Intense and Tunable Electrochemiluminescence of Corannulene - The

Oct 26, 2010 - Electrochemiluminescence of corannulene in acetonitrile by the use of coreactants is reported here for the first time. The investigatio...
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J. Phys. Chem. C 2010, 114, 19467–19472

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Intense and Tunable Electrochemiluminescence of Corannulene Giovanni Valenti,† Carlo Bruno,† Stefania Rapino,† Andrea Fiorani,† Edward A. Jackson,‡ Lawrence T. Scott,‡ Francesco Paolucci,† and Massimo Marcaccio*,† Dipartimento di Chimica “G. Ciamician”, UniVersita` di Bologna, Via Selmi 2, 40126 Bologna, Italy, and Chemistry Department, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467-3860, United States ReceiVed: August 23, 2010

Electrochemiluminescence of corannulene in acetonitrile by the use of coreactants is reported here for the first time. The investigation has been carried out utilizing both benzoyl peroxide as a sacrificial coreactant and arylamine derivatives as nonsacrificial species to perform mixed-annihilation processes. The electrochemiluminescence produces an intense blue light. The use of different tris(arylamines) as mixed-annihilation coreactants gives rise to exciplexes with corannulene that emit at different wavelengths, thus allowing the emission color to be tuned by changing the coreactant molecule. This opens a potential pathway to exploitation of such an intriguing carbon structure for building efficient blue-light emitting devices. Introduction

Experimental Section

The electrogenerated chemiluminescence (ECL) of polyaromatic hydrocarbons has been widely investigated in the past years.1 A pivotal role was played by 9,10-diphenylanthracene (DPA), rubrene, and pyrene, owing to their excellent combination of photonic and redox properties, which typically comprise very high luminescence quantum efficiencies and radical anions that are sufficiently long-lived to generate ECL. Corannulene is a polynuclear aromatic hydrocarbon with a geodesic carbon framework recalling the buckminsterfullerene surface. In fact, the molecular structure of corannulene (CA) may be viewed as precisely one-third of a C60 molecule (i.e., a fullerene cap), with the residual valences saturated by hydrogen atoms.2 CA fluoresces at room temperature, and the well-defined emission band, exhibiting some vibronic structure, has a maximum at 423 nm. The intensity is not very high and about 1 order of magnitude lower (Φem ) 0.07) than that of 9,10-diphenylanthracene (Φem ) 0.88).3 Recently, Wu et al. showed that arylethynyl-CA derivatives can display high emission quantum yields and tunable emission wavelengths that depend on the nature of the substituent moiety.4 Moreover, corannulene is a useful building block for the synthesis and construction of large molecular and supramolecular architectures,5 such as pentagonal dendrimers,6 liquid crystals,7 and single-walled carbon nanotubes,8 and it might also be used in organic light-emitting devices (OLEDs).9 We report herein, for the first time, on the ECL of corannulene. This luminescence could be generated only by the socalled coreactant method10 because of unfavorable electrochemical properties of corannulene that make generation by cationanion annihilation unviable. Different coreactants have been used: both benzoyl peroxide as a sacrificial coreactant and nonsacrificial species for mixed-annihilation processes based on tris(arylamines). In particular, the use of a selection of nonsacrificial mixed-annihilation coreactants has allowed the tuning of the color emission from green to blue.

Chemicals and Electrochemical Measurements. Corannulene was synthesized according to the three-step procedure by Scott et al.11 Tetrabutylammonium hexafluorophosphate (TBAH; from Fluka), as supporting electrolyte, was used as received. Dry acetonitrile (ACN) was successively refluxed over, and distilled from, CaH2 and activated alumina super I neutral (ICN Biomedicals), and it was stored in a specially designed Schlenk flask over 3 Å activated molecular sieves, protected from light.12 Shortly before performing the experiment, the solvent was distilled through a closed system into an electrochemical cell containing the supporting electrolyte and the species under examination. Electrochemical experiments were carried out in an airtight single-compartment cell described elsewhere13 by using platinum as working and counter electrodes and a silver spiral as a quasi-reference electrode. The cell containing the supporting electrolyte and the electroactive compound was dried under vacuum at about 110 °C for at least 60 h before each experiment. All the E1/2 potentials have been directly obtained from cyclic voltammetric curves as averages of the cathodic and anodic peak potentials for one-electron peaks and by digital simulation for those processes closely spaced in multielectron voltammetric peaks. The E1/2 values are referred to an aqueous saturated calomel electrode (SCE) and have been determined by adding, at the end of each experiment, ferrocene as an internal standard and measuring them with respect to the ferrocinium/ ferrocene couple standard potential. Voltammograms were recorded with a custom-made fast potentiostat14 controlled by an AMEL model 568 programmable function generator. The potentiostat was interfaced to a Nicolet model 3091 digital oscilloscope, and the data were transferred to a personal computer through the program Antigona.15 The minimization of the uncompensated resistance effect in the voltammetric measurements was achieved by the positive-feedback circuit of the potentiostat. Photophysics. Adsorption spectra were measured on a UV-vis-NIR Varian Cary 5 spectrophotometer and were baseline corrected. Steady state emission spectra were recorded on a Varian Cary Eclipse spectrofluorimeter.

* To whom correspondence should be addressed. E-mail: massimo. [email protected] (M.M.). † Universita` di Bologna. ‡ Boston College.

10.1021/jp107964r  2010 American Chemical Society Published on Web 10/26/2010

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Luminescence quantum yields (Φem) were measured in optically dilute solutions (OD < 0.1 at excitation wavelength) and compared to reference emitters by the following equation

[ ][ ][ ][ ]

Ar(λr) Ir(λr) n2x Dx Φx ) Φr Ax(λx) Ix(λx) n2 Dr r

where A is the absorbance at the excitation wavelength (λ), I is the intensity of the excitation light at the excitation wavelength (λ), n is the refractive index of the solvent, D is the integrated intensity of the luminescence, and Φ is the quantum yield. The subscripts r and x refer to the reference and the sample, respectively. The quantum yield was performed at identical excitation wavelengths for the sample and the reference, canceling the I(λr)/I(λx) term in the equation. The refractive indices were nr ) 1.359 (ethanol) and nx ) 1.342 (acetonitrile). The quantum yields were measured against 9,10-diphenylanthracene (DPA) in degassed ethanol as the reference (Φ ) 0.88).16 Experimental uncertainties are estimated to be (10% for emission quantum yields. Electrochemiluminescence. The ECL measurements were carried out in ACN solution with TBAH as supporting electrolyte, under the same strictly aprotic conditions as described above, in the electrochemical subsection. A one-compartment, three-electrode airtight cell, with high-vacuum O-rings and glass stopcocks, was used for ECL measurements.17 The working electrode consisted of a platinum side-oriented 2 mm diameter disk sealed in glass, while the counter electrode was a platinum spiral and the reference electrode was a quasi-reference silver wire. In a given solution, two or three records were made to check the temporal stability of the system investigated. The compounds benzoyl peroxide (BPO), tris(4-bromophenyl)ammine (TBrPA), and N,N,N′,N′-tetramethyl-p-phenylendiamine (TMPDA) were used as coreactants as received from Aldrich, and all were analytical grade. Tris(2,4-dibromophenyl)amine (TBr2PA) and tris(2-bromo-4-methylphenyl)amine (TBrMePA) were synthesized following the method reported by Steckhan.18 For ECL generation by coreactant and mixedannihilation, these compounds were added at the concentration of 10 and 1 mM, respectively. The ECL signal generated by performing the potential step program was measured with a photomultiplier tube (PMT, Hamamatsu R4220p) placed a few millimeters from the cell, and in front of the working electrode, inside a dark box. A voltage in the range 250-750 V was supplied to the PMT. The light/current/voltages curves were recorded by collecting the preamplified PMT output signal (by a ultralow noise Acton research model 181) with the second input channel of the ADC module of the AUTOLAB instrument. ECL spectra have been recorded by inserting the same PMT in a dual-exit monochromator (ACTON RESEARCH model Spectra Pro2300i) and collecting the signal as described above. Photocurrent detected at PMT was accumulated for 1-3 s, depending on the emission intensity, for each monocromator wavelength step (usually 1 nm). Entrance and exit slits were fixed to the maximum value of 3 mm. The ECL yield is defined as the photons emitted per redox event, which is related to the total electrical charge involved in the generation of the reactants. Thus, the ECL efficiency can be rigorously estimated by the annihilation method and obtained by chronoamperometric experiment using the following expression19

ΦECL ) Φ°ECL(IQ◦ /I◦Q)

Valenti et al. where Φ°ECL is the ECL efficiency of the standard under the same experimental conditions, I and I° are the integrated ECL intensity of the species and the standard systems, and Q and Q° the faradaic charges (in coulombs) passed for the investigated species and the standard species, respectively. It has been estimated that the ECL efficiency can be confidently given with an error of (15%. The ECL yields by coreactant methods cannot be directly obtained,20 but it would be more appropriate to compare the integrated ECL intensity of the species with that of a reference compound, whose ECL efficiency by annihilation is known. Therefore, the measurements of a standard ECL system (i.e., 9,10-diphenylanthracene, which is among the most efficient ECL system19,21,22) in ACN solution containing BPO as a coreactant, under the same experimental conditions as those used for corannulene, were performed (Figure S2 in the Supporting Information), and the ECL intensity ratio (Icoran/IDPA) was determined. From such an ECL intensity ratio, using the value of ECL annihilation efficiency of DPA (whose value, under similar experimental conditions, is reported to be 11%),22b the ECL yield of corannulene can be indirectly obtained. Results and Discussion Corannulene may undergo several redox processes:23 within the millisecond cyclic voltammetry time scale, up to the generation of a trianion has been obtained, through a proper choice of solvent and supporting electrolyte.23c In acetonitrile (ultradry conditions), a solvent suitable for the simultaneous generation of radical cation and radical anion, corannulene displays two reductions: the first process is reversible with a E1/2 ) -1.90 V vs SCE, while the second one is affected by a follow-up reaction, which makes the voltammetric peak irreversible (Ep ) -2.77 V; Figure 1).23c The peak remains irreversible even at low temperature and higher scan rates (up to 200 V/s). The comparison between the first and second voltammetric peak shows that the second one is nearly two consecutive one-electron processes. The digital simulation of the cyclic voltammetric curves, at different scan rates and over a range of 2 orders of magnitude, indicates an ECE mechanism for the second irreversible peak, and a value E1/2 ) -2.61 V for the second reduction has been determined. It should be noted that the first reduction becomes less reversible, evidenced by the increased cathodic-anodic peak separation, when the potential is swept to include the second

Figure 1. Cyclic voltammetric curves (two single scans) of corannulene 1 mM in a 0.07 M TBAH/ACN solution. Working electrode Pt disk 125 µm (diameter); reference electrode SCE; scan rate ) 10 V/s, T ) 25 °C.

Electrochemiluminescence of Corannulene

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CHART 1: Structural Formulas of the Mixed-Annihilation Coreactants and Acronyms Used in This Work

reduction. This is a consequence of the follow-up reaction coupled to the second reduction process; moreover, an anodic peak appears at about -0.9 V on the reverse scan due to the follow-up reaction. On the other hand, in the positive potential region, CA is oxidized at about 1.9 V, as shown in Figure 1. The oxidation is represented by an irreversible multielectron anodic peak with a number of electrons exchanged between 2 and 3. Such a peak, due to a not well-identified follow-up chemical reaction, remains irreversible even at scan rates up to 200 V/s. In a typical electrochemiluminescence experiment, the energy to generate the emitting excited state comes from the homogeneous electrochemical reaction between the electrochemically generated oxidized and reduced species; thus, the energy requirements must be fulfilled by the electrochemical data. The annihilation reaction enthalpy, ∆H°ann ) ∆G°ann + T∆S°, must be exoergonic enough to generate the emitting state. ∆G°ann is obtained from the half-wave potential difference between the first oxidation and first reduction wave in the cyclic voltammogram (∆E1/2(ox-red)) which is, for corannulene, 3.8 V. If we consider an entropy contribution ≈0.1 eV,19,24 we obtain a minimum value of 3.7 eV for |∆H°ann | which is larger than the minimum energy of the singlet excited state of corannulene, estimated as 2.93 eV from the emission spectrum. From all of the above, the annihilation process involving the electrochemically generated cations and anions of corannulene would be sufficiently energetic to populate the emitting excited state. However, the anodic oxidation of CA is a totally irreversible process as already reported;23 the reaction responsible for the multielectron anodic peak (Figure 1) brings about the rapid fouling and passivation of the electrode surface even after the first scan, thus making ECL generation undetectable. Taking advantage of the rather negative potential for the first reduction of CA and of the very high stability of CA radical anion, ECL generation has been obtained by the use of a coreactant, such as benzoyl peroxide (BPO), operating in the reductive-oxidation mode.10b In typical experiments, ECL was obtained by pulsing or linearly scanning the working electrode potential (between 0 and -2.2 V) while simultaneously measuring the cell current and the emitted light intensity. ECL is generated by reaction of the reduced forms of BPO and CA, according to the well-established mechanism depicted in Figure 2.25 In fact, BPO is irreversibly reduced in acetonitrile (with a cathodic peak potential Ep ) -1.23 V vs SCE, labeled as I in Figure 2a), because of a fast follow-up reaction that generates the strongly oxidizing benzoyloxy radical (PhCOO•) species,

as sketched in Figure 2b. The redox potential E° for the couple PhCOO•/PhCO2- has been reported to be 1.5 V vs SCE.26 The benzoyloxy radical reacts with the corannulene radical anion, as soon as the latter is generated at the electrode surface, i.e., at potentials more negative than -1.90 V (peak labeled as II in Figure 2a), yielding the benzoate anion and the excited state of pristine CA, respectively (Figure 2b). The observation of an intense blue luminescence, easily visible by the naked eye, at potentials more negative than -1.90 V (red trace in Figure 2a) confirms the above mechanism. The

Figure 2. (a) Cyclic voltammetric and ECL/potential curves of 1 mM corannulene in a 0.07 M TBAH/ACN solution containing 10 mM BPO. (b) Sketch of the main mechanism for the ECL generation of corannulene (H atoms not shown) with BPO as coreactant. See also Scheme S1 in the Supporting Information.

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Figure 3. Corannulene spectra in acetonitrile solution: (a) photoluminescence (λexc ) 285 nm) and (b) ECL with BPO as coreactant; double potential step program: E1 ) 0 V, E2 ) -2.2 V (first reduction), pulse width t1 ) t2 ) 0.2 s, accumulation time 1 s, PMT bias 550 V.

ECL spectrum was also recorded (Figure 3b) and was found to be superimposable on the photoluminescence (PL) spectrum (compare Figures 3a and 3b);27 whence, an ECL efficiency of about 10% of that of the very efficient DPA28 was calculated (see also the Experimental Section). The inference therefore is that both ECL and PL are generated from the same excited state. ECL was also generated by the mixed-annihilation1 method using tris(4-bromophenyl)amine (TBrPA) as an oxidizing annihilation partner. In this case, both a double potential step and a potential sweep were applied at the working electrode, between the potential of the first reversible oxidation of the TBrPA29 (E1/2 ) 1.05 V vs SCE) and that of the first reduction of CA (Figure 4). Thus, ECL was obtained by the energy-sufficient homogeneous electron transfer occurring between the electrochemically generated CA radical anion and the radical cation of the nonemitting partner TBrPA. Since the high potentials for CA oxidation were not reached in such experiments, electrode surface passivation was also avoided. It is especially noteworthy that the ECL spectrum (see Figure 5, trace a) differs greatly in the present case from that obtained using BPO as the source of the oxidant: in particular, a much broader emission band was recorded, the maximum of which was significantly red-shifted (by ∼100 nm) with respect to that in the PL spectrum. It should be noted that the PL spectrum did not show any appreciable shift under the same experimental conditions (i.e., in the presence of tris(4-bromophenyl)amine even at much higher concentrations), and this casts doubt on the hypothesis that photoemission may involve an exciplex species. The formation of a tight ion pair between the CA radical anion and the TBrPA radical cation just prior to electron

Figure 4. (a) Sketch of the mixed-annihilation mechanism for the ECL of corannulene and TBrPA. The potential is cycled or switched between the first reduction of corannulene and the oxidation of TBrPA. The homogeneous electron transfer occurs between the oxidized and the reduced species to generate the excited corannulene which emits. (b) Cyclic voltammetric curve (black trace) and ECL intensity profile (red trace) of 0.7 mM corannulene and 0.9 mM TBrPA in 0.07 M TBAH/ ACN electrolyte solution. Working electrode Pt 2 mm diameter; reference electrode SCE; T ) 25 °C; PMT bias 750 V. The potential is first scanned to negative potentials and then to positive ones.

transfer,30 as schematically shown in Figure 5, on the other hand, would bring about a stabilization of the CA excited state, thus explaining the observed different influence of the two different coreactants. It is worth noting that the electrochemical behavior of CA shows no difference when it is performed in the presence and absence of TBrPA. Also, no ECL spectrum change was observed in the experiments carried out with higher concentrations of electrolytes (up to 0.3 M), which would disrupt the CA-TBrPA ion pair. Further experimental and theoretical work is currently ongoing. One strategy to prevent the exciplex formation during the homogeneous electron transfer reaction relies on the use of reactants with greater steric hindrance, in order to impede the molecular stacking. Thus, the tris(2,4-dibromophenyl)amine (TBr2PA) coreactant was used, as the Br substituents in the ortho positions constrain the phenyl rings in a more tilted conformation, thus making the molecule less prone to produce an exciplex with corannulene. The species TBr2PA maintains most of the characteristics of the TBrPA, apart from the standard redox potential, which moves up to 1.58 V (vs SCE). However, the ECL spectrum obtained with TBr2PA (Figure 5, trace b) shows an emission for which the maximum is still red-shifted (to 485

Electrochemiluminescence of Corannulene

Figure 5. ECL spectra of corannulene in acetonitrile solution with TBrPA (trace a, black), TBr2PA (trace b, blue), and TBrMePA (trace c, red) as coreactants. A sketch of the possible structure of the exciplex between the TBrPA and corannulene is also shown.

nm), although by only 50 nm with respect to that with BPO. Moreover, the ECL efficiency is lower than those achieved by the other two coreactants, in part because the oxidation process of TBr2PA is so positive that the cation partially oxidizes the corannulene, which produces a passivating film on the electrode surface that can be clearly observed at the end of the experiments. A further modification of the tris(arylamine), designed to have the oxidation less positive, led us to use the tris(2-bromo-4methylphenyl)amine (TBrMePA), whose oxidation occurs at 1.25 V (vs SCE; see Figure S3 in the Supporting Information). As expected, the ECL emission was as intense as that with BPO, and the spectrum showed the maximum at the same wavelength (Figure 5, trace c), indicating that exciplex formation does not occur or the interaction between the two partners is negligible. In this case, the vibrational structure of the ECL spectrum is basically the same as that of the photoluminescence one, and, unexpectedly, it appears to be even more resolved, as observed for the fluorescence spectra recorded in less polar solvents reported in literature.3 Thus, the use of the three coreactants by the mixed-annihilation method has made possible the generation of ECL emission with three different colors. Finally, mixed-annihilation ECL was also observed for the energy-deficient system, represented by corannulene and N,N,N′,N′-tetramethyl-p-phenylendiamine (TMPDA), although the signal was much weaker (Figure 6) and the spectrum could not be recorded. Figure 6 shows the chronoamperometry and ECL/time curves obtained by switching the potential between the oxidation of TMPDA (+0.15 V vs SCE) and the first reduction of corannulene. It is worth observing that the ECL signal is much more intense when the reduction of corannulene is performed in comparison with the oxidation of the TMPDA, and this indicates that the corannulene anion is less stable than the TMPDA cation. Concerning the excited state generation mechanism, according to the photophysical data available,3 the most plausible hypothesis involves the formation of the triplet state of corannulene through the cross-reaction between the TMPDA cation and the CA anion, followed by the triplet-triplet annihilation (TTA) which brings on the formation of the emitting state. This is known as TTA mechanism in the ECL,19 often invoked for energy-deficient systems, and its global efficiency is comprehensibly rather low.

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Figure 6. Chronoamperometry (black signal) and ECL/time (red signal) curves of 0.5 mM corannulene 0.07 M TBAH/ACN solution containing 1 mM TMPDA. PMT bias 750 V. The potential program is switched between 0.45 and -2.1 V.

Conclusions In summary, the ECL of the intriguing corannulene species was obtained and found to produce an intense blue light emission using a coreactant. Most important, the investigation shows also that utilizing a suitable nonsacrificial coreactant with corannulene (or corannulene derivatives) would allow the development of efficient light-emitting devices whose color can be tuned by modulating the molecular geometry (steric hindrance) of the coreactant itself. In particular, to enhance the efficiency, the use of highly fluorescent corannulene-based species, such as ethynyl derivatives,4,9 would be of great impact. Such devices could potentially be exploited, for example, by embedding the CA in a polymeric matrix or using either the corannulene itself or appropriately functionalized derivatives as the backbone of a highly luminescent polymeric material. Acknowledgment. This research was supported by the University of Bologna, the United States Department of Energy, and the Italian Ministry of University and Research (MIURs project PRIN 2008). G.V. thanks the “SENSTOX” project of “Regione Friuli Venezia-Giulia” for a fellowship grant. The authors also thank Dr. Marco Bandini for helpful discussions. Supporting Information Available: Picture of the ECL cell during emission; chronoamperometric and ECL/time curves of corannulene in TBAH/ACN solution with coreactant BPO; cyclic voltammetric curves and ECL/potential profiles of corannulene in TBAH/ACN solution with coreactant TBrMePA. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Forry, S. P.; Wightman, M. R. In Electrogenerated Chemiluminescence; Bard, A. J., Ed.; Marcel Dekker: New York, 2004; pp 273-299. (2) (a) Scott, L. T.; Hashemi, H. M.; Meyer, D. T.; Warren, H. B. J. Am. Chem. Soc. 1991, 113, 7082–7084. (b) Rabideau, P. W.; Abdourazak, A. H.; Folsom, H. E.; Marcinow, Z.; Sygula, A.; Sygula, R. J. Am. Chem. Soc. 1994, 116, 7891–7892. (c) Rabideau, P. W.; Sygula, A. Acc. Chem. Res. 1996, 29, 235–242. (d) Wu, Y.-T.; Siegel, J. S. Chem. ReV. 2006, 106, 4843–4867. (3) (a) Dey, J.; Will, A. Y.; Agbaria, R. A.; Rabideau, P. W.; Abdourazak, A. H.; Sygula, R.; Warner, I. M. J. Fluoresc. 1997, 7, 231– 236. (b) Yamaji, M.; Takehira, K.; Mikoshiba, T.; Tojo, S.; Okada, Y.; Fujitsuka, M.; Majima, T.; Tobita, S.; Nishimura, J. Chem. Phys. Lett. 2006, 425, 53–57.

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Valenti et al. (17) Zanarini, S.; Della Ciana, L.; Marcaccio, M.; Marzocchi, E.; Paolucci, F.; Prodi, L. J. Phys. Chem. B. 2008, 112, 10188–10193. (18) Schmidt, W.; Steckhan, E. Chem. Ber. 1980, 113, 577–582. (19) Electrogenerated Chemiluminescence; Bard, A. J., Ed.; Marcel Dekker; New York, 2004. (20) Choi, J.-P.; Bard, A. J. J. Electroanal. Chem. 2004, 573, 215–225. (21) Sutin, N. Acc. Chem. Res. 1982, 15, 275–282. (22) (a) Beideman, F. E.; Hercules, D. M. J. Phys. Chem. 1979, 83, 2203–2209. (b) Maness, K. M.; Wightman, R. M. J. Electroanal. Chem. 1995, 396, 85–95. (23) (a) Janata, J.; Gendell, J.; Ling, C.-Y.; Barth, W.; Backes, L.; Mark, H. B., Jr.; Lawton, R. G. J. Am. Chem. Soc. 1967, 89, 3056–3058. (b) Seiders, T. J.; Baldridge, K. K.; Siegel, J. S.; Gleiter, R. Tetrahedron Lett. 2000, 41, 4519–4522. (c) Bruno, C.; Benassi, R.; Passalacqua, A.; Paolucci, F.; Fontanesi, C.; Marcaccio, M.; Jackson, E. A.; Scott, L. T. J. Phys. Chem. B 2009, 113, 1954–1962. (24) Faulkner, L. R.; Tachikawa, H.; Bard, A. J. J. Am. Chem. Soc. 1972, 94, 691–699, and references therein. (25) Lai, R. Y.; Fleming, J. J.; Merner, B. L.; Vermeij, R. J.; Bodwell, G. J.; Bard, A. J. J. Phys. Chem. A 2004, 108, 376–383. (26) Chandross, E.; Sonntag, F. J. Am. Chem. Soc. 1966, 88, 1089– 1096. (27) The 12 nm shift of the ECL maximum with respect to that from photoluminescence falls within the 5-15 nm shift range usually observed in electrochemiluminesce experiments. (28) Maness, K. M.; Wightman, R. M. J. Electroanal. Chem. 1995, 396, 85–95. (29) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877–910. (30) Choi, J.-P.; Wong, K.-T.; Chen, Y.-M.; Yu, J.-K.; Chou, P.-T.; Bard, A. J. J. Phys. Chem. B 2003, 107, 14407–14413, and references therein.

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