Ultrafast dynamics of the indoline dye D 149 on electrodeposited ZnO and sintered ZrO 2 and TiO 2 thin films

The ultrafast photoinjection and subsequent relaxation steps of the indoline dye D149 were investigated in detail for a mesoporous electrodeposited ZnO thin film and compared with experiments on sintered TiO2 and ZrO2 thin films, all in contact with air, using pump–supercontinuum probe (PSCP) transient absorption spectroscopy in the range 370–770 nm. D149 efficiently injects electrons into the ZnO surface with time constants from r70 fs (time-resolution-limited) up to 250 fs, without the presence of slower components. Subsequent spectral dynamics with a time constant of 20 ps and no accompanying change in the oscillator strength are assigned to a transient Stark shift of the electronic absorption spectrum of D149 molecules in the electronic ground state due to the local electric field exerted by the D149 + radical cations and conduction band electrons in ZnO. This interpretation is consistent with the shape of the relaxed PSCP spectrum at long times, which resembles the first derivative of the inverted steady-state absorption spectrum of D149. In addition, steady-state difference absorption spectra of D149 + in solution from spectroelectrochemistry display a bleach band with distinctly different position, because no first-order Stark effect is present in that case. Interference features in the PSCP spectra probably arise from a change of the refractive index of ZnO caused by the injected electrons. The 20 ps component in the PSCP spectra is likely a manifestation of the transition from an initially formed bound D149 –electron complex to isolated D149 + and mobile electrons in the ZnO conduction band (which changes the external electric field experienced by D149) and possibly also reorientational motion of D149 molecules in response to the electric field. We identify additional spectral dynamics on a similar timescale, arising from vibrational relaxation of D149 + by interactions with ZnO. TiO2 exhibits similar dynamics to ZnO. In the case of ZrO2, electron injection accesses trap states, which exhibit a substantial probability for charge recombination. No Stark shift is observed in this case. In addition, the spectroelectrochemical experiments for D149 + in dichloromethane and acetonitrile, which cover the spectral range up to 2000 nm, provide for the first time access to its complete D0 D1 absorption band, with the peak located at 1250 and 1055 nm, respectively. Good agreement is obtained with results from DFT/TDDFT calculations of the D149 + spectrum employing the MPW1K functional.


Introduction
2][3] The top efficiency of such devices has currently reached 12.3%, employing a mesoporous TiO 2 electrode co-sensitized with Zn-porphyrin and cyclopentadithiophene dyes and a cobalt(II/III) based redox electrolyte. 4,57][8] Direct low-temperature fabrication of highly porous crystalline ZnO films from aqueous solution has been achieved by co-electrodeposition with water soluble dyes, e.g.0][11] Subsequent readsorption of another dye with a broad absorption band and a high absorption coefficient results in the formation of a sensitized ZnO electrode, which currently reaches efficiencies of up to 5.6% in DSSCs, with the added advantage that the low-temperature process opens up applications for flexible plastic-based devices. 12For sensitization, metal-free dyes have come into the focus as an alternative for the frequently used ruthenium complexes.][15] The timescale of photoinduced electron injection into the mesoporous semiconductor oxide surface is a key parameter for the efficiency of a DSSC.7][18] For sintered ZnO surfaces several studies exist, [19][20][21][22][23][24][25][26] however, electron injection has not yet been investigated for electrodeposited ZnO.In addition, information on the ultrafast dynamics of indoline dyes on ZnO surfaces is lacking, and there has been so far only one study of the femto-to picosecond dynamics of D149 on TiO 2 . 27ere we characterize the ultrafast photoinjection and subsequent relaxation steps of the indoline dye D149 on an electrodeposited ZnO surface using ultrafast broadband transient absorption spectroscopy from the UV to the near IR and compare it with the dynamics on ZrO 2 and TiO 2 .We find evidence for the transient build-up of a local Stark effect, [28][29][30] where the electric field exerted by the D149 + radical cations and injected electrons influences the ground-state absorption spectrum of D149.In addition, undulatory structure in the transient spectra can be traced back to injected electrons modifying the optical response of the thin film.We also characterize the differential absorption spectrum of D149 + by spectroelectrochemistry up to 2000 nm.

Experimental
2.1 Preparation of ZnO, TiO 2 and ZrO 2 thin films Details of the preparation procedure for the ZnO films can be found in previous publications. 9,31,32Briefly, glass substrates coated with fluorine-doped tin oxide (FTO, 10 O square À1 , Asahi Glass) were first cleaned with acetone, ethanol and distilled water and then etched in 2 M nitric acid for 2 min.ZnO/eosin Y (= EY) hybrid films were then electrodeposited from an oxygen-saturated aqueous solution (T = 70 1C), which contained 0.1 M KCl, 5 mM ZnCl 2 and 80 mM eosin Y, onto the FTO substrate employing a rotating disk setup (300 rpm) featuring an active area of ca. 2 cm 2 .A Zn wire and an Ag/AgCl electrode were employed as counter and reference electrode, respectively.Deposition for 30 min at À0.91 V vs. Ag/AgCl (= À1.00 V vs. SCE) resulted in a film thickness of ca.2.5 mm, with the ZnO c-axis preferentially oriented perpendicular to the substrate surface.The films were briefly rinsed with distilled water and dried in a stream of air.EY was desorbed by placing the as-deposited ZnO/EY films in an aqueous KOH solution (pH 10.5) for 24 h.For dye sensitization, substrates were dried at 120 1C for 1 h and immediately immersed in a 0.5 mM solution of the indoline dye D149 (Inabata UK Ltd.) 33 in acetonitrile/tert-butanol (1 : 1 by volume) for 30 min.This solution also contained 1 mM cholic acid as a co-adsorbant to avoid dye aggregation.The resulting sensitized ZnO film had an OD of 1.2 at the peak of the D149 absorption spectrum.
For preparation of the TiO 2 thin films, 1.2 g of TiO 2 paste (Dyesol DSL 90T) was stirred in 600 mg isopropanol and then sonicated for 20 minutes.Afterwards, doctor blading on 1 mm thick microscope slides was performed.Subsequent sintering at 450 1C resulted in highly transparent films with a thickness of ca. 2 mm, as determined from the interference structure of the thin film.Films were then immersed in a 0.2 mM solution of D149 in acetonitrile for 10 minutes, rinsed with fresh acetonitrile to remove unbound D149, and dried afterwards in an oven at 60 1C for 30 minutes, which had no impact on the shape of the absorption spectra.ZrO 2 films were prepared in the same manner.The required ZrO 2 paste was obtained from A. Hagfeldt as a generous gift and used as received.The typical S 0 -S 1 peak absorption of the D149-sensitized sintered films was in the range of OD 0.6-1.0.

Pump-SuperContinuum Probe (PSCP) spectroscopy
5][36] An overview of the principle of PSCP spectroscopy can be found elsewhere. 37The sample was excited using a NOPA with a center wavelength of 510 or 630 nm and probed by a supercontinuum (370-770 nm) generated in a 1 mm thick CaF 2 plate.Pump and probe pulses (beam diameter 100 and 50 mm, respectively; pump beam energy ca.1.5 mJ pulse À1 ) were focused onto the sample, which was mounted on an x/y translation stage and permanently moved in a plane perpendicularly to the probe beam propagation direction so that a fresh spot on the sample was used for each laser shot.The pump-probe intensity cross-correlation time was ca.70 fs as determined by using the ultrafast response of a ZnO thin film without adsorbed D149.The accuracy of the time zero determination was ca. 10 fs.Because of scattering of the thin films, the signal-to-noise ratio in a range of ca.AE15 nm around the respective pump wavelengths is reduced (e.g.around 510 nm in Fig. 3).No degradation of the ZnO samples was observed during the experiments: kinetic traces over the whole spectral range were identical in amplitude and shape and the absorption spectrum of the film remained unchanged even after a sequence of measurements over a period of weeks.Kinetic traces for the ZrO 2 films showed a minor reduction in amplitude (ca.1% per scan), however the observed kinetics was reproducible.Photodegradation on the TiO 2 films was more severe (ca.5% per scan), yet kinetics were still reproducible and corrected by rescaling.We tentatively ascribe the reduced stability on TiO 2 and ZrO 2 to secondary reactions of the long-lived D149 + radical cations.The differences in stability of the sensitized films will be the subject of a separate future study.

Spectroelectrochemical measurements
Spectroelectrochemistry of D149 in solution was performed using a Metrohm Autolab PGSTAT 101 potentiostat and an optically transparent thin-layer electrochemical (OTTLE) cell with 200 mm path length, featuring a Pt-disk working electrode, a Pt-gauze auxiliary electrode and an Ag-wire as a pseudoreference electrode. 38The N 2 -saturated solutions had an OD of 0.15 in acetonitrile and 0.90 in dichloromethane at the S 0 -S 1 band maximum.Tetrabutylammonium perchlorate (100 mM) was used as a supporting electrolyte.UV-Vis-NIR absorption spectra were recorded on a Varian Cary 5000 spectrometer over the range 200-2000 nm.No additional features were discovered in the spectral range up to 3300 nm.

DFT/TDDFT calculations
We performed complementary DFT/TDDFT calculations to analyze the D149 + radical cation difference absorption spectra from spectroelectrochemistry and PSCP experiments.The equilibrium structure of the D149 + doublet ground state D 0 was optimized by density functional theory (DFT) using the B3LYP functional 39 and a 6-31G(d) basis set.Subsequently, time-dependent density functional theory (TDDFT) 40,41 was applied to this geometry for calculating the first 20 excited doublet electronic states at vertical excitation using MPW1K, 42 B3LYP and PBE0 43 functionals in combination with a 6-31+G(d) basis set.Calculations were carried out using the Gaussian 09 package. 44

Results and discussion
3.1 Steady-state absorption spectra Fig. 2 depicts normalized steady-state absorption spectra of D149.The peak position of the S 0 -S 1 band in organic solvents shows only a weak variation (530 nm in acetonitrile and 543 nm in dichloromethane, both solvents containing tetrabutylammonium perchlorate at a concentration of 0.1 M for the spectroelectrochemical measurements reported in Section 3.4), in agreement with our previous study. 34For all thin films, the spectra have a blue-shifted maximum (518 nm in the case of ZnO and ZrO 2 , 524 nm for TiO 2 ) and exhibit a significant broadening which is probably due to heterogeneous binding environments and a certain fraction of D149 aggregates.In all cases, the background absorption of the unsensitized thin films was subtracted, which features a slow rise towards the UV (due to scattering).In addition, a pronounced increase starts at around 390 nm in the case of ZnO and TiO 2 (band gap 3.2 eV).An undulatory structure is superimposed on the absorption spectra, which is much less pronounced for ZrO 2 .It can be assigned to interference features of the thin films.

Transient absorption spectra of D149 on ZnO
Fig. 3 contains PSCP spectra of D149 adsorbed on ZnO upon excitation to the S 1 state at 510 nm (very similar results are obtained for the TiO 2 films).At early times (upper panel, À0.30 to 0.05 ps), negative S 0 -S n ground state bleaching (GSB) occurs which resembles the inverted steady-state absorption spectrum of D149 (Fig. 2).In addition, S 1 -S 0 stimulated emission (SE) is observed at around 540 nm.At the same time, transient excited state absorption (ESA) appears at around 430 nm and above 600 nm.The shape resembles transient spectra of the S 1 state of D149 in organic solvents at early times measured in our previous PSCP study. 34Yet, the appearance of characteristic spectral features in the range 600-750 nm already signals the very fast formation of an additional species, which has an absorption resembling the doublet ground state D 0 of the D149 + radical cation produced after electron injection. 28,45Therefore, it is reasonable to assume that a substantial part of electron injection already occurs within the 70 fs time resolution of the current experiments.In fact, as will be shown below, the superimposed regular structure arises from such photoinjected electrons modifying the refractive index of the mesoporous ZnO. 46n the middle panel (0.1-1.0 ps) of Fig. 3, the SE quickly decays, and this is accompanied by a decay of the ESA band above 600 nm.The essentially unchanged GSB below 410 nm suggests that this decay cannot be due to recombination of a fraction of cations with some of the already injected electrons reforming D149 in the ground electronic state S 0 .Instead, the global analysis of the PSCP spectra presented below is consistent with the interpretation that additional slower electron injection processes are present which result in a decrease of the GSB amplitude at around 540 nm due to superimposed absorption of the D149 + radical cation in this This journal is c the Owner Societies 2012 spectral range.Thus, the ESA decay above 600 nm must be due to weaker ESA of D149 + compared to the ESA of the S 1 state of D149.The global analysis (Section 3.3) provides a time constant of 250 fs for this slower part of electron injection.
On longer timescales (bottom panel) of Fig. 3, we observe interesting additional dynamics: an increase of absorption in the range 450-540 nm and a decrease of absorption above 570 nm.Notably, the band integral covering the range 370-770 nm stays constant, and also the energetic positions of the peaks above 580 nm remain almost unchanged.As will be discussed in the following section, these dynamics are most likely due to a transient Stark shift of the S 0 -S n spectrum of D149 with additional spectral contributions from collisional cooling of vibrationally excited D149 + at larger wavelengths.
In Fig. 4, we compare representative kinetic traces of D149 on ZnO with our previous experiments for D149 in the solvent methanol to highlight the completely different dynamics. 34At 581 nm (A), the methanol transient first exhibits formation of S 1 -S n ESA.The characteristic curvature around the peak is due to solvation dynamics arising from the transient red-shift of the superimposed weaker S 1 -S 0 SE.The signal then decays to zero due to internal conversion (IC, S 1 -S 0 ) with a time constant of 99 ps.In contrast, for ZnO (581 nm) we see an instantaneous drop due to formation of GSB.The origin of the curvature (Stark shift and radical cation cooling) will be discussed in Section 3.3.The GSB then stays at a constant level showing that the original D149 population was depleted by electron injection and does not recover on this timescale.
At 693 nm (B), S 1 -S n ESA in methanol is formed initially which is then transformed into S 1 -S 0 SE because of the transient Stokes shift of the SE band.Finally, the signal decays due to IC from S 1 to S 0 .In contrast, ZnO stimulated emission of D149 is never observed in this spectral region because the S 1 state is so short-lived and dipolar solvation dynamics is of course absent on ZnO.Consequently, we first see S 1 -S n ESA of D149 which is quickly decaying due to electron injection.Afterwards, the same type of curvature is found as in (A), and the absorption then stays at a constant level (arising from D149 + ).

Global analysis of transient spectra of D149 on ZnO
The transient absorption spectra in Fig. 3 were analyzed globally using the scheme The experiments were carried out on a sensitized ZnO film in air, which is not in contact with a redox electrolyte.In this case, the recombination rate constant k rec of D149 + with electrons in ZnO in step ( 3) is apparently slow (c1 ns), because no decay of the PSCP transients is seen up to 1 ns.Possible spectral contributions of the photoinjected electrons will be discussed below. 23,46,47n the current study, all species-associated spectra were represented by a sum of Gaussian functions.A time-independent spectrum was sufficient to describe the S 1 state of D149 due to its very short lifetime on the ZnO thin film.Because of this ultrashort lifetime and the fact that no solvent molecules are in contact with the dye, it is also reasonable to assume that the S 1 spectrum of D149 on the ZnO thin film closely resembles earlytime S 1 spectra previously measured in our PSCP experiments of D149 in organic solvents. 34These are characterized by weak stimulated emission appearing at around 540 nm (slightly to the red of the peak for the ground state bleach) and by broad unstructured excited-state absorption below 500 nm and above 560 nm (Fig. 5).
To understand the slower dynamics in the bottom panel of Fig. 3, it is necessary to consider two main processes which will likely influence the transient spectra: Cappel et al. reported that local electric fields originating from the D149 + radical cations and photoinjected electrons induce a Stark effect which shifts the S 0 -S 1 absorption band of D149 to shorter wavelengths.This was confirmed in photoinduced absorption (PIA) experiments for D149 on mesoporous TiO 2 films, which exhibited a characteristic bleach feature centered at ca. 600 nm. 28 similar spectral feature was observed before in the study of Snaith et al. 48The effect is also supported by DFT/TDDFT calculations, in which a characteristic blue-shift of the D149 absorption spectrum in the presence of an external electric field was found. 28he second process to be considered is collisional cooling of vibrationally excited D149 + radical cations which are produced after electron injection.Because injection will occur from the neutral D149 geometry, the resulting D149 + in the D 0 state will certainly be produced in a non-equilibrium configuration, i.e., vibrationally excited.Afterwards, this vibrational excess energy needs to be dissipated by interactions with the ZnO surface.
In our global analysis we used a simplified approach to describe these processes by employing a time-dependent D149 + radical cation spectrum.Transient changes in this spectrum will therefore reflect changes of the radical cation spectrum due to vibrational relaxation and superimposed changes in the S 0 -S n spectrum of D149 molecules induced by the electric field exerted by these radical cations and the injected electrons.
In the fitting procedure, the time constant for the injection step (2) t inj = k inj À1 and for the subsequent changes of the D149 + spectrum (t relax = k relax

À1
) were optimized to obtain the best fit with a minimum set of parameters.The spectra obtained from the global analysis are presented in Fig. 5.The initially broad D149 + spectrum shows a decrease above 560 nm and an increase between 430 and 560 nm, with an isosbestic point at 560 nm.Spectral development occurs with a time constant t relax E 20 ps.This means that one observes an increase in absorption slightly to the blue of the S 0 -S 1 absorption maximum of D149, and a loss of amplitude on the red edge of the S 0 -S 1 band of D149.The resulting spectrum looks also different from the D149 + spectra obtained from spectroelectrochemistry, which will be discussed in Section 3.4.This leads us to the conclusion that these spectral dynamics are likely dominated by the ''local Stark effect'' proposed by Cappel et al. 28 and Meyer and co-workers. 29,30Further support comes from the study of Cappel et al. 28 They showed that PIA spectra of D149 on TiO 2 were dominated by the firstorder Stark effect: for the limiting case of aligned D149 molecules and an electric field acting normal to the surface, the change in absorbance (DOD) should be proportional to the first derivative of the steady-state absorption spectrum of D149. 28The scaled derivative of the inverted steady-state absorption spectrum is depicted in Fig. 3 (bottom panel) as a dashed green line.Its shape is remarkably similar to the ''relaxed'' PSCP spectrum at 450 ps, yet it is clearly further red-shifted.This indicates that complete uniform alignment of all D149 molecules on the ZnO surface is not achieved, and this is reasonable, because electrodeposited mesoporous ZnO does not represent a flat surface. 12t is interesting that the blue-shift of the S 0 -S 1 band does not appear instantaneously upon photoinjection, but gradually builds up with a time constant of 20 ps.Earlier studies for two dyes on ZnO suggest that after photoinjection first a bound electron-cation complex is quickly formed which possesses a ''cation-like'' spectrum.Afterwards, this complex dissociates into the cation and a mobile electron in the ZnO conduction band. 21,22,25,26This mechanism is supported by the delayed appearance of mobile electrons (on the several 10 ps timescale) in transient THz experiments. 26We speculate that such a process could be one reason for the gradual establishment of the local Stark field.In addition, the dipolar D149 molecules will need some time to align to this external electric field, which will require reorientational motion.
Particularly interesting is also the fact that the PSCP spectra in the bottom panel of Fig. 3 show the same undulatory structure as the first derivative of the steady-state absorption spectrum (see e.g. the region above 580 nm) and that these peaks in the PSCP spectra are already present at early times (Fig. 3, top) and essentially do not shift with time.The structure of the derivative arises from the weak interference pattern in the steady-state absorption spectrum of the ca.2.5 mm thick ZnO film.This journal is c the Owner Societies 2012 Normally, this structure should cancel out in differential absorption spectra.The fact that it is however still seen in the PSCP spectra suggests that there must be a subtle change in the refractive index of ZnO after photoexcitation of the dye.To explain this, we refer to an earlier study of Bauer et al. 46 In that case, a similar interference pattern was observed in differential absorption spectra from spectroelectrochemistry of a mesoporous ZnO electrode (without attached dye) upon application of a negative potential.The result was interpreted in terms of a flow of electrons into the electrode, which change the optical properties (i.e.refractive index) of the thin film.As we see a very similar structure already in the early PSCP spectra (Fig. 3, top), we take this as additional evidence that the photoinduced injection of electrons into ZnO is extremely fast.Beside the undulatory structure and weak absorption above 620 nm, Bauer et al. also observed a pronounced Moss-Burstein bleach feature of the ZnO thin film below ca.400 nm. 46Although this is already close to the lowest probe wavelengths covered in our current PSCP experiments, it appears as if there is not such a strong bleach feature for our D149/ZnO sample.
We note that to some extent the transient spectra in Fig. 5 also indicate relaxation of vibrationally ''hot'' D149 + on the ZnO surface: particularly, the absorption decay in the range above 650 nm, where there is no appreciable influence of the S 0 -S 1 spectrum of D149, can be hardly explained by a local Stark effect.0][51][52] In the present case, vibrational relaxation will occur by interaction of the attached D149 + with the ZnO surface (vibration-tophonon energy transfer, both ''intermolecular'' as well as by coupling through the carboxyl anchor group) and with adjacent D149 molecules.We did not attempt to further deconvolve the transient spectra in Fig. 5 into a ''pure D149 + spectrum'' and a ''Stark-shifted S 0 -S n spectrum''.
We also note that similar PSCP spectra and simulations are obtained for excitation on the red edge of the S 0 -S 1 band (630 nm), the only difference being that the broadening of the transient ''D149 + spectrum'' was less pronounced than in Fig. 5: this is understandable, because excitation at 630 nm with the same pulse energy excites less D149 molecules (smaller absorption coefficient, see Fig. 2), resulting in a smaller amount of cations/electrons and thus a less pronounced Stark effect.Also, the initial vibrational excitation of the generated radical cations is ca.3700 cm À1 smaller for excitation at 630 nm, resulting in narrower ''hot'' D149 + spectra.

D149 + radical cation spectrum
We also performed spectroelectrochemical experiments in dichloromethane and acetonitrile solutions to determine steady-state difference absorption spectra of D149 + for comparison with our transient PSCP data.These are depicted in Fig. 6 as red and blue lines, respectively.Studies of D149 + on TiO 2 over the spectral range 400-1100 nm were reported previously. 28,45Our current experiments extend the spectral range considerably, providing for the first time access to the complete D 0 -D 1 absorption band of D149 + located in the near IR.Its peak is located at 1250 nm in dichloromethane and at 1055 nm in acetonitrile which suggests a quite substantial influence of solvent polarity on the D 0 -D 1 transition energy.
We carried out DFT/TDDFT calculations to assign the electronic transitions in the D149 + spectrum.Best agreement with the experimental data was achieved for the MPW1K functional, whereas calculations employing B3LYP and PBE0 functionals appear to underestimate the transition energies systematically (see Table 1 for a summary of the 20 lowest electronic transitions).Stick spectra for the MPW1K results are overlaid in Fig. 6.The D 0 -D 1 transition wavelength is very well reproduced.The small broad peak at around 870 nm in the dichloromethane spectrum can be assigned to the D 0 -D 2 transition, with a tail extending down to 650 nm.Further to the blue, a group of three peaks dominated by D 0 -D 5 emerges, which is located in the bleach region of the PSCP spectrum (dotted green line).Another group of peaks (D 0 -D 10,12,15,16 ) is responsible for the prominent absorption feature in the 410-470 nm region on top of the D149 bleach.
Particularly interesting is the comparison between the spectroelectrochemistry results and the ''relaxed'' PSCP spectrum at 450 ps.The bleach region of the PSCP spectrum is redshifted by about 600 cm À1 with respect to the dichloromethane spectrum (and also the spectra of D149 + on TiO 2 published previously 28,45 ).In addition, the PSCP ESA band centered at ca. 460 nm has an increased amplitude.As mentioned above, we take both observations as a strong indication of the presence of a local Stark effect in the PSCP spectra, which shifts the S 0 -S 1 transition of D149 to shorter wavelengths.We also carried out experiments for D149 adsorbed on ZrO 2 , which represents a semiconductor oxide surface with a much larger band gap (ca. 5 eV) than ZnO (ca.3.2 eV).Fig. 7 contains PSCP results for the dynamics of D149 on ZrO 2 after photoexcitation at 510 nm, where spectra were recorded at the same time delays as in Fig. 3.The dynamics up to the first picosecond (top and middle panels) resemble those of ZnO.The fast decay of the GSB and ESA bands above 450 nm (ca.250 fs time constant) in the middle panel is accompanied by essentially no change in spectral features in the 390-420 nm region.Therefore we assign these dynamics to the primary electron injection step (formation of a cation-electron complex), as in ZnO.
The quite similar early-time behavior of ZrO 2 and ZnO might be at first sight surprising.In the case of bulk ZrO 2 , the conduction band edge is reportedly 1.0-1.5 eV higher than for TiO 2 (anatase), however the difference was estimated to be much smaller in the case of the nanocrystalline systems, about 0.5 eV. 53In addition, it can be estimated that vertical S 0 -S 1 excitation provides access to D149 levels which should be located at ca. 0.3 eV above the conduction band edge of ZnO. 54][57] The dynamics at longer times can be found in the bottom panel and is different from ZnO in two respects: here, one does not observe such distinct rise and decay features in the ESA (430 nm) and GSB (530 nm) bands (appearing like a red-shift of the whole spectrum in Fig. 3).This suggests, that separation of the initially formed close complexes of trapped electrons and cations does not easily occur, because of the unfavorable energetic location of the conduction band of ZrO 2 .The buildup of a local Stark field acting on ground-state D149 (S 0 ) therefore appears to be largely suppressed.There are also no clear interference features of the thin film in these PSCP spectra.
One reason for this is the fact that the undulations in the steadystate absorption spectrum for ZrO 2 are much weaker than in the case of ZnO, which will result in a much smaller amplitude of such features.Also, the refractive index change caused by electron-cation complexes might be considerably reduced in the case of ZrO 2 (Fig. 7).
Also, one observes a clear decay (time constant ca.30 ps), most easily seen in the GSB band at around 540 nm.A reasonable explanation is the recombination of a fraction of the electron-cation complexes reforming D149 in S 0 . 55The red-shift of the ESA band above 640 nm looks similar to the spectral development on ZnO (and also TiO 2 ) in this wavelength region.We therefore assign this process to vibrational relaxation of the electron-cation complex, which should have a spectral signature similar to isolated D149 + . 26its in this wavelength region provide a time constant of 25 ps (for a comparison of ZrO 2 , ZnO and TiO 2 see Fig. 8).For ZrO 2 , in this spectral region we therefore observe a decay of the electron-cation complexes due to charge recombination and also vibrational cooling, both with a similar time constant.We also note, that only for ZrO 2 there is a very slow decay of the whole spectrum with small amplitude, roughly with a time constant of 1 ns (not reliably measurable within our time window).This could be due to recombination of longer-lived This journal is c the Owner Societies 2012 electron-cation complexes or internal conversion of a smaller fraction of D149 molecules in S 1 which did not undergo electron injection.For the latter process, the lifetime would be in line with those previously measured by us in less polar solvents (e.g.720 ps in THF). 34

Conclusions
In summary, we have shown that the indoline dye D149 very efficiently injects electrons on an ultrafast timescale (r250 fs) into a highly porous electrodeposited ZnO thin film, initially forming an e À -D149 + complex.The same is observed for sintered ZrO 2 and TiO 2 surfaces, where only in the former case electrons are obviously trapped and have the chance to recombine to repopulate D149 in S 0 on longer timescales.
For ZnO (and TiO 2 ), the e À -D149 + complex dissociates and forms mobile electrons in the conduction band.We believe that we track this formation of mobile electrons in terms of a characteristic spectral evolution in the PSCP spectra with a time constant of ca.20 ps, which signals the build-up of a local electric field (Stark effect) [28][29][30] acting on unexcited D149 molecules in S 0 .This is also supported by the shape of the relaxed PSCP spectrum which is similar to that of the first derivative of the steady-state (bleach) absorption spectrum.The spectral dynamics possibly also contain contributions of D149 molecules on the thin film, which reorient in response to the newly forming electric field.Additional undulatory structure in the PSCP spectra is likely due to the change of the refractive index of ZnO due to injection of electrons. 46 spectral component with a similar time constant observed in the red region of the spectrum (where a Stark shift cannot be present) is likely due to vibrational cooling of D149 + by interactions with the surface (collisional or through-bond energy transfer to ZnO).We believe that it is of general importance to consider the possible presence of such processes on the several picosecond timescale in the interpretation of any ultrafast transient absorption experiment dealing with electron injection from dye molecules into thin films.
The very efficient injection of electrons into highly porous electrodeposited ZnO suggests a negligible disturbance of the injection process by D149 aggregates. 19,20,58Obviously, aggregation of the dye molecules on the ZnO surface was successfully suppressed by the addition of cholic acid as a co-adsorbent in the dye solution.We plan to study the influence of the co-adsorbant on the dynamics of the three thin film systems in a more systematic way in a forthcoming publication.
A substantial loss of injected electrons by recombination in the picosecond range was reported previously, e.g., 75% in the study of Sundstro¨m and co-workers for a Zn-porphyrin dye on ZnO nanoparticles. 26In that case, the initially formed e À -dye complex obviously had a high chance of recombination.Such a behavior is not seen in the current study.This is most likely due to favorable electronic properties of the indoline dye, with the LUMO located close to the anchoring group (resulting in facile electron injection) and the HOMO also more delocalized toward the ''opposite end'' of the dye, decreasing the possibility for charge recombination by ''hole trapping''. 3,54,59n future investigations, it will also be interesting to probe the population of mobile charge carriers in D149-sensitized electrodeposited ZnO films using time-resolved THz spectroscopy: this way, one could test if one observes the appearance of mobile electrons on the 20 ps timescale. 25,26It will also be worthwhile to extend the PSCP investigations toward ZnO thin films in contact with an electrolyte solution to investigate its influence on the ultrafast electron injection dynamics and transient Stark shift contributions to the PSCP spectra.

Fig. 2
Fig.2Normalized steady-state absorption spectra of D149, adsorbed on an electrodeposited ZnO thin film (red line) and on sintered TiO 2 (green line) and ZrO 2 thin films (blue line), dissolved in acetonitrile (black solid line) and in dichloromethane (black dashed line).In the latter two cases, the solutions contained additional tetrabutylammonium perchlorate as a supporting electrolyte for spectroelectrochemistry (100 mM).

Fig. 3
Fig. 3 Transient PSCP absorption spectra of indoline D149 on an electrodeposited ZnO thin film.Laser excitation at 510 nm.The dashed blue line in the bottom panel represents the scaled and inverted steady-state absorption spectrum of D149 on ZnO from Fig. 2. The dashed green line is the scaled first derivative of the inverted steadystate absorption spectrum.

Fig. 4
Fig. 4 Selected kinetic traces for the dynamics of D149 on ZnO (red) and in methanol (black) at (A) l probe = 581 nm, (B) l probe = 693 nm.l pump = 510 nm.The insets show magnifications at early times.Solid lines are results from the global kinetic analysis.

Fig. 5
Fig. 5 Species-associated spectra from global analysis of the ultrafast transient absorption data for D149 on ZnO.The time-dependent spectrum termed ''D149 + radical cation'' features a Stark shift of the S 0 -S n band of D149 and vibrational relaxation of D149 + .

Fig. 6
Fig.6Differential absorption spectra of the D149 + radical cation from spectroelectrochemistry in dichloromethane (red line) and acetonitrile (blue line) compared with the ''relaxed'' PSCP spectrum on ZnO at 450 ps (dotted green line).Grey columns mark the positions of the D 0 -D n electronic transitions obtained from DFT/TDDFT calculations employing the MPW1K functional (Table1), where the relative amplitudes are proportional to the respective oscillator strengths.

Table 1
Results of DFT/TDDFT calculations for the D149 + radical cation: wavelengths of the first 20 vertical electronic transitions using B3LYP, PBE0 and MPW1K functionals, and oscillator strengths f for calculations using the MPW1K functional D 0 -D n l/nm (B3LYP) l/nm (PBE0) l/nm (MPW1K) f (MPW1K) Transient PSCP absorption spectra of indoline D149 on a ZrO 2 thin film.Laser excitation at 510 nm.The dashed line in the bottom panel represents the scaled and inverted steady-state absorption spectrum of D149 on ZrO 2 from Fig. 2. Published on 15 October 2012.Downloaded by University of Goettingen on 12/06/2014 15:32:42.