An excellent ultraviolet nonlinear optical crystal derived from the polar methylphosphonate unit†

Ultraviolet (UV) nonlinear optical (NLO) crystals that have the capability of converting the wavelength of lasers into the UV region have attracted a lot of research interests. However, the exploration of UV NLO crystals is limited by highly transparent functional units. Herein, the polar methylphosphonate unit with large polarizability and polarizability anisotropy, as well as a wide HOMO–LUMO gap, has been used as a new UV NLO building unit for the first time. An excellent UV NLO crystal, [C(NH₂)₃]₂[CH₃PO₃], has been rationally obtained by tuning the polar [CH₃PO₃] units into an ordered arrangement with the π-conjugated [C(NH₂)₃]⁺ cations. Remarkably, [C(NH₂)₃]₂[CH₃PO₃] exhibits good balance among three key NLO properties, including a short UV cutoff edge extending to 207 nm, a moderate phase-matching second-harmonic generation (SHG) response of 1 × KH₂PO₄ (KDP), and a suitable birefringence of 0.053@1064 nm. Besides, [C(NH₂)₃]₂[CH₃PO₃] possesses a very high laser damage threshold of 326.8 MW cm⁻², a wide high transparency window spanning over 224 to 1425 nm and enhanced thermal stability up to 180 °C under an air atmosphere. More importantly, the [C(NH₂)₃]₂[CH₃PO₃] crystal can easily grow up to a large size of 5.4 × 3.8 × 3 mm³ in view of its facile crystal growth behavior. Both crystal structure and theoretical calculations demonstrate that the SHG response of [C(NH₂)₃]₂[CH₃PO₃] mainly originates from the ordered arrangement of the polar [CH₃PO₃] units. This research confirms that the polar [CH₃PO₃] group is an intriguing SHG-active unit that will contribute to the discovery of more UV NLO crystals in future.