Excitation Wavelength Dependence of Excited State Intramolecular

Article pubs.acs.org/JPCB

Excitation Wavelength Dependence of Excited State Intramolecular Proton Transfer Reaction of 4′‑N,N‑Diethylamino-3-hydroxyflavone in Room Temperature Ionic Liquids Studied by Optical Kerr Gate Fluorescence Measurement Kayo Suda,*,† Masahide Terazima,† Hirofumi Sato,‡ and Yoshifumi Kimura*,§ †

Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto Daigaku Katsura, Kyoto, 615-8510, Japan § Department of Molecular Chemistry and Bioscience, Faculty of Science and Engineering, Doshisha University, Kyoto 610-0321, Japan ‡

S Supporting Information *

ABSTRACT: Excited state intramolecular proton transfer reactions (ESIPT) of 4′-N,Ndiethylamino-3-hydroxyflavone (DEAHF) in ionic liquids have been studied by steady-state and time-resolved fluorescence measurements at different excitation wavelengths. Steady-state measurements show the relative yield of the tautomeric form to the normal form of DEAHF decreases as excitation wavelength is increased from 380 to 450 nm. The decrease in yield is significant in ionic liquids that have cations with long alkyl chains. The extent of the decrease is correlated with the number of carbon atoms in the alkyl chains. Time-resolved fluorescence measurements using optical Kerr gate spectroscopy show that ESIPT rate has a strong excitation wavelength dependence. There is a large difference between the spectra at a 200 ps delay from different excitation wavelengths in each ionic liquid. The difference is pronounced in ionic liquids having a long alkyl chain. The equilibrium constant in the electronic excited state obtained at a 200 ps delay and the average reaction rate are also correlated with the alkyl chain length. Considering the results of the steady-state fluorescence and time-resolved measurements, the excitation wavelength dependence of ESIPT is explained by state selective excitation due to the difference of the solvation, and the number of alkyl chain carbon atoms is found to be a good indicator of the effect of inhomogeneity for this reaction.

1. INTRODUCTION Ionic liquids provide unique solvation properties, and there have been many studies on solvation related for the use of ionic liquids as chemical reaction media. Various kinds of traditional solvatochromism parameters such as ET(30), π*, α, and β have been studied by many researchers.1,2 It is known that ET(30) values of ionic liquids are close to those of methanol and dimethyl sulfoxide, that π* values are close to 1, and that α and β values are strongly dependent on the anionic counterion of the ionic liquid. The local interactions between solute molecules and the cation and anion of the ionic liquids have also been investigated by using vibrational spectroscopy such as IR3 and Raman spectroscopy.4−6 In relation with the dynamical aspects of the solvation, the dynamic fluorescence Stokes shift of photoexcited molecules in ionic liquids has been studied using time-resolved fluorescence spectroscopy under various conditions using a variety of probe molecules.7−20 One important feature of solvation dynamics in ionic liquids is the wide distribution of solvation time scales from sub-picosecond to nanosecond. The ultrafast solvation component is independent of solvent viscosity, while the longer solvation time is linearly correlated with the viscosity. Theoretical © 2013 American Chemical Society

calculations suggest the sub-picosecond solvation dynamics are related to translational motion of the solvent cations and anions in the vicinity of the solute.21−23 The longer time scale dynamics are thought to be related to diffusive motion of the cation and anion. Recently, a relationship between solvation dynamics and conductivity of ionic liquids has been reported.24 Other unique spectroscopies have been applied to investigate the solvation dynamics of ionic liquids. For example, Muramatsu et al. investigated ultrafast solvation observed by three-pulse photon echo peak shift measurements.25 From these detailed studies of the solvation and its dynamics of ionic liquids, it is now widely recognized that heterogeneous solvation exists in ionic liquids. One example is the excitation wavelength dependence of fluorescence spectra, so-called rededge effect. Samanta et al. found that the fluorescence of 2amino-7-nitrofluorene (ANF) showed a lower energy shift when excitation wavelength was shifted to the red edge of the absorption spectrum in ionic liquids.26 Later, Maroncelli et al. Received: June 4, 2013 Revised: August 23, 2013 Published: September 17, 2013 12567

dx.doi.org/10.1021/jp405537c | J. Phys. Chem. B 2013, 117, 12567−12582

The Journal of Physical Chemistry B

Article

Scheme 1. Reaction Scheme for Excited State of DEAHF

gated spectroscopy.54 The reaction rate of ESIPT was also understood from the solvation dynamics of the excited state, and the reaction rate was correlated with the polarity of the ionic liquid scaled by the ion concentration. Very recently, our group has presented a letter on the excitation wavelength dependence of ESIPT of DEAHF in various liquids mainly using the steady-state fluorescence spectroscopy.55,56 We evaluated the yield of the tautomer in several ionic liquids, and found that the yield of the tautomer significantly decreases with increasing the excitation wavelength to the red in ionic liquids that have long alkyl chains such as trihexyl(tetradecyl)phosphonium bis(trifluoromethanesulfonyl)amide ([P6,6,6,14][NTf2]).55,56 We also measured the reaction dynamics excited at the very red-edge wavelength (470 nm) for two typical ionic liquids, and found that the reaction rate is slower at 470 nm excitation. Although we interpreted that the excitation wavelength dependence is due to the heterogeneous solvation on DEAHF in ionic liquids, the number of the ionic liquids and the studies on the dynamics in the previous studies were quite limited and the discussions on the excitation wavelength dependence in relation with the solvation were not enough. In this paper, we present the detailed study on the ESIPT of DEAHF in ionic liquids with different alkyl chain lengths at different excitation wavelengths. In addition to 1-ethyl-3methylimidazolium ([EMIm]+) and [BMIm]+ previously used,53−56 we used 1-hexyl-3-methylimidazolium ([HMIm]+) and 1-octhyl-3-methylimidazolium ([OMIm]+) to investigate the effect of the alkyl-chain length of the imidazolium-cation based ionic liquids. We have analyzed the excitation wavelength dependence of the yield of the tautomer estimated by the steady-state fluorescence measurement in a quantitative manner. We have found that the number of carbon atoms in alkyl chains of cation is a good indicator of the excitation wavelength dependence of the yield by comparing the results in imidazolium-cation based ionic liquids with phosphoniumcation based ionic liquids. We also performed the detailed study on the initial reaction dynamics at different excitation wavelengths (400 nm, 430 nm, and 450 nm) by integrating a stable OPA system to an optical Kerr gate system previously built by our group. At first we will discuss the alkyl chain length dependence of the initial dynamics obtained at 400 nm excitation. Then we will discuss the excitation wavelength dependence in various ionic liquids. By comparing the reaction rates obtained at different excitation wavelengths with the solvation dynamics, we concluded that the heterogeneous solvation due to the different alkyl-chain lengths is the origin of the excitation wavelength dependence of ESIPT of DEAHF.

found an excitation wavelength dependence of the dynamic Stokes shift of Coumarin 153 (C153) in ionic liquids.18 Later, Hu and Margulis clarified the existence of a local heterogeneity of the electric field around ANF in 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm][PF6]) by molecular dynamic simulations.27 Other spectroscopic methods have also been applied to reveal the heterogeneous solvation in ionic liquids. Fujisawa et al. reported the existence of heterogeneous solute solvation in ionic liquids through excitation wavelength dependent Raman spectra of phenol blue.4 Kimura et al. found an excitation wavelength dependence of the Raman shift of a NO2 stretching mode of N,N,-dimethylamino-p-nitroaniline in ionic liquids.5 These results are interpreted by selective excitation of a molecule in a different solvation environment by different excitation wavelengths. However, it is not clearly understood how solvation heterogeneity4,5,18,26−32 may be related to the structural heterogeneity reported for ionic liquids.33−38 Based upon small angle neutron scattering and molecular dynamics studies, it has been proposed that ionic liquids with long alkyl chains have a segregated structure with polar and nonpolar domains. Mizoshiri et al. reported that the permittivity of ionic liquids may be interpreted from the mixture of polar and nonpolar domains.39 The spectroscopic observation of the heterogeneity may be related to the domain structure. Although various types of ultrafast reaction such as chargetransfer dynamics have been investigated in ionic liquids,40−46 there have been few systematic studies of the effect of heterogeneous solvation on reaction dynamics and the effects of heterogeneous solvation on the reaction rate have hardly been discussed.47 In this paper, we will present the detailed study on the effect of the heterogeneous solvation on the excited state intramolecular proton transfer reaction (ESIPT) of 4′-N,N-diethylamino-3-hydroxyflavone (DEAHF) in ionic liquids (see Scheme 1). There are many reports on ESIPT processes of DEAHF and related compounds in conventional liquids.48−52 In the electronic ground state, DEAHF exists in the normal form, and upon photoexcitation ESIPT occurs and a tautomer is formed in an electronic excited state. Both the normal form and the tautomeric form show fluorescence and the reaction dynamics can be monitored by the fluorescence dynamics. Chou et al. investigated the ESIPT dynamics of DEAHF by the up-conversion fluorescence spectroscopy,49 and found that the proton transfer process occurs on two time scales (sub-ps and tens of ps) in polar liquids. The bimodal reaction rates of ESIPT were interpreted by solvation of the normal form in the excited state. Our group has investigated ESIPT of DEAHF in various ionic liquids53,54 and found that the ESIPT occurs on three different time scale (system response, a few ps, and a few tens of ps) by using optical Kerr 12568



Article

Scheme 2. Structures and Viscosities of Ionic Liquids and Triacetin

2. EXPERIMENTAL SECTION 2.1. Samples and Steady-State Measurement. DEAHF and 4′-N,N-diethylamino-3-methoxyflavone (DEAMF, the methoxy derivative of DEAHF) were synthesized and purified according to the literature.54,57 We used several ionic liquids composed of different cations with the same anion [NTf2]−, and also [BMIm][PF6] having the PF6− anion. A series of ionic liquids with the [NTf2]− anion included an imidazolium-cation such as [EMIm]+, [BMIm]+, [HMIm]+, and [OMIm]+. Another series of ionic liquids with the [NTf2]− anion were phosphonium and ammonium cations such as triethyloctylphosphonium ([P 2,2,2,8 ] + ), tributylmethylphosphonium ([P 4,4,4,1 ] + ), [P 6,6,6,14 ] + , and tributylmethylammonium ([N4,4,4,1]+). For comparison with the ionic liquids, triacetin was used as a less polar (εr = 7.1158) but relatively viscous conventional solvent. Structures and viscosities of all ionic

liquids and triacetin are shown in Scheme 2 (see reference of viscosity1,59,60). [EMIm][NTf2], [BMIm][NTf2], [BMIm][PF6], [P2,2,2,8][NTf2], and [P4,4,4,1][NTf2] were purchased from KantoKagaku. [HMIm][NTf2], [OMIm][NTf2], and [N4,4,4,1][NTf2] were purchased from iolitec, and [P6,6,6,14][NTf2] was purchased from Cytec. Triacetin was purchased from Nakarai Tesque. [EMIm][NTf 2], [BMIm][NTf2], [BMIm][PF6], [P2,2,2,8][NTf2], and [N4,4,4,1][NTf2] were used without purification. [P6,6,6,14][NTf2] was washed by distilled water to remove chloride ion contamination. The washing was repeated until there was no precipitation from the addition of an aqueous solution of AgNO3 to the water washings. The received [HMIm][NTf2] and [OMIm][NTf2] appeared colored, and these ionic liquids were purified by addition of activated carbon in an acetonitrile solution. The solution was 12569



Article

Figure 1. Schematic illustration of the experimental system.

an OPA (TOPAS-C, Spectra-Physics, Light Conversion). In these experiments, the wavelengths of the output were adjusted to 860 or 900 nm pulses and frequency doubled using a BBO crystal for optical pumping at 430 or 450 nm. The fluorescence of the sample solution excited by the pump pulse was collected by an off-axes parabolic mirror and focused into an optical Kerr medium by a lens after passing through the first polarizer (wiregrid type). A cutoff filter was placed before the first polarizer, to reduce light from the excitation pulse. A sharp cut filter centered at 450 nm was used for the 430 nm excitation, and an edge filter centered at 465 nm was used for the 450 nm excitation. The polarization of the pump pulse was rotated to the magic angle of the first polarizer to reduce the rotational effect of the fluorescence. For the optical Kerr gate pulse, the residual fundamental pulse was further split to reduce the power, and focused on the Kerr medium (liquid benzene, enclosed in a 2 mm path length quartz cell) after passing through an optical delay stage. The fluorescence after the second polarizer (double Glan-laser polarizer) was detected by an intensified CCD (ICCD) camera with a 300 ps gate width (LaVision, Picostar HR) attached to a spectrometer (Chromex 250I). The fluorescence image at each delay time of the gate pulse was corrected for the group velocity dispersion of light and color sensitivity of the system using standard dye solutions as reported previously.54,61,62 Corrections for reabsorption of fluorescence by the sample were performed as reported previously.54 A flow type optical cell with optical path lengths of 0.5 mm or 1 mm was used for the measurement. The sample solution (ca. 6−8 mL) was circulated using a microgear pump (mzr2905, HNP Mikrosysteme) at a flow rate of 1 or 2 mL min−1. All measurements were performed at room temperature (around 21.0 °C). The absorbance of the sample solution in 1 mm path length cells were as follows: the solutions excited at 400 nm had absorbance of 0.43−1.04 at 400 nm; solutions excited at 430 nm had absorbance of 0.165−1.65 at 430 nm; solutions excited at 450 nm had absorbance of 0.56−1.8 at 450 nm. The water content before and after optical Kerr gate experiments were measured by Karl Fisher moisture titration

allowed to stand for several days, and activated carbon was removed by filtering and acetonitrile was removed under reduced pressure for more than 24 h at 80 °C. Fluorescence from the purified [HMIm][NTf2] and [OMIm][NTf2] was negligible compared with that of the sample solution, and there was no effect on the optical Kerr gate measurement at the excitation wavelengths used in this study. Other ionic liquids in this study showed no fluorescence at the excitation wavelengths used in this study and spectroscopic measurements were unaffected by fluorescence from contamination. Water contamination in triacetin was removed by addition of molecular sieves and allowing the solution to stand for several days. After purification of the ionic liquids, DEAHF was dissolved in each ionic liquid by stirring under vacuum overnight at 50 °C. After filtering the sample solution with a membrane filter (45 μm), the sample solution placed under reduced pressure for more than 24 h at 60 °C before use. Sample preparation of the triacetin solution was different from that of the ionic liquids. DEAHF was dissolved with stirring in purified triacetin. After filtering, the sample solution was immediately used for the measurement. The procedures for preparing solutions of DEAMF were the same as those used to prepare the DEAHF solutions. The DEAMF reference solution will be described in detail in section 2.2. Steady-state absorption and fluorescence spectra of DEAHF were measured using a quartz cell (optical path length of 1 mm) using standard spectrometers (Shimadzu Co., UV2500PC and JASCO, FP-6500). Steady state fluorescence measurements were performed at 25 °C. The color-sensitivity of the fluorometer was corrected using a calibrated light source spectrum pattern file (ESC-333, JASCO). 2.2. Time-Resolved Fluorescence Measurement Using an Optical Kerr Gate Measurement at Different Excitation Wavelengths. For measurements of the timeresolved fluorescence with excitation at several different wavelengths, we incorporated an OPA system into the optical Kerr gate system reported previously (see Figure 1).54 Briefly, the fundamental output (800 nm, ca. 120 fs, 2 W, 1 kHz) was split into two beams by a beam splitter. One beam (1 W) drove 12570



Article

nm.55,56 This excitation wavelength dependence could not be attributed to contamination of the ionic liquids.55,56 Comparing the fluorescence spectrum from excitation at 400 nm in [EMIm][NTf2] with that of [HMIm][NTf2], the relative intensity of the fluorescence from the tautomeric form of DEAHF to that from the normal form is larger in [HMIm][NTf2]. The excitation wavelength dependence of the relative intensity is more significant in [HMIm][NTf2] compared with that in [EMIm][NTf2]. These results suggest that the yield of the tautomer and the excitation wavelength dependence are affected by the alkyl chain length of imidazolium-cation based ionic liquids. The spectral changes were more apparent in [P2,2,2,8][NTf2] in comparison of the imidazolium cation-based ionic liquids. As reported previously,55,56 for [P6,6,6,14][NTf2], which has long alkyl chains, the spectra of the tautomeric form of DEAHF showed more remarkable changes than in [P2,2,2,8][NTf2]. These results suggest that the fluorescence from the tautomeric form of DEAHF is strongly affected by alkyl chain length. To analyze these results quantitatively, the ratio of the fluorescence intensities from the tautomeric form to the normal form excited at a certain wavelength (λex) (the ratio, R(λex) = tautomer/normal) are estimated from the peak height of each fluorescence line shape and plotted against excitation wavelength in Figure 3. The figure is almost the same as Figure 2 in ref 55 (see also ref 56) except for the additional ionic liquids used in this study. For the imidazolium-cation based ionic liquids with the [NTf2]− anion, the values of the ratio (1.2− 2.0) are similar to those of conventional polar molecular liquids such as acetonitrile and dimethyl sulfoxide (about 1.7−2.3).55,56 The ratio is larger for imidazolium cations with longer alkyl chain lengths when compared at the same excitation wavelengths. It is apparent that the ratio in each solvent decreases as the excitation is shifted to longer wavelengths. The excitation wavelength dependence is greater for the imidazolium-based cation with longer alkyl chains. The decrease of this ratio at longer excitation wavelengths for phosphonium and ammonium-cation based ionic liquids is remarkable compared with imidazolium-cation based ionic liquids. The ratios at shorter excitation wavelengths (e.g., 400 nm) are larger compared with conventional polar molecular liquids such as acetonitrile, and are close to the values obtained in less polar solvents such as triacetin and ethyl acetate.55,56 The ratio decreases as the excitation is shifted to longer wavelengths and the values at the red absorption edge are similar to those found in conventional polar solvents. Although we tried to correlate R(λex) with the conventional polarity scale of ionic liquids, it does not help with understanding the excitation wavelength dependence and the relative yields of the tautomer and normal molecular forms. For example, the π* values of the ionic liquid studied here are quite similar to one another, ranging from 0.87 to 1.05, while the value of acetonitrile is 0.46 and that of ethyl acetate is 0.23.1,6 The values of β are also similar among the ionic liquids with [NTf2]− anion. Considering that the alkyl chain length has an effect on the value of R(λex), we tried to make the correlation of R(λex) with the alkyl chain length. Figure 4a shows the correlation between R(400) and the number of carbon atoms in alkyl chains. We consider that the number of carbon atoms in alkyl chain represents the index of nonpolar domain of ionic liquids. Figure 4b shows the correlation between the wavelength dependence and the alkyl chain length. The wavelength dependence is estimated by taking the ratio R(400)/R(450). As

(MKC-501, Kyoto Electronics Manufacturing Co. Ltd.). The water content of the each sample solution was typically less than 100 ppm before the measurement, and increased by ca. 200−400 ppm after the optical Kerr gate experiment.

3. RESULTS AND DISCUSSIONS 3.1. Steady-State Fluorescence Spectra of DEAHF in Ionic Liquids. Figure 2 shows the fluorescence line shape

Figure 2. Absorption and steady-tate fluorescence spectra of DEAHF at 25 °C in (a) [EMIm][NTf2], (b) [HMIm][NTf2], and (c) [P2,2,2,8][NTf2]. The excitation wavelengths of fluorescence spectra are 400, 430, and 450 nm.

functions (fluorescence intensity I(ν) divided by ν3) of DEAHF obtained at different excitation wavelengths in (a) [EMIm][NTf2], (b) [HMIm][NTf2], and (c) [P2,2,2,8][NTf2] together with the absorption spectrum of DEAHF in each solvent. The fluorescence spectra at each excitation wavelength are normalized to the higher wavenumber shoulder peak. The water contents of the solvents were (a) under 10 ppm, (b) 40 ppm, and (c) 82 ppm. As reported previously,49,55,56 the high energy (blue) fluorescence was assigned to the fluorescence from the normal excited state (around 19 000−20 000 cm−1) and the low energy (red) fluorescence is assigned to tautomer fluorescence (around 17 000 cm−1), respectively,49 and the fluorescence intensity of the tautomeric form of DEAHF clearly decreases as excitation wavelength is shifted from 400 to 450 12571



Article

Figure 3. Ratio of the tautomeric form to the normal form in various liquids (R(λex)) plotted against the excitation wavelength (λex). The vertical scale is logarithmic.

Figure 4. (a) R(400) plotted against the number of carbon atoms in alkyl chains of ionic liquids. (b) Ratio of R(400) to R(450) plotted against the number of carbon atoms in alkyl chains of ionic liquids. (c) R(400) plotted against the ion concentration of ionic liquids. (d) Ratio of R(400) to R(450) plotted against the ion concentration of ionic liquids. Symbols are categorized as shown in the legends of figure.

shown in Figure 4a and b, both increase linearly with the number of carbon atoms in alkyl chains except for [BMIm][PF6] and [N4,4,4,1][NTf2]. This result suggests that the number of carbon atoms in alkyl chains of cation is a good indicator for

the local heterogeneity or local polarity, as we consider that the excitation wavelength dependence arises from the heterogeneous distribution of the solute molecule in ionic liquids. The difference between [P2,2,2,8][NTf2] and [P4,4,4,1][NTf2] and the 12572



Article

Figure 5. Time-resolved fluorescence line shapes obtained at 400 nm excitation in (a) [EMIm][NTf2], (b) [HMIm][NTf2], (c) [P4,4,4,1][NTf2], and (d) [N4,4,4,1][NTf2]. The solid lines show the spectral fitting results using eq 1.

Although the polarity scale of [BMIm][PF6] is similar to that of [BMIm][NTf2], the hydrogen-bonding acceptor basicity of [BMIm][PF6] is quite small compared to [BMIm][NTf2].3 For [N4,4,4,1][NTf2] and [P4,4,4,1][NTf2], one of different properties is their viscosity, with [N4,4,4,1][NTf2] considerably more viscous than [P4,4,4,1][NTf2]. The relatively viscous molecular solvent triacetin shows some excitation wavelength dependence, while nonviscous polar solvents (acetonitrile, DMSO) do not. This suggests that dynamic properties may contribute to the excitation wavelength dependence. If true, the difference between [BMIm][NTf2] and [BMIm][PF6] can be explained by the higher viscosity of [BMIm][PF6]. In the next section, we discuss the dynamics of DEAHF in each solvent at different excitation wavelengths, and how dynamic solvent properties are related to these phenomena.

somewhat different slope between the ionic liquids with the imidazolium and phosphonium cations may reflect how the alkyl chain is distributed in the ionic liquids. Figure 4c and d show plots of R(400) and their ratio (R(400)/R(450)) against the ion concentration of the ionic liquids, respectively. The ion concentration of ionic liquids represents the index of polarity of ionic liquids. Both plots show linear correlations, although the trends do not appear as strong as for the comparison with alkyl chain length. This suggests that ion concentration may not affect local heterogeneity as strongly as the alkyl chain length. Before discussion of the time-resolved spectrum, we discuss the differences between the ionic liquids that have the cation with the same alkyl chain length; [BMIm][NTf2] and [BMIm][PF6], [N4,4,4,1][NTf2] and [P4,4,4,1][NTf2]. For the former case, the different anion species are likely to affect the normal/tautomer ratios and excitation wavelength dependence. 12573



Article

Figure 6. Time-resolved fluorescence line shapes in [BMIm][PF6] and [P2,2,2,8][NTf2] obtained at 400, 430, and 450 nm excitations. The solid lines show the spectral fitting results using eq 1.

3.2. Time-Resolved Fluorescence Spectra of DEAHF Obtained by Optical Kerr Gate Measurement. a. Alkyl Chain Length Dependence of Time-Resolved Fluorescence Spectra of DEAHF in Ionic Liquids Obtained at 400 nm Excitation. Figure 5 shows the time-resolved fluoresce spectra obtained using the optical Kerr gate method for DEAHF in (a) [EMIm][NTf2], (b) [HMIm][NTf2], (c) [P4,4,4,1][NTf2], and (d) [N4,4,4,1][NTf2]. The results in other RTILs are presented in the Supporting Information (Figure S1). The spectral dynamics were similar to those reported previously;54 the fluorescence from the normal excited state (N*) is initially observed, and shifted to the lower energy side due to the solvation dynamics; a simultaneous rise of the tautomer fluorescence is also observed around 17 000 cm−1. Although the spectral dynamics in [HMIm][NTf2] are qualitatively similar to that in [EMIm][NTf2], the spectral line shape at 200 ps in [HMIm][NTf2] is different from that in [EMIm][NTf2]; that is, the relative yield of the tautomeric form to the normal

form in [HMIm][NTf2] is larger than that in [EMIm][NTf2] at a 200 ps delay. As mentioned in previous sections, the relative yield of the tautomer obtained from steady state fluorescence in [HMIm][NTf2] is larger than that in [EMIm][NTf2]. These results suggest that the yield of the tautomer is affected by dynamics within a few hundred ps. The spectral dynamics in [P4,4,4,1][NTf2] are similar to those in [N4,4,4,1][NTf2] as shown in Figure 4c and d. The relative intensities of the normal and tautomeric forms in [P4,4,4,1][NTf2] at 200 ps are similar to those in [N4,4,4,1][NTf2], although the steady-state fluorescence ratio is larger in [N4,4,4,1][NTf2]. These results further suggest that the alkyl chain length of the ionic liquids is a major factor in determining the initial dynamics of the reaction, and that other factors such as the viscosity may affect the reaction dynamics over the longer time scale. b. Excitation Wavelength Dependence of Time-Resolved Fluorescence Spectra. Figure 6 shows that the time-resolved spectra of DEAHF in [BMIm][PF6] and [P2,2,2,8][NTf2] 12574



Article

peak height were the only adjustable parameters. Figure 7 shows that the peak shift of DEAMF in [BMIm][NTf2]

obtained from excitation at 400 nm, 430 nm, and 450 nm. In each ionic liquid, the dynamics of DEAHF are similar to those from 400 nm excitation presented in the previous section. However, as shown in Figure 6, the reaction dynamics up to 200 ps show some differences from one another. The rate of increase of the tautomeric form of DEAHF becomes slower as the excitation wavelength increases. As a result, the spectra at a 200 ps delay show significantly different features, at different excitation wavelengths. Namely, the fluorescence from the tautomeric form of DEAHF decreases with increasing excitation wavelength. These results mean that dynamics within a few hundred picoseconds are crucial for determining the relative yield of the tautomer to the normal forms of DEAHF in ionic liquids. To analyze the complicated effects of ionic liquid cation alkyl chain length and excitation wavelength on the relative yield of tautomeric form to normal form of DEAHF, time-resolved fluorescence spectrum at each delay time were analyzed, using previously reported methods.54 Initially, the fluorescence spectra at each delay time were fitted by the sum of two lognormal functions representing the normal fluorescence and the tautomer fluorescence as Ifl(t , ν) = N *(t , ν) + T *(t , ν)

Figure 7. Fluorescence peak shifts of DEAMF at 400 and 430 nm excitations in [BMIm][NTf2].

obtained at 400 and 430 nm excitations. The initial peak position from the excitation at the longer wavelength (430 nm) shows a smaller energy, and the peak positions obtained at different excitation wavelengths merges into the same at the delay of hundreds of ps. The observation is the same as previously observed for the ionic liquids used in ref 18. Therefore we consider that the excess energy of DEAHF in ionic liquids does not significantly affect the bandwidth parameters such as Δ and γ. In this study, in order to reduce the number of parameters and maintain the quality of fitting, we used the same fixed values of Δ and γ of the normal from and the same fixed values of Δ, γ, and νp of the tautomeric form to fit the fluorescence spectra obtained at different excitation wavelength in one solvent ionic liquid. The height and the peak position of the normal form and the height of tautomeric form were considered to be free parameters in this analysis method. This method was applied to all samples in different ionic liquids. The details of the parameter fixing are given in the Supporting Information. The solid lines in Figures 5 and 6 are the results of the spectral fitting described above. As shown in the figure, all spectra were reasonably represented by our method. Figure 6 includes some data presented in the previous manuscript,54 and the quality of the fitting is not worse than the previous one. As is shown in Figure 6, for [BMIm][PF6], the spectral dynamics of DEAHF vary slightly with shifting excitation from 400 to 450 nm, with little change between excitation at 430 and 450 nm. Therefore, we will discuss the difference of the two (400 and 450 nm) excitation wavelength for the imidazolium-cation based ionic liquids. In the case of the phosphonium-cation based ionic liquids, it proved difficult to fit the spectra obtained from excitation at 450 nm, because the edge filter used for cutting off the excitation pulse also let the early fluorescence peak unresolved (see Figure 6). Therefore the peak height of the normal fluorescence was overestimated for the data at an earlier state of the excitation as will be discussed in the next section. c. Population Dynamics of DEAHF in Ionic Liquids Obtained at Different Excitation Wavelengths. Figure 8 shows the normalized time profiles of population dynamics of DEAHF in [HMIm][NTf2] obtained from excitation at 400 and 450 nm, and in [P2,2,2,8][NTf2] obtained from excitation at

(1)

where N*(t,ν) and T*(t,ν) are the fluorescence line shape functions from the normal and tautomer excited states at the delay time t, respectively. Here we assumed that both line shape functions are given by a log-normal function as A(t , ν) = h(t ) 2 ⎧ ⎪ exp[ − ln(2){ln(1 + α(t ))/ γ (t )} ] α(t ) > − 1 ×⎨ ⎪ α(t ) ≤ − 1 0 ⎩ (2)

where α(t) = 2γ(t)(ν − νP(t))/Δ(t), h(t) is the scaling factor, νP(t) is the peak position, γ(t) is the asymmetric parameter, and Δ(t) is the bandwidth parameter, and these parameters were assumed to be dependent on time t in the previous literature,54 respectively. In a previous report,54 to separate the population dynamics of tautomer and normal species, we employed information from the spectral dynamics of DEAMF that does not show ESIPT. For fluorescence from the normal form, we used the parameters Δ and γ obtained by fitting to the spectrum of DEAMF excited at the same wavelength (400 nm). This excitation wavelength corresponds to a low energy (long wavelength) side of the DEAMF absorption. Previous studies have suggested that the fluorescence dynamics of charge transfer molecules can depend upon excitation wavelength in ionic liquids because of inhomogeneity of solvation and the initial excess energy in the solvation.18 Therefore in our study, the reference spectrum of the normal form may be dependent on the excitation wavelength. Although we tried to measure the DEAMF fluorescence dynamics by excitation at 430 nm, the spectra were not well resolved, particularly at the initial ultrafast time scale (several ps), because an edge filter was used to cut the excitation pulse and the fluorescence wavelength of DEAMF was shorter than that of DEAHF. It was impossible to obtain reliable value of Δ and γ from the spectral fitting. When we analyzed DEAMF fluorescence obtained from excitation at 430 nm using the parameters of Δ and γ obtained from excitation at 400 nm, the fluorescence dynamics were adequately simulated by assuming that peak position and the 12575



Article

the ESIPT process of this reaction can be described by Scheme 1, where kPT, k−PT, kN, and kT are the ESIPT rate from the normal form, the reverse ESIPT rate, the total (radiative and nonradiative) relaxation rate of the normal form and that of the tautomeric form, respectively. By assuming that kPT, k−PT ≫ kN, kT, time profiles of the excited state populations of the normal and tautomeric forms ([N*] and [T*], respectively) are described as follows:49,50 [N *](t ) = A e−k fastt + B e−kslowt

(3)

[T *](t ) = C e−k fastt + D e−kslowt

(4)

k fast = kPT + k −PT

(5)

kslow =

k TkPT kNk −PT + kPT + k −PT kPT + k −PT

(6)

The pre-exponential factors A, B, C, and D are given A=

[N*]0 kPT − [T*]0 k −PT , kPT + k −PT

B=

C=− D= Figure 8. Comparison of time profiles of the population dynamics of DEAHF (top) in [HMIm][NTf2] at 400 and 450 nm excitations and (bottom) in [P2,2,2,8][NTf2] at 400, 430, and 450 nm excitations. Each figure is normalized at each initial rise of normal form.

([N *]0 + [T *]0 )k −PT kPT + k −PT [N *]0 kPT − [T *]0 k −PT = −A , kPT + k −PT ([N *]0 + [T *]0 )kPT kPT + k −PT

where [N*]0 and [T*]0 are the initial populations of the excited states of the normal and tautomeric forms after the photoexcitation. Using these pre-exponential factors, the equilibrium constant in the excited state (Keq) is given by the following expression:

400, 430, and 450 nm. The time profiles of the populations of the normal and tautomeric forms in the excited state were evaluated by integration of the line shape function at each delay time in several solvents. In each solvent, the effect of excitation wavelength is significant within several tens of ps and generation of the tautomeric form decreases with increasing excitation wavelength (see Figures S4 and S5). As mentioned previously, because of the fitting artifact induced by the edge filter, the time profile of the dynamics in phosphonium-cation based ionic liquids at the 450 nm excitation showed a rapid decay of the normal fluorescence directly after photoexcitation. Therefore for the data obtained from excitation at 450 nm, we tried to extract the time profile of the tautomer only by evaluating the tautomeric fluorescence (around 17 000 cm−1) because the peak position of the normal and tautomeric forms were well separated in phosphonium-cation based ionic liquids. Figure S6 show the time profile of the tautomeric form population dynamics obtained by the spectral fitting and the time profile of fluorescence intensity from the tautomeric form obtained from excitation at 430 nm. Both of these are similar in each phosphonium-cation based ionic liquid. Therefore the dynamics of the generation of the tautomeric form at 450 nm excitation were evaluated by from the time profile of fluorescence intensity around 17,000 cm−1. To analyze the time profiles of population dynamics obtained at different excitation wavelengths in each solvent in a quantitative manner, at first we refer to the conventional scheme for ESIPT processes of DEAHF.49,50 We assume that

Keq = −

A D B C

(7)

According to this model, the population dynamics can be expressed using two exponential functions. However, as is mentioned in previous work, at least three rise components and one decay component were required to simulate the population dynamics of the tautomer.54 Accordingly, eqs 3 and 4 were modified to include additional dynamics as follows: [N *](t ) = N1 e−k1t + N2 e−k 2t + N3 e−k3t

(8)

[T *] = {T0 + T1(1 − e−k1t ) + T2(1 − e−k 2t )}e−k3t ≅ −T1e−k1t − T2e−k 2t + (T0 + T1 + T2)e−k3t = −T1e−k1t − T2e−k 2t + T3e−k3t

(9)

where k1, k2 ≫ k3, T3 = T0 + T1 + T2, and all parameters Ni and Ti (i = 0, 1, 2, 3) are positive. Here the terms T0, T1, and T2 represent the rises of the tautomer with different time scales, the response function limited, the faster (k1), and the slower (k2) components, respectively. As is described in previous work,54 the pre-exponential factors N1, N2, T1, and T2 satisfy the relation N1/N2 = T1/T2, and the average of k1 and k2 is considered to correspond to the kfast process, and the slowest component (k3) to the population decay in the excited state (kslow). 12576



Article

Figure 9. Time profiles of the populations of the normal form (red) and the tautomeric form (green) in the excited state evaluated by the integrations of the line shape function at each delay time at different excitation wavelengths in [HMIm][NTf2] and [P2,2,2,8][NTf2]. The solid lines represent the fitting lines to eqs 8 and 9.

Figure 10. Peak shift of the normal form fluorescence of DEAHF in several ionic liquids at different excitation wavelengths; Imidazolium-based ionic liquids obtained at (a) 400 nm and (b) 450 nm excitations; (c) [P2,2,2,8][NTf2] obtained at 400 and 430 nm excitations; (d) [P4,4,4,1][NTf2] and [N4,4,4,1][NTf2] obtained at 400 nm excitation.

12577



Article

Figure 11. Keq* and ⟨τave⟩ obtained at 400 nm excitation plotted against two parameters. Left: the number of carbon atoms in alkyl chains of cation. Right: ion concentration of ionic liquid.

10 shows the fluorescence peak shift of the normal form in the excited state in each ionic liquid obtained at different excitation wavelengths, evaluated by the spectral fitting. Figure 10a and b shows the results of peak shift in imidazolium-cation based ionic liquids at 400 and 450 nm excitation. It is clearly noted that the initial peak position from excitation at 400 nm is higher than those from excitation at 450 nm if compared within the same solvent ionic liquid, and that the peak shift within the initial 100 ps delay is faster at 400 nm excitation than that at 450 nm excitation. On the other hand, the peak position at a delay of 200 ps shows less variation with excitation wavelength. The same stories are true for the phosphonium-cation based ionic liquids as is shown in Figure 10c. These results are considered to reflect the inhomogeneous solute distribution in ionic liquids and the selective excitation of the solute as reported previously.4,5,18,26,27 We consider that the difference in solvation of the normal form at the initial stage is key to understanding the excitation wavelength dependence of the reaction dynamics, which will be discussed in the next section. It may be noted that the peak shifts of fluorescence from the normal form in [P4,4,4,1][NTf2] and [N4,4,4,1][NTf2] are parallel within several hundred ps although there is a large difference in viscosities of solutions. Therefore the relaxation within a few hundred ps region cannot be explained only by the viscosity. e. Excited State Equilibrium and Reaction Rate at the Different Excitation Wavelengths. At first we consider the origin of nonsingle exponential behavior of the reaction process from the normal to the tautomer form. Recently, Hayaki et al. presented theoretical calculations on the energy surface of a similar proton transfer reaction in the ionic liquid [BMIm][PF6] using RISM-SCF-SEDD.63 According to their calculations, directly after photoexcitation, the normal form exists in an energetically unfavorable solvation state, and the reaction

Based on eqs 8 and 9, both time profiles of the normal and tautomeric forms were simultaneously simulated and convoluted with the system response function. In the fitting, the system response function was assumed as to be a Gaussian function with the time zero and the width as adjustable parameters. The slow time decay constant, k3, was fixed to a value estimated from the decay of the integrated fluorescence intensity measured by a streak camera. As the fluorescence lifetime is long (around 3 ns) in ionic liquids, the estimation of this value did not affect the rate constants k1 and k2. Typical examples of the fits are shown by the solid lines in Figure 9, which illustrates the time profiles of the population of normal (red) and tautomeric (green) forms of the excited state evaluated by integration of the line shape function at each delay time in [HMIm][NTf2] and [P2,2,2,8][NTf2]. The simulated lines reproduced the time profiles well at all excitation wavelengths. Other examples of the fittings are shown in the Supporting Information (Figures S7−S10). The time profile of the tautomeric form in phosphonium-cation based ionic liquids excited at 450 nm were evaluated by fitting the fluorescence intensity around 17 000 cm−1 using a three-exponential function. Figure S11 show the fluorescence intensity of the tautomeric form of DEAHF in phosphonium-cation based ionic liquids obtained from excitation at 450 nm. The obtained parameters are listed in Supporting Information Tables 1−3. Some of the results obtained from excitation at 400 nm are reevaluation of the previously reported results, and almost similar values were obtained in comparison with the previously reported values.54 d. Solvation Dynamics at the Different Excitation Wavelengths. Before discussion of the population dynamics, we comment on the excitation wavelength dependence of the solvation dynamics of the normal form in ionic liquids. Figure 12578



Article

Now we will discuss the excitation wavelength dependence of Keq* and ⟨τave⟩. As shown in Supporting Information Tables 1− 3, the values of Keq* decrease with increasing excitation wavelength and the decrease is more apparent in ionic liquids having longer alkyl chains. The values of ⟨τave⟩ increase in all ionic liquids with increasing excitation wavelength, as may be expected from the lower tautomer yield at longer excitation wavelengths. The longer time scale component of the rate becomes significantly slower in imidazolium-cation based ionic liquids with longer alkyl chains. The excitation wavelength dependence of Keq* (the ratio Keq* for the 400 nm excitation to that for the 450 nm excitation) is plotted against the number of carbon atoms in alkyl chains in Figure 12. Although the

barrier to the tautomeric form is small. This is mainly because the magnitude and the direction of the dipole moment of the normal excited state are quite different from those of the normal ground state while those of the tautomeric excited state are close to those of the normal ground state, as was predicted by the semiempirical calculations on the electronic state.48 With the solvation of normal form, the reaction barrier becomes high. This model calculation suggests that the reaction rate for transition from the normal to tautomeric forms should be time dependent. Although we used three components for fitting the reaction rate, these may represent a single continuous process with a continuous change of the reaction barriers. Therefore, the following averaged value of the rate constants is appropriate for the discussion of the proton transfer rate: ⟨τ ⟩ave = k1−1T1 + k 2−1T2

(10)

The initial system response limited component was omitted because of inability of the estimation of the fastest rate constant. Further, in order to estimate the excited state equilibrium constant we used the following simplified equation considering that the equilibrium between the normal and the tautomeric forms in the excited state is essential attained at 200 ps in each solvent at all excitation wavelengths as shown in Figures 8 and 9,

Keq* =

T3 N3

(11)

Figure 12. Ratio of Keq* at 400 nm excitation to that at 450 nm excitation plotted against the number of carbon atoms in alkyl chains of cation.

The asterisk indicates that the value is not exactly the same as that determined by eq 7. In Supporting Information Tables S1−3, we have listed the values, which show remarkable dependence on the series of ionic liquids cation alkyl chain lengths, and excitation wavelength. First, we will discuss the solvent species dependence of Keq* and ⟨τave⟩ obtained from excitation at 400 nm. In the previous paper, we have mentioned the excited state equilibrium constant (estimated by the different way from the present method) shows the linear correlation with the ionic concentration.54 The ionic concentration is an index of polarity of ionic liquid. In the present paper, we present another index of ionic liquid, the number of carbon atoms in alkyl chains of cation. We consider that this parameter is an index of the nonpolar domain of ionic liquid, and may serve as the index of the heterogeneity of ionic liquid. Figure 11 shows plots of Keq* and ⟨τave⟩ obtained from excitation at 400 nm against the number of carbon atoms and ion concentration. As is the case for the steady state fluorescence analysis, Keq* at the 400 nm excitation is linearly correlated with increase of number of carbon atoms in alkyl chains of cation. The correlation of Keq* with the ionic concentration is also good, although a stronger trend is clear from the plot with the carbon number, and especially from the imidazolium-cation based ionic liquids. These results suggest that the polarity and/or nonpolarity of the ionic liquids dominate the ESIPT reaction of DEAHF. The correlation of ⟨τave⟩ obtained from excitation at 400 nm excitation with the number of carbon atoms in alkyl chains is less clear, although ⟨τave⟩ shows a slight decrease with increasing number of carbon atoms in alkyl chains. Considering the errors in the rate and the fact that an ultrafast component (T0) is not included in this plot, it can be safely said that the initial reaction dynamics within several hundred ps are correlated with the polarity scale (ion concentration) and the nonpolarity scale (the number of carbon atoms in alkyl chains).

correlation is somewhat poor in comparison with the steadystate fluorescence analysis (Figure 4b), a good linear correlation is found between the excitation wavelength dependence of Keq* and the number of carbon atoms in alkyl chains. Therefore, it is concluded that the effect of the excitation wavelength dependence of DEAHF in ionic liquids, originates from dynamics within a few hundred ps of excitation, and that the number of carbon atoms in alkyl chains is a good indicator of these processes. Figure 13 shows a schematic illustration of the energy surface in the excited state and reaction pathways at different excitation

Figure 13. Schematic illustration of the excited energy surface and reaction pathways at different excitation wavelengths in ionic liquids.

wavelengths in ionic liquids. According to the model theoretical calculations,63 the solvation coordinate of the Franck−Condon state of the normal excited state is energetically unstable, although the tautomeric excited state is well solvated at this position of the solvent coordinate. Therefore, as solvation of 12579



Article

of carbon atoms in the cation alkyl chains. The ionic concentration is also found to be a good indicator. From the time-resolved fluorescence spectra, the population dynamics of the normal and tautomeric forms were extracted from spectral fitting using the sum of log-normal functions. The ESIPT rates at different excitation wavelengths were evaluated by analyzing the time profiles of the normal and the tautomer populations in the excited state simultaneously. The ESIPT rate is found to depend strongly on the number of carbon atoms of alkyl chain of ionic liquids; the initial dynamics of ESIPT represented by Keq* and ⟨τave⟩ are well correlated with the number of alkyl carbon atoms. Considering the results of steady-state and timeresolved fluorescence measurements, it is concluded the initial dynamics of DEAHF are important for the excitation wavelength dependence of ESIPT, and alkyl chain length is a good indicator of the inhomogeneity of the solvation. Although the dynamics beyond 250 ps were not surveyed in this paper, the slower dynamics are also considered important. For example, the values of Keq* from excitation at 400 nm in [P4,4,4,1][NTf2] and [N4,4,4,1][NTf2] are similar to each other (Figure 11a), although there are some differences of R(400) in the analysis of the steady state fluorescence (see Figure 4a); that is, only the initial dynamics does not represent the full time results. This suggests that dynamics over a longer time scale may also affect the reaction. This point should be investigations further in the future.

the normal form progresses, the reaction barrier becomes high as is illustrated by the changes of the reaction paths from blue to orange. If the initial solvation coordinate in the normal excited state moves from the blue circle to the green circle indicated in the figure by the change of excitation wavelength from the shorter (blue) to the longer (red) wavelength, the initial barrier of ESIPT is raised because of the difference in solvation of the excited state. Although it was not confirmed by the theoretical calculation, the differences of the initial solvation dynamics at the different excitation wavelengths are shown by the experimentally observed solvation dynamics within a few hundred ps region having a strong dependence on the excitation wavelength (see Figure 9). As shown in the previous section, by excitation of the long wavelength portion (red edge) of the absorption, the initial fluorescence peak position shifts to a longer wavelength and the solvation dynamics become slow. Therefore the molecules excited at the longer (red) wavelength see the larger potential barrier to the tautomer at the initial stage of the reaction. As shown in Figure 9c, the difference of the solvation state becomes negligible by ca. 100 ps, suggesting that in the longer time range the difference between the redexcited molecules and the blue-excited molecule becomes negligible. These differences of the solvation dynamics lead to the difference of the reaction dynamics at the initial stage after the photoexcitation at different excitation wavelength, resulting in the different yield of the tautomer. Correlations of the excitation wavelength dependences of R and Keq* with the number of carbon atoms in alkyl chains of cation and the ionic concentration suggest that solvation inhomogeneity that affects the ESIPT is strongly related to these properties. As mentioned previously, the number of carbon atoms in alkyl chains reflects the nonpolar domain. According to the scattering study, no unique domain structure (a scattering peak near 3 nm−1) was not observed for [EMIm][NTf2] and [BMIm][NTf2].34,35 In the present case, however, the effect of the solvation heterogeneity continuously changes from [EMIm][NTf2] to [HMIm][NTf2] as represented by Figure 4b. Therefore, although the contribution of the nonpolar alkyl chain of the cation induces the solvation heterogeneity of the dissolved solute, the segregation of the domain part is not necessarily required for the observation of the excitation wavelength dependence. Ion concentration may work another index for the heterogeneity of ionic liquid, although the correlation with the excitation wavelength dependence is less poor than the number of carbon atoms of alkyl chains. Ion concentration represents how many charges are present in per volume, and can be the index of the polarity of ionic liquid. However, it does not contain the information of the “size of polar part” of cation and anion molecules, which may be the reason for the somewhat poor correlation with the local heterogeneity.

■

ASSOCIATED CONTENT

S Supporting Information *

Detailed procedure of the spectral fitting, tables listing the peak of the tautomeric form and the parameters obtained by the fit of the spectral line shape obtained at different excitation wavelengths to eqs 8−10; figures of time-resolved fluorescence spectra in ionic liquids and of the time profiles of the populations of the normal and the tautomeric forms obtained at different excitation wavelengths; the time profiles of the fluorescence intensity of the tautomeric form in the phosphonium-cation based ionic liquids. These materials are available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-774-65-6561. Fax:+81-774-65-6803. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work supported by the Grant-in Aid for Scientific Research from JSPS (No. 23350006), and by the research fellowship of Global COE program, International Center for Integrated Research and Advanced Education in Material Science, the Core-stage program from Kyoto University.

4. SUMMARY We investigated the excitation wavelength dependence of the ESIPT of DEAHF in two series of ionic liquids by steady-state fluorescence and time-resolved fluorescence measurements using the optical Kerr gate method. Steady-state fluorescence spectra showed that the relative intensity of the tautomer fluorescence decreased compared with the normal form as excitation wavelength was increased from 380 to 450 nm in ionic liquids. It is found that the ratio of tautomer to normal fluorescence from excitation at 400 nm divided by the ratio obtained from excitation at 450 nm correlates with the number

■

REFERENCES

(1) Science of Ionic Liquids; Nishikawa, K. Ouchi, Y., Itoh, T. Watanabe, M., Eds.; Maruzen: Japan, 2012. (2) Ionic Liquids in Synthesis, 2nd ed.; Wasserschid, P., Welton, T., Eds.; Wiley-VCH Verlag GmBH & Co. KGaA, Weinheim, 2008. (3) Crowhurst, L.; Mawdsley, P. R.; Perez-Arlandis, J. M.; Slater, P. A.; Welton, T. Solvent-Solute Interaction in Ionic Liquids. Phys. Chem. Chem. Phys. 2003, 5, 2790−2794. 12580



Article

Transfer Reaction in Room-Temperature Ionic liquids. Acc. Chem. Res. 2007, 40, 1130−1137. (24) Zhang, X.-X.; Liang, M.; Ernsting, N. P.; Maroncelli, M. Conductivity and Solvation Dynamics in Ionic Liquids. J. Phys. Chem. Lett. 2013, 4, 1205−1210. (25) Muramatsu, M.; Nagasawa, Y.; Miyasaka, H. Ultrafast Solvation Dynamics in Room Temperature Ionic Liquids Observed by ThreePulse Photon Echo Peak Shift Measurements. J. Phys. Chem. A 2011, 115, 3886−3894. (26) Mandal, P. K.; Sarkar, M.; Samanta, A. Excitation-WavelengthDependent Fluorescence Behavior of Some Dipolar Molecules in Room-Temperature Ionic Liquids. J. Phys. Chem. A 2004, 108, 9048− 9053. (27) Hu, Z.; Margulis, C. J. Heterogeneity in a Room-Temperature Ionic Liquid: Persistent Local Environments and the Red-Edge Effect. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 831−836. (28) Paul, A.; Mandal, P. K.; Samanta, A. On the Optical Properties of the Imidazolium Ionic Liquids. J. Phys. Chem. B 2005, 109, 9148− 9153. (29) Samanta, A. Dynamic Stokes Shift and Excitation Wavelength Dependent Fluorescence of Dipolar Molecules in Room Temperature Ionic Liquids. J. Phys. Chem. B 2006, 110, 13704−13716. (30) Mandal, P. K.; Paul, A.; Samanta, A. Excitation Wavelength Dependent Fluorescence Behavior of the Room Temperature Ionic Liquids and Dissolved Dipolar Solutes. J. Photochem. Photobiol., A 2006, 182, 113−120. (31) Santhosh, K.; Banerjee, S.; Rangaraj, N.; Samanta, A. Fluorescence Response of 4-(N,N′-Dimethylamino)benzonitrile in Room Temperature Ionic Liquids: Observation of Photobleaching under Mild Excitation Condition and Multiphoton Confocal Microscopic Study of the Fluorescence Recovery Dynamics. J. Phys. Chem. B 2010, 114, 1967−1974. (32) Zhao, Y.; Hu, Z. Structural Origin of Energetic Heterogeneity in Ionic Liquids. Chem. Commun 2012, 48, 2231−2233. (33) Lopes, J. N. C.; Pádua, A. A. H. Nanostructural Organization in Ionic Liquids. J. Phys. Chem. B 2006, 110, 3330−3335. (34) Triolo, A.; Russina, O.; Bleif, H.-J.; Cola, E. D. Nanoscale Segregation in Room Temperature Ionic Liquids. J. Phys. Chem. B 2007, 111, 4641−4644. (35) Triolo, A.; Russina, O.; Fazio, B.; Cola, E. D. Morphology of 1Alkyl-3-Methylimidazolium Hexafluorophosphate Room Temperature Ionic Liquids. Chem. Phys. Lett. 2008, 457, 362−365. (36) Pison, L.; Lopes, J. N. C.; Rebelo, L. P. N.; Padua, A. A. H.; Gomes, M. F. C. Interactions of Fluorinated Gases with Ionic Liquids: Solubility of CF4, C2F6, and C3F8 in Trihexyltetradecylphosphonium Bis(trifluoromethylsulfonyl)amide. J. Phys. Chem. B 2008, 112, 12394−12400. (37) Annapureddy, H. V. R.; Kashyap, H. K.; Biase, P. M. D.; Margulis, C. J. What is the Origin of the Prepeak in the X-ray Scattering of Imidazolium-Based Room-Temperature Ionic Liquids? J. Phys. Chem. B 2010, 114, 16838−16846. (38) Yamamuro, O.; Yamada, T.; Kofu, M.; Nakakoshi, M.; Nagao, M. Hierarchical Structure and Dynamics of an Ionic Liquid 1-Octyl-3Methylimidazolium Chloride. J. Chem. Phys. 2011, 135, 054508 (1−7). (39) Mizoshiri, M.; Nagao, T.; Mizoguchi, Y.; Yao, M. Dielectric Permittivity of Room Temperature Ionic Liquids: A Relation to the Polar and Nonpolar Domain Structures. J. Chem. Phys. 2010, 132, 164510 (1−7). (40) Castner, E.; Margulis, C. J.; Maroncelli, M.; Wishart, J. F. Ionic liquids: Structure and Photochemical Reaction. Annu. Rev. Phys. Chem. 2011, 62, 85−105. (41) Nagasawa, Y.; Itoh, T.; Yasuda, M.; Ishibashi, Y.; Ito, S.; Miyasaka, H. Ultrafast Charge Transfer Process of 9,9′-Bianthryl in Imidazolium Ionic Liquids. J. Phys. Chem. B 2008, 112, 15758−15765. (42) Nagasawa, Y.; Oishi, A.; Itoh, T.; Yasuda, M.; Muramatsu, M.; Ishibashi, Y.; Ito, S.; Miyasaka, M. Dynamic Stokes Shift of 9,9′Bianthryl in Ionic Liquids: A Temperature Dependence Study. J. Phys. Chem. C 2009, 113, 11868−11876.

(4) Fujisawa, T.; Fukuda, M.; Terazima, M.; Kimura, Y. Raman Spectroscopic Study on Solvation of Diphenylcyclopropenone and Phenol Blue in Room Temperature Ionic Liquids. J. Phys. Chem. A 2006, 110, 6164−6172. (5) Kimura, Y.; Hamamoto, T.; Terazima, M. Raman Spectroscopic Study on the Solvation of N,N-Dimethyl-p-nitroaniline in RoomTemperature Ionic Liquids. J. Phys. Chem. A 2007, 111, 7081−7089. (6) Kobayashi, A.; Osawa, K.; Terazima, M.; Kimura, Y. Solute− Solvent Hydrogen-Bonding in Room Temperature Ionic Liquids Studied by Raman Spectroscopy. Phys. Chem. Chem. Phys. 2012, 14, 13676−13683. (7) Karmakar, R.; Samanta, A. Solvation Dynamics of Coumarin-153 in a Room-Temperature Ionic Liquid. J. Phys. Chem. A 2002, 106, 4447−4452. (8) Ingram, J. A.; Moog, R. S.; Ito, N.; Biswas, R.; Maroncelli, M. Solute Rotation and Solvation Dynamics in a Room-Temperature Ionic Liquid. J. Phys. Chem. B 2003, 107, 5926−5932. (9) Arzhantsev, S.; Ito, N.; Heitz, M.; Maroncelli, M. Solvation Dynamics of Coumarin 153 in Several Classes of Ionic Liquids: Cation Dependence of the Ultrafast Component. Chem. Phys. Lett. 2003, 381, 278−286. (10) Ito, N.; Arzhantsev, S.; Heitz, M.; Maroncelli, M. Solvation Dynamics and Rotation of Coumarin 153 in Alkylphosphonium Ionic Liquids. J. Phys. Chem. B 2004, 108, 5771−5777. (11) Karmakar, R.; Samanta, A. Dynamics of Solvation of the Fluorescent State of Some Electron Donor−Acceptor Molecules in Room Temperature Ionic Liquids, [BMIM][(CF3SO2)2N] and [EMIM][(CF3SO2)2N]. J. Phys. Chem. A 2003, 107, 7340−7346. (12) Mandal, P. K.; Samanta, A. Fluorescence Studies in a Pyrrolidinium Ionic Liquid: Polarity of the Medium and Solvation Dynamics. J. Phys. Chem. B 2005, 109, 15172−15177. (13) Lang, B.; Angulo, G.; Vauthey, E. Ultrafast Solvation Dynamics of Coumarin 153 in Imidazolium-Based Ionic Liquids. J. Phys. Chem. A 2006, 110, 7028−7034. (14) Paul, A.; Samanta, A. Solute Rotation and Solvation Dynamics in an Alcohol-Functionalized Room Temperature Ionic Liquid. J. Phys. Chem. B 2007, 111, 4724−4731. (15) Seth, D.; Chakraborty, A.; Setua, P.; Sarkar, N. Dynamics of Solvent and Rotational Relaxation of Coumarin-153 in RoomTemperature Ionic Liquid 1-Butyl-3-methyl Imidazolium Tetrafluoroborate Confined in Poly(oxyethylene glycol) Ethers Containing Micelles. J. Phys. Chem. B 2007, 111, 4781−4787. (16) Arzhantsev, S.; Jin, H.; Baker, G. A.; Maroncelli, M. Measurement of the Complete Solvation Response in Ionic Liquids. J. Phys. Chem. B 2007, 111, 4978−4989. (17) Jin, H.; Baker, G. A.; Arzhantsev, S.; Dong, J.; Maroncelli, M. Solvation and Rotational Dynamics of Coumarin 153 in Ionic Liquids: Comparisons to Conventional Solvents. J. Phys. Chem. B 2007, 111, 7291−7302. (18) Jin, H.; Xiang, Li.; Maroncelli, M. Heterogeneous Solute Dynamics in Room-Temperature Ionic Liquids. J. Phys. Chem. B 2007, 111, 13473−13478. (19) Maroncelli, M.; Zhang, X.-X.; Liang, M.; Roy, D.; Ernsting, C, P. Measurements of the Complete Solvation Response of Coumarin 153 in Ionic Liquids and the Accuracy of Simple Dielectric Continuum Predictions. Discuss. Faraday Soc. 2012, 154, 409−414. (20) Zhang, X.-X.; Liang, M.; Ernsting, N. P.; Maroncelli, M. The Complete Solvation Response of Coumarin 153 in Ionic Liquids. J. Phys. Chem. B 2013, 117, 4291−4304. (21) Shim, Y.; Choi, M. Y.; Kim, H. A Molecular Dynamics Computer Simulation Study of Room-Temperature Ionic Liquids. II. Equilibrium and Nonequilibrium Solvation Dynamics. J. Chem. Phys. 2005, 122, 044511(1−12). (22) Kobrak, M. N. Characterization of the Solvation Dynamics of a Room-Temperature Ionic Liquid via Molecular Dynamics Simulation. J. Chem. Phys. 2006, 125, 064502 (1−11). (23) Shim, Y.; Jeong, D.; Manjari, S.; Choi, M. Y.; Kim, H. J. Solvation, Solute Rotation and Vibration Relaxation, and Electron12581



Article

(61) Arzhabtsev, S.; Maroncelli, M. Design and Characterization of a Femtosecond Fluorescence Spectrometer Based on Optical Kerr Gating. Appl. Spectrosc. 2005, 59, 206−220. (62) Gardecki, J. A.; Maroncelli, M. Set of Secondary Emission Standards for Calibration of the Spectral Responsivity in Emission Spectroscopy. Appl. Spectrosc. 1998, 52, 1179−1189. (63) Hayaki, S.; Kimura, Y.; Sato, H. Ab initio Study on an ExcitedState Intramolecular Proton Transfer Reaction in Ionic Liquid. J. Phys. Chem. B 2013, 117, 6759−6767.

(43) Kotni, S.; Samanta, A. Modulation of the Excited State Intramolecular Electron Transfer Reaction and Dual Fluorescence of Crystal Violet Lactone in Room Temperature Ionic Liquids. J. Phys. Chem. B 2010, 114, 9195−9200. (44) Kotni, S.; Samanta, A. What Determines the Rate of ExcitedState Intramolecular Electron-Transfer Reaction of 4-(N,N′dimethylamino)benzonitrile in Room Temperature Ionic Liquids? A Study in [bmim][PF6]. ChemPhysChem 2012, 13, 1956−1961. (45) Li, X.; Liang, M.; Chakraborty, A.; Kondo, M.; Maroncelli, M. Solvent-Controlled Intramolecular Electron Transfer in Ionic Liquids. J. Phys. Chem. B 2011, 115, 6592−6607. (46) Yoshida, K.; Iwata, K.; Nishiyama, N.; Kimura, Y.; Hamagaguchi, H. Local Structures in Ionic Liquids Probed and Characterized by Microscopic Thermal Diffusion Monitored with Picosecond Timeresolved Raman Spectroscopy. J. Chem. Phys. 2012, 136, 104504 (1− 8). (47) Sahu, K.; Kern, S. J.; Berg, M. A. Heterogeneous Reaction Rates in an Ionic Liquid: Quantitative Results from Two-Dimensional Multiple Population-Period Transient Spectroscopy. J. Phys. Chem. A 2011, 115, 7984−7993. (48) Chou, P. T.; Huang, C. H.; Pu, S. C.; Cheng, Y. M.; Yu, W. S.; Yu, Y. C.; Wang, Y.; Chen, C. T. The Dipolar Functionality Tuning Excited-State Charge/Proton Transfer Coupled Reaction. J. Phys. Chem. A 2004, 108, 6452−6454. (49) Chou, P. T.; Pu, S. C.; Cheng, Y. M.; Yu, W. S.; Yu, Y. C.; Hung, F. T.; Hu, W. P. Femtosecond Dynamics on the Excited-State Proton/ Charge Transfer Coupled Reaction in 4′-N,N-Diethylamino-3Hydroxyflavones. J. Phys. Chem. A 2005, 109, 3777−3787. (50) Cheng, Y. M.; Pu, S. C.; Yu, Y. C.; Chou, P. T.; Huang, C. H.; Chen, C. T.; Li, T. H.; Hu, W. P. Spectroscopy and Femtosecond Dynamics of 7-N,N-Diethylamino-3-hydroxyflavone; Generalization of Dipolar Tuning Excited-state Proton/Charge Transfer Reaction. J. Phys. Chem. A 2005, 109, 11696−11706. (51) Organero, J. A.; Tormo, L.; Sanz, M.; Roshal, A.; Douhal, A. Complexation Effect of γ-Cyclodextrin on a Hydroxyflavone Derivative: Formation of Excluded and Included Anions. J. Photochem. Photobiol., A 2007, 188, 74−82. (52) Demchenko, A. P.; Tang, K. C.; Chou, P. T. Excited-State Proton Coupled Charge Transfer Modulated by Molecular Structure and Media Polarization. Chem. Soc. Rev. 2013, 42, 1379−1408. (53) Fukuda, M.; Terazima, M.; Kimura, Y. Study on the Excited State Intramolecular Proton Transfer of 4′-N,N-diethylamino-3hydroxyflavone in Imidazolium-Based Room Temperature Ionic Liquids. Chem. Phys. Lett. 2008, 463, 364−368. (54) Kimura, Y.; Fukuda, M.; Suda, K.; Terazima, M. Excited State Intramolecular Proton Transfer Reaction of 4′-N,N-Diethylamino-3hydroxyflavone and Solvation Dynamics in Room Temperature Ionic Liquids Studied by Optical Kerr Gate Fluorescence Measurement. J. Phys. Chem. B 2010, 114, 11847−11858. (55) Suda, K.; Terazima, M.; Kimura, Y. Excitation Wavelength Dependence of Photo-Induced Intramolecular Proton Transfer Reaction of 4′-N,N-diethylamino-3- hydroxyflavone in Various Liquids. Chem. Phys. Lett. 2012, 531, 70−74. (56) Suda, K.; Terazima, M.; Kimura, Y. Erratum to “Excitation wavelength dependence of photo-induced intramolecular proton transfer reaction of 4′-N,N-diethylamino-3-hydroxyflavone in various liquids”. Chem. Phys. Lett. 2013, 582, 169. (57) Ormson, S. M.; Brown, R. G.; Vollmer, F.; Rettig, W. Switching between Charge- and Proton-Transfer Emission in the Excited State of a Substituted 3-Hydroxyflavone. J. Photochem. Photobiol., A 1994, 81, 65−72. (58) Handbook of Chemistry and Physics, 89th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2008−2009. (59) Kunugi, Y.; Hayakawa, H.; Tsunashima, K.; Sugiya, M. DyeSensitized Solar Cells Besed on Quaternary Phosphonium Ionic Liquids as Electrolytes. Bull. Chem. Soc. Jpn. 2007, 12, 2473−2475. (60) Rodriguez, M.; Galan, M.; Munoz, M. J.; Martin, R. Viscosity of Triglycerides + Alcohols from 278 to 313 K. J. Chem. Eng. Data 1994, 39, 102−105. 12582


Excitation Wavelength Dependence of Excited State Intramolecular

Recommend Documents