Anomalies within Alkyl and Alkoxy Substituted Bis (1,3-diphenylpropane - 1,3 dionato) Copper (II) metallomesogens from X-ray and EPR Studies

Abstract

Our previous EPR studies dealt with the contrasting behaviour of the lamellar C₈OCu, as against the columnar C₈Cu/C₁₀Cu. X-ray and EPR studies of C _n , Cu(n = 6–12) and C_nOCu(n = 6–12) presented here, however, indicate that some members exhibit anomalous behaviour within each series. Thus C₆Cu, C₈Cu, C₁₀Cu and C₁₂Cu (even members) show a single exchange-narrowed strong asymmetric g⊥ along with a weak g‖ in pure solids. Interestingly, C₇Cu and C₉Cu (odd members) show hyperfine quartets in g‖, indicating weak or no exchange. C₇Cu shows a single g⊥, conforming to the axial symmetry in C_nCu. However, C₉Cu is rather unique: gives three different mesophases and exhibits non-axial behaviour. Crystal structure for C₉Cu and C₇Cu reported here for the first time, indicate that the former has a structure similar to that of C₇Cu. However, C₇Cu, which also has z = 1 and belongs to the same triclinic space group, has a close interaction between two molecules at the opposite corners of the ab plane in the unit cell, through chain – chain intermolecular interaction and possibility π – π interaction among phenyl-rings in adjacent molecules. This is reflected in the ‘c’ parameter, which is 10.066 Å, as compared to 5.863 Å in C₆Cu. However, this interaction in C₇Cu vanishes in the mesophase, as indicated by the collapse of the quartet. Weak exchange in C₇Cu is also reflected in the proton relaxation time T ₁ (68.9 msecs), as compared 1.9 msecs for C₆Cu. In C _n OCu, C₇OCu behaves like the dimer C₈OCu, with a complex hyperfine structure including quadrupole forbidden transitions after g ₁ along with a broad g ₂ and a weak g ₃. The dimer in C₈OCu is strong and the quartet persists in the mesophase. Other members, possibly monomers viz., C₆OCu, and C₁₀OCu, give a hyperfine quartet in g ₁ (while in C₁₂OCu it is after g ₁), followed by a strong single g2 line and a weak g3, conforming to the non-axial symmetry of the lamellar complex. Thus, chain length does play a role but not in a linear fashion, because of the anomalies presented above.

Key Words:

• Copper metallomesogen
• X-ray
• EPR