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

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 CnCu(n = 6 - 12) and CnOCu(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(perpendicular to) along with a weak g(parallel to) in pure solids. Interestingly, C7Cu and C9Cu (odd members) show hyperfine quartets in g(parallel to), indicating weak or no exchange. C7Cu shows a single g(perpendicular to), conforming to the axial symmetry in CnCu. However, C9Cu is rather unique : gives three different mesophases and exhibits non-axial behaviour. Crystal structure for C6Cu and C7Cu reported here for the first time, indicate that the former has a structure similar to that of C8Cu. 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 pi-pi interaction among phenyl-rings in adjacent molecules. This is reflected in the 'c' parameter, which is 10.066 angstrom, as compared to 5.863 angstrom 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 CnOCu, 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 g(2) line and a weak g(3), 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.