Towards rationalizing photoswitchable behavior of Cu$^{\mathrm{II}}_{2}$Mo$^{\mathrm{IV}}$ cyanido-bridged molecule

TL;DR

This study investigates the photoinduced magnetic behavior of a Cu-Mo cyanido-bridged molecule, revealing two independent mechanisms—charge transfer and spin crossover—that explain its complex magnetic responses upon light irradiation.

Contribution

It introduces a detailed model explaining the dual photoinduced processes in a Cu-Mo cyanido-bridged molecule, combining experimental magnetization and susceptibility data analysis.

Findings

Irradiation induces charge transfer, transforming Cu(II) to Cu(I) and Mo(IV) to Mo(V).

A spin crossover occurs, creating an excited Mo(IV)* state with spin 1.

Both mechanisms are supported by fitting experimental data to the proposed model.

Abstract

[Cu$^{\mathrm{II}}$(enpnen)]$_{2}$[Mo$^{\mathrm{IV}}$(CN)$_{8}$]$\cdot$7H$_{2}$O (enpnen = N,N$^\prime$-bis(2-aminoethyl)-1,3-propane-diamine) molecular cluster compound was subject to a series of irradiations with the light of 405 nm. On irradiation isothermal magnetization at 1.8 and 5 K in the field range 0-70 kOe as well as magnetic susceptibility in the temperature range of 2-300 K were subsequently detected. Both types of magnetic signals were next analyzed assuming that the irradiation triggers two independent processes: the metal to metal charge transfer (MMCT) leading to a state with the Arrhenius-type relaxation and the spin crossover (SC) transition ending in a state whose relaxation displays a threshold behavior. The first mechanism leads to an electron from the spinless Mo(IV) configuration being transferred to one of the Cu(II) ions transforming the trimer into the state Cu(II)-N-C-Mo(V)-C-N-Cu(I), with spin 1/2 on the Mo(V) ion and the spinless Cu(I) ion. The other mechanism gives rise to an excited paramagnetic Mo(IV)$^{*}$ linked to two paramagnetic Cu(II) centers with a possible superexchange interaction. The spin of the excited Mo(IV)$^{*}$ species is equal to 1 and associated to a disruption of the 5s-electronic pair. A reasonable result of simultaneous fitting the full series of susceptibility and magnetization data to the model taking into account both mechanisms corroborates their presence.