Unusually Long Lifetime of Excited Charge-Transfer State of All

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J. Phys. Chem. C 2010, 114, 9167–9172

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Unusually Long Lifetime of Excited Charge-Transfer State of All-Inorganic Binuclear TiOMnII Unit Anchored on Silica Nanopore Surface Tanja Cuk,† Walter W. Weare,‡ and Heinz Frei*,‡ Materials Sciences DiVision and Physical Biosciences DiVision, Lawrence Berkeley National Laboratory, UniVersity of California, Berkeley, California 94720 ReceiVed: February 16, 2010; ReVised Manuscript ReceiVed: April 14, 2010

The lifetime and back electron transfer kinetics of an all-inorganic, oxo-bridged heterobinuclear TiIVOMnII group covalently anchored on a silica nanopore surface was investigated by transient optical absorption spectroscopy. Mesoporous silica particles of type SBA-15 loaded with TiOMn sites (1 wt %) were suspended in an index matching liquid for performing spectroscopy in transmission mode. Upon excitation of the TiIVOMnII f TiIIIOMnIII metal-to-metal charge-transfer transition (MMCT) by a visible laser pulse of 8 ns duration, a transient bleach was observed whose intensity versus pump wavelength dependence agreed with the MMCT absorption profile. The decay kinetics is well described by a superposition of first-order rates with a mean time constant of 1.8 ( 0.3 µs (room temperature). The dispersion of γ ) 2 ( 0.2 (Albery model) is attributed to variations in the local silica coordination environment reflecting the disordered, amorphous nature of the silica nanopore surface. The result constitutes the first observation of the electron transfer kinetics of an all-inorganic heterobinuclear group. It is proposed that the microsecond lifetime, unusually long for such as small charge transfer chromophore, originates from strong polarization of the local and remote silica environment upon light-triggered electron transfer from Mn to Ti. This results in a substantial reorganization barrier for back electron transfer. The long lifetime makes oxo-bridged heterobinuclear units anchored on silica surfaces efficient visible light photocatalysts and suitable as charge-transfer chromophores for driving multielectron catalysts in artificial photosynthetic systems. 1. Introduction The recent development of oxo-bridged heterobinuclear units anchored on nanoporous silica supports has opened up access to robust visible light charge-transfer chromophores with selectable redox potentials.1-8 Photoexcitation of the broad metal-to-metal charge-transfer absorptions, typically encompassing a large fraction of the visible spectrum have been shown to induce hydrocarbon oxidation (TiOMnII,8 TiOCeIII4) or splitting of CO2 to CO (ZrOCuI site2). Other binuclear chromophores were engaged as single photon, single electron pumps for driving multielectron catalysts, which is particularly relevant for artificial photosynthesis. For example, TiOCrIII-Ir oxide nanocluster units in MCM-41 silica pores evolve O2 from water at neutral pH with 13% quantum efficiency (460 nm).6 A unique aspect of using heterobinuclear units as visible light charge-transfer chromphores is the synthetic flexibility in selecting donor or acceptor metal centers with properly tuned redox potential, a prerequisite for matching the potential of donor/acceptor with that of the catalyst. Redox potential matching is the key for achieving thermodynamic efficiency, i.e., conversion of the maximum fraction of the photon energy to chemical energy of a product. A critical factor for developing efficient photocatalytic units capable of driving multielectron redox chemistry is knowledge of the competing rates of electron transfer within the heterobinuclear charge transfer chromophore one the one hand and between donor (or acceptor) center and the catalyst core on the other. While the observed photochemical reactions suggest that * To whom correspondence should be addressed. † Materials Sciences Division. ‡ Physical Biosciences Division.

lifetimes of excited MMCT states are reasonably long and almost certainly not ultrafast, no measurements of the electron transfer kinetics have been reported thus far. Our ultimate goal is to understand the photoinitiated charge transport kinetics of a complete photocatalytic unit at a level of detail described recently, for example, by Mallouk and collaborators for a Ru sensitizer complex coupled to an Ir oxide nanocluster catalyst for water oxidation.9 Here, we describe the excited state lifetime and back electron transfer kinetics of TiOMnII units anchored on silica nanopore surfaces of SBA-15 silica material using transient optical absorption spectroscopy with 8 ns resolution. The TiOMn unit was chosen for this study because of the low intensity of the MnII ligand field absorptions. As a consequence, the visible spectral region is dominated by the TiIVOMnII f TiIIIOMnIII charge-transfer chromophore thus facilitating detection of transient ground state depletion and recovery.8 The results constitute the first observation of charge transfer kinetics of an all-inorganic heterobinuclear group. 2. Experimental Section SBA-15 mesoporous silica materials containing binuclear TiOMn groups (TiMn-SBA-15) and monometallic Ti-SBA-15 or Mn-SBA-15 were prepared according to synthetic procedures described in a previous paper.8 Samples with Ti:Mn ratio of 1:1.60 (Ti:Si ) 1:67) per ICP-MS analysis were used. The concentration of Mn was the same within error limits for TiMn and Mn-SBA-15 samples. For transient absorption spectroscopy of photoexcited TiMnSBA-15 and Mn-SBA-15 samples, an Edinburgh Instruments model LP920 transient absorption spectrometer equipped with

10.1021/jp101444z  2010 American Chemical Society Published on Web 04/28/2010

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Figure 1. (a) Ru(bpy)3 loaded in SBA and then slurried in chloroform is pumped at 450 nm and probed at the indicated wavelengths. The inset shows the curve at 500 nm, where Ru(bpy)3 does not absorb, isolating the extrinsic bleach due to electrostriction. (b) Data normalized by the 500 nm probe wavelength curve (by subtracting a scaled 500 nm curve from the time response at each probe wavelength) show the intrinsic Ru(bpy)3 bleach centered at 450 nm. All the curves are fit by an exponential of 380 ns, with a probe-dependent coefficient graphed in the inset. The same probe-dependent coefficient is obtained if Ru(bpy)3 is dissolved in water.

a pulsed Xe probe lamp was used in conjunction with a Nd: Yag laser pumped tunable Optical Paramagnetic Oscillator (Continuum model Surelite II and Surelite OPO Plus) as excitation source. Laser pulse width was 8 ns, typical pulse energy 22 mJ cm-2, and the repetition rate was typically 1 Hz. Signals showed linear dependence with pump power in this regime. Note that while the instrument resolution is 8 ns, transient absorption measurement at times shorter than 20 ns was obscured by oxygen defect fluorescence associated with the silica.10 Because of strong visible light scattering of powders or pressed wafers of mesoporous silica particles, transient absorption measurements were conducted on TiMn-SBA or Mn-SBA15 powder suspended in index matching liquids (typically 17 mg of SBA-15 powder per milliliter of index matching liquid). Chloroform (η ) 1.44) and mineral oil (η ) 1.47) have refractive indexes very close to silica (η ) 1.46), which allowed us to conduct measurement in transmission mode. Nevertheless, the very small differences of refractive indexes of silica and matching liquid give rise to a long-lived (∼100 µs) signal attributed to dipole-dipole interactions of different silica particles. Upon excitation, the light absorbing units constitute small dipoles inside the micrometer-sized silica particle causing the silica particles to interact, move within the solvent background, and relax on the time scale of diffusion. Electric field induced optical effects of this kind have been referred to in the literature as electrostriction.11 Since all of the scattered Xe probe light is not collected in the experimental setup, a change in the index of refraction generated by the motion of silica particles within the probe beam results in an apparent absorption (or bleach) depending on the sign of the refraction index difference between silica and the liquid. To validate our procedure for correcting the decay traces for the long-lived electrostriction signal, we recorded the well established transient absorption spectrum of Ru2+(bpy)3 (Aldrich) with the complex loaded into SBA-15 and the powder suspended in chloroform. Loading of the Ru complex into the mesoporous silica particles was conducted in aqueous solution. Figure 1a shows the transient signal at various probe wavelengths between 400 and 500 nm following pulsed laser excitation at 450 nm, the peak of the Ru2+(bpy)3 ground state absorption band.12 At 400 nm, the initial amplitude is positive

because of transient excited triplet charge-transfer absorption of the Ru complex, while all other probe wavelengths show negative amplitude due to ground state depletion. The biphasic decay at all probe wavelengths consists of a ∼300 ns component originating from the excited Ru complex and an approximately 100 µs long decay due to the electrostriction effect. Because Ru2+(bpy)3 does not absorb at 500 nm, the signal at that probe wavelength is exclusively due to the effect of the index matching liquid. As can be seen from the inset of Figure 1a, this signal exhibits a rise of about 1 µs, much slower than the instrumentlimited rise of the excited Ru2+(bpy)3. Since the amplitude of the slow signal scales with the absorption intensity of Ru2+(bpy)3 at the laser excitation wavelength, we determined the corrected decay traces by scaling and then subtracting the 500 nm trace from the observed signal at each probe wavelength. Figure 1b shows the corrected traces in the 400-500 nm region. Global fit for all probe wavelengths to a first-order decay gave a 1/e time of 380 ( 5 ns and initial amplitude as shown in the inset. As indicated in the same plot, the probe wavelength dependence of the amplitude agrees well with the transient absorption observed for Ru2+(bpy)3 dissolved in aqueous solution. Moreover, as can be seen from Figure 2, transient absorption spectroscopy of the Ru complex loaded into an optically transparent (nonscattering) mesoporous silica membrane of 100 µm thickness13 showed identical spectral behavior as in Figure 1b with a first order decay of 280 (5 ns. In particular, no long decay was observed. This observation confirms the assignment of the long signal to an electrostriction effect associated with the index-matching liquid. In light of the quantitative agreement between corrected signal and the spectral and kinetic behavior in the absence of an index matching liquid, we adopted the same correction procedure to transient measurements of TiMn-SBA15 material. All transient absorption data shown in this paper are corrected for the electrostriction signal. UV-vis diffuse reflectance spectra (DRS) were recorded with a Shimadzu UV-2100 spectrometer equipped with integrating sphere model ISR-260. Barium sulfate was used as reference. The materials were pressed into pellets and mounted in a homebuilt stainless steel vacuum cell. L-edge X-ray absorption spectra were recorded using total electron yield at the Advanced Light Source at LBNL (beamline 7.0).

Excited-State Lifetimes of TiOMnII Units

Figure 2. Ru(bpy)3 loaded in a transparent silica membrane (“monolith”) is pumped at 450 nm and probed across the width of the bleach. Due to the solid, transparent membrane, only the intrinsic bleach of Ru(bpy)3 is observed. Exponential fits give a time constant of 283 ns and the spectrum shown in the inset.

Figure 3. Diffuse reflectance spectra of the TiMn oxo-bridged binuclear units grafted in SBA (a). Also shown are Mn (b) and Ti (c) only grafted in SBA. The inset shows the metal-to-metal charge transfer (MMCT) spectrum, obtained by subtracting (b) and (c) from (a).

3. Results Figure 3 shows the static optical spectra of TiOMn units anchored on SBA-15 (trace a), the monometallic Mn-SBA-15 material (trace b), and the monometallic Ti-SBA-15 material (trace c). Both materials (a) and (b) were heated to 250 C under vacuum in order to remove organic base, triethanol amine, which was utilized during the anchoring procedure. The steep absorption increase at wavelengths shorter than 330 nm is due to the TiIVOII ligand-to-metal charge-transfer absorption while the broad band centered at 465 nm originates from the 6A1 g f 4T2 IV II III III g ligand field transition of Mn(II). The Ti OMn f Ti OMn charge-transfer absorption is given by subtracting traces (b) and (c) from trace (a) and is separately displayed in the inset of Figure 3. The metal-to-metal charge-transfer transition results from the overlap of Ti(e) and Mn(t2 g) 3d orbitals according to spectroscopic analysis of mixed metal oxide minerals.14-16 As reported previously,8 optical and EPR spectroscopy of TiMnMCM-41 samples show that Mn is in oxidation state II and

J. Phys. Chem. C, Vol. 114, No. 19, 2010 9169 distorted octahedral coordination, while Ti(IV) is tetra- or pentahedrally coordinated. Compelling supporting evidence is provided by X-ray L-edge absorption measurements on TiMnSBA-15. The Ti L2,3 edge spectrum, Figure 4a, is identical with the spectrum of fresnoite, a pentacoordinate center. The spectrum is very characteristic and distinct from octahedrally coordinated Ti.17 Absence of contribution from octahedrally coordinated Ti strongly supports other spectroscopic data indicating that no TiO2 clusters are present. Likewise, the L2,3 edge X-ray absorption profile of the Mn center of TiMn-SBA15 shows the distinct bands of Mn(II), and no indication of contamination by higher oxidation states.17 Excitation of the TiOMn unit at 400 nm revealed transient bleach at all visible probe wavelengths examined, as shown in Figure 5a. The rise of the signal reflects the time resolution of the spectrometer (8 ns) while the bleach recovers within a few microseconds. Note that excitation at wavelengths longer than 400 nm avoids light absorption by Ti or Mn ligand-to-metal charge-transfer bands (LMCT), which lie at much shorter wavelengths in the UV. This was confirmed by control experiments with monometallic Ti-SBA-15 or MnII-SBA-15 samples; excitation at 400 nm or longer wavelengths resulted in a very short-lived, 20 ns, fluorescence with a spectrum characteristic for oxygen defects sites in the silica framework (peaks at 420 and 450 nm10) and a residual, long-lived electrostriction signal (Figure 5a includes the trace of monometallic Mn-SBA-15 material, which lacks the transient bleach observed for TiMnSBA-15). A plot of the initial amplitude of the bleach at constant probe wavelength (400 nm) versus pump wavelength, Figure 5b, is in good agreement with the absorption profile of the TiOMnII MMCT band. This implies that the data represent the excitation spectrum of the TiIVOMnII f TiIIIOMnIII transition. We conclude that the observed bleach of a few microseconds duration for the TiMn-SBA-15 sample is due to ground-state depletion of the TiOMnII unit, and the recovery of the bleach reveals the kinetics of back electron transfer TiIIIOMnIII f TiIVOMnII. The observed decays do not fit a single first-order rate law, but they are well described by a superposition of first-order rates with a spread of rate constant. The Albery model of dispersive first-order decays is appropriate for this case.18 In this model, the energy change of the system associated with the photoexcitation has a Gaussian distribution of width γ and the decay of the excited state an average rate constant k′, leading to a k ) k′ exp(γx) dispersion. The decay of the excited-state concentration is then given by c(t) ) c0 ∫ exp(-k′ exp(γx)t) dx. The result is a decay time 1/k′ ) 1.8 ( 0.3 µs with a dispersion γ ) 2 ( 0.2 for all wavelengths examined. The fits are shown as solid traces for various pump wavelengths in Figure 5a. The coefficient A0 for each pump wavelength (A0 ) c0εd, absorbance at t ) 0; c0, concentration at t ) 0) is plotted in Figure 5b. The same spectral dependence can be obtained in the absence of fits by averaging the time points in the entirety of the bleach at each pump wavelength. Furthermore, since the extrinsic, electrostriction effect is also proportional to the light absorbed, we obtain the same, MMCT spectral dependence without normalizing the data for electrostriction and simply average the time points (also plotted in Figure 5b). In contrast to the pump wavelength dependence of the bleach amplitude at constant probe wavelength, which clearly mimics the MMCT absorption profile, the amplitude plotted as a function of probe wavelength at constant pump wavelength is more complex (Figure 6). At each probe wavelength, the absorbance change not only reflects depletion of the MMCT

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Figure 4. Ti (a) and Mn (b) L-edges of the TiMn units grafted in SBA: (a) exhibits the characteristic L-edge spectrum for a penta-coordinated Ti(IV); (b) shows the characteristic L-edge spectrum of octahedral Mn(II).

Figure 5. (a) Normalized time response of the TiMn units grafted in SBA when pumping at the wavelengths indicated and probing at 400 nm, near the maximum of the MMCT band. Solid traces are fits to the heterogeneous Albery model with an average time constant of 1.8 µs and dispersion of 2. Gray trace is the normalized time response of the single metal centers, Mn, grafted in SBA when pumping with 400-500 nm; Ti-SBA does not absorb in this wavelength region. (b) Graphs of the pump-dependent coefficients of the kinetic fits in (a) are shown and compared with: (1) the static DRS spectra (dotted line) and (2) the time average (10 µs) of the un-normalized data inclusive of electrostriction (gray dots).

Figure 6. (a) Normalized time response of the TiMn units grafted in SBA, probing at the wavelengths indicated and pumping at 420 nm, near the maximum of the MMCT band. Solid traces show fits to the heterogeneous Albery model with an average time constant of 1.6 µs and dispersion of 1. (b) Graphs of the probe-dependent coefficients of the kinetic fits in (a).

ground state but also contains contribution from any excited state absorption or ground state bleach of other ligand field transitions at that wavelength. Likely contributors are the OMnIII LMCT absorption around 400 nm, the bleach of the MnII ligand field transition in the 400-600 nm region, the TiIII ligand field transition around 550-600 nm,19 and the new MMCT absorption TiIIIOMnIII f TiIIOMnIV. Therefore, we do not expect the probe wavelength dependence to merely mimic the MMCT

absorption profile. As can be seen from Figure 6b, the depletion amplitude covers broadly the spectral range between 400 and 600 nm suggesting the confluence of different transient absorptions and bleaches. We conclude that the probe wavelength dependence of the transient absorption further supports our assignment of the observed process. In the future, the development of transparent membranes will allow more quantitative measurements to unravel the different components of this

Excited-State Lifetimes of TiOMnII Units

Figure 7. Cartoon showing envisioned changes of the silica ligand coordination sphere upon photoexcitation of the MMCT transition.

spectrum. The current normalization required due to the extrinsic electrostriction effect, which is a larger fraction of the transient signal in the probe dependence than in the pump dependence, likely washes out important spectral features. 4. Discussion While there is no precedent for excited-state lifetimes of metal-to-metal charge-transfer states for all-inorganic oxobridged heterobinuclear units to our knowledge, a back electron transfer time of 1.8 µs (room temperature) might seem unusually long. Measurements of excited state lifetimes for the closest MMCT analogues that are available feature electron-rich cyano bridges and range from ultrafast to picoseconds.20-23 In organometallic structures, for which the metal centers are linked through organic structures (e.g., porphyrins, pyrazine), time scales for back electron transfer range from picoseconds to microseconds and can be strongly influenced by solvent polarity.24-26 The very large spread of back electron transfer rates reflects variation in the specific nature and ordering of adjacent electronic states and the magnitude of coupling among them. We are not aware of any studies of the excited MMCT lifetime of oxo-bridged heterobinuclear units of molecular systems that would provide precedents for such a weakly coupled system. A qualitative difference between molecular binuclear systems reported in the literature and the all-inorganic heterobinuclear chromophore anchored on a silica surface as part of a macroscopic particle discussed here is the presence of a sphere of oxygen ligands with a wide range of bond interaction energies. The silica coordination sphere may include strong, covalent Mn-O-Si (Ti-O-Si) linkages along with electrostatic or weaker general Lewis acid/base interactions with oxygens of SiOH (silanol), SiOSi (siloxane), or SiO- (siloxy) groups (symbolically sketched in Figure 7). Distances between metal centers and O lone electron pairs would adjust in response to the significant local electrostatic changes upon conversion of Mn(II) to Mn(III), or Ti(IV) to Ti(III), thereby stabilizing the system following electron transfer. Very likely, distances between metal and O would get shorter upon increase of the charge on Mn by a full unit, while opposite structural changes are expected for the ligand sphere of Ti. Our finding that the decay kinetics exhibits a spread of first-order rate constants is

J. Phys. Chem. C, Vol. 114, No. 19, 2010 9171 not surprising because the silica nanopore surface is disordered; each TiOMn group has a slightly different silica coordination environment resulting in dispersion in the magnitude of the barrier to back electron transfer. We propose that such structural changes impose a reorganization barrier to back electron transfer for this weakly coupled adiabatic system that may extend the lifetime of the MMCT state to microseconds.27 A related phenomenon has been used to explain prolonged back electron transfer in biological systems. For example, Warshel has explained slow back electron transfer in Photosystem II of bacterial photosynthesis by structural rearrangement of the local protein environment in response to photoinduced charge transfer among chlorophyll components.28 We cannot rule out other possible explanations such as spin crossing effects. However, the finding that heterobinuclear units as diverse as TiOMn, TiOCe, TiOCr, or ZrOCu exhibit photochemical reactivity and, hence, excited MMCT lifetimes of nanoseconds or longer point to a common mechanism that does not depend on the specific electronic state configuration of each system. Determination of the reorganization barrier and electronic coupling matrix constant within the framework of Marcus theory requires measurement of the temperature dependence of the back electron transfer kinetics.27 Transient absorption spectroscopy of these systems using TiMn-SBA-15 powders suspended in index-matching liquids is not suitable for such experiments because of the temperature sensitivity of the liquid properties. Instead, optically transparent mesoporous silica membranes that afford measurements without index matching liquid are needed. Preparation of binuclear units in such materials is in progress in our laboratory. 5. Conclusions In summary, lifetime and back electron transfer kinetics of an all-inorganic heterobinuclear charge-transfer chromophore anchored on nanoporous silica have been determined for the first time. Transient optical absorption spectroscopy of TiIVOMnII f TiIIIOMnIII units in SBA-15 reveals dispersive first-order decay of the excited state with a time constant of about 2 µs. We propose that the unusually long lifetime of the excited MMCT state, which explains the demonstrated utility of these units for visible light-driven redox chemistry, is caused by structural reorganization of the silica coordination sphere upon charge transfer that imposes a significant barrier to back electron transfer. The heterogeneity of the kinetics reflects the amorphous nature (disorder) of the silica nanopore surface. Determination of the magnitude of the reorganization barrier and other Marcus parameters requires measurements on nonscattering mesoporous silica samples. Preparation of such materials is in progress. The excited-state lifetime of microseconds renders this chargetransfer chromophore suitable as single photon, single electron pump for driving a multielectron catalyst. Such photocatalytic units are the key to progress in artificial photosynthesis where the single photon nature of the solar flux needs to be reconciled with the multielectron nature of water oxidation to O2 or the reduction of CO2 to a liquid fuel. With an initial example of such a unit for visible light water oxidation demonstrated,6 the photophysical insights uncovered here point to a general use of heterobinuclear chromophore on silica surfaces as photocatalysts in artificial photosynthetic systems. Acknowledgment. This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Chemical, Geological and Biosciences of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. T.C.,

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Miller postdoctoral fellow, acknowledges support by the Miller Institute, University of California, Berkeley. The authors thank Dr. Marisa MacNaughtan for the synthesis of transparent mesoporous silica membranes and Dr. Jinghua Guo for assistance at BL 7 or the Advanced Light Source at LBNL. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. References and Notes (1) Lin, W.; Frei, H. J. Phys. Chem. B 2005, 109, 4929. (2) Lin, W.; Frei, H. J. Am. Chem. Soc. 2005, 127, 1610. (3) Han, H.; Frei, H. Microporous Mesoporous Mater. 2007, 103, 265. (4) Nakamura, R.; Okamoto, A.; Osawa, H.; Irie, H.; Hashimoto, K. J. Am. Chem. Soc. 2007, 129, 9596. (5) Han, H.; Frei, H. J. Phys. Chem. C 2008, 112, 8391. (6) Han, H.; Frei, H. J. Phys. Chem. C 2008, 112, 16156. (7) Okamoto, A.; Nakamura, R.; Osawa, H.; Hashimoto, K. Langmuir 2008, 24, 7011. (8) Wu, X.; Weare, W.; Frei, H. Dalton Trans. 2009, 10114. (9) Youngblood, W. J.; Lee, S. H. A.; Kobayashi, Y.; Hernandez-Pagan, E. A.; Hoertz, P. G.; Moore, T. A.; Moore, A. L.; Gust, D.; Mallouk, T. E. J. Am. Chem. Soc. 2009, 131, 926. (10) Gimon-Kinsel, M. E.; Groothuis, K.; Balkus, Jr, K. J. Microporous Mesoporous Mater. 1998, 20, 67. (11) Vicari, L. Eur. Phys. J. E 2002, 9, 335.

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