The Induced Current Strengths And Aromatic Pathways Of Heteroporphyrins And Their Antiaromatic Derivatives

Over the last century, aromaticity has become one of the most important andfrequently used concepts in modern chemistry, and the aromaticity degree has beencontinuously tried to characterized by various criteria including: bond lengthequalization and planarity, energetic stabilization, low-field1H NMR chemical shifts,lowered reactivity and electrophilic aromatic substitution, enhanced anisotropy ofdiamagnetic susceptibility. Porphyrins and their derivatives have been extensivelyinvestigated during the past decades owing to their important roles in coordinationchemistry, biology, optical power limiting and other practical applications. All ofthese technological applications are obviously related to the aromaticity of porphyrinsand porphyrinoids. Therefore, it is indispensable to elucidate the aromatic propertiesof various porphyrinoids.This study aims to elucidate the influence of the substitution of NH by oxygen andsulfur atoms on the aromatic (antiaromatic) character and how the heteroatoms (N, O,and S) affect the induced current strengths as well as the current pathways in differentporphyrinoids. In this work, we carried out a theoretical analysis on the aromaticity ofporphyrin and its classical substituted derivatives with O and S heteroatoms. Theformation of22,24-dideazaheteroporphyrins can be prepared by removing nitrogensof pyrroles without an inner hydrogen in heteroporphyrins. Interestingly,heteroporphyrins are macrocyclic4n+2π electron conjugated porphyrinoids, while22,24-dideazaheteroporphyrins become4n π electron porphyrinoids afterdenitrification. We performed a comparative study on heteroporphyrins and 22,24-dideazaheteroporphyrins based on the induced current strengths and currentpathways. The NICS calculations were also employed to perform this aromaticityinvestigation.The magnetically induced current density as well as nucleus independent chemicalshifts (NICS) of aromatic heteroporphyrins and antiaromatic22,24-dideazaheteroporphyrins have been studied theoretically. All molecularstructures were optimized at the density functional theory (DFT) level using thewidely used B3LYP hybrid functional with the6-31G(d) basis set in Gaussian09program. Vibrational frequencies of all the studied structures were calculated at thesame theory level to characterize them as minima on the potential energy surface.Nuclear magnetic shieldings have been calculated at the B3LYP/def2-TZVP level inTURBOMOLE. NICS computations were performed at the B3LYP/6-31G(d) levelusing the NBO6.0program. The induced current strengths and pathways are obtainedby numerical integration of the induced current density following the specificchemical bonds employing the gauge including magnetically induced current (GIMIC)method. Heteroporphyrins sustain a diatropic induced current while22,24-dideazaheteroporphyrins reveal paratropic ring current, the total currentstrengths of which are about27and-21nA/T, respectively. The substitution of pyrroleNH groups by O and S atoms seems to have little influence on the macrocyclicaromaticity (antiaromaticity) of porphyrins, because the total current strengths andtotal NICS(0)πzz values are almost unchanged.In all investigated porphyrinoids, the induced currents around the molecularmacroring split at the heterocycles (pyrrole, furan, and thiophene) into the inner andouter routes. For aromatic heteroporphyrins, the heteroatoms (N, O, and S) have highresistance and consequently lead to a weaker induced current strength than the onepassing the outer route. In sharp contrast, the heteroatoms enhance the currentstrength in antiaromatic22,24-dideazaheteroporphyrins and change the main currentpathway into the inner route. The induced current strength following the NH moiety isstronger than the one passing the oxygen moiety of furan ring and the sulfur moiety ofthe thiophene ring in both heteroporphyrins and22,24-dideazaheteroporphyrins.