Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India.
J Phys Chem B. 2012 Oct 4;116(39):12030-7. doi: 10.1021/jp3080817. Epub 2012 Sep 24.
The effect of addition of water-miscible ionic liquids (ILs), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF(4)]), 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim][OTf]), and 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate ([bmpyrr][OTf]), on photophysical properties of 2-naphthol in water at various pHs is reported. Electronic absorbance behavior of 2-naphthol dissolved in aqueous mixtures of ILs is observed to be similar to that found in water at different pHs. The excited-state properties, however, are changed dramatically as the IL is added to the milieu. The presence of imidazolium IL results in significant quenching of the fluorescence emission from 2-naphtholate. On the contrary, pyrrolidinium IL does not quench the fluorescence from the anionic species. The quenching of 2-naphtholate fluorescence by aromatic imidazolium cations in aqueous IL mixtures is found to follow simple Stern-Volmer behavior. The aromatic imidazolium cation acts as an electron/charge acceptor during the quenching process where formation of a weakly fluorescent complex between the imidazolium cation and the excited 2-naphtholate anion possibly involving the acidic C2 proton of imidazolium is proposed. Because of the absence of such an acidic proton, the nonaromatic pyrrolidinium cation is not able to form a complex with the excited 2-naphtholate and cannot act as an electron/charge acceptor. Excited-state emission intensity decay data further corroborate this hypothesis as the intensity decay fits well to a single-exponential decay with no change in recovered lifetimes as [bmpyrr][OTf] is added; a double-exponential decay is required to satisfactorily fit the decay data in the presence of [bmim][BF(4)], hinting at the presence of a weakly fluorescent complex. The uniqueness of ILs in affecting excited-state properties of the 2-naphthol system is demonstrated through comparison with NaBF(4), NaCl, and polyethylene glycol with an average molecular weight of 200, respectively, as additives.
本文报道了水溶性离子液体（ILs）1-丁基-3-甲基咪唑四氟硼酸盐([bmim][BF4])、1-丁基-3-甲基咪唑三氟甲磺酸盐([bmim][OTf])和 1-丁基-1-甲基吡咯烷三氟甲磺酸盐([bmpyrr][OTf])对 2-萘酚在不同 pH 值下水溶液中光物理性质的影响。观察到 2-萘酚在含有 IL 的水溶液中的电子吸收行为与在不同 pH 值下的水中相似。然而，随着 IL 的加入，激发态性质发生了显著变化。含咪唑基 IL 的存在显著猝灭了 2-萘酚阴离子的荧光发射。相反，吡咯烷基 IL 不会猝灭阴离子物种的荧光。在水相 IL 混合物中，芳香咪唑阳离子猝灭 2-萘酚荧光被发现遵循简单的 Stern-Volmer 行为。在猝灭过程中，芳香咪唑阳离子作为电子/电荷接受体，形成 2-萘酚阴离子与激发态的弱荧光配合物，可能涉及咪唑的酸性 C2 质子。由于不存在这样的酸性质子，非芳香吡咯烷阳离子不能与激发态的 2-萘酚形成配合物，也不能作为电子/电荷接受体。激发态发射强度衰减数据进一步证实了这一假设，因为强度衰减很好地符合单指数衰减，随着[bmpyrr][OTf]的加入，恢复寿命没有变化；在存在[bmim][BF4]的情况下，需要双指数衰减才能满意地拟合衰减数据，暗示存在弱荧光配合物。通过与 NaBF4、NaCl 和平均分子量为 200 的聚乙二醇分别作为添加剂进行比较，证明了 ILs 在影响 2-萘酚体系的激发态性质方面的独特性。
