From (Sub)Porphyrin to (Sub)Phthalocyanine: Aromaticity Signatures in the UV-Vis Absorption Spectra

Abstract

Understanding the structure-property relationship in (sub)porphyrin and (sub)phthalocyanine derivatives facilitates the design of new materials on demand. One of the main features of these systems are their optical properties, i.e. characteristic Soret and Q absorption bands, which depend on π-delocalization. A possible way to characterize differences in the π system is the theoretical description of the aromaticity, previously used to rationalize the non-optical properties of octaphyrins [1]. However, the high complexity given by the topological flexibility and the existence of multiple π-electron circuits makes it a challenging task [2]. In this work, we analyze the influence structural differences on the aromatic character and the UV/vis absorption spectra of eight (sub)porphyrinoids (Figure 1), including phthalocyanine and subpthalocyanine. We performed calculations at CAM-B3LYP/cc-pVTZ level of theory followed by global and local aromaticity descriptors, providing a qualitative relation between aromaticity and UV-vis absorption spectra. We found that the red shifts in Soret and Q bands observed for tetra/tri-isoindole systems compared to pyrrole analogs correlate with the decrease of global aromaticity due to the inclusion of the fused benzene units. Figure 1