Magnetic and Luminescent Binuclear Double-Stranded Helicates

Three new binuclear helicates, [M2L2]·3DMF
(M = Co­(II), 1, Zn­(II), 3) and [Cu2L2]·DMF·0.4H2O (2),
have been assembled using the helicand H2L that results
from the 2:1 condensation reaction between o-vanillin
and 4,4′-diaminodiphenyl ether. The metal ions within the binuclear
helicates are tetracoordinated with a distorted tetrahedral geometry.
Direct current magnetic characterization and EPR spectroscopy of the
Co­(II) derivative point to an easy axis type anisotropy for both Co­(II)
centers, with a separation of at least 55 K between the two doublets.
Dynamic susceptibility measurements evidence slow relaxation of the
magnetization in an applied dc field. Since the distance between the
cobalt ions is quite large (11.59 Å), this is attributed in a
first instance to the intrinsic properties of each Co­(II) center (single-ion
magnet behavior). However, the temperature dependence of the relaxation
rate and the absence of slow dynamics in the Zn­(II)-doped sample suggest
that neither the simple Orbach mechanism nor Raman or direct processes
can account for the relaxation, and collective phenomena have to be
invoked for the observed behavior. Finally, due to the rigidization
of the two organic ligands upon coordination, the pure zinc derivative
exhibits fluorescence emission in solution, which was analyzed in
terms of fluorescence quantum yields and lifetimes.