Isoreticular Design of KTiOPO 4 -Like Deep-Ultraviolet Transparent Materials Exhibiting Strong Second-Harmonic Generation.

Second-harmonic generation (SHG) is of great technological importance for applications in nonlinear optics, but it remains challenging to engineer large SHG responses in the short-wavelength ultraviolet (UV) region owing to competing microstructure requirements. Herein, we report the first examples of d 0 transition-metal-based (TM-based) deep-UV-transparent nonlinear optical (NLO) crystals MOF 4 H 2 (M = Zr (ZOF), Hf (HOF)), which exhibit unprecedented short UV absorption edges (below 190 nm). Evolving from the KTiOPO 4 (KTP) structure by an isoreticular node substitution strategy, the three-dimensional frameworks of ZOF and HOF consist of corner-sharing [MO 2 F 6 ] moieties that are new functional units in deep-UV NLO material design, conferring wide UV transparency and strong phase-matchable SHG response (2.2 × KH 2 PO 4 (ZOF) and 1.8 × KH 2 PO 4 (HOF) at 1064 nm). Such d 0 -TM-based [MO 2 F 6 ] polyhedra preclude deleterious d-d electronic transitions, resulting in significantly blue-shifted UV absorption edges of ZOF and HOF (<190 nm). The d 0 -TM-based [MO 2 F 6 ] polyhedra introduced in this work offer a new perspective in the construction of deep-UV transparent NLO materials, demonstrating the feasibility of an isoreticular design strategy in developing functional NLO materials.