Hydrogen‐Bonded Assembly of Functional Units for Constructing Linear and Nonlinear Optical Crystals

Abstract The ideal foundation for constructing nonlinear optical (NLO) and birefringent materials is provided by planar π‐conjugated units with high hyperpolarizability and optical anisotropy. However, the optical bandgap diminishes with the number of functional units joined by covalent bonds, making their employment challenging in the short‐wavelength spectral region. To overcome this limitation, a strategy is devised that assembles different functional units via hydrogen bonding. By using [C 3 H x O 4 ] ( x = 2–4), [CN 4 H 7 ], and [CN 3 H 6 ] as the fundamental building blocks and omitting the metal cations, six compounds are effectively synthesized. [CN 3 H 6 ][C 3 H 3 O 4 ] exhibits a moderate second harmonic generation (SHG) response (1.2 × KDP) along with a shortest phase‐matching wavelength of 255 nm. Additionally, [CN 4 H 7 ][C 3 H 3 O 4 ] and [CN 3 H 6 ][C 3 H 4 O 4 ] 2 [C 3 H 3 O 4 ] display a significant birefringence of 0.209 and 0.182 at 1064 nm, respectively, and possess short cutoff edges at 233 and 200 nm, indicating their potential as ultraviolet NLO and birefringent materials. This study confirms the role of hydrogen bonding in structural rearrangement, which thereby reinforces the optical properties.