Solid nonlinear optical (NLO) materials with switchable second-harmonic generation (SHG) on/off states show strong potential applications in photo-electronic devices. A sustained progress has been made through the persistent exploration of NLO materials, and numerous switchable crystals viewed as single-step SHG switching have been reported. However, the multi-step switching with SHG “off–on–off” states has rarely been reported, which severely restricts the development of novel NLO sensors and lasers. Herein, we report an organic–inorganic hybrid crystal with unconventional two-step SHG transitions. It shows excellent high contrast “off–on–off” SHG conversion in the range of around room temperature. Most notably, its SHG-active state covers a wide temperature range from 304 to 354 K, which is superior to most unconventional SHG switches. The structure–activity relationship analysis shows an unusual symmetry reduction feature triggered by molecular motion, that is, the dual centro–noncentro–centrosymmetric transition, producing unusual two-step SHG switching. This study will promote the diversified development of switchable NLO materials.

中文翻译：

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由逆对称破缺触发的具有宽 SHG 激活阶跃的热驱动异常双 SHG 开关

具有可切换二次谐波产生 (SHG) 开/关状态的固体非线性光学 (NLO) 材料在光电器件中显示出强大的潜在应用。通过对 NLO 材料的持续探索取得了持续的进展，并且已经报道了许多被视为单步 SHG 切换的可切换晶体。然而，SHG“关-开-关”状态的多步切换很少被报道，这严重限制了新型NLO传感器和激光器的发展。在这里，我们报告了一种具有非常规两步 SHG 跃迁的有机-无机杂化晶体。它在室温附近显示出出色的高对比度“关-开-关” SHG 转换。最值得注意的是，其 SHG 激活状态覆盖了从 304 到 354 K 的宽温度范围，优于大多数非常规 SHG 开关。构效关系分析显示了由分子运动触发的不寻常的对称性降低特征，即双中心-非中心-中心对称转变，产生不寻常的两步 SHG 转换。本研究将促进可切换非线性光学材料的多元化发展。