Remarkable Birefringence Enhancement Enabled by π-Conjugated Cyanoguanidine Group Based on Anisotropic Molecule Coordination Approach

Birefringent crystals find widespread applications due to their ability to manipulate polarized light. The development of birefringent materials largely relies on the creation and identification of the promising birefringence "gene". Herein, a planar π-conjugated group, cyanoguanidine (C₂H₄N₄, cnge), was identified as a new birefringence "gene", inspired by an idea of chemically tailoring the typical birefringence unit guanidine with a linear -C≡N unit. Importantly, based on an anisotropic molecule coordination substitution strategy, harnessing the affinity of coordination of cnge to Cd²⁺ achieves the first cnge-based birefringent crystal, Cd(cnge)Cl₂. Cd(cnge)Cl₂ exhibits an enhanced birefringence (0.241 at 550 nm), which is 8.2 times that of the parent compound CdCl₂ and is superior to most guanidine-based crystals. It also possesses characteristics of remarkable thermal stability (>250 °C), wide UV transparency range (cut-edge of 227 nm), and easiness for crystal growth, endowing it with a promising short-wave UV birefringent crystal. Systematic theoretical calculations indicated that cnge has large polarizability anisotropy, and this birefringence-active unit and its favorable arrangement are the main causes of the enhanced optical property. This work provides not only an innovative functional "gene" but also an effective way to find novel UV optical crystals with superior comprehensive performance.