Dynamics of a Covalently Conjoined FRET Dye Ensemble for Electron

Article pubs.acs.org/JPCC

Dynamics of a Covalently Conjoined FRET Dye Ensemble for Electron Injection into ZnO Nanorods Robert Schütz,†,∥ Shashwat Malhotra,‡,∥ Inara Thomas,‡ Christian Strothkam ̈ per,† Andreas Bartelt,† Klaus Schwarzburg,† Thomas Hannappel,§ Carlo Fasting,‡ and Rainer Eichberger*,† †

Helmholtz Center Berlin for Materials and Energy, Hahn-Meitner-Platz 1, 14109 Berlin, Germany Institut für Chemie und Biochemie, Freie Universität Berlin, Takustrasse 3, Berlin 14195, Germany § Ilmenau University of Technology, Gustav-Kirchhoff-Straße 5, 98693 Ilmenau, Germany ‡

ABSTRACT: A low molecular weight purely organic Förster resonance energy transfer (FRET) sensitizer system combining multiple chromophores into a single molecule via covalent attachment is designed to adjust the ratio and relative position of the fluorol donor and coumarin acceptor units. A phenyl based scaffold accommodates both FRET partners and is connected to a carboxylic anchor group for adsorption on a semiconductor surface. The functionality of the complete ensemble is demonstrated by UV−vis and luminescence lifetime measurements. The energy and charge transfer dynamics of the FRET assemblies in solution and adsorbed on ZnO nanorods are measured by femtosecond transient absorption spectroscopy. Optical pump THz probe (OPTP) spectroscopy is applied to detect the injected electrons in the ZnO electrode, confirming the full electronic pathway from FRET light harvesting to subsequent charge carrier injection.

1. INTRODUCTION Ever since the breakthrough in conversion efficiency (∼11%) with ruthenium-based photosensitizers,1−3 dye-sensitized solar cells (DSC) have attracted much attention in the past 2 decades owing to their low production cost (compared to silicon based solar cells).4 Also, dye sensitized hydrogen evolution from water5 using molecular light absorbers has recently become a topic, complementing solar light absorption with the possibility of fuels generation and energy carrier storage. By use of TiO2 colloidal networks or ZnO nanorod6,7 electrodes that exhibit enhanced effective surface area, only a monolayer of molecular absorbers is needed for light absorption. However, because of limited resources, toxicity issues, and high production costs associated with Ru metal itself, Ru-free organic dyes have been considered to be a potential alternative for the development of low-cost DSCs.8−12 Organic sensitizers such as coumarins, indolines, cyanines, and others for DSCs in this context are an attractive alternative to Ru sensitizers with properties rather easily tunable by facile structural modifications. In addition, they generally have much higher extinction coefficients when compared to Ru-based photosensitizers, often higher than 100 000 M−1 cm−1.13 However, there are a number of issues that limit the use of organic dyes such as narrow absorption bands, aggregation, long-term stability, and less availability of organic dyes absorbing in the red spectrum. To expand the absorption bandwidth of organic based DSCs, cosensitization using different complementary absorbing dyes has been applied. The major drawback of simultaneous application of several dyes is the competition of individual dyes with the limited binding sites on the semiconductor.14 To address this problem, dyes with different anchoring groups have been investigated that do © 2014 American Chemical Society

not compete for identical absorption sites, as was reported just recently by Ogura et al.15 Alternatively, linearly linked dye systems have been reported to essentially avoid the need for more than one binding site.16−18 Another approach involves relay systems where unattached donor dyes solubilized in the electrolyte transfer their absorbed energy via fluorescence resonance energy transfer (FRET, Förster resonance energy transfer)19−22 to a sensitizing dye, which also occupies only one binding site on the semiconductor material. Following this concept of noncovalently coupled dye systems, high concentrations of soluble donor dyes are required to achieve an efficient FRET. This implicates the need of substantial amounts of materials especially in larger solar cell modules, which is not desirable. Along with this, there could be issues with solubility and aggregation behavior using low-boiling polar solvents. In the present study, we propose a unification of multiple chromophores into a single molecule via covalent attachment to control the ratio of donor and acceptor FRET dyes and also their relative positions to each other (Figure 1). Absorption of photons of a broader energy range by such a conjoined dye system implies a more efficient light harvesting compared to a simple mixture of separate dyes. The donor dye contributes with an additional spectral input from the higher energy photons and feeds supplemental energy via FRET into the acceptor dye which finally injects electrons into the semiconductor. Only the acceptor dye has to be regenerated by a suitable redox system in a working DSC. Received: January 30, 2014 Revised: April 7, 2014 Published: April 9, 2014 9336

dx.doi.org/10.1021/jp501062e | J. Phys. Chem. C 2014, 118, 9336−9345

The Journal of Physical Chemistry C

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molecular donor and acceptor to subsequent electron injection into the metal oxide electrode via the FRET acceptor unit. We believe that these results may open new avenues for understanding the related mechanisms in energy transfer based optical devices and DSCs.

2. EXPERIMENTAL SECTION 2.1. Synthesis of FRET System and Reference Compounds. Regarding the synthesis of the FRET probe, we had first started the synthesis of donor azide. Commercially available 4-(N,N-dimethylamino)-1,8-naphthalic anhydride was reacted under refluxing conditions with 3-azidopropylamine using tert-butanol as a solvent to obtain donor azide in 70% yield. We then proceeded with the synthesis of acceptor alkyne. Commercially available coumarin-153 was first formylated via Vilsmayer−Hack aldehyde synthesis using N,N′-dimethylformamide and POCl3 at 80 °C to obtain formyl coumarin-153. This was then reduced to the primary alcohol using sodium borohydride in methanol at room temperature, and the crude product was directly used for the synthesis of coumarin-153 phosphonium bromide salt by reacting it at room temperature with carbon tetrabromide and triphenylphosphine in chloroform. In parallel, commercially available 3-formylbenzoic acid was brominated at meta-position using NBS followed by tertbutyl protection by di-tert-butyl dicarbonate and 4-N,N′dimethylaminopyridine in tert-butanol to obtain tert-butyl 3bromo-5-formylbenzoate. We then applied a Sonogashira reaction with this compound using mono-TMS protected acetylene, which was followed by Wittig reaction with the initially prepared coumarin-153 phosphonium bromide salt. The TMS protecting group was removed by alkali treatment to furnish the acceptor alkyne. Both donor azide and acceptor alkyne were then coupled at 70 °C using the copper-catalyzed assisted azide−alkyne cycloaddition (CuAAC, “click reaction”),28,29 followed by tertbutyl deprotection in TFA−DCM mixture (1:1) to furnish the final FRET compound sys-3. Besides the synthesis of FRET probe sys-3, we also synthesized a donor reference (don-1, truncated system with donor fluorol dye only) and an acceptor reference (acc-2, truncated system with coumarin-153 acceptor dye only) as shown in Figure 1. The structures of already known compounds were confirmed by comparing their spectroscopic data reported in the literature. The structures of all new intermediate and final compounds used in this study were fully characterized and analyzed by spectroscopic techniques. The synthesis will be published elsewhere. 2.2. Preparation of Dye-Sensitized Nanorod Substrates. Electrodes with a dense package of ZnO nanorods were grown on a ∼40 nm ZnO seeding layer applied by spray pyrolysis using an alcoholic Zn(NO3)2 solution. As substrates, 50 μm alkaline-free glass, 500 μm alkaline-free glass, and 1 mm crystalline quartz were used for femtosecond transient absorption, stationary spectroscopy, and time-resolved THz spectroscopy, respectively. The nanorods were prepared using two cycles of chemical bath deposition in an aqueous solution of Zn(NO3)2 (0.01 M) and NaOH (0.4 M) at 80 °C. Their average length and mean diameter were about 2.0 μm and 60 nm, respectively, with the nanostructured electrode yielding a surface magnification of roughly 50 compared to a planar electrode. ZnO nanorods prepared by chemical bath deposition from NaOH solution grow as hexagonal crystals elongated

Figure 1. (Top) Scheme of the covalently conjoined FRET dye ensemble adsorbed on ZnO nanorods. (Bottom) Structural formulas of the coupled dye system sys-3, donor reference don-1, and acceptor reference acc-2.

We hereby report a coumarin-based donor−acceptor FRET light absorber probe by covalently linking the donor dye to the acceptor in 1:1 ratio. This is accomplished by attaching both molecular units to a phenyl based scaffold which in turn is equipped with a carboxylic anchor group that binds the ensemble to a semiconductor surface and allows for electron transfer from the acceptor excited state. ZnO is considered a potential replacement for TiO2 in DSCs, since it has higher bulk mobility and can be grown in the form of well-aligned crystalline nanorods having direct contact to the back electrode, facilitating carrier collection compared to the random colloidal TiO2 structures.23,24 Recent studies have shown that overall DSC performance can de facto be boosted by specially designed energy coupled antenna dyads.25,26 This study focuses on the intramolecular FRET-driven energy flux upon photoexcitation using model chromophores that allow for time-resolved spectroscopic studies. The novel scaffold design carrying the antenna system is well suited for the substitution of the model compounds against efficient donor−acceptor combinations. For a combined investigation of energy transfer via FRET and sequential electron injection into a ZnO nanorod electrode we performed steady-state UV−vis, picosecond fluorescence lifetime, and femtosecond transient absorption (TA) measurements. Complementary optical pump THz probe (OPTP) spectroscopy27 was applied to detect the injected electrons in the semiconductor conduction band. The dynamics of the FRET system was measured in solution and for adsorption on ZnO and compared to the dynamics of truncated precursor systems containing solely either the donor or acceptor unit. The time-resolved studies reveal for the first time the complete pathway from energy transfer between the 9337



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Figure 2. Absorption and emission spectra in CHCl3: (a) absorbance spectra of sys-3, don-1, and acc-2 reference compounds; (b) spectral overlap of don-1 and acc-2; (c) fluorescence spectra of don-1 and acc-2 in a 1:1 molar ratio; (d) fluorescence spectra of pure sys-3 at different excitation wavelengths; (e) normalized absorbance spectra of the 1:1 mix and sys-3.

along the c-axis.30 The dyes adsorb on the nonpolar (1010̅ ) surface, which is the most exposed surface of the hexagonal ZnO structures.31,32 The dye sensitization of the ZnO nanorod samples was done in a wet-chemical process. Samples were heated in a muffle furnace in air at ∼430 °C to guarantee a clean and water-free surface. Still hot, the samples were quickly transferred into nitrogen atmosphere for cooling and then put into lockable vessels with CHCl3 (5 × 10−5 M, spectroscopic grade, Uvasol, Merck) dye solutions without further contact with ambient air. The vessels containing the dye solutions and the samples were kept in darkness on a shaker for about 1 h. The colored samples were subsequently rinsed with pure CHCl3 for 5 min to avoid unattached dye molecules laying on the ZnO surface and then dried with a nitrogen stream. The freshly prepared samples were stored in a dark inert atmosphere for a few days only or directly transferred to the experiments to avoid degradation of the samples. 2.3. Time Resolved Spectroscopy. Transient absorption was performed using a Coherent RegA 9050 regenerative amplifier laser system as a front end pulsed light source that was distributed by beam splitters to the different nonlinear wavelength conversion setups used for the pump/probe experiments. The fundamental at 800 nm provided pulses with a fwhm of 50 fs and 7 μJ intensity at 150 kHz repetition rate. A home-built near-infrared noncollinear optical parametric amplifier (NOPA) in combination with a subsequent harmonic generation stage was used to produce the pump wavelength at 430 nm.33 Transient absorption was measured by lock-in with photodiode detection using a monochromator for wavelength selection. A small part of the regenerative amplifier output was split off and focused into a 3 mm sapphire crystal to generate a 450−1000 nm white light continuum (WLC) as probe pulse when broadband probing was applied. The time resolution was

around 50−100 fs fwhm depending on the probe wavelength. For optical pump terahertz probe (OPTP) spectroscopy the fundamental beam was split into three branches to generate THz radiation by optical rectification in a 1 mm thick ZnTe crystal to excite the sample using a third home-built NOPA and to detect the THz absorption change by electro-optic sampling.34 The THz radiation was collected and collimated by parabolic mirrors and focused to a spot of about 1.2 mm at the sample position. TA measurements were performed in UHV in absence of any dye regenerating species normally present in working DSCs to avoid degradation of the molecular compound during laser illumination. Time resolved photoluminescence with a resolution of about 100 ps was recorded in a home-built apparatus applying single photon counting (SPC) using an avalanche photodiode detector module from Micro Photon Devices.

3. RESULTS AND DISCUSSION 3.1. Steady-State Optical Characterization. The absorption and emission spectra of all studied dyes measured in a 10−5 M fresh CHCl3 solution are shown in Figure 2. The spectrally extended and higher absolute absorbance of sys-3 as depicted in Figure 2a results from a proportional superposition of the individual spectral parts belonging to the donor and acceptor moiety. At the individual absorption maxima the extinction of the don-1 reference dye is about 2−3 times lower than the acceptor reference acc-2 and would rather be situated in the range of metal−organic ruthenium based dyes.16 Preferably a higher extinction in the range of the acceptor unit would be desirable, but nevertheless, a clear enhancement of the overall absorption characteristics can be seen for the model compound sys-3. Figure 2b graphically shows the predicted useful spectral area for energy transfer resulting from 9338



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the spectral overlap of the don-1 emission and acc-2 absorption bands. A red shift of about 40 nm for the acceptor band would lead to the desired broader spectral absorption range. However, the design of the scaffold body allows for relatively uncomplicated substitution of the chromophore system with superior dye combinations considered for future synthesis. An improvement could be expected by substitution of the donor− acceptor units by, for instance, perylene derivatives. Efficient FRET has recently been reported for a linear arrangement of a perylene bisimides based dyad.35 Another promising approach would be to attach a second light absorbing donor unit to the phenyl scaffold. Also a greater spectral separation for the quite closely neighboring donor and acceptor bands would be eligible for an increased light absorption and expected efficient FRET. But here we stress that our model compounds were first of all synthesized for probing the dynamics of the FRET reaction and the ability of our covalently linked system to transfer its excitation energy to a semiconductor electrode by subsequent electron transfer. This will be discussed later in the timeresolved characterization section. Emission spectra were recorded in CHCl3 (5 × 10−6 M) by exciting at different wavelengths starting from the higher absorption wavelength at 530 nm down to 350 nm. Figure 2c shows a 1:1 binary mixture of don-1 and acc-2 CHCl3 giving rise to two separate fluorescence peaks at 507 and 564 nm according to the excitation wavelength which could be assigned to the emission of the individual compounds. On the other hand, excitation of sys-3 from 350 to 530 nm (see Figure 2d) showed only a single fluorescence peak at 564 nm in the emission spectrum being identical with the emission maximum of acc-2, while emission in the region of the emission maximum of don-1 (507 nm) is suppressed. This suggests that virtually all of the fluorol donor chromophore fluorescence is quenched because of FRET to the coumarin acceptor chromophore for the covalently coupled dye system sys-3. Other quenching mechanims such as intramolecular electron or hole transfer are rather unlikely because of the spatially separated molecular orbitals of either donor and acceptor moiety in this molecular design (DFT calculations, data not shown). A putative hole transfer mechanism would not explain generation of acceptor emission at solely donor exitation wavelengths. Also, donor to acceptor electron transfer within the compound is ruled out for the same reason. The normalized absorbance of the covalently coupled FRET system is practically identical with the absorbance of the 1:1 mixture of the reference dyes as shown in Figure 2e. This means that the individual reference contributions are conserved and electronically decoupled in the assembled donor−acceptor compound. However, as would be expected, sys-3 has a higher combined absorbance than both individual reference compounds and a potentially improved spectral utilization of the sunlight. The freshly prepared dye-sensitized ZnO nanorod substrates were characterized by stationary UV−vis spectroscopy applying an integrating sphere. The absorbance spectra (Figure 3) were calculated from transmission and reflection measurements of the individual dye-loaded samples, while the absorbance of a clean ZnO nanorod sample was subtracted, since the absorption of ZnO itself (Figure 3, inset) dominates in the UV region. From these spectra an improvement of light harvesting efficiency can be seen for the coupled dye system sys-3 in comparison to the individual compounds don-1 and acc-2: first, an enhancement of absorbance from about 0.35 for the individual compounds to approximately 0.5 for sys-3;

Figure 3. Absorbance of dyes attached to the ZnO nanorod surface corrected by the absorbance of the ZnO nanorods themselves. The inset shows the absorption properties of the prepared ZnO nanorod films without dyes.

second, by the broadening of the absorption region combining the absorption properties of don-1 and acc-2. Even though an addition of the optical densities of the individual compounds could be expected, the slightly reduced absorbance compared to the anticipated most possible absorbance may be due to a reduced surface dye load for sys-3 connected to its spacial extension. A slightly widened absorption spectrum compared to the compounds in solution can be seen that points to a strong electronic coupling to the semiconductor surface presumably accompanied by inhomogeneous broadening.36,37 Such strong coupling is known for dyes anchored to the semiconductor surface by a carboxylic acid group that is believed to deprotonate and bind in a bidentate way.38,39 On the semiconductor surface both don-1 and acc-2 absorption maxima are blue-shifted by about 30 nm similar to reported shifts for a number of dyes adsorbed on ZnO via a carboxylic anchor. The actual position of the absorption maxima might also be disguised by the absorption due to the ZnO band edge. Even though the maxima seem to be shifted a little bit toward higher energies, the order of the absorption maxima is kept, so FRET capability is still a given. 3.2. Time Resolved Characterization. Fluorescence lifetime measurements (Figure 4) of sys-3 and the equally concentrated truncated reference compounds don-1 and acc-2, respectively, were performed in CHCl3 by single photon counting (SPC) as described in the Experimental Section. Excited by a high repetitive laser diode at 405 nm, the truncated donor unit exhibits a lifetime of 9.4 ns in solution,

Figure 4. Fluorescence lifetime in CHCl3 measured by SPC showing the fluorescence quenching of the FRET donor moiety within sys-3 compared to the lifetimes of the truncated donor and acceptor species don-1 and acc-2, respectively. The inserted semilogarithmic plot indicates that the singular compounds do not aggregate. 9339



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Figure 5. (Top) 2D color coded transient absorption plots of don-1 (a), acc-2 (b), and sys-3 (c) in CHCl3 at identical concentrations of 10−5 M, excited at 430 nm. Red regions indicate positive, blue regions negative, and green regions no change in absorption. (Bottom) 2D color coded transient absorption plots of don-1 (d), acc-2 (e), and sys-3 (f) on ZnO nanorods in UHV, excited at 430 nm. Red regions indicate positive, blue regions negative, and green regions no change in absorption.

explained by the conformational and chemical environment differences of this system that covalently combines both donor and acceptor moieties. A minor residual trace at around 780 nm attributed to donor excited state absorption is however present, but the lifetime is considerably shorter compared to the lifetime of the isolated donor don-1 in solution. In CHCl3 not all of the excited species completely recover within the pulse repetition time of the laser system as can be seen from the pedestals at negative delay times. Such a pedestal before time zero is missing in the 780 nm range within the top plot of Figure 5 showing sys-3, which means that the lifetime of the excited donor integrated in this compound is considerably shortened. From these results it can be concluded that FRET from the donor to the acceptor unit is also the main reason for the decrease of the donor excited state signal observed in the TA measurements in solution. This agrees well with the fluorescence lifetime measurements showing the dynamic quenching of the donor fluorescence. The long recovery times found in CHCl3 are possibly due to exciplex or excimer formation.40,41 Since no significant pedestals at negative times were observed for the molecules adsorbed on the ZnO, this behavior is considered a peculiarity in solution only and will not further be discussed. All species in solution exhibit a strong (blue coded) signal at shorter wavelengths which is attributed to transient ground state bleaching (GSB) occurring mainly in the spectral vicinity of the exciting laser pulse at 430 nm. When covalently bound to the ZnO nanorods, the studied three molecular species exhibit new spectral features in addition to the excited state absorption found in solution. As shown in the bottom part of Figure 5, these are attributed to cation absorption bands that appear as electron transfer proceeds into the semiconductor when the chromophores are excited by the laser pulse. It is no surprise that don-1 can alternatively also

while the lifetime of the system complex incorporating both donor and acceptor moieties is almost identical to the lifetime of the truncated acceptor of about 3.9 ns. In agreement with the static fluorescence quenching we therefore conclude that most of the donor fluorescence is suppressed by the acceptor moiety through direct energy transfer by dipole−dipole interaction in the sys-3 complex as a consequence of efficient FRET. To study both energy and electron transfer dynamics in the truncated reference molecules and the fully assembled FRET system, we adsorbed the respective molecules onto the ZnO nanorod electrodes from the same batch as described in the technical section. When attached to the semiconductor the molecules can perform ultrafast electron transfer from their excited state, which is much faster than the fluorescence process on the one hand. On the other hand fluorescence lifetime of remaining noninjecting dye molecules is reduced far below the time resolution of the SPC apparatus because of the changed chemical environment of the molecules. We therefore performed femtosecond transient absorption (fs TA) measurements comparing the dynamics of all the three species adsorbed on the ZnO nanorods as well as in solution. A broad femtosecond white light continuum (WLC, see Experimental Section) was used here as the probe pulse to first locate the spectral position of the excited state absorption in CHCl3 in the absence of interfacial electron transfer. Figure 5 depicts color coded 2D plots containing both the dynamics within the first 5 ps and the corresponding spectral behavior that was simultaneously recorded after excitation at 430 nm. The plots reveal excited states (red spectral regions) centered at 780 and 570 nm for don-1 and acc-2, respectively, while the difference in intensity correlates well with the weaker extinction of the donor as discussed above. Sys-3 shows a strong signal essentially in the same spectral region as of the truncated acceptor excited state but with a slight blue shift that can be 9340



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spectral regions while keeping the pump wavelength fixed at 430 nm. Transients at probe wavelengths of 550 and 680 nm shown in Figure 6 within a 1.5 ns time window exhibit the

perform electron transfer into the semiconductor, since the energy accepting molecular FRET partner is missing. This is evidenced as a decrease of the excited state signal in conjunction with the coupled appearance of a cationic spectral band at around 680 nm. By design, the donor chromophore is linked to the phenyl scaffold and carboxylic anchor ensemble only via a single C−C bond with the intention to retard a possible donor electron injection into the semiconductor, whereas the acceptor unit, which itself is connected by a double CC bond, facilitates ultrafast electron transfer. It is wellknown that the sp2-hybridized nodal planes of the double bonds allow for a better electronic coupling with the semiconductor surface than sp3-hybridized single bonds.42−45 By this, we expect a slower direct electron injection pathway of the donor into ZnO, intentionally favoring energy transfer to the molecular acceptor moiety instead. The signal around 900 nm for don-1 on ZnO is believed to be related to a broadened excited state absorption due to electronic coupling to the semiconductor and is therefore also present in the corresponding sys-3 plot, whereas acc-2 does not show this spectral feature. For acc-2 in Figure 5 there is mainly only one spectral band present around 550 nm. However, small spectral tails of the cationic and excited state absorption extend to longer wavelengths that are not visible within the color code resolution of the 2D plot obtained from the WLC probe measurement. As will be discussed later, this band comprises a mixing of both the excited and cationic state absorption and possibly a small spectral overlap with the ground state absorption. The strong blue shift compared to the literature is a little surprising, as other groups have reported oxidized states for coumarin based dyes in the spectral range from 600 to 650 nm when adsorbed on a variety of metal oxide electrodes.46−48 On ZnO sys-3 exhibits a broad absorption band that incorporates both spectral features from don-1 and acc-2. However, the absorption signal around 680 nm that we relate to the cationic state of a direct electron injecting fraction of the donor moiety within sys-3 shows a very fast decay. Also, there is no sign of a longer lived excited state in this compound that could be related to the donor. The lack of a pronounced excited state absorption of the donor and no major trace of a cation absorption also suggest that on the ZnO nanorod electrode FRET is the main excited state deactivation mechanism for the donor in sys-3. However, the presence of a cationic trace that almost completely decays in this time window indicates that some donors can still inject electrons directly into the ZnO substrate without participation in FRET. Probably the configurations of some of the molecules on the semiconductor surface that are oriented unfavorably do not contribute to dipole−dipole interactions. However, this may be regarded as a virtue of the molecular antenna system, since potentially more charges can be collected by the ZnO substrate. As mentioned above, on ZnO nanorods the maxima of the cationic and excited state absorption bands for the truncated donor don-1 reference compound are spectrally separated whereas for acc-2 only one single spectral band is apparent implying state mixing. This considerably complicates the disentanglement of the electron injection dynamics. As shown in the bottom part of Figure 5, it consists of a superposition of the cation rise and the correlated excited state decay signals for the latter sample. We therefore performed TA measurements with a higher time resolution and a longer time window at specific probe wavelengths to better distinguish between these

Figure 6. Transient absorption of sys-3 and the reference compounds don-1 and acc-2 measured on ZnO nanorods at 550 and 680 nm probe and 430 nm pump wavelengths.

superimposed dynamics of the ground, cationic, and excited states. At 550 nm the negative don-1 signal is dominated by GSB with an instant rise and a slow recovery that is interpreted as a slow electron back-reaction due to the intentional poor electronic coupling via its single C−C bond attachment. At the same probe wavelength the sys-3 and acc-2 traces behave almost identically within the first 10 ps with a small deviation reflecting slightly different recombination kinetics at longer times probably owing to a negative contribution of directly injecting donors. The TA signals of sys-3 and acc-2 mainly show a fast positive rise that slows and reaches a maximum at around 1 ps. This behavior is typical for a signal with a high cationic state contribution.49 At a wavelength of 680 nm all probe traces show positive absorption and exhibit a very fast rise as well as a fast decay time that is typical for a transient dominated by the excited state.50,51 Although all studied molecular species can inject electrons into the semiconductor, the similar temporal behavior of sys-3 and acc-2 in comparison to the clearly slower decay time measured for don-1 indicates that the dynamics of sys-3 are dominated by electron injection from the acceptor moiety therein. This strengthens the assumption that in sys-3 most of the donor excitation energy is routed to the semiconductor in a sequential pathway consisting of FRET to the acceptor chromophore and electron transfer from the latter. The small differences between sys-3 and acc-2 indicate that only a minor fraction of donor excited state is converted into direct electron transfer circumventing FRET. This is also emphasized by the zero crossing and a subsequent negative signal at around 100 ps for sys-3 at 550 nm that is due to the prevailing GSB component of the donor in this complex. However, the selective comparison of sys-3 with its truncated references allows for the conclusion that the main electron injection pathway is via the coumarin unit in the fully assembled FRET absorber. 9341



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Figure 7. Early femtosecond transient absorption spectra with revealed excited (orange) and cationic (green) state dynamics from the global fitting (black) procedure for acc-2 (left and center) and don-1 (right).

values. An echo related to a second excitation from backreflected light from the glass substrates can be seen as a second signal rise after ∼0.8 ps. This inherent phenomenon is present in all TA measurements with dye coated ZnO nanorods that act as linear waveguides partially reflecting the pump pulse at the glass/ZnO interface. This effect is accounted for by expanding the global fitting model by means of an adopted extra G(t) term containing a second extenuated Gaussian-shaped excitation pulse. For the retrieval of the electron injection dynamics of acc-2, transients at 550 and 680 nm were used assuming that a potential GSB background would not contribute to significant dynamics changes in the early time window. The color coded 2D measurements (Figure 5) demand a trade-off in terms of signal sensitivity across the entire spectrum recorded with the WLC probe, and the signal contribution of acc-2 around 680 nm is hardly visible in these experiments. But the higher sensitivity for transients recorded with a narrowed spectrum at this wavelength reveals small tails of both the excited and cationic states for acc-2, allowing for a more precise determination of the injection constants (Figure 7). For don1 only the transient at 680 nm probe wavelength was considered, since this species is subject to strong GSB at 550 nm. The injection time constant for acc-2 was determined to be about 300 fs which is in the expected order of magnitude for heterogeneous electron transfer of an electronic strongly coupled dye to ZnO, while the injection constant of the electronically weaker coupled don-1 is 3−4 times slower (∼1100 fs).46,52−54 An analogous treatment for sys-3 was not considered because of the higher number of necessary parameters needed for a retrieval of this much more complex system incorporating both the donor and acceptor units. However, it is reasonable to assume that the injection dynamics of the latter compound should be similar to acc-2 with some minor deviation caused by slower injecting don-1 species that do not participate in FRET. OPTP measurements were also performed to verify that indeed electron transfer occurs from the molecular species into the ZnO nanorods upon excitation by the laser pump pulse. The terahertz probe pulse then measures the conduction band conductivity change due to the arrival of mobile bulk electrons. Figure 8 shows similar rise times for acc-2 and sys-3 according to single exponential fits on the order of 10 ps. The OPTP signals are much slower than the ultrafast appearance of the cationic state observed in the TA measurements representing the molecular view of the electron transfer reaction. This discrepancy is described as a delayed bulk injection mechanism with a slow release of surface electrons originally transferred from the absorber molecules to intermediate semiconductor

At wavelengths where GSB is not significant the femtosecond transient absorption spectra are considered a superposition of only two signals originating from the interacting excited and cationic states. The injection rate constants can then be best derived from the different transients probing either a spectral region where mostly excited or oxidized molecules are probed. A simple approach to obtain injection constants for don-1 and acc-2, respectively, is to describe this coupled state mixing by elementary rate equations. Charge carriers being located in the ground state N are transferred to the excited state N* instantaneously upon excitation of the laser pump pulse. Subsequently the electrons are injected from these excited states N* into the ZnO semiconductor, leaving behind the cationic states N+ of the dye molecules which finally recombine with the previously injected electrons to the corresponding ground states N again. This simple model disregarding direct excited state recombination is illustrated by eq 1 N → N* → N + → N

(1)

leading to a system of coupled differential equations dN * = G(t ) − rinjN * dt

(2)

dN + = rinjN * − rrecN + dt

(3)

with an excitation term G(t) and rinj representing electron injection from the excited dye and rrec representing the return to the ground state, respectively. G(t) is described as a Gaussian-shaped pulse with a fwhm corresponding to the crosscorrelation of pump and probe pulse. By introduction of a mixing coefficient α, the resulting single state signals can be superimposed defining the measured signal N(t) according to eq 4. N (t ) = A[αN * + (1 − α)N +]

(4)

Herein A is a scaling factor taking into account that different fwhm values result in different amplitudes for G(t) while N(t) is normalized to 1. By application of this model to the femtosecond transient absorption spectra, the time constants for injection and recombination processes can be derived via τinj ≈ 1/(rinj) and τrec ≈ 1/(rrec) and the separated excited and cationic state dynamics are revealed. Figure 7 shows the results of such a disentanglement of the excited and cationic state dynamics within an early 5 ps time window, applying a simplified but reliable and converging global fitting adaption. A least-squares fitting based on a quasi-Newton algorithm with predefined boundaries was used to determine the requested 9342



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probe spectroscopy was applied to detect the arrival of transferred electrons in the ZnO substrate, confirming the full energetic pathway from FRET light harvesting to subsequent charge carrier injection into the semiconductor. Significant insight is obtained from the ultrafast measurements carried out in vacuum. A future study in electrolyte using a hole scavenger may have even further impact. We believe that these results may open new routes for the understanding and future design of energy transfer based DSCs and related optical devices.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions ∥

R.S. and S.M. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Ursula Michalzik for ZnO sample preparation. This work was supported by the German Federal Ministry of Education and Research (Grant BMBF 03SF0339G).

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Figure 8. OPTP measurements at a pump wavelength of 490 nm showing the rise of the ZnO bulk conductivity due to injected electrons from acc-2 and sys-3 and single exponential fits (top), and the conductivity decay of acc-2 and don-1 (bottom) related to recombination of the electrons with the cations forming the neutral ground state. The linear fits are meant as a guide for the eye.

REFERENCES

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surface states on a femtosecond time scale.49,55,56 While a delayed electron transfer has also been proposed for TiO2 based electrodes,57 it is more pronounced for ZnO substrates. Such long-lived surface states are considered as detrimental for DSCs because interfacial recombination can be severely enhanced when the injected electron does not delocalize fast enough from the vicinity of the oxidized dye. Also shown in Figure 8 within a 1 ns time window is the OPTP signal depicting the decay of don-1 electrons compared to acc-2. The measured signal intensities reflect the about 3:1 difference in extinction ratio between the two species. Because of sensitivity reasons the signals are perturbed by some oscillatory noise. The decay of don-1 appears to be slower which points to a longer recombination time that is in agreement with the slow GSB recovery observed in the TA measurements due to the intentional weaker electronic coupling to the phenyl scaffold.

4. CONCLUSION A novel pure organic bischromophoric model FRET sensitizer was developed with the donor and acceptor moieties both covalently but individually linked to a phenyl based scaffold that keeps the dyes in proximity. The scaffold extended with a carboxylic anchor group serves as a conducting bridge facilitating ultrafast electron transfer from the FRET acceptor chromophore excited state into a semiconductor surface. Efficient energy transfer in solution was demonstrated in detail with steady-state UV−vis spectroscopy, picosecond fluorescence lifetime, and femtosecond transient absorption measurements. The assemblies were adsorbed onto a ZnO nanorod electrode maintainig FRET action under illumination, and the concatenated energy transfer and electron injection processes were probed with ultrafast spectroscopy. Optical pump THz 9343



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