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 tetraphenyl 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 π-π* 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 π-π* 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 as well as determining novel strategies for the optical read-out of spin states.