Optical absorption spectroscopy of the tetranuclear compound [Mn{Cu(oxpn)}(3)](ClO4)(2)center dot 2H(2)O (oxpn equals N,N'-bis(3-aminopropyl)oxamide): Complementarity with magnetic properties

The optical absorption spectroscopy of the tetranuclear compound [Mn{Cu(oxpn)}(3)](ClO4)(2) . 2H(2)O, With oxpn standing for N,N'-bis(3-aminopropyl) oxamide, has been investigated in the 4-300 K temperature range. The central Mn-II ion is linked to three Cu(oxpn) complex ligands, so the molecular symmetry may be defined as D-3. The spectra, in addition to a d-d transition at 19 230 cm(-1) due to the Cu-II ion in square planar surroundings, exhibit narrow and intense Mn-II spin-forbidden transitions in the 24 300-28 750 cm(-1) range. These transitions are activated by an exchange mechanism. The temperature dependence of the main feature corresponding to the (6)A(1) --> (4)A(1), E-4(G) Mn-II transition was investigated. Using a model proposed first by Tanabe and co-workers and adapted to the MnCu3 topology, a theoretical expression for the temperature dependence of the intensity of the transition has been established and compared with the experimental data. The only parameter of this expression is the interaction parameter J between the local ground states (H = -J Sigma(i)S(Mni). S-Cui), which has been found to be -33.8 cm(-1). The energy of the transition has been found to be shifted by 47 cm(-1) toward the high energies as the temperature was lowered. A theoretical expression for the energy shift of the transition has been given. It depends on both J and the interaction parameter J* between the Cu-II ions in their ground states and the Mn-II ion in the spin flip excited state. The comparison with the experimental data has led to a negative J* value of the same order of magnitude as the J value. These results have been discussed in relation with the information deduced from magnetic measurements.