An Ultra-Wide Bandgap Sulfonate with Two-Step Second-Harmonic Generation Switching

Nonlinear optical (NLO) switches featuring multistate stability and external-stimuli responsiveness represent an important class of functional materials for photonics and sensing applications. However, their practical application has been seriously impeded by their narrow bandgaps, which limit transmission in the (deep-)ultraviolet (UV) spectral region. Herein, we report the first organic deep-UV-transparent NLO switch [NH(CH₃)₃][CH₃SO₃], which exhibits a thermally activated reversible two-step switching of second-harmonic generation (SHG) via low-, no-, and high-response states over the temperature range 293–428 K, and with remarkable SHG contrasts that span many orders of magnitude. The temperature activated SHG is responsive in the UV–visible wavelength region, reaching 2.6 × KH₂PO₄ at 1064 nm, setting a new record for deep-UV-transparent NLO switches. Detailed experimental and mechanistic studies demonstrate that the reversible single-crystal-to-single-crystal structure transformation responsive to thermal stimulation arises from the translation and rotation of the [NH(CH₃)₃]⁺ and [CH₃SO₃]⁻ primitives driven by the rearrangement of intermolecular hydrogen bonds, and the progressive disordering of these primitives at elevated temperatures.