Reversible photomagnetic properties of the molecular compound [{CuII(bipy)2}2{MoIV(CN)8}]·9H2O·CH3OH

Abstract

The optical and photomagnetic properties of [{Cu II (bipy) 2 } 2 {Mo IV (CN) 8 }]$9H 2 O$CH 3 OH (1) have been reinvestigated. A comparison between spectra in solution and in the solid state revealed the presence of an intervalence band (or MetaleMetal Charge Transfer, hereafter noted MMCT) at 570 nm. The photomagnetic properties have been performed in a Superconducting QUantum Interference Device at 10 K with irradiation in the range of the MMCT: 488 nm, 520 nm and 647 nm at 10 K. An important increase of the magnetic signal has been measured after 1 h of irradiation at 488 nm, whereas a weaker increase has been obtained for the irradiation at 520 nm in the same conditions. Moreover, after an excitation at 488 nm, an irradiation at 647 nm has induced a decrease of the magnetic moment, which corresponds to a partial deexcitation. The complete characterization of the photoproduct has been realised after an irradiation of 4 h at 488 nm. The photomagnetic properties have shown an increase of the paramagnetism of 1 at low temperature. After a thermal heating at 300 K, the material goes back to its initial state before irradiation. It is the first time that a fully reversible photomagnetic behaviour for the compound [{Cu II (bipy) 2 } 2 {Mo IV (CN) 8 }]$9H 2 O$CH 3 OH has been described. The observed properties have been discussed in terms of an electron transfer mechanism Mo / Cu.

On the mechanism of the photoinduced magnetism in copper octacyanomolybdates

“…This is significantly higher than the energy used in the experiments to induce the magnetism. On the other hand, CASPT2 calculations with an extended CAS (11,12), which includes all the occupied metal orbitals, show that the MMCT state does not appear at energies lower than 4 eV. Consequently, the MMCT state is probably not populated at the initial stages of the photomagnetic process.…”

“…5 For this reason they are used as building blocks of magnetic clusters and networks and are especially important for the development of single molecule magnets. 3,6 Besides, some compounds of this family display photomagnetic behavior, [7][8][9][10][11][12][13][14][15][16][17] which turns them into promising materials, since their magnetic properties can be controlled by external stimuli. Moreover, in some cases the inverse reaction can be induced thermally or by irradiation with light.…”

“…Moreover, in some cases the inverse reaction can be induced thermally or by irradiation with light. [7][8][11][12][13][14] Switching of the magnetic state has been explained by a metal to metal electron transfer that changes the electronic state, and hence the spin, on the interacting metallic centers, then giving rise to a high molecular magnetic moment. 4,7,11,18,19 Specifically, for compounds of the type [Mo(CN) 2 (CN-CuL) 6 ] 8+ (L = 2,2 0 -bipyridine, tris(2-aminoethyl)amine, N,N 0 -dimethylethylenediamine), electron transfer in one of the Mo IV -Cu II pairs would lead to a situation with one Mo V -Cu I pair and five Mo V -Cu II magnetic pairs, resulting in a strong ferromagnetic interaction (J > 100 cm À1 ).…”

Experimental and theoretical insights into the photomagnetic effects in trinuclear and ionic Cu(ii)–Mo(iv) systems

“…Up to now, photomagnetic processes in Cu(II)-Mo(IV) systems have been considered in terms of two possible mechanisms: metal-to-metal charge transfer (MMCT): paramagnetic Cu II (S = 1/2)⋯Mo IV-LS (S = 0)⋯Cu II (S = 1/2) → ferromagnetic Cu I (S = 0)⋯[Mo V-LS -Cu II ](S total = 1), 27,[29][30][31]33,36,[38][39][40][41]43,44,[46][47][48][49][50][51][52][53][54][55] and a Light-Induced Excited Spin-State Trapping (LIESST) effect on the Mo(IV) centre: paramagnetic Cu II (S = 1/2) ⋯Mo IV-LS (S = 0)⋯Cu II (S = 1/2) → ferromagnetic [Cu II -Mo IV-HS -Cu II ](S total = 2). [27][28][29][31][32][33]35,37,42,45 The latter case may result from the photoinduced formation of an intermediate geometry between the ideal geometries of TDD-8 and SAPR-8 (Fig.…”

“…[13][14][15][16][17][18][19][20][21][22][23][24] The greatest successes in developing photomagnetic materials were achieved for octacyanidometallate based systems, 25,26 in particular for copper(II)-molybdenum(IV) ones. 27 Furthermore, it was found that Cu II -Mo IV photomagnetic assemblies can be based on polynuclear molecules, [28][29][30][31][32][33][34][35][36][37][38][39][40][41] chains, 33,[42][43][44] layers 33,[45][46][47] and three-dimensional networks 33,[48][49][50][51][52][53][54][55] with at least one bridging cyanide per single copper(II) centre. Surprisingly, ionic systems with an isolated [Mo(CN) 8 ] 4− anion were not considered, assuming that the photomagnetic phenomenon in Cu II -Mo IV compounds originates from the light-induced MMCT mechanism.…”