Luminescence Oxygen Sensor Based on a Ruthenium (II) Star

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Luminescence Oxygen Sensor Based on a Ruthenium(II) Star Polymer Complex Sarah J. Payne, Gina L. Fiore, Cassandra L. Fraser, and J. N. Demas* Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904 A novel quenchometric oxygen sensor based on a low polydispersity (PDI) star polymer [Ru(bpyPS2)3](PF6)2 (bpy ) 2,2′-bipyridine, PS ) polystyrene) is reported. The synthesis, characterization, photophysics, and oxygen sensing properties are examined. Combining the polystyrene support with the oxygen sensing ruthenium complex provides much higher doping levels without microcrystallization of the complex than traditional two-component sensors. The single molecule approach also avoids sensor leaching. While the polydispersity was 1.10, indicating a very tight distribution of molecular weights, sensor heterogeneity was not completely eliminated, as the luminescence decays were still multiexponentials. The likely source of this heterogeneity and possible methods for generating more homogeneous materials are discussed. Luminescent compounds have been widely used as sensors.1,2 For practical reasons, sensing molecules are generally incorporated into solid substrates such as polymer films.3 These films find use in pressure sensitive paints,4 in vivo oxygen sensors,5 and metal ion and pH sensors.6 The transition from well-behaved solution sensors to supported ones is not without difficulties. In our area, transition metal complexes imbedded in polymers can undergo metal complex leaching, inefficient excitation of low optical density films due to low doping levels, and precipitation or aggregation when concentrations are raised. They can also suffer from multiexponential decays, as well as nonlinear Stern-Volmer quenching plots (SVQPs) in quenchometric sensors.7,8 It has been suggested that the nonlinearities of SVQPs are caused by varying microenviron* Author to whom correspondence should be addressed. E-mail: [email protected]. (1) DeGraff, B. A.; Demas, J. N. Luminescence-Based Oxygen Sensors. Reviews in Fluorescence; Geddes, C., Lakowicz, J. R., Eds.; Springer Science: New York, 2005; Vol. 2, pp 125-151. (2) Callan, J. F.; de Silva, A. P.; Magri, D. C. Tetrahedron 2005, 61, 8551– 8588. (3) Bacon, J. R.; Demas, J. N. Anal. Chem. 1987, 59, 2780–2785. (4) Engler, R. H.; Klein, C.; Trinks, O. Meas. Sci. Technol. 2000, 11, 1077– 1085. (5) del Hierro, A. M.; Kronberger, W.; Hietz, P.; Offenthaler, I.; Richter, H. J. Exp. Bot. 2002, 53, 559–563. (6) Price, J. M.; Xu, W.; Demas, J. N.; DeGraff, B. A. Anal. Chem. 1998, 70, 265–270. (7) Carraway, E. R.; Demas, J. N.; DeGraff, B. A.; Bacon, J. R. Anal. Chem. 1991, 63, 337–342. (8) Demas, J. N.; DeGraff, B. A.; Xu, W. Anal. Chem. 1995, 67, 1377–1380. 10.1021/ac9020837  2010 American Chemical Society Published on Web 01/05/2010

Figure 1. Representative Ru(II) tris(bipyridine)-centered polystyrene complex.

ments within the polymer substrates.8 This heterogeneity causes downward curved SVQPs that can be fit using a multisite model.9 Some success in alleviating these problems has been achieved by covalently linking the transition metal complexes to the polymer. Winnik has succeeded in attaching a tris(polypyridine) ruthenium complex to a polymer chain via a reactive pendant group on one ligand.10 This gave a reasonably high doping concentration, mildly curved SVQPs, and freedom from leaching. However, the luminescence decays were complex with up to three exponentials being required to fit them. To provide for a more homogeneous system, we proposed, as an alternative route, a star polymer as shown in Figure 1. These have been synthesized via metalloinitiation as well as macroligand chelation utilizing controlled polymerization, and can be produced with a very low polydispersity (PDI), as near as one can get to a polymeric single molecule by chain growth methods.11 We anticipated that these star polymers would have several advantages over doped systems. Leaching is a nonissue. Significantly higher concentrations of the sensor molecule can be achieved without precipitation or aggregation. Since the complex is essentially a pure molecule, it was hoped that each sensing molecule would be in a similar environment. We will show that some, but not all, of these features were realized. Here we investigate [Ru(bpyPS2)3](PF6)2 via its excited state lifetime, SVQPs, as well as diffusion coefficients toward the (9) Demas, J. N.; DeGraff, B. A.; Xu, W. Anal. Chem. 1995, 67, 1377–1380. (10) Winnik, M. A.; Manners, I.; Wang, Z.; McWilliams, A. R.; Evans, C. E. B.; Lu, X.; Chung, S. Adv. Funct. Mater. 2002, 12, 415–419. (11) Fraser, C. L.; Smith, A. P. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 4704–4716.

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goal of developing systems that are free from complex decays and have linear SVQPs. While PS is not an optimum polymer because of its low diffusion coefficient and rather poor quenching, it makes for a good model system to test the potential of macromolecular metal complex sensors. MATERIALS AND METHODS Materials. The hexafunctional metalloinitiator, [Ru{bpy(CH2Cl)2}3](PF6)2, was prepared as previously described.12 Styrene was dried over CaH2 and distilled under reduced pressure prior to use. All other reagents were used as received. Instrumentation. Absorption spectra were taken on a PerkinElmer Lambda 25 UV-vis spectrometer. A Spex Fluorolog 2 + 2 spectrofluorimeter was used to obtain luminescence spectra, intensity Stern-Volmer quenching plots, and diffusion data. The diffusion data were obtained using a gas flow chamber and a manual three-way valve to switch between N2 and O2. The films were excited at 450 nm and observed at 610 nm. Lifetimes were obtained via a home-built time-domain lifetime system with a pulsed 337 N2 laser.13 Decays (400 averages) were recorded on a Tektronix TDS-540 digital oscilloscope and then transferred to a PC and analyzed using a nonlinear least-squares fitting routine programmed in Mathcad 12. The fitting routine is based on the multiexponential approximation seen in eq 114 N

I(t) )

∑R · e

-t/τj

j

(1)

j)0

where N (N ) 1, 2, or 3) is the number of exponents to be fit and R is the fractional preexponential contribution. For the multiexponential decays, the lifetimes reported are the pre-exponential weighted lifetime τprexp given by N

τprexp )

∑Rτ

j j

(2)

j)1

which is directly comparable to intensities in the Stern-Volmer expression.15 An Olympus IX-70 widefield microscope was used to observe the polymer films to ensure the absence of microcrystal formation. A 60× oil immersion objective was employed for all observations. Molecular weights were determined by gel permeation chromatography (GPC) (THF, 25 °C, 1.0 mL/min) using multiangle laser light scattering (MALLS) (λ ) 633 nm, 25 °C) and refractive index (RI) (λ ) 633 nm, 40 °C) detection. A Polymer Laboratories 5 µm mixed-C guard column and two GPC columns along with Wyatt Technology Corp. (Optilab DSP interferometric refractometer, DAWN DSP laser photometer), Agilent Technologies instrumentation (series 1100 HPLC), and Wyatt Technology software (ASTRA) were used in GPC analysis. (12) Collins, J. E.; Lamba, J. J. S.; Love, J. C.; McAlvin, J. E.; Ng, C.; Peters, B. P.; Wu, X.; Fraser, C. L. Inorg. Chem. 1999, 38, 2020–2024. (13) Kneas, K. A.; Xu, W.; Demas, J. N.; DeGraff, B. A. Appl. Spectrosc. 1997, 51, 1346–1351. (14) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 2nd Ed.; Kluwer Academic/Plenum: New York, 1999; p 98. (15) Carraway, E. R.; Demas, J. N.; DeGraff, B. A. Langmuir 1991, 7, 2991– 2998.

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Synthesis of [Ru{bpyPS2}3](PF6)2. The six-arm Ru polystyrene complex was prepared as previously described with the following modifications.16 In a glovebox, a stock solution of CuBr (0.004 g, 0.027 mmol), pentamethyldiethylenetriamine [PMDETA] (5.7 µL, 0.027 mmol), styrene (1.25 mL, 10.85 mmol), and DMF (0.145 mL; 10% v/v vs styrene) were combined and stirred until homogeneous. The CuBr/PMDETA solution was transferred to a Kontes flask charged with [Ru{bpy(CH2Cl)2}3](PF6)2 (0.010 g, 0.009 mmol). The flask was sealed, removed from the glovebox, and immersed in an oil bath maintained at 110 °C. The reaction was stirred for 2 h and then quenched by immersion in an ice bath. The crude product was diluted in THF and passed through a neutral alumina plug to remove the Cu catalyst. The solution was concentrated in vacuo and precipitated into cold MeOH (-78 °C) to yield an orange product: 0.139 g (12%, ∼20% conversion, uncorrected for monomer conversion). The 1H NMR spectrum is in accord with previously reported data.17 1 H NMR (300 MHz, CDCl3): δ ) 7.34-6.85 (br m), 6.85-6.25 (br m), 1.98-1.16 (br m). Mw (MALLS) ) 42 800, PDI ) 1.10. UV-vis (CH2Cl2) λmax (
) ) 459 nm (24 700 M-1 cm-1). Sensor Preparation. The films were cast in one of two ways: (1) by dropping a solution of [Ru{bpyPS2}3](PF6)2 in dichloromethane or chloroform onto glass coverslips, (2) by spincasting the solution on no. 1 glass coverslips (VWR, cat. no. 48393 084). The solutions used were 2% by mass ruthenium polymer complex. A Dremel 395 VS MultiPro rotary tool was used as a spin coater with speeds varied using a Variac; the rpm measured with a strobe lamp was about 450 rpm. The film thickness for the diffusion experiment was measured using a micrometer that was sensitive to ±0.001′′. For all measurements films remained adhered to the glass coverslip. Stern-Volmer Quenching Plots. Stern-Volmer quenching plots are common tools for viewing and analyzing luminescence lifetime and intensity quenching data.18 Equation 3 gives the relationship between the pre-exponential weighted lifetime (τprexp) and quencher concentration [O2] τ0prexp I0 ) ) 1 + Ksv[Q] I τprexp

(3)

where τ0prexp is the lifetime and I0 is the intensity in the absence of quencher, τprexp is the lifetime and I is the intensity at different quencher concentrations [Q], and Ksv is the SternVolmer quenching constant. For single-component collisional quenching systems, the linear form is obeyed. Though multicomponent systems frequently do not obey this linear form, our systems do. The intensity measurements were made using a flow cell utilizing oxygen, air, or nitrogen. The data were fit using the linear form to obtain Ksv. Diffusion Coefficient Measurements. Diffusion coefficient measurements were made in the gas flow cell by following the emission intensity with a step change in the oxygen concentra(16) Wu, X.; Collins, J. E.; McAlvin, J. E.; Cutts, R. W.; Fraser, C. L. Macromolecules 2001, 34, 2812–2821. (17) Wu, X.; Fraser, C. L. Macromolecules 2000, 33, 4053–4060. (18) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 2nd ed.; Kluwer Academic/Plenum: New York, 1999; p 240.

Figure 2. 3-D plot of GPC elution volume versus inline-diode array UV-vis spectra for [Ru{bpyPS2}3](PF6)2 six-arm star showing that the metal complex is associated with the eluting polymer.

tion.19 The system was equilibrated by flowing nitrogen through it. The gas flow then was switched to oxygen via a three-way valve system. The film was excited at 450 nm, and emission intensity was monitored at 610 nm as a function of time using the fluorimeter with a 1 s integration time. The data were analyzed as discussed earlier.19 RESULTS AND DISCUSSION Ruthenium tris(bipyridine)-centered polystyrene materials ([Ru(bpyPS2)3](PF6)2) were prepared by atom transfer radical polymerization (ATRP) of styrene from a hexafunctional Ru complex initiator, [Ru{bpy(CH2Cl)2}3](PF6)2. Polystyrene was selected over other pendant groups such as polymethylmethacrylate (PMMA) since oxygen quenching is much better in PS than in PMMA where the diffusion coefficient D is roughly an order of magnitude lower.20 The polymer product was analyzed by NMR, UV-vis, and luminescence spectroscopy. As previously described for this method, the polymerization of styrene from a metal complex is controlled at low monomer conversions, and can result in materials with broad molecular weight distributions at higher conversions.16 This is especially true in the preparation of six-armed RuPS materials. When DMF is used as a solvent and the reaction is quenched at ∼20% monomer conversion, a polymer product with narrow molecular weight distribution is obtained (Mn ) 42 800 g/mol, PDI ) 1.10). This low PDI material was used in all measurements. The 1H NMR, absorption, and emission spectra and ε are in agreement with previously published values.17 Figure 2 shows the three-dimensional plot of GPC elution volume versus inline-diode array UV-vis spectra for (19) Kneas, K. A.; Demas, J. N.; Nguyen, B.; Lockhart, A.; Xu, W.; DeGraff, B. A. Anal. Chem. 2002, 74, 1111–1118. (20) Charlesworth, J. M.; Gan, T. H. J. Phys. Chem. 1996, 100, 14922–14927.

Figure 3. Absorbance and emission spectra of [Ru(bpyPS2)3](PF6)2 in a dichloromethane solution.

[Ru(bpyPS2)3](PF6)2. The coincidence of the 460 and 290 nm absorptions of the Ru(bpy)32+ with the polystyrene absorption at 260 nm establishes that the metal complex is associated with the eluting polymer. There is some tailing in the 260, 290, and 450 nm regions. The 260 nm absorption indicates the presence of polystyrene, and the 290 and 450 nm absorptions arise from lower molecular weight [Ru(bpyPS2)3](PF6)2. Nevertheless, the low polydispersity suggests that the complex is very close to being a single distinct species, compared to conventional radical polymerization methods. The solution UV-vis spectrum and the luminescence spectrum of the film (Figure 3) are typical of ruthenium complexes.21 The UV-vis absorption of the film is broader and less structured due to scattering compared with that of the solution, which lessens the definition of the spectrum, but the λmax of the 450 nm metal (21) Shi, S.; Zhou, J.; Zong, R.; Ye, J. J. Lumin. 2007, 122-123, 218–220.

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to ligand charge transfer (MLCT) bands are very similar and differ by less than 5 nm. The solution emission is 2 nm blueshifted compared to the film but is otherwise virtually identical. In a 0.003 ± 0.002 cm thick film, the absorbance was 0.73, which gave ε ≈ 12 000 cm-1 M-1. Given the errors in the very thin film thickness, this is similar to the solution value. As expected, both dropped and spun cast films showed no microscopic evidence of microcrystallization. This is in marked contrast to a number of discrete complexes dissolved in polymer where it is easy to exceed the solubility of the complex in the polymer and end up with unquenched crystals. The result is not surprising given the homogeneous nature of the star polymer and the inherent close interaction between the complex and the covalently attached polymer. In the polymer, the Ru(II) concentration was 0.027 M, which is generally far in excess of what can be achieved in doped polymers. Winnick’s single component system used a loading about a factor of 30 below our levels, although it appears that they could synthesize polymers with higher loadings.10 In good solvents for PS such as THF and dichloromethane, a single component accounts for the 96-98% emission intensity for [Ru(bpyPS2)3](PF6)2 with lifetimes of 1240 ns (major) and 280 ns (minor) in deoxygenated solvents. This could be due either to impurities such as shorter chain complexes or other ruthenium species or some type of persistent heterogeneity. The GPC suggests small amounts of low molecular weight ruthenium complexes which could account for the second component. However, due to the large difference in lifetimes we favor low levels of some unknown ruthenium species. Treating it as a two-component system, both the long- and short-lived components give linear SV quenching constants with values of 14 and 1 atm-1, which is consistent with similar bimolecular constants. Regardless, the impurity is a minor emission component. In contrast to the microscopy results, lifetime data on the polymer film shows evidence for nonexponential decays. In nitrogen, there is a long-lived 1660 ns feature comprising 73% of the emission intensity, a 660 ns component comprising 25%, and a minor 2% comprising 100 ns decay. This is a clear indication of heterogeneous environments around the metal centers, well above the impurity levels suggested by the solution data. Clearly the multiple lifetimes indicate multiple environments. The best fits to the data utilize a triple-exponential approximation, but a fourth would be required to fit the very weak residual tail. Figure 4 illustrates the combined lifetime and intensity SVQPs for [Ru{bpyPS2}3](PF6)2 films which, within experimental error, are linear. The averaged quenching constant is 0.35 ± 0.04 atm-1, which is consistent with values obtained for Ru(bpy)32+ dissolved in polystyrene at much lower concentrations (0.150.45 atm-1).22 While the intensity SVQP appears linear, this does not establish a homogeneous system. For low degrees of quenching, curvature can be greatly attenuated and not easily detected even for highly inhomogeneous systems. The lifetime measurements, which are much more sensitive to heterogeneity, unambiguously establish multiple environments. The poor quenching cannot be attributed to a short excited state lifetime (22) Fuller, Z. J. Masters Thesis, University of Virginia, Charlottesville, VA, 2002, pp 48-59.

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Figure 4. Stern-Volmer quenching plot for [Ru(bpyPS2)3](PF6)2 in 1 atm N2, 0.21 atm O2, and 1 atm O2. f and b are the SVQPs of the dropped film for the intensity (I) and the lifetime (τ), respectively. 9 and 2 are the SVQPs of the spun cast film for the intensity (I) and the lifetime (τ), respectively. The linear fit for all the data is I(τ)0/I(τ) ) 0.35 atm-1 [O2] ( 1.003.

Figure 5. Fit of the diffusion data to quenching diffusion equations in ref 19. Actual data (2), fit to data (s), (20% of best fit D to show reliability (f).

from self-quenching; the lifetime of [Ru{bpyPS2}3](PF6)2 is longer than that for dilute [Ru(bpy)3](PF6)2 in PS. The diffusion constant for the star polymer, 2.2 ± 0.7 × 10-7 2 cm /s, is in line with the values reported earlier for ruthenium complex doped polystyrene at 2.3 ± 0.8 × 10-7 cm2/s.19 The consistent diffusion constants show that incorporating the complex into the star polymer has little effect on its characteristics with respect to oxygen diffusion. A representative fit is shown in Figure 5. The line is the best fit to the actual data points. The outer lines are the calculated curve if the diffusion constant is varied from the best-fit value by ±20%, which gives a measure of the reliability of the measurement. In keeping with the low D, the response time for even this thin film is not good (about 1 min). Even with the low polydispersity, we found significant heterogeneity. A possible issue is due to molecular weight control in the styrene polymerization. Although ATRP is controlled and can result in well-defined materials, it is also characterized in some instances by side reactions such as chain termination and chain-chain coupling that can lead to polymer products with broad molecular weight distributions. This is particularly so for

multifunctional initiators. Although the reaction was quenched at low monomer conversion in an effort to obtain materials with low molecular weight distribution, this does not guarantee an entirely homogeneous polymer sample. Another possible explanation is that polystyrene has a glass transition temperature (Tg) of ∼95 °C,23 and at room temperature the sensor molecules are locked into different environments that cannot change during their excited state lifetime. We favor this interpretation. We made measurements at higher temperatures to test this hypothesis. With our system we were able to make measurements in nitrogen up to 80 °C. However, there appeared to be no improvement in the nonexponentiality, which is not a surprise as we were still well below Tg. We would expect that this problem could be corrected by using a pendant polymer with a lower Tg. This could also enhance quenching due to the greater oxygen mobility. A primary advantage of these systems would still be the very high concentration of Ru complex (∼30 mM) that allows good signals with very thin films or nanoparticles.

avoid or minimize problems associated with multicomponent sensors. The star polymer sensor concept tested for [Ru{bpy(PS)2}3](PF6)2 generated by copper catalyzed ATRP provides a very promising route to aggregation-free high concentration sensors. It did not solve the heterogeneity issue as ascertained by very sensitive luminescence lifetime measurements. However, variation of the star polymer arms to give lower Tg values and better controlled polymerization methods for synthesis should solve the heterogeneity issue in future studies. ACKNOWLEDGMENT We thank the National Science Foundation for support through NSF CHE 0410061 (J.N.D.) and CHE 0718879 (C.L.F.), and Zachary J. Fuller for preliminary measurements on the star polymer. We thank A. Periasamy for use of the microscope in the Keck Imaging Center. We thank the Physics Department for the loan of the strobe light.

CONCLUSIONS There was hope that a star polymer sensor molecule would provide a homogeneous environment for the sensing moiety and

Received for review September 16, 2009. Accepted December 15, 2009.

(23) Fox, T. G.; Flory, P. J. J. Polym. Sci. 1954, 14, 315–319.

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