases as we move from six membered phenyl groups to five membered thienyl and furyl groups, which explains the bathochromic shifts observed in absorption and fluorescence bands. Results on structure and electronic properties based on DFT studies are in agreement with experimental observations.