Spin-crossover in phenylazopyridine-functionalized Ni-porphyrin: trans-cis isomerization triggered by π-π interactions

Identifier:  PC:1175

Handle: https://hdl.handle.net/20.500.11797/PC1175

Authors:  Coen de Graaf; Gerard Alcover-Fortuny; Rosa Caballol

Abstract:
Reversible, room-temperature light-induced spin-crossover has been reported in a Ni–porphyrin functionalized with a phenylazopyridine (PAPy) ligand (Venkataramani et al., Science, 2011, 331, 445). Upon light irradiation (500 nm), the azopyridine moiety induces a change in the Ni(II) coordination sphere from square planar (n = 4) to square pyramid (n = 5), leading to a change in the total spin of the molecule from S = 0 to S = 1. The trans– cis isomerization in the azopyridine ligand has been proposed to trigger the spin-crossover effect. However, the radiation used to induce the HS state is about 135 nm red-shifted with respect to the radiation used for trans– cis isomerization of the NQN double bond in other compounds. To elucidate the light-induced spin-crossover mechanism of this Ni(II) compound, a combined DFT/CASSCF/CASPT2 study has been performed to determine the most stable cis and trans conformers with n = 4 or n = 5, and to characterize the excitation that triggers the SCO process. π–π interactions between porphyrin and PAPy are shown to play an essential role in the spin crossover.