Selective Quenching of 2-Naphtholate Fluorescence by Imidazolium Ionic Liquids
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- 作者：Vinod Kumar ; Siddharth Pandey
- 刊名：The Journal of Physical Chemistry B
- 出版年：2012
- 出版时间：October 4, 2012
- 年：2012
- 卷：116
- 期：39
- 页码：12030-12037
- 全文大小：496K
- 年卷期：v.116,no.39(October 4, 2012)
- ISSN：1520-5207
文摘
The effect of addition of water-miscible ionic liquids (ILs), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), 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鈥揤olmer 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][BF4], 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 NaBF4, NaCl, and polyethylene glycol with an average molecular weight of 200, respectively, as additives.