Electron Injection Efficiency in Ru-Dye Sensitized TiO2 in the

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Electron Injection Efficiency in Ru-Dye Sensitized TiO in Presence of Room Temperature Ionic Liquid Solvents Probed by Femtosecond Transient Absorption Spectroscopy: Effect of Varying Anions Subrata Mahanta, Akihiro Furube, Hiroyuki Matsuzaki, Takurou N Murakami, and Hajime Matsumoto J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp306138j • Publication Date (Web): 04 Sep 2012 Downloaded from http://pubs.acs.org on September 9, 2012

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The Journal of Physical Chemistry

Electron Injection Efficiency in Ru-Dye Sensitized TiO2 in Presence of Room Temperature Ionic Liquid Solvents Probed by Femtosecond Transient Absorption Spectroscopy: Effect of Varying Anions.

Subrata Mahanta1, Akihiro Furube*1, Hiroyuki Matsuzaki1, Takurou N. Murakami1, and Hajime Matsumoto2

1

National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan

2

National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31, Mirodigaoka, Ikeda, Osaka, 563-8577, Japan

AUTHOR EMAIL ADDRESS: [email protected]

1

ACS Paragon Plus Environment


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Abstract: We investigated the electron injection efficiency of Ru-complex dye (N719) sensitized TiO2 system using femtosecond transient absorption in presence of two different room temperature ionic liquids (ILs) varying their anionic counterparts (tetracyanoborate [TCB]−

and

dicyanoamide

[DCA]−),

keeping

the

cationic

counterpart

(ethylmethylimidazolium [EMIm]+) invariant. We also investigated the effect of varying IL environments on the metal-ligand charge transfer (MLCT) band of the adsorbed N719 on TiO2 film using steady state absorption spectroscopy and the effect on cyano (CN) stretching frequency of ILs in presence of TiO2 films using attenuated total reflectance (ATR)-IR method. The downshift of conduction band (CB) edge energy of TiO2 nanoparticles in presence of [EMIm][TCB] IL environment has recently been reported (J. Phys. Chem. C 2011, 115, 816-822). Based on CB edge energy modulation and transient absorption spectroscopic data, we found that the electron injection efficiency is higher in N719/TiO2 system in presence of [EMIm][TCB] IL environment than in presence of [EMIm][DCA] IL environment by ~20%.

KEYWORDS: transient absorption spectroscopy, room temperature ionic liquid, N719, ATR-IR spectroscopy, electron injection efficiency.

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1. Introduction

Dye-sensitized solar cells (DSSCs) have been an attractive and promising technology for the conversion of solar energy to electricity at very low cost and this has become environment friendly in alternative of fossil fuel energy for scientific and technological community. DSSC is based on a mesoporous titanium dioxide (TiO2) network with a monolayer of dye molecules adsorbed on the surface.1-2 The dye (sensitizer) molecules absorb visible photons and inject electrons into the TiO2. Thin film solar cells constitute one of the future technological solutions for sustainable energy supply, DSSCs being one of the low cost high efficiency solar cells belonging to this category. The elementary steps involved in the photoconversion process are light absorption to generate excited states of a donor dye molecule adjacent to an acceptor, (TiO2 etc.) and “direct electron injection” from the excited dye molecule to the conduction band of the TiO2 type semiconductors.3-6 In order to regenerate the oxidized form of the dye molecule, an electron is transferred from the iodine/iodide electrolyte resulting in its conversion to triiodide. Ideally, this electron transfer to the dye molecules occurs much faster thus, preventing the bimolecular recombination of the electron with the oxidized dye molecule.4 Very recently, Grätzel group reported a DSSC using porphyrin dye and cobalt based electrolyte with over 12% efficiency.7 Early this year Han’s group reported 11.4% efficiency of DSSCs using a novel co-adsobant.8 These results are definitely a big boost for DSSC scientific and technological community. Use of volatile organic solvents cause serious performance deterioration of DSSCs by solvent evaporation and thus limits their large-scale implementation due to their poor long term stability and the necessity of complex sealing process. For this, one has to find 3



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out alternative ways to get rid of such degradation issues. Room temperature ionic liquid (IL) is one of the ways to compensate these issues because of the negligible vapor pressure, high viscosity, low flammability, high ionic conductivity, and electrochemical stability and hence can be used as an electrolyte solution.9-13 Many DSSC research groups across the globe have been using different kinds of ILa as electrolytes to monitor the DSSCs performances.14-18 Recently Wang’s group studied the DSSC efficiency in presence of [EMIm][TCB] and [EMIm][DCA] IL environments using organic dyes and they found the efficiency of DSSC in presence of [EMIm][TCB] IL environment to be the best due to the upshift of CB edge energy of TiO2 nanoparticles in presence of [DCA]− in comparison to [TCB]− anionic counterpart of these two ILs.19 Very little work on electron injection efficiency in DSSCs have been studied in presence of IL environments.20 The best established way of investigating the nature of the photoexcitation and the following femto- and picosecond kinetic evolution is transient absorption (TA) spectroscopy.4 A first pulse excites the system and a second one is used with a variable delay to probe the evolution. TA spectroscopy is widely used to directly observe the electron injection process in dye-sensitized semiconductor films, and the reaction mechanism has been understood deeply as a function of the sensitizer dye (with different LUMO levels, different distances to the semiconductor surface, and different charge transfer characters), semiconductor, and environment (e.g., simply in air, in solution, and in electrolyte).4, 21-24 Also, additives in the electrolyte and coadsorbates are known to affect the electron injection dynamics.21, 25-26 Because of the variety of factors that influence the reaction, it is impossible to predict the reaction rate and efficiency in the actual solar cells simply from the device design. 4


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In this paper, we report the effect of variation of the anionic counterpart of the IL solvents ([EMIm][TCB]/[DCA]) on the electron injection efficiency in N719/TiO2 system based on femtosecond transient absorption measurements. The anions, [TCB]− and [DCA]− differing in their structures, dipole moments (DM), and donor numbers (DN) have an impact on the CB energy level, thereby, modulating the band gap and hence the electron injection efficiency. Here, we also investigated the effect of varying IL environments on the metal-ligand charge transfer (MLCT) band of the adsorbed N719 on TiO2 film using steady state absorption spectroscopy and the effect on CN stretching frequency of ILs in presence of TiO2 films using attenuated total reflectance (ATR)-IR method. Wang’s group recently reported the downshift of CB edge energy of TiO2 nanoparticles in presence of [EMIm][TCB] IL environment.19 Based on transient absorption spectroscopic data, we found that the electron injection efficiency is higher in N719/TiO2 system in presence of [EMIm][TCB] IL environment than in presence of [EMIm][DCA] IL environment by ~20% due to modulation of CB edge energy of TiO2 nanoparticles.

2. Experiment 2.1 Samples Nano-crystalline TiO2 films were prepared from a commercially available TiO2 paste (Solaronix SA, T/SP). The paste was painted on a glass plate substrate with a screen printer. Nano-crystalline films were prepared by calcination of the painted substrate for 3 hours at 450 °C. The films obtained had an area of 10 × 10 mm2 and a thickness of ~6 µm. Di-tetrabutylammonium

cis-bis(isothiocyanato)bis(2,2’-bipyridyl-4,4’-

dicarboxylato)ruthenium(II) (N719) (Solaronix) was dissolved at a concentration of 0.3 5



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mM in 1:1 tert-butylalcohol:acetonitrile. The nanocrystalline TiO2 films were immersed in the dye solution for 18 hours, so that the dye could adsorb onto the semiconductor surface. The films then were rinsed with acetonitrile (ACN) and dried in air. [EMIm][TCB] (Merck) and [EMIm][DCA] (IoLiTec) were used in this study. The chemical structures of the cation and anions of the ILs are shown in Scheme 1.

2.2 Steady state absorption and ATR-IR spectroscopy. Steady state absorption measurements were carried out with an absorption spectrophotometer (Shimadzu, UV-3101PC). The attenuated total reflection (ATR)-IR measurement was done in ambient air at room temperature. The IR absorbance spectrum of RTILs and ACN was recorded from 650 to 4000 cm-1 with 4 cm-1 resolution and averaged over 10 scans. Dried TiO2 films were taken and a drop of IL was added onto it and allowed to stand for ~ 10 minutes to ensure adsorption of the IL on the film. This procedure was carried out for different ILs before ATR measurements were conducted. The IR spectra from 650 to 4000 cm-1 were recorded with a Perkin Elmer Spectrum 100 FTIR with Pike MIRacle ATR attachment with a resolution 2 cm-1. The device operates in attenuated total reflection (ATR) mode. For this purpose it is equipped with a diamond crystal at whose surface the evanescent electromagnetic field interacts with the sample.

2.3. Femtosecond transient absorption spectroscopy The details of the femtosecond transient absorption spectrometer have already been discussed elsewhere.27 Briefly, the light source for the femtosecond pump–probe transient 6


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absorption measurements was a regenerative amplifier system consisting of a Ti:sapphire laser (800 nm wavelength, 130 fs FWHM pulse width, 0.8 mJ/pulse intensity, 1 kHz repetition; Spectra Physics, Hurricane) combined with two optical parametric amplifiers (OPAs; Quantronix, TOPAS). To probe IR wavelengths, differential frequency generation method of the OPA was used. For a pump pulse, the output of the OPA at a wavelength of 532 nm with an intensity less than 1 microjoules per pulse at a 500-Hz modulation frequency was used; and for a probe pulse, the output of the other OPA or the white-light continuum generated by focusing the fundamental beam (800 nm) onto a sapphire plate (2mm thick) was used. The probe beam was focused at the center of the pump beam on the sample (Scheme 2), and the transmitted probe beam was then detected by means of a InGaAs or photoconductive mercury cadmium telluride (MCT) photodetector after passing through a bandpass filter or monochromator (Acton Research, SpectraPro-150), respectively. The time resolutions of the measurements were about 250 fs. All measurements were performed at 295 K.

3. Results and Discussions 3.1 Physical properties of ([EMIm][TCB] and [EMIm][DCA]) ILs Imidazolium-based room temperature ILs have been widely used as solvents for organic synthesis owing to their excellent physical properties, such as low viscosities and high thermal and aqueous stability. Data pertaining to the liquid densities (ρ), shear viscosities (η), surface tension (γ), donor number (DN), calculated dipole moments (DM) of the anionic counterpart of the ILs ([TCB]− and [DCA]−), refractive indices (n) are summarized in Table 1. DN of [EMIm][TCB] was estimated by solvatochromism of 7



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[Cu(acac)(tmen)][BPh4] (acac:acetylacetate; tmen:N,N,N',N'-tetramethylethylenediamine) in ILs. The absorption maximum of the complex in [EMIm][TCB] was 573 nm, which corresponds to donor number of 25.32 As can be seen from Table 1, the densities, viscosities and surface tensions are not hugely different from each other for these ILs ([EMIm][TCB] and [EMIm][DCA]). [EMIm][TCB] IL has higher viscosity (21cP) and surface tension (48.20) compared to [EMIm][DCA] IL. The noticeable differences are in donor number (DN) and dipole moment (DM) (calculated value using MP2/ 6-311G**level of theory using Gausian 03) of these two ILs where [EMIm][DCA] possesses higher DN as well as DM (anionic counterpart)

3.2 Steady state absorption and ATR-IR spectra The steady state absorption spectra of N719, adsorbed on TiO2 film (Figure 1) measured in [EMIm][TCB] and [EMIm][DCA] room temperature ILs and ACN environments, shows a single broad absorption band at ~530 nm which is attributed to the metal-to-ligand charge transfer (MLCT) band.35 However, as seen in Figure 1, the MLCT absorption band maxima of N719 in [EMIm][DCA] IL environment is slightly blue-shifted to ~526 nm compared to [EMIm][TCB] (~528 nm) IL and ACN (~528 nm) environments, but their shape is

essentially the same in all ILs and ACN environments, suggesting slight interaction of [EMIm][DCA] with N719 dyes in the ground state. In general it is found that the MLCT absorption band of N719 is blue-shifted with increasing solvent polarities.35-36 This suggests that the polarity of [EMIm][DCA] is the slightly higher compared to [EMIm][TCB] IL, as being reasonable from the fact that DM is larger for [EMIm][DCA]. 8


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Hypsochromic shift of the MLCT band of N719 is directly proportional to the polarity of the ILs i.e. higher the polarity, greater is the blue shift. This signifies that the HOMOLUMO energy gap of adsorbed N719 is slightly higher in case of [EMIm][DCA] IL environment. Attenuated total reflection (ATR) infrared (IR) spectroscopy, which is a powerful tool for investigation of liquid samples, allows detection of liquid-phase products (for online reaction monitoring) and investigation of species adsorbed on metal oxides the presence of strongly adsorbing solvents.37-39 Figure 2 depicts the ATR-IR spectra of room temperature ILs ([EMIm][TCB] and [EMIm][DCA]) and ACN in the CN stretching frequency (ν(CN)) region, after each of ILs and ACN was placed on the diamond ATR crystal exposed to the atmosphere. In this study we concentrated on the ν(CN) region. The position of ν(CN) of the two different ILs and ACN appears at different regions reflecting their structural anomaly and different orientation of anions and cations. The ν(CN) of ACN,40 [EMIm][TCB] and [EMIm][DCA] appears at 2254cm-1, 2223 cm-1 and 2125 cm-1 (highest transmittance band) respectively (Table 2). Figure 2 shows the redshift of CN stretching frequency in [EMIm][DCA] IL is due to the elongation of CN bond because of strong van der Waals interaction between alkyl chains of [EMIm]+ with [DCA]−. Another noticeable change observed is the transmittance change at CN frequency range. Figure 2 shows the transmission change at CN frequency range of [EMIm][DCA] is the higher compared to [EMIm][TCB] IL. This may be attributed to the more symmetric nature of the later. Unlike EMIm][TCB] IL, two additional distinct bands appear in case of [EMIm][DCA] at 2195cm-1 and 2228cm-1 (Table 2). The peak of highest transmittance in 9



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the spectrum of [EMIm][DCA], found at 2125 cm-1 corresponds to the antisymmetric CN stretch, while the peak at 2195 is assigned to the symmetric CN stretch. The peak at 2228 cm-1 can be assigned to a combination band of the symmetric and antisymmetric CN stretching (Table 2).41 The IR spectrum of the ILs in presence and absence of TiO2 film in the ν(CN) region presented in Figure 3 illustrated the effect of the shifting of ν(CN) in presence of TiO2 films. [EMIm][DCA] exhibits maximum blue-shifted of the ν(CN) whereas [EMIm][TCB] exhibits no change of ν(CN) in the presence of semiconductor TiO2 film (Table 2). The blue shift observed for the ν(CN) when adsorbed on the TiO2 surface basically hints at the fact that the cyanide group is involved in the adsorption. We do have another possibility of adsorption via the central N atom but, this would have led to a red shift in the ν(CN) due to reduction of its triple bond character. Moreover, Yates group studied the adsorption and photoexcitation of ACN on TiO2 surface using FTIR spectroscopy and reported a similar observation.42 They have also reported that the adsorption occurs via the cyanide group of ACN on TiO2 surface. It should be mentioned here that TiO2 film was heated at 4500C just to ensure that there is least amount of water molecules in TiO2 films. From the magnitude of the blue-shifted value it is confirmed that [EMIm][DCA] IL interacts more with the TiO2 film compared to [EMIm][TCB]. It should also be noted that the magnitude of blue-shift decreased when we used different TiO2 nanocrystalline electrodes with larger pore size, indicating that the spectral shift surely includes the contribution of IL molecules attached on TiO2 surface. Recently Carper et.al reported the interactions between different types of ILs varying, both cationic and anionic counterparts 10


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with aluminum oxide using theoretical calculations and they showed that anionic counterpart of an IL interacts mainly with aluminum oxide to give the most stable conformation.43-44.

3.3 Femtosecond transient absorption To analyze the dynamics of the electron injection process in N719/TiO2, the spectra of the excited (N719*) and cation (N719+) states of N719 are necessary. It is already established for N719 adsorbed on nano-crystalline TiO2 films that N719* gives a transient absorption band at around 700 nm (due to the lowest triplet state after ultrafast intersystem crossing) and N719+ at around 800 nm.22 It is also known that there is another additional spectrum of N719* in the near-IR region around 1200-1600 nm.45 Figure 4 presents the transient absorption spectra of N719/TiO2 in the visible and near-IR region (between 630 to 1400 nm) at delay times of 2 ps, 100 ps, 500 ps and 1000 ps in presence of ILs environment under the 532 nm laser-pulse excitation. Immediately after excitation (2 ps), transient absorption signals were strong in the near-IR region (from 1000 nm to 1400 nm), whereas those in the visible region (from 700 nm to 900 nm) were rather weak. A brief discussion of transient spectrum at 2 ps of N19/TiO2 is needed to assign the origin of the two main absorption bands (~800 nm and ~1400 nm). As already mentioned, the 800 nm absorption band is attributed to N719+ (Figure 4).20,22 According to the picosecond dynamics, the near-IR band around 1400 nm is considered as an intermediate state in the course of electron transfer from N719* to the CB of TiO2. Considering that the intermediate has an absorption band with a maximum and is generated very rapidly after 11



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photoexcitation of N719 adsorbed on the TiO2 surface, the temporal variation of the transient absorption spectra in near-IR region can be attributed to the electron injection from the triplet state to the CB of TiO2 semiconductor.20,22 Another interesting observation in Figure 4 is that the change of absorbance value in the near-IR region (1000-1400 nm) is more prominent of N719/TiO2 in [EMIm][TCB] IL environment (Figure 4a) compared to [EMIm][DCA] IL environment (Figure 4b). This reflects that electron injection from N719* to CB of TiO2 is more efficient in presence of [EMIm][TCB] compared to [EMIm][DCA] IL environments. In contrast, the absorbance at around 800 nm increases with increasing delay time (Figure 4). This supports the generation of N719+ on electron donation from N719* to the CB of TiO2. Figure 5 shows the transient time profile of adsorbed N719 in two different IL environments using 532 nm laser-pulse excitation and monitored at two different probe wavelengths of 800 nm and 3440 nm. The choice of 800 nm as the probe wavelength was motivated by the observation of the absorption band of oxidized dye molecules at 800 nm in the transient absorption spectra (Figure 4).20 3440 nm was selected as the second probe wavelength based on the literature reports of the position of the absorption of CB electron of TiO2 semiconductor nanoparticles.45 Here all measurements were carried out under the same optical configuration and the steady-state absorption intensities at the excitation wavelength were the same between the two films; therefore the TA signals are proportional to the efficiency of electron injection in each set of sample. Figure 5 shows that the electron injection efficiency from N719* to the CB of TiO2 is higher for [EMIm][TCB] than for [EMIm][DCA]. Figure 4a (probed at 800 nm) also shows the dynamics of N719+ generation in two IL environments. Interestingly, we found a single exponential rise time in 12


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both the IL environments, however in [EMIm][DCA] the N719+ formation being slower. The time constants observed in [EMIm][TCB] and [EMIm][DCA] IL environments are 47 ±7 ps and 67±6 ps respectively. Estimated rate constants, k[TCB]= 2.1×1010 sec-1 and k[DCA]= 1.5×1010 sec-1 indicate a relatively fast formation of the dye cations in [EMIm][TCB] environments (~ 1.4 times faster). Also the TA signal amplitude in the saturated time region (> 200 ps) for [EMIm][TCB] environment is higher than for [EMIm][DCA]. From the above transient absorption data it is clear that the electron injection efficiency from N719 dye to the CB of TiO2 is highest in [EMIm][TCB] IL environment. The main driving force for higher electron injection efficiency is the Gibbs free energy change (∆G), which is difference between the LUMO of dye molecule and conduction band edge energy of TiO2 nanoparticles. In [EMIm][TCB] IL environment the ∆G value is relatively higher compared to [EMIm][DCA] IL environment of N719/TiO2 due to upshift of CB energy of TiO2 in [EMIm][DCA] IL environments.19 Many groups have reported the dependency of electron injection efficiencies and rates from the excited state dye to the CB of TiO2 in different solvent environments due to modulation of CB edge energy.46-47 In Wang's paper, they drew plots of interfacial charge recombination resistance (Rct) vs the Fermi level difference of the titania film and redox electrolyte (E1 and E2) and found the CB edge energy to be 39 mV higher in presence of E2 electrolyte which is basically of [EMIm][DCA] based electrolyte.19 From the result of ATR-IR, it was suggested that [DCA]− adsorb on TiO2 surface more favorably than TCB anions. These results indicate a more negatively charged condition for TiO2 surface in the case of [EMIm][DCA] IL.

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In general, ∆G is not the only parameter that influences the rate of electron transfer, other factors such as the electronic coupling and reorganization energy must also be considered. Since the steady state absorption band of the N719 (MLCT transition) was nearly identical for both [EMIm][TCB] and [EMIm][DCA] environments, the electronic coupling strength should be similar between the two. The reorganization energy may be larger for [EMIm][DCA] because of the non-zero dipole moment (0.84 Debye) of DCA anion, but at present we believe this effect is minor since the common cation part, [EMIm]+ has a larger dipole moment (1.67 Debye). Inhomogeneity at the dye and TiO2 interface should also play a vital role in electron injection efficiency. As we discussed in our earlier reports,48-49 ∆G has inhomogeneity of a few hundred meV probably mainly due to a variety of surface nature on semiconductor nanoparticles. When the CB edge level shifts upward by the effect of DCA anions, a portion of dye molecules which was weakly interacting with TiO2 in the case of [EMIm][TCB] environment loses electron acceptor states. This translates into the decreased amplitude of transient absorption at 800 nm.

4. Conclusions In this paper, we present the ground state properties of adsorbed N719 dyes on TiO2 film in presence of different IL environments using steady state absorption spectroscopy and the effect on CN stretching frequency of ILs in presence of TiO2 film using ATR-IR spectroscopy. Simultaneously we also studied electron injection efficiencies in N719/TiO2

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system in presence of different IL environments using femtosecond transient absorption spectroscopy. Absorption spectra show only a little blueshift of MLCT band of N719 in presence of [EMIm][DCA] IL compared to [EMIm][TCB] IL, indicating their weak interaction with dye molecules. From ATR-IR spectra it is clear that [EMIm][DCA] interacts more strongly with TiO2 nanoparticle compared to other IL as is evident from the significant blue-shift of CN stretching frequency in presence of TiO2 nanoparticles. The upshift in CB energy of TiO2 nanoparticle in presence of [EMIm][DCA] IL also reinforces our conclusion from ATR-IR results about strong interactions with semiconductor nanoparticles (TiO2). We have also studied the electron injection efficiency in N719/TiO2 with variation of the anionic counterpart of the room temperature ILs solvents based on femtosecond transient absorption measurements. The electron injection efficiency from excited N719 dye to the CB of TiO2 semiconductor is higher in case of [EMIm][TCB] IL environment and lower in case of [EMIm][DCA] IL environment. The time constants observed in [EMIm][TCB] and [EMIm][DCA] IL environments are 47 ± 7 ps and 67 ± 6 ps, respectively. The reduced efficiency for the latter was derived from the slower injection from the triplet state of N719 and presence of non-injecting dyes.

Acknowledgements: This work was supported by the project ‘Research and development of high-efficiency and low-cost dye-sensitized solar cells and their mass production technologies based on the 15



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three phase-harmonized interface’ of the New Energy and Industrial Technology Development Organization (NEDO), Japan.

References: (1) Oregan, B.; Gratzel, M. Nature 1991, 353, 737-740. (2) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Y. Jpn. J. Appl. Phys., Part 2 2006, 45, L638-L640. (3) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Chem. Rev. 2010, 110, 6595-6663. (4) Listorti, A.; O'Regan, B.; Durrant, J. R. Chem. Mater. 2011, 23, 3381-3399. 16


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(19) Zhou, D.; Bai, Y.; Zhang, J.; Cai, N.; Su, M.; Wang, Y.; Zhang, M.; Wang, P. J. Phys. Chem. C 2011, 115, 816-822. (20)

Furube, A.; Wang, Z.-S.; Sunahara, K.; Hara, K.; Katoh, R.; Tachiya, M. J. Am.

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Table 1. Formula Weight (FM), Liquid density (ρ in g/cm-3), Donor Number (DN), Viscosity (η in cP) at 298K, Surface Tension (γ in mN/m) at 200C, Dipole Moment (DM in Debye) obtained from theoretical calculations, refractive index (n) at 200C and ET (30) of [EMIm][TCB] and [EMIm][DCA] ILs ILs

FW

ρ

DN

Η

γ

DM(D)*

n

ET(30)

[EMIm][TCB]

226.05 1.04a

25

21a 48.20b

0.00

1.45a

-

[EMIm][DCA]

177.21 1.08c

41

17c 44.14d

0.84

1.51e

51.7c

20


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a

Reference28, bReference29, cReference30-32, dReference33, eReference34. * (Calculated value using MP2/ 6-311G(d,p) level of theory of [TCB]− and [DCA]−).

Table 2: CN stretching frequencies (in cm-1) of ILs in presence or absence of TiO2 film. ν(CN)

ν(CN)antisymm

ν(CN)symm

ν(CN)combi

[EMIm][TCB]

2223

-

-

-

TiO2+[EMIm][TCB]

2223

-

-

-

[EMIm][DCA]

-

2125

2193

2228

TiO2+[EMIm][DCA]

-

2130

2198

2236

Cation

Anions CN

(-)

B

(+)

NC

CN CN

N + N

[TCB]-

[EMIm]+

(-)

N CN

NC [DCA]21



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

Scheme 1: Structures of the cationic and anionic counterparts of [EMIm][TCB] and [EMIm][DCA] room temperature ILs.

Scheme 2: Schematic representation of transient absorption set up and sample cell.

22


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Figure 1: Steady state absorption spectra of adsorbed N719 on TiO2 film in presence of [EMIm][TCB], [EMIm][DCA] and ACN environments at room temperature.

Figure 2: The ν(CN) stretching frequency of [EMIm][TCB] and [EMIm][DCA] ILs and pure ACN. The CN stretching frequencies of ACN, [EMIm][TCB] and [EMIm][DCA] are 2254 cm-1, 2223 cm-1 and 2125 cm-1 respectively at room temperature. 23



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

Figure 3: The ν(CN) stretching frequency of (a) [EMIm][TCB] and (b)[EMIm][DCA] ILs with and without TiO2 film measured at room temperature and in ambient air.

24


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-3

∆OD

12x10

2ps 100ps 500ps 1000ps

(a)

8 4 0 800

1000

1200

1400

Wavelength (nm)

-3

12x10

∆OD

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


(b)

2ps 100ps 500ps 1000ps

8 4 0 800

1000

1200

1400

Wavelength (nm) Figure 4: Transient absorption spectra of N719/TiO2 system at 2 ps (red line), 100 ps (blue line), 500 ps (green line) and 1000 ps (brown line) in different IL environments (a)[EMIm][TCB] and (b)[EMIm][DCA]. The excitation wavelength was 532 nm.

25



-3

16x10

14

(b)

Probe Wavelength=3440nm

12

∆OD

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

10 8 6

N719+TiO2+[EMIm][TCB] N719+TiO2+[EMIm][DCA]

4 2 0 1

10

100

Delay time/ps Figure 5: Femtosecond time profile of N719/TiO2 in different ILs environment at room temperature, excitation wavelength 532 nm (a) probe wavelength 800 nm (Excitation energy=0.2mW), (b) 3440nm (Excitation energy=0.3mW).

26


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TOC graphics

27


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