Intramolecular Hydrogen Bonds Enhance Structure Coplanarity, Resulting in Significant Birefringence in Bridged-Bipyridine Halides

The interactions among monomers in an expanded π-conjugated group directly influence the geometry and, consequently, the macroscopic performance of the resulting crystalline material. Therefore, investigating the interaction mechanisms that impact the geometry of the expanded π-conjugated group is a crucial issue. Herein, we report three bridged-bipyridine halides (2,2′-dipyridylamine, 2,2′-dipyridylsulfonamide), denoted as (C₅H₄N)NH(C₅H₄NH)Cl·2H₂O (1, Cc), (C₅H₄N)NH(C₅H₄NH)Br·2H₂O (2, Cc), and (C₅H₄NH)₂SBr₂ (3, I4₁cd), to demonstrate the influence of intramolecular hydrogen bonds (HBs) on controlling the coplanarity of the two linked pyridine rings, thus impacting the macroscopic optical isotropy. Single crystal diffraction data reveal that the presence of different bridging atoms (S in 3 vs N in 1 and 2) led to distinct dihedral angles of 64.4 versus 2.1 and 1.8°, respectively. Experimental studies indicate that while compounds 1–3 all exhibit moderately strong second harmonic generation (0.32–1.1 × KDP), their birefringence (Δn) varies significantly. Compound 3 has a very small (Δn_cal.; _obv.) value of (0.03_cal.; 0.048_obv.), whereas 1 and 2 have values 1 order of magnitude larger (0.26_cal.; 0.25_obv.)/1 and (0.30_cal.; 0.28_obv.)/2, at 550 nm. In-depth analyses demonstrate that this difference is attributed to the nearly coplanar alignment of the bridged-pyridine rings in 1 and 2, which is achieved by the intramolecular HBs that restrict the rotation of the N–C single bond.