Changing the Sign of Exchange Interaction in Radical Pairs to Tune

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Changing the Sign of Exchange Interaction in Radical Pairs to Tune Magnetic Field Effect on Electrogenerated Chemiluminescence Haiping Pan,† Yan Shen,† Lin Luan,† Kai Lu,† Jiashun Duan,† and Bin Hu*,†,‡ †

Wuhan National Laboratory for Optoelectronics and School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China ‡ Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States S Supporting Information *

ABSTRACT: Two different electrogenerated chemiluminescence (ECL) systems, Ru(bpy)32+/TPrA and Ru(bpy)32+/C2O42−, are chosen to study the relationship between the sign of exchange interaction in radical pairs and magnetic field effects (MFEs) on electrogenerated chemiluminescence intensity (MFEECL). A positive MFEECL up to 210% is observed for the Ru(bpy)32+/TPrA system, while a negative MFEECL of only −33% is observed based on the Ru(bpy)32+/C2O42− system. The significant difference on MFEECL is ascribed to different signs of exchange interaction in radical pairs [Ru(bpy)33+···TPrA•] and [Ru(bpy)33+··· CO2−•] because they have a distant and proximate separation distance between two radicals of a pair, which result in different magnetic-field-induced intersystem crossing directions between singlet and triplet states. The experimental results suggest that an applied magnetic field can enhance the singlet → triplet conversion rate in radical pairs [Ru(bpy)33+···TPrA•] while facilitating an inverse conversion of triplet → singlet in radical pairs [Ru(bpy)33+···CO2−•]. The increase/decrease of triplet density in radical pairs stimulated by an applied magnetic field leads to an increase/decrease on the density of light-emitting triplets of Ru(bpy)32+*. As a consequence, we can tune MFEECL between positive and negative values by changing the sign of exchange interaction in radical pairs during an electrochemical reaction.

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INTRODUCTION Electrogenerated chemiluminescence (ECL) involves electron transfer between electrochemically generated radical ions (A− and D+) resulting in an excited species (A* or D*) that emits light.1−3 An external magnetic field can exert unusual effects on the intensity of electrogenerated chemiluminescence from several electrochemical systems, which was first reported in the early 1970s.4 Since then magnetic field effects (MFEs) on electrochemical reactions have been widely studied, and several mechanisms have been proposed. The first one is magnetic field sensitive intersystem crossing (ISC) between singlet and triplet radical pairs [A−···D+] after they formed from two electrochemical generated radical ions A− and D+. An applied magnetic field can change the rate of the 1[A−···D+] ↔ 3 − [A ···D+] transition5−7 in radical pairs [A−···D+] (reactants) before the electron transfer step between them. Furthermore, the spin states of reactants are identical to that of the products (light-emitting states) during the electron transfer generation of ECL emission states (because of spin conservation rules). Therefore, the singlet to triplet ratio in light-emitting states can be changed by an applied magnetic field. As a result, the ECL emission intensity in the electrochemical cell is magnetic field sensitive. The second path is magnetic field dependent triplet− triplet annihilation. In the years prior to this early work, the enhancement of ECL intensity in the magnetic field of rubrene, tetracene, and phenanthrene anion−cation reactions was observed and ascribed to the magnetic field reducing triplet− triplet annihilation rate.8−10 The third route for magnetic field effects on electrochemical systems is magnetic field sensitive © XXXX American Chemical Society

triplet-ion reaction. The light-emitting triplet states formed after the electron transfer process can be quenched by adjacent radical ions (3A*+A−/D+ → A+A−/D+). An external magnetic field can inhibit the quenching process11,12 and generate MFEs on light emission intensity. Besides the above routes to generate MFEs on ECL intensity, if one of the ECL reaction steps (e.g., the oxidation of reactants) can be changed by an external magnetic field, then the ECL intensity is magnetic field dependent. In this paper, the MFEs on ECL intensity (MFEECL), photoluminescence intensity (MFEPL), and electrical current (MC) are exploited to elucidate the key factors that determine ECL intensity in electrochemical cells. The ECL system based on Ru(bpy)32+ is widely studied because this system forms the basis of commercial systems for immunoassay and DNA analysis.13,14 Although Ru(bpy)32+ systems with coreactants, such as tripropylamine (TPrA) and oxalate ion (C2O42−), have been of practical importance, details of the kinetics and mechanisms of the reactions involved have been sparse. The optimization of these systems clearly needs a better understanding of the ECL reactions and how they affect the efficiency of light emission. Received: February 14, 2015 Revised: March 21, 2015

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DOI: 10.1021/acs.jpcc.5b01541 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (a) MFEECL from two different ECL systems: (Ru(bpy)32+ + TPrA) system and (Ru(bpy)32+ + C2O42−) system; MFEPL of 1 mM Ru(bpy)3Cl2 in water. The (Ru(bpy)3Cl2 + TPrA/C2O42−) systems contain 1 mM Ru(bpy)3Cl2 and 0.1 M TPrA/Na2C2O4 as reactants. (b) Electrogenerated chemiluminescence spectra and photoluminescence spectrum from the above systems. (c) Magnetic field sensitive reaction mechanism of electrochemiluminescence process. When J > 0 (or J < 0), the applied magnetic field can enhance S → T (or T → S) conversion in radical pairs.

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EXPERIMENTAL AND THEORETICAL METHODS Chemical tris(2,2′-bipyridine)ruthenium(II) dichioride (Ru(bpy) 3 Cl 2 ), tripropylamine (TPrA), sodium oxalate (Na2C2O4), and sodium dihydrogen phosphate (NaH2PO4) were all purchased from Aldrich and used as received. The reactant Ru(bpy)32+ with TPrA and Na2C2O4 were dissolved in deionized water. The final ECL solution concentration was 1 mM Ru(bpy)32+ + 0.1 M TPrA (Na2C2O4) + 0.1 M NaH2PO4 in water. All solutions were deoxygenated by bubbling with nitrogen for at least 15 mins before the measurement. A threeelectrode configuration was employed in all experiments with two flat Pt foil plate electrodes (10 mm × 15 mm × 0.3 mm) that served as the working electrode and counter electrode; a silver chloride (Ag/AgCl) electrode served as the reference electrode. An electrochemical cell was placed in a magnetic field generated by an electromagnet. Electrode potential was supplied, and the current was recorded by a CHI 750D electrochemical workstation. The electrogenerated chemiluminescence spectrum and intensity were characterized by an FLS920 fluorescence spectrometer (Edinburgh Instrument) equipped with an optical fiber connection. A schematic experimental setup for the measurements is shown as Figure S1 in the Supporting Information. The amplitude of MFE is given by relative change in percentage, MFE = (SB − S0)/S0 × 100%, where SB and S0 are the signal intensities with and without a magnetic field, respectively.

chemical system was ascribed to the reasons as described in the Introduction. In our system, the light emission is clearly attributable to the excited triplets, Ru(bpy)32+*, since the spectral peak of ECL at 610 nm (Figure 1b) is characteristic of phosphorescence from the triplet states of the Ru(bpy)32+*.15 To determine the key factors that account for the MFEECL in our system, the triplet−triplet annihilation process was studied separately by photoluminescence experiment on Ru(bpy)3Cl2 dissolved in water. Nevertheless, there is no detectable MFEPL observed from the former system (shown in Figure 1a), and therefore the triplet−triplet annihilation could be excluded. For the triplet−ion reaction, it is possible that the external magnetic field inhibits the triplet quenching process by decreasing the reaction rate constant.12 However, this consideration only takes into account the positive MFEECL. It cannot account for the negative MFEECL based on the Ru(bpy)32+/C2O42− system. In addition, it has been observed in solid states that an external magnetic field can only change the triplet−ion reaction rate by a few percent, as indicated by the studies of magnetic field effect on photocurrent.16 Clearly, the triplet−ion reaction is not sufficient to generate the giant MFEECL (210%) observed from the Ru(bpy)32+/TPrA system. Here, we suggest that magnetic field-induced change on intersystem crossing (1[A−···D+] ↔ 3 − [A ···D+]) rates in radical pairs is the main reason for the positive and negative MFEECL based on Ru(bpy)32+/TPrA and Ru(bpy)32+/C2O42− systems. As shown in Figure 1c, before the electron transfer generates light-emitting excited states A*, activated radicals A− and D+ must be formed by electrochemical oxidations and reductions. Each of the activated radicals has a single unpaired electron with a spin state. Thus, the activated radicals form a doublet pair with two spin configurations [the singlet 1(A−···D+) and the triplet 3(A−···D+)]. Therefore, the intersystem crossing between 1(A−···D+) and 3(A−···D+) is a general phenomenon17

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RESULTS AND DISCUSSION Figure 1a shows the MFEECL from Ru(bpy)32+/TPrA and Ru(bpy)32+/C2O42− systems and the MFEPL of Ru(bpy)3Cl2 in water. A positive MFEECL of 210% was observed for the Ru(bpy)32+/TPrA system, a negative MFEECL of −33% for the Ru(bpy)32+/C2O42− system, and no MFEPL for only Ru(bpy)3Cl2 in water. Traditionally, the MFEECL in an electroB

DOI: 10.1021/acs.jpcc.5b01541 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 2. (a) MC from three different electrochemical systems: (Ru(bpy)3Cl2 + TPrA) system, (Ru(bpy)3Cl2 + C2O42−) system, and TPrA system. The Ru(bpy)3Cl2 + TPrA/C2O42− systems contain 1 mM Ru(bpy)3Cl2 and 0.1 M TPrA/Na2C2O4 as reactants. The TPrA system contains only 0.1 M TPrA as reactants. (b) Effect of an external magnetic field on radical pair intersystem crossing and recombination rate of a chemical reaction. RC− H denotes tripropylamine.

TPrA•] and negative for [Ru(bpy)33+···CO2−•]. Therefore, an external magnetic field can enhance the singlet → triplet transition in radical pairs [Ru(bpy)33+···TPrA•] but triplet → singlet transition in radical pairs [Ru(bpy)33+···CO2−•] due to different signs of inter-radical exchange interaction J. The density of triplet radical pairs is proportional to that of lightemitting triplet Ru(bpy)32+* (as shown in Figure 1c). Hence an applied magnetic field can increase/decrease the density of triplet Ru(bpy)32+* based on Ru(bpy)32+/TPrA and Ru(bpy)32+/C2O42− systems by magnetic field-induced change on intersystem crossing rate. As a consequence, positive and negative MFEECL can be separately observed for Ru(bpy)32+/ TPrA and Ru(bpy)32+/C2O42− systems. Thus, we can tune MFEECL between positive and negative values by changing the sign of exchange interaction in intermediate radical pairs. To further understand the significant difference of the MFEECL based on Ru(bpy)32+/TPrA and Ru(bpy)32+/C2O42− systems, the MCs have been studied separately. Figure 2a shows that an applied field of 0.95 T can generate 5.4% MC based on the Ru(bpy)32+/TPrA system, 1.5% MC based on the TPrA system, and hardly any MC based on the Ru(bpy)32+/ C2O42− system. Both the Ru(bpy)32+/TPrA and Ru(bpy)32+/ C2O42−systems involve the oxidation of Ru(bpy)32+ to Ru(bpy)33+. This indicates it cannot be the primary factor in the generation of MFE, and this is proved by no MC based on the Ru(bpy)32+/C2O42− system once again in Figure 2a. Therefore, the key factor for the generation of MFEECL is suggested to be the electron transfer process between Ru(bpy)33+ and TPrA•/ CO2−• to form light-emitting triplet Ru(bpy)32+*. TPrA•, a highly reducing species involved in the electron transfer process to form Ru(bpy)32+*, is generated by the oxidation and the following deprotonation of TPrA. It is proposed that the oxidation and subsequent deprotonation of the amine are the critical steps in ECL reactions.30 The oxidation and subsequent deprotonation of TPrA can be enhanced by an applied magnetic field, which is verified by the positive MC of 1.5% based on the system containing only TPrA as reactants (red line in Figure 2a). The positive MC suggests the following process: first, during the deprotonation process of the TPrA radical cation, the cleavage of the α-C−H bond31,32 leads to a radical pair with a singlet state in the reaction zone (shown in Figure 2b). Furthermore, the generated singlet radical pair can recombine to reform an α-C−H bond. Second, an applied magnetic field can enhance the singlet → triplet conversion rate33,34 in the initially formed singlet radical pairs. In addition, the triplet radical pairs cannot recombine to form a chemical bond due to the Pauli exclusion principle. Thus, triplet radical pairs tend to dissociate and escape from the reaction zone. As a consequence, the magnetic field enhanced

that occurs before the intrinsic electron transfer forms excited states of A*. An external magnetic field can influence the intersystem crossing rate between 1(A−···D+) and 3(A−···D+) due to magnetic field-induced change on the radical pair (A−··· D+) Hamiltonian H0. Generally, the 1(A−···D+) ↔ 3(A−···D+) interconversion rate is determined by magnetic interactions, depending on the difference between precessional frequencies (Δω) of the electron spin vectors. The Δω will be given by5,18 Δω = ω1 − ω2 = (μB /ℏ)[ΔgB0 ] + Δ ∑ aimi

(1)

where ΔgB0 arises from the Zeeman interaction for cases in which the g factors of the two electrons are different and ΔΣaimi arises from the differences caused by different hyperfine fields applied onto different electrons. When the two radicals of a pair experience different magnetic fields or have different g factors, the spin precessions in the two radicals can change between parallel and antiparallel alignments, causing 1(A−···D+) ↔ 3(A−···D+) transitions. In our electrochemical systems, the radical pairs [Ru(bpy)33+···TPrA •]15 and [Ru(bpy)33+··· CO2−•]19,20 are first formed from the radicals generated by electrochemical oxidation. According to eq 1, increasing an external magnetic field B0 can further increase the spin flipping and enlarges the 1(A−···D+) ↔ 3(A−···D+) conversion rate. This is consistent with magnetic field-induced Zeeman splitting decreasing the singlet−triplet energy difference (2J) and increasing singlet−triplet conversion rate. Thus, an external magnetic field can enhance the singlet → triplet (when J > 0) or triplet → singlet (when J < 0) transition in these radical pairs (shown in Figure 1c). Though the inter-radical exchange interaction J is always considered as negative (J < 0), positive J has also been proposed for some radical pairs.21−25 It is suggested that the exchange interaction J in a radical pair is negative for proximate radicals but positive for more distant ones (Figure 1c) because the intervening solvent molecules become involved in the exchange interaction giving rise to J > 0 via the superexchange process.26−28 Chemically induced dynamic electron polarization of some radical pairs indicates that the solvent molecules contain nonbonding orbitals to overlap with the orbitals of the radicals, which can transfer an indirect exchange interaction in radical pairs.27,29 This indirect exchange interaction is likely to generate a positive contribution to the total exchange interaction, resulting in J > 0. We should note that in our ECL system the separation distance between two radicals in radical pairs [Ru(bpy)33+···TPrA•] is larger than that in [Ru(bpy)33+···CO2−•] because the latter has stronger Coulombic attraction between two oppositely charged Ru(bpy)33+ and CO2−• ions. Thus, it is proposed that the interradical exchange interaction J is positive for [Ru(bpy)33+··· C

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Figure 3. MFEECL with different applied electrode potentials from two different systems: (a) (Ru(bpy)32+ + TPrA) system and (b) (Ru(bpy)32+ + C2O42−) system. The ECL solution concentration is 1 mM Ru(bpy)3Cl2 in water with 0.1 M TPrA or Na2C2O4 and 0.1 M NaH2PO4 as supporting electrolyte.

conversion of singlet → triplet in radical pairs tends to increase the amount of escape products from the reaction zone. Thus, the oxidation and deprotonation rates increase with an external magnetic field. Accordingly, the density of TPrA• radicals in the reaction zone is enhanced by an external magnetic field. Therefore, the density of light-emitting Ru(bpy)32+* greatly increases with an applied magnetic field since TPrA• is a critical reactant in the electron transfer process to generate Ru(bpy)32+*.30 For the Ru(bpy)32+/C2O42− system, no detectable MC was observed. This indicates that the formation of the CO2−• radical cannot be affected by an external magnetic field. Thus, the amplitude of MFEECL based on the Ru(bpy)32+/ C2O42− system is much smaller than the Ru(bpy)32+/TPrA system because the density of the CO2−• radical cannot be increased by an applied magnetic field. Now we discuss the MFEECL with different applied electrode potentials to further understand the significant difference of MFEECL between Ru(bpy)32+/TPrA and Ru(bpy)32+/C2O42− systems. Figure 3a shows that the amplitude of MFEECL changes greatly (from 18% to 210%) when the applied electrode potentials increase from 1.2 to 1.5 V, with the same applied magnetic field based on the Ru(bpy)32+/TPrA system. However, the amplitude of MFEECL does not change much (from −27% to −33%) when applied electrode potentials increase from 1.1 to 1.3 V based on the Ru(bpy)32+/C2O42− system (Figure 3b). We should mention that the oxidation potentials for Ru(bpy)32+/TPrA and Ru(bpy)32+/C2O42− systems are slightly different (Figure S3 in Supporting Information). Thus, the selected electrode potentials for the MFEECL experiment of the two systems are slightly different. In general, the amplitude of MFEECL can be expressed as MFE ECL =

⎛I ⎞ IB − I0 × 100% = ⎜ B − 1⎟ × 100% I0 ⎝ I0 ⎠

As a result, the amplitude of MFEECL is increased with increasing applied electrode potential due to the external magnetic field enhancing the intersystem crossing rate to form triplet radical pairs 3[Ru(bpy)33+···TPrA•]. For the Ru(bpy)32+/ C2O42− system, only Ru(bpy)32+ is directly oxidized at the working electrode. Thus, changing the applied electrode potential from 1.1 to 1.3 V does not cause an appreciable change on the density of active radicals CO2−•. Consequently, the amount of radical pairs [Ru(bpy)33+···CO2−•] that participates in the magnetic field-facilitated intersystem crossing stays almost unchanged when applied electrode potential increases. Namely, IB stays almost unchanged. Therefore, the amplitude of MFEECL changes little as the applied electrode potential increases. In addition, increasing applied electrode potential can only change the density of radical pairs, leaving the separation distance between two radicals of a pair unchanged. In this situation, the sign of exchange interaction in radical pairs stays unchanged upon increasing electrode potential. As a consequence, the MFEECL based on Ru(bpy)32+/ TPrA and Ru(bpy)32+/C2O42− systems keeps positive and negative values, respectively, while increasing applied electrode potentials.

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CONCLUSION We studied MFEECL and MC based on the ECL reaction of Ru(bpy)32+ with different coreactants TPrA and C2O42−. A large positive MFEECL of 210% and MC of 5.4% are observed for the Ru(bpy)32+/TPrA system, while a small negative MFEECL of −33% and no detectable MC is observed for the Ru(bpy)32+/C2O42− system. Here, we suggest that an external magnetic field could primarily affect the intersystem crossing direction between singlet and triplet radical pairs based on the significant difference between MFEECL and MC of the Ru(bpy)32+/TPrA and Ru(bpy)32+/C2O42− systems. An external magnetic field can enhance singlet → triplet transition in radical pairs [Ru(bpy)33+···TPrA•] but promote triplet → singlet transition in radical pairs [Ru(bpy)33+···CO2−•]. This occurs due to the fact that the radical pairs [Ru(bpy)33+··· TPrA•] have positive spin exchange interaction (J > 0), while the radical pairs [Ru(bpy)33+···CO2−•] have negative spin exchange interaction (J < 0) because specific solvent effects in radical pairs [Ru(bpy)33+···TPrA•] would switch J sign. In addition, the magnetic field could enhance the oxidation process of TPrA but has hardly any effect on the oxidation of C2O42−. Thus, the MFEECL based on the Ru(bpy)32+/TPrA system is much larger than the Ru(bpy)32+/C2O42− system. Furthermore, the MFEs on ECL intensity can be exploited as a diagnostic tool not only for the sign of exchange interaction in

(2)

where IB and I0 refer to the ECL intensity with and without a magnetic field, respectively. It should be noted that with a more positive applied electrode potential the oxidation of Ru(bpy)32+ and TPrA is more favorable. Therefore, when applied electrode potentials increase from 1.2 to 1.5 V, more active Ru(bpy)33+ and TPrA• radicals are generated in the reaction zone near the working electrode. When the density of Ru(bpy)33+ and TPrA• radicals increases, the amount of radical pairs [Ru(bpy)33+··· TPrA•] that participates in the magnetic field-facilitated intersystem crossing to generate the triplet radical pairs is enhanced. Thus, the density of triplet Ru(bpy)32+* is enhanced since it is proportional to the density of the triplet radical pairs. Namely, IB increases when applied electrode potential increases. D

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intermediate radical pairs but also for the entire reaction mechanism of which they are a part.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

B. H. guided the research. H. P. did the experimental measurements and data analysis. B.H. and H.P. wrote the main manuscript text. Y.S., L.L., K. L., and J. D. made contributions to the discussion of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by International Cooperation and Exchange Program (Gant No. 21161160445) and photovoltaic project (Grant Nos. 61077020, 61205034, 61475051) funded by the National Natural Science Foundation of China. The authors acknowledge the support from National Significant Program (2013CB922104 and 2014CB643506). The author (B.H.) acknowledges the Air Force Office of Scientific Research (AFOSR) under the grant number FA9550-11-1-0082 and from NSF Under grant number ECCS-0644945 and CBET1438181in USA.

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