Coordination-Site Engineering Enables Synergistic π-Coupling in Peroxomolybdates

Achieving constructive coupling among multiple second-harmonic-generation (SHG)-active units remains a major challenge in the development of high-performance nonlinear optical (NLO) crystals. Herein, we introduce a coordination-site occupation strategy that enables precise regulation of the geometric arrangement and electronic coupling among three SHG-active components: a delocalized π-conjugated organic ligand, a d0 Mo6+ center, and a localized π-conjugated peroxide ligand. Application of this strategy to a peroxomolybdate system affords three new complexes with progressively increasing coordination-site occupancy. The optimized compound, [MoO(O2)(H2O)(C7H3NO4)] (III), crystallizes in a noncentrosymmetric structure and exhibits a record powder SHG response of 34.9 × KH2PO4 at 1300 nm, together with phase-matching behavior and an impressive laser-induced damage threshold of 5.8 GW/cm2, representing the highest SHG performance reported to date for a peroxomolybdate complex. Computational analyses reveal that the nearly parallel alignment between the organic π-conjugated plane and the Mo-peroxide plane maximizes electronic coupling and hyperpolarizability, thereby giving rise to the exceptional second-order NLO performance. These findings establish coordination-site occupation as a general crystal-engineering strategy for synergistically integrating multiple SHG-active units and provide a rational framework for the design of next-generation high-performance nonlinear optical materials.