Hindered Isomerization at the Silica/Aqueous Interface: Surface Polarity or Restricted Solvation?
Hindered Isomerization at the Silica/Aqueous Interface: Surface Polarity or Restricted Solvation?
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DOI：
10.1021/acs.langmuir.8b02299
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发表时间：
2018-07
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影响因子：
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通讯作者：
Grace E Purnell;R. Walker
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文献类型：
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作者：
Grace E Purnell;R. Walker
在全内反射 (TIR) 几何结构中进行时间分辨荧光测量，检查了吸附到二氧化硅/水界面的香豆素 152 (C152) 的光物理行为。结果表明，界面 C152 与其本体溶液值相比，具有显着不同的光致异构化速率。 C152 在大量水中的荧光主要是短的亚纳秒发射寿命，因为溶质很容易形成非发射、扭曲的分子内电荷转移 (TICT) 状态。来自二氧化硅/水界面的时间分辨 TIR 数据表明，C152 发射包含来自较长寿命状态 (τ = 3.5 ns) 的贡献，该贡献与 C152 在非极性溶剂中的荧光寿命相匹配，在非极性溶剂中不形成光激发 TICT 状态。这种长寿命的激发态归因于界面区域中溶剂化的 C152，其中强的底物-溶剂氢键可防止水性溶剂稳定 C152 的 TICT 异构体。 C152 在冷冻水中也观察到了类似的结果，这强调了二氧化硅表面限制溶剂流动性并改变本体溶液限制的界面溶剂化和反应性的能力。
Time-resolved fluorescence measurements performed in a total internal reflection (TIR) geometry examined the photophysical behavior of coumarin 152 (C152) adsorbed to a silica/aqueous interface. Results imply that interfacial C152 has a remarkably different photoisomerization rate compared to its bulk solution value. C152's fluorescence in bulk water is dominated by a short, sub-nanosecond emission lifetime as the solute readily forms a nonemissive, twisted, intramolecular charge transfer (TICT) state. Time-resolved-TIR data from the silica/aqueous interface show that C152 emission contains a contribution from a longer-lived state (τ = 3.5 ns) that matches C152's fluorescence lifetime in nonpolar solvents where a photoexcited TICT state does not form. This long-lived excited state is assigned to C152 solvated in the interfacial region, where strong substrate-solvent hydrogen bonding prevents the aqueous solvent from stabilizing C152's TICT isomer. Similar results are observed for C152 in frozen water, emphasizing the silica surface's ability to restrict solvent mobility and change the interfacial solvation and reactivity from bulk solution limits.