Aromaticity and ring currents in boron-doped porphyrins

Abstract

The electron delocalization and aromaticity of a series of boron-doped porphyrins were investigated computationally using the ring current criterion. Magnetically induced current densities, ring current strengths, and aromatic ring current pathways were calculated at the density functional theory (DFT) level using the B3LYP and BHandHLYP functionals. The results demonstrate that boron coordination with O/Cl, as well as two-electron oxidation of B₂(porphin) disrupt current pathways, significantly altering the aromatic character and magnetic properties of the molecules. Neutral B₂(porphin) sustains a strong paratropic ring current of −39.1 nA T⁻¹ leading to its paramagnetism (χ = 15.6 a.u.). In contrast, its double charged forms adopt aromatic character with diatropic ring current (27.3 nA T⁻¹ and 28.3 nA T⁻¹ for dication and dianion, respectively) and diamagnetic susceptibility (χ = −77.2 a.u. and χ = −88.9 a.u. for dication and dianion, respectively). The introduction of electron-deficient boron atoms into the conjugated system of aromatic porphin and antiaromatic isophlorins reduces electron delocalization, as evidenced by significantly weaker ring current strengths compared to their parent counterparts without boron atoms. Charge-dependent ring current behavior was also observed for meso-boron-doped porphin, tetraoxa-isophlorin, and tetrathia-isophlorin. Notably, the B-tetrathia-isophlorin dication sustains a strong diatropic ring current of 13.1 nA T⁻¹ despite its distorted geometry, confirming its aromatic character. These findings highlight the tunable electronic and magnetic properties of boron-doped porphyrins, providing key insights for designing functional molecular materials.