J. Phys. Chem. B 2001, 105, 9011-9015
9011
Solvent Viscosity and Interrelated Effects on Electrochemiluminescence Intensity of Tris(2,2′-bipyridyl)ruthenium(II)† Amy M. Scott and Radha Pyati* Department of Chemistry, UniVersity of Colorado at Colorado Springs, Colorado Springs, Colorado 80933-7150 ReceiVed: April 2, 2001; In Final Form: July 4, 2001
We report the electrochemiluminescence (ECL) intensity of tris(2,2′-bipyridyl)ruthenium(II) [Ru(bpy)32+] as a function of varying solution viscosity in mixtures of N,N-dimethylformamide and glycerin. Diffusion coefficient (D), electron-transfer rate (k) of Ru(bpy)32+/3+, and luminescence intensity are measured and correlated with viscosity and ECL intensity (Iecl). Iecl decreases with increasing solution viscosity and tracks diffusion and electron transfer until it reaches an upper limit. This limit is reached at D ) 5 × 10-7 cm2/s and k ) 0.03 cm/s in this system. Normalizing Iecl for diffusion reveals a weak enhancement of ECL with increasing viscosity.
Introduction The electrochemiluminescence (ECL) of tris(2,2′-bipyridyl)ruthenium(II) [Ru(bpy)32+, R2+] has been studied under many conditions and for a variety of applications.1 The mechanism accepted in the literature1b is the following: k2/3
R2+ 98R 3++ek2/1
R2+ + e- 98 R+ kannihilation
R3+ + R+ 98 *R2+ + R2+ *R2+ f R2+ + hν Clearly several processes occur in the mechanism, and each is dependent upon solvent properties to a certain degree. The combined impact of these processes produces a complex dependence of ECL intensity on solvent. Although ECL behavior has been related to solvent effects,2 ECL has not yet been systematically described as a function of solvent viscosity. The significance of this problem lies in the development of ECLbased solid-state light-emitting devices.3 To predict ECL behavior as liquid solvents give way to solid ones, a description of viscosity effects on ECL becomes important. Solvent viscosity maps the macroscopic transition between the liquid and solid states and affects each component process of ECL. An understanding of the ECL behavior of Ru(bpy)32+ in this arena can point to the factor that most strongly controls ECL so that ultimately solid-state devices can be improved. The ECL of Ru(bpy)32+ in the solid state has been investigated in several configurations,4 but viscosity effects have not been explicitly examined. This study reports the first description of the ECL intensity of Ru(bpy)32+ as a function of solution viscosity (η). Mixtures of N,N-dimethylformamide and glycerin were utilized to vary the viscosity across 2 orders of magnitude in the liquid range. †
Part of the special issue “Royce W. Murray Festschrift”. * Corresponding author. E-mail:
[email protected]; fax (719) 2623047.
Intuition suggests that increased viscosity should depress ECL intensity; this study presents the correlation of that decrease with changes in diffusion coefficient, electron-transfer rate constant of Ru(bpy)32+/3+, and photoluminescence (PL). Experimental Section Chemicals and Electrodes. N,N-Dimethylformamide(DMF, UV, Burdick & Jackson) and lithium trifluoromethanesulfonate (Aldrich, 99,995%) were used as received, and LiCF3SO3 was stored in a desiccator. Tris(2,2′-bipyridyl)ruthenium(II) hexafluorophosphate, [Ru(bpy)3](PF6)2, was metathesized from [Ru(bpy)3]Cl2 (Strem, 98%) and KPF6 (Johnson Matthey Electronics, 98%) in water, and the product was dried overnight in a vacuum oven at 82 °C. Glycerin (J. T. Baker Chemical Co.) was used in w/w percentage amounts with DMF to vary viscosity. Viscosities were measured using a Brookfield Dial Viscometer, model LVTCP under temperature control at 25 °C. Solutions were deoxygenated with argon for 20 min before voltammetric, ECL, and PL measurements. Microelectrodes were prepared by encasing a 25-micron diameter platinum wire in a glass capillary tube, soldering the Pt wire to a copper wire for electrical contact, and polishing the assembly with sandpaper, Meta-Di diamond paste, and 0.05µm alumina (Buehler). Microelectrodes and a 3.0-mm diameter glassy carbon electrode (Bioanalytical Systems, Inc., BAS) were polished with 0.05-µm alumina and rinsed before each experiment. A silver-silver chloride electrode (Ag/AgCl/3M NaCl, +0.207 V vs NHE, BAS) and Pt wire (Strem Chemicals, 99.95%) served as reference electrodes; the auxiliary electrode was a Pt wire. Diffusion Coefficient and Electron-Transfer Rate Measurements. All cyclic voltammetry (CV) was performed on a CH-620 electrochemical analyzer (CH Instruments, Austin, TX). Diffusion coefficients (D) of Ru(bpy)32+ were calculated from sigmoidal CVs taken at microelectrodes, using the equation ilim ) 4nFrDC.5 Electron-transfer rate constants of Ru(bpy)32+/3+ were measured using either the Nicholson method6 for more viscous solutions or a mixed radial-linear diffusion method for less viscous solutions. The mixed diffusion method is based upon an oblate spheroidal space coordinate-expanding time grid
10.1021/jp0112323 CCC: $20.00 © 2001 American Chemical Society Published on Web 08/02/2001
9012 J. Phys. Chem. B, Vol. 105, No. 37, 2001 and uses the Hopscotch algorithm and resultant working curves to measure rate constants faster than those usually accessible by the Nicholson method.7 The accuracy of this method was determined by measuring the electron-transfer rate of ferrocene, which matched that of literature7 within experimental error. In addition, the rate constant of Ru(bpy)32+/3+ in the 40% glycerin solution was determined by both methods and was identical within experimental error. Resistance compensation was done for all rate measurements by measuring Rsoln through the potentiostat and compensating for Rsoln in each subsequent voltammogram used for kinetics. Each rate measurement was done at seven scan rates. After resistance compensation, the rate constants showed no trend with increasing scan rates, indicating that rate constants reflect electron-transfer kinetics and not solution resistance. Luminescence Measurements. Solutions of 0.1 mM Ru(bpy)32+ were excited at 452 nm in an F-3010 Hitachi fluorescence spectrophotometer, and emission spectra were recorded from 500 to 700 nm. The intensity scale was held constant through the set of measurements in order to compare intensities among solutions. ECL Intensity Measurements. ECL intensities were measured with a Hamamatsu 4632 photomultiplier tube (PMT; Bridgewater, NJ) operating at -700 V provided by a Bertan series 230 power supply (Hicksville, NY). The cell and the PMT were placed under a black felt-lined box and covered with more black felt; the room was dark during measurements. Photocurrents were collected by a Hamamatsu 3866 photon counter, powered by an Elenco XP-765 power supply. Pulses from the photon counter were attenuated from 5 V to 300 mV using a home-built voltage divider, and 300-mV pulses were counted in 1000 bins of 1.3107-ms duration by a Stanford Research Systems multichannel scaler SR430. ECL intensity was measured by applying a 10-Hz square wave from a Wavetek 171 function generator for 1.31 s to a solution of 3.75 mM Ru(bpy)32+ in 0.375 M LiCF3SO3. The voltages of the square wave were set to the voltages that gave the highest ECL intensity in the most viscous solution, thus accounting for solution resistance and resultant potential drops that might not achieve the appropriate ECL potentials. These voltages were +3.6 to -3.4 V. This potential program did not appear to produce Ru(bpy)30 because three consecutive runs were reproducible and no electrode filming was observed; the possible generation of Ru(bpy)3- does not compromise emission intensity.1h The potential program also lay within solvent limits throughout the solution series. The cell was constructed so that only the working electrode and a small ring of solution around it were visible to the PMT. Integrated ECL intensities were calculated by summing the counts per second across a single pulse. These sums were an average of four pulses early in the 1.31-s experiment, across three experiments. Results and Discussion Cyclic Voltammetry, Diffusion Coefficient, and ElectronTransfer Rate Constant. Figure 1 shows the cyclic voltammetry of Ru(bpy)32+ in DMF and in a 30% glycerin solution. The Ru(bpy)32+/3+ wave appears clearly, but the Ru(bpy)32+/+ is somewhat obscured by background processes, which are attributed to proton reduction from glycerin or water carried into solution by glycerin. Despite the presence of these processes, ECL in these solutions was consistently observed and quantified. Figure 2 is an example of a microelectrode voltammogram of Ru(bpy)32+/3+ in DMF. Such voltammograms are used to calculate diffusion coefficient; at faster scan rates,
Scott and Pyati
Figure 1. Cyclic voltammograms of 7.5 mM Ru(bpy)32+ and 0.75 M LiCF3SO3 at a 3-mm diameter glassy carbon working electrode, scan rate V ) 100 mV/s. (a) 30% glycerin/70% DMF; (b) 100% DMF.
Figure 2. Cyclic voltammetry at a 25-mm diameter Pt disk of 7.5 mM Ru(bpy)32+, 0.75 M LiCF3SO3, 100% DMF, scan rate ) 5 mV/s.
they yield peaks in a mixed diffusion profile that allow the calculation of rate constants of Ru(bpy)32+/3+. Table 1 shows the diffusion coefficient (D) and rate constant (k) for each solution. Both obey linear trends with 1/η. The diffusion coefficients follow the Stokes-Einstein equation (D ) kBT/ 6πrhη) for Newtonian fluids, and the slope of the equation gives a hydrodynamics radius (rh) for Ru(bpy)32+ of 7 Å. This value is comparable to similar calculations elsewhere,8 based on the supposition that Ru(bpy)32+ and Co(bpy)32+ have similar radii because the metal ions have similar radii.9 The rate constants, shown in Figure 3, are comparable to literature values10 and follow Marcus theoretical prediction9,11 described by the equation
(-∆G* RT )
k ) AτL-1exp
in which A is a constant describing precursor complex formation, reaction adiabaticity, and outer-sphere reorganizational energy; τL is the solvent longitudinal relaxation time; and ∆G* is the sum of the outer- and inner-sphere reorganizational energies of the electron transfer. The τL term is raised to the -1 power assuming that the reaction is adiabatic and that inner-sphere reorganizational energy is much less than outer-sphere reorganizational energy. The τL term can be related to the reciprocal of η:12
τL )
()
∞ 4πR3 τD where τD ) η s kBT
ECLIntensity of [Ru(bpy)32+]
J. Phys. Chem. B, Vol. 105, No. 37, 2001 9013
TABLE 1: Viscosity, Diffusion Coefficient, Electron Transfer Rate Constant, Photoluminescence Intensity, and ECL Results for Seven DMF/Glycerin Mixtures % glycerina
η (cP)
D (cm2/s)b
k (cm/s)b
PL intensityc (% FS)
Iecl (kc)d
DNI (kc-s1/2/cm)e
0 10 20 30 40 50 60
1.39 3.16 6.21 11.91 35.35 54.45 121.46
2.3 ((0.6) × 10-6 1.1 ((0.6) × 10-6 7.0 ((0.05) × 10-7 3.5 ((0.2) × 10-7 2.0 ((0.07) × 10-7 5.4 ((0.2) × 10-8 1.8 ((0.9) × 10-8
9.5 (( 3.9) × 10-2 4.0 (( 1.8) × 10-2 1.9 (( 0.8) × 10-2 1.1 (( 0.6) × 10-2 2.1 (( 0.7) × 10-3 4.9 (( 2.0) × 10-4 3.0 (( 2.4) × 10-4
2.3 3.8 3.1 2.9 4.7 3.1 3.2
223.9 (( 64.3) 239.9 (( 54.6) 239.9 (( 65.3) 157.8 (( 32.1) 100.0 (( 22.1) 135.2 (( 30.0) 66.6 (( 18.3)
1.47 (( 0.46) × 105 2.34 (( 0.85) × 105 3.48 (( 0.95) × 105 2.66 (( 0.55) × 105 2.79 (( 0.62) × 105 5.82 (( 1.25) × 105 4.16 (( 1.35) × 105
a Percent glycerin is w/w. b Solutions are 7.5 mM [Ru(bpy) ](PF ) /0.75 M LiCF SO . c Data reported as percent of full scale (% FS) PL intensity 3 6 2 3 3 recordable by spectrofluorimeter. Emission maximum is 619 nm. Luminescence spectra were taken in solutions of 0.1 mM [Ru(bpy)3](PF6)2/0.01 M LiCF3SO3. d Solutions are 3.75 mM [Ru(bpy)3](PF6)2/0.375 M LiCF3SO3. Unit is kilocounts. e DNI is diffusion-normalized intensity, or Iecl/D1/2. Error (σDNI) is calculated as follows:
σDNI )
x( ) ( σIecl
D1/2
Figure 3. Electron transfer rates measured for each glycerin/DMF solution containing 7.5 mM Ru(bpy)32+ and 0.75 M LiCF3SO3. The mixed-diffusion method at 25-µm diameter Pt disk or the Nicholson method was applied at a 3-mm diameter glassy carbon electrode.
where ∞ is the high-frequency dielectric constant, s is the static dielectric constant, τD is the Debye relaxation time and R is solvent radius. Substituting the latter two equations into k reveals a linear relationship between k and 1/η. This relationship relies upon the assumption that static and high-frequency dielectric constants do not vary across the solvent range selected here. This assumption is valid because the static dielectric constants of DMF and glycerin are very close (DMF ) 36.7,13 gly ) 42.514) and electrolyte concentrations are constant throughout the solution set. Observation of a linear trend here indicates that the electrontransfer rate of Ru(bpy)32+/3+ is controlled by solvent dynamics. This echoes other work that has shown solvent dynamics control of electron transfer,9,15 even on a Ru(II/III) couple,16 although work on solvent dynamics control of Ru(bpy)32+/3+ has not appeared in the literature. Photoluminescence. Literature indicates that in solutions ranging from fluid to rigid states, the luminescence lifetime of Ru(bpy)32+ is longer in more viscous solutions, because nonradiative relaxation of the excited state occurs to a lesser extent in rigid solvents.17 Intensity is thus postulated to be somewhat higher in the solid state.17 Table 1 shows that PL intensity of Ru(bpy)32+ does not vary significantly with η. However, the fact that intensity is constant is likely due to the η range used in this study. This constant PL intensity indicates that despite the expected decrease in ECL intensity with solution η, the emission does not contribute to such a decrease in this system.
2
+
)
-IeclD-3/2σD 2
2
Figure 4. Solid line: ECL emission of 3.75 mM Ru(bpy)32+ in 30:70 glycerin/DMF with 0.375 M LiCF3SO3 at a 3-mm diameter glassy carbon disk cycled at 10 Hz. Emission is displayed as counts per bin with bin width ) 1.3107 ms. Dashed line: Voltage applied to solution by function generator.
ECL Intensity. Figure 4 is an example of an ECL intensitytime trace. Light is observed on only one pulse per cycle, and emission occurs throughout the duration of the pulse. The singlepulse light behavior is attributed to instability of Ru(bpy)3+. We speculate that light is emitted on the negative voltage pulse during Ru(bpy)3+ production, which reacts immediately with Ru(bpy)33+. Yet insufficient Ru(bpy)3+ persists in solution long enough to generate light during the next positive-going pulse. In addition, the decrease in ECL intensity with repetitive pulses is likely due to the instability of Ru(bpy)3+ as well; loss of Ru(bpy)3+ near the electrode surface results in a net loss of *Ru(bpy)32+ over the course of the experiment. This does not affect a consecutive run because the surface region of solution was refreshed by gentle agitation. The integrated light intensities (Iecl) of single pulses are reported in Table 1. Clearly Iecl drops with increasing η, as intuition predicts. Figure 5 shows this correlation in Panel A; however, this correlation does not elucidate the underlying causes of this behavior. Thus the relationships between Iecl, diffusion, and electron transfer are explored further. To isolate the effect of diffusion, Figure 5 Panel B shows Iecl correlated with D1/2. Iecl rises and reaches a limiting value above a diffusion coefficient of 5 × 10-7 cm2/s, suggesting that increasing diffusion coefficient above that value in this solvent system does not improve ECL. Theory predicts that ECL intensity follows the square root of diffusion coefficient;18 yet
9014 J. Phys. Chem. B, Vol. 105, No. 37, 2001
Scott and Pyati
Figure 6. ECL intensity normalized for D1/2 shown as a function of reciprocal viscosity.
Figure 5. Integrated counts calculated from an average of three repetitive runs. (A) ECL intensity as a function of viscosity. (B) ECL intensity as a function of the square root of diffusion.
the trend is not observed here. This is likely due to the nonideality of the system evidenced earlier by the single-pulse ECL trace. This unusual trace precluded the possibility of obtaining meaningful ECL efficiencies. ECL efficiency (φecl) reflects photos emitted per electron transferred; electrons transferred are often represented by current passed on a potential step. Since current depends on diffusion coefficient, ECL efficiency is commonly determined by comparing integrated intensities with those of a known system [Ru(bpy)32+/CH3CN, φecl ) 0.051b] under the same conditions of electrode, electrolyte, and potential program frequency, and then accounting for the difference in Ru(bpy)32+ diffusion coefficient between CH3CN and the sample solution.19 The ECL traces in this study do not conform to expectation; nevertheless, the ability to correct Iecl for D allows one to approximate efficiency by correcting intensity for the transport rate that brings material to the surface. Thus, a diffusion-normalized intensity (DNI, Iecl/D1/2) was calculated, as shown in Table 1. Figure 6 shows DNI as a function of 1/η, and DNI increases very slightly with decreasing 1/η. The trend is not strong and could simply be a representation that DNI remains constant, as per theory. However, this slight increase in more viscous solutions could indicate the potentially helpful effect that slowed diffusion may have on ECL. Consider the transport processes occurring during a voltage cycle in ECL. The 2+ state diffuses to the electrode to make the 1+ on the negative pulse and the 3+ on the positive pulse. Also, the 1+ and 3+ states must encounter one another in order to undergo annihilation. Certainly slowing diffusion in these processes will diminish ECL. However, slowing other transport processes can ostensibly enhance ECL. The 1+ state diffuses to the electrode during the positive pulse to form the 2+ or 3+
Figure 7. ECL intensity as a function of the log of electron transfer rate (k).
states, and the 3+ undergoes an analogous process during the negative pulse. These reactions consume the ECL reactants before they have the chance to annihilate one another; thus, retarding their transport to the electrode will leave more 1+ and 3+ intact. Also, both the 1+ and 3+ states diffuse to the bulk solution, improving their chances of an encounter as transport is slowed. In these situations, slow diffusion enhances ECL, and it is speculated that our system exhibits this slight increase in DNI due to the impact of these processes. It is conceivable that the slight rise in DNI with increasing viscosity results from a retarded rate of Ru(bpy)3+ reaction with impurities, preserving more Ru(bpy)3+ for the ECL reaction. However, the DNI trend cannot be directly attributed to rates of reaction with impurities, because varying solvent composition could introduce changes in impurity concentration that counteract reaction rate effects. To isolate the effect of electron-transfer kinetics on Iecl, Figure 7 displays Iecl correlated with k. Here as well, Iecl rises and reaches a limiting value at k ) 0.03 cm/s. This suggests that elevating k above this value will not improve ECL considerably. Overall it appears that k has the strongest effect on Iecl and this is strengthened by the correction of Iecl for diffusion. PL intensities show no correlation with Iecl because they remain constant over the range of solutions. Conclusions In a series of glycerin/DMF mixtures, the ECL intensity of Ru(bpy)32+ decreases with increasing solution viscosity and follows diffusion and electron-transfer rate until it reaches a limiting value. Normalizing Iecl for diffusion shows a slight
ECLIntensity of [Ru(bpy)32+] increase in intensity as solutions grow viscous; this may indicate improved opportunities for reactant encounters near the electrode surface. Ongoing work in our laboratory addresses other factors that control ECL intensity, such as the rate of ion annihilation between Ru(bpy)3+ and Ru(bpy)33+. In addition, the extension of this work to higher viscosity solvents is interesting and leads ultimately to making measurements in the solid state. It is hoped that further study will lay out a detailed picture of the interrelated effects of solvent viscosity on ECL emission, to provide a road map for efficient optimization of ECL-based solid-state lightemitting devices. Acknowledgment. We thank Research Corporation and the University of Colorado at Colorado Springs for support. References and Notes (1) (a) Tokel, N.; Bard, A. J. J. Am. Chem. Soc. 1972, 95, 6582. (b) Tokel-Takvoryan, N. E.; Hemingway, R. E.; Bard, A. J. J. Am. Chem. Soc. 1973, 94, 2862. (c) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; Von Zelewsky, A.; Coord. Chem. ReV. 1988, 84, 85. (d) Leland, J. K.; Powell, M. J. J. Electrochem. Soc. 1990, 137, 3127. (e) Jamieson, F.; Sanchez, R. I.; Dong, L.; Leland, J. K.; Yost, D.; Martin, M. T. Anal. Chem. 1996, 68, 1298. (f) Workman, S.; Richter, M. M. Anal. Chem. 2000, 72, 5556. (g) McCall, J.; Alexander, C.; Richter, M. M. Anal. Chem. 1999, 71, 2523. (h) Wallace, W. L.; Bard, A. J. J. Phys. Chem. 1979, 83, 1350. (2) (a) Collinson, M. M.; Wightman, R. M.; Pastore, P. J. Phys. Chem. 1994, 98, 11942. (b) Maness, K. M.; Bartelt, J. E.; Wightman, R. M. J. Phys. Chem. 1994, 98, 3993. (3) Pei, Q.; Yu, G.; Zhang, C.; Yang, Y.; Heeger, A. J. Science 1995, 269, 1086. (4) (a) Collinson, M. M.; Novak, B.; Skylar, M. A.; Taussig, J. S. Anal. Chem. 2000, 72, 2914. (b) Khramov, A. N.; Collinson, M. M. Anal. Chem. 2000, 72, 2943. (c) Maness, K. M.; Terrill, R. H.; Meyer, T. J.; Murray, R. W.; Wightman, R. M. J. Am. Chem. Soc. 1996, 118, 10609. (d) Maness, K. M.; Masui, H.; Wightman, R. M.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 3987. (5) Wightman, R. M.; Wipf, D. O. In Electroanalytical Chemistry: A Series of AdVances; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15, p 267.
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