Designing Birefringent Materials: A Crystal Engineering Approach

This thesis focuses on the rational design of birefringent materials via the use of crystal engineering methodologies. Birefringence (∆n) arises when the refractive index differs in different directions of a material. The ligands targeted have a high polarizability anisotropy and are (mainly) based on 2,6-bis(benzimidazole)pyridine (BBP). By incorporating these highly anisotropic ligands into well-ordered networks, formed through coordination and/or hydrogen bonds, the ligands are oriented parallel to one another to gain additive polarizability anisotropy, resulting in birefringent materials. Initial studies targeted complexes of zinc and manganese with BBP incorporated into coordination polymers, as well as supramolecular hydrogen bonding networks. The resulting materials highlight the factors affecting the measurable birefringence: crystal quality; crystal growth direction; and alignment of BBP units, with ∆n values ranging from 0.08(1) to the extremely high 0.69(2) for Mn(BBP)[Au(CN)2 ]2 - ·H2O and Mn(BBP)Cl2 (MeOH)·MeOH respectively. Attempts to remove the NH hydrogen bonding donor groups on BBP by substituting BBP for 2,6-bis(Nmethyl-benzimidazole)pyridine (MBBP) or 2,6-bis(benzathiazole)pyridine (BBTP), as well as removing hydrogen bonding solvents, resulted in a poorer alignment of the anisotropic ligands and lower ∆n values (0.23(3) for Mn(MBBP)Br2 ). By incorporating highly fluorescent lanthanide ions with BBP units into coordination polymers of the form Ln(BBP)(NO3 )2 [Au(CN)2 ]·(CH3CN), multifunctional materials that are both highly birefringent (∆n = 0.57(3)) and emissive were synthesized. Structural parameters to describe the orientation of the anisotropic ligands with respect to the primary crystal growth face were outlined and applied to a series of coordination polymers containing 2-(2-pyridyl)- 1,10-phenanthroline (phenpy). This methodology enabled the elucidation of reasons for differences in ∆n values (0.37(2) vs 0.59(6) for Cd(phenpy)[Au(CN)2 ]2 and Zn(phenpy)(H2O)[Au(CN)2 ]2 ·2H2O respectively). Finally, BBP derivatives with higher polarizability anisotropies: 2,6-bis(5/6-halo-benzimidazole)pyridine (diXBBP), 2,6-bis(5,6-dichloro-benzimidazole)pyridine (TClBBP), 2,6-bis(5,6-dimethyl-benzimidazole)- pyridine (TMBBP) and 2,6-bis(napthoimidazole)pyridine (BNP) were also incorporated into materials in an attempt to make crystals with even higher birefringence. Eu(diClBBP)(NO3 )2 [Au(CN)2 ]·(CH3CN) is isostructural with Ln(BBP)(NO3 )2 [Au(CN)2 ]·(CH3CN) and the crystal growth face can be tuned to be more favourable by making a solid solution of the BBP and diXBBP ligands within the structure. Initial results with TClBBP formed well aligned structures and led to ∆n values as high as 0.912(12) for Pb(TClBBP−H)(MeOH)[Au(CN)2 ], which is among the highest values reported worldwide.