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J. Phys. Chem. B 2001, 105, 2998-3004
Time-Resolved Optical Spectroscopy of Heterosupramolecular Assemblies Based on Nanostructured TiO2 Films Modified by Chemisorption of Covalently Linked Ruthenium and Viologen Complex Components Alan Merrins,† Cees Kleverlaan,‡ Geoffrey Will,† S. Nagaraja Rao,† Franco Scandola,‡ and Donald Fitzmaurice*,† Department of Chemistry, UniVersity College Dublin, Belfield, Dublin 4, Ireland, and Dipartimento di Chimica, UniVersita` di Ferrara, Via Luigi Borsari 46, 44100 Ferrara, Italy ReceiVed: NoVember 30, 2000
Described is the preparation of transparent nanostructured TiO2 (anatase) and Sb-doped SnO2 films supported on F-doped conducting glass. Also described is the modification of these films by chemisorption of RV, bis(4,4′-bis(phosphonomethyl)-2,2′-bipyridine)(5-((1′-ethyl-4,4′-bipyridinediium-1-yl)butyl)-2,2′-bipyridine)ruthenium(II) tetrakis(hexafluorophosphate), to form the covalent heterosupramolecular assemblies TiO2-RV and SnO2RV, respectively. Visible light excitation of the ruthenium complex component, R, results in electron injection into the TiO2 or SnO2:Sb nanocrystal component (60%) and electron transfer to the viologen component (40%), V. In the case of TiO2-RV, however, long-lived formation of the radical cation of the V component, V•+, is observed. In the case of SnO2-RV, long-lived formation of V•+ is not observed. These findings, based on a detailed study of the time-resolved optical spectroscopy of TiO2-RV and SnO2:Sb-RV, are accounted for by proposing that long-lived V•+ is formed by TiO2 nanocrystal mediated electron transfer to the V component.
Introduction Supramolecular chemistry is concerned with the assembly of molecular components through covalent or noncovalent interactions where the intrinsic properties of the resulting supermolecule are not a simple superposition of the molecular components; i.e., there exists a supramolecular function.1 Analogously, assembly of condensed phase and molecular components yields heterosupermolecules which, when organized, offer the prospect of heterosupramolecular devices addressable on the nanometer scale. Reported previously was the covalent assembly of heterosupermolecules from a TiO2 nanocrystal (TiO2), a ruthenium complex (R), and a viologen (V) component.2 Also described was their covalent organization to yield a heterosupramolecular assembly denoted TiO2-RV. It has been shown that effective function modulation is possible. Specifically, upon application of a sufficiently positive potential to the TiO2 nanocrystal component, visible light excitation of the ruthenium complex component results in electron transfer to the TiO2 nanocrystal component with 95% efficiency. Electron transfer to the viologen component occurs with a much lower efficiency (5%) (Scheme 1a). Upon application of a sufficiently negative potential to the TiO2 nanocrystal component, however, visible light excitation of the ruthenium complex component results in electron transfer to the TiO2 nanocrystal component with 48% efficiency and to the viologen component with 52% efficiency (Scheme 1b). In short it is possible to potentiostatically modulate the direction of light-induced electron transfer. * To whom correspondence should be addressed. † University College Dublin. ‡ Universita ` di Ferrara`.
SCHEME 1: (a, Top ) Light-Induced Electron Transfer to the TiO2 Nanocrystal Component (95%) of TiO2-RV at Positive Applied Potentials and (b, Bottom) Light-Induced Electron Transfer to the TiO2 Nanocrystal Component (48%) and Viologen Component (52%) of TiO2-RV at Negative Applied Potentials
It should be noted that visible light-induced electron transfer from the ruthenium complex component, at either positive or negative applied potentials, does not result in long-lived charge
10.1021/jp004343v CCC: $20.00 © 2001 American Chemical Society Published on Web 03/21/2001
Spectroscopy of Heterosupramolecular Assemblies SCHEME 2: Optical “Write-Read-Reset” Device Based on the Heterosupramolecular Assembly TiO2-RV “Written” to Using Blue-Green (488 and 514 nm) Light, “Read” Using Red (632 nm) Light, and “Reset” upon Application of a Positive Potential
separation. This is because electron transfer from the electronically excited ruthenium complex component to the viologen component is followed by back electron transfer from the radical cation of the viologen component to the oxidized ruthenium complex. In a subsequent study,3 however, it has been shown that applying a potential close to that of the conduction band edge of the TiO2 nanocrystal component leads to long-lived charge separation. Briefly, visible light-induced electron transfer from the ruthenium complex component to the TiO2 nanocrystal component is followed by electron transfer from the TiO2 nanocrystal to the viologen component on the same or an adjacent TiO2 nanocrystal. The radical cation of the viologen formed is not covalently linked to an oxidized ruthenium complex component and, therefore, is long-lived. In this study it was also shown that subsequent application of a sufficiently positive potential leads to the reoxidation of any long-lived radical cation. In a further subsequent study, the above heterosupramolecular assembly was incorporated as the working electrode in a sealed two-electrode cell, to yield a heterosupramolecular optical write-read-reset device which can be “written” to using bluegreen light, “read” using red light, and “reset” upon application of a sufficiently positive potential4 (Scheme 2). In order, however, to fully realize the potential of this and related heterosupramolecular assemblies in optical write-readreset and related devices, it will be necessary to have a deeper understanding of the mechanisms outlined above. In particular, it will be necessary to measure the rate constants for each of the electron-transfer steps and to understand the factors which govern the values of these constants. The present study represents a first step toward these ends.
J. Phys. Chem. B, Vol. 105, No. 15, 2001 2999 Experimental Section Preparation of Transparent Nanostructured TiO2 Films. Transparent nanoporous-nanocrystalline TiO2 films (4 µm thick, 10 nm diameter nanocrystals), supported on fluorinedoped SnO2 glass (0.5 µm thick, 11 Ω/square, supplied by Nippon Sheet Glass Co., Tokyo, Japan), were prepared by following the method described by Gratzel and co-workers.5 Briefly, a colloidal dispersion of TiO2 nanocrystals was prepared by hydrolysis of titanium isopropoxide. The dispersion was autoclaved at 200 °C for 12 h and concentrated (160 g dm-3), and Carbowax 20 000 (40 wt % equivalent of TiO2) was added to yield a white viscous sol. This sol was spread on a conducting glass substrate masked with Scotch tape using a glass rod and allowed to dry in air for 1 h. The resulting gel film was fired, also in air at 450 °C for 2 h. All films were stored in a darkened vacuum desiccator until required for use. The resulting transparent nanostructured TiO2 films are 4 µm thick and have a surface roughness of about 1000. Preparation of Transparent Nanostructured SnO2 Films. Transparent nanostructured Sb-doped SnO2 (SnO2:Sb) films were prepared on F-doped tin oxide glass substrates (0.5 µm thick, 11 Ω/square, supplied by Nippon Sheet Glass Co., Tokyo, Japan) were prepared largely as described elsewhere.6 Briefly, 10 drops of acetic acid (2.0 mol dm-3) were added with stirring to an aqueous dispersion (50 g) of 5 nm diameter Sb-doped SnO2 nanocrystals (15% weight equivalent SnO2:Sb, supplied by Alfa). The gel which formed immediately was diluted by addition of water (15 mL) and autoclaved at 200 °C for 12 h. Addition of Carbowax 20 000 (3.75 g) with stirring for 8 h yields an amber colored viscous paste which was diluted with water (10 mL) to make it suitable for spreading. This paste was spread using a glass rod on the conducting glass substrate masked by Scotch tape. Following drying in air for 1 h, the film was fired, also in air, at 450 °C for 2 h. The resulting transparent nanostructured SnO2:Sb films are 3.0 µm thick and have a surface roughness of about 1000. Preparation of Molecular Components. The molecular components tris(4,4′-bis(phosphonomethyl)-2,2′-bipyridine)ruthenium(II) bis(hexafluorophosphate) (R) and bis(4,4′-bis(phosphonomethyl)-2,2′-bipyridine)(5-((1′-ethyl-4,4′-bipyridinediium-1-yl)butyl)-2,2′-bipyridine)ruthenium(II) tetrakis(hexafluorophosphate) (RV) were prepared and characterized as described in detail elsewhere.2,3 Preparation of Heterosupramolecular Assemblies. Transparent nanostructured TiO2 and SnO2:Sb films were immersed for 3 h in a methanolic solution (1 × 10-4 mol dm-3) of R and RV acidified (pH 2) by addition of perchloric acid (0.01 mol dm-3). In this context, it is noted that molecules incorporating phosphonic acid groups chelate surface Ti4+ or Sn4+ sites and are strongly chemisorbed at the surface of the nanostructured metal oxide films.6 The resulting heterosupramolecular assemblies denoted TiO2-R and TiO2-RV or SnO2-R and SnO2RV (Scheme 3) were washed thoroughly with ethanol and stored in a darkened vacuum desiccator until required for use. UV-Visible Optical Absorption and Emission Spectroscopy. UV-visible absorption spectra were recorded using a Hewlett-Packard 8452A diode array or a Perkin-Elmer Lambda 40 spectrophotometer. UV-visible emission spectra were recorded using a SPEX Fluoromax 2 spectroflourometer equipped with a Hamamatsu R3896 photomultiplier tube (PMT). In all cases the background spectrum measured was that for the conducting glass substrate. Transient UV-Visible Optical Absorption and Emission Spectroscopy. Transient UV-visible absorption and emission
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SCHEME 3: Heterosupramolecular Assemblies TiO2-R, TiO2-RV, SnO2:Sb-R and SnO2:Sb-RV
spectra were measured by irradiating the sample with a 7 ns pulse at 532 nm from a Continuum Surelight Nd:Yag laser and using a pulsed Xe lamp aligned perpendicularly to the beam as a probe source. The 150 W Xe lamp, equipped with an Applied Photophysics model 408 power supply and Applied Photophysics model 410 pulsing unit, allowed generation of 0.5 ms pulses. An Oriel model 71445 shutter was placed between the lamp and the sample and was opened for 100 ms to prevent fatigue of the PMT. Band-pass filters, both pre- and postcutoff, were used to minimize the adverse effects of scattered light. The samples were placed at an angle of 45° with respect to the laser and probe light and set up in such a way that the scattered light was directed away from the detector. By doing this, it was possible to record in the early time domain (t < 50 ns) without measuring artifacts due to scattered light. Sample rate was kept relatively low (10 s intervals) to prevent electron accumulation in the semiconductor materials. All plotted transients are the average of 10 individually recorded transients. Light was collected in a LDC analytical monochromator, detected by a Hamamatsu R928 PMT, and recorded on a Le Croy 9360, 600 MHz oscilloscope. The laser oscillator, Q-switch, lamp, shutter, and trigger were externally controlled using a digital logic circuit that allowed for synchronous timing. The setup, as described above, was also employed for the time-resolved emission experiments with the exception that the probe lamp was not used. Results and Discussion Presented is a detailed description of the steady-state and timeresolved UV-visible absorption and emission spectroscopy of the following heterosupramolecular assemblies: TiO2-R; TiO2RV; SnO2:Sb-R; SnO2:Sb-RV. 1. UV-Visible Absorption and Emission Spectra of R and RV. Shown in Figure 1 are the steady-state and time-resolved UV-visible absorption and emission spectra of R in acetonitrile. The steady-state optical absorption spectrum of R is characterized by the broad absorption band at 465 nm seen in Figure 1a. This band is assigned to a spin-allowed metal-to-ligand charge transfer (MLCT) transition from a d-orbital on the ruthenium atom to a vacant π*-orbital on a bipyridine ligand. Also observed is a long wavelength tail extending to about 600 nm assigned to a spin-forbidden MLCT d-π* transition. The UV-visible absorption spectra of R and RV are indistinguishable. This is a consequence of there being no measurable absorbance which may be assigned to the V component.
Figure 1. (a) UV-visible optical absorption spectra of R in MeCN. (b) Transient optical absorption spectra of R in MeCN at the indicated times I-V (10, 25, 75, 150, and 250 ns, respectively), following irradiation at 532 nm using the pulsed output of a Nd:Yag laser (7 ns, average of 10 transients at 10 s intervals, 7 mJ/pulse). (c) Steady-state optical emission spectrum of R in MeCN.
Figure 1b shows the time-resolved UV-visible absorption spectra of R recorded following excitation by the pulsed output of a Nd:Yag laser at 532 nm (7 mJ/pulse). Visible excitation results in a transient absorption spectrum that is assigned to the complex in the lowest excited triplet state, 3R*.7 Specifically, the absorption increase at 375 nm is assigned to the reduced bipyridine ligand, while the absorbance decrease (bleach) at 460 nm is assigned to a decrease in intensity of the ground-state MLCT transition of the complex.8 Time-resolved absorption spectra were recorded for RV under the same conditions (not shown) but did not show an observable transient. The steady-state UV-visible emission spectrum of R in acetonitrile exhibited a broad emission band centered at 645 nm and is shown in Figure 1c. This emission is assigned to a radiative decay of the triplet excited state, 3R*, formed following visible excitation of R. The steady-state optical emission spectrum of RV in acetonitrile (not shown) exhibited only a very weak band at the same wavelength. It is known that the electronically excited electron in 3R* hops between the three bipyridine ligands in R. It is assumed this is also the case in RV.9 In the case of R, as discussed above, 3R* undergoes radiative decay to give rise to an emission spectrum with a maximum at 645 nm.10 In the case of RV, 3R*V undergoes an initial intramolecular electron transfer to form the charge-separated state, R+V-, and a subsequent and second rapid intramolecular electron transfer to regenerate the groundstate complex, RV. In this context it is noted for a supermolecule closely related to RV a rate constant of 6.55 × 108 s-1 for electron transfer to the V component, corresponding to lifetime of 1500 ps, and a rate constant 3.24 × 109 s-1 for electron transfer from the reduced viologen to the oxidized ruthenium complex, corresponding to a lifetime of 300 ps, have been measured by Mallouk and co-workers.11,12 This accounts for the fact no steady-state emission spectrum is measured for RV
Spectroscopy of Heterosupramolecular Assemblies
Figure 2. UV-visible optical absorption spectra of (a) TiO2-R and (b) TiO2-RV in MeCN. In each case the contribution to the measured spectrum by TiO2 is indicated.
and, also, for the fact that on the nanosecond time scale no transient absorption spectrum is measured for this complex. 2. Steady-State UV-Visible Absorption Spectroscopy of TiO2-R and TiO2-RV. The heterosupramolecular assemblies TiO2-R and TiO2-RV were prepared by chemisorbing the molecular components R and RV, respectively, at the surface of the constituent nanocrystals of a transparent nanostructured TiO2 film supported on conducting glass. Molecules possessing phosphonic acid groups are strongly chemisorbed at the surface of a TiO2 nanocrystal by chelating to the surface Ti4+ atoms.13 The presence of these groups promotes electronic coupling between the π*-orbitals of the bipyridine ligand and the conduction band states of the semiconductor. These and closely related heterosupramolecular assemblies exhibit long-term stability,14-17 and the properties of these and similar films have been extensively studied.5,6 Shown in Figure 2 are the steady-state UV-visible absorption spectra of transparent nanostructured TiO2 films prior to and following adsorption of R and RV, respectively. It can be seen that the absorption maximum for R and RV on the metal oxide surface is centered at 465 nm (broad) which is the same as that observed for R and RV in solution. As discussed above, this band is assigned to a spin-allowed d-π* MLCT transition. Also observed is a long wavelength tail extending to about 600 nm. Also as discussed above, this band is assigned to a spinforbidden d-π* MLCT transition. The heterosupramolecular assemblies SnO2:Sb-R and SnO2: Sb-RV were prepared by adsorbing the molecular components R and RV, respectively, at the surface of the constituent nanocrystals of a transparent nanostructured SnO2:Sb film supported on conducting glass. The UV-visible absorption spectra measured for SnO2:Sb-R and SnO2:Sb-RV are qualitatively and essentially quantitatively similar to those measured for TiO2-R and TiO2-RV and are accounted for as described above. It is noted that on adsorption of R and RV at a nanostructured SnO2:Sb film, there is a slight bathochromic shift (∼1 nm). This observation is in agreement with those previously reported.18 Finally, it is noted that all heterosupramolecular assemblies for which results are reported were optically matched at 532 nm to ensure that the same number of molecules were excited by each laser pulse and that, as a consequence, the same number of ruthenium complex components were optically excited. 3. Time-Resolved UV-Visible Absorption Spectra of TiO2-R and TiO2-RV. Shown in Figure 3 are the time-resolved optical absorption spectra of TiO2-R and TiO2-RV. These spectra were measured using optically matched samples in
J. Phys. Chem. B, Vol. 105, No. 15, 2001 3001
Figure 3. (a) Transient absorption spectra of TiO2-R in MeCN with added LiClO4 (0.1 mol dm-3) at the indicated times I-V (100, 500, 1000, 2000, and 5000 ns, respectively), following irradiation at 532 nm using the pulsed output of a Nd:Yag laser (7 ns, average of 10 transients at 10 s intervals, 6 mJ/cm2). (b) Transient absorption spectra of TiO2-RV in MeCN with added LiClO4 (0.1 mol dm-3) at the indicated times I-V (100, 500, 1000, 2000, and 5000 ns, respectively), following irradiation at 532 nm using the pulsed output of a Nd:Yag (7 ns, average of 10 transients at 10 s intervals, 6 mJ/cm2).
acetonitrile containing added LiClO4 (0.1 mol dm-3) following excitation at 532 nm by the pulsed output of a Nd:Yag laser (6 mJ/cm2). The spectra measured for TiO2-R, shown in Figure 3a, are characterized by a strong absorbance decrease (bleach) between 360 and 550 nm assigned to a depletion of the ground state of R. No absorption increase between 360 and 400 nm that could be assigned to formation the excited state, 3R*, is observed. Accordingly, there is no evidence of emission between 600 and 700 nm which would be expected following radiative decay of 3R*. It has been established that electron injection by 1R* in TiO21R* occurs on the femtosecond time scale with lifetimes of between 25 and 150 fs being reported for 1R* in a similar ruthenium complex.19 It should be noted that the sensitizers used in this study incorporate a methylene group between the phosphonic acid linker groups and the bipyridine ligands. This may result in a reduced electronic coupling between the π* orbital and the TiO2, as a consequence slowing the rate of electron injection. It has also been established that 1R* undergoes intersystem crossing to form TiO2-3R* on the femtosecond time scale in solution with a lifetime of 300 fs being reported. Intersystem crossing would be expected to lead to radiative decay of 3R* with a quantum efficiency of approximately 0.04 and emission on the nanosecond time scale between 600 and 700 nm, unless injection by 3R* is also possible. Since no absorption or emission by 3R* is observed, it is concluded that injection by 1R* is so efficient that intersystem crossing to form 3R* is a minor pathway and/or that injection by 3R* is also very efficient. The spectra measured for TiO2-RV, shown in Figure 3b, are characterized by a bleach between 360 and 550 nm assigned to a depletion of the ground state of R. As for TiO2-R, there is no evidence for formation of an excited state, 3R* or its subsequent decay by radiative emission to R. Significantly, two absorption bands at 390 and 600 nm, assigned to the radical cation of the viologen component, V•+. While a significant fraction of V•+ formed is present after 100 ns, formation clearly continues on the microsecond time scale and is long-lived. As discussed in detail elsewhere,4,20 photoexcitation of TiO2RV results in formation of TiO2-1R*V and in electron injection into the conduction band states of a constituent TiO2 nanocrystal
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SCHEME 4: Light-Induced Electron Transfer to the TiO2 Nanocrystal (60%) Component and Electron Transfer to the Viologen Component (40%) in TiO2-RV
SCHEME 5: Trap-Mediated Electron Transfer from the TiO2 Nanocrystal Component to the Viologen Component in TiO2-RV, Resulting in Long-Lived Reduced Viologen
by 1R* on the femtosecond time scale19 to form TiO2(e-)-R+V. Following photoexcitation, TiO2-1R*V may also undergo intersystem crossing, also on the femtosecond time scale, to form TiO2-3R*V and subsequently either inject an electron to form TiO2(e-)-R+-V or transfer an electron on the picosecond time scale (approximately 1500 ps) to the V component to form TiO2-R+-V•+. However, V•+ formed by this pathway is not longlived as TiO2-R+-V•+ is converted, as a result of back electron transfer, to TiO2-RV on the picosecond time scale (approximately 300 ps).11,12 In a recent study,2 it was reported that 95% of TiO2-1R*V injected an electron into TiO2 to form TiO2(e-)-R+-V or underwent intersystem crossing to form TiO2-3R*V and subsequently injected an electron into TiO2 to form TiO2-R+-V. The remaining 5% of TiO2-1R*V underwent, first, intersystem crossing to form TiO2-3R*V and, second, intramolecular electron transfer to form TiO2-R+-V•+, by back electron transfer to form TiO2-RV. Furthermore, it was observed that applying a sufficiently negative potential to the nanostructured TiO2 film significantly changed the above branching ratio. Specifically, 48% of TiO2-1R*V or TiO2-3R*V injected an electron into TiO2 to form TiO2(e-)-R+-V while the other 52% transferred an electron to TiO2-R+-V- and then TiO2-RV. This change in branching ratio, it was suggested, could be accounted for in one of two ways. First, application of a sufficiently negative potential leads to occupation of the available conduction band states and inhibits electron injection by 1R* or 3R* into TiO2. Second, that electron accumulation in the constituent nanocrystals of the nanostructured TiO2 film leads to reduced electronic coupling, possibly as a consequence of partial desorption of the RV moiety and inhibition of electron injection. As pointed out above, the heterosupramolecular assemblies TiO2-R and TiO2-RV were optically matched. As a consequence, a comparison of the relative magnitudes of the bleaches at 465 nm assigned to a depletion of the ground state of R in TiO2-R (0.060 au) and TiO2-RV (0.037 au) yields the branching ratio between charge injection (60%) and electron transfer (40%) by either the 1MLCT or 3MLCT state of the electronically excited ruthenium complex to the viologen moiety in TiO21R*V (Scheme 4). The fact that these values are similar to those previously reported for TiO2-RV at negative applied potentials in an aqueous solution at pH 2 2 can be explained by the expected shift in the energy of the conduction band edge to more negative potentials in an aprotic solvent such as acetonitrile. The fact, however, that the branching ratio indicates that 40% of TiO23R*V transfers an electron to form TiO -R+-V•+ does not 2 account for the formation of long-lived radical cation of the viologen. This is because formation of TiO2-R+-V•+ is followed
on the picosecond time scale by intramolecular back electron transfer to regenerate TiO2-RV. The question arises, therefore, by what mechanism is the longlived radical cation of the viologen component in TiO2-RV, clearly observed in the spectra shown in Figure 3b, formed. A possible answer to this question is that long-lived reduced viologen is formed as a consequence of electron transfer from the TiO2 nanocrystal to the viologen ligand.4 Results reported in a previous study show that prolonged irradiation of TiO2RV results in the formation of long-lived radical cation of the viologen component.4 The same study suggested that trap state mediated electron transfer from a TiO2 nanocrystal was responsible for formation of the radical cation of the viologen component observable on the nanosecond time scale (Scheme 5). 4. Incident Light Intensity Dependence of Time-Resolved Transient Kinetics of TiO2-R and TiO2-RV. Shown in Figure 4 are the time-resolved optical absorption transients of TiO2RV. These spectra were measured using optically matched samples in acetonitrile containing added LiClO4 (0.1 mol dm-3) following excitation at 532 nm using the pulsed output of a Nd:Yag laser (0.4 and 10 mJ/cm2). The absorbance transients measured for TiO2-RV at 450 nm under the conditions outlined above are shown in Figure 4a. At low laser intensities (0.4 mJ/cm2) a negative transient (bleach) assigned to the formation of TiO2-R+-V is observed. This transient is clearly long-lived on the microsecond time scale. At high laser intensities (10 mJ/cm2) a strong bleach, also assigned to formation of TiO2-R+-V, is observed. This transient is clearly short-lived on the microsecond time scale. The recovery of the above transients are assigned to back electron transfer from the TiO2 nanocrystal component to the oxidized R component. In general, the rate of recovery of the above transients increases as the intensity of the laser pulse used to excite the ruthenium complex of TiO2-RV increases. The decay kinetics of the transient measured following high-intensity excitation can be fit using a biexponential function giving rate constants ka and kb of 2.4 × 106 s-1 (50%) and 2.0 × 105 s-1 (50%), respectively, at 10.0 mJ/cm2. Shown in Figure 4b are the absorbance transients measured for TiO2-RV at 390 nm under the conditions outlined above. At low laser intensity (0.4 mJ/cm2), a weak bleach assigned to formation of TiO2-R+-V is again observed. Clearly this transient is long-lived on the microsecond time scale. It should be noted,
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J. Phys. Chem. B, Vol. 105, No. 15, 2001 3003
Figure 5. (a) Transient absorption spectra of SnO2-R in MeCN with added LiClO4 (0.1 mol dm-3) at the indicated times I-VI (10, 25, 50, 100, 1000, and 2000 ns, respectively), following irradiation at 532 nm using the pulsed output of a Nd:Yag laser (7 ns, average of 10 transients at 10 s intervals, 6 mJ/cm2). (b) Transient absorption spectra of SnO2RV in MeCN with added LiClO4 (0.1 mol dm-3) at the indicated times I-VI (10, 25, 50, 100, 1000, and 2000 ns, respectively), following irradiation at 532 nm using the pulsed output of a Nd:Yag laser (7 ns, average of 10 transients at 10 s intervals, 6 mJ/cm2).
Figure 4. (a) Transient kinetics at 450 nm measured for TiO2-RV in MeCN with added LiClO4 (0.1 mol dm-3) following irradiation at 532 nm using the pulsed output of a Nd:Yag laser (7 ns, average of 10 transients at 10 s intervals, 0.4 and 10 mJ/cm2). (b) Transient kinetics at 390 nm of TiO2-RV in MeCN with added LiClO4 (0.1 mol dm-3) following irradiation at 532 nm using the pulsed output of a Nd:Yag laser (7 ns, average of 10 transients at 10 s intervals, 0.4 and 10 mJ/ cm2).
however, that no component of the measured transient could be assigned to reduced viologen. At high laser intensities (10 mJ/cm2), a large bleach, also assigned to formation of TiO2-R+-V, is observed. Clearly, this transient is not long-lived on the microsecond time scale. Significantly, this transient also shows formation of a new species on the microsecond time scale, which on the basis of the spectra in Figure 3 is assigned to formation of the radical cation of the viologen component. Further support for this assignment is the fact that, as expected, formation of this species is observed at 390 nm and not at 450 nm. The fact that formation of the radical cation of the viologen component of TiO2-RV is on the microsecond time scale strongly supports the view that the radical cation is formed by electron transfer from a TiO2 nanocrystal component to a V component adsorbed at the same or another TiO2 nanocrystal. 5. Time-Resolved Transient Absorption Spectroscopy of SnO2:Sb-R and SnO2:Sb-RV. The heterosupramolecular assemblies SnO2:Sb-R and SnO2:Sb-RV were prepared by chemisorbing the molecular components R and RV, respectively, at the surface of the constituent nanocrystals of a transparent nanostructured SnO2:Sb film supported on conducting glass. In the case of R and RV chemisorbed at the surface of the constituent nanocrystals of a transparent nanostructured SnO2: Sb film, the following are noted: First, the exact nature of the interaction between the phosphonic acid groups of R and RV and the surface of the constituent SnO2:Sb nanocrystals is not known. Second, the strength of the electronic coupling between the π*-orbitals on the bipyridine ligands and the available states of the Sb-doped SnO2 nanocrystal is not known. Third, the presence of the dopant, Sb, in the constituent nanocrystals endows the nanostructured film with a conductivity that has
not been fully quantified.6 Work is currently in progress to better understand this and related issues. As was the case for TiO2-R and TiO2-RV, spectra were measured for the heterosupramolecular assemblies SnO2:Sb-R and SnO2:Sb-RV, using optically matched samples in acetonitrile containing added LiClO4 (0.1 mol dm-3). Shown in Figure 5 are the time-resolved optical absorption spectra recorded for SnO2:Sb-R and SnO2:Sb-RV following excitation at 532 nm using the pulsed output of a Nd:Yag laser (6 mJ/cm2). The transient absorption spectra measured for SnO2:Sb-R, shown in Figure 5a, are characterized by a strong bleach between 360 and 550 nm assigned to a depletion of the ground state of R. These spectra are also characterized by a monotonically increasing absorption band in the near-infrared and assigned to injected electrons in the conduction band states of the nanostructured SnO2:Sb film.21 There is no transient that may be assigned to the triplet excited state of the sensitizer, 3R*. The transient absorption spectra measured for SnO2:Sb-RV, shown in Figure 5b, are characterized by a strong bleach between 360 and 550 nm assigned, as for SnO2:Sb-R, to a depletion of the ground state of R. These spectra are also characterized by a monotonically increasing absorbance in the near-infrared assigned, as for SnO2:Sb-R, to injected electrons in the conduction band states of the nanostructured SnO2:Sb film.21 No transients that may be assigned to either 3R* or the radical cation of the viologen component, V•+, are observed. Comparing the magnitude of the bleach at 465 nm, assigned to a depletion of the ground state of R in SnO2:Sb-R (0.026 au) and SnO2:Sb-RV (0.015 au), yields information concerning the branching ratio for formation of SnO2:Sb(e-)-R+V (60%) and SnO2:Sb(e-)-RV•+ (40%) by SnO2:Sb-1R*-V and SnO2: Sb-3R*-V. It is assumed that the precursor to 3R*, namely 1R*, is quenched by electron injection into SnO2:Sb and that any 3R* formed by intersystem crossing is quenched either by electron injection or by intramolecular injection to the covalently linked viologen component.18,22,23 This latter electron transfer, for reasons discussed in detail in relation to TiO2-RV, does not yield a long-lived radical cation state. In the case of TiO2(e-)-R+-V, it was observed that trap mediated transfer of the injected electrons to the viologen component of TiO2-RV led to the formation of long-lived radical
3004 J. Phys. Chem. B, Vol. 105, No. 15, 2001 cation TiO2-R-V•+. An analogous process in SnO2:Sb(e-)-R+V is not observed as the injected electrons are not localized in surface trap states as in TiO2 but move freely throughout the conducting nanostructured film in the absence of low-energy states capable of acting as traps.24 Consequently, an injected electron diffuses quickly through the substrate toward the back contact, as a consequence of which there is no localized shift of the quasi-Fermi level to more negative potentials, and hence, no detectable electron transfer to the viologen component by this route is observed. Conclusions The covalent assembly of a TiO2 nanocrystal, a ruthenium complex, and a viologen complex component to form the heterosupramolecular assembly TiO2-RV has been described. The covalent assembly of an analogous heterosupramolecular assembly, SnO2:Sb-RV, has also been described. It was shown that visible excitation of the above heterosupramolecular assemblies at 532 nm resulted in formation of the excited state of the ruthenium complex component, 1R*, and in electron injection by 1R* or 3R* into the nanocrystal (TiO2 or SnO2:Sb) component and electron transfer to the viologen component. Specifically, formation of the excited R component resulted in electron injection into the TiO2 or SnO2:Sb nanocrystal 60% of the time and electron transfer to the V component 40% of the time. In the case of TiO2-RV, long-lived formation of the radical cation of the V component is observed but not in the case of SnO2:Sb-RV. The findings reported in this study establish that formation of the long-lived radical cation of V is on the microsecond time scale. On this basis, it is concluded that formation of long-lived V•+ in TiO2-RV is mediated by the TiO2 nanocrystal. An understanding of the electron-transfer mechanisms occurring in these systems furthers progress toward the development and optimization of efficient optical storage devices based on organized heterosupramolecular assemblies.4 As reported elsewhere, the above heterosupramolecular assemblies can be written to using green light, read using red light, and reset upon application of a sufficiently positive potential.4 References and Notes (1) Balzani, V.; Scandola, F. In Supramolecular Photochemistry; Horwood: New York, 1991.
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