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 (Δn) 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 Δn 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.
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