Electrogenerated Chemiluminescence of Bithiophenes with Methylthio

Article pubs.acs.org/JPCC

Electrogenerated Chemiluminescence of Bithiophenes with Methylthio Functionalities Nicole L. Ritzert, Thanh-Tam Truong, Geoffrey W. Coates, and Héctor D. Abruña* Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States S Supporting Information *

ABSTRACT: The photoluminescence and electrogenerated chemiluminescence (ECL) of 5,5′-bis(methylthio)-2,2′-bithiophene (BMTbT) have been studied and compared to those of 2,5-bis(methylthio)thieno[3,2-b]thiophene (f-BMTbT) and 5,5′-diethyl-2,2′-bithiophene (DEbT), the fused-ring and alkyl analogues of BMTbT, respectively. All of these compounds emitted in the blue/violet region of the visible spectrum, with a fluorescence quantum yield of ca. 0.03. In acetonitrile, BMTbT and f-BMTbT each exhibited an irreversible reduction wave in addition to the well-documented two-electron oxidation processes. ECL was observed from solutions of BMTbT and of fBMTbT via annihilation of the electrogenerated anion and dication, with emission following only the reduction step, likely due to an unstable anion species. Lack of solvent stability at potentials required to reduce or oxidize DEbT in acetonitrile or tetrahydrofuran, respectively, precluded annihilation ECL in these solvents. For all three compounds, sweeping to negative potentials in the presence of the coreactant benzoyl peroxide produced ECL. Emission from BMTbT was detected for over 1000 cycles using benzoyl peroxide. Herein, it is demonstrated that thioether functionalities may provide a stable alternative to alkyl side chains in oligothiophenes in light-emitting applications.

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INTRODUCTION Electronic and optical devices based on organic materials, including π-conjugated thiophene systems, have received much attention due to their ease of chemical modification, stability upon oxidation, high charge transport, simple processing, and self-assembling properties.1−5 Recent investigations for applications of thiophene and thiophene derivatives include photovoltaic devices,6,7 field-effect transistors,8,9 optical displays,10 potential cathode materials in electrical energy storage devices,11−13 and studies of fundamental charge transfer in single molecules.14 Much work has been performed on controlling the electronic and optical properties of oligothiophenes. Approaches include changing the type and/or position of functional side groups6,9,15−18 and the structure of the oligothiophene backbone itself.10,15,19 Attaching various functional groups has been demonstrated to improve the stability of oligothiophenes, which often exhibit ill-behaved chemistry and electrochemistry in their bare and alkylated forms.15,18,20−22 For example, polymerization of shorter oligothiophenes was prevented by adding methylthio groups at the α-site21,22 or using 4,5,6,7-tetrahydrobenzo[b]thiophene as an end cap.23 In addition to enhancing the stability, the selective choice of functional groups can be used to tune the emission wavelength of oligothiophenes.18,24 Electrogenerated chemiluminescence (ECL) has been used to study the electrochemical and optical properties of thiophene-related compounds.10,17,20,25−27 ECL is a useful method for studying the properties of fluorescent species, providing information such as formal potential, aggregation and excimer formation, evidence for reaction mechanisms, and © 2013 American Chemical Society

chemical stability. Excellent reviews and descriptions of ECL are available in the literature.28−31 Two modes of generating ECL are annihilation and coreactant ECL. Briefly, in annihilation ECL, anions and cations of an electroactive, luminescent species are electrogenerated in close proximity, such as by applying alternating reducing and oxidizing potentials to an electrode R + e− → R•−

(1)

R → R•+ + e−

(2)

When these species react, sufficient energy is released to produce an excited electronic state that subsequently emits light R•− + R•+ → R* + R

(3)

R* → R + hν

(4)

In contrast to annihilation ECL, coreactant ECL is induced by a single potential step or a linear potential sweep. In this case, a coreactant is used to generate a very reactive species such as a radical ion that provides the energy needed to excite the emitting species. An example is benzoyl peroxide (BPO), where a very oxidizing species is produced upon reduction. Chandross and Sonntag proposed the following mechanism28,32 R•− + BPO → R + BPO•−

(5)

BPO•− → C6H5CO2− + C6H5CO2•

(6)

Received: December 10, 2013 Published: December 22, 2013 924

dx.doi.org/10.1021/jp412100x | J. Phys. Chem. C 2014, 118, 924−932

The Journal of Physical Chemistry C

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Figure 1. Structures of (a) 5,5′-bis(methylthio)-2,2′-bithiophene (BMTbT), (b) 2,5-bis(methylthio)thieno[3,2-b]thiophene (f-BMTbT), and (c) 5,5′-diethyl-2,2′-bithiophene (DEbT).

R•− + C6H5CO2• →1 R* + C6H5CO2−

compared to that of a reference compound with a similar excitation wavelength.37,38 Φf of BMTbT (λex = 350 nm) and DEbT (λex = 325 nm) was determined using DPA, with Φf = 0.97 at 355 nm39 and Φf = 0.90 at 325 nm.40 Naphthalene, with Φf = 0.23 at 270 nm,39 was the reference for determining Φf of f-BMTbT (λex = 276 nm). To avoid any complications from differences in refractive index, the solvent for all fluorescence yield measurements was cyclohexane. Electrochemical and ECL Measurements. Except for those at ultramicroelectrodes (vide infra), electrochemical measurements were performed using an EG&G Princeton Applied Research (Princeton, NJ) model 173 PotentiostatGalvanostat with a model 175 Universal Programmer and model 176 Current Follower, and data were recorded using a custom LabVIEW (National Instruments, Austin, TX) program and a National Instruments USB-6210 DAQ device. ECL emission was detected using a Hamamatsu model R928 photomultiplier tube (PMT) operated at −750 V using a model 556 high-voltage power supply from EG&G Ortec (Oak Ridge, TN). The PMT was connected to the DAQ device using an Ithaco (Ithaca, NY) model 1211 current preamplifier. An Oriel (Stratford, CT) model 7240 monochromator with 10 nm slits was used to obtain ECL spectra, which were not corrected for the PMT response. Mercury lines from a Model 6291 Hg/ Xe lamp (Oriel) were used to calibrate the monochromator. A custom-made electrochemical cell of threaded Pyrex (transmittance above 300 nm) with a Teflon cap (Ace Glass Inc., Vineland, NJ) was used for making electrochemical and ECL measurements at the bench while keeping the solutions under an oxygen- and water-free argon atmosphere.30 Electrical contacts were made to the electrodes inside the cell using threaded stainless-steel rods driven through the cap and connecting the electrodes inside using pin- and socket-type connections. The working electrode, area of 0.023 cm2, was fabricated by sealing a platinum bead in soft glass bent at 90° and then exposing and polishing the platinum disk using sandpaper of successively finer grit. Before each measurement, the working electrode was polished with 1.0 μm and then 0.3 μm alumina on polishing cloth (Buehler, Lake Bluff, IL) and then sonicated in water. A large coiled platinum wire and a silver wire were used as the auxiliary and quasi-reference electrodes (QREs), respectively. These three electrodes were sonicated in acetone for at least 10 min and were dried in an oven at 95 °C for at least one hour prior to use. A drybox (Vacuum Atmospheres Co., Hawthorne, CA) was used to prepare all solutions and to seal the electrochemical cell under an argon atmosphere. When detecting ECL emission, care was taken to block any light emitted from any side reactions at the auxiliary electrode by placing a window made of glass (transmittance above 350 nm) and aluminum foil next to the electrochemical cell to expose only the working electrode to the PMT. Each measurement was performed at least three times,

(7)

Other proposed mechanisms involving triplet states have been presented in the literature.33,34 Coreactant ECL is useful in systems where the solvent is unstable at potentials necessary to generate the reactive species or if one of the electrogenerated species is unstable. We have investigated the photoluminescence and ECL properties, both through annihilation and with the coreactant BPO, of 5,5′-bis(methylthio)-2,2′-bithiophene (BMTbT) (Figure 1a). Its solubility in common electrolyte solutions makes BMTbT, with methylthio functionalities, an attractive small molecule in place of those requiring long alkyl groups.17,20,35 Although the oxidation of BMTbT has been well-characterized using cyclic voltammetry,11,12,22,24 UV−vis spectroscopy,11,12,22 and ESR,22 its reduction has not been reported. Comparisons of BMTbT to 2,5-bis(methylthio)thieno[3,2-b]thiophene (f-BMTbT) (Figure 1b) and 5,5′diethyl-2,2′-bithiophene (DEbT) (Figure 1c), the fused-ring and alkyl analogues of BMTbT, respectively, confirmed that substituent groups and structure are important in the emission energy and stability of thiophenes.

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EXPERIMENTAL METHODS Materials. Purified BMTbT12 and f-BMTbT13 were from available batches. DEbT was synthesized according to the literature (Figure S1, Supporting Information).36 Sigma-Aldrich (St. Louis, MO) was the source of BPO (97%), 9,10diphenylanthracene (DPA), ferrocene (98%), acetonitrile (MeCN, 99.8%, anhydrous), and tetrahydrofuran (THF, +99.9%, anhydrous, inhibitor free). Naphthalene (99.8%) and tetra-n-butyl ammonium hexafluorophosphate (TBAPF6, 98%) were from Alfa Aesar (Ward Hill, MA). J. T. Baker (Avantor Performance Materials, Center Valley, PA) was the source of cyclohexane (ACS grade). DPA and TBAPF6 were purified by recrystallization from ethanol, and ferrocene was sublimed. All other chemicals were used as received. Water (18.2 MΩ·cm) purified using a Millipore (Billerica, MA) Milli-Q system was used to clean all glassware. All solids were placed under vacuum at room temperature overnight, and all glassware was placed in an oven at 95 °C overnight before being introduced into the drybox. MeCN and THF were opened and stored in the drybox. Photoluminescence Measurements. Photoluminescence measurements were performed in a 1 cm quartz cuvette. Absorbance spectra were obtained using a Hewlett-Packard 8453 UV−vis spectrophotometer with slit width of 1 nm. A FluoroLog-3 fluorometer (HORIBA Scientific, Edison, NJ), with a slit width of 1 nm, was used to obtain emission spectra. Fluorescence quantum yields, Φf, were determined by plotting the integrated emission spectrum of each compound as a function of absorbance, with absorbance values less than 0.1 to avoid inner filter effects.37,38 The slope of this plot was 925



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and no correction was made for any iR loss in electrochemical measurements. Platinum ultramicroelectrodes (UMEs) were used with a model 900 potentiostat (CH Instruments, Austin, TX) to determine formal potential, E0′, and diffusion coefficient, D, values. The UMEs were fabricated by sealing either 15 or 25 μm platinum wire (Goodfellow Corporation, Oakdale, PA) in a glass capillary tube and making back-contact with gallium− indium eutectic.41 Before each measurement, the UME was polished using 1 and 0.3 μm alumina on polishing cloth and then sonicated for five minutes each in water and then acetone and dried in the oven before being transferred to the drybox. E0′ was approximated as the potential at one-half (i.e., the halfwave potential, E1/2) of the steady-state current, iss. Potentials were calibrated using ferrocene, which has a standard potential of +0.34 V vs SCE in acetonitrile.42 Because the reduction wave of DEbT overlapped with the reduction of the supporting electrolyte in MeCN, the difference in E0′ for the oxidation and reduction of DEbT was estimated in 0.1 M TBAPF6 in THF. Then, the E0′ of the oxidation step was calibrated in MeCN using ferrocene. D was determined using iss and the equation iss = 4nFaDC, where n is the number of electrons transferred per molecule; F is Faraday’s constant; a is the radius of the electrode; and C is the concentration.43 D values of species listed in Table 2 were determined in 0.1 M TBAPF6 in MeCN.

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RESULTS AND DISCUSSION Photoluminescence Behavior. Figure 2 presents the absorbance and emission spectra for BMTbT, f-BMTbT, and DEbT, and their photoluminescence properties are listed in Table 1. Molar absorptivity, ε, and Φf values are reported within 95% confidence. Absorbance spectra for BMTbT11,12,22,24 and f-BMTbT13 were similar to those reported in the literature. ε of DEbT was similar to that of 5,5′-dihexyl-2,2′-bithiophene, its dihexyl equivalent.20 The reported ε of BMTbT and other bithiophene-based compounds10,20,24 was of the same order of magnitude as those reported here. Each of these species emitted in the blue/violet region of the spectrum. Values of Φf were ca. 0.03 for all compounds. Reported Φf values of other bithiophene compounds are about an order of magnitude higher, in the case with benzothiophene9 and phenyl10 functionalities, or an order of magnitude lower, as in the case of alkyl side chains.20 BMTbT absorbed and emitted at a lower energy than fBMTbT and DEbT (Figure 2). Using the particle-in-a-box model, a higher number of double bonds impart a longer conjugation length, leading to lower energies involved in molecular electronic transitions.44,45 Due to its fused thiophene rings and one fewer double bond, the conjugation length of fBMTbT is shorter than that of BMTbT, thus increasing the energy involved with electronic transitions in f-BMTbT relative to BMTbT. Comparing BMTbT and DEbT, a similar slight red shift (less than 30 nm) was observed upon addition of methoxy groups on bithiophene compounds,46 perhaps reflecting overlap of the lone pair sulfur orbitals with the thiophene π-system.15 Electrochemical Behavior. A summary of electrochemical data for BMTbT, f-BMTbT, and DEbT is presented in Table 2. The two electrochemical couples at +0.84 and +1.02 V vs SCE in the cyclic voltammogram of BMTbT (Figure 3a) correspond to BMTbT/BMTbT•+ and BMTbT•+/BMTbT2+.11,12,22,24 A more complete description of these couples is presented in the literature,11,12,22,24,47 as well as for f-BMTbT13 (vide infra). The ratio of the cathodic and anodic peak currents, ipc/ipa, was unity

Figure 2. Absorbance and emission spectra of (a) BMTbT, (b) fBMTbT, and (c) DEbT in acetonitrile. Slit width was 1 nm; excitation wavelengths were (a) 350 nm, (b) 276 nm, and (c) 313 nm.

for both couples for sweep rates ranging from 20 to 1000 mV/s (Figure S2, Supporting Information), indicating that these processes were chemically reversible under these experimental conditions,48 in agreement with reported studies.12,22 Of interest in this discussion is the reduction peak near −2.2 V vs SCE (Figure 3a) corresponding to a one electron transfer per molecule, indicating that this wave is due to the formation of a radical anion. No accompanying anodic wave was observed for sweep rates between 20 and 1000 mV/s (Figure S2, Supporting Information), suggesting that this reduction process is completely chemically irreversible,48 likely due to instability of the radical anion. At higher sweep rates, a peak appeared near −0.15 V vs SCE, possibly due to side reactions. The voltammetric profile of f-BMTbT (Figure 3b) was similar to that of BMTbT, with two oxidation waves and one reduction wave. The first oxidation appears electrochemically reversible; however, ipc/ipa was less than unity for the second oxidation at sweep rates between 50 and 1000 mV/s (Figure S3, Supporting Information), indicating that a chemical process likely follows this oxidation step,48 as demonstrated by RDE measurements.13 As for BMTbT, the reduction wave at −2.4 V 926



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Table 1. Photoluminescence Properties of Bithiophene Compoundsa compound

λpeak,abs (nm)

λpeak,em (nm)

λem (eV)

BMTbT f-BMTbT

345 309 289 (sh) 315 322 (sh)

423 360

2.93 3.44

380

3.26

DEbT a

ε (M−1 cm−1) 1.9 8.5 7.2 1.6 1.5

(±0.5) (±0.5) (±0.5) (±0.3) (±0.2)

× × × × ×

Φf 0.035 ± 0.006 0.029 ± 0.011

104 103 103 104 104

0.027 ± 0.004

Intervals are reported within 95% confidence.

Table 2. Electrochemical Properties of Bithiophene Compoundsa

a

compound

E0′R/R−b

E0′R/R+b

E0′R+/R2+b

ΔE1 (eV)

ΔE2 (eV)

BMTbT

−2.15

+0.84

+1.02

2.99 ± 0.03

3.18 ± 0.02

f-BMTbT

−2.33

+0.87

+1.20

3.20 ± 0.05

3.53 ± 0.05

DEbT

−2.7e

+1.05f

―

3.8g

―

D (cm2 s−1) 2.4 2.1 2.7 2.4 3.0

(±0.1) (±0.2) (±0.2) (±0.8) (±1.5)

× × × × ×

10−5c 10−5d 10−5c 10−5d 10−5c

b

Intervals are reported within 95% confidence. Potential vs SCE in 0.1 M TBAPF6/MeCN, except as noted. cNeutral species. dCation. ePotential vs SCE in 0.1 M TBAPF6/THF. fPeak potential of anodic wave vs SCE in 0.1 M TBAPF6/MeCN. gEstimated difference in formal potential between the oxidation and reduction waves in 0.1 M TBAPF6/THF.

vs SCE was also irreversible, and the peak at −0.15 V became more pronounced at higher sweep rates (Figure S3, Supporting Information). Note that E0′ values of the first oxidation and reduction of f-BMTbT were more positive and negative, respectively, than those of BMTbT, resulting in a larger ΔE1 (i.e., the energy difference between R/R•+ and R/R•−) for fBMTbT. Similar to the photoluminescence spectra (vide supra), this difference could reflect the shorter conjugation length of f-BMTbT, which would increase the energy difference between the HOMO and LUMO orbitals, represented by the oxidation and reduction potentials, respectively, of the neutral species.28 In contrast to BMTbT and f-BMTbT, DEbT exhibited only one well-behaved oxidation wave in addition to the one reduction step under these conditions (Figure 3c). This response is similar to that of its hexyl analogue,20 although two oxidation steps were observed for 5,5′-dimethyl-2,2′bithiophene.49 Additional nonreproducible oxidation waves of DEbT in MeCN were observed at higher potentials. The cyclic voltammetry of DEbT was complicated by solvent instability at reducing and oxidizing potentials in MeCN and THF, respectively. In MeCN, DEbT exhibited one irreversible oxidation wave at +1.1 V vs SCE, with an accompanying cathodic wave near 0 V vs SCE (Figures 3c and Figure S4, Supporting Information). A quasi-reversible couple was observed upon reduction of DEbT in THF (Figure 3c and Figure S5, Supporting Information). An irreversible oxidation wave also appeared in THF; however, this wave overlapped with the oxidation of THF. Annihilation ECL. Annihilation ECL measurements of BMTbT and f-BMTbT were performed by alternating the applied potential just beyond the reduction wave and the first or second oxidation waves. For BMTbT and f-BMTbT, emission was detected upon the formation of the dication and only following the negative step (Figures 4a and 4c). Little or no ECL was detected upon formation of the cation (Figures 4b and 4d). If both ions participating in the ECL process are stable, emission of equal intensity is expected upon both oxidizing and reducing potential steps.50 One reason that emission may not occur upon both potential steps, even though the energy

supplied is sufficient (vide infra), is that one of the species is unstable or side reactions, such as film formation on the electrode, occur.51 Cyclic voltammetry revealed that the reduction of BMTbT was chemically irreversible (Figure 3a), likely due to an unstable anion. Thus, at negative potentials, this anion was able to react with the dication immediately as the anion was generated. However, upon the positive potential step, no anion was available in the vicinity of the electrode surface to react with the dication; thus, no ECL emission was observed. As indicated by data presented in the literature, similar behavior was observed for a silole compound with thiophene end groups, which also exhibited an irreversible reduction step.26 Linear plots of i as a function of t−1/2, where t is time, are indicative of behavior under diffusion control.28 As shown in Figure S6 (Supporting Information), although a plot of faradaic current vs t−1/2 was linear, ECL intensity vs t−1/2 was not, suggesting that the annihilation ECL process was complicated by factors such as kinetics or side reactions.28 The higher ECL emission following dication formation likely reflects the energy available between the cationic and anionic species.27,28 The enthalpy involved in ECL emission can be calculated from a thermodynamic cycle.28,52,53 Two enthalpies, one for the cation and the other for the dication, to consider in the ECL process of BMTbT are −ΔH10 = ΔE1 − T ΔS 0

(8)

ΔE1 = E 0(BMTbT/BMTbT•+) − E 0(BMTbT/BMTbT•−)

(9)

and −ΔH2 0 = ΔE2 − T ΔS 0

(10)

ΔE2 = E 0(BMTbT•+/BMTbT2 +) − E 0(BMTbT/BMTbT•−)

(11)

The energy difference between the ground and first singlet state was 2.93 eV, calculated from the peak in the emission spectrum of BMTbT at 423 nm (Figure 2a and Table 1) and the relationship E = hc/λ.28,44 If the entropy term is taken as 0.1 eV at room temperature,28,53,54 the potential difference needed to 927



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Figure 4. Typical annihilation ECL transients and corresponding faradaic current and applied potential of (a,b) 1.9 mM BMTbT and (c,d) 0.43 mM f-BMTbT in 0.1 M TBAPF6/MeCN. ECL was generated by alternating the potential at 1 Hz just beyond the reduction and (a,c) second or (b,d) first oxidation step.

be 2.99 and 3.18 eV, respectively (Figure 3a and Table 2). Therefore, it is likely that only the energy involved with the dication produces ECL. An analogous argument can be made for f-BMTbT, where the energy difference between the ground and first excited singlet state was 3.44 eV (Figure 2b and Table 2), suggesting that a ΔE of at least 3.5 eV was required. The energy associated with formation of the cation was insufficient, but the dication provides sufficient energy because ΔE1 and ΔE2 were 3.20 and 3.53 eV, respectively. Although sufficient energy (3.8 eV) was available to electrochemically populate the first excited state (3.26 eV) of DEbT, no annihilation ECL emission was observed for DEbT in MeCN or THF, likely due to the limited stability of electrogenerated species, particularly the cation radical. Within five minutes of stepping between oxidizing and reducing potentials in solutions of BMTbT or f-BMTbT, the oxidation waves became ill-defined, and a visible film formed on the working electrode surface, indicating that a chemically irreversible process occurred. Therefore, to obtain more reproducible results and improve the ECL emission lifetimes, coreactant ECL was pursued. Coreactant ECL. Benzoyl peroxide (BPO) was used as the coreactant20,28,32−34 to generate ECL emission from BMTbT, fBMTbT, and DEbT. Upon reduction, it is proposed that BPO produces an oxidizing radical (eqs 6 and 7) involved in forming the excited state of the fluorescing species. Upon sweeping the potential negative in a solution of BMTbT with BPO (Figure 5a), visible, violet-colored light was produced. In contrast to

Figure 3. Typical cyclic voltammograms of (a) 1.9 mM BMTbT in 0.1 M TBAPF6/MeCN; sweep rate, 50 mV/s, and (b) 2.2 mM f-BMTbT in 0.1 M TBAPF6/MeCN; sweep rate, 100 mV/s. In (c), two supporting electrolytes are shown: 2.3 mM DEbT in 0.1 M TBAPF6/ THF and 2.5 mM DEbT in 0.1 M TBAPF6/MeCN; sweep rate, 100 mV/s. Note that the potential axis is different in (c). Electrode area, 0.023 cm2.

excite this state electrochemically (i.e., ΔE1 or ΔE2) is ca. 3.0 eV. Assuming that ΔE0′ is similar to ΔE0, ΔE1 and ΔE2 would 928



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Figure 6. (a) ECL intensity as a function of cycle number for 2.2 mM BMTbT with 10 mM BPO in 0.1 M TBAPF6/MeCN. ECL transients are shown at the (b) beginning and (c) end of cycling. The potential was stepped at 1 Hz between −0.2 and −2.3 V. One cycle represents two pulses: the step to −0.2 V and the other to −2.3 V.

The coreactant ECL emission intensity was typically an order of magnitude lower for f-BMTbT than for BMTbT (Figure 5b). This difference may reflect effects such as the slightly lower Φf of f-BMTbT compared with BMTbT (Table 1) and available energy. As a conservative estimate using +0.8 V vs SCE as the value of E0 for the C6H5CO•/C6H5CO2− couple,33 ΔE between C6H5CO•/C6H5CO2− and f-BMTbT/f-BMTbT•− would be ca. 3.1 eV, which would be insufficient to produce ECL, although using a value of +1.5 V vs SCE for the C6H5CO•/C6H5CO2− couple32 would yield a sufficient ΔE of 3.8 eV. Similar to BMTbT, the ECL process of f-BMTbT also appeared to be diffusion-limited (Figure S7b, Supporting Information). Although the potential range of the solvent (viz., MeCN or THF) was too narrow for annihilation ECL of DEbT, emission was observed upon a negative sweep of DEbT with BPO in THF (Figure 5c). This emission was less intense than that of BMTbT. ECL Spectrum of BMTbT. The ECL spectrum (Figure 7) of BMTbT was obtained using 10 mM BPO and stepping the potential between −0.2 and −2.3 V vs SCE. The wavelength of the main ECL emission peak (430 nm) was similar to the photoluminescence emission peak (423 nm), indicating that this peak was due to the emitting monomer. At higher concentrations, a second peak at lower energy emerged near 505 nm, possibly due to emission from an aggregated species. Additional peaks at lower energies have been observed in the ECL spectra of dyes based on 4,4-difluoro-4-bora-3a,4a-diaza-sindacene (BODIPY),51,56 attributed to dimers formed by chemical and electro-oxidation methods, and aromatic compounds,57,58 attributed to excimers or side products. In the case of BMTbT, it is possible that π-stacked species formed under these conditions.11,21,22,47 Note that the peak at 505 nm was absent in the photoluminescence spectra, albeit the concentrations used in the photoluminescence measurements were typically at least 2 to 3 orders of magnitude lower than those in the electrochemical ones. However, if aggregation was electro-

Figure 5. Typical cyclic voltammograms and corresponding ECL emission of (a) 2.2 mM BMTbT with 3.2 mM BPO, (b) 1.5 mM fBMTbT with 10 mM BPO, and (c) 2.3 mM DEbT with 10 mM BPO. Insets show ECL transients produced by stepping the potential between (a) −0.2 and −2.3 V, (b) 0.0 and −2.5 V, and (c) and 0.0 and −2.7 V. Supporting electrolyte, (a,b) 0.1 M TBAPF6/MeCN, (c) 0.1 M TBAPF6/THF; sweep rate, 20 mV/s; electrode area, 0.023 cm2. Note that the potential axis is different in (c).

annihilation ECL, the cyclic voltammetric profile for the oxidation waves was similar before and after coreactant ECL, indicating that no significant electrode fouling occurred, perhaps due to fewer side reactions involved,55 although the solution sometimes turned yellow after several cycles. For similar concentrations of BMTbT, the coreactant ECL intensity was typically 1 to 2 orders of magnitude greater than that produced through annihilation. A plot of ECL intensity as a function of t−1/2 yielded a linear relationship (Figure S7a, Supporting Information), indicating that the coreactant ECL process may be diffusion-limited,28 in contrast to annihilation ECL (Figure S6, Supporting Information). When the potential was stepped at1 Hz between −0.2 and −2.3 V vs SCE, over 1000 cycles were detectable with the PMT under quiescent conditions (Figure 6). 929



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AUTHOR INFORMATION

Corresponding Author

*Phone: 607-255-4720. Fax: 607-255-9864. E-mail: hda1@ cornell.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported as part of the Energy Materials Center at Cornell (EMC2), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001086. The authors thank Dr. Stephen Burkhardt for sharing his observation that BMTbT fluoresces, Dr. Sean Conte for providing samples of BMTbT and f-BMTbT, and Prof. William Dichtel’s group for use of their fluorometer. N.L.R. gratefully acknowledges useful ́ conversations with Prof. Joaquiń Rodriguez López regarding ECL and Prof. Fernando Uribe-Romo regarding fluorescence and assistance from Dr. Eric Rus with the electronic equipment and LabVIEW program.

Figure 7. Normalized ECL spectra of BMTbT at a series of concentrations with 10 mM BPO in 0.1 M TBAPF6/MeCN. Monochromator slit width was 10 nm. ECL emission was generated by stepping the potential at 1 Hz, except for 2.2 mM, which was at 0.5 Hz, between −0.2 and −2.3 V vs SCE.

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chemically driven, a peak at longer wavelengths would be expected at all concentrations, suggesting that aggregation is a function of BMTbT concentration, as is often the case.

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CONCLUSIONS The bithiophene compounds BMTbT, f-BMTbT, and DEbT were characterized using UV−vis spectroscopy, fluorescence, cyclic voltammetry, and ECL. Experimentally estimated HOMO−LUMO gaps corresponded with the second oxidation of BMTbT and f-BMTbT. Cyclic voltammetry revealed that BMTbT and f-BMTbT had an irreversible reduction step, possibly due to a chemically unstable species, which affected their ECL transient profiles. DEbT exhibited irreversible oxidation and quasi-reversible reduction steps in acetonitrile and THF, respectively. BMTbT and f-BMTbT produced ECL emission through annihilation of the anionic and dicationic species, and annihilation with the cation was insufficient in energy. Using BPO as a coreactant produced ECL with all three compounds. Over 1000 cycles of detectable emission were obtained with BMTbT and BPO. ECL spectra of BMTbT indicated formation of an aggregated species at concentrations above 1.1 mM in acetonitrile. Although the fluorescence quantum yield was lower than those reported for other oligothiophenes, these studies demonstrate that thioether groups may provide a more stable alternative to alkyl side chains in oligomeric thiophene compounds. Future studies include investigating the effects of heteroatoms and adding electron-donating and -withdrawing groups on the emission wavelength of BMTbT.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

NMR spectrum of DEbT. Cyclic voltammograms at several sweep rates for BMTbT, f-BMTbT, and DEbT. Plots of faradaic current and ECL intensity as a function of t−1/2 for annihilation ECL of BMTbT and for coreactant ECL of BMTbT and f-BMTbT. This material is available free of charge via the Internet at http://pubs.acs.org. 930



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Electrogenerated Chemiluminescence of Bithiophenes with Methylthio

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