Multidimensional Bidirectional Functional Ordering Strategy for Deep-Ultraviolet Nonlinear Optical Crystals

Abstract Spatial ordering of functional units is a central yet poorly controllable factor in the chemical design of deep-ultraviolet (deep-UV) nonlinear optical (NLO) crystals, where wide bandgaps often compromise structural anisotropy. Here we introduce a bidirectional functional ordering strategy that couples zero-dimensional π-conjugated cations with two-dimensional fluorooxoborate layers, allowing weak directional interactions to actively regulate layer topology and stacking coherence. In crystal [C(NH<2sub>)<3sub>]B<6sub>O<9sub>F (GBOF), the guanidinium cations simultaneously adapt and reshape the [B<6sub>O<9sub>F] layer. Crystal exhibits deep-UV transparency (6.66 eV), enhanced birefringence (0.133 @ 1064 nm), and phase matching wavelength to 196 nm. Systematic modulation of interlayer stacking further reveals functional ordering as an independent chemical variable, enabling continuous tuning of optical properties across a GBOF-i (i=1-30) family of low-energy structures. This work establishes sequence-controlled functional ordering as a general design principle for deep-UV NLO crystals.