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J. Phys. Chem. B 2008, 112, 1906-1912
Comparison of Photophysical Properties of the Hemicyanine Dyes in Ionic and Nonionic Solvents Taekyu Shim,†,‡ Myoung Hee Lee,† Doseok Kim,*,†,‡ and Yukio Ouchi| Department of Physics, Sogang UniVersity, Seoul 121-742, Korea, Interdisciplinary Program of Integrated Biotechnology, Sogang UniVersity, Seoul 121-742, Korea, and Department of Chemistry, Nagoya UniVersity, Chikusa-ku, Nagoya 464-8602, Japan ReceiVed: August 23, 2007; In Final Form: NoVember 15, 2007
The fluorescence properties of 4-[4-(dimethylamino)styryl]-1-n-alkylpyridinium bromide (hemicyanine) dissolved in solvents of different polarities and viscosities (methanol, ethylene glycol, tetra-ethylene glycol, glycerol, benzyl alcohol, pyridine, and two ionic liquids, 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIM]BF4, and 1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM]PF6) were investigated. Significant increase in the fluorescence quantum yield and the fluorescence decay lifetime was observed with the increase in the viscosity of the solvent medium. It is because the intramolecular rotational motion of the molecule becomes more difficult in viscous liquid, which leads to a decrease in the nonradiative decay processes. The fluorescence quantum yields for all of the solutions followed a semiempirical law that depends only on the solvent viscosity. The correlation function C(t) was obtained for each solution by joining fluorescence decay curves measured at different wavelengths. From the fitted results of C(t), we observed the distinctive feature unique to the ionic liquids, in which the correlation functions for ionic liquid solutions are fitted to be biphasic, while they are monophasic for other solvents. The fluorescence maximum of hemicyanine dissolved in these ionic liquids red-shifted following the increase in the excitation wavelength.
I. Introduction Ionic liquids are organic salts composed totally of ions and are in liquid phase at room temperature unlike common inorganic salts. Recently, they have been investigated intensively as substitutes for conventional organic solvents in various applications such as chemical synthesis, extraction, and separation.1-7 One of the attractive properties of ionic liquids is that their chemical or physical properties can be tuned easily over a wide range by a small change of functional groups or constituents.8 For example, dielectric response and solvation dynamics of ionic liquids were shown to change sensitively for ionic liquids having slightly different ionic species.9,10 Thus, over the past few years, several groups have investigated solvation dynamics in a number of different ionic liquids.11-34 They attempted to assign the observed response times in terms of the motions of the cations and the anions consisting the liquid. However, even when the same ionic liquid was used as a solvent, the solvation time was quite different depending upon the solutes used, coumarin, prodan, 4-dimethylamino-4′-cyanostilbene (DCS), and 4-aminophthalimide (4-AP).13,14,17-19,23,28 Differences in experimental time resolution, analysis method, and trace amount of impurities in the sample might have contributed to the observed variations in previous reports. It might help to understand the phenomena if we compare the similarities and the differences in the solvation dynamics of standard nonionic solvents with ionic liquids consisting only of charged species. * Corresponding author. E-mail:
[email protected]. † Department of Physics, Sogang University. ‡ Interdisciplinary Program of Integrated Biotechnology, Sogang University. | Nagoya University.
Hemicyanine (4-[4-(dimethylamino)styryl]-1-n-alkylpyridinium bromide) and its derivatives have been used in various applications such as frequency converter,35,36 fluorescence marker,37 and membrane-voltage-sensing probe.38,39 The unique photophysical property of hemicyanine chromophore is strongly related to its chemical structure (shown in Figure 1a), in which an electron donor (dimethylamino) group is attached on one end and an electron acceptor (alkylpyridinium) on the other end across π-conjugated bonds. Upon photoexcitation, the molecule can make an internal rotation with respect to the anilineethylene bond that leads to the twisted intramolecular charge transfer (TICT) state. This TICT state is known to be nonradiative and has been used to explain the fast fluorescence decay.40-42 The intramolecular rotational motion of the dye molecule is dependent mainly on the environmental conditions such as polarity and viscosity of the solvent.42 As the fluorescence from hemicyanine reflects sensitively the properties of the solvent, it would be applied to study solvation dynamics in ionic liquids and conventional solvents. We report in this Article the study of the fluorescence properties of hemicyanine molecules influenced by the surrounding solvent molecules by monitoring fast fluorescence decay dynamics. We used archetypal 1-butyl-3-methylimidazolium tetrafluoroborate and hexafluorophosphate ([BMIM]BF4 and [BMIM]PF6, chemical structures shown in Figure 1b) for ionic liquid solvents. Methanol, ethylene glycol (EG), tetraethylene glycol (tetra-EG), glycerol, benzyl alcohol, and pyridine were used for nonionic solvents of varying viscosities. The relative fluorescence quantum yield and decay lifetimes were compared for these solvents covering a wide range of polarities and physical properties. While the fluorescence quantum yield and decay lifetimes were extremely sensitive to the viscosity of the medium, the fluorescence decay lifetimes for all of the
10.1021/jp076757v CCC: $40.75 © 2008 American Chemical Society Published on Web 01/26/2008
Hemicyanine Dyes in Ionic and Nonionic Solvents
Figure 1. Molecular structures of (a) hemicyanine and (b) ionic liquid [BMIM]BF4 and [BMIM]PF6.
solvents investigated could be fitted well with a single empirical formula.43 On the other hand, the fluorescence correlation function showed a distinctive characteristic only for the molecules in ionic liquid environments. Another unique phenomenon observed in ionic liquid was the peak-shift of fluorescence maximum following the change in the excitation wavelength. II. Experimental Section The hemicyanine molecules were synthesized as follows. The acetonitrile solution (25 mL) of (4-[4-(dimethylamino) styryl] pyridin (224.3 mg, 1.0 mM) and 1-bromocaalkane (279 mg, 1.0 mM) was refluxed under vigorous stirring for 6 days. After the mixture was cooled to room temperature, acetonitrile was removed in vacuo. Ethyl acetate (100 mL) was introduced into the reaction flask containing the residue. The undissolved salt was filtered and washed with ether. Recrystallization from methanol/ether yielded the desired compound as red products.44 For the preparation of the hemicyanine solution, methanol, ethylene glycol, tetra-ethylene glycol, glycerol, benzyl alcohol, pyridine, and two ionic liquids [BMIM]BF4 and [BMIM]PF6 are used. [BMIM]BF4 and [BMIM]PF6 (ultrahigh grade, water contents
