Molecular Geometry and Electronic Structure of Copper Corroles

Copper corroles are known for their unique multiconfigurational electronic structures in the ground state, which arise from the transfer of electrons from the π orbitals of the corrole to the d orbital of copper. While density functional theory (DFT) provides reasonably good molecular geometries, the determination of the ground spin state and the associated energetics is heavily influenced by functional choice, particularly the percentage of Hartree-Fock exchange. Using extended multireference perturbation theory methods (XMS-CASPT2), functional choice can be eliminated. The molecular geometries and electronic structures of both the unsubstituted and meso-triphenyl copper corroles were investigated. A minimal active space was employed for structural characterization, while larger active spaces are required to examine the electronic structure. The XMS-CASPT2 investigations conclusively identify the ground electronic state as a multiconfigurational singlet (S0) with three dominant electronic configurations, in its lowest energy and characteristic saddled structure. In contrast, the planar geometry corresponds to the triplet state (T0), which is approximately 5 kcal/mol higher in energy compared to the S0 state for both the bare and substituted copper corroles. Notably, the planarity of the T0 geometry is reduced in the substituted corrole compared to the unsubstituted one. By analyzing the potential energy surface (PES) between the S0 and T0 geometries using XMS-CASPT2, the multiconfigurational electronic structure is shown to transition towards a single electron configuration as the saddling angle decreases (i.e., as one approaches the planar geometry). Despite the ability of the functionals to reproduce the minimum energy structures, only the TPSSh-D3 PES is reasonably close to the XMS-CASPT2 surface. Significant deviations along the PES are observed with other functionals.