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J. Phys. Chem. C 2010, 114, 13459–13464

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Luminescence of a Ruthenium Complex Monolayer, Covalently Assembled on Silica Substrates, upon CO Exposure Fabio Lupo,† Maria E. Fragala`,† Tarkeshwar Gupta,‡ Antonino Mamo,§ Alessandro Aureliano,§ Marco Bettinelli,| Adolfo Speghini,⊥ and Antonino Gulino*,† Department of Chemistry, UniVersity of Catania and INSTM UdR of Catania, Viale Andrea Doria 6, 95125 Catania, Italy; Department of Chemistry, UniVersity of Delhi, North Campus, 110007 Delhi, India; Department of Physical Methods and Chemical Engineering, UniVersity of Catania, Viale Andrea Doria 6, 95125 Catania, Italy; Laboratory of Solid State Chemistry, DB, UniVersity of Verona and INSTM, UdR of Verona, Ca’ Vignal, Strada Le Grazie 15, I-37134 Verona, Italy; and Department of Science, Technology and Markets of the Vine and Wine, Verona and INSTM, UdR of Verona, UniVersita` di Verona, Strada Le Grazie 15, 37134 Verona, Italy ReceiVed: March 31, 2010; ReVised Manuscript ReceiVed: July 5, 2010

High quality silica and Si(100) substrates were functionalized with a covalent 4-ClCH2C6H4SiCl3 monolayer. Additional covalent bonding of an OH-phenol functionalized asymmetric ruthenium complex [Ru(bpy)2L](PF6)2 to the silylated substrates was further achieved, thus fabricating a new monolayer of ruthenium complex molecules on both silica and Si substrates. The chemical and spectroscopic characterization was carried out by X-ray photoelectron and UV-visible spectroscopy measurements. The optical properties of this robust monolayer were studied at room temperature by luminescence measurements in controlled atmosphere. Results grant the system recognition properties for CO at ppm levels. The adopted synthetic procedure has proven to be effective in transferring molecular properties to the solid state thus obtaining a photoluminescent device. Introduction Syntheses based on covalent assembly of appropriate molecules on suitable inorganic substrates represent one of the most powerful approaches in the perspective of fabrication of functional molecular architectures. These hybrid inorganic/ organic nanomaterials and supramolecular systems are of value for devices showing specific single molecule properties.1-9 Several [Ru(bpy)3]2+ (bpy )2,2′-bipyridine) complexes and their derivatives show low-lying metal-to-ligand charge transfer (MLCT) excited states with long luminescence lifetimes.10,11 In addition, stability toward photobleaching and transition energy levels can be tailored through synthetic efforts. As a result of these photophysical characteristics, polypyridine ruthenium(II) derivatives are excellent candidates for photosensitization reactions as the quenching of their MLCT excited states by “external stimuli”. In fact, polypyridine ruthenium(II) complexes have found extensive applications as photoprobes, sensors, photooxidizers, photocatalysts, light emitters, and so forth.12-22 To these purposes, ruthenium complexes have been supported on inorganic assemblies,5,23-25 polymer matrixes,26 and other hosts20-22 in order to fabricate specific functional systems. In this large field, selective detection of chemicals at low concentrations is a hot topic27-37 and molecular recognition by solids that employ monolayer strategies offers facile routes to practical probes for monitoring analytes.33-37 * To whom correspondence should be addressed. Tel. +39-095-7385067. Fax: +39-095-580138. E-mail: [email protected]. † Department of Chemistry, University of Catania and INSTM UdR of Catania. ‡ Department of Chemistry, University of Delhi. § Department of Physical Methods and Chemical Engineering, University of Catania. | Laboratory of Solid State Chemistry, DB, University of Verona and INSTM, UdR of Verona, Ca’ Vignal. ⊥ Department of Science, Technology and Markets of the Vine and Wine Verona and INSTM, UdR of Verona, Universita` di Verona.

Figure 1. Schematic representation of the ruthenium complex.

In general, designing device-quality monolayer-based sensors requires not only selectivity and sensitivity toward a given analyte, but also a high degree of stability and a fast, nondestructive read-out process. Sensor regeneration may be another requirement. Another major challenge is to maintain and enhance the desired molecular properties of a compound at the solid-state interface. For these reasons, the Ru complex shown in Figure 1 could be an excellent candidate for the selectively detection of given gas molecules in air. The advantages of Ru monolayer-based sensors include: (i) only a small amount of complex is needed to generate a large active surface, (ii) no sensing material is consumed, and (iii) there are no diffusion limitations because the surface-confined molecules are in direct contact with their environment. Fabrication of single layered pyridine ruthenium(II) systems has rarely been explored.5,20,22,25 Therefore, herein we

10.1021/jp1028917  2010 American Chemical Society Published on Web 07/22/2010

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Figure 2. Schematic representation of the Ru-SAM.

present synthesis, characterization, and optical properties of a new ruthenium quinoline-dipyridine molecular monolayer. In addition, selective detection of ppm levels of CO using this Ru-based self-assembled monolayer (hereafter Ru-SAM) covalently bound to quartz substrates (Figure 2) has been demonstrated. Experimental Section Aldrich reagents and solvents, some of them packed under nitrogen, were used throughout all present syntheses. The synthesis of the [Ru(bpy)2L](PF6)2 complex (L ) 4-p-hydroxyphenyl-6-bromo-2-(2′-pyridyl)quinoline) has been accomplished according to a general procedure described in the literature,38 and the details on synthesis and characterization are reported as Supporting Information. Fused silica (quartz) substrates were cleaned by immersion in “piranha” solution (c H2SO4: 30% H2O2 70:30 v/v) for 1 h and then left to cool to room temperature. Subsequently, substrates were repeatedly rinsed with double-distilled water and immersed in a H2O: 30% H2O2: NH3 5:1:1 v/v/v mixture at room temperature for 35 min.39-41 Then, they were washed with double-distilled water and dried under vacuum immediately before the deposition of the coupling agent. Si(100) substrates, obtained from ST Microelectronics (Catania, Italy), were first cleaned with “piranha” solution at room temperature for 15 min, rinsed in double-distilled water for 3 min, etched in 2.5% hydrofluoric acid for 100 s, washed with double-distilled water, and accurately dried with prepurified N2. Subsequently, they were treated for 10 min with UV and ozone using the OzoneGenerator (Fisher 500) system in order to obtain a 20 Å SiO2 thin layer.42 All the following sample manipulations were performed in a glovebox under N2 atmosphere. In particular, freshly cleaned substrates were immersed, at room temperature for 15 min, in a 0.5:100 (v/v) n-pentane solution of the chemisorptive trichloro[4-(chloromethyl)phenyl]silane, (siloxane) to afford a monolayer of the coupling agent (CA).35,36,39-41 Then they were washed with copious amounts of pentane, sonicated in pentane for 4 min to remove any physisorbed CA,

Lupo et al. immersed in a 1.0 × 10-3 M CH3CN/Toluene 50:50 v:v solution of the Ru complex, and heated up to 90 °C under stirring for 80 h in the dark. Finally, the substrates bearing the covalently self-assembled Ru complex monolayer were cooled to room temperature and sonicated with CH3CN, toluene, and THF to remove any residual unreacted Ru complex. The films strongly adhere to the substrates and, when stored in a desiccator with the exclusion of light, were stable for months, as evidenced by UV/vis spectroscopy. Neither washing nor sonication with CH3CN, toluene, benzene, acetone, isopropyl alcohol, or THF removed the films from the surface. Angle resolved X-ray photoelectron spectra (AR-XPS) of the Ru-SAM on Si(100) were measured at 80°, 45°, 30°, 15°, and 5° relative to the surface plane with a PHI 5600 Multi Technique System which offers a good control of the photoelectron takeoff angle (base pressure of the main chamber 1.5 × 10-10 Torr).42-47 The analyzer acceptance angle and the precision of the sample holder concerning the angle are (3° and (1°, respectively. The spectrometer is equipped with a dual anode X-ray source, a spherical capacitor analyzer (SCA) with a mean diameter of 279.4 mm, and an electrostatic lens system Omni Focus III. Freshly synthesized samples were mounted on Pt sampleholders and quickly transferred from the glovebox under N2 atmosphere to the XPS main chamber. Spectra were excited with monochromatized Al-KR X-ray radiation. The XPS peak intensities were obtained after Shirley background removal.45 No relevant charging effect was observed. Experimental uncertainties in binding energies lie within (0.3 eV.42 UV-visible absorption measurements were performed using a UV-vis V-650 Jasco spectrophotometer. Experimental uncertainty lies within (0.2 nm. Photoluminescence (PL) measurements were measured using the 488.0 nm line (100 mW) of an Ar laser as excitation source, equipped with a monochromator and a charge-coupled device (CCD) as detector. The emission was measured in backscattering geometry with respect to the exciting line beam. The quartz Ru-SAM was placed in a quartz cuvette closely sealed with a rubber septum lid and equipped with two needles for gas inand outlet. Controlled atmosphere measurements were performed by allowing different gas mixtures to flow in the cuvette. In particular, starting reference measurements were performed in air. For the purpose of comparison, PL measurements were also performed in pure N2 and O2 atmospheres (see below). Then, mixtures of air and 50-1000 ppm of each of the following gases: N2, CO2, H2, Ar, NOx, and CO were continuous leaked for 1 m prior to collect emissions (mixture flow rate ) 50 sccm). Flow rates were controlled within (2 sccm using MKS flow controllers and a MKS 147 Multi gas Controller. Regeneration of the sensor after CO gas exposure required heating at 90 °C for 5 min under air flow. Atomic force microscopy (AFM) measurements were performed with a Solver P47 NTD-MDT instrument in semicontact mode (resonance frequency 150 Hz). The monolayer surface was flat and homogeneous with no pinholes. The observed average roughness is 0.664 Å. Infrared transmittance spectra of the monolayer was recorded using a Jasco FT/IR-430 spectrometer, equipped with a Harrick GATR germanium single reflection ATR (attenuated total reflectance) accessory. One hundred scans (scan range 400-4000 cm-1, resolution 4 cm-1) per spectrum were collected. IR(ν/ cm) of the Ru-SAM: 1608 (w), 1541 (s), 1507 (s), 1498 (s), 1488 (s), and 1473 (s), 1457 (s), 1436 (m), 1419 (m).

Luminescence of a Ruthenium Complex ML with CO

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TABLE 1: XPS Derived Atomic Concentration Analysis For The Ru-SAM take-off angle

5°

15°

30°

45°

80°

Si 2p O 1s C 1s N 1s Cl 2p F 1s Br 3d Ru 3d5/2 or 3p3/2 P 2p

6.6 18.2 66.0 3.2 2.4 2.4 0.3 0.4 0.4

10.1 19.1 62.5 3.3 2.4 1.9 0.3 0.4 0

16.0 24.1 52.5 2.7 2.5 1.6 0.2 0.3 0

22.5 25.4 45.2 2.3 2.3 1.8 0.2 0.3 0

26.0 26.7 41.7 2.1 2.0 1.1 0.2 0.2 0

Results and Discussion Synthesis of the Monolayer. The Ru-SAM was synthesized by covalent grafting of the Ru complex to quartz substrates that were previously cleaned, hydroxylated, and silylated.35,36,39-41 In particular, the silylation was performed under N2, with the trichloro[4-(chloromethyl)phenyl]silane, 4-ClCH2C6H4SiCl3, a bifunctional coupling agent that bonds both to the substrate and to the Ru molecules.35,36,39-41 This Ru-SAM strongly adheres to the substrate, is robust, insoluble in common organic solvents, stable for months, and cannot be removed by the “Scotch-tape decohesion” test5,40 nor by mechanical abrasion with a task wipe, as evidenced by both UV-visible and XPS measurements. XPS of the Monolayer. The molecular monolayer characterization of the Ru-SAM on Si(100) was carried out with X-ray photoelectron spectroscopy. This technique gives information on the bonding states of the grafted molecules,35,36,39,42 and allows estimation of the surface elemental composition, making due allowance for the relevant atomic sensitivity factors.42-49 The reaction between Ru complex and the chlorobenzylterminated monolayer is not quantitative due to its high molecular footprint. The observed Cl/Ru ratio ) 7.1 (Table 1), obtained by considering the entire analyzed takeoff angles, indicates a yield of ∼12.5%. Figure 3 shows the XPS spectrum of the Ru-SAM in the Ru 3d-C 1s energy region. The spectral resolution is good and, as known, there is overlap between the Ru 3d3/2 and the C 1s bands. The Ru 3d5/2 state lies at 281.4 eV and unambiguously confirmed the presence of the Ru complex on the substrate surface. A careful inspection of Figure 3 reveals two additional components. That centered at 285.0 eV is due to both aliphatic and aromatic backbones,44 while the component at 286.5 eV, in agreement with literature data, is due to the carbon of the unreacted benzyl chloride moiety of the

Figure 4. XPS atomic concentrations (IC/ISi) vs the photoelectron takeoff angle of the Ru-SAM on Si(100). The R2 value of the fit is 0.983.

Cl-CH2C6H4- fragment and to the C-N groups.43,44 Finally, aromatic carbon, usually shows XPS shakeup satellites at 6-7 eV to higher binding energy with respect to the main peak. In Figure 3 there is evidence of very weak πfπ* shakeup features, centered at 291.5 eV, whose intensity is 3-4%, with respect to the main peak. These overall values are in agreement with previously reported XPS data for similar systems.44 A further experiment was made to rule out any alternative surface grafting. A hydrophilic SiO2 terminated surface was exposed to a 1 × 10-3 M CH3CN/toluene Ru complex solution for 60 h and then rinsed. No XPS evidence of Ru was found, thus excluding the presence of Ru complex on the surface. Figure 4 shows the AR-XPS angular dependence of the IC/ISi intensity ratios (IC and ISi are the total intensities of carbon and silicon, respectively) vs the photoelectron takeoff angle for the Ru-SAM. The ratios exponentially decrease upon increasing the photoelectron takeoff angle θ, consistently with the presence of a carbonaceous overlayer C of thickness d on Si. The IC/ISi intensity ratio of AR-XPS signals can be modeled as follows:42

I∞C (1 - e-d/λC1ssenθ) IC ) C ISi I∞Sie-d/λSi2psenθ C

C C where λSi2p and λC1s are the mean free paths of Si 2p and C 1s photoelectrons in a carbonaceous overlayer.46,47 This equation can be adopted to fit experimental data and provides an estimation of the thickness of the carbonaceous overlayer.42 The obtained d value of 2.5 nm is strongly consistent with the presence of a monolayer of Ru-complex in a perpendicular grafting geometry, with respect to the silicon surface. Using eq 2 where nN represents the number of N-containing molecules/ cm3 in the monolayer; σ is the photoelectron cross-section; λ is the inelastic mean free path; T(E) is the analyzer transmission function of the XPS instrument; d is the monolayer thickness (2.5 nm), θ is the photoelectron takeoff angle, it was possible to estimate nN corresponding to the surface coverage of the RuSAM. Taking into account 6 nitrogen atoms per molecule, a value of 1.2 × 1014 Ru-molecules was obtained.

IN Figure 3. Monochromatized Al-KR excited XPS of the Ru-SAM on Si(100) substrate, in the Ru 3d-C 1s energy region, at 45° electron takeoff angle.

(1)

ISi

)

nN(atom/cm3)σNλN/monolayerT(EN)(1 - edN/monolayer/λN/monolayercos θ) nSi(atom/cm3)σSiλSi/SiT(ESi)(1 - edN/monolayer/λSi/monolayercos θ)

(2)

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Figure 5. UV-vis spectra of the CH3CN 5.897 × 10-4 solution of the Ru complex (dotted red line) and of a representative Ru-SAM on quartz (solid black line).

Absorbance Spectra of the Monolayer. Figure 5 shows the room temperature absorption UV-visible spectra of both RuSAM on a quartz substrate (solid black line) and of the Ru complex 5.897 × 10-4 M solution in CH3CN (red dotted line). The UV/vis absorption spectrum of the monolayer shows a ligand-centered (LC) π-π* transition band peaked at 290 nm while the singlet metal-to-ligand charge-transfer bands (1MLCT) are visible in the 420-550 nm range.50 The absorption spectrum for the Ru-SAM sample is very similar to that observed for the Ru complex solution, apart from a small red shift of 6.8 nm for the ligand-centered π-π* transition. Using the Beer-Lambert law (A ) εlc, where A is the absorbance and ε, l, and c are the molar extinction coefficient, the thickness of the film, and the concentration of the ruthenium complex molecules in the film, respectively), it is possible to obtain the surface coverage, dsurf ) Aε-1 (number of ruthenium complex molecules/cm2 for the Ru-SAM).35,39 In fact, from the absorption spectrum of the Ru complex in CH3CN solution an ε value of 63033 L mol-1cm-1, at 284 nm, can be obtained. Then, the calculated density value results to be 1.3 × 1014 molecules/cm2, a value that is practically identical, considering the experimental uncertainties, to that derived by XPS, thus giving a molecular footprint of 72 Å2. This footprint value is in agreement with a full surface coverage and very close to similar values (60-65 Å2) obtained for an Rh-complex monolayer.35 IR of the Monolayer. Also ATR-FTIR measurements provide further information useful to confirm the monolayer formation. According to already reported results on a similar system, both bpy and Ru-bpy bands are in the 1600-1420 cm-1 region.51 Luminescence Properties of the Monolayer. The room temperature emission spectrum for the Ru-SAM in controlled atmosphere was first measured in air. The spectrum shows a broad band extending from 600 to 850 nm with PL features at 680, 720, and 760 nm (Figure 6). The luminescence is attributed to typical 3RuL-CT bands.51 Moreover, significant overlapping structures are evident. These features can be due to different transitions starting from a set of closely spaced levels in thermal equilibrium. These “clusters” of luminescent excited states most probably have substantial triplet character, as evidenced for [Ru(bpy)3]2+ complexes.52 For the sake of comparison, PL spectra in pure N2 and O2 atmospheres were also measured. Thus, a quartz cuvette containing the Ru-SAM and closely sealed with a rubber septum lid was leaked for 30 m with N2 or O2. The spectrum in N2 shows minor variation in the abovementioned structures with respect to that of the Ru-SAM in air, apart from overall stronger emission intensity. After leaking O2 for 30 m inside the cuvette, an intensity decrease with no spectral shape variation was observed, thus indicating that O2 gives rise

Figure 6. Luminescence spectra of the Ru-SAM: in pure N2 (top intensity, black line), in air or O2 (middle intensity, black line) and after 1 m leaking of 50 (red line), 100 (green line), 150 (blue line), and 200 (pale blue line) ppm of CO in air. Inset: Stern-Volmer plot using PL intensities at 680 nm.

to significant emission quenching. Interestingly, the PL intensities in O2 are identical to those observed in the air atmosphere. The initial emission intensity observed in N2 was completely restored by fluxing the Ru-SAM under nitrogen. An identical behavior for the PL intensity variation was observed on going along the N2, air, O2 gas sequence. No intensity difference was observed in PL spectra on fluxing O2 after air. It results evident that there are no differences between the spectra recorded in air or in pure oxygen, thus inferring that the oxygen content in air (∼21%) is above that for the maximum luminescence quenching for the Ru-SAM. Therefore, the emission intensity obtained in air was used as reference for the following measurements. In fact, the capability of the present Ru-SAM to behave as a sensor for other small gas molecules has been investigated by further luminescence measurements. Thus, the quartz cuvette containing the Ru-SAM was leaked for 1 min with different gas/air mixtures. Specifically, 1000 ppm of N2, CO2, H2, Ar, NOx, or CO were allowed to flow in the cuvette. Air was used to dilute these gases. Further reduced emission intensities upon laser excitation at 488.0 nm, become evident only upon CO exposure, compared to the reference initial emission in air (I0) (Figure 6). All of the other gases did not substantially affect the initial emission intensity in air. In particular, PL intensities decrease upon increasing the CO/air concentration in the 0-200 ppm range and do not change after a prolonged (>1 min) flow of a given CO/air mixture. These experiments clearly show the dependence of the Ru-SAM emission intensity in air upon the CO concentration and an 80% emission (I200) was observed when the Ru-SAM sample was excited in an air atmosphere containing 200 ppm of CO. Importantly, the system can be reversibly brought to the reference air intensity value by heating at 90 °C for 5 min under air flow. In this perspective, it is crucial to evaluate the sensitivity level of the sensor. PL measurements upon exposure to different air/CO mixtures showed intermediate PL intensities ranging between I0 and I200 (Figure 6) and stable PL intensity emission values are attained after only 1 m exposure. The CO concentration range in air revealed by the Ru-SAM lies in the 50-200 ppm range. Repeated experiments within this CO range are highly reproducible, reversible, and indicate a good response dynamics. Inset of Figure 6 clearly shows a PL linear behavior of the Stern-Volmer plot vs the CO concentration. In the present system, the Ru-SAM consists of a single layer of Ru-

Luminescence of a Ruthenium Complex ML with CO

Figure 7. Ru-SAM ∆ PL intensity variations, measured at 680 nm, during cycling 100 ppm CO and the restoring process.

based luminescent molecules and, therefore, the quenching process occurs in the unique 2D sensing surface. Both the formation of an adduct in the ground state or, more likely, the dynamic PL quenching (exciplex formation) involve linear Stern-Volmer plots. In the case of the exciplex, the quenching could be attributed to the creation of an excited complex in which the CO molecule interacts with the metallic center, upon the weakening of the Ru-N bonds. In this case, it is reasonable to expect that in the exciplex, the high vibration frequency of the CO moiety can promote radiationless decay channels to the luminescent Ruthenium complex. The observed behavior implies a residual luminescence not quenched by air or CO. Therefore, it is conceivable that some of the Ru-based molecular building blocks are less accessible to the quencher because of the high packing of the Ru-complex molecules in the Ru-SAM (molecular footprint of 72 Å2). Prolonged air/CO flux in the Ru-SAM containing cuvette did not significantly change the PL intensities. The monolayer was exposed several times to 100 ppm of CO followed by the restoration process. Figure 7 shows relative PL intensity variation in terms of percentage between the starting (100%) or air restored Ru-SAM and the final saturation values (the lowest of which has been chosen as 0%).35 It is evident from the results that cycling does not affect the performance. No hysteresis was observed, and shapes and peak positions of PL bands remained unchanged. Remarkably, heating the Ru-SAM at 180 °C in air for 8 days does not affect its performance. The temporal stability of the system was also demonstrated in air for, at least, 7 months. The formation of real monolayer-based sensing devices requires the combination of many properties, including selectivity, reversibility, and stability. For instance, the response to CO needs to be consistent in the presence of other gaseous compounds. The Ru-SAM is highly selective toward CO in air because no PL variations were observed after fluxing with mixtures of air and 1000 ppm of N2, NOx, H2, CO2, or Ar. Only exposure to CO resulted in a positive response, as judged by PL emission. Conclusions The novel [Ru(bpy)2L](PF6)2 molecular monolayer, covalently assembled to engineered silica and Si(100) substrates, has been synthesized, characterized, and its luminescent behavior has been exploited. This latter result confirms the transferring of the photoluminescence molecular property to the solid state. In general, systems like this can be used for the design of molecular electronics, molecular machines, and motors, for the fabrication of dye-sensitized solar cells, in artificial photosynthesis, in light-

J. Phys. Chem. C, Vol. 114, No. 32, 2010 13463 to-chemical energy conversion, as light harvesting antennas, in nonlinear optics, as DNA probes and as building blocks for macromolecular assemblies that are of interest in biochemistry and chemical diagnosis.21 Our present results indicate that the Ru-SAM is a solid chromophore also useful for chemical sensors. Photoluminescence measurements performed on the RuSAM at room temperature indicate that the system is suited for CO optical recognition in air, therefore in real life conditions, yielding emission in the visible range. Interestingly, in the present case, sensor regeneration is straightforward. In fact, exposing this monolayer-based CO sensor for only a few minutes to air at 90 °C is sufficient to fully reset the system. The demonstrated response time coupled with nearly immediate optical read-out is short (1 min). The structure and function of the sensor apparently are not affected by thermal stress tests, thus placing the present monolayer in a rare class of functional monolayer-based assemblies that are highly stable. Acknowledgment. The authors thank the MIUR, Roma, for financial supports (PRIN 2009). Supporting Information Available: Details of the synthesis of the [Ru(bpy)2L](PF6)2 complex (Scheme S1, L ) 4-p-hydroxyphenyl-6-bromo-2-(2′-pyridyl)quinoline), elemental analysis, and 1H NMR spectrum, accomplished by using a combination of one- and two-dimensional NMR techniques (Figure S1), are reported as Supporting Information. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Shi, F. N.; Cunha-Silva, L.; Sa´ Ferreira, R. A.; Mafra, L.; Trindade, T.; Carlos, L. D.; Almeida Paz, F. A.; Rocha, J. J. Am. Chem. Soc. 2008, 130, 150. (2) Massue, J.; Quinn, S. J.; Gunnlaugsson, T. J. Am. Chem. Soc. 2008, 130, 6900. (3) Crivillers, N.; Mas-Torrent, M.; Perruchas, S.; Roques, N.; VidalGancedo, J.; Veciana, J.; Rovira, C.; Basabe-Desmonts, L.; Ravoo, B. J.; Crego-Calama, M.; Reinhoudt, D. N. Angew. Chem., Int. Ed. 2007, 46, 2215. (4) Wang, L.; Yoon, M.-H.; Lu, G.; Yang, Y.; Facchetti, A.; Marks, T. J. Nat. Mater. 2006, 5, 893. (5) Shukla, A. D.; Das, A.; van der Boom, M. E. Angew. Chem., Int. Ed. 2005, 44, 3237. (6) Yoon, M.-H.; Facchetti, A.; Marks, T. J. Proc. Acad. Nat. Sci. U.S.A. 2005, 102, 4678. (7) Liu, Z.; Yasseri, A. A.; Lindsey, J. S.; Bocian, D. F. Science 2003, 302, 1543. (8) Hatzor, A.; Weiss, P. S. Science 2001, 291, 1019. (9) Maoz, R.; Matlis, S.; DiMasi, E.; Ocko, B. M.; Sagiv, J. Nature 1996, 150, 384. (10) Liu, F.; Meyer, G. J. Inorg. Chem. 2005, 44, 9305. (11) Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin Complexes; Academic Press: London, 1992. (12) Collinson, M. M.; Novak, B.; Martin, S. A.; Taussig, J. S. Anal. Chem. 2000, 72, 2914. (13) Baker, G. A.; Wenner, B. R.; Watkins, A. N.; Bright, F. V. J. SolGel Sci. Technol. 2000, 17, 71. (14) Ohsaka, T.; Yamagishi, Y.; Oyama, N. Bull. Chem. Soc. Jpn. 1990, 63, 2646. (15) Fiaccabrino, G. C.; Koudelka-Hep, M.; Hsueh, Y.-T.; Collins, S. D.; Smith, R. L. Anal. Chem. 1998, 70, 4157. (16) Xu, X.; Bard, A. J. J. Am. Chem. Soc. 1995, 117, 2627. (17) Li, C.; Hoffman, M. Z. J. Phys. Chem. A 2000, 104, 5998. (18) Shklover, V.; Ovchinnikov, Y. E.; Braginsky, L. S.; Zakeeruddin, S. M.; Gra¨tzel, M. Chem. Mater. 1998, 10, 2533. (19) Walt, D. R. Acc. Chem. Res. 1998, 31, 267–278. (20) Chu, B. W.-K.; Yam, V. W.-W. Langmuir 2006, 22, 7437. (21) Derossi, S.; Brammer, L.; Hunter, C. A.; Ward, M. D. Inorg. Chem. 2009, 48, 1666. (22) Harriman, A.; Khatyr, A.; Ziessel, R. Dalton Trans. 2003, 2061. (23) Szulbinski, W. S.; Kincaid, J. R. Inorg. Chem. 1998, 57, 859. (24) Khairoutdinov, R. F.; Doubova, L. V.; Haddon, R. C.; Saraf, L. J. Phys. Chem. B 2004, 108, 19976.

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