pH-Dependent Switching Between Nonlinear-Optical-Active Nitrate-Based Supramolecular Polymorphs

Tailored syntheses of non-centrosymmetric (NCS) second-order nonlinear optical (NLO) crystals, particularly those of metastable NCS polymorphs, remain extremely challenging due to the complex interactions of their constituent primitives. In this work, we report the first successful synthesis of a set of three nitrate supramolecular polymorphs (C₂H₅N₄)(NO₃) (α-, β-, and γ-phases) by a pH-modulation secondary-bond strategy, via the assembly of two types of π-conjugated planar primitives. Solutions of different pH result in differing orientation of secondary bonds between [C₂H₅N₄] and [NO₃] primitives. Secondary bonds dominate the packing of primitives in crystal lattices; varying these secondary bonds can afford NCS or centrosymmetric nitrate supramolecular polymorphs, with 2D α-phase irreversibly transformed to 2D β-phase in a solution of appropriate pH. These two polymorphs display distinctly different SHG responses and birefringences, ascribed to variation in stacking of 2D hydrogen-bond layers resulting from the differing environmental pH. Supramolecular crystal structures comparison and theoretical studies confirm the crucial role played by secondary-bond interactions between adjacent primitives in the rare nitrate supramolecular polymorphs. This work paves the way in elucidating the supramolecular polymorph transition-mechanisms and correlating the polymorph structures and their NLO properties, and thereby discloses a new paradigm for development of high-performance NCS materials with tailored optical properties.