Discovery of NLO Semiorganic (C₅H₆ON)⁺(H₂PO₄)⁻: Dipole Moment Modulation and Superior Synergy in Solar-Blind UV Region

Abstract

We herein report a novel semiorganic NLO material, (C₅H₆ON)⁺(H₂PO₄)^-, 4HPP, showing promising excellent properties in the important solar-blind UV region where LAP and its deuterated form DLAP are the only commercialized semiorganic materials. For the first time, the 4-hydroxypyridine (4HP⁺, (C₅H₆ON)⁺) cation is identified as NLO active and how to eliminate the dipole-dipole interaction to avoid the unwanted center-symmetry-trap caused by the polar-induced susceptibility is well demonstrated. Remarkably, 4HPP exhibits competitive and even better properties compared with LAP that include better thermal stability (decomposition at 166 vs 112 °C of LAP); wider transparency range (0.26-1.50 μm); very strong SHG response (3 × KDP); a suitable large birefringence (Δ<i>n</i>_cal = 0.25 vs 0.075 of LAP); and a high laser-induced damage threshold (2.2 × KDP). First-principles calculations show that the π-conjugated organic (4HP)⁺ cation governs the optical anisotropy, whereas the synergy of the organic and inorganic moieties dominates the SHG process. Our discovery points out a new path for the rational design of high performance semiorganic materials that require an acentric structure.