OPTICAL-PROPERTIES OF AND SPIN INTERACTION IN THE TRINUCLEAR COMPOUND ([MN(ME6-[14]ANE-N4)]2CU(PBA))(CF3SO3)2.2H2O

The goal of this paper is to study to what extent information on the interaction between magnetic centers in a Mn(II)Cu(II)Mn(II) molecular compound can be deduced from the optical spectra. The compound of interest is {[Mn(Me6-[14]ane-N4)]2Cu(pba)}(CF3SO3)2.2H2O with Me6-[14]ane-N4 = (+/-)-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane and pba = 1,3-propylenebis(oxamato). A central Cu(II) ion is bridged by oxamato groups to two peripheral Mn(II) ions. The absorption spectra have been recorded both in pyridine solution at room temperature and in KBr pellets in the 10-290 K temperature range. These spectra, in addition to a d-d transition at 613 nm due to Cu(II) in square planar surroundings, exhibit narrow and rather intense Mn(II) spin-forbidden transitions in the 360-424-nm range. These transitions are activated by an exchange mechanism. The temperature dependence of the intensity of the Mn(II) 6A1(6)A1 --> 6A(1)4A1 excitation corresponding to the spin flip of an electron on its orbital was investigated. It has been shown that this transition is the envelope of 16 subtransitions, the relative intensities of which have been calculated using a model proposed by Tanabe and co-workers. A theoretical expression for the temperature dependence of the intensity of this transition has then been derived; this expression only depends on the J interaction parameter with Mn(II) and Cu(II) ions in their local ground states. By least-squares fitting of the experimental data, J has been found to be equal to -28.2 cm-1. The energy of this transition is shifted by 68 cm-1 toward the high energies as the temperature is lowered. This energy shift depends on both J and the interaction parameter J* between the Cu(II) ion in its ground state and the Mn(II) ions in the spin-flip excited state. By simulation of the spectra, J* has been found as 45 cm-1. These results have been discussed, and the consistency and the complementarity with the information deduced from magnetic properties have been emphasized.