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

Abstract

Striking differences occur between alkyl and alkoxy substituted transition metal β 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 C_nCu, 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 (g_⊥ ) in C_nCu 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 _⊥, 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 ∼ 70:30). However. g _⊥ 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 C_nCu and C_nOCu, due to the absence of the strong exchange-interaction. However, in the solid, the strong exchange-interaction in C_nCu modifies the packing situation, leading to different structures, columnar in C_nCu but lamellar in C_nOCu.

Key Words:

• EPR
• discotic
• copper complex