Charge-Transfer Electronic Absorption Spectra of 1-Ethylpyridinium

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Charge Transfer Electronic Absorption Spectra of 1-Ethylpyridinium Cation and Halogen Anion Pairs in Dichloromethane and as Neat Ionic Liquids Takahiro Ogura, Nobuyuki Akai, Kazuhiko Shibuya, and Akio Kawai J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 21 Jun 2013 Downloaded from http://pubs.acs.org on June 22, 2013

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The Journal of Physical Chemistry

Charge Transfer Electronic Absorption Spectra of 1-Ethylpyridinium Cation and Halogen Anion Pairs in Dichloromethane and as Neat Ionic Liquids

Takahiro Ogura, Nobuyuki Akai†, Kazuhiko Shibuya, Akio Kawai *

Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 H-89, Ohokayama, Meguro-ku, Tokyo 152-8551, Japan

*Corresponding author: Tel and Fax: +81-3-5734-2231.

E-mail address: [email protected] (A. Kawai).

† Present address: Graduate School of Bio-Applications and Systems Engineering,

Tokyo University of Agriculture and Technology 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

The charge transfer (CT) absorption bands of ion pairs composed of 1-ethylpyridinium (Epy+) and halogen anions (X−: Cl, Br, or I) were measured in dichloromethane solutions of EpyX. The CT band of the Epy+I− ion pair shows clear splitting because of spin-orbit interaction in the excited state. The CT transition energy of an Epy+X− ion pair in a dichloromethane solution is related to electron affinity of X, which is in accordance with the Mulliken theory for CT bands. Extinction coefficients for the CT bands of the Epy+X− ion pairs in dichloromethane were determined using the measured absorbance, and the ion pair concentration was estimated on the basis of electro-conductivity. Structures of Epy+X− ion pairs were also evaluated on the basis of both quantum chemical calculations and NMR spectroscopy. In addition, in the absorption spectrum measured for neat EpyI liquid, a broad band appeared at a longer wavelength side of the S1(ππ*) band. This new band has been assigned to the CT band of the Epy+I− ion pair formed in neat EpyI liquid.

KEYWORDS

Ion Pair, conductivity, charge transfer to solvent, Mulliken theory,

absorption spectroscopy. 2


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INTRODUCTION Ionic liquids (ILs) are a new class of solvent attracting broad interest since the late 20th century.1,2 ILs consist of ion molecules, and the intermolecular interaction between the components is characterized by strong Coulomb forces, as well as hydrogen bonding and van der Waals forces. Various macroscopic properties of ILs originate from these microscopic molecular interactions.3-17 Moreover, a hypothetical inhomogeneous structure formed in ILs has been proposed as another key factor that may explain the physicochemical properties of ILs.3,18

Compared to conventional molecular solvents, ILs are unique because the same number of anion and cation molecules coexists in most ILs, and additional intermolecular forces are induced by electric charge transfer (CT). This type of interaction is hardly found in molecular solvents that are composed of one identical molecule. To date, many different anion species have been examined for use in ILs, including those that are characterized by strong electron-donating ability.19 Therefore, CT interactions may play an additional role in determining the solvent properties of ILs.

Takahashi and Katoh et al. studied a solvated electron produced by the photoionization of I− dissolved in ILs by absorption spectroscopy.15 They reported some optical transition bands that were induced through charge transfer to solvent (CTTS) from I−. In particular, 3



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Katoh et al. reported broad absorption bands in addition to the CTTS bands for some ILs, including 1,3-butylmethylimidazolium iodide (BmimI) .16 This optical transition was attributed to the CT band of an ion pair complex, Bmim+I−, formed in the BmimI solvent. Unfortunately, the band was broad and overlapped with the broad S1(ππ*) band of the imidazolium ion. Therefore, it seems rather difficult to examine the CT nature of BmimI on the basis of spectroscopic parameters such as CT band energy and spin-orbit components of iodide. However, it should be noted that this pioneering study is important because for the first time, the existence of CT interactions in room temperature ILs was suggested.

The present study aims to understand CT interactions between ion pairs in ILs from the analysis of the CT absorption bands. We used pyridinium-based ILs such as 1-ethylpyridinium iodide (EpyI) instead of BmimI because 1-ethylpyridinium is known to exhibit well-structured S1(ππ*) and S2(ππ*) bands in the UV wavelength region.17 This spectral characteristic of sharp ππ* band makes it easy to analyze the CT bands of ion pairs composed of Epy+ and halogen anion X− (X: Cl, Br, or I) as compared to Bmim+ based ILs which have broad ππ* band. To evaluate the strength of CT interaction in the Epy+X− ion pair, the electronic absorption spectrum of EpyX was measured in dilute solutions of various molecular solvents and ILs. The CT bands were successfully recorded in dichloromethane

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solution, and the extracted spectroscopic information enabled evaluation of the CT interactions in the Epy+X− ion pair.

EXPERIMENTAL SECTION

Samples. A series of colorless pyridinium salts, EpyI (Iolitec, >98%), EpyBr (Tokyo Chemical Industry, >98%), EpyCl (Tokyo Chemical Industry, >98%), EpyPF6 (Iolitec, >99%), and EpyTf2N (Tf2N= bis(trifluoromethylsulfonyl)amide, Tokyo Chemical Industry, >98%), were used as received. IL samples used as solvents were TmpaTf2N (Kanto Chemical), BmpyrrolTf2N (Kanto Chemical), and BmimTf2N (Kanto Chemical) which were used as received. The structures of these ions such as Epy+ and Tf2N anion are shown in Chart 1. Ultrapure water, acetonitrile, and dichloromethane (Kanto Chemical) were used as solvents for UV/VIS absorption and conductivity measurements. Deuterated water and dichloromethane (Acros Organics) were used as NMR spectroscopy solvents.

Spectroscopic measurements. Steady state absorption spectra were measured using a UV/VIS spectrophotometer (Shimadzu, UV-2450) with a wavelength resolution of 0.1 nm. The temperature of the sample cell was controlled using a flow of temperature-adjusted water in a cell holder. Cubic cells made of fused silica with 1 cm or 0.2 cm optical path lengths were used for solution samples. For neat EpyI, neat EpyTf2N and their mixture, thin layer of 5



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the samples were placed between two fused silica plates. The thickness of the thin liquid layer was on the order of micrometers. 1H-NMR spectra were measured using a JEOL JMTC-400 NMR spectrometer (400-MHz).

Conductivity measurements. Conductivities were measured using a conductivity meter (TOA, CM-20S) with a custom-made cell whose inner volume was 50 cm3. The cell was placed in a water bath, and the temperature was adjusted to 298.0 ± 0.2 K. The cell constant was determined as 0.976 cm−1 at 298.0 K. A series of conductivity measurements for various ion concentrations were conducted beginning with a sample of the highest concentration (20 cm3 volume) that was then diluted in the cell by stepwise addition of a certain amount of solvent.

Quantum chemical calculations. Density functional theory (DFT) calculations were performed using the Gaussian09 program.20 The geometries of the Epy+Cl− ion pair in a dichloromethane solution were optimized using the DFT method at the RB3LYP/6-311++G(3df,3pd) level of theory. A solvation effect was included in the calculations within the framework of a polarized continuum model using the integral equation formalism model.

RESULTS AND DISCUSSION 6


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Absorption spectra of 1-ethylpyridinium-based ILs dissolved in solvents

Figure 1 shows absorption spectra of pyridinium-based ILs dissolved in water and in acetonitrile [dielectric constant (D) = 78.4 and 37.5, respectively] measured in a wavelength region of 220–300 nm. The spectra are dominated by the S1(ππ*)←S0 electronic transition of the pyridinium cation with a peak wavelength, λmax, of 258.2 nm. The molar absorption coefficient of the S1(ππ*) band was determined to be 4–5 × 103 dm3 mol−1 cm−1, as listed in Table 1. An additional band appears in the higher energy region in the EpyI (in water and acetonitrile) and EpyBr (in acetonitrile) spectra. These bands were attributed to an electronic transition due to CTTS from the halide anions.21 A peak wavelength of the CTTS band of the iodide anion shows a spectral redshift as the solvent polarity decreases from water to acetonitrile, which agrees with previously reported results.21

Figure 2 shows the absorption spectra of pyridinium-based ILs dissolved in dichloromethane (D = 9.1). The spectra of EpyTf2N and EpyPF6 are well characterized by the S1(ππ*)←S0 electronic transition, and the spectral parameters such as λmax and ε(λmax) are almost the same as those in water and acetonitrile (Table 1). On the contrary, the spectra of EpyCl, EpyBr, and EpyI show additional broad bands (indicated as arrows) in the lower energy regions of the corresponding S1(ππ*) bands; in the particular case of EpyI, dual bands appear at 4.30 (288.0 nm) and 3.35 eV (369.6 nm) with a separation energy of 0.95 eV. Table 7



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2 summarizes the peak energies of the broad bands of EpyX. Because halide anions are strong electron donors, these bands were assigned to the CT electronic transition induced within the ion pair complexes of Epy+ and X−. This assignment is also supported by the following detailed arguments about (1) energy separation of 0.95 eV for EpyI and (2) spectral shift of the CT bands for EpyCl, EpyBr and EpyI.

The separation energy of the dual CT bands for EpyI, 0.95 eV, is in accordance with the literature value of an energy splitting between the spin-orbit sublevels of the iodine atom, I 2

P1/2 and I 2P3/2, which was determined in the gas phase (∆ESO = 0.942 eV, Table 2). This

observation suggests that the spectral nature of the dual bands includes the neutral iodine atom in these excited states. The CT transition of the Epy+I− ion pair accompanies electron

.

.

transfer from I− to Epy+, which generates the EpyI radical pair of Epy and I . Hence, the dual bands are reasonably attributed to the two CT transitions terminating on the radical pair, including I 2P1/2 and I 2P3/2. As for EpyBr, the spin-orbit splitting between Br 2P1/2 and Br 2P3/2 is 0.459 eV (Table 2). Although the dual bands are expected to appear below 300 nm in addition to the observed 304.0 nm band, no clear band is recognized in the EpyBr spectrum in Figure 2, possibly because of overlap with the strong S1(ππ*) band. Additional evidence supporting the assignment of the observed bands to the CT transition lies in the fact that the band origin shifts to the blue side starting from EpyI (369.6 8


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nm, 3.35 eV) to EpyBr (304.0 nm, 4.08 eV) to EpyCl (∼271 nm, ∼4.6 eV). According to the Mulliken theory22, the CT transition energies (hνCT) of the ion pairs of Epy+X− are approximated by the following equation: X Epy , (1) where I(X−) stands for the ionization potential of each halide anion and EA(Epy+) for the electron affinity of Epy+. The symbols Gi, Xi, and Soli (i = g or e) represent Coulomb potential, CT interaction, and solvation energy, respectively, for the Epy+X− ion pair in the ground state (i = g) or for the EpyX radical pair in the excited state (i = e). The Coulomb interaction is almost constant for all of the EpyX complexes, and the CT interaction may be smaller than the first term of eq (1). Thus, the value is dominated by two terms: X Epy and solvation energy. Because I(X−) is essentially equivalent to EA(X), the spectral shift is related to EA(X). This consideration is in agreement with the experimental results listed in Table 2; the value increases as EA(X) increases from I (3.06eV) to Br (3.36eV) to Cl (3.61eV).

Oscillator strength of the CT transitions of Epy+X− ion pairs in dichloromethane

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In this section, we determine the oscillator strength of the CT absorption bands of the Epy+X− ion pairs to evaluate the strength of the CT interactions. First, the concentration of the ion pairs in dichloromethane was evaluated on the basis of electric conductivity measurements, and then the absorption coefficients of the CT bands were determined according to the following procedure.

Figure 3 shows the absorption spectra of (a) EpyCl, (b) EpyBr, and (c) EpyI recorded at various concentrations in dichloromethane. As the molar concentration increases, the absorption intensity of the CT bands also increases, while the ππ* bands lose intensity. There are isosbestic points at 269.5 nm for EpyCl, 271.0 nm for EpyBr, and 273.0 nm for EpyI. These results suggest that there is an equilibrium between the ion pair and isolated ions.

Next, the concentrations of the ion pairs and isolated ions were measured in the solutions with various EpyX concentrations. For this purpose, molar conductivity measurements were carried out, and the ion association constant (KA) was determined. The Shedlovsky method for 1:1 electrolytes was applied23 by using the following equations:

!"

#

"

$% #

&

' ( ) *+ ,

) -, .1

"& 0

,

1

(2)

(3)

10


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)

2 # 3√5 #

6/&

,

(4)

where Λ0 stands for the limiting molar conductivity, Λ for the molar conductivities, and c for the molar concentration of the separated salt. *+ , , 8, and : values were calculated from the experimental conditions, as explained in the supporting information.

The experiment provides sets of Λ and c values. In the beginning of the analysis, the Λ0 value was estimated using the so-called limiting Onsager equation.23 With this roughly estimated Λ0 value, the KA and Λ0 values were determined by the analysis of the experimental values for Λ and c using eq (2) combined with eqs (3) and (4). Then, iterative analysis using the Λ0 value obtained in each step was carried out until a unique set of KA and Λ0 values were obtained. Figure 4 shows examples of the fitting lines with the experimental plots. From these analyses, the KA values were determined, as listed in Table 3. For determination of molar absorption coefficients, it is necessary to know the concentration of the ion pairs, which can be calculated from KA. For this calculation, the following equation23 was introduced:

Charge-Transfer Electronic Absorption Spectra of 1-Ethylpyridinium

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