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J. Phys. Chem. B 2000, 104, 1198-1205
Electron Injection and Recombination in Dye Sensitized Nanocrystalline Titanium Dioxide Films: A Comparison of Ruthenium Bipyridyl and Porphyrin Sensitizer Dyes Yasuhiro Tachibana, Saif A. Haque, Ian P. Mercer, James R. Durrant,* and David R. Klug Centre for Photomolecular Sciences, Departments of Chemistry and Biochemistry, Imperial College, London SW7 2AY, United Kingdom ReceiVed: August 5, 1999; In Final Form: NoVember 3, 1999
This paper is concerned with the parameters influencing the interfacial electron transfer kinetics, and therefore the sensitizing efficiency, for different sensitizer dyes adsorbed to nanocrystalline titanium dioxide films. We consider three sensitizer dyes: Ru(2,2′-bipyridyl-4,4′-dicarboxylate)2-cis-(NCS)2 (Ru(dcbpy)2(NCS)2) and zinc and free base tetracarboxyphenyl porphyrins (ZnTCPP & H2TCPP). These dyes were selected as they exhibit large differences in their oxidation potentials and photophysics, while retaining similar carboxylate groups for binding to the TiO2 surface. For example, whereas the photophysics of Ru(dcbpy)2(NCS)2 in solution is dominated by ultrafast (1 ns) π* singlet excited states and only weak singlet/triplet mixing. The ground and excited-state oxidation potentials also differ by up to 600 mV between these different dyes. Remarkably, we find that the large differences in these dyes’ photophysics and redox chemistry have rather little influence upon the interfacial electron transfer kinetics observed following adsorption of these dyes to the nanocrystalline TiO2 films. The kinetics of electron injection into the TiO2 conduction band following pulsed optical excitation of the adsorbed sensitizer dyes are found to be indistinguishable for all three sensitizer dyes. For all three dyes, the kinetics are ultrafast and multiexponential, requiring a minimum of three time constants ranging from 95% by comparison of the optical absorption spectrum with literature data,43,44 indicating that contribution of impurities to the transient data is only 2-3%. Anatase nanocrystalline TiO2 films were prepared as previously17 (average particle diameter, 15 nm; film thickness, 8 µm). Sensitization of TiO2 films by all three dyes was performed by immersion of the films in ethanolic dye solutions in room temperature overnight. The sensitizing solution concentration was adjusted to yield optical densities of ∼0.3 for all three dyes at the excitation wavelengths used. After sensitization, the films were washed thoroughly with fresh ethanol, immediately covered with a 1:1 ethylene carbonate/propylene carbonate (EC/PC) solution and thin cover glasses, and then stored under dry, dark conditions to avoid dye degradation.17,39,45,46 Spectroscopy. Steady state UV/visible absorption spectra were obtained on a Shimadzu UV1601 spectrometer. Emission spectra were collected on a Perkin-Elmer LS50 luminescence spectrometer. The details of the transient absorption spectrometer for the ultrafast time scale measurements will be given elsewhere.27 Excitation pulses were provided by an optical parametric amplifier (OPA) at 560 nm for Ru(dcbpy)2(NCS)2 or ZnTCPP and at 645 nm for H2TCPP with excitation intensities of typically 200-800 µJ cm-2. Experiments were carried out with the magic-angle configuration (ca. 54.7°) between excitation and probe pulses with a repetition rate of 1 kHz at 20 °C. Transient absorption spectra were collected with the films covered in EC/PC using a white light probe pulse and multichannel detector.47 The instrument response of the spectrometer was 100-250 fs depending upon probe wavelength. Analyses were conducted without deconvolution of the instrument response, and therefore only for time delays greater than the pump/probe pulse overlap. Nanosecond-millisecond transient absorption experiments were conducted with external bias control of the TiO2 potential; these experiments were conducted
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with a three electrode photoelectrochemical cell with an Ag/ AgCl reference electrode and a 0.1 M lithium perchlorate/0.1 M tetrabutylammonium perchlorate/distilled acetonitrile electrolyte as detailed elsewhere.29 In all experiments, the excitation intensity was attenuated such that e1 dye molecule was excited per pulse per TiO2 particle. The stability of all samples was monitored by the change of the signal size during all of the transient absorption experiments and by the change of the ground state absorption maximum before and after the transient absorption experiments. Normalized electron injection kinetics were calculated using the following equation:
φ ) ∆ODfilm(λ,t) - ∆ODsol(λ,0) where ∆ODfilm and ∆ODsol correspond to the absorption change observed for the dye sensitized TiO2 films and dyes in solution, respectively, normalized to the same density of absorbed photons. The probe wavelength, λ, was selected for each sensitizer dye to be indicative of electron injection, as detailed below; moreover, at these probe wavelengths, dye excited state dynamics contribute negligibly to the calculation. The resulting electron injection transients were further normalized at 25 ps to facilitate comparison of the kinetics between different dyes (the unnormalized kinetics were in any case in agreement to within (5% at 25 ps, consistent with the high injection yields observed for all three sensitizer dyes). Results Dye Photophysics in Solution. Figure 2 shows steady-state absorption and emission spectra of Ru(dcbpy)2(NCS)2, ZnTCPP, and H2TCPP dissolved in ethanol. Ru(dcbpy)2(NCS)2 exhibits a broad absorption band (peak 538 nm) assigned to a t2 f π* MLCT transition, and a weak absorption tail extending to 750 nm associated with lower energy electronic states. The dye exhibits a very low emission yield (Φem ) 4 × 10-3 at 125 K), and the emission is moreover strongly red shifted with an emission maximum at 755 nm.8 The low yield and large red shift of the emission is consistent with rapid relaxation of the initially formed singlet excited state to lower energy, weakly emissive states. Indeed, we are unable to resolve any stimulated emission from this dye in solution,17,48 indicating that this relaxation process was complete in less than our 100 fs instrument response. Such a fast relaxation time is consistent with recent studies of analogous ruthenium bipyridyl dyes.49 The resultant weakly emissive excited state of this dye decays to the ground state on the nanosecond time scale.17 In contrast to Ru(dcbpy)2(NCS)2, both the porphyrin dyes exhibit a series of relatively narrow visible absorption bands, corresponding to π - π* transitions of the conjugated macrocycles (“Q" absorption bands) and their vibrational sidebands. For ZnTCPP, the lowest energy Q absorption band is observed at 598 nm, with its vibrational sideband at 558 nm.44 The reduction of overall symmetry for H2TCPP results in distinct Qx and Qy absorption bands and associated vibrational sidebands.44 Both porphyrin dyes exhibit relatively small Stokes shifts (11 nm for ZnTCPP and 5.5 nm for H2TCPP) with high fluorescence intensities, consistent with their relatively long singlet excited-state lifetimes (approximately 3 ns for ZnTCPP and 9 ns for H2TCPP). For both porphyrins, vibronic emission bands can be observed (at 660 nm for ZnTCPP and at 720 nm for H2TCPP). Transient Absorption Spectra. Figure 3 compares transient absorption spectra obtained for Ru(dcbpy)2(NCS)2 (a), ZnTCPP
Figure 2. Steady state absorption (‚ ‚ ‚) and emission (s) spectra of Ru(dcbpy)2(NCS)2 (a), ZnTCPP (b), and H2TCPP (c) in ethanol. Excitation wavelengths for emission were 531, 560, and 589 nm for Ru(dcbpy)2(NCS)2, ZnTCPP and H2TCPP, respectively. The inserts show the structure of each dye.
Ruthenium Bipyridyl and Porphyrin Sensitizer Dyes
Figure 3. Picosecond absorption difference spectra obtained for Ru(dcbpy)2(NCS)2 (a), ZnTCPP (b), or H2TCPP (c) in ethanol solution (‚ ‚ ‚) or sensitized TiO2 films (s). The spectra for Ru(dcbpy)2(NCS)2 were taken from our previous report.17 The spectra for the dye excited state and the sensitized TiO2 films were taken at time delays of 5 and 5, 6 ps (solution data) and 60, and 60 and 600 ps (film data) with excitation wavelengths of 605, 560, 645 nm for Ru(dcbpy)2(NCS)2 (a), ZnTCPP (b), and H2TCPP (c), respectively.
(b), or H2TCPP (c) in ethanol and for the corresponding dye sensitized TiO2 films. The spectra for Ru(dcbpy)2(NCS)2 were taken from our previous report.17 As discussed previously, electron injection into the TiO2 results in a red shift of the induced infrared absorption from ∼720 nm for the dye excited state to 800 nm for the dye+(TiO2)- state.50,51 The spectra for ZnTCPP and H2TCPP in ethanol exhibit prominent stimulated
J. Phys. Chem. B, Vol. 104, No. 6, 2000 1201 emission bands at 660 and 720 nm, respectively (cf. Figure 2), confirming the assignment of these spectra to the singlet excited states of these dyes. Adsorption of these dyes to the TiO2 results in quenching of the stimulated emission, consistent with electron injection into the TiO2 resulting in formation of dye+(TiO2)-. This assignment is confirmed by both (i) the observation of micro- to millisecond kinetics consistent with dye+(TiO2)recombination (see below) and (ii) the observation that the product state spectra observed following stimulated emission decay are characteristic of the formation of a high yield of porphyrin cation states. In particular, the transient spectrum observed at 60 ps observed for ZnTCPP sensitized TiO2 films (Figure 3b solid line) exhibits a broad absorption maximum peaking at ∼650 nm superimposed upon bleaching of the porphyrin ground state absorption at 600 nm, in good agreement with a previous study of the oxidation of zinc porphyrin in solution.52 It should, however, be noted that a small residual stimulated emission feature was observed for both porphyrin dyes adsorbed to TiO2 (7% and 16% of the amplitude in solution for ZnTCPP and H2TCPP, respectively), probably resulting from subpopulations of dyes which are unable to inject electrons. Electron Injection Kinetics. We have previously studied the kinetics of electron injection for TiO2 sensitized by Ru(dcbpy)2(NCS)2.17 We found that, following excitation at 605 nm, the electron injection kinetics were at least biphasic with time constants of
