EPR Studies of Alkyl Substituted Aryl β Diketonates of Discotic Copper Complexes 2: Concentrated Versus Dilute Systems

Abstract

Restricted Access.Striking differences occur between alkyl and alkoxy substituted transition metal b.beta diketonates. Thus pure C8Cu exhibit polymorphic discotic phases, whereas C8OCu shows a single mesophase. The length of the chain and the nature of the metal atom also play an important role. Thus C10Cu exhibits a single discotic phase, whereas C10Pd is a polymorphic discotic. Magnetic resonance studies indicate the presence of strong long-range antiferromagnetic exchange-interaction in CnCu, as against no such interaction in C8OCu. However, C8Cu does not order antiferromagnetically, even at liquid He temperature. A comparative 1H NMR study of diamagnetic C10Pd against C8Cu reveals interesting features of the phase transitions. The highly asymmetric, strongly exchanged-narrowed EPR line (gbottom) in CnCu becomes somewhat wider in the mesophase (D1). broadens significantly (decreases in intensity) in the D2 phase and remains almost unchanged in the isotropic phase. However, in dilute systems viz., C8Cu doped in C8Pd and C10Cu doped in C10Pd, the results are very different. The gbottom, line now splits up into a quartet (I = 3/2 for Cu), with the outer lines clearly exhibiting a doublet structure from the isotopes of Cu (viz., 63Cu and 65Cu in the ratio of their concentration of sim 70:30). However. gbottom exhibits the quadrupole-split pattern, as in C8OCu, with loss of axial symmetry. The frozen solution spectra of C8Cu and C10Cu in CHCI3 at 77 K are very similar to that in the doped solid. Thus, at the molecular level, there is virtually no difference between CnCu and CnOCu, due to the absence of the strong exchange-interaction. However, in the solid, the strong exchange-interaction in CnCu modifies the packing situation, leading to different structures, columnar in CnCu but lamellar in CnOCu