DOI: 10.1002/chem.201402534
Full Paper
& Photochemical Properties
Characterization of a Novel Intrinsic Luminescent RoomTemperature Ionic Liquid Based on [P6,6,6,14][ANS]** Joana M. Delgado,[a, b] Anabela Raymundo,[c] Mrcia Vilarigues,[b] Lus C. Branco,*[a] and Csar A. T. Laia*[a]
Abstract: Intrinsically luminescent room-temperature ionic liquids (RTILs) can be prepared by combining a luminescent anion (more common) or cation with appropriate counter ions, rendering new luminescent soft materials. These RTILs are still new, and many of their photochemical properties are not well known. A novel intrinsic luminescent RTIL based on the 8-anilinonaphthalene-1-sulfonate ([ANS]) anion combined with the trihexyltetradecylphosphonium ([P6,6,6,14]) cation was prepared and characterized by spectroscopic techniques. Detailed photophysical studies highlight the in-
fluence of the ionic liquid environment on the ANS fluorescence, which together with rheological and 1H NMR experiments illustrate the effects of both the viscosity and electrostatic interactions between the ions. This material is liquid at room temperature and possesses a glass transition temperature (Tg) of 230.4 K. The fluorescence is not highly sensitive to factors such as temperature, but owing to its high viscosity, dynamic Stokes shift measurements reveal very slow components for the IL relaxation.
Introduction
ment of photochemically active materials. The usual method would be to dissolve photochromic[3] or photoluminescent compounds[4] in neat ILs. However, some groups have described the possible combination of specific molecules such as metal complexes[5–7] or fluorescein[8] with appropriate organic cations, giving intrinsic luminescent ILs, and anthracene derivatives, which in this case gives molecular liquids.[9] Potentially, these ILs may have applications in, for example, foldable lightemitting devices, solar cells, or fluorescence sensors, avoiding the use of volatile solvents and allowing strong interactions between luminescent compounds because of their high concentrations, thus increasing the yields of charge recombination in, for instance, light-emitting devices.[7] In this work, an intrinsically luminescent room-temperature ionic liquid (RTIL) was prepared through adequate combination of a phosphonium cation with a very common and largely studied fluorescent molecule, 8-anilinonaphthalene-1-sulfonate (ANS) (Scheme 1). ANS is used as a fluorescence probe of biological macromolecules such as proteins,[10] thanks to its sensitivity to solvent effects such as polarity and viscosity.[11–23] The charge-transfer emission band shifts the ANS fluorescence
In the past few years, ionic liquids (ILs) have been studied extensively, and their large range of possible applications has been explored widely. Ionic liquids are composed of an organic cation and an organic or inorganic anion; they are liquid at temperatures under 100 8C and present some peculiar and tunable characteristics such as negligible vapor pressure, high ionic conductivity, high chemical and thermal stability, and considerable capacity to solubilize organic, inorganic, and polymeric materials.[1] In recent years, novel generations of ILs have been developed, in which a cation or anion is chosen to obtain ILs with specific physical, chemical, or biological functions, usually called task-specific ionic liquids.[1, 2] Within this framework, intrinsically luminescent ionic liquids have also been developed, in which the anion is usually the luminescent molecule. These ILs represent a new approach for the develop[a] J. M. Delgado, Dr. L. C. Branco, Dr. C. A. T. Laia REQUIMTE, Chemistry Department Universidade Nova de Lisboa, 2829-516, Caparica (Portugal) E-mail: [email protected] [email protected] [b] J. M. Delgado, Dr. M. Vilarigues Vicarte—Vidro e Cermica para as Artes Universidade Nova de Lisboa, 2829-516, Caparica (Portugal) [c] Dr. A. Raymundo CEER - Biosystems Engineering Instituto Superior de Agronomia, Universidade de Lisboa Tapada da Ajuda, 1349-017 Lisboa (Portugal) [**] [P6,6,6,14][ANS] = (trihexyltetradecylphosphonium 8-anilinonaphthalene-1-sulfonate). Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201402534. Chem. Eur. J. 2015, 21, 726 – 732
Scheme 1. Novel luminescent RTIL based on [P6,6,6,14][ANS] structure.
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Full Paper strongly to red upon increasing the solvent polarity, namely from about 463.2 nm in solvents such as dioxane to 530 nm in water.[17, 20] In addition, the fluorescence quantum yield and excited-state lifetimes decrease with increasing solvent polarity. ANS was previously used as a solvation dynamics probe by studying the shifts in the emission spectra with time-resolved fluorescence techniques.[24, 25] The excited-state dynamics are dominated by a locally excited state (LE) with rather low Stokes shift dominant in non-polar solvents and a chargetransfer (CT) emission dominant in polar solvents, responsible for the solvatochromic effect of the fluorescence.[16, 17, 19, 20] Nevertheless, single-exponential fluorescence decays are only observed in low-viscosity solvents in which the solvation dynamics are on the nanosecond timescale, analogous to the ANS excited-state lifetime. Therefore, a priori a luminescent ionic liquid with ANS as the anion may reveal some of the general properties of ILs such as viscosity or polarity. To illustrate the potential of this new luminescent RTIL, we performed photophysical studies, complementary temperature-dependence 1H NMR spectroscopy, and rheological studies to better understand the ion–ion interactions (such as electrostatic or hydrogen bonding) and their impact on either the macroscopic properties (viscosity) or molecular properties (in this case, ANS photophysics behavior).
RTIL. The NMR spectra provided evidence of the expected stable structure as well as the adequate cation/anion (1:1) proportion. In the FTIR spectra, some characteristic bands were identified, such as that located at 1238 cm1 and attributed to the sulfonated group from the anion, indicating specific electrostatic interactions with cations.
DSC and rheology studies DSC measurements revealed a glass transition temperature Tg of 230.4 K with a rate of 10 8C min1 (Figure 2 A), which did not change with the number of DSC runs to which the sample was submitted. It is important to note that no crystallization (melting temperature) was observed in different measurement experiments. In this context, [P6,6,6,14][ANS] can be defined as a room-temperature ionic liquid. DSC measurements reveal, however, some interesting features when DCp is calculated during the glass transition. In the first DSC run, the values are the same for both cooling and heating cycles (0.23 J K1 g1), whereas the values become significantly different when the sample is submitted again to new DSC runs (see Figure 2 A). Finally, DCp calculated from the cooling cycle is around 0.27 J K1 g1, whereas from the heating cycle it reaches a plateau around 0.48 J K1 g1, in the third DSC run (see Supporting Information). On the other hand, the rheological studies also revealed some interesting features. [P6,6,6,14][ANS] exhibits Newtonian behavior (n = 1, from power
Results and Discussion Synthesis [P6,6,6,14][ANS] as an RTIL was obtained in quantitative yield by using an optimized ion-exchange methodology. In particular, the 1-anilino-sulfonate ammonium salt (NH4[ANS]) was dissolved in dichloromethane and then added to trihexyltetradecylphosphonium chloride ([P6,6,6,14]Cl) solution at room temperature. The purification method is efficient with the simple filtration of undissolved inorganic salts (e.g., ammonium chloride) in the same organic solvent. The pure product was obtained as a dark green and viscous RTIL displaying turquoise blue photoluminescence under UV light, as shown in Figure 1 (chromaticity constants x = 0.198, y = 0.321; see Figure 1). The refraction index at 300 K is 1.534, and the density is 0.9984 g cm3 at 298.4 K. 1H or 13C NMR and FTIR techniques as well as elemental analysis were used to characterize the novel luminescent
Figure 2. A) DSC measurements of [P6,6,6,14][ANS] and B) viscosity of [P6,6,6,14][ANS] in heating (circles) and cooling (triangles) cycles. Viscosity values of the heating cycle were fitted with the VTF equation (see text), giving the parameters shown in the figure.
Figure 1. [P6,6,6,14][ANS] in daylight and under UV light. Chem. Eur. J. 2015, 21, 726 – 732
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Full Paper law equation), corresponding a viscosity value of 4.87 104 Pa s at 25 8C. The heating experiments from 5 8C to 80 8C (Figure 2 B) showed the usual viscosity profile for glass-forming liquids, and the data could be fitted with the Vogel–Tammann–Fulcher (VTF) equation [Eq. (1)], in which h is the liquid viscosity, and h0, B’ and T0 are empirical parameters.[26] h ¼ h0 expð
B0 Þ T T0
ð1Þ
The fitting parameters give TgT0 = 35.8 K, which is in fair agreement with the general empirical expectation for this value (40 K). The RTIL displays Newtonian behavior as well (see Supporting Information, SM1). However, the cooling cycle yielded an unexpected result: at around 20 8C the measured viscosities diverged from the heating cycle, reaching values one order of magnitude greater than those from the heating cycle at 0 8C and deviating clearly from the VTF equation. This experiment was repeated three times, and exactly the same results were obtained for both cycles. On the cooling cycle, the shear viscosity increased with decreasing temperature, showing a marked hysteresis, higher viscosity values for temperatures below 20 8C, compared with the heating curve. This behavior suggests that the heat treatment leads to the formation of a more compact structure, probably resulting from a temperature-induced molecular rearrangement, revealed especially at lower temperatures. Hints for this behavior are shown below in the NMR results.
Figure 3. 1H NMR spectra of pure [P6,6,6,14][ANS] in heating (black lines) and cooling (gray lines) cycles, containing aromatic (A) and aliphatic (B) resonances.
[P6,6,6,14][ANS], electrostatic cation/anion interactions are mainly expected. H-bonding or p–p interactions characteristic of many classes of ILs are not presented in this case. The peak widths, which were obtained by fitting with a Lorentzian function (see Supporting Information), are linearly correlated with the solvent viscosity divided by temperature (h/T) in the heating cycle, but in the cooling cycle the widths are larger, as if the pure IL had a higher local viscosity. The NMR measurements show, therefore, a molecular rearrangement at high temperatures, namely, a tighter electrostatic interaction between cation and anion (presumably an ion pair). As the temperature is lowered, this high-temperature memory is preserved, giving rise to the hysteresis effect shown dramatically in the viscosity experiment, at least until a more stable arrangement is reached. Therefore, during cooling the system is trapped in the high-temperature interactions. Eventually, the supercooled liquid rearranges, but no crystallization occurs (so neither crystallization processes nor the melting point are observed).
NMR studies The 1H NMR spectra of the RTIL [P6,6,6,14][ANS] in deuterated solvent showed two separate spectral windows: one (4 to 0 ppm) containing peaks of the aliphatic protons from the long alkyl chain of the [P6,6,6,14] cation with a pattern which was, in all samples, similar to that appearing for the starting pure [P6,6,6,14] [Cl]; and the other (9 to 6 ppm) displaying the aromatic protons from the ANS anion structure. The ratio of the integrated intensities of the 1H (aliphatic/aromatic) signals was approximately 1:1, as predicted, although in chloroform the value was somewhat affected by putative resonances hiding under the solvent peak. Very broad signals in the 1H NMR spectra of the pure IL were observed below 25 8C, and no significant conclusions may be drawn. However, above 25 8C, the peaks became narrower with increasing temperature, with a concomitant shift of the chemical shifts downfield of both the aromatic and aliphatic regions, in accordance with a more compact structure. This chemical shift is as large as 0.5 ppm from 25 to 85 8C, especially in the protons of the aromatic ring close to the sulfonate group, indicating a decreasing electron density of all protons from both cation and anion. Furthermore, as in the rheological measurements, a hysteresis is observed upon comparing the heating and cooling cycles. Figure 3 shows the 1 H NMR spectra of pure [P6,6,6,14][ANS] for heating (black lines) and cooling (gray lines) cycles comparing the aromatic resonance (Figure 3 A; from ANS anion) and aliphatic resonances (Figure 3 B; from [P6,6,6,14] cation). In the case of pure Chem. Eur. J. 2015, 21, 726 – 732
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UV/Vis absorption spectroscopy and emission spectroscopy studies [P6,6,6,14][ANS] was dissolved in acetonitrile and methanol, enabling the measurement of the absorption spectra (see Table 1 and Figure 4). The results are similar to those reported previously for ANS sodium salts.[15, 19–22] The emission spectra are also similar to other ANS salts, which indicates that the 728
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Full Paper Table 1. UV/Vis spectroscopy measurements of [P6,6,6,14][ANS] IL.
acetonitrile methanol solvent-free IL
labsmax [nm]
labsmax [nm]
Stokes shift [cm1]
364 370 376
477 481 479
6510 6240 5720
Figure 4. [P6,6,6,14][ANS] UV/Vis absorption and fluorescence spectra (lex = 370 nm). Also shown are spectra of the IL dissolved in acetonitrile and methanol for comparison (105 m). Figure 5. A) [P6,6,6,14][ANS] temperature dependence of the fluorescence spectra (lex = 370 nm), and B) variation of the fluorescence intensity (normalized at 20 8C) with temperature, adjusted with an Arrhenius equation.
ANS spectra are not affected significantly by the counter ion when dissolved in those polar solvents (i.e., there is no indication of the formation of ion pairs). As a solvent-free IL, a redshift is observed in the absorption spectra, rendering a significantly smaller Stokes shift (Table 1). The presence of aggregates (such as ANS dimers) could partially explain this result. The emission spectrum also indicates a polarity similar to the molecular solvents.[15] The absolute fluorescence quantum yield of the pure ionic liquid was also measured with an integrated sphere accessory. The value of the fluorescence quantum yield was 1.4 % with excitation at 370 nm (in aerated conditions, it is 30 % in ethanol). This rather low fluorescence quantum yield is indicative of nonemissive species absorbing light at 370 nm, as the fluorescence lifetime is not particularly low (see time-resolved fluorescence experiments). Again, this indicates the presence of nonemissive ANS aggregates. The solvent-free [P6,6,6,14][ANS] emission spectra show some temperature dependence (see Figure 5), namely in their intensity, which drops by about 50 % on going from 15 8C to 80 8C. The band shape does not change with temperature; in fact, the normalized spectra overlap completely at all temperatures used in this study. Therefore, there is no indication of, for example, changes in the local polarity of the IL with temperature. The activation energy from the intensity decrease is rather low (about 670 cm1, comparable with values obtained by Nakamura and Tanaka in molecular solvents[16]). There is no indication of effects related to those described in the rheology and NMR studies in these temperature studies.
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[P6,6,6,14][ANS] time-resolved luminescence The fluorescence decays of [P6,6,6,14][ANS] in aerated methanol and acetonitrile (105 m) are single-exponential (see Table 2), with decay times similar to those reported in the literature.[14–20] Again, no significant effect from the cation is observed, indicating that the ions are well separated in polar solvents. However, the RTIL exhibits a highly nonexponential decay that is dependent on the emission wavelength (see
Table 2. UV/Vis spectroscopy measurements of [P6,6,6,14][ANS]. Solvent a1 t1 a2 t2 a3 t3 c2 5 (10 m) [0.02] [ns, 0.1] [0.02] [ns, 0.3] [0.002] [ns, 10] CH3CN 1.0 ethanol 1.0
6.96 8.84
– –
– –
– –
– –
1.166 1.126
ANS ionic liquid t1 a2 t2 a3 t3 c2 l [nm] a1 [0.02] [ns, 0.1] [0.02] [ns, 0.3] [0.002] [ns, 10] 420 430 450 460 470 485 510 530 550 570
729
0.682 0.660 0.561 0.685 0.556 0.385 0.508 0.212 0.251 0.412
1.16 1.07 1.43 1.10 1.85 0.76 2.27 1.16 1.40 0.47
0.313 0.331 0.430 0.311 0.439 0.489 0.454 0.662 0.687 0.537
4.01 3.79 4.15 3.71 4.68 2.94 4.64 3.68 4.23 3.85
0.005 0.009 0.009 0.003 0.005 0.126 0.037 0.126 0.062 0.051
23.9 12.6 14.5 19.9 23.9 6.4 10.2 8.2 10.8 11.3
1.012 1.107 0.963 1.115 0.992 1.109 1.204 1.007 0.983 1.081
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Figure 6. A) Example of a solvent-free [P6,6,6,14][ANS] fluorescence decay at room temperature measured with the time-correlated single-photon counting technique. IRF is the instrument response function, and the fluorescence decay is presented with the best fitting with a multiexponential function (see Table 2). The residual of the fitting is inserted at the top of the figure. B) Solvent-free [P6,6,6,14][ANS] phosphorescence decay at room temperature (lex = 355 nm and lem = 615 nm) fitted with exponential kinetics (lifetime equal to 360 ns).
Figure 7. A) [P6,6,6,14][ANS] time-resolved fluorescence spectra reconstructed from fluorescence decays at several emission wavelengths using the method described in reference [25] between 500 ps and 10 ns (arrow indicates time evolution). B) The dynamic Stokes shift shows an exponential relaxation with a decay time equal to 1.87 ns followed by a very slow relaxation (above 20 ns, difficult to retrieve accurately from experimental data).
280 cm1. It is impossible to determine the very long relaxation accurately with this setup, but it is certainly above 20 ns and will give rise to emission centered above 500 nm. This very slow effect is here assigned to the diffusion of phosphonium cations stabilizing the ANS large excited-state dipole, which may be connected with a charge-transfer state, as proposed by Kosower[18] and later confirmed by other authors.[20–22] The “short” component should be related to a local reorganization of the environment surrounding ANS involving nontranslational relaxation components. The fluorescence decays depend only slightly on temperature (Figure 8), being more pronounced for the longer components of the multiexponential analysis. The average fluorescence lifetime indeed decreases, with an activation energy similar to that found in the steady-state fluorescence experiments. As the solvent becomes less viscous, a faster relaxation to the CT state may give rise to shorter excited-state lifetimes. Phosphorescence appears in the decays as a tail, which may be resolved at higher temperatures, and here, indeed, a larger temperature effect is observed. The combination of dynamic Stokes shift and temperature measurements reveal, therefore, a fluorescence behavior in which the conversion to a relaxed CT state is hindered by the slow environment response to the large difference in the ANS excited-state dipole moment. The very slow dynamics hint at a local polarity smaller than that which would occur if the IL
Figure 6). At blue emission wavelengths, the decay is considerably shorter than those obtained at higher wavelengths. This is a strong indication that homogenous solvation of the ANS anion occurs, either through a distribution of local environments surrounding the fluorophore or through slow solvation dynamics. Both explanations have been used previously to explain multiexponential features in the fluorescence decays of other molecules dissolved in common ionic liquids.[27, 28] Besides this effect, a very long component is also seen, especially at high emission wavelengths. The nature of this component was confirmed on Laser Flash Photolysis equipment: a very long luminescence decay was observed with a decay time equal to 360 ns (Figure 6 B), and is assigned to ANS phosphorescence.[21, 23] The observation of phosphorescence is interesting, but is far from being the main component of the ANS photoluminescence. To understand its nature, time-resolved emission spectra were constructed following the method described by Fleming and Maroncelli.[25] It is already known that the viscosity is very high at room temperature (about 10 Pa s, see Figure 2), so it is rather unsurprising that a dynamic Stokes shift is observed in the IL on the nanosecond timescale. Figure 7 A shows the effect here described, and Figure 7 B describes how the emission maximum shifts with time. A “short” nanosecond component is observed (1.87 ns) with an energy relaxation equal to Chem. Eur. J. 2015, 21, 726 – 732
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Full Paper rescence intensity as being caused by the faster formation of the relaxed CT state. The phosphonium cation used here, nevertheless, renders an IL in which the local environment is rather nonpolar. It is also concluded that the ANS probe may provide an interesting means to analyze the effect of different cations on an IL; this work is currently in progress.
Experimental Section Reagents All solvents were used as supplied; methanol (spectroscopic grade), acetonitrile p.a., and dichloromethane p.a. were purchased from Sigma–Aldrich. Trihexyltetradecylphosphonium chloride ([P6,6,6,14][Cl]) was purchased from Cytec (98 %), and 8-anilino-1naphthalenesulfonic acid ammonium salt ([NH4][ANS], 99 %) was obtained from Sigma–Aldrich. Deuterated NMR solvents were purchased from Euro-Isotop. Type I water was obtained from a Watermax purification station (Diwer Technologies). All glassware was cleaned with a mixture of concentrated H2SO4/H2O2 (1:1), rinsed thoroughly, oven dried, and cooled in a desiccator prior to use.
Figure 8. [P6,6,6,14][ANS] fluorescence decays as a function of temperature (21, 50, 67.5, and 85 8C) with lex = 373 nm and lem = 460 nm. The time delay scale is logarithmic above 20 ns.
relaxed completely within a fraction of the excited-state lifetime. Furthermore, the very slow component in the solvation dynamics (longer than 20 ns), here assigned to the cation translational diffusion, would indicate an important electrostatic interaction between the phosphonium positive charge and the ANS excited-state dipole. Such a result indicates that the charge/dipole interactions are an important part of the polarity effects in ILs. Intrinsically luminescent ILs such as [P6,6,6,14][ANS] may therefore be important for revealing general properties of these fluids. In terms of applications, this type of IL may be used in organic light-emitting diodes (OLEDS), as they have the potential to replace organic semiconductors in which radicals should be formed in the electrodes and recombine through diffusion. The viscosity effects reported here are rather large, which may have an impact on the future design of devices employing this type of material. Other possible applications include the conversion of UV light into visible light, which is important for phosphors or solar cells, giving their strong absorption in the UV. The low fluorescence quantum yield, however, might be a problem. The presence of anionic groups may also be useful in the future for applications such as sensors for the dissolution of cations in ILs and/or other applications such as cleaning fluids for surfaces. Some of the authors are employing this IL for such applications, and CaCO3 indeed has high solubility in [P6,6,6,14][ANS]. This work will be reported in the future.
Synthesis of [P6,6,6,14][ANS] [P6,6,6,14]Cl (1.37 g, 2.63 103 mol) was dissolved in dichloromethane (80 mL), and [NH4][ANS] (1.00 g, 3.16 103 mol) was added to the reaction mixture. The reaction mixture was stirred at room temperature for 24 h. Then, the solution was filtrated to remove the NH4Cl formed, the solvent was removed, and the final product was dried in vacuum for 24 h. For elucidation of the absence of residual chloride, additional washing of the product with water followed by the addition of silver nitrate to the water phase was also performed. The desired product was obtained as a dark green viscous liquid (2.20 g; 99 %). 1H NMR (400.13 MHz, CDCl3) d = 8.48 (d, J = 8.00 Hz,1 H), 7.81 (d, J = 8.00 Hz, 1 H), 7.61 (d, J = 8.01 Hz, 1 H), 7.45 (d, J = 4.02 Hz,1 H), 7.32 (t, J = 4.00 Hz, 2 H), 7.28 (t, J = 8.00 Hz, 2 H), 7.18 (t, J = 8.01 Hz, 2 H), 6.82 (t, J = 8.01 Hz, 1 H), 2.04 (m, 8 H), 1.28–1.22 (m, 48 H), 0.87 ppm (m,12 H); 13C NMR (100.62 MHz, CDCl3) d = 144.57, 141.55, 136.88, 132.02, 128.86, 128.09, 125.81, 124.11, 123.30, 122.84, 120.03, 118.06, 31.86, 31.06, 30.35, 30.15, 29.64, 29.37, 28.93, 22.74, 22.32, 21.79, 19.06, 18.53, 14.13, 13.94 ppm; FTIR (NaCl): 3267, 3052, 2926, 2858, 2361, 1927, 1599, 1561, 1497, 1452, 1350, 1309, 1234, 1181, 1109, 1033, 873, 819, 751, 700, 636, 581 cm1; elemental analysis calcd (%) for C48H80NO3SP.H2O: C 72.00, H 10.25, N 1.75; found: C 72.37, H 10.31, N 1.93.
Characterization NMR spectroscopy: NMR spectra were obtained on a Bruker AMX 400 instrument operating at 400.13 MHz (1 H) and 100.61 MHz (13 C). Temperature effects were monitored by submitting the sample to successive heating steps up to 85 8C and subsequent similar stepwise cooling. The temperatures quoted were those presented in the spectrometer temperature control unit.
Conclusion An intrinsic luminescent ionic liquid was synthesized by combining a phosphonium cation and ANS fluorophore. This material is liquid at room temperature and has a glass temperature of 223.7 K. The viscosity follows the VTM equation, showing the usual Newtonian behavior of glass-forming liquids. The fluorescence is not highly sensitive to effects such as temperature; however, because of its high viscosity, the dynamic Stokes shift measurement reveals very slow components for the IL relaxation. Temperature effects show a faster relaxation at higher temperature, explaining part of the decrease in fluoChem. Eur. J. 2015, 21, 726 – 732
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FTIR spectroscopy: FTIR spectra were recorded on a Bruker Tensor 27 using NaCl cells for the deposition of the RTIL as a stable film. Elemental analysis: Elemental (C, H, N) analysis was performed on a Thermofinnigan Flash EA 1112 Series instrument by the Laboratrio de Anlises at REQUIMTE, FCT/UNL. Differential scanning calorimetry (DSC): DSC analysis was performed with a Setaram model DSC 131 with a refrigerated cooling
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Full Paper system. The temperature range was from 100 to 100 8C and the scanning rate was 10 8C min1. The resolution was 0.2 mW. The sample was purged continuously with nitrogen (50 mL min1). Approximately 5–10 mg of salt was crimped in an aluminum standard sample pan with a lid. Glass transition temperatures (Tg) were determined in the second heating. Rheology experiments: Steady-state flow measurements were performed in a controlled-stress rheometer (RS-750) from Haake (Germany), coupled with a Peltier system for temperature control. Flow curves were obtained at 25 8C with a serrated plate–plate sensor system (20 mm) to prevent wall-slip phenomena.[29] The power law was adjusted to obtain the consistency (k) and flow index (n) from h = K y(n1). These rheological parameters were used to classify the type of IL flow. The flow behavior of ILs with temperature was evaluated in the temperature range 5 8C to 80 8C at a constant rate of 2 8C min1. Spectroscopic measurements: UV/Vis absorbance spectra were obtained with a UV/Vis/NIR spectrophotometer (Varian Cary 5000; spectral range 300 to 800 nm). Luminescence spectra were measured using a SPEX Fluorolog-3 Model FL3–22 spectrofluorimeter, with 2 nm slits and the temperature controlled with an external bath. Phosphorescence lifetime measurements were run on an LKS.60 nanosecond laser photolysis spectrometer from Applied Photophysics, with a Brilliant Q-Switch Nd:YAG laser from Quantel, using the second harmonic (lexc = 355 nm, laser pulse half-width equal to 6 ns). An optical cutoff filter (570 nm) for the emitted light was used to avoid scattering light contamination. Fluorescence lifetimes (t) were measured through the time-correlated singlephoton-counting (TCSPC) technique using home-built equipment. The samples were excited at 373 nm with a nanoLED (IBH). The electronic start pulses were shaped in a constant fraction discriminator (Canberra 2126) and directed to a time-to-amplitude converter (TAC, Canberra 2145). The emission wavelength was selected by a monochromator (Oriel 77250) imaged in a fast photomultiplier (PM, 9814B Electron Tubes Inc.); the PM signal was shaped as before and delayed before entering the TAC as stop pulses. The analog TAC signals were digitized (ADC, ND582) and stored in a multichannel analyzer installed in a PC. Temperature control was performed with a water bath connected to the spectrophotometer, with an approximate accuracy of 1 8C.
[2]
[3]
[4] [5] [6] [7] [8] [9]
[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Acknowledgements
[21] [22]
This work was supported by Fundażo para a CiÞncia e a Tecnologia through grant no. PEst-C/EQB/LA0006/2013. Financial support was received from Fundażo para a CiÞncia e a Tecnologia (FCT-MCTES) (projects PTDC/CTM-NAN/120658/2010, PTDC/ CTM/103664/2008 and SFRH/BD/72808/2010). The NMR spectrometers are part of The National NMR Facility, supported by Fundażo para a CiÞncia e a Tecnologia (RECI/BBB-BQB/0230/ 2012).
[23] [24] [25] [26] [27] [28] [29]
Keywords: ANS · fluorescence · ionic liquids · photophysics · rheology
Received: March 10, 2014 Revised: June 11, 2014 Published online on August 14, 2014
[1] a) M. Freemantle, An Introduction to Ionic Liquids. Royal Society of Chemistry, 2009; b) L. C. Branco, G. V. S. M. Carrera, J. Aires-de-Sousa, I. L. Martin, R. Frade, C. A. M. Afonso “Physico-Chemical Properties of
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Task-Specific Ionic Liquid” in Ionic Liquids, Theory and Applications (Chapter 3), Intech Press, 2011; c) P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis Wiley-VCH, 2007; d) K. Y. Yung, A. J. Schadock-Hewitt, N. P. Hunter, F. V. Bright, G. A. Baker, Chem. Commun. 2011, 47, 4775 – 4777; e) A. J. Boydston, C. S. Pecinovsky, S. T. Chao, C. W. Bielawski, J. Am. Chem. Soc. 2007, 129, 14550 – 14551; f) Q. Zhang, B. Yang, S. Zhang, S. Liu, Y. Deng, J. Mater. Chem. 2011, 21, 16335 – 16338. a) A. Kokorin, Ionic Liquids: Applications and Perspectives, InTech 2011; b) A. Branco, L. C. Branco, F. Pina, Chem. Commun. 2011, 47, 2300 – 2302; c) A. Branco, J. Belchior, L. C. Branco, F. Pina, RSC Adv. 2013, 3, 25627 – 25630; d) L. C. Branco, F. Pina, Chem. Commun. 2009, 6204 – 6206; e) R. Ferraz, L. C. Branco, C. Prudencio, J. P. Noronha, Z. Petrovski, ChemMedChem 2011, 6, 975 – 985; f) J. P. Hallett, T. Welton, Chem. Rev. 2011, 111, 3508 – 3576; g) S. S. Babu, T. Nakanishi, Chem. Commun. 2013, 49, 9373. a) L. C. Branco, F. Pina, in Ionic Liquids, Theory and Applications, Intech 2011; b) F. Pina, A. J. Parola, M. J. Melo, C. A. T. Laia, C. A. M. Afonso, Chem. Commun. 2007, 1608 – 1610. a) R. Karmakar, A. Samanta, J. Phys. Chem. A 2002, 106, 4447 – 4452; b) Z. Hu, C. J. Margulis, Proc. Natl. Acad. Sci. USA 2006, 103, 831 – 836. S. Tang, A. Babai, A.-V. Mudring, Angew. Chem. 2008, 120, 7743 – 7746; Angew. Chem. Int. Ed. 2008, 47, 7631 – 7634. C. C. L. Pereira, S. Dias, I. Coutinho, J. P. Leal, L. C. Branco, C. A. T. Laia, Inorg. Chem. 2013, 52, 3755 – 3764. a) S. Gago, L. Cabrita, J. C. Lima, L. C. Branco, F. Pina, Dalton Trans. 2013, 42, 6213 – 6218. C. A. B. Rodrigues, C. GraÅa, E. MaÅas, A. Fedorov, C. A. M. Afonso, J. M. G. Martinho, J. Phys. Chem. B 2013, 117, 14108 – 14114. S. S. Babu, M. J. Hollamby, J. Aimi, H. Ozawa, A. Saeki, S. Seki, K. Kobayashi, K. Hagiwara, M. Yoshizawa, H. Mçhwald, Takashi Nakanishi Nature Communications 2013, 4, Article number: 1969 doi:10.1038/ ncomms2969. L. Brand, J. R. Gohlke, Annu. Rev. Biochem. 1972, 41, 843 – 868. C. J. Seliskar, L. Brand, J. Am. Chem. Soc. 1971, 93, 5405 – 5414. E. M. Kosower, H. Dodiuk, K. Tanizawa, M. Ottolenghi, N. Orbach, J. Am. Chem. Soc. 1975, 97, 2167 – 2178. R. P. DeToma, J. H. Easter, L. Brand, J. Am. Chem. Soc. 1976, 98, 5001 – 5007. G. W. Robinson, R. J. Robbins, G. R. Fleming, J. M. Morris, A. E. W. Knight, R. J. S. Morrison, J. Am. Chem. Soc. 1978, 100, 7145 – 7150. P. J. Sadkowski, G. R. Fleming, Chem. Phys. 1980, 54, 79 – 89. H. Nakamura, J. Tanaka, Chem. Phys. Lett. 1981, 78, 57 – 60. D. Huppert, H. Kanety, E. M. Kosower, Chem. Phys. Lett. 1981, 84, 48 – 53. E. M. Kosower, Acc. Chem. Res. 1982, 15, 259 – 266. E. M. Kosower, H. Kanety, J. Am. Chem. Soc. 1983, 105, 6236 – 6243. J. Drew, P. Thistlethwaite, G. Woolfe, Chem. Phys. Lett. 1983, 96, 296 – 301. T. W. Ebbesen, C. A. Ghiron, J. Phys. Chem. 1989, 93, 7139 – 7143. A. Upadhyay, T. Bhatt, H. B. Tripathi, D. D. Pant, J. Photochem. Photobiol. A 1995, 89, 201 – 207. A.-J. Tong, Y.-G. Wu, L.-D. Li, Anal. Chim. Acta 1996, 322, 91 – 97. B. Bagchi, D. W. Oxtoby, G. R. Fleming, Chem. Phys. 1984, 86, 257 – 267. E. W. Castner, M. Maroncelli, G. R. Fleming, J. Chem. Phys. 1987, 86, 1090 – 1097. R. Bçhmer, K. L. Ngai, C. A. Angell, D. J. Plazek, J. Chem. Phys. 1993, 99, 4201 – 4209. J. Y. Ye, T. Hattori, H. Nakatsuka, Y. Maruyama, M. Ishikawa, Phys. Rev. B 1997, 56, 5286 – 5296. E. W. Castner Jr., J. F. Wishart, H. Shirota, Acc. Chem. Res. 2007, 40, 1217 – 1227. P. K. Mandal, S. Saha, R. Karmakar, A. Samanta, Curr. Sci. 2006, 90, 301 – 310.
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& Photochemical Properties
Characterization of a Novel Intrinsic Luminescent RoomTemperature Ionic Liquid Based on [P6,6,6,14][ANS]** Joana M. Delgado,[a, b] Anabela Raymundo,[c] Mrcia Vilarigues,[b] Lus C. Branco,*[a] and Csar A. T. Laia*[a]
Abstract: Intrinsically luminescent room-temperature ionic liquids (RTILs) can be prepared by combining a luminescent anion (more common) or cation with appropriate counter ions, rendering new luminescent soft materials. These RTILs are still new, and many of their photochemical properties are not well known. A novel intrinsic luminescent RTIL based on the 8-anilinonaphthalene-1-sulfonate ([ANS]) anion combined with the trihexyltetradecylphosphonium ([P6,6,6,14]) cation was prepared and characterized by spectroscopic techniques. Detailed photophysical studies highlight the in-
fluence of the ionic liquid environment on the ANS fluorescence, which together with rheological and 1H NMR experiments illustrate the effects of both the viscosity and electrostatic interactions between the ions. This material is liquid at room temperature and possesses a glass transition temperature (Tg) of 230.4 K. The fluorescence is not highly sensitive to factors such as temperature, but owing to its high viscosity, dynamic Stokes shift measurements reveal very slow components for the IL relaxation.
Introduction
ment of photochemically active materials. The usual method would be to dissolve photochromic[3] or photoluminescent compounds[4] in neat ILs. However, some groups have described the possible combination of specific molecules such as metal complexes[5–7] or fluorescein[8] with appropriate organic cations, giving intrinsic luminescent ILs, and anthracene derivatives, which in this case gives molecular liquids.[9] Potentially, these ILs may have applications in, for example, foldable lightemitting devices, solar cells, or fluorescence sensors, avoiding the use of volatile solvents and allowing strong interactions between luminescent compounds because of their high concentrations, thus increasing the yields of charge recombination in, for instance, light-emitting devices.[7] In this work, an intrinsically luminescent room-temperature ionic liquid (RTIL) was prepared through adequate combination of a phosphonium cation with a very common and largely studied fluorescent molecule, 8-anilinonaphthalene-1-sulfonate (ANS) (Scheme 1). ANS is used as a fluorescence probe of biological macromolecules such as proteins,[10] thanks to its sensitivity to solvent effects such as polarity and viscosity.[11–23] The charge-transfer emission band shifts the ANS fluorescence
In the past few years, ionic liquids (ILs) have been studied extensively, and their large range of possible applications has been explored widely. Ionic liquids are composed of an organic cation and an organic or inorganic anion; they are liquid at temperatures under 100 8C and present some peculiar and tunable characteristics such as negligible vapor pressure, high ionic conductivity, high chemical and thermal stability, and considerable capacity to solubilize organic, inorganic, and polymeric materials.[1] In recent years, novel generations of ILs have been developed, in which a cation or anion is chosen to obtain ILs with specific physical, chemical, or biological functions, usually called task-specific ionic liquids.[1, 2] Within this framework, intrinsically luminescent ionic liquids have also been developed, in which the anion is usually the luminescent molecule. These ILs represent a new approach for the develop[a] J. M. Delgado, Dr. L. C. Branco, Dr. C. A. T. Laia REQUIMTE, Chemistry Department Universidade Nova de Lisboa, 2829-516, Caparica (Portugal) E-mail: [email protected] [email protected] [b] J. M. Delgado, Dr. M. Vilarigues Vicarte—Vidro e Cermica para as Artes Universidade Nova de Lisboa, 2829-516, Caparica (Portugal) [c] Dr. A. Raymundo CEER - Biosystems Engineering Instituto Superior de Agronomia, Universidade de Lisboa Tapada da Ajuda, 1349-017 Lisboa (Portugal) [**] [P6,6,6,14][ANS] = (trihexyltetradecylphosphonium 8-anilinonaphthalene-1-sulfonate). Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201402534. Chem. Eur. J. 2015, 21, 726 – 732
Scheme 1. Novel luminescent RTIL based on [P6,6,6,14][ANS] structure.
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Full Paper strongly to red upon increasing the solvent polarity, namely from about 463.2 nm in solvents such as dioxane to 530 nm in water.[17, 20] In addition, the fluorescence quantum yield and excited-state lifetimes decrease with increasing solvent polarity. ANS was previously used as a solvation dynamics probe by studying the shifts in the emission spectra with time-resolved fluorescence techniques.[24, 25] The excited-state dynamics are dominated by a locally excited state (LE) with rather low Stokes shift dominant in non-polar solvents and a chargetransfer (CT) emission dominant in polar solvents, responsible for the solvatochromic effect of the fluorescence.[16, 17, 19, 20] Nevertheless, single-exponential fluorescence decays are only observed in low-viscosity solvents in which the solvation dynamics are on the nanosecond timescale, analogous to the ANS excited-state lifetime. Therefore, a priori a luminescent ionic liquid with ANS as the anion may reveal some of the general properties of ILs such as viscosity or polarity. To illustrate the potential of this new luminescent RTIL, we performed photophysical studies, complementary temperature-dependence 1H NMR spectroscopy, and rheological studies to better understand the ion–ion interactions (such as electrostatic or hydrogen bonding) and their impact on either the macroscopic properties (viscosity) or molecular properties (in this case, ANS photophysics behavior).
RTIL. The NMR spectra provided evidence of the expected stable structure as well as the adequate cation/anion (1:1) proportion. In the FTIR spectra, some characteristic bands were identified, such as that located at 1238 cm1 and attributed to the sulfonated group from the anion, indicating specific electrostatic interactions with cations.
DSC and rheology studies DSC measurements revealed a glass transition temperature Tg of 230.4 K with a rate of 10 8C min1 (Figure 2 A), which did not change with the number of DSC runs to which the sample was submitted. It is important to note that no crystallization (melting temperature) was observed in different measurement experiments. In this context, [P6,6,6,14][ANS] can be defined as a room-temperature ionic liquid. DSC measurements reveal, however, some interesting features when DCp is calculated during the glass transition. In the first DSC run, the values are the same for both cooling and heating cycles (0.23 J K1 g1), whereas the values become significantly different when the sample is submitted again to new DSC runs (see Figure 2 A). Finally, DCp calculated from the cooling cycle is around 0.27 J K1 g1, whereas from the heating cycle it reaches a plateau around 0.48 J K1 g1, in the third DSC run (see Supporting Information). On the other hand, the rheological studies also revealed some interesting features. [P6,6,6,14][ANS] exhibits Newtonian behavior (n = 1, from power
Results and Discussion Synthesis [P6,6,6,14][ANS] as an RTIL was obtained in quantitative yield by using an optimized ion-exchange methodology. In particular, the 1-anilino-sulfonate ammonium salt (NH4[ANS]) was dissolved in dichloromethane and then added to trihexyltetradecylphosphonium chloride ([P6,6,6,14]Cl) solution at room temperature. The purification method is efficient with the simple filtration of undissolved inorganic salts (e.g., ammonium chloride) in the same organic solvent. The pure product was obtained as a dark green and viscous RTIL displaying turquoise blue photoluminescence under UV light, as shown in Figure 1 (chromaticity constants x = 0.198, y = 0.321; see Figure 1). The refraction index at 300 K is 1.534, and the density is 0.9984 g cm3 at 298.4 K. 1H or 13C NMR and FTIR techniques as well as elemental analysis were used to characterize the novel luminescent
Figure 2. A) DSC measurements of [P6,6,6,14][ANS] and B) viscosity of [P6,6,6,14][ANS] in heating (circles) and cooling (triangles) cycles. Viscosity values of the heating cycle were fitted with the VTF equation (see text), giving the parameters shown in the figure.
Figure 1. [P6,6,6,14][ANS] in daylight and under UV light. Chem. Eur. J. 2015, 21, 726 – 732
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Full Paper law equation), corresponding a viscosity value of 4.87 104 Pa s at 25 8C. The heating experiments from 5 8C to 80 8C (Figure 2 B) showed the usual viscosity profile for glass-forming liquids, and the data could be fitted with the Vogel–Tammann–Fulcher (VTF) equation [Eq. (1)], in which h is the liquid viscosity, and h0, B’ and T0 are empirical parameters.[26] h ¼ h0 expð
B0 Þ T T0
ð1Þ
The fitting parameters give TgT0 = 35.8 K, which is in fair agreement with the general empirical expectation for this value (40 K). The RTIL displays Newtonian behavior as well (see Supporting Information, SM1). However, the cooling cycle yielded an unexpected result: at around 20 8C the measured viscosities diverged from the heating cycle, reaching values one order of magnitude greater than those from the heating cycle at 0 8C and deviating clearly from the VTF equation. This experiment was repeated three times, and exactly the same results were obtained for both cycles. On the cooling cycle, the shear viscosity increased with decreasing temperature, showing a marked hysteresis, higher viscosity values for temperatures below 20 8C, compared with the heating curve. This behavior suggests that the heat treatment leads to the formation of a more compact structure, probably resulting from a temperature-induced molecular rearrangement, revealed especially at lower temperatures. Hints for this behavior are shown below in the NMR results.
Figure 3. 1H NMR spectra of pure [P6,6,6,14][ANS] in heating (black lines) and cooling (gray lines) cycles, containing aromatic (A) and aliphatic (B) resonances.
[P6,6,6,14][ANS], electrostatic cation/anion interactions are mainly expected. H-bonding or p–p interactions characteristic of many classes of ILs are not presented in this case. The peak widths, which were obtained by fitting with a Lorentzian function (see Supporting Information), are linearly correlated with the solvent viscosity divided by temperature (h/T) in the heating cycle, but in the cooling cycle the widths are larger, as if the pure IL had a higher local viscosity. The NMR measurements show, therefore, a molecular rearrangement at high temperatures, namely, a tighter electrostatic interaction between cation and anion (presumably an ion pair). As the temperature is lowered, this high-temperature memory is preserved, giving rise to the hysteresis effect shown dramatically in the viscosity experiment, at least until a more stable arrangement is reached. Therefore, during cooling the system is trapped in the high-temperature interactions. Eventually, the supercooled liquid rearranges, but no crystallization occurs (so neither crystallization processes nor the melting point are observed).
NMR studies The 1H NMR spectra of the RTIL [P6,6,6,14][ANS] in deuterated solvent showed two separate spectral windows: one (4 to 0 ppm) containing peaks of the aliphatic protons from the long alkyl chain of the [P6,6,6,14] cation with a pattern which was, in all samples, similar to that appearing for the starting pure [P6,6,6,14] [Cl]; and the other (9 to 6 ppm) displaying the aromatic protons from the ANS anion structure. The ratio of the integrated intensities of the 1H (aliphatic/aromatic) signals was approximately 1:1, as predicted, although in chloroform the value was somewhat affected by putative resonances hiding under the solvent peak. Very broad signals in the 1H NMR spectra of the pure IL were observed below 25 8C, and no significant conclusions may be drawn. However, above 25 8C, the peaks became narrower with increasing temperature, with a concomitant shift of the chemical shifts downfield of both the aromatic and aliphatic regions, in accordance with a more compact structure. This chemical shift is as large as 0.5 ppm from 25 to 85 8C, especially in the protons of the aromatic ring close to the sulfonate group, indicating a decreasing electron density of all protons from both cation and anion. Furthermore, as in the rheological measurements, a hysteresis is observed upon comparing the heating and cooling cycles. Figure 3 shows the 1 H NMR spectra of pure [P6,6,6,14][ANS] for heating (black lines) and cooling (gray lines) cycles comparing the aromatic resonance (Figure 3 A; from ANS anion) and aliphatic resonances (Figure 3 B; from [P6,6,6,14] cation). In the case of pure Chem. Eur. J. 2015, 21, 726 – 732
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UV/Vis absorption spectroscopy and emission spectroscopy studies [P6,6,6,14][ANS] was dissolved in acetonitrile and methanol, enabling the measurement of the absorption spectra (see Table 1 and Figure 4). The results are similar to those reported previously for ANS sodium salts.[15, 19–22] The emission spectra are also similar to other ANS salts, which indicates that the 728
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Full Paper Table 1. UV/Vis spectroscopy measurements of [P6,6,6,14][ANS] IL.
acetonitrile methanol solvent-free IL
labsmax [nm]
labsmax [nm]
Stokes shift [cm1]
364 370 376
477 481 479
6510 6240 5720
Figure 4. [P6,6,6,14][ANS] UV/Vis absorption and fluorescence spectra (lex = 370 nm). Also shown are spectra of the IL dissolved in acetonitrile and methanol for comparison (105 m). Figure 5. A) [P6,6,6,14][ANS] temperature dependence of the fluorescence spectra (lex = 370 nm), and B) variation of the fluorescence intensity (normalized at 20 8C) with temperature, adjusted with an Arrhenius equation.
ANS spectra are not affected significantly by the counter ion when dissolved in those polar solvents (i.e., there is no indication of the formation of ion pairs). As a solvent-free IL, a redshift is observed in the absorption spectra, rendering a significantly smaller Stokes shift (Table 1). The presence of aggregates (such as ANS dimers) could partially explain this result. The emission spectrum also indicates a polarity similar to the molecular solvents.[15] The absolute fluorescence quantum yield of the pure ionic liquid was also measured with an integrated sphere accessory. The value of the fluorescence quantum yield was 1.4 % with excitation at 370 nm (in aerated conditions, it is 30 % in ethanol). This rather low fluorescence quantum yield is indicative of nonemissive species absorbing light at 370 nm, as the fluorescence lifetime is not particularly low (see time-resolved fluorescence experiments). Again, this indicates the presence of nonemissive ANS aggregates. The solvent-free [P6,6,6,14][ANS] emission spectra show some temperature dependence (see Figure 5), namely in their intensity, which drops by about 50 % on going from 15 8C to 80 8C. The band shape does not change with temperature; in fact, the normalized spectra overlap completely at all temperatures used in this study. Therefore, there is no indication of, for example, changes in the local polarity of the IL with temperature. The activation energy from the intensity decrease is rather low (about 670 cm1, comparable with values obtained by Nakamura and Tanaka in molecular solvents[16]). There is no indication of effects related to those described in the rheology and NMR studies in these temperature studies.
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[P6,6,6,14][ANS] time-resolved luminescence The fluorescence decays of [P6,6,6,14][ANS] in aerated methanol and acetonitrile (105 m) are single-exponential (see Table 2), with decay times similar to those reported in the literature.[14–20] Again, no significant effect from the cation is observed, indicating that the ions are well separated in polar solvents. However, the RTIL exhibits a highly nonexponential decay that is dependent on the emission wavelength (see
Table 2. UV/Vis spectroscopy measurements of [P6,6,6,14][ANS]. Solvent a1 t1 a2 t2 a3 t3 c2 5 (10 m) [0.02] [ns, 0.1] [0.02] [ns, 0.3] [0.002] [ns, 10] CH3CN 1.0 ethanol 1.0
6.96 8.84
– –
– –
– –
– –
1.166 1.126
ANS ionic liquid t1 a2 t2 a3 t3 c2 l [nm] a1 [0.02] [ns, 0.1] [0.02] [ns, 0.3] [0.002] [ns, 10] 420 430 450 460 470 485 510 530 550 570
729
0.682 0.660 0.561 0.685 0.556 0.385 0.508 0.212 0.251 0.412
1.16 1.07 1.43 1.10 1.85 0.76 2.27 1.16 1.40 0.47
0.313 0.331 0.430 0.311 0.439 0.489 0.454 0.662 0.687 0.537
4.01 3.79 4.15 3.71 4.68 2.94 4.64 3.68 4.23 3.85
0.005 0.009 0.009 0.003 0.005 0.126 0.037 0.126 0.062 0.051
23.9 12.6 14.5 19.9 23.9 6.4 10.2 8.2 10.8 11.3
1.012 1.107 0.963 1.115 0.992 1.109 1.204 1.007 0.983 1.081
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Figure 6. A) Example of a solvent-free [P6,6,6,14][ANS] fluorescence decay at room temperature measured with the time-correlated single-photon counting technique. IRF is the instrument response function, and the fluorescence decay is presented with the best fitting with a multiexponential function (see Table 2). The residual of the fitting is inserted at the top of the figure. B) Solvent-free [P6,6,6,14][ANS] phosphorescence decay at room temperature (lex = 355 nm and lem = 615 nm) fitted with exponential kinetics (lifetime equal to 360 ns).
Figure 7. A) [P6,6,6,14][ANS] time-resolved fluorescence spectra reconstructed from fluorescence decays at several emission wavelengths using the method described in reference [25] between 500 ps and 10 ns (arrow indicates time evolution). B) The dynamic Stokes shift shows an exponential relaxation with a decay time equal to 1.87 ns followed by a very slow relaxation (above 20 ns, difficult to retrieve accurately from experimental data).
280 cm1. It is impossible to determine the very long relaxation accurately with this setup, but it is certainly above 20 ns and will give rise to emission centered above 500 nm. This very slow effect is here assigned to the diffusion of phosphonium cations stabilizing the ANS large excited-state dipole, which may be connected with a charge-transfer state, as proposed by Kosower[18] and later confirmed by other authors.[20–22] The “short” component should be related to a local reorganization of the environment surrounding ANS involving nontranslational relaxation components. The fluorescence decays depend only slightly on temperature (Figure 8), being more pronounced for the longer components of the multiexponential analysis. The average fluorescence lifetime indeed decreases, with an activation energy similar to that found in the steady-state fluorescence experiments. As the solvent becomes less viscous, a faster relaxation to the CT state may give rise to shorter excited-state lifetimes. Phosphorescence appears in the decays as a tail, which may be resolved at higher temperatures, and here, indeed, a larger temperature effect is observed. The combination of dynamic Stokes shift and temperature measurements reveal, therefore, a fluorescence behavior in which the conversion to a relaxed CT state is hindered by the slow environment response to the large difference in the ANS excited-state dipole moment. The very slow dynamics hint at a local polarity smaller than that which would occur if the IL
Figure 6). At blue emission wavelengths, the decay is considerably shorter than those obtained at higher wavelengths. This is a strong indication that homogenous solvation of the ANS anion occurs, either through a distribution of local environments surrounding the fluorophore or through slow solvation dynamics. Both explanations have been used previously to explain multiexponential features in the fluorescence decays of other molecules dissolved in common ionic liquids.[27, 28] Besides this effect, a very long component is also seen, especially at high emission wavelengths. The nature of this component was confirmed on Laser Flash Photolysis equipment: a very long luminescence decay was observed with a decay time equal to 360 ns (Figure 6 B), and is assigned to ANS phosphorescence.[21, 23] The observation of phosphorescence is interesting, but is far from being the main component of the ANS photoluminescence. To understand its nature, time-resolved emission spectra were constructed following the method described by Fleming and Maroncelli.[25] It is already known that the viscosity is very high at room temperature (about 10 Pa s, see Figure 2), so it is rather unsurprising that a dynamic Stokes shift is observed in the IL on the nanosecond timescale. Figure 7 A shows the effect here described, and Figure 7 B describes how the emission maximum shifts with time. A “short” nanosecond component is observed (1.87 ns) with an energy relaxation equal to Chem. Eur. J. 2015, 21, 726 – 732
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Full Paper rescence intensity as being caused by the faster formation of the relaxed CT state. The phosphonium cation used here, nevertheless, renders an IL in which the local environment is rather nonpolar. It is also concluded that the ANS probe may provide an interesting means to analyze the effect of different cations on an IL; this work is currently in progress.
Experimental Section Reagents All solvents were used as supplied; methanol (spectroscopic grade), acetonitrile p.a., and dichloromethane p.a. were purchased from Sigma–Aldrich. Trihexyltetradecylphosphonium chloride ([P6,6,6,14][Cl]) was purchased from Cytec (98 %), and 8-anilino-1naphthalenesulfonic acid ammonium salt ([NH4][ANS], 99 %) was obtained from Sigma–Aldrich. Deuterated NMR solvents were purchased from Euro-Isotop. Type I water was obtained from a Watermax purification station (Diwer Technologies). All glassware was cleaned with a mixture of concentrated H2SO4/H2O2 (1:1), rinsed thoroughly, oven dried, and cooled in a desiccator prior to use.
Figure 8. [P6,6,6,14][ANS] fluorescence decays as a function of temperature (21, 50, 67.5, and 85 8C) with lex = 373 nm and lem = 460 nm. The time delay scale is logarithmic above 20 ns.
relaxed completely within a fraction of the excited-state lifetime. Furthermore, the very slow component in the solvation dynamics (longer than 20 ns), here assigned to the cation translational diffusion, would indicate an important electrostatic interaction between the phosphonium positive charge and the ANS excited-state dipole. Such a result indicates that the charge/dipole interactions are an important part of the polarity effects in ILs. Intrinsically luminescent ILs such as [P6,6,6,14][ANS] may therefore be important for revealing general properties of these fluids. In terms of applications, this type of IL may be used in organic light-emitting diodes (OLEDS), as they have the potential to replace organic semiconductors in which radicals should be formed in the electrodes and recombine through diffusion. The viscosity effects reported here are rather large, which may have an impact on the future design of devices employing this type of material. Other possible applications include the conversion of UV light into visible light, which is important for phosphors or solar cells, giving their strong absorption in the UV. The low fluorescence quantum yield, however, might be a problem. The presence of anionic groups may also be useful in the future for applications such as sensors for the dissolution of cations in ILs and/or other applications such as cleaning fluids for surfaces. Some of the authors are employing this IL for such applications, and CaCO3 indeed has high solubility in [P6,6,6,14][ANS]. This work will be reported in the future.
Synthesis of [P6,6,6,14][ANS] [P6,6,6,14]Cl (1.37 g, 2.63 103 mol) was dissolved in dichloromethane (80 mL), and [NH4][ANS] (1.00 g, 3.16 103 mol) was added to the reaction mixture. The reaction mixture was stirred at room temperature for 24 h. Then, the solution was filtrated to remove the NH4Cl formed, the solvent was removed, and the final product was dried in vacuum for 24 h. For elucidation of the absence of residual chloride, additional washing of the product with water followed by the addition of silver nitrate to the water phase was also performed. The desired product was obtained as a dark green viscous liquid (2.20 g; 99 %). 1H NMR (400.13 MHz, CDCl3) d = 8.48 (d, J = 8.00 Hz,1 H), 7.81 (d, J = 8.00 Hz, 1 H), 7.61 (d, J = 8.01 Hz, 1 H), 7.45 (d, J = 4.02 Hz,1 H), 7.32 (t, J = 4.00 Hz, 2 H), 7.28 (t, J = 8.00 Hz, 2 H), 7.18 (t, J = 8.01 Hz, 2 H), 6.82 (t, J = 8.01 Hz, 1 H), 2.04 (m, 8 H), 1.28–1.22 (m, 48 H), 0.87 ppm (m,12 H); 13C NMR (100.62 MHz, CDCl3) d = 144.57, 141.55, 136.88, 132.02, 128.86, 128.09, 125.81, 124.11, 123.30, 122.84, 120.03, 118.06, 31.86, 31.06, 30.35, 30.15, 29.64, 29.37, 28.93, 22.74, 22.32, 21.79, 19.06, 18.53, 14.13, 13.94 ppm; FTIR (NaCl): 3267, 3052, 2926, 2858, 2361, 1927, 1599, 1561, 1497, 1452, 1350, 1309, 1234, 1181, 1109, 1033, 873, 819, 751, 700, 636, 581 cm1; elemental analysis calcd (%) for C48H80NO3SP.H2O: C 72.00, H 10.25, N 1.75; found: C 72.37, H 10.31, N 1.93.
Characterization NMR spectroscopy: NMR spectra were obtained on a Bruker AMX 400 instrument operating at 400.13 MHz (1 H) and 100.61 MHz (13 C). Temperature effects were monitored by submitting the sample to successive heating steps up to 85 8C and subsequent similar stepwise cooling. The temperatures quoted were those presented in the spectrometer temperature control unit.
Conclusion An intrinsic luminescent ionic liquid was synthesized by combining a phosphonium cation and ANS fluorophore. This material is liquid at room temperature and has a glass temperature of 223.7 K. The viscosity follows the VTM equation, showing the usual Newtonian behavior of glass-forming liquids. The fluorescence is not highly sensitive to effects such as temperature; however, because of its high viscosity, the dynamic Stokes shift measurement reveals very slow components for the IL relaxation. Temperature effects show a faster relaxation at higher temperature, explaining part of the decrease in fluoChem. Eur. J. 2015, 21, 726 – 732
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FTIR spectroscopy: FTIR spectra were recorded on a Bruker Tensor 27 using NaCl cells for the deposition of the RTIL as a stable film. Elemental analysis: Elemental (C, H, N) analysis was performed on a Thermofinnigan Flash EA 1112 Series instrument by the Laboratrio de Anlises at REQUIMTE, FCT/UNL. Differential scanning calorimetry (DSC): DSC analysis was performed with a Setaram model DSC 131 with a refrigerated cooling
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Full Paper system. The temperature range was from 100 to 100 8C and the scanning rate was 10 8C min1. The resolution was 0.2 mW. The sample was purged continuously with nitrogen (50 mL min1). Approximately 5–10 mg of salt was crimped in an aluminum standard sample pan with a lid. Glass transition temperatures (Tg) were determined in the second heating. Rheology experiments: Steady-state flow measurements were performed in a controlled-stress rheometer (RS-750) from Haake (Germany), coupled with a Peltier system for temperature control. Flow curves were obtained at 25 8C with a serrated plate–plate sensor system (20 mm) to prevent wall-slip phenomena.[29] The power law was adjusted to obtain the consistency (k) and flow index (n) from h = K y(n1). These rheological parameters were used to classify the type of IL flow. The flow behavior of ILs with temperature was evaluated in the temperature range 5 8C to 80 8C at a constant rate of 2 8C min1. Spectroscopic measurements: UV/Vis absorbance spectra were obtained with a UV/Vis/NIR spectrophotometer (Varian Cary 5000; spectral range 300 to 800 nm). Luminescence spectra were measured using a SPEX Fluorolog-3 Model FL3–22 spectrofluorimeter, with 2 nm slits and the temperature controlled with an external bath. Phosphorescence lifetime measurements were run on an LKS.60 nanosecond laser photolysis spectrometer from Applied Photophysics, with a Brilliant Q-Switch Nd:YAG laser from Quantel, using the second harmonic (lexc = 355 nm, laser pulse half-width equal to 6 ns). An optical cutoff filter (570 nm) for the emitted light was used to avoid scattering light contamination. Fluorescence lifetimes (t) were measured through the time-correlated singlephoton-counting (TCSPC) technique using home-built equipment. The samples were excited at 373 nm with a nanoLED (IBH). The electronic start pulses were shaped in a constant fraction discriminator (Canberra 2126) and directed to a time-to-amplitude converter (TAC, Canberra 2145). The emission wavelength was selected by a monochromator (Oriel 77250) imaged in a fast photomultiplier (PM, 9814B Electron Tubes Inc.); the PM signal was shaped as before and delayed before entering the TAC as stop pulses. The analog TAC signals were digitized (ADC, ND582) and stored in a multichannel analyzer installed in a PC. Temperature control was performed with a water bath connected to the spectrophotometer, with an approximate accuracy of 1 8C.
[2]
[3]
[4] [5] [6] [7] [8] [9]
[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Acknowledgements
[21] [22]
This work was supported by Fundażo para a CiÞncia e a Tecnologia through grant no. PEst-C/EQB/LA0006/2013. Financial support was received from Fundażo para a CiÞncia e a Tecnologia (FCT-MCTES) (projects PTDC/CTM-NAN/120658/2010, PTDC/ CTM/103664/2008 and SFRH/BD/72808/2010). The NMR spectrometers are part of The National NMR Facility, supported by Fundażo para a CiÞncia e a Tecnologia (RECI/BBB-BQB/0230/ 2012).
[23] [24] [25] [26] [27] [28] [29]
Keywords: ANS · fluorescence · ionic liquids · photophysics · rheology
Received: March 10, 2014 Revised: June 11, 2014 Published online on August 14, 2014
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