Ru(bpy)₃²⁺ → Ru(bpy)₃³⁺ + e^- (1)

TPrA → TPrA^+• + e^-→ TPrA^•+ H⁺ (2a)

Ru(bpy)₃³⁺ + TPrA → Ru(bpy)₃²⁺ + TPrA^+• → TPrA^•+ H⁺ (2b)

Ru(bpy)₃³⁺ + TPrA^• → Ru(bpy)₃^2+• + products (3)

Ru(bpy)₃^2+• → Ru(bpy)₃²⁺ + h (4)

Scheme 1. Plausible Ru(bpy)3II-TPrA ECL reaction sequence.

The tris(2,2’)bipyridylruthenium(II)/tripropylamine (Ru(bpy)₃²⁺-TPrA) system has been widely used over the past one and a half decades in trying to enhance and better understand the phenomenon of chemiluminescence. The mechanism behind light emission is quite complex and still not completely understood. A very common scheme describing the event shows how tris(2,2’)bipyridylruthenium(II) is electrochemically oxidized (eq 1) and tripropylamine (TPrA) serves the role of co reactant. TPrA oxidation is also believed to occur electrochemically and the newly formed tripropylamine radical (eq 2a) is now able to reduce Ru(bpy)₃³⁺ to a radical (eq 3). This excited radical species relaxes (eq 4) and gives off energy in form of light in the 700 nm range. TPrA oxidation becomes quite important at the low ruthenium(II) concentration usually employed in ECL experiments.

(A) CV scan for the Ru(bpy)₃²⁺ (10-5M), TPrA (0.05M) ECL solution indicating the potential range of interest on bare electrode; (B) CV-ECL oxidation runs; upper line represents 16 layers or PDADMA/PSS; middle line represents P4VMP/PSS, 16 layers and lower line denotes the ECL current signal on bare electrode. In both multilayers the outermost layer was PSS (negative). Conditions: 50 mV s-1 scan rate, 1 cm² electrode area vs. SCE reference electrode. The electrolyte composition consisted of 0.2M NaH₂PO₄ with no extra NaCl added. ECL plot displays the oxidation runs only.

Cyclic voltammetry was used to determine the voltage inducing the highest luminescent levels on 16 layers of PDADMA/PSS and P4VMP/PSS respectively and Figure 1 shows the results. Repeated scans established that the system is brightest at approximately 1.19V on both the oxidation and reduction runs. This voltage corresponds to an otherwise uneventful region when plotted against i_PEMU. This can be attributed to the fact that the phenomenon of ECL is not entirely due to the electrical component. The oxidized Ru(III) has to be reduced and excited by the TPrAH^+• coreactant in order to become luminescent. TPrA has also to be oxidized to TPrAH^+• in order to trigger the ECL sequence. This effect along with a loading of the PEMU is responsible for the large (30 times more luminescence) ECL enhancement, panel B for the PDADMA/PSS multilayer vs. the bare electrode.

ECL behavior vs. TPrA concentration of a 16 layer PSS-PDADMA PEMU. Panel A shows the cell diffusion currents for 16 layers (∆), with PSS on top and 17 layers (○) with PDADMA on top. The ECL of the bare electrode is depicted by (◊). Panel B shows the ECL currents and panel C compares relative efficiencies. Data was recorded at E = 1.199 V, on a 1 cm² platinum electrode, with a saturated KCl reference electrode. ECL solution consisted of 10 M Ru(bpy)₃²⁺ and 0.05 M TPrA at pH =7.2, kept by 0.2 M phosphate buffer.

Our data also show an increase in i_ECL with increasing TPrA concentration. The relationship is not linear however, as i_ECL levels off in the PSS-PDADMA multilayer and even decreases at higher TPrA concentrations in the PSS-P4VMP film. The constant increase in i_ECL on the bare electrode provides ground for the TPrA in the polyelectrolyte film. Since luminescence is achieved among other steps by the reaction of Ru(bpy)₃³⁺ and TPrA, the leveling off of i_ECL can be explained by this overloading with TPrA, a case when there is too much TPrA and not enough Ru(bpy)₃³⁺ to react with it, and light emission is ultimately limited by the Ru(bpy)₃³⁺ concentration. This suggests that in our case, reaction steps 2a and 2b are rate limiting up to 0.4 M TPrA.

LBL-ECL; (◊) represents PDADMA/PSS ECL current signal upon LBL deposition and (□) represents P4VMP ECL current signal upon LBL deposition. Panel (A) shows membrane diffusion current through the PEMU and panel (B) shows the ECL currents. ECL was recorded at E = 1.199 V, on a 1 cm² platinum electrode, with a saturated KCl reference electrode. ECL solution consisted of 10^-5 M Ru(bpy)₃²⁺ and 0.05 M TPrA at pH =7.2, kept by 0.2 M phosphate buffer. This potential provides the highest ECL levels and a steady state for the diffusion current. The thicknesses of PDADMAC/PSS and P4VMP/PSS PEMUs at 16 layers were 669 Å and 285 Å respectively.

The LBL approach to ECL shows an oscillatory behavior which reinforces the permeability properties of the multilayer. There is more ruthenium(II) and TPrAH⁺ diffusing when a negative polyelectrolyte is on top of the PEMU due to charge-charge interactions. The positive charges on the coreactant molecules promote their interaction with the negatively charged PSS top layer and increases ECL levels. This is clearly visible in both the cell diffusion plot and the ECL current plot. The addition of a positive layer greatly reduces the number of Ru(bpy)₃²⁺ and TPrAH⁺ molecules diffusing through and is followed by a decrease in ECL.