From Low to Very High Birefringence in Bis(2‐pyridylimino)isoindolines: Synthesis and Structure–Property Analysis

A series of new substituted 1,3‐bis(2‐pyridylimino)isoindolines—1,3‐bis(2‐pyridylimino)‐5,6‐bis(2,6‐diisopropylphenoxy)isoindoline ( 2 b ), 1,3‐bis(2‐pyridylimino)‐5,6‐bis(4‐ tert ‐butylphenyl)isoindoline ( 2 c ), and 1,3‐bis(2‐pyridylimino)‐5‐ tert ‐butylisoindoline ( 2 d )—were synthesized and structurally characterized by single‐crystal X‐ray diffraction. The birefringence (Δ n ) of the crystals of unsubstituted 1,3‐bis(2‐pyridylimino)isoindoline ( 2 a ), 2 b , 2 c , and 2 d were measured and found to vary greatly, with Δ n values of 0.0654(3), 0.0629(17), 0.588(10), 0.701(12), respectively. A structure–property relationship for the birefringence values of 2 a – 2 d was outlined and indicated that the anisotropy of the polarizability of the molecules plays a crucial role in the birefringence of the crystals. The greatest birefringence values are achieved when the molecules are oriented in a face‐to‐face configuration intermolecularly, and along the crystallographic face being measured.