Insights into magnetically induced current pathways and optical properties of isophlorins.

The magnetically induced current density of tetraoxa-isophlorin and dioxa-dithia-isophlorin have been studied at the density functional theory (DFT) level using the gauge including magnetically induced current method (GIMIC). The current density calculations show that the studied isophlorins with formally 28 π electrons are strongly antiaromatic, sustaining paratropic ring currents of -48.5 and -58.3 nA/T, respectively. All chemical bonds of the porphyrinoid macroring participate in the transport of the paratropic ring current. Calculations of excitation energies at the time-dependent density functional theory (TDDFT) level and at correlated ab initio levels show that tetraoxa-isophlorin and dithia-isophlorin have small optical gaps of 0.82-1.34 and 0.90-1.25 eV, respectively. The transition to the lowest excited state, which belongs to the Bg irreducible representation, is dipole forbidden and has therefore not been observed in the spectroscopical studies. For dioxa-dithia-isophlorin, the excitation energies of the Q and B bands calculated at the TDDFT level agree rather well with experimental values with deviations in the range of [-0.15, +0.27] eV, whereas corresponding excitation energies obtained at the ab initio levels are 0.14-0.51 eV too large as compared to experimental values. For tetraoxa-isophlorin, the deviations of the TDDFT excitation energies from experimental values are in the range [-0.10, +0.50] eV and the ab initio values are 0.53-0.97 eV larger than the experimental values due to the significant double excitation character of the excited states.