From (Sub)Porphyrins to (Sub)Phthalocyanines: Aromaticity Signatures in the UV-Vis Absorption Spectra

Abstract

The development of novel synthetic methods has greatly expanded the toolbox available to chemists for engineering porphyrin and phthalocyanine derivatives with precise electronic and optical properties. In this study, we focus on the UV-vis absorption characteristics of substituted phthalocyanines and their contracted analogs, subphthalocyanines, which feature nonplanar, bowl-shaped geometries. These macrocycles, which are central to numerous applications in materials science and catalysis, possess extensive π-conjugated systems that drive their unique electronic properties. We explore how the change from a metalloid (B) to a metal (Zn) and the resulting coordination environments influence the aromaticity and, consequently, the spectroscopic features of these systems. A combined computational and experimental approach reveals a direct correlation between the aromaticity of the external conjugated pathways and the Q bands in the UV-vis spectra. Our findings highlight key structural modifications that can be leveraged to fine-tune the optical properties of porphyrinoid systems, offering new pathways for the design of advanced materials and catalysts with tailored functionalities.

Scheme 1. Metallo or B–X Coordinated (Sub)Porphyrins and (Sub)Phthalocyanines Included in This Study

The structural differences that relate porphyrin with phthalocyanine and subphthalocyanine are highlighted in dark blue (N–meso) and turquoise (fused 6-MR). In subporphyrin, we considered both central Zn and B–Cl coordination.

Figure 1

Spatial representation of frontier a_2u, a_1u, and e_g (or a₁, a₂, and e in C_3v) molecular orbitals, with an isocontour of 0.02 a.u., from top to bottom for P, Pc, SubP, and SubPc. In the case of SubPc, the a₁ orbital corresponds to the HOMO – 3.

Scheme 2. Possible Routes to Follow, i Inner, o Outer, and b Benzo, at Each Pyrrole or Isoindole Moiety, Which Define the Closed Pathways along the Molecule

Three examples are the bbb(b), ooo(o), and iii(i) pathways in blue, red, and green, respectively.

Figure 2

Energy of the frontier orbitals (in eV), and and (or and ) (in eV) at the CAM-B3LYP/cc-pVTZ level of theory for phthalocyanines (left) and subphthalocyanines (right). In the case of SubPc, the orbital with a₁ symmetry is H – 3 instead of H – 1. Further details are given in Tables S17–S19.

Figure 3

Relationship between (a) Q-band energy and () and (b) B-band energy and ().

Figure 4

Net current strengths (in nA·T^–1) passing through selected bonds in the S₀ state. In the case of subphthalocyanines, the values within parentheses represent the calculated current strengths when an external magnetic field is oriented perpendicular to the plane defined by the pyrrole or isoindole ring (refer to Section S4.1 for details).

Figure 5

Aromaticity values of iii(i), ooo(o), and bbb(b) circuits in each system according to (a) AV1245 and (b) EDDB_P (normalized according to the number of atoms in the circuit) aromaticity measures. The darker filled bars represent the (a) AV_min and (b) limit of EDDB_P.

Figure 6

Relationship between (a) Q-band energy and AV1245 of the ooo(o) pathway and (b) B-band energy and AV1245 of the iii(i) pathway.