EPR studies of alkyl substituted aryl beta diketonates of discotic copper complexes .2. Concentrated versus dilute systems

Striking differences occur between alkyl and alkoxy substituted transition metal 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 H-1 NMR study of diamagnetic C10Pd against C8Cu reveals interesting features of the phase transitions. The highly asymmetric, strongly exchanged-narrowed EPR line (g(perpendicular to)) in CnCu becomes somewhat wider in the mesophase (D-1), broadens significantly (decreases in intensity) in the D-2 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 g(parallel to) line now splits up into a quarter (I = 3/2 for Cu), with the outer lines clearly exhibiting a doublet structure from the isotopes of Cu (viz., (CU)-C-63 and Cu-65 in the ratio of their concentration of similar to 70:30). However, g(perpendicular to) exhibits the quadrupole-split pattern, as in C8OCu, with loss of axial symmetry. The frozen solution spectra of C8Cu and C10Cu in CHCl3 al 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 bur lamellar in CnOCu.