Femto - American Chemical Society

Article pubs.acs.org/JPCC

Femto- to Millisecond Photophysical Characterization of IndoleBased Squaraines Adsorbed on TiO2 Nanoparticle Thin Films G. de Miguel,† M. Marchena,† M. Ziółek,†,§ S. S. Pandey,‡ S. Hayase,‡ and A. Douhal*,† †

Departamento de Química Física, Facultad de Ciencias Ambientales y Bioquímica and INAMOL, Universidad de Castilla-La Mancha, Avenida Carlos III, S.N., 45071 Toledo, Spain ‡ Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu, Kitakyushu, 808-0196, Japan S Supporting Information *

ABSTRACT: In this paper, we address femto- to millisecond transient absorption studies of TiO2 nanoparticle (NP) thin films sensitized with four squaraine (SQ) molecules, with and without a deaggregating agent, chenodeoxycholic acid (CDCA). On the femto- to picosecond time scale, we determined the presence of three transient species by using singular value decomposition (SVD) analysis, i.e., S1 of the SQ monomers, S1 of the SQ Haggregates, and the SQ radical cation formed after the electron injection. Both monomers and H-aggregates are proven to inject electrons to the TiO2 conduction band, being 5 times faster in the monomers (e.g., keimon = 5.1 × 1011 s−1 and keiH‑agg = 1.1 × 1011 s−1 for SQ 41). Besides, the undesired singlet−singlet annihilation is an active process in these samples, constituting the drain of a high percentage of the absorbed photons. The coadsorption of CDCA on the TiO2 NP avoids the formation of H-aggregates, and therefore, only two transient species are present in these samples: S1 of the monomer and the SQ radical cation with keimon = 6.7 × 1011 s−1 for SQ 41. On the microsecond scale, we only observed the transient feature of the radical cation of the SQ that permits one to study its recombination dynamics. Similar lifetimes (94− 150 μs) of the four SQ radial cations are obtained when only monomers are present in the sample. In the absence of CDCA, the presence of H-aggregates contributes to shorten the lifetime of the radical cation (e.g., from 110 to 45 μs in the case of SQ 41). This fact can be explained by considering a stronger electronic coupling of H-aggregates/TiO2 surface with respect to the monomers. These results explore the photodynamics of this family of SQs adsorbed on TiO2 NP in a very large time window and will enable a better understanding of the influence of aggregates in the kinetics of these SQs used as sensitizers in DSSCs. DSSCs.13 Particularly, in this last application, SQs are emerging as an ideal candidate to extend the photon collection efficiency to the red part of the solar spectrum.14−17 Furthermore, oligomerization of the monomer units, extension of the πconjugation by means of different side groups, and other strategies have resulted in successful lowering of the band gap (Eg) of the SQs up to a value of 1 eV,18 which is comparable with that of the silicon-based solar cells.19 This makes the LUMO energy close to the conduction band of TiO2 and/or the HOMO energy is also near to the redox potential, Eredox, of the electrolyte (i.e., energy pairing), causing a decrease in the driving force for the electron injection and/or the regeneration processes. Because of that, it is highly desirable to elucidate the photodynamics of these processes using low band gap materials to further improve the energy pairing by synthetically engineering the dye energy levels. Aggregation of dyes upon adsorption to the TiO2 nanoparticle (NP) surface is a common phenomenon that needs to be evaluated in relation to the efficiency of the DSSCs.20−30 There is an open discussion about how the formation of aggregates influences the efficiency of the solar cell.31,32 On one

1. INTRODUCTION Since the advent of dye-sensitized solar cell (DSSC) technology, an impressive progress has been made to overcome the limiting issues that hinder the transference of this technology to commercial application.1,2 Recently, a DSSC device has been reported to exhibit a stunning record efficiency of 12.3%, having its photon harvesting window mainly in the visible region of the solar spectrum.3 In this respect, the absorption of infrared photons up to 1 eV is fundamental to compete with the 20% average efficiency of silicon-based solar cells.4 Thus, there is an increasing amount of work on DSSCs using red-absorbing organic dyes aimed to improve sunlight collection in the IR region.5−9 Squaraine (SQ) dyes comprise a rich family of molecules characterized by their intense absorption (ε > 200 000 M−1 cm−1) in the low-energy region of the visible spectrum (600− 750 nm).10,11 This beneficial property for DSSCs is determined by the extended π-electron delocalization throughout the conjugated squaric acid backbone and the electron-donating side groups. In this respect, indole-based SQs have been considered as one of the more versatile type of SQs, due to their high photostability with regard to other SQs.12 These cyanine-type SQs have been widely investigated as active materials in multiple applications, from labeling proteins to photosensitizing the wide band gap of semiconductors in © 2012 American Chemical Society

Received: March 27, 2012 Revised: May 16, 2012 Published: May 18, 2012 12137

dx.doi.org/10.1021/jp302928f | J. Phys. Chem. C 2012, 116, 12137−12148

The Journal of Physical Chemistry C

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Figure 1. Molecular structure of the four studied SQ compounds.

vs 9.1 × 103 s−1 with CDCA for SQ 41), which corroborates their detrimental effect on the photon-to-current efficiency. We believe that this detailed study sheds light on the behavior of SQ aggregates in DSSCs and probably on their working scheme in other applications.

hand, self-quenching processes, like the singlet−singlet exciton annihilation, are thought to be responsible for the slower injection from the aggregates in a multitude of dyes.20−24 Thus, competition between electron injection and exciton annihilation provokes losses of absorbed photons in the cell. The main used strategy to prevent aggregation has been the coadsorption of the dye with nonchromophoric adsorbates, such as deoxycholic or chenodeoxycholic acid (CDCA), which localize between the dye molecules.33−38 On the other hand, photocurrent efficiencies of H- and J-aggregates of certain cyanine and rhodamine dyes were reported to be comparable to or greater than those of monomers, in addition to the extension of the photon absorption to a wider range.25−30 Hence, there is a necessity to gain a deep insight into the influence of the aggregates in the working scheme of DSSCs. In this work, we report on the photodynamics on the femtoto millisecond time scale of four indole-based SQs adsorbed on TiO2 nanoparticle (NP) thin films with and without the CDCA coadsorber. Femtosecond transient absorption spectroscopy and nanosecond−millisecond flash photolysis were employed to determine the transient spectra and related dynamics of the different involved species. We have detected three transient species in the samples without CDCA: the excited state of the monomers and of the H-aggregates and the radical cation of the SQs. The presence of H-aggregates resulted in a negative effect on the efficiency of the cell, since the exciton annihilation process leads to 15−25% losses of the absorbed photons. This quenching mechanism is strongly active in the SQ dyes due to a large overlap between the absorption and emission spectra, which favors the Förster-type energy transfer mechanism responsible of the annihilation process.39 Addition of the CDCA molecule to the system prevents the formation of Haggregates, and therefore, the majority of the absorbed photons can be injected into the TiO2 conduction band. The charge recombination rate constant (kCR) was measured to be greater in the presence of H-aggregates (2.2 × 104 s−1 without CDCA

2. EXPERIMENTAL SECTION The four studied SQ molecules (Figure 1) were synthesized as previously described.36,40,41 To fabricate the TiO2 thin films, TiNanoxide HT paste (Solaronix SA) was employed. First, a conductive fluorine-doped tin oxide (FTO) glass (Pilkinton NSG TEC 8A 2 × 3 cm2) was cleaned with acetone, with water, and finally with a detergent in ion-exchanged water using an ultrasonic bath during 10 min each. Afterward, the FTO glass was washed again with water and finally immersed in 2propanol and sonicated for 30 min. After drying the FTO glass, it was coated with a layer of the TiO2 paste using the doctor blade technique with the help of two parallel adhesive Scotch tapes. The substrate was then baked at 450 °C in a furnace to fabricate TiO2 layers of about 5 μm thickness. The substrate was dipped in an ethanol solution (Scharlau SA, extra pure) containing the SQ dye (0.25 mM) in the presence or absence of the CDCA coadsorber (2.5 mM) for 3−4 min at room temperature to obtain an absorbance of ∼0.7 at the maximum of the absorption peak. It is worth noting the short dipping time compared to other dyes. This is dictated by the special requirements of the samples to be measured in transient absorption spectroscopy. Thus, we need to utilize thin and highly transparent TiO2 NP layers (4−5 μm) and the absorbance of the SQ has to be not higher than 0.7 to avoid the probe beam being completely scattered or absorbed by the sample, respectively. However, when preparing the samples to measure the solar cell efficiency of SQ dyes, we used nanoporous TiO2 NP films with a thickness of about 10−12 μm. In this situation, we keep the dye adsorption time of 3−4 h from about 0.25 mM dye solution, which is sufficient to give the 12138



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recorded in the spectral range of 500−750 nm at 10 nm intervals, and the time-resolved absorption spectra were constructed from the kinetic traces. The quality of the fits was checked by examining the residual distribution and the χ2 value. The pump pulse energy density was 40−80 μJ/cm2. All the experiments were done at 293 K.

best efficiency. In fact, we have carried out the dye adsorption for 24 h, but it led to a decrease in the photoconversion efficiency as compared to the 4 h dipping time. Thus, the dipping time is only 2−3 times shorter than for other dyes. UV−visible steady-state absorption spectra were measured with a JASCO V-670. The absorption spectrometer is equipped with a 60 mm integrating sphere (ISN-723) that allowed measuring the diffuse transmittance spectra. For the diffuse transmittance, the Kubelka−Munk remittance function is used: F(R) = (1 − R)2/2R, where R is the diffuse reflectance intensity from the sample. Fourier transform infrared (FTIR) spectra were measured using a JASCO FT/IR-4100 spectrophotometer in the range 4000−400 cm−1. The samples were measured in attenuated total reflectance (ATR) mode. Femtosecond (fs) transient UV−visible absorption spectra were measured using a two-channel detection system described previously.42 It consists of a Ti:sapphire oscillator (TISSA 50, CDP Systems) pumped by a 5 W diode laser (Verdi 5, Coherent). The seed pulse (30 fs, 450 mW at 86 MHz) centered at 800 nm wavelength is directed to an amplifier (Legend-USP, Coherent). The amplified fundamental (50 fs, 1 W at 1 kHz) was then directed through optical parametric amplifier for wavelength conversion (CDP Systems) to a femtosecond transient absorption spectrometer (CDP Systems), where it is split into two beams. The first one is frequency-doubled to give 630−670 nm for exciting the sample. The pump energy was kept constant at 100 μJ·pulse−1·cm−2. The remaining fundamental beam goes through a delay line and is focused on a rotating 3 mm thick BaF2 plate to generate the white light continuum. The produced white light is split into two parts to form probe and reference beams, which are directed to the cell, where the probe and the pump beams are overlapped. The sample is placed in a holder connected to a pair of translation stages (MTS series, Thorlabs) which moves in the x−y plane perpendicular to the pump beam, preventing it from photodegradation during the measurements. The polarization of the pump is set to the magic angle with respect to the probe. The transmitted light is focused to light guides, directed to a spectrograph, and collected by a pair of photodiode arrays. The obtained signal, as a difference in the optical density of the sample probed by the white light continuum with and without pump excitation, is processed by ExciPRO software (CPD Systems). The zero time of all the analyzed spectra was corrected for the group velocity dispersion effect, according to the standard numerical scheme.43 The chirp of the white light continuum was obtained by measuring two-photon absorption (TPA) in a very thin (150 mm) BK7 glass plate. Singular value decomposition (SVD) 44−46 and global analysis of the experimental data were carried out by using the Globe Population Dynamics Modeling Toolbox integrated in a Matlab environment.47 The UV−visible nanosecond−second flash photolysis setup consists of an LKS.60 laser flash photolysis spectrometer (Applied Photophysics), Vibrant (HE) 355 II laser (Opotek) as a pump pulse source (5 ns time duration), and a 150 W xenon arc lamp as a probe. The signal from OPO (355 nm pumped by Q-switched Nd:YAG laser, Brilliant, Quantel) at 640 nm was used for the sample excitation. The probing light transmitted through the sample (held by tweezers) was dispersed by a monochromator and detected by a photomultiplier coupled to a digital oscilloscope (Agilent Infiniium DS08064A, 600 MHz, 4 GSa/s). The pump energy pulse (5 mJ/pulse) was attenuated by the pair of half-waveplates and a polarizer. The kinetics was

3. RESULTS AND DISCUSSION 3.1. Steady-State Absorption Spectroscopy. Figures 2A,B and S1A,B (Supporting Information) show the

Figure 2. Normalized steady-state absorption spectra of SQ 41 (A) and SQ 26 (B) in solution (dotted lines), adsorbed in a TiO2 NP thin film without the CDCA coadsorber (dashed lines) and with the CDCA coadsorber (solid lines).

normalized absorption spectra of SQ 41, SQ 26, SQ 4, and SQ 2 adsorbed on the TiO2 NP thin films with (solid lines) and without CDCA (dashed lines), respectively. The absorption spectra in an acetonitrile (ACN) solution have been included for comparison, showing maxima at 640, 643, 647, and 644 nm for SQ 41, SQ 26, SQ 4, and SQ 2, respectively. The weak shoulders at the blue side (∼600 nm) correspond to highenergy vibrational transitions. In a previous work, we have excluded the formation of aggregates in solution, since in concentration-dependent experiments we did not observe changes in the absorption spectra and fluorescence anisotropy decays.48 On the other hand, in the TiO2 NP thin films without the CDCA molecules, the absorption spectra show a small blueshift of the wavelength with the highest absorbance (λmax) in relation to the spectra in solution, 3 and 5 nm for SQ 41 and SQ 26, respectively. This is in opposition to the behavior found in the solid-state samples (without the TiO2 NP) prepared by the spin-coating method,39 with a 33 and 36 nm red-shift for SQ 41 and SQ 26, respectively. The former was attributed to the formation of J-aggregates, which, in the light of the absence of a red-shift, can be ruled out in the samples with the TiO2 nanoparticles. The second important feature of the absorption 12139



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Figure 3. Femtosecond transient absorption spectra of SQ 41 (A) and SQ 26 (B) adsorbed on a TiO2 NP thin film (without CDCA coadsorber) at four time delays. Decay of the transient absorption intensity of the same samples, SQ 41 (C) and SQ 26 (D), with best fits at the time window of 20 ps. Table 1 gives the fit results. The excitation wavelength was 650 nm.

spectra is the high intensity of the absorption signal around 600 nm, being closer to the signal at λmax, as happened in the solidstate samples.39 This band was assigned to the formation of Haggregates, while the main peak around 636 nm for both SQs originated from the monomer species. Co-adsorption of CDCA and SQ dye on the surface of TiO2 NPs is a well-know procedure to prevent the aggregation of the adsorbed dyes. Thus, the absorption spectra of these samples become narrower, and the signal around 600 nm is comparable to that of the absorption spectra in solution. This confirms the deaggregating effect of the CDCA, which is localized in between the SQ molecules adsorbed on the TiO2 NP surface, hindering the π−π interactions that form aggregates. Moreover, we have measured the absorption spectra of TiO2 NP thin films sensitized with SQ 41 and SQ 26 (with and without CDCA) at different dipping times (Figure S2 and S3, Supporting Information). In the samples with CDCA, we did not observe the formation of H-aggregates (shoulder at the short wavelengths) upon increasing the dipping time, unlike the situation in the samples without CDCA. Nevertheless, a redshift of the λmax of 10 and 14 nm in relation to the spectra in solution for SQ 41 and SQ 26, respectively, is observed in samples containing CDCA (Figure 2). These shifts without significant changes in the full width at half-maximum (fwhm) of the main band are normally attributed to the different environment that SQs molecules feel on the surface of the TiO2 NPs compared to that in solution. The red-shift of the λmax is opposite to that in the samples without CDCA. The explanation is that the absorption of H-aggregates in the samples without CDCA overlaps with the monomer spectrum, leading to an apparent shift of the λmax of the monomer absorption band (Figure S4, Supporting Information). FTIR measurements were carried out to investigate the type of binding between the SQs and the surface of the TiO2

nanoparticles. Figure S5 (Supporting Information) shows the FTIR spectra for SQ 26 adsorbed on TiO2 NP, with and without CDCA coadsorber. The spectrum of SQ 26 in powder is also shown for comparison. We observed that the symmetric stretching mode (1702 cm−1) of the free carboxylic acid completely disappears in both samples, with and without CDCA coadsorber. This clearly indicates that both carboxylic groups are anchored to the surface of the TiO2 nanoparticles, and therefore, the SQs are strongly bound to the surface of the TiO2 NP in both situations.49,50 We have also determined the difference between the carboxylate asymmetric (νas) and symmetric (νs) bands, since this has also been used to distinguish between bidentate or unidentate coordination.51 We found the same values for both samples (with or without CDCA coadsorber), 1602 and 1383 cm−1 for the carboxylate asymmetric (νas) and symmetric (νs) bands, respectively. The 219 cm−1 difference has been ascribed to bidentate bridging of the carboxylate unit of the SQs with metal atoms on the TiO2 surface.51 We concluded that the coupling strength and sites are similar in both samples, independently of the addition or not of CDCA as coadsorber, and therefore, we can compare the aggregation effect between both samples. 3.2. Femtosecond Transient UV−Visible Absorption Studies. 3.2.1. SQs Adsorbed on TiO2 Nanoparticle Thin Films. To unravel the deactivation processes following electronic excitation of the SQ molecules adsorbed on the TiO2 nanoparticle thin films, femtosecond (fs) pump−probe absorption experiments were performed on these samples. The excitation wavelength was fixed at 640 nm and the optical density was ∼0.7 for a thickness of 5 μm. First, a short description of the femtosecond studies of solely SQs molecules deposited as thin films on quartz substrates39 is made, since these results will be valuable to understand the photodynamics of the used SQs adsorbed on TiO2 NP thin 12140



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films. A global analysis of the femtosecond transient absorption data obtained from the thin films of SQs revealed the existence of two transient species in the excited state. The fastest component (with transient absorption maximum at 500 nm) was assigned to the singlet excited state of the SQ monomer, which deactivates with a time constant of about 100 fs. An energy transfer process to the aggregates is thought to be responsible for this fast deactivation. On the other hand, the second component (with a broad transient absorption band) was ascribed to the singlet excited state of J- and H-aggregates, with lifetimes of 2.0, 1.0, 1.5, and 1.8 ps for SQ 41, SQ 26, SQ 4, and SQ 2, respectively. The strong dependence of the kinetics of this second component on the pump fluence indicates a singlet−singlet annihilation process as the most likely mechanism for the deactivation of the excited state of the aggregates. In conclusion, the close proximity of the SQ molecules in the thin films gives rise to rapid deactivation channels, like energy transfer from the monomer excited state to the ground state of the J-aggregates or singlet−singlet exciton annihilation between the excited states of the aggregates. These channels are responsible for the short lifetimes of the excited states when compared to those in solution (roughly several hundreds of picoseconds).48 When the SQ are adsorbed on TiO2 nanoparticle thin films, those two processes might be also active due to relatively close packing, as indicated by the formation of H-aggregates reflected in the absorption spectra (Figure 2). To strengthen the previous results, we have also done femtosecond absorption experiments of the SQ 41 adsorbed on Al2O3 nanoparticle thin films. The Al2O3 NPs exhibit dye binding properties similar to those of TiO2 but have a conduction band edge potential about ∼4 V more negative than that of TiO2, preventing electron injection from the excited state of the SQs. Figure S6A (Supporting Information) shows the UV−visible diffuse absorption spectra of SQ 41 adsorbed onto Al2O3 and TiO2 NPs thin films. Clearly, both spectra are very similar to each other, which ensure the same type of aggregates. Figure S6B (Supporting Information) illustrates the femtosecond decay of the transient absorption intensity at 500 nm of SQ 41 adsorbed on Al2O3 or TiO2 NPs thin films and as a thin film on a quartz substrate. It is worthy noting that the three decays were measured at the same pump fluence, 0.25 μJ/pulse. When comparing the decays of the samples with Al2O3 NP and the thin films of only SQs, we observed no changes, which indicate that similar deactivation channels are expected in both cases. Thus, the singlet−singlet annihilation process that takes place in the thin films of SQs deposited on quartz substrates is also expected to occur in the samples with Al2O3 NPs and therefore in the samples with TiO2 NPs. Finally, when comparing the decays of the two samples with the nanoparticles, the deactivation of the singlet excited state is slower in the sample with Al2O3 than with TiO2, which suggests that an extra deactivation channel is active in the samples with TiO2. Figures 3A,B and S7A,B (Supporting Information) exhibit transient absorption spectra registered at different time delays for the four SQs adsorbed on the TiO2 nanoparticle thin films. At early times, these spectra show a broad transient absorption band with a maximum for the positive signal around 530 nm, which resembles the initial transient features in SQ thin films. The latter is assigned to a signal from the combination of the singlet excited states of the SQ monomers and H-aggregates. Additionally, a negative signal in the region of the ground-state absorption (575−675 nm) is observed, which confirms the

ground-state depopulation of both monomers and Haggregates. The strong light scattering of the pump beam at 650 nm prevents the measurement of the transient signal at the maximum of the absorption peak. Thus, the minima of the transient signal around 600 nm correspond to the bleach of the H-aggregates. The initial transient absorption features evolve during the first 10 ps in a red-shifted peak centered at 567, 552, 574, and 547 nm for SQ 41, SQ 26, SQ 4, and SQ 2, respectively. This latter band extends to the near-IR region (730−780 nm) and retains the same shape and intensity until the end of our 1.7 ns experimental time window. It is worth noting that this band was not present in the corresponding femtosecond transient absorption experiments of SQs thin films.39 Moreover, these SQs in solution did not show an active intersystem crossing process; therefore, triplet states can be precluded as the origin of this signal.48 Thus, the long-living band is ascribed to a radical cation of the SQ, SQ•+, which is formed after an electron is injected to the TiO2 conduction band, in agreement with other works.6,52 Pulse radiolysis experiments on similar SQs obtained absorption spectra for the SQ•+ comparable to those presented in Figures 3 and S7 (Supporting Information).53,54 To know the number of involved transient species in the signals, singular value decomposition (SVD) analysis along with a global analysis of the data were performed. We calculated the first six singular values for the studied SQs adsorbed on the TiO2 films. For SQ 41, we obtained 10.4, 1.6, 1.1, 0.5, 0.3, and 0.2. The spectral and kinetics vectors associated with the first three singular values show clear peaks and decays, while the same vectors associated with the fourth and following singular values present a random structure or no decay at all. This behavior indicates that only the first three singular values are considerably different from the following ones, reflecting that only three transient species (monomers, radical cations, and Haggregates) are present in the sample. Knowing this information, we have then carried out a global analysis of the data. Three exponential functions were used to get an accurate fit, since an extra exponential function gave rise to the same associated transient spectrum. Figure 4 illustrates the associated transient spectra of SQ 41 and SQ 26. When comparing these associated spectra with the spectral vectors obtained from the SVD analysis, we noticed that they largely coincide between each other. Transient bands centered at 500 nm were observed for the fastest component (∼500 fs), which matches up with the transient features for the singlet excited state of the monomer in solution.48 The second component (∼10 ps) presents a rather broad associated spectrum, as it was observed for the singlet excited state of the SQ aggregates in the SQ thin films.39 Finally, the long-living component (∼10 ns) was assigned to the radical cation of the SQ, since similar transient spectra were previously reported.6,52−54 Local analysis at different probing wavelengths was carried out to further elucidate the relaxation dynamics in these samples (Table 1 and Figures 3C,D and S7C,D, Supporting Information). First, we focused on the blue part of the spectrum (510 nm), where there is no appreciable contribution to the transient signal from the radical cation. Thus, a twoexponential function was needed to get an accurate fit of the data, resulting in lifetimes of 0.26−0.43 and 4.6−6.2 ps. Comparison of these lifetimes with those obtained for the SQs thin films (0.1−0.2 and 1−2 ps) revealed longer times for the samples with TiO2 NPs. It is important to identify which processes are responsible for the deactivation of the transient 12141



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Figure 5. Decays of the transient absorption intensity of SQ 41 adsorbed on a TiO2 NP thin film (without CDCA coadsorber) using different fluence energies, 0.085, 0.2, and 1 μJ/pulse, and best fits of the experimental data by using the annihilation model. Decays of the transient absorption intensity at 0.085 and 0.2 μJ/pulse were normalized to the 1 μJ/pulse. The excitation and observation wavelength were 650 and 500 nm, respectively.

favors their deactivation, as happens in the singlet−singlet annihilation mechanism. However, the longer lifetimes found for the singlet excited states in the SQs adsorbed on the TiO2 nanoparticle thin film with regard to the SQ thin films suggest dissimilar rate constants in both samples for this process; it is slower in the samples with TiO2 thin films. The former is explained on the basis of the different packing, i.e., distinct separation between the SQ molecules and almost no formation of J-aggregates on the TiO2 nanoparticle thin films. In line with these results, the exciton annihilation process in H-aggregates is known to be less effective than that in J-aggregates, which would explain the longer lifetimes in the samples with TiO2 thin films. Förster-type dipole−dipole interactions depend on the orientation factor k2, which, in turn, is dependent on the relative angle between the transition dipole moments of donor and acceptor. This factor is larger for J-type aggregates than for H-type ones.55 Analyzing the kinetics at the bleaching band, 610 nm, resulted in slightly longer lifetimes than those obtained at the blue side (Table 1). Initially, this negative band was attributed to the ground-state depopulation, as was mentioned earlier. However, at longer time delays the positive transient absorption signal of the radical cation of the SQs overlaps with the bleach. Thus, the kinetics in this region involves both the ground-state recovery and the formation of the radical cation of the SQs. Finally, we analyzed decays at 570 nm, where the initial negative signal, due to the ground-state depopulation, develops with a rise to give a positive signal corresponding to the radical cation of the SQs. Thus, again the overlap of the two transient signals precludes the straightforward determination of

Figure 4. Associated transient absorption spectra of SQ 41 (A) and SQ 26 (B) adsorbed on a TiO2 NP thin film (without CDCA coadsorber) obtained from a multiexponential global analysis of the data. Time constants for each spectrum are shown in the legend. Transient absorption spectra in ACN solution are shown as a comparison (dotted lines).

signals of SQs in the TiO2 thin films to make a comparison of the different lifetimes between both samples. On the one hand, it is expected that the energy transfer process is not a significant quenching process for the deactivation of the singlet excited state of the monomers in the SQs adsorbed on the TiO2 nanoparticle thin film. This assumption is based on the dependence of the energy transfer yield on the spectral overlap between the donor-emission and the acceptor-absorption spectra. In these samples, there is almost no overlap between the monomers emission and the Haggregates absorption, and therefore, we have ruled out the energy transfer process to occur in the SQs adsorbed on the TiO2 nanoparticle thin films. On the other hand, we have carried out fluence-dependence experiments for SQ 41 to evaluate if the singlet−singlet annihilation process is active. Figure 5 shows that upon increasing the pump fluence from 0.085 to 1 μJ/pulse the transient absorption kinetics are much faster, which proves that much higher density of excited states

Table 1. Lifetimes and Relative Amplitudes Obtained from Exponential Local Analyses of the Decays of the Transient Absorption Intensity at Different Probing Wavelengths of the SQ 41, SQ 26, SQ 4, and SQ 2 Adsorbed on a TiO2 NP Thin Film SQ 41

SQ 26

SQ 4

SQ 2

λobs/nm

τ1/ps

a1/%

τ2/ps

a2/%

τ1/ps

a1/%

τ2/ps

a2/%

τ1/ps

a1/%

τ2/ps

a2/%

τ1/ps

a1/%

τ2/ps

a2/%

470 510 570 610 665

0.77 0.43 0.45 1.1 0.30

55 60 −42 −46 −65

9.5 5.6 6.4 19 4.3

45 40 −58 −54 −35

0.26 0.34 0.34 0.31 0.41

65 65 −54 −42 −46

4.6 4.6 8.8 4.9 6.4

35 35 −46 −58 −54

0.30 0.26 0.33 0.55 0.44

71 77 −43 −48 −54

7.1 4.9 4.5 11 15

29 23 −57 −52 −46

0.30 0.28 0.42 0.48 0.50

75 77 −64 −87 −75

7.3 6.2 19 14 15

25 23 −36 −13 −25

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the rate constants for the electron injection, kei. We have not considered the analysis of the signal appearing in the near-IR region (730−780 nm), as it is again a mixture of the signal from the radical cation formation, stimulated emission, and singlet excited state decay. An interesting point to examine here is whether the electron injection from SQs to the TiO2 nanoparticles takes place exclusively from the monomers and to what extent the Haggregates could be involved in the electron injection. To explore both possibilities, we carried out a global analysis of the data (pump fluence = 0.2 μJ/pulse) using two models that describe the two situations. It is important to note that we only made the global analysis of the data at 0.2 μJ/pulse fluence, which corresponds to ∼100 mW·cm−2. By using that pump fluence, we are very near to the operating conditions of a solar cell (simulated solar irradiation of 100 mW/cm2 at AM 1.5). S1(monomer) → S•+ + 1e− electron injection from the monomer 2S1(H‐aggregate) → S2 + S0

annihilation process

(1) (2)

S1(H‐aggregate) → S•+ + 1e− electron injection from the H‐aggregate

Figure 6. Decays of the transient absorption intensity of the SQ 41 (A) and SQ 26 (B) adsorbed in a TiO2 NP thin film (without CDCA coadsorber) at 500 nm (red squares) using an energy of 0.2 μJ/pulse. Solid and dashed lines are the fitted curves obtained from the global analysis by using model II (eqs 1−3) and model I (eqs 1 and 2), respectively. The excitation wavelength was 650 nm.

(3)

The radiative and nonradiative deactivation channels of the involved species to the ground state are not considered here, since the time constants are known to be much longer (hundreds of picosecond) than the electron injection time. In the first model (I), H-aggregate excited state deactivates only through the singlet−singlet annihilation process (eq 2), while the monomer excited state decays via the electron transfer process to the TiO2 conduction band (eq 1). In the second model (II), in addition to the two previous reactions, electron injection can also occur from the H-aggregate excited state (eq 3), competing with the annihilation process and increasing the percentage of absorbed photons that are converted into electrons in the TiO2 conduction band. To analyze the resulted complex dynamics, the two electron injection processes are represented by stretched exponential functions (eq 4, vide infra). For the rate law governing the annihilation process, the rate constant is time-dependent according to the equation, γ = γ′t−1/2.39 Parts A and B of Figure 6 illustrate the fitting results at one representative wavelength (500 nm) by using the two models for SQ 41 and SQ 26, respectively. Model I clearly fails to account for the experimental points, which indicates that the electron injection from the H-aggregate is needed to understand the observed behavior. On the other hand, model II accurately describes the experimental data, giving rate constant values of 5.1 × 1011 and 9.1 × 1011 s−1 for the electron injection from the monomers for SQ 41, and SQ 26, respectively. Timeindependent spectra for each species obtained from model II are shown in Figure S8 (Supporting Information). The three spectra turned out to be the expected ones for the three species: excited state of the monomers and of the H-aggregates and radical cation of the SQs. Regarding the injection from the H-aggregates, rate constants of 1.1 × 1011 and 1.9 × 1011 s−1 for SQ 41 and SQ 26, respectively, were deduced from the fits. It is remarkable that the electron injection is about 5 times faster for the monomers in relation to the H-aggregates. This behavior has been described in the literature,33 which was explained on

the basis of a poorer electronic coupling with the TiO2 or a weaker driving force for the injection reaction in the Haggregates. We find that reactions 2 and 3 are coupled, making it difficult to get an accurate value for γ′ [(5−30) × 10−15 cm3·s−1/2]. In the global analysis, we have considered that the initial concentration of H-aggregates is 2 times higher than that of the monomers, which is deduced from the deconvolution of the steady-state absorption spectra by using two Gaussian functions. We estimate that approximately between 15 and 25% of the total absorbed photons are lost due to the annihilation process.56 In the hypothetical situation in which this relative ratio was much higher or even no monomers were remaining, the percentage of losses due to the annihilation process could be as large as 40%, supposing that the same rate constants are still correct in this new situation. It is worth remarking that the higher the concentration of aggregates in the sample, the closer the aggregates are, resulting in an acceleration of the annihilation process and therefore an increase in the percentage of losses. Thus, despite that electron injection has been demonstrated to occur also through the excited state of the Haggregates to the TiO2 NP, the fast annihilation process is responsible for a high percentage of losses in DSSCs and, therefore, is intended to be avoided to increase the photon-toelectron conversion efficiency. Thus, addition of a deaggregating agent, like CDCA, is compulsory to minimize the losses caused by the annihilation mechanism. Nevertheless, for other organic sensitizers, formation of aggregates does not always result in a negative effect for the efficiency of the solar cell.31,32 The key point is whether the aggregates are able to inject electrons to the TiO2 conduction band or whether there is a 12143



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Figure 7. Femtosecond transient absorption spectra of SQ 41 (A) and SQ 26 (B) adsorbed on a TiO2 nanoparticle thin film with the CDCA coadsorber at four time delays. Decays of the transient absorption intensity of the same samples, SQ 41 (C) and SQ 26 (D), with best fits at the time window of 50 ps. The excitation wavelength was 640 nm.

competitive process (annihilation) that might reduce the efficiency of this process, provoking losses in the injection. To summarize this section, SVD analysis of the femtosecond transient absorption data allowed us to identify the transient signatures for three species in the SQs adsorbed on TiO2 NP thin films: singlet excited state of the monomers and of the Haggregates and the radical cation of the SQs. The latter species implies that the electron injection to the conduction band of the TiO2 takes place in these samples. We also observed a singlet−singlet annihilation process between the H-aggregates, since the kinetics is faster upon increasing the pump fluence. A global analysis of the data was performed on the basis of two reacting models, confirming that the electron injection also takes place from the H-aggregates, but with a smaller kei compared to that from the monomers. Particularly, we obtained kei = 5.1 × 1011 s−1 from the monomer and 1.1 × 1011 s−1 from the H-aggregates for SQ 41. We estimated that over 15−25% of the absorbed photons are wasted through the annihilation reaction, which is competing with the injection from the Haggregate. 3.2.2. SQs Adsorbed on TiO2 Nanoparticle Thin Films with CDCA. In this section, we discuss the ultrafast dynamics of the SQ molecules adsorbed on TiO2 NP thin films in the presence of CDCA as coadsorber. The steady-state absorption spectra showed that the CDCA deaggregating agent prevents the formation of SQ aggregates. In these time-resolved absorption experiments, we confirmed the absence of excited H-aggregates and assessed the observed electron injection photodynamics exclusively from the SQ monomers. To begin with the results, Figures 7A,B and S9A (Supporting Information) illustrate the transient absorption spectra gated at four different times for SQ 41, SQ 26, and SQ 4, respectively. We observed positive transient absorption features for both SQs in the region between 450 and 575 nm. In particular, at earlier times (0−4 ps) we observed an asymmetric peak with an

intensity maximum at 510 nm and a shoulder around 475 nm for SQ 41 and a symmetric peak with the maximum at 500 nm for SQ 26 and SQ 4. These signals are assigned to the singlet excited states of the monomers, since they are coincident with those obtained in solution.48 However, the shape of the initial transient absorption features is not changing with the pump− probe time delay, unlike the behavior for the samples without CDCA. This implies that the initial singlet excited state of the monomers directly and solely decays to produce the radical cation without intermediate species or parallel reactions. In the near-IR region, we clearly observed two negative signals: a strong peak around 655 nm and a week band centered at 710 nm. The former is ascribed to the ground-state bleach of the monomers, since this matches the steady-state absorption band, while the latter is attributed to a stimulated emission from the S1 state of the monomers, as their emission takes place at these wavelengths. Evolution of those signals gave rise to an asymmetric band peaking around 580 nm (dotted-dashed line in Figures 7A,B and S9A, Supporting Information) for the studied SQs. We attributed these signals to the radical cation of the SQs (which is photoinduced by an electron injection from the SQ excited state to the conduction band of the TiO2 NP), the same as observed in the previous section without CDCA. We have also applied the singular value decomposition analysis to these data. Clearly, only the kinetics and spectral vectors associated with the first two singular values depict real behaviors, since the other vectors show either unstructured shape or noisy distribution. We also performed global analysis of the data by using a two-exponential function and an extra component as an offset to account for the signal that does not decay in the experimental time window. Parts A and B of Figure 8 show the associated spectra for each component for SQ 41 and SQ 26, respectively. In both SQs the two fast components exhibit similar spectra, which are not different from the transient ones of the singlet excited state of the monomers, 12144



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exponential function, which is accounted for the exponential density of states of the TiO2 conduction band: f (x) = A exp[( −t /τ )β ]

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where τ is a characteristic (mean) time of the decay and β (typically 0.3−0.65)57 reflects the heterogeneity of the sample related to the exponential distribution of trap states in the TiO2 (mathematically, this parameter can have values between 0 and 1; β = 1 is for a perfect homogeneous system, and a smaller value indicates more dispersive kinetics). The nonexponential kinetics can be also attributed to a distribution of different sensitizer binding modes. The fit of the data at 520 nm using eq 4 gave lifetimes of 1.5, 1.3, 1.4, and 1.5 ps for SQ 41, SQ 26, SQ 4, and SQ 2, respectively, with a β value of 0.45. The inverse of the times gives the mean values of the electron injection rate constants, kei, 6.7 × 1011, 7.7 × 1011, 7.1 × 1011, and 6.7 × 1011 s−1, respectively. Recently, Kamat et al. observed for a similar indole-based SQ the transient features of the radical cation within 1 ps following the laser pulse excitation.54 The latter result gave an electron injection rate constant of ∼1012 s−1. In our case, we were able to deduce an accurate value of kei from the decay of the singlet excited state for each SQ. On the other hand, the driving force for the electron injection, ΔGei, has been reported to vary largely among the four studied SQs (from −0.15 to −0.8 eV for SQ 26 and SQ 4, respectively) due to the different energy of the LUMO orbital.36,39,40 According to the Marcus theory, given the same semiconductor acceptor species (TiO2), the kei should only depend on the ΔGei and the electronic coupling, V, between the LUMO orbital of the SQs and the TiO 2 conduction band.58 However, we did not observe significant changes in kei for the different SQs: a 0.1 eV more negative value of ΔGei should give an ∼3-fold increase in injection dynamics.58 We believe that, in our SQs, a strong electronic coupling governs the dynamics of the electron injection, which is then no longer dependent on the energy of the injecting state. This situation has been previously reported for the injection from the singlet excited state of Ru complexes (kei ≈ 1012 s−1), and it was assigned to a barrierless electron transfer in the adiabatic limit.59 In our samples, kei is not as large as in the Ru complexes, which suggest that even though the coupling is strong enough to govern the injection, we are in an intermediate situation between the adiabatic and nonadiabatic limit. This result is particularly important for the development of sensitizers with red-shifted absorption spectra, like our SQs. In these sensitizers, the low optical band gap usually entails a small difference between the energy of the excited state of the dye and the conduction band of the TiO2, which therefore slows down the electron injection.58 However, the strong electronic coupling in our SQs implies an ultrafast electron

Figure 8. Associated transient absorption spectra of SQ 41 (A) and SQ 26 (B) adsorbed on a TiO2 NP thin film with the CDCA coadsorber obtained from a multiexponential global analysis of the data. Time constants for each spectrum are shown in the legend. Transient absorption spectra in ACN solution are shown as a comparison (dotted lines).

while the long-living component spectrum is identical to that of the SQs radical cation. Therefore, we conclude that only two transient species are present in these samples: the singlet excited state of the monomers and the corresponding radical cation. The two-exponential function fit is used to “simulate” the nonexponential (stretched exponential) decay of the monomer (vide supra). Clearly, the presence of the CDCA is preventing the formation of the SQ aggregates. This was confirmed by transient absorption experiments on complete cells sensitized with SQ 41 having CDCA using different pump fluences (0.1−1 μJ/pulse), where we observed no singlet− singlet annihilation process (Figure S10, Supporting Information). Table 2 shows the lifetime values obtained from the fit of the decays at five representative wavelengths (Figures 7C,D and S9B, Supporting Information). The multiexponential behavior indicates a complex mechanism for the electron injection process, a common situation in these heterogeneous systems. In fact, electron injection is usually modeled by a stretched

Table 2. Lifetimes and Relative Amplitudes Obtained from Exponential Local Analyses of the Decays of the Transient Absorption Intensity at Different Probing Wavelengths of the SQ 41, SQ 26, SQ 4, and SQ 2 Adsorbed on a TiO2 NP Thin Film with the CDCA Coadsorber SQ 41

SQ 26

SQ 4

SQ 2

λobs/nm

τ1/ps

a1/%

τ2/ps

a2/%

τ1/ps

a1/%

τ2/ps

a2/%

τ1/ps

a1/%

τ2/ps

a2/%

τ1/ ps

a1/ %

τ2/ps

a2/%

470 510 570 620 665

0.8 1.3 0.8 1.0 1.1

58 67 −55 −51 −65

9.0 13 7.5 10 12.5

42 33 −45 −49 −35

1.0 1.0 0.8 1.4 3.8

70 60 −63 −51 −54

16 14 17 15 26

30 40 −37 −49 −56

1.0 1.3 1.1 0.7 0.9

51 58 −54 −58 −55

7.7 8.2 10 11 9.1

49 42 −46 −42 −45

1.4 1.5 1.3 1.7 1.8

75 73 −61 −81 −84

10 20 14 30 16

25 27 −39 −19 −16

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injection independently on the ΔGei. In fact, a value as small as −0.15 eV in SQ 26 led to an efficient electron injection. Finally, we noticed that the kei are similar to those obtained from monomer injection in the samples without CDCA, reflecting that the presence of H-aggregates did not modify the electronic coupling (i.e., orientation/organization) of the SQ monomers. It should be pointed out that the kei values correspond to the electron injection of the SQs adsorbed on TiO2 NP thin films without the presence of the redox electrolyte or additives (Li+ or tert-butylpyridine, TBP), which can slightly modify the kinetics.60 In summary, for the studied SQs coadsorbed with CDCA on TiO2 NP thin films, we have detected the transient absorption features of the singlet excited state, S1, of the monomers and the corresponding radical cation. The fast decay of the S1 gave the rate constants for the electron injection, 6.7 × 1011, 7.7 × 1011, 7.1 × 1011, and 6.7 × 1011 s−1 for SQ 41, SQ 26, SQ 4, and SQ 2, respectively. These values of kei coincide with those obtained for injection from the monomer in the samples without the CDCA, which confirms that the strong electronic coupling of monomers is not affected by the presence of the Haggregates. Finally, the wide variation of ΔGei in the studied SQ molecules (from −0.15 to −0.8 eV) is not reflected in large changes of kei, owing to the key factor of the electron injection being the strong electronic coupling. 3.3. Nanosecond Flash Photolysis Measurements. The transient feature of the radical cation of the SQs is the only signal detected at the end of the time window (1.7 ns) of the femtosecond transient absorption experiments discussed in the previous section. Thus, in the absence of other reducing species, the disappearance of the oxidized SQ molecules (SQ•+) occurs through the recombination reaction with the electrons located in the conduction band and trap states of the TiO2 nanoparticles (eq 5). This process normally happens in the nanosecond−millisecond time regime, and therefore we studied it by using the flash photolysis technique. Investigating the samples without the I−/I3− electrolyte is necessary to determine the charge recombination dynamics, since the presence of the electrolyte in a complete solar cell regenerates more rapidly the SQ•+, hampering the determination of the rate constant for the charge recombination (kCR). First, we consider the simplest scenario, i.e., samples with the CDCA molecule coadsorbed onto the TiO2 surface, which ensures the absence of Haggregates. SQ•+ + e TiO2 → SQ

Figure 9. (A) Nanosecond transient absorption spectra of SQ 41 adsorbed on a TiO2 NP thin film with CDCA at 1, 20, and 80 μs time delay after the laser pulse. (B) Decay of the transient absorption intensity at 540 nm of the previous sample with the four SQs and best fits of the experimental data. The excitation wavelength was 640 nm.

wavelengths resulted in no change in the kinetics, which confirms the exclusive recombination channel for the studied SQs. The use of a stretched function for the fits is consistent with a recombination mechanism that involves direct electron transfer from an exponential distribution of energy states in the conduction band and trap states within the band gap, in addition to different orientations of the SQs onto the TiO2 surface. The lifetimes obtained for the different SQs are similar to each other: 150, 115, 110, and 94 μs, reflecting kCR of 6.7 × 103, 8.7 × 103, 9.1 × 103, and 10.6 × 103 s−1 for SQ 26, SQ 4, SQ 41, and SQ 2, respectively. In a first approximation, the similar recombination kinetics could be surprising, regarding the different values for the driving force (ΔGCR) for each SQ, −1.7, −1.5, −1.4, −1.1 eV for SQ 26, SQ 41, SQ 2, and SQ 4, respectively.36,39,40 However, it has been previously found that the recombination dynamics can be relatively insensitive to changes in ΔGCR when it is controlled by the trapping and detrapping events occurring in the conduction band; i.e., the electron transport in the TiO2 nanoparticle is the limiting step for the recombination.61 Another possible explanation is that the electronic coupling between the trap states/conduction band in the TiO2 and the HOMO orbital in the SQs is the factor governing the process, being very similar for the four studied SQs. Now, we consider the samples without the CDCA coadsorber and, therefore, with the presence of H-aggregates. We registered transient absorption spectra similar to those of the samples with the CDCA. Thus, the same type of radical cation of the SQs is formed with and without the deaggregating agent. Parts A and B of Figure 10 illustrate the decay profiles at 540 nm for the SQ 41 and SQ 26 adsorbed on the TiO2 nanoparticle thin film with (blue squares) and without (green

(5)

Figure 9A shows the transient absorption spectra at three different time delays of SQ 41 adsorbed on the TiO2 NP thin films following excitation at 640 nm. These spectra resemble the ones obtained previously at long time delays in the femtosecond transient absorption experiments. The bleaching band at 625 nm reflects the signature of the steady-state absorption peak of the SQs. The two positive absorption bands with maxima at 540 and 730 nm are therefore assigned to the radical cation of the SQs. It is worth noting that the bleaching band is clearly overlapping with the transient signature of the radical cation. Thus, the two positive bands correspond to the side regions of a larger peak, the radical cation of the SQ, which should be centered at the bleaching region. Figure 9B depicts the decays for the different SQs recorded at 540 nm together with the best fits (solid lines) by using stretched exponential functions (eq 4). Fitting the data at other 12146



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SQs. The electron injection was proved to occur from monomers but also from H-aggregates, e.g., keimon = 5.1 × 1011 s−1 and keiH‑agg = 1.1 × 1010 s−1 for SQ 41. We conclude that around 15−25% of the absorbed photons are wasted through the annihilation reaction, which competes with the injection from the H-aggregates. It is noteworthy here that the annihilation process is strongly active in the SQ molecules due to the good overlap between absorption and emission spectra, which favors the Förster-type energy transfer mechanism responsible for the annihilation process. Thus, formation of aggregates in SQs involves a negative effect in the power conversion of the DSSC. In the samples coadsorbed with CDCA, we have only detected the presence of two transient species: the singlet excited state of the monomers and radical cation of the SQs. The rate constants for the electron injection, kei, are very similar for the different SQs, despite the different driving force (ΔGei), e.g., 6.7 × 1011 and 7.7 × 1011 s−1 for SQ 41 (ΔGei = −0.45 eV) and SQ 26 (ΔGei = −0.15 eV), respectively. We believe that a strong electronic coupling governs the dynamics of the electron injection, which is then no longer dependent on the energy of the injecting state. This result is particularly important for the development of sensitizers with red-shifted absorption spectra, since the LUMO energy is normally close to the conduction band of the TiO2. Thus, the strong electronic coupling in our SQs implies an ultrafast electron injection independent of the ΔGei. In fact, a value as small as −0.15 eV in SQ 26 led to an ultrafast electron injection. We obtained similar values for the charge recombination rate constants, kCR, among the studied SQs, despite the different ΔGCR, for example, 6.7 × 103 and 9.1 × 103 s−1 for SQ 26 and SQ 41 adsorbed on TiO2 NP thin films with CDCA, respectively. The former could be attributed to the electronic coupling parameter governing the kinetics of recombination or the electron transport in the TiO2 nanoparticle being the limiting step. Finally, kCR for the samples with H-aggregates, without the CDCA coadsorber, is 2−3 times larger for the studied SQs compared to the samples with CDCA. This is ascribed to a stronger electronic coupling between the HOMO orbital of the H-aggregates and the trap states/conduction band of the TiO2 NP in comparison with samples containing only monomers.

Figure 10. Decays of the transient absorption intensity at 540 nm of the SQ 41 (A) and SQ 26 (B) adsorbed on a TiO2 NP thin film with CDCA (blue squares) and without CDCA (green diamonds) and best fits of the experimental data. The excitation wavelength was 640 nm.

diamonds) the CDCA coadsorber. It is quite interesting that kinetics are faster in the samples without the CDCA. In particular, we measured lifetimes of 30, 77, 45, and 84 μs for SQ 26, SQ 4, SQ 41, and SQ 2, respectively. It is worth noting that in Figure 10, the recombination dynamics of SQ 26 is more sensitive to the coadsorption of CDCA, which is explained by the larger concentration of H-aggregates in SQ 26 compared to that in SQ 41 (Figure 2). This shortening in the recombination lifetimes in the presence of the H-aggregates might be explained by changes in the oxidation potential of the H-aggregates or a different coupling between the HOMO orbital and the trap states/conduction band of the TiO2 nanoparticles, consistent with a higher delocalization of excitons and different dye coverage.62 Concluding this part, the transient signature of the radical cation of the SQs is detected as two peaks with maxima at 540 and 730 nm. We found no significant differences in the kCR among the studied SQs adsorbed on TiO2 NP thin films despite the different ΔGCR. The former could be attributed to the electronic coupling parameter governing the kinetics of recombination or the electron transport in the TiO2 nanoparticle being the limiting step. Regarding the samples having the H-aggregates, the recombination kinetics are 2−3 times faster, a result that is explained in terms of a stronger electronic coupling.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S10. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address §

Faculty of Physics, A. Mickiewicz University, Umultowska 85, 61−614 Poznan, Poland. Notes

The authors declare no competing financial interest.

4. CONCLUSION We found that SQ molecules appear as monomers in the samples with the CDCA coadsorber but with about 66% percentage of H-aggregates when CDCA is not added. In the samples without CDCA, we identified three transient species from the femtosecond transient absorption experiments: S1 of the monomers and of H-aggregates and the radical cation of the

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ACKNOWLEDGMENTS This work was supported by the MICINN through project PLE2009-0015. M.Z. thanks the support from the European Community’s Seventh Framework Programme (FP7/20072013) under grant agreement no. 235286 (NANOSOL) and 12147



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G.M. is grateful to MICINN for a “Juan de la Cierva” contract. We thank Dr. M. Zitnan for his help at the beginning of the experimental part.

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