A Universal Design Strategy Achieving Large Birefringence and Short Phase‐Matching Capability of Optical Materials via Dissimilating Non‐π‐Conjugated Units

Abstract Optical materials with non‐π‐conjugated units usually can exhibit large bandgaps, which are conducive to a wide ultraviolet (UV) transparency window. However, the relatively small structural anisotropy hinders the application of these materials as nonlinear optical (NLO) and birefringent materials. Herein, a universal strategy of dissimilating non‐π‐conjugated units is proposed to enhance birefringence, optimize phase‐matching (PM) wavelength, and maintain a large bandgap. The functionality of the series of hetero‐anionic units is distinguished in terms of optimizing birefringence and PM wavelength. In particular, the dimeric hetero‐anionic unit N(SO 2 F) 2 − with the dual‐heteroatom dissimilation is found to be a novel superior optical hereto‐gene. In addition, AN(SO 2 F) 2 (A = Li, Na‐I, K‐I/II, H) and Ba(SO 3 CF 3 ) 2 are identified with large birefringence of approximately 0.051–0.106 at 546 nm, which are much larger than those of corresponding sulfates. HN(SO 2 F) 2 exhibits a short PM wavelength (208 nm), a large NLO coefficient (≈ 2 × KDP), and an extremely short UV cutoff edge (149 nm). Experimental polarizing microscope measurement confirms that the birefringence of Pbca ‐KN(SO 2 F) 2 is significantly enhanced to 0.06 at 546.1 nm, which is 12 times higher than that of K 2 SO 4 . Finally, this strategy offers direction for the design and synthesis of novel NLO or birefringent materials in the future.