Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India.
J Phys Chem B. 2009 Dec 10;113(49):15901-6. doi: 10.1021/jp907902t.
Solvation dynamics in Nafion membrane is studied using the well-known solvation probe, coumarin 102 (C102). In native Nafion membrane, the fluorescence maximum of C102 occurs at 525 nm. The decays recorded at different wavelengths are superimposable. There is no time-dependent Stokes shift (TDSS) in the time scale of the experiment. This is rationalized in light of the strongly acidic environment in Nafion membrane, which causes the C102 molecules to become protonated. The protonated molecules are bound tightly to the negatively charged sulfonate groups. In Na(+)- and Me(4)N(+)-exchanged Nafion membranes, the fluorescence gets blue-shifted by 65 nm, indicating the deprotonation of the cation and formation of neutral C102 in these membranes. TDSS is observed in the picosecond-nanosecond time scale, in the cation-exchanged Nafion membranes, although the amount of Stokes' shift is rather small, as compared to that observed in organic solvents, indicating that a significant amount of the solvation is ultrafast and is missed in the present experiment. The observed solvation dynamics is bimodal with fast ( approximately 1 ns) and slow (>10 ns) components. The ultraslow component is ascribed to the quasi-static water molecules in the Nafion membrane. The difference in the extents of apparently missing ultrafast components, between Me(4)N(+)- and Na(+)-substituted membranes is rationalized by a model involving the difference in distributions of the cations in the water channel.
使用著名的溶剂探针香豆素 102（C102）研究了 Nafion 膜中的溶剂化动力学。在天然 Nafion 膜中，C102 的荧光最大发射波长出现在 525nm。在不同波长下记录的衰减可以叠加。在实验的时间尺度内没有时间相关的斯托克斯位移（TDSS）。这可以根据 Nafion 膜中强酸性环境来解释，该环境导致 C102 分子质子化。质子化分子与带负电荷的磺酸盐基团紧密结合。在 Na(+)和 Me(4)N(+)交换的 Nafion 膜中，荧光蓝移了 65nm，表明阳离子去质子化并在这些膜中形成中性 C102。在 picosecond-nanosecond 时间尺度上观察到 TDSS，尽管与在有机溶剂中观察到的相比，斯托克斯位移的量相当小，这表明大量溶剂化是超快的，并且在当前实验中被忽略了。观察到的溶剂化动力学呈双峰分布，具有快速（约 1ns）和缓慢（>10ns）组分。超慢速组分归因于 Nafion 膜中的准静态水分子。Me(4)N(+)和 Na(+)取代膜之间明显缺少超快成分的程度差异可以通过涉及水通道中阳离子分布差异的模型来解释。
