Sub-100 fs Charge Separation and Subsequent Diffusive Solvation

Jun 15, 2016 - Femtosecond transient absorption (TA) and picosecond time-resolved fluorescence (TRF) spectroscopies were applied to the charge separat...
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Sub-100 fs Charge Separation and Subsequent Diffusive Solvation Observed for Asymmetric Bianthryl Derivative in Ionic Liquid Eisuke Takeuchi,† Masayasu Muramatsu,† Tetsuro Katayama,‡ Yusuke Yoneda,† Syoji Ito,† Yutaka Nagasawa,*,§,∥ and Hiroshi Miyasaka*,† †

Division of Frontier Materials Science, Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan ‡ Institute for NanoScience Design, Osaka University, Toyonaka Osaka 560-8531, Japan § Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu 525-8577, Japan ∥ PRESTO, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan S Supporting Information *

ABSTRACT: Femtosecond transient absorption (TA) and picosecond time-resolved fluorescence (TRF) spectroscopies were applied to the charge separation (CS) dynamics of 10cyano-9,9′-bianthryl (CBA) in a normal polar solvent, acetonitrile (Acn), and in a highly viscous room temperature ionic liquid (IL), N,N-diethyl-N-methyl-N-(methoxyethyl)ammonium tetrafluoroborate (DemeBF4). The primary CS took place within the ultrafast sub-100 fs time range in both solvents, which was completely independent of diffusive solvation. Subsequent viscosity-dependent spectral evolution was observed by the TA measurement in the picosecond range which was ascribed to the structural relaxation. A red shift of the TRF spectrum in the picosecond to nanosecond range was observed in DemeBF4 which was due to the diffusive solvation in the CS state. Interestingly, integrated fluorescence intensity decayed more rapidly than TA in the IL, while they decayed simultaneously in Acn. It was concluded that diffusive solvation decreases the radiative transition rate of the CS state through the temporal evolution of the CS state electronic structure.

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INTRODUCTION Electron and charge transfer (ET/CT) are the important basic processes in a number of chemical reactions and also in lightenergy conversion systems such as solar cells and natural/ artificial photosynthesis. From the vast number of experimental and theoretical studies accumulated to date, the energy gap between the reactant and the product states and the inter- and intramolecular reorganization energies have been deduced as essential factors regulating the rate constants of ET reactions.1−5 In order to increase the efficiency of light-energy conversion, a large reaction rate constant is required for photoinduced ET reaction, because it takes place in the finite lifetime of an electronically excited state. Moreover, a high energy level of the ET state is preferable to secure abundant energy for the subsequent processes. This condition requires a small energy gap between the excited state and the ET state which according to Marcus theory3 prohibits a rapid ET. The energy-gap dependence of ET restricts the simultaneous optimization of the rate constant and the energy level of the ET state. In natural photosynthetic systems, an energy diagram known as Z scheme is utilized, where the optimization is provided only for the rate of the primary ET process, while the high energy level of the © 2016 American Chemical Society

ET state is acquired through accumulation of multiple photoexcitation.6 In polar solutions, energy fluctuation along the reaction coordinate is induced by the random motion of solvent molecules surrounding the solute. Thus, solvation dynamics is considered to be the rate limiting factor for the strongly coupled adiabatic ET reaction.7−13 In the framework of the solvation-controlled ET, a large ET rate constant is hardly expected in viscous solutions such as room temperature ionic liquids (ILs)14−17 because the high viscosity of solvents reduces the rate of solvation. It is worth noting, however, that rapid ET reactions could occur even in environments where large molecular motions are restricted, such as in the solid phase.18−21 Even in a rigid medium where rapid solvent fluctuation is absent, the solute itself could fluctuate within the microscopic cavity and induce ET. Or otherwise, small structural reorganization could derive ET, if intramolecular contributions dominate the reorganization energy.22,23 Hence, elucidation of factors enabling ET to transcend the borders set Received: April 8, 2016 Revised: June 13, 2016 Published: June 15, 2016 14502

DOI: 10.1021/acs.jpcc.6b03593 J. Phys. Chem. C 2016, 120, 14502−14512

Article

The Journal of Physical Chemistry C

Picosecond TA Measurements. Picosecond TA spectra were measured by using a custom-built mode-locked Nd3+:YAG laser.43,44 The third harmonic (355 nm) with a fwhm of 15 ps was employed as an excitation pulse, and a white-light supercontinuum as a probe pulse was generated by focusing the fundamental pulse (1064 nm) into a binary mixture of D2O and H2O (3:1) contained in a cell with an optical length of 10 cm. To neglect the anisotropic response of the sample, the fundamental pulse was converted into circular polarization by a quarter-wave-plate just before the supercontinuum generation. The sample was set in a quartz cell with 1.0 cm optical length and the laser pulses were focused into a spot with a diameter of ca. 1.5 mm. The signal was detected by two sets of multichannel photodiode-array system (MOS multichannel detector C4351, Hamamatsu Photonics) with a polychromator (Monospec27, Jarrell Ash Corp.). To ensure the alignment of the delay line for ns measurement, the intensity of the Tn ← T1 absorption spectrum of benzophenone in n-hexane solution was used as a reference. Picosecond Time-Resolved Fluorescence Measurements. The excitation light source for the time-correlated single-photon-counting (TCSPC) measurement was the second harmonic (420 nm) of a Ti:sapphire laser (Tsunami, Spectra-Physics).28 The repetition rate was reduced to 4 MHz with a power of 5−10 μW by an EO modulator (M360-80 with 25D amplifier and 305 countdown electronics, Conoptics). Emission was detected at the magic angle configuration and a quartz sample cell with optical length of 1.0 cm was used. A photomultiplier-tube (R3809U-50, Hamamatsu Photonics) with an amplifier (C5594, Hamamatsu Photonics) and a counting board (PicoHarp 300, PicoQuant) were used for the signal detection. A monochromator (Oriel 77250, Newport) was placed in front of the photomultiplier-tube. The instrumental response function was estimated by the fwhm of the scattered light from a colloidal solution for the excitation light pulse. In the present measurements, it was ca. 30 ps. Time-resolved fluorescence (TRF) spectrum was constructed from the time profiles of the emission detected at various wavelengths.28 The time-integrated intensity of the fluorescence at each wavelength was assumed to be proportional to the intensity of the steady-state fluorescence at the same wavelength. The solvation correlation function, S(t), was constructed from the fluorescence peak shift obtained by leastsquares-fitting of the TRF spectrum utilizing a log-normal function.7 Prior to the construction of S(t) and integrating the emission intensity over frequency, ν, the emission spectra were divided by the cube of the frequency (ν3) to remove the effect of frequency dependent radiative rate. Chemicals. CBA (Chart 1) was synthesized according to the reported method45 and purified by column chromatography

by the framework of Marcus theory, i.e., energy-gap dependent solvent-controlled ET reaction, is of crucial importance to achieve advanced utilization of photoinduced ET in various systems. With the aim along these viewpoints, we have been investigating the photoinduced ET processes in room temperature ILs for some time.16,24−28 IL is expected to be a new type of solvent for a wide range of chemical applications.29−32 They are, however, highly viscous (≥50 cP), and consequently, their diffusive solvation is much slower than that of the normal organic solvents, i.e., the average solvation times are in the order of sub-ns to ns.15,17 Hence, in the framework of the solvent-limited ET reaction, the time constant for the strongly coupled adiabatic ET reaction in IL could be comparable or longer than the fluorescent lifetimes of common organic molecules and efficient photoinduced ET with high quantum yield is hardly expected. Several experimental investigations have been performed so far on photoinduced ET processes in ILs,14,16 and some researchers reported that the ET reaction was completely dependent upon solvation dynamics.33,34 To incorporate IL with solar-energy-harvesting devices,35−37 it is necessary to explore ultrafast driving force that can overcome this limit. Here, we report the ultrafast photoinduced charge separation (CS) of 10-cyano-9,9′-bianthryl (CBA), in an IL, N,N-diethylN-methyl-N-(methoxyethyl)ammonium tetrafluoroborate (DemeBF4), by means of femtosecond-nanosecond transient absorption (TA) and time-resolved fluorescence (TRF) spectroscopies. Because CBA possesses an electron-withdrawing cyano-group, the CS is expected to be faster than that of the symmetric 9,9′-bianthryl (BA).38−40 We will discuss the mechanism of the ultrafast CS reaction of CBA in the IL by comparing with that in a conventional fluid solvent, acetonitrile (Acn), where extremely rapid ET reaction is reported.40

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EXPERIMENTAL SETUP Femtosecond TA Measurements. For the femtosecond transient absorption (TA) measurements, a Ti:sapphire laser system combined with noncollinear optical parametric amplifiers (NOPA) was provided as a pulsed light source.41,42,7,38 The output of a Ti:sapphire laser (Tsunami, Spectra-Physics) pumped by second harmonic of a cw Nd3+:YVO4 laser (Millennia V, Spectra-Physics) was regeneratively amplified with 1 kHz repetition rate (Spitfire, SpectraPhysics). The amplified pulse with energy of 1 mJ/pulse and 85 fs fwhm was divided into two pulses with the same energy and guided into a pair of NOPA systems (TOPAS-white, LightConversion). One of the outputs of NOPA systems was converted into a second harmonic at 420 nm with power of 200 μW and utilized as a pump pulse. White-light supercontinuum was generated by focusing the output of the other NOPA at 1000 nm with power of 0.5−1.0 mW into a 2 mm CaF2 plate and utilized as a probe pulse. Polarization of the two pulses was set at the magic angle for all the measurements. The signal and the reference pulses were detected with multichannel diode array systems (PMA-10, Hamamatsu Photonics) and sent to a computer for further analysis. Group velocity dispersion was measured by optical Kerr effect (OKE) between the pump pulse and the supercontinuum and the result was utilized to calibrate the wavelength-dependent delay of the TA spectra. The pulse duration of the pump pulse was determined to be ca. 30 fs from the self-diffraction frequency-resolved optical gating (FROG) measurement.

Chart 1. Molecular Structures of 10-Cyano-9,9′-bianthryl (CBA) and N,N-Diethyl-N-methyl-N(methoxyethyl)ammonium Tetrafluoroborate (DemeBF4)

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DOI: 10.1021/acs.jpcc.6b03593 J. Phys. Chem. C 2016, 120, 14502−14512

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The Journal of Physical Chemistry C

absorption of the bianthryls can be expressed as a superposition of the corresponding anthracenes, we think that such contribution is weak. Kovalenko et al. stated that CS character was undetectable for the absorption spectrum of CBA, because the first absorption band was independent of the solvent polarity.40 Meanwhile, the fluorescence spectrum and the quantum yield are strongly dependent on the solvent polarity. The emission in n-hexane solution is safely ascribable to the locally excited (LE) state, while red-shifted broad spectra due to the CS state are observed in polar solutions. The intensity of the red-shifted emission from CS state is significantly weaker than that from the LE state. The fluorescence quantum yield in Hex solution is reported to be 0.89, while that in Acn is only 0.01.46 Interestingly, the excited state lifetime in Hex is only ∼2.4 times longer than that in Acn solution, i.e., 13.8 and 5.6 ns, respectively.46 Thus, radiative decay rate is expected to be ∼36 times larger for the LE state (6.4 × 107/s) compared to that for the CS state (1.8 × 106/s). Generally, no fluorescence is expected from an ion pair (anion and cation) produced by a complete unit CS. In the case of BA derivatives, transferred charge from donor (D) to acceptor (A) is less than unity, i.e., D+δ-A−δ with δ < 1, thus the CS state is weakly fluorescent, because the wave function of the CS state is a mixture of those of the fluorescent LE state and the nonfluorescent ion pair. Fluorescence intensity can be considered as a measure of the amount of transferred charge. The excited state lifetime and fluorescence quantum yield of BA in Hex and Acn are 7.3 ns, 0.94 and 25.8 ns, 0.22, respectively.46 Thus, the radiative rate constant of BA in Hex (13 × 107/s) is ∼15 times larger than that in Acn (8.5 × 106/s) and the amount of separated charge is expected to be smaller compared to that of CBA. A smaller Stokes shift between the absorption and fluorescence spectrum for BA compared to CBA also supports this conclusion. The emission maximum in DemeBF4, ca. 580 nm, is almost the same with that in Acn. Slightly broader band shape in DemeBF 4 , especially in shorter wavelength region, is attributable to the slower solvation time for the CS state. Details of the CS and the solvation dynamics will be discussed in the following sections. Femtosecond TA Measurements. Prior to the results of CS dynamics in IL, we show the results in Acn as a reference. Figure 2a,b show transient absorption (TA) spectra of CBA in Acn, excited by the laser pulse with a duration of ca. 30 fs at 420 nm. Upon photoexcitation, the absorption maximum around 550 nm appears together with sharp dips at 440 and 470 nm corresponding to stimulated Raman scatterings induced by the pump pulse. The absorption band around 550 nm is safely attributable to the LE state of the cyano-anthryl moiety, because the absorption maximum (ca. 550 nm) and spectral band shape closely resemble to those of CBA in n-hexane at 10 ps (Figure 2c) where the steady-state emission spectrum is dominated by that from the LE state.46 With increasing delay time to 150 fs, new bands appear at ca. 610 and 680 nm. These absorption bands are ascribed to the CS state in CBA on the basis of the band shapes and absorption maximum of the reference spectra of the anion radical of 9cyanoanthracene48 and the cation radical of 9-methylanthracene.49 The CS state of BA also peaks at ca. 680 nm ascribable to the cationic state of the anthryl moiety.27,50 Temporal evolution in the initial 200 fs following the excitation shows that the LE state in the cyano-anthryl moiety produced by the photoexcitation undergoes CS reaction. As shown in the time

followed by recrystallization in a binary solution of methanol chloroform (2:1). Coumarin 153 (C153) was purchased from Exciton Chemicals and used after recrystallization in ethanol. DemeBF4 (Chart 1) was purchased from Kanto Chemical and purified by the reported method.26,30 The water content and the viscosity of the IL was determined to be 20 ppm and 590 cP (295 K) by a Karl Fischer titrator (831 KF Coulometer, Metrohm) and a viscometer (DV-I Prime, Brookfield), respectively. After adding the solute into the IL, the solution was put into a tightly sealed container with a glass-covered magnetic stirrer and kept under vacuum (usually overnight) until dissolution of the solute was completed. Modest heat (ca. 40 °C) was added for dehydration and to hasten the dissolution. The sample solution was always prepared in a dispensable glovebag filled with dried nitrogen gas to avoid moisture. Steady-State Measurements. Absorption and fluorescence spectra were measured by Hitachi U-3500 spectrophotometer and F850 spectrofluorometer, respectively. Absorption measurement of the IL solution was carried out in a homemade fused-silica cell with an optical length of 2.0 mm and the absorbance was set to ca. 1.0. For fluorescence measurement, the wavelength dependence of the throughput of the monochromator and the sensitivity of the detector was calibrated by measuring blackbody radiation from the standard lamp (018-0081, Hitachi). The absorbance of the sample was reduced to ≤0.2 to avoid self-absorption.

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RESULTS AND DISCUSSION Steady-State Spectra. Figure 1 shows steady-state absorption and fluorescence spectra of CBA in n-hexane

Figure 1. Steady-state absorption and fluorescence spectra of CBA in n-hexane (Hex), ethyl acetate (EtAc), acetonitrile (Acn), and in DemeBF4.

(Hex), ethyl acetate (EtAc), acetonitrile (Acn), and DemeBF4 solutions. Absorption maxima and band shapes are nearly independent of the solvent with only a slight red-shift of ∼5 nm from Hex to DemeBF4 solution. The absorption band shape of CBA is similar to the superposition of those of BA and 10,10′dicyano-9,9′-bianthryl.46 The maximum at the longest wavelength of ∼410 nm is characteristic of cyano-substituted anthryl moiety, while the absorption bands of both moieties overlap at shorter wavelengths. Thus, excitation wavelengths longer than 400 nm can selectively generate excited state localized at cyanosubstituded anthryl moiety, while those at shorter wavelengths can generate those at either side. In general, transition with CS character can appear in the absorption;47 however, because the 14504

DOI: 10.1021/acs.jpcc.6b03593 J. Phys. Chem. C 2016, 120, 14502−14512

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The Journal of Physical Chemistry C

Figure 2. Femtosecond transient absorption (TA) spectra of CBA in acetonitrile (Acn) in time ranges of (a) −100 to 200 fs and (b) 0.5 to 10 ps with the excitation wavelength centered at 420 nm. (c) TA spectra of CBA in n-hexane at 10 ps.

Figure 3. Femtosecond-picosecond TA spectra of CBA in DemeBF4 in the time range of (a) −100 to +150 fs and (b) 150 fs to 900 ps.

stabilization of the CS state by a diffusive solvation and/or by a torsional motion of the anthryl moieties within the CS state. In the present experiment, we have observed the formation of the CS state as an appearance of the band at 670 nm for the first time, followed by an increase of absorbance around 720 nm which is similar to those observed by Martin and co-workers. Kovalenko et al. also reported the dynamic behaviors of photoinduced CS of CBA in Acn solution, as observed by the TA measurements covering the wavelength range of 280−620 nm after the excitation with a 60 fs laser pulse.41 From the analysis of the TA in the UV region, they deduced the time constant of 0.15 ps for the CS process of CBA in Acn. Although they did not mention the appearance of the new band at 720 nm owing to their limited spectral coverage of 280−620 nm, they found that a substantial amount of the CS state was already populated within the experimental time-resolution and concluded that the rapid CS leading to an equilibrium between the LE and CS states evolved into larger amount of the CS state

profile of the differential absorbance (Δabs) at 670 nm (Figure 4a), the rate constant of the CS reaction was obtained to be (43 ± 4 fs)−1. With a further increase in the delay time, the TA spectrum gradually broadens in the wavelength region 1.0 ns were obtained (Figure S2a), while in the DemeBF4 solution, four components with time constants of 78 fs, 550 fs, 150 ps, and >1.0 ns were obtained (Figure S2b). The spectra indicate that, in Acn, the primary CS process is somewhat mixed with the secondary process, while in the IL, they are more separated in time. The time constants of 78 and 82 fs are longer than that obtained by the single wavelength probing at 670 nm (Figure 4a), because they are averaged out for the entire wavelength range of 430−850 nm by the global analysis. The secondary process is characterized by the rise of the peak at 710 nm in Acn with a time constant of 700 fs (Figure S2a), while it is separated into two processes in DemeBF4 with time constants of 550 fs and 150 ps reflecting the multiexponential nature of the dynamics in the viscous ionic liquid. For more detail, see the SI. Picosecond−Nanosecond TA Measurements. To elucidate the dynamics after the CS process in short time region, TA spectroscopy was utilized in a time-window of 0 to 16 ns after the excitation with a picosecond laser pulse at 355 nm. Figure 5a shows TA spectra of CBA in Acn, with the spectra at time delays of 100−500 ps being almost the same with those observed at a few ps with femtosecond pulse excitation (Figure 2b). In Acn solution, the TA bands ascribable to the CS state in the wavelength region of 450−800 nm are nearly independent of time in the range of