The hydrogen‐bonded complexes of fluorophenylacetylenesexhibit unusual and interesting fluorescence turn ON/OFF behaviour following excitation to ¹ππ* (S₁) state. The fluorescence switching behaviour can be realized by (i) “change in the intermolecular structure, (ii) change in the position of fluorine substitution and (iii) change in the hydrogen bonding partner or a combination thereof. Experiments indicate that the ≡C−H⋅⋅⋅X (X=O, N) hydrogen bonding with the acetylenic group plays a pivotal role in this switching behaviour. Intriguingly, weaker ≡C−H⋅⋅⋅X hydrogen bonding leads to fluorescence OFF state, which is turned ON by stronger hydrogen bonding. The observed fluorescence this switching behaviour is rationalized on the basis of a phenomenological model which suggests a coupling between the initially excited S₁ state and a dark S_n state in the Franck‐Condon region with limited window controlled by the ≡C−H⋅⋅⋅X hydrogen bonding as a crucial parameter. Such fluorescence switching behaviour in hydrogen‐bonded complexes is unprecedented and these intriguing results hopefully will stimulate theoreticians to test ′state of the art′ theories to explain these observations in a consistent manner.

中文翻译：

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氟苯基乙炔的氢键配合物：要发荧光还是不发荧光？

以下激发的fluorophenylacetylenesexhibit不寻常的和有趣的荧光转ON / OFF行为氢键络合物¹ ππ*（S ₁）状态。荧光转换行为可以通过（i）“分子间结构的改变，（ii）氟取代的位置的改变和（iii）氢键配偶体的改变或其组合来实现。实验表明，≡C-H⋅⋅⋅X（X = O，N）与炔基的氢键在这种转换行为中起关键作用。有趣的是，≡C-H⋅⋅⋅X氢键较弱会导致荧光关闭状态，而荧光会由于较强的氢键而开启。在现象学模型的基础上，观察到的这种切换行为的荧光是合理的，该现象表明初始激发的S ₁状态和暗色S _n之间存在耦合弗兰克-康登区的氢键状态，由windowC-H⋅⋅⋅X氢键作为关键参数控制的窗口有限。氢键配合物中的这种荧光转换行为是前所未有的，这些有趣的结果有望激发理论家测试“最新技术”理论，以一致的方式解释这些观察结果。