A multireference picture of electronic excited states in vanadyl and copper tetraphenyl porphyrin molecular qubits

The nature of electronic excited states has a deep impact on the dynamics of molecular spins, but remains poorly understood and characterized. Here we carry out a thorough multiconfigurational investigation for two prototypical molecular qubits based on vanadyl and copper tetra-phenyl porphyrins. State-average CASSCF and NEVPT2 calculations have been employed with four different active spaces of growing complexity to account for the d-d, second d-shell, ligand-to-metal charge transfer states and $\pi-\pi^*$ excited states, revealing an in-depth picture of low-lying excited states in agreement with experimental observations. The largest active spaces attempted, (13,14) for the vanadyl and (17,12) for the copper compounds, reveal that the lowest-lying excited states originate from $\pi-\pi^*$ quartet excitations. These findings shed light on the nature of the excited states of molecular qubits, taking an important step toward elucidating their role in molecular spin dynamics.