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J. Phys. Chem. B 2002, 106, 6088-6095
Selective Population of Excited States during Electrogenerated Chemiluminescence with 10-Methylphenothiazine Andrew F. Slaterbeck, Timothy D. Meehan, Erin M. Gross, and R. Mark Wightman* UniVersity of North Carolina at Chapel Hill, Department of Chemistry, Chapel Hill, North Carolina 27599-3290 ReceiVed: July 18, 2001; In Final Form: October 2, 2001
Electrogenerated chemiluminescence of 10-methylphenothiazine (10MP) is demonstrated using the radical cation of 10MP and a variety of radical anions which provided a range of energies from insufficient to form the triplet state of 10MP through sufficient to form the first excited singlet state. When naphthalene was used as the radical anion, the reaction was sufficiently energetic to populate the singlet state of 10MP and the expected spectrum was obtained. When benzophenone was used as the radical anion, the sufficiency of the reaction energy depended upon the dielectric constant of the solvent. At high values ( ) 20) the reaction was energy insufficient and no emission was observed, but at low values ( ) 7) the reaction was sufficiently energetic to populate the triplet and emission was observed at wavelengths significantly longer than that of the excited singlet. Identical emission from 10MP was formed with the radical anion of 1,6-diphenyl-1,3,5hexatriene. The emission results were inconsistent with an exciplex but the available energetics from the radical ion reactions and the susceptibility to quenching suggest an excited triplet state of 10MP. Efforts to corroborate this state photochemically were inconclusive since phosphorescence could only be obtained at 77 K and the spectrum was not identical to the room-temperature electrogenerated chemiluminescence. When three different anthracene derivatives were employed (9,10-diphenylanthracene, 9-phenylanthracene, and 9,10-dimethylanthracene) as the radical anion, the triplet-like emission was observed from 10MP and also singlet emission from the anthracenes, resulting in a bright, white or pale blue light. This mixed emission has been observed in solid-state systems, but never before in solution ECL.
Introduction Electrogenerated chemiluminescence (ECL) involves the electrochemical generation of radical ions that undergo a homogeneous electron transfer and populate the excited state of one of the species, which can then emit light.1 This chemistry has long been of interest, as it provides a simple mechanism for photon production and also provides some insight into electron-transfer reactions. Recently, several papers have shown that solution ECL may follow the same mechanisms as are found in solid-state organic light-emitting devices (OLEDs), suggesting that solution ECL might be an effective tool with which to investigate the mechanisms of light generation within the solidstate display devices.2,3 Population of an excited state in an ECL system is dependent upon the amount of energy released during the electron-transfer reaction; the energy available can be approximated by the ∆E1/2 of the compounds employed (the difference in the EOX 1/2 of the 4 Typiof the anion precursor). cation precursor and the ERED 1/2 cally, the goal of ECL is to populate the excited singlet state of a molecule whose subsequent emission should match the fluorescence of that molecule. For ECL involving radical anions and cations, there are two main routes by which this excited singlet can be produced. If the energy provided by the electron transfer is sufficient to populate the singlet state directly, i.e., ∆E1/2 > ∆E0,0 (where ∆E0,0 is the energy difference between the first excited singlet state, S1, and the ground state, S0) the process is known as an S-route. If, however, the electron transfer is not energy sufficient to populate directly the excited singlet state, there may be sufficient energy to populate an excited triplet
state, the subject of several studies.4-8 Almost exclusively the triplet has been seen as a “byway” to the singlet state; two triplets can interact through a triplet-triplet annihilation reaction (TTA), pooling their energy, and thereby providing sufficient energy to populate the singlet state.9 Typically, population of the ground state is negligible due to the Marcus prediction that ground state formation is sufficiently exothermic that the process lies in an inverted kinetic region.10 Even though there is significant evidence for the role of triplets in light generation by ECL, for only one organic molecule (benzophenone) has direct emission from the triplet state been documented in the literature.11 Metal-containing complexes are known to emit from the triplet (e.g., Ru(bpy)32+), but are excluded from this discussion as, in these cases, the radiative lifetimes are decreased by the presence of the heavy metal atom. Benzophenone was perhaps the ideal candidate with which to demonstrate ECL phosphorescence due to its high intersystem crossing rate. Photochemical studies have shown benzophenone to be essentially nonfluorescent; 90% of the photons it absorbs could be accounted for by its phosphorescence.12 In this paper, electrogenerated chemiluminescence from 10methylphenothiazine (10MP), which forms a stable radical cation, is demonstrated with a variety of radical anions which provide a range of ∆E1/2 from insufficient for the triplet state through sufficient for the first excited singlet state. 10MP is unique in that no evidence of room temperature phosphorescence was found in the literature, and all efforts to photochemically generate the phosphorescence at room temperature gave no
10.1021/jp0127824 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/15/2002
Chemiluminescence with 10-Methylphenothiazine emission. For several of the anions employed, both emission from what is presumed to be the 10-MP triplet and from the reaction partner singlet was observed, resulting in a bright emission (visually observable) which, due to the wide range of wavelengths covered by the multiple emission pathways, appears pale blue or white. This behavior has been demonstrated in solidstate devices, but never before in solution ECL. The examples presented here illustrate the significant role ECL can play in the investigation of excited states of molecules. Many of the excited states investigated here are difficult to populate photochemically, but are readily accessible to ECL excitation. Experimental Section Apparatus. ECL experiments were run on a system previously described.13 It consisted of a stainless steel syringe pump (ISCO), 0.5 mL loop injector, and channel type electrochemical flow cell. The channel height of the flow cell, 150 µm, was established with a polyethylene gasket. The bottom of the flow cell was fabricated from epoxy, into which was imbedded a 52 µm radius gold wire (working electrode) encircled by a silver band (pseudo-reference electrode). A glass window was mounted in the top of the flow cell opposite the working electrode for observation of light generated near the working electrode surface. Photon-counting experiments used a photomultiplier tube (PMT, Hamamatsu, R4632), operated at -900 V and connected to a preamplifier (EG&G Ortec, VT120A) and monitored with a multichannel scaler (EG&G Ortec, T-914). ECL emission spectra were collected with a 600 µm optical fiber attached to a CCD array spectrometer (Ocean Optics, S2000FL). The spectrometer was calibrated with a NIST standardized light source (Ocean Optics, LS-1-CAL) for absolute intensity measurements. Fluorescence measurements were made using a xenon arc lamp (Varian) excitation source, excitation wavelengths were selected with a monochromator (Instruments SA, H20), and the fluorescence detected with the S2000FL system. To monitor electrochemical currents, the working electrode was connected to a fast current to voltage converter built in-house, the output of which was recorded with a digital oscilloscope (Lecroy, 9450). During light-generating experiments the working electrode was connected directly to ground. The electrochemical cell potential was controlled with an arbitrary waveform generator (Hewlett-Packard, 33120A). Phosphorescence measurements were made with a steady-state spectrofluorometer (Photon Technology International, QM-1). Molecular modeling was performed using HyperChem (Hypercube, Inc, v 6.0). Chemicals. Acetonitrile (ACN, UV grade, Burdick-Jackson), benzene (BZ), and ethylene glycol dimethyl ether (DME, Aldrich) were dried on an activated alumina column before use. 10-methylphenothiazine (10MP), 9,10-diphenylanthracene (DPA), 9-phenylanthracene (9PA), 9,10-dimethylanthracene (DMA), and 1,6-diphenyl-1,3,5-hexatriene (DPH) were purchased from Aldrich, twice recrystallized from ethanol and dried overnight in a vacuum oven prior to use. Naphthalene (NP), benzophenone (BZO), fluoranthene (FA), and the supporting electrolyte tetrabutylammonium hexafluorophosphate (TBAPF6) were purchased in high purity (>99%) from Fluka and were dried in a vacuum oven overnight before use. All solutions were deoxygenated with solvent saturated nitrogen before use. Methods. The solvent system most commonly employed was 0.1M TBAPF6 in 50/50 ACN/BZ; exceptions are noted. The solvent was deoxygenated, loaded into the syringe pump, and delivered through the flow system at 200 µL/min; sample solutions were introduced to the flow system through the loop
J. Phys. Chem. B, Vol. 106, No. 23, 2002 6089 injector. The E1/2 values were obtained from cyclic voltammograms obtained in the flow cell, but without the solution flowing. E1/2 values are reported vs Ag/AgCl. For light generating experiments the waveform potential limits were set ∼200 mV beyond the E1/2 values for each compound and a continuous square waveform was applied to a silver counter electrode at frequencies from 62.5 Hz up to 1 kHz. The emission was therefore observed under electrochemical steady-state conditions. Photon-Counting Measurements. PMT counts were collected in 1 µs bins and the response of 1000 cycles summed. The number of bins (length of acquisition time) depended upon the frequency of the waveform employed. An appropriate number of bins were used to ensure data was collected only over one full cycle of the applied potential waveform. Given the wide range of solution dielectric constants employed, varying degrees of ohmic drop affected the applied electrochemical potentials. For a given solvent system, the required overpotential could be estimated from a known system, 0.1 mM DPA.13 Compensation for the ohmic losses determined from the DPA experiments was applied to other systems investigated under the same conditions. ECL Spectra. Spectra were recorded using electrochemical conditions optimized during photon counting experiments. Reproducible alignment of the fiber optic over the electrode was ensured by calibration with a standard DPA solution. For each experiment, DPA was injected prior to the system of interest and the fiber position was adjusted until a maximum signal was obtained. A blank was obtained both by potential cycling in a solution containing only supporting electrolyte and with the cell at open circuit. Since both methods gave the same response, the open circuit response was used. The integration time of the spectrometer was varied from 1 to 60 s depending upon the intensity of the emission. When absolute intensities are reported, the spectrometer was calibrated with the NIST source immediately prior to data capture. Results Selective population of 10MP excited states. 10MP-Naphthalene. The compound 10MP forms a stable radical cation at 0.57 V vs Ag/AgCl in 50/50 ACN/BZ (the standard solvent used in these studies). When combined with the naphthalene radical anion (∆E1/2 was equal to 3.4 V in 50/ 50 ACN/BZ as determined from cyclic voltammograms), the electron transfer reaction was energy sufficient to directly populate the excited singlet of 10MP (∆E0,0 ) 3.0 eV).5 The resulting emission, Figure 1a, with a λmax ) 450 nm correlates exactly with the singlet emission from 10MP. See Table 1 for a summary of the relevant experimental values. The energy of the naphthalene singlet, reported as ∆E0,0 ) 3.99 eV,14 is inaccessible in this reaction and therefore does not emit. While the triplet of naphthalene is energy accessible to this reaction, the rate of any mechanism involving the triplet state would be sufficiently slow relative to singlet emission from 10MP that its participation in emission can also be excluded. 10MP-Benzophenone. The ∆E1/2 of the 10MP radical cation-BZO radical anion system is very close to the energy of the 10MP triplet (estimated in the literature as 2.4 or 2.7 eV).15;16 In 75/25 ACN/BZ ( ) 20), the ∆E1/2, 2.54 V, was apparently less than either triplet energy because no emission was observed. When the same experiment was repeated in DME ( ) 7), the ∆E1/2, 2.73 V, was sufficient to populate the 10MP triplet, and light was generated, Figure 1b. The emission did not originate from direct singlet formation or the TTA route since it occurred with a λmax red shifted from the fluorescence of 10MP by ∼150 nm occurred (Table 1).
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Figure 2. Fluorescence spectra of DPH ()) and 10MP (O); ECL emission spectra of 10 mM 10MP, 1 mM DPH in ACN (black solid line) and DME (gray solid line) containing 0.01 M TBAPF6. Applied potentials adjusted to compensate for ohmic losses, 125 Hz, r ) 52 µm gold disk. Light intensity integrated over 60 s, and the data digitally filtered with a first-order Chebychev filter (sampling frequency ) 2 Hz, cutoff frequency ) 0.05 Hz, band-pass ripples ) 1 dB). Inset: Effect of DPH concentration on 10MP-DPH ECL intensity. 1 mM 10MP, 0.01 M TBAPF6 in DME for all solutions, 1.8 to -2.3 V continuous square wave, 125 Hz.
Figure 1. ECL emission spectra of 10 mM 10MP with (a) 1 mM Naphthalene in 50/50 ACN/BZ, 0.1M TBAPF6, (b) 1mM Benzophenone in DME, 0.1M TBAPF6, and (c) 1 mM Fluoranthene in 50/50 ACN/BZ, 0.1 M TBAPF6.
TABLE 1: Summary of Electrochemical and Emission Data for the Chemical Systems employed system
∆E1/2 (V)a
λmax (nm)
intensity (cd/m2 x 103)b
10MP-NAP 10MP-FA 10MP-BZO
3.32 2.59 2.73 (DME) 2.54 (75/25) 2.65 3.17 3.10 3.27 2.67 2.11 2.79 2.72
450 440, 690 640
3.58 1.58 .647
640 450 410,550 423, 500 450, 640 450 410, 580 423c
0.507 16.8 18.4 13.3 6.49 4.59 4.88 3.45
10MP-DPH DPA DMA 9PA 10MP-DPA TMPD-DPA 10MP-DMA 10MP-9PA
a Electrochemical data obtained from steady-state cyclic voltammetry, 1 mM concentrations, 0.1 M TBAPF6, 50/50 ACN/BZ except where noted, 5 µm Au electrode, Ag/AgCl reference. b Intensity given as candela per surface area of fiber optic probe (detector). If the intensity values are normalized to the surface area of the emitter, these values compare with published data for OLED devices, see discussion. c There is significant emission around 600 nm, but a distinct maximum is not discernible.
10MP-Diphenylhexatriene. DPH has been investigated photochemically for its fluorescence and for its participation in
exciplexes.17-21 On the time scale of electrochemical experiments (ms), DPH forms an unstable radical cation (E1/2 ) 0.97 vs Ag/AgCl) and a stable radical anion (-2.08 V vs Ag/AgCl). The ∆E1/2 (2.65 V) of the 10MP radical cation-DPH radical anion system in 50/50 ACN/BZ is energy insufficient to populate the singlet state of either 10MP or DPH (∆E0,0 ) 3.11 eV).19 Generation of the DPH radical cation was avoided since it fouled the electrode surface resulting in diminishing ECL with time. The spectrum of the emission from the 10MP radical cationDPH radical anion system generated with a 125 Hz continuous square wave was a broad, featureless wave, of low intensity and with the same λmax as that found with the 10MP-BZO system, Figure 2. The λmax was insensitive to solvent dielectric constant over a range from 7 (DME) to 37 (ACN), Figure 2. Again, because the emission spectrum is distinct from either the 10MP or DPH fluorescent spectra, Figure 2, the emission does not result from direct or indirect (via TTA) singlet formation. Several other possibilities were considered as sources of the emission. The flow cell employed for these studies prevents the accumulation of electrochemical byproducts, thereby ensuring that the emission originates from one of the principal species in solution. This was confirmed by intentionally overdriving the electrochemical potential with the solution flow turned off. Emission was observed under these conditions, but was distinctly different from that shown in Figure 2 (data not shown). The triplet of DPH (1.09 eV) would be expected to emit in the near-infrared and, consequently, not be observed in these experiments. Only two remaining possibilities could explain the observed emission. An exciplex, an emissive charge-transfer complex, between the 10MP and DPH would be expected to emit in a broad, featureless wave ∼150 nm red shifted from the singlet emission. While this describes the emission seen in Figure 2, the λmax of an exciplex would be expected to show a sensitivity to the dielectric constant of the medium employed.14 The observed emission exhibited no such sensitivity, Figure 2. The fact that identical emission is obtained from 10MP-BZO also provides evidence against the exciplex hypothesis. The energy of exciplex emission is affected by the energy difference
Chemiluminescence with 10-Methylphenothiazine
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Figure 3. (a) Effect of potential cycling frequency on ECL emission intensity. 1 mM 10MP, 10 mM DPH, 0.01 M TBAPF6 in DME for all solutions, 1.8 to -2.3 V continuous square wave, 52 µm gold disk (O). 0.1 mM DPA in 0.1M TBAPF6 in 50/50 ACN/BZ 1.9 to -2.1 V continuous square wave, 52 µm gold disk (0). DPA data deviates from linearity at high cycling frequencies due to ohmic losses. (b) Comparison of phosphorescence ()) and ECL (- -) spectra of 10MP in ACN.
between electrochemical oxidation and reduction potentials of the radical cation and radical anion employed as well as structural differences of the different radical ion pairs employed.14;22-24 Another alternate source of the emission is the triplet state of 10MP (estimated in the literature as 2.4 or 2.7 eV)15,16 which coincides with the emission energy (∼2.6 eV, calculated from the emission spectrum). Triplets are known to be quenched by paramagnetic species.9;25 Previous work from this group has demonstrated that triplets can be eliminated during ECL by excess radical ions of the nonemissive species.9 In contrast, an emission from an exciplex will reach a plateau regardless of which species is in excess.26 For 1 mM 10MP, as the concentration of DPH was increased from 0 to 5 µM, the emission intensity reached a peak value, Figure 2, inset.27 As the concentration of DPH increased, the light intensity decreased, until the saturation limit of DPH in solution was reached, behavior expected for radical anion quenching of 10MP emission. When the concentration of 10MP was increased from 0 to 100 mM in the presence of 1 mM DPH, the intensity reached a plateau at 25 mM 10 MP (data not shown). It should be noted that even at 100 mM 10MP no evidence of TTA was found. Phosphorescence is characterized, in part, by its long lifetime. The lifetime of the 10MP excited triplet state has been reported as 0.6 ms at 77 K, significantly longer than singlet lifetimes.16 The sensitivity of the 10MP-DPH emission intensity to potential cycling frequency suggests that the lifetime of the emissive state is longer than that of excited singlets. For all concentrations of 10MP-DPH, the emission exhibited the frequency dependence shown in Figure 3a. At very low frequencies, low intensities per unit time were observed; at these frequencies the radical ions are produced infrequently. After the intensity reached a maximum (125 Hz), the light intensity per unit time steadily decreased as the potential cycling frequency was increased, Figure 3a. Thus, unlike DPA, whose emission increased linearly with Hz1/2, consistent with diffusion controlled production of the excited singlet state, Figure 3a, the low efficiency of 10MP emission at high frequencies is consistent with a long lived excited state that can be quenched by the more frequently formed radical ions.
To compare the emission to that of the photochemically generated triplet, the phosphorescence of 10MP was obtained in ACN at 77K and compared to the ECL emission obtained in ACN, Figure 3b. The onset of emission in both the photochemical experiment and the ECL experiment overlap, indicating that the ∆E0,0 is similar.28 The shape of the two spectra is significantly different, however, with the λmax of the ECL data red shifted from the 77K phosphorescence. The differences in the spectra may be due to the different temperatures at which the data was collected. Freezing the solvent is known to “lock” a molecule in its ground-state equilibrium solvent cage and molecular conformation, thereby preventing solvent cage relaxation. This solvent cage relaxation, and its stabilizing effect on the excited state of a molecule, contributes to the wavelength shifts observed between absorbance and fluorescence.29 Molecular modeling indicates that the ground-state singlet of 10MP is bent along the N-S axis, having a narrow band of vibrational energies while the radical cation is flat with a more broad vibrational spectrum. Given these differences, one would expect differences in the emission spectrum collected at room temperature, with molecular and solvent rearrangement, as compared to the spectrum collected at 77K, with no rearrangement. Unfortunately, efforts to collect the photochemically generated phosphorescence at room temperature were unsuccessful. Transient absorbance studies performed on the 10MP radical cation and excited triplet state offer insufficient evidence to decide this issue.30 Although definitive declaration of the source of emission cannot be made due to the discrepancies in the spectra shown in Figure 3b, the other results are consistent with triplet phosphorescence from the 10MP. 10MP-Fluoranthene. The ∆E1/2 for the 10MP-FA system is insufficient to form either singlet, and the triplet energy of FA (2.3 eV) is less than that of 10MP. Thus the energy from the electron-transfer reaction can populate the FA triplet. The observed emission was characteristic of the FA singlet, Figure 1c. These results corroborate the previous work that showed that the FA singlet was populated via TTA.15 There is an additional peak not previously reported, λmax ) 690 nm. The emission was present under all applied potentials that allowed the primary emission band to be observed (i.e. unlikely to be the result of a byproduct), the λmax was insensitive to changes
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Figure 4. (a) DPA ECL, 1mM DPA, 0.1 M TBAPF6, 50/50 ACN/BZ. Light intensity integrated over 1 s. (b) TMPD-DPA ECL, 1 mM TMPD, 10 mM DPA, 0.1 M TBAPF6, 50/50 ACN/BZ, integrated 10 s. (c) 10MP-DPA ECL, 10 mM 10MP, 1 mM DPA, 0.1 M TBAPF6, 50/50 ACN/BZ, integrated 10 s. (d) Comparison of data from (c), solid line, with a normalized sum of DPA ECL and 10MP-DPH luminescence (O).
in dielectric constant, and its energy (∼ 2.25 eV, estimated from the onset of the emission spectrum) is consistent with the FA triplet. However, further experiments are required before a definitive assignment can be made. Multiple Emission Pathways. 10MP-Anthracenes. Radical anions from three anthracene derivatives were investigated for their ability to induce emission from 10MP: 9,10-diphenylanthracene (DPA), 9,10-dimethylanthracene (DMA), and 9-phenylanthracene (9PA). All three form stable radical anions and cations at similar potentials. Alone in solution, the ∆E1/2 of each compound was energy sufficient to populate its own excited singlet state, Table 1, and the predictable fluorescence-like emission was observed for each, Figures 4a, 5a, and 5d. In the case of DMA and 9PA, there was an additional peak, for DMA λmax ) 550 nm, Figure 5a, for 9PA λmax ) 500 nm, Figure 5d. In both cases, this peak has been attributed to excimer emission;4 presumably the formation of a DPA excimer is sterically hindered. Reaction of the DPA radical anion with the 10MP radical cation resulted in two peaks. The first, λmax ) 450 nm, was consistent with the shape, amplitude, and wavelength of the singlet emission from DPA (although the 10MP singlet emits at the same wavelength, vide infra), while the second, λmax ) 640 nm, Figure 4c, is similar to that of the 10MP-BZO and 10MP-DPH emission. An additional peak was regularly observed at 850 nm in this system and with the other anthracenes. Experiments using cutoff filters and a different monochromator ruled out the possibility that this peak was a high order mode of the grating, however insufficient data exists at this time to suggest a mechanism for this emission. The ∆E1/2 for 10MP-DPA, 2.67 V, provides insufficient energy to form the singlet state of either 10MP or DPA. Therefore, the low wavelength emission must have arisen via TTA. Figure 4b
shows the ECL spectrum obtained from DPA with N,N,N′,N′tetramethyl-1,4-phenylenediamine (TMPD), a system known to populate the DPA excited singlet via TTA.9 The 10MP-DPA spectrum was compared to a normalized sum of the DPA ECL spectrum and the 10MP-DPH ECL spectrum, and these spectra overlap exactly, Figure 4d. The time course of ECL during each potential step also indicated relatively slow processes for light production. When the potential range was adjusted to oxidize only the 10MP, the emission profile was that of a slow process,9 Figure 6a. Both TTA and 10MP phosphorescence are slow relative to the S-route, leading to this behavior. As the potential range was expanded to include generation of the DPA radical cation in addition to the 10MP radical cation, the intensity increased and the time course decreased, exhibiting sharp features consistent with emergence of direct S-route population of the DPA excited singlet, Figure 6b. The spectra of 10MP-DMA and 10MP-9PA ECL also show multiple components, Figure 5b,e. As with the 10MP-DPA, when the anthracene radical cation was excluded by appropriate potential selection, the electrochemical reactions did not provide sufficient energy to directly populate the excited singlet state directly, indicating TTA. The emitting species were determined by comparing the complex emission to normalized contributions from each of the participating species. If one compares the emission from the 10MP-DMA ECL with the normalized sum of DMA ECL (which includes singlet and excimer emission) and 10MP-DPH ECL, the spectra appear identical, Figure 5c. Similarly, if one compares a normalized sum of 9PA fluorescence, 9PA excimer, and 10MP-DPH ECL spectra with the 10MP-9PA ECL spectrum, they appear identical, Figure 5f. Thus, both of these systems seem to involve three modes of
Chemiluminescence with 10-Methylphenothiazine
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Figure 5. 10MP-anthracene ECL emission. (a) DMA ECL, 1mM DMA, 0.1 M TBAPF6, 50/50 ACN/BZ. Light intensity integrated over 1 s. (b) 10MP-DMA, 10 mM 10MP, 1 mM DMA, 0.1 M TBAPF6, 50/50 ACN/BZ. Light intensity integrated over 10 s. (c) Comparison of data from (b), solid line, with a normalized sum of DMA ECL and 10MP phosphorescence (O). (d) 9PA ECL, 1mM 9PA, 0.1 M TBAPF6, 50/50 ACN/BZ. Light intensity integrated over 1 s. (e) 10MP-9PA, 10 mM 10MP, 1 mM 9PA, 0.1 M TBAPF6, 50/50 ACN/BZ. Light intensity integrated over 10 s. (f) Comparison of data from (e), solid line, with a normalized sum of 9PA fluorescence, 9PA excimer emission, and 10MP-DPH luminescence (O).
Figure 6. 10MP-DPA ECL intensity profiles during the negative potential step from photon counting experiments. 100 mM 10MP with 10 mM DPA, 0.01 M TBAPF6 in DME, 125 Hz. 3.5 Vpp (a) and 4.1 Vpp (b). Data were collected in 1 µs bins for 1000 cycles of the applied square wave (8 s total collection time).
emission: singlet and excimer emission from the anthracene derivative and phosphorescence from 10MP. Discussion The results presented here reveal a previously unreported emission from 10MP at room temperature and show that this
excited state of 10MP can be populated by a variety of electron donors. The results also show that multiple excited species can be formed and emit in solution. Historically, the excited states involved in these studies have been probed photochemically. However, direct photochemical population of triplet states is “spin forbidden”, i.e., the simultaneous excitation and spin
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inversion required to create the excited triplet state from the ground singlet state is highly improbable. ECL faces no such limitation since the electron that enters an elevated molecular orbital originates from a different molecule. Indeed, population of an excited triplet state is the predominant result of ECL; spin statistics predict that 75% of the states produced by the electron transfer reaction will be triplet. The primary role that triplets have played in ECL generation, however, has been through TTA, thereby populating the excited singlet state that subsequently emits. This is because once the triplet is formed it has a slow emissive rate so it can be quenched by other species in solution before emitting. Quenching can be especially problematic with ECL since the chemical precursors for light emission are electrochemically generated radical ions, and these paramagnetic species are highly efficient triplet quenchers. Historically, the time scale of ECL experiments was sufficiently long that the radical ions could participate in side reactions rather than those that generate light. By using microelectrodes, however, one can employ high frequency potential waveforms, decreasing the time scale of the experiment and limiting the probability of side reactions. High frequency ECL has also been shown to generate more ECL reagents per given unit time. These benefits, combined with the absence of any background light (contrast with photochemical studies), suggest that ECL should be especially well suited to the investigation of excited states with low emission efficiencies or that are difficult to observe photochemically. Results from the 10MP-DPH ECL suggest formation of the triplet state and demonstrate the utility of the electrochemical generation for the generation of unusual excited states. In addition to the increased probability of triplet formation, ECL may also provide information regarding the lifetime and energy of the state under investigation. The 10MP-DPH system exhibited a sensitivity to cycling frequency. With each step of the electrode potential, a local high concentration of radical ions is generated at the electrode surface. Under typical ECL conditions, these radical ions diffuse away from the electrode, encounter radical ions generated on the previous step, form an encounter complex, transfer an electron, and emit light (assuming S-route). In the case of 10MP phosphorescence, if the duration of one step of the potential waveform is shorter than lifetime of the excited triplet, the radical ions can encounter the 10MP excited triplet and quench it, having the same result as that described for Figure 2b. For the data shown in Figure 3, the maximum intensity was observed at 125 Hz. After accounting for the time of each step and the rise time of the electrode, one can estimate the lifetime of the excited triplet as ∼1 ms, in close agreement with the reported value of 0.6 ms.16 The results from the 10MP-BZO system suggest ECL may be a way to determine the energy of the excited state. The energy available to populate an excited state is related to the ∆E1/2 of the precursors employed, which itself can be affected by the dielectric constant of the solvent. As the dielectric constant decreases, the solvation energy of the cation and anion radicals formed during electrolysis increases. Weller published equations which allow one to calculate the effect of dielectric constant on the ∆E1/2 of the radical ions which participate in the annihilation reaction:31-33
(
∆E1/2 ) -IP + EA +
) (
)
z- e2 z+e2 1 1 1 + - e2 + 2r2r+ 2r- 2r+ (1)
where IP is the ionization potential, EA is the electron affinity,
r+/- is the radius of the reactant, z+/- is the charge of the reactant, e is the charge of an electron, and is the dielectric constant of the solvent. The energy available to populate the excited state is related to the ∆E1/2 by
∆GEC ) ∆E1/2 - w
(2)
in which ∆GEC is the energy of the encounter complex and w is the Coulombic work term
w)
z-z+e2 a
(3)
where z-/+ is the charge of the ions, e is the electron charge, a is the intermolecular radius, and the dielectric constant of the solvent. This work term gives the energy required to bring the ions together into the encounter complex in which the electron transfer takes place. By comparing the energy available from the electron transfer, ∆GEC, to the energy of the state one hopes to populate, ∆E0,0, one can determine the ∆G of the overall process:
∆GPROCESS ) ∆E0,0 + ∆GEC
(4)
If ∆GPROCESS is positive, the reaction is insufficient, and direct population will not occur; only when ∆GPROCESS is negative can the electron transfer populate the state. Typically, the reactions which are energy insufficient are significantly so, and the effect of solvent dielectric constant is negligible. For the 10MP-BZO system, however, this energy change appears significant. In 75/25 ACN/BZ ( ) 20, ∆E1/2 ) 2.5 V), the energy calculated from these equations for ∆GEC (2.42 eV) was not sufficient to generate light. When the same system was examined in DME ( ) 7, ∆E1/2 ) 2.7 V), the energy available (2.54 eV) was sufficient to generate light, suggesting the triplet energy for 10MP ∼2.5 eV. This value agrees with what can be estimated from the spectra (∼2.6 eV) and falls between the values reported for 10MP in the literature (2.4 and 2.7 eV). The utility of this method for determining lifetime and energy values for an excited state is dependent upon the availability of molecules with the desired electrochemistry, structure, and energy levels. A surprising outcome of these experiments was the emission from multiple excited states. Previous work with ECL has demonstrated dual emission from singlet states and excimers or exciplexes,14 but the results from both 10MP-DMA and 10MP-9PA show three modes of emission: triplet-like emission from 10MP and both singlet fluorescence and excimer emission from the anthracene. Due to the long lifetime of the 10MP triplet, the excited states may become saturated, allowing the available energy to excite the anthracene derivatives instead. The resulting emission occurs over a wide range of wavelengths (∼400 nm for 10MP-DPA) and appears white (10MP-DPA) or pale blue (10MP-DMA and 10MP-9PA). The emission is bright enough to see clearly with the room lights dimmed. The intensities shown in Table 1 for these systems do not account for the small surface area of the emitter. While mixed emitting systems have been demonstrated in solid-state devices,34-38 this appears to be the first example in solution ECL. Acknowledgment. This research was supported by the National Science Foundation. We thank Dr. Jack Saltiel of Florida State University for donating the DPH that initiated these studies and Dr. Kevin Belfield of the University of Central Florida for performing the phosphorescent studies of 10MP.
Chemiluminescence with 10-Methylphenothiazine References and Notes (1) Faulkner, L. R.; Bard, A. J. Electrogenerated Chemiluminescence. In Electrochemical Methods; John Wiley & Sons: New York, 1980; pp 621-629. (2) Gross, E. M.; Anderson, J. D.; Slaterbeck, A. F.; Thayumanavan, S.; Barlow, S.; Zhang, Y.; Marder, S. R.; Hall, H. K.; Flore Nabor, M.; Wang, J. F.; Mash, E.; Armstrong, N. R.; Wightman, R. M. J. Am. Chem. Soc. 2000, 122, 4972-4979. (3) Anderson, J. D.; McDonald, E. M.; Wightman, R. M.; Armstrong, N. R.; et al. J. Am. Chem. Soc. 1998, 120, 9646-9655. (4) Weller, A.; Zachariasse, K. Chem. Phys. Lett. 1971, 10, 197-200. (5) Freed, D. J.; Faulkner, L. R. J. Am. Chem. Soc. 1972, 94, 47904792. (6) Faulkner, L. R.; Tachikawa, H.; Bard, A. J. J. Am. Chem. Soc. 1972, 94, 691-699. (7) Freed, D. J.; Faulkner, L. R. J. Am. Chem. Soc. 1971, 93, 20972102. (8) Michael, P. R.; Faulkner, L. R. J. Am. Chem. Soc. 1977, 99, 77547761. (9) Ritchie, E. L.; Pastore, P.; Wightman, R. M. J. Am. Chem. Soc. 1997, 119, 11920-11925. (10) Sutin, N. Acc. Chem. Res. 1982, 15, 275-282. (11) Park, S. M.; Bard, A. J. Chem. Phys. Lett. 1976, 38, 257-262. (12) Ma, Y.; Lai, T.; Wu, Y. AdV. Mater. 2000, 12, 433-436. (13) Collinson, M. M.; Wightman, R. M. Anal. Chem. 1993, 65, 25762582. (14) Bard, A. J.; Park, S. M. J. Am. Chem. Soc. 1975, 97, 2978-2985. (15) Freed, D. J.; Faulkner, L. R. J. Am. Chem. Soc. 1971, 93, 35653568. (16) Barra, M.; Calabrese, G. S.; Allen, M. T.; Redmond, R. W.; Sinta, R.; Lamola, A. A.; Small, R. D.; Scaiano, J. C. Chem. Mater. 1991, 3, 610-616. (17) Saltiel, J.; Crowder, J. M.; Wang, S. J. Am. Chem. Soc. 1999, 121, 895-902. (18) Schael, F.; Lohmannsroben, H.-G. Chem. Phys. 1996, 206, 193210. (19) Schael, F.; Kuster, J.; Lohmannsroben, H.-G. Chem. Phys. 1997, 218, 175-190. (20) Saltiel, J.; Wang, S.; Ko, D. H.; Gormin, D. A. J. Phys. Chem. A 1998, 102, 5383-5392. (21) Dutta, R.; Basu, S.; Chowdhury, M. Chem. Phys. Lett. 1991, 182, 429-434. (22) Park, S. M.; Paffett, M. T.; Daub, G. H. J. Am. Chem. Soc. 1977, 99, 5393-5399. (23) Weller, A. Z. Phys. Chem. 1982, 133, 93-98.
J. Phys. Chem. B, Vol. 106, No. 23, 2002 6095 (24) Cosa, G.; Chesta, C. J. Phys. Chem. A 1997, 101, 4922-4928. (25) Beldeman, F. E.; Hercules, D. M. J. Phys. Chem. 1979, 83 (17), 2203-2209. (26) Rathore, R.; Hubig, S. M.; Kochi, J. K. J. Am. Chem. Soc. 1997, 119, 11468-11480. (27) The results from this experiment when performed in 50/50 ACN/ benzene ( ) 15) gave remarkably different results than when performed in DME ( ) 7). In 50/50 the emission reached a plateau at low concentrations of DPH; even when no DPH was present, emission was observed. Upon examination of the solution by cyclic voltammetry, it was found that 10MP formed a dication (E1/2 ∼ 1.2 V vs Ag/AgCl), unstable over the time course of these experiments, whose degradation created a reducible species (E1/2 ∼ -2.3 V vs Ag/AgCl) which could participate in the ECL process. Fortunately, the spectrum of this emission was distinctly different from that of the 10MP emission of interest so its contribution could be monitored. In solvents of low dielectric constant this was less of a problem as the potentials were shifted beyond normal operating potential ranges. With less aggressive compensation for the ohmic losses, this extraneous emission was not an issue. Only when extreme ranges were employed (BZO, NAP, and FLO experiments) was this phenomenon problematic. (28) Turro, N. J. Modern Molecular Photochemistry; Benjamin/Cummings: Menlo Park, CA, 1978; p 114. (29) Schulman, S. G. Fluorescence and Phosphorescence Spectroscopy: Photochemical Principles and Practice; Pergamon Press, Ltd.: Elmsford, New York, 1977. (30) Guo, Q. X.; Liang, Z. X.; Liu, B.; Yao, S. D.; Liu, Y. C. J. Photochem. Photobiol. A 1996, 93, 27-31. (31) Weller, A.; Zachariasse, K. Chemiluminescence from Radical Ion Recombination VI. Reactions, Yields, and Energies. In Chemiluminescence and Bioluminescence; Cormier, M. J., Hercules, D. M., Lee, J., Eds.; Plenum Press: New York-London, 1973; pp 169-208. (32) Maness, K. M.; Bartelt, J. E.; Wightman, R. M. J. Phys. Chem. 1994, 98, 3993-3998. (33) Kapturkiewicz, A. J. Electroanal. Chem. 1994, 372, 101-116. (34) O’Brien, D. F.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 1999, 74, 442-444. (35) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nature 2000, 403, 750-753. (36) Xie, Z. Y.; Huang, J. S.; Li, C. N.; Liu, S. Y.; Wang, Y.; Li, Y. Q.; Shen, J. C. Appl. Phys. Lett. 1999, 74, 641-643. (37) Kido, J.; Shionoya, H.; Nagai, K. Appl. Phys. Lett. 1995, 67, 22812283. (38) Kalinowski, J.; Di Marco, P.; Fattori, V.; Giulietti, L.; Cocchi, M. J. Appl. Phys. 1998, 83, 4242-4248.
