Porphyrins exhibit tunable optical properties on fusing with aromatic systems. In the present study, optical properties of four distinct configurations of helicene-fused porphyrins are explored using density functional theory (DFT) with the dispersion corrected B3LYP functional and the def2-TZVP basis set. The multiple helicene-fused porphyrin derivatives are modeled based on the position and direction of helicene rings, such that they are pointed above and below the plane of porphyrin in the cis and trans configurations. The TDDFT analysis revealed a significant red shift in absorption bands, with Q-bands extending into the near-infrared (NIR) region due to extended π-conjugation. The positioning of helicenes modifies the spectral profile such that the trans configurations exhibit charge transfer between porphyrin and helicene, whereas cis configurations have an additional helicene-to-helicene charge transfer band. Further, the magnetic properties are calculated to assess the aromaticity of the porphyrin derivatives via nucleus-independent chemical shifts (NICS) and magnetically induced current density (MICD). The above properties are found to be varied with the orientation of helicenes such that higher aromaticity for the trans configuration compared to that for cis.
Kalavadia et al. (Thu,) studied this question.