Susceptibility of Polycyclic Aromatic Hydrocarbons in Oxidative Voltammetry: Unveiling the Effect of Electrolyte-coordination

Abstract

Redox behavior is a fundamental and fascinating feature of polycyclic aromatic hydrocarbons (PAHs). Cyclic voltammetry (CV) measurements are commonly performed to estimate the electronic structure of PAHs and to determine the stability of their oxidation and reduction states. However, the influences of electrolytes on electrochemically oxidized/reduced PAHs have rarely been discussed. In this note, we report voltammetric analyses of five PAHs (anthracene, 9,10-dimethylanthracene, phenanthrene, pyrene, and perylene) in Bu₄NB(C₆F₅)₄/CH₂Cl₂ and Bu₄NTfO/CH₂Cl₂, respectively, to highlight how the electrolyte-coordination affects the oxidative voltammetric behavior of PAHs. In most cases, reversible voltammetric responses were obtained with Bu₄NB(C₆F₅)₄/CH₂Cl₂, suggesting that this electrolyte is enough weakly coordinating to investigate its intrinsic oxidation behavior. On the other hand, irreversible voltammetric responses were obtained with Bu₄NTfO/CH₂Cl₂, indicating that the presence of a relatively coordinating anion, TfO⁻, destabilizes the radical cation species and induces further chemical and electrochemical processes. This study provides hints for rational electrolyte design to properly understand the redox behavior of molecules and maximize the potential of functional molecules for applications related to redox chemistry.

1. Introduction

Polycyclic aromatic hydrocarbons (PAHs) show unique electronic, optical, and redox properties owing to their extended π-system, thus playing a vital role in organic electronics such as light-emitting diodes, semiconductors, photovoltaics, and batteries.^1–3 The development of synthetic organic chemistry enabled the address to various conceptually novel and structurally defined PAHs, including nano-graphenes,⁴ bucky-bowls,⁵ and carbon nanobelts.⁶ Voltammetry measurements significantly contribute to the research of structurally defined PAHs to estimate their electronic structures, i.e., highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), as well as obtaining a sense of the redox stability of a molecule of interest. In many cases, voltammetry measurements are performed using CH₂Cl₂ as a solvent due to its capability to solubilize PAHs, and Bu₄NX as a supporting electrolyte, where X stands for anions such as TfO⁻, ClO₄⁻, BF₄⁻, PF₆⁻ (TfO⁻: trifluoromethanesulfonate anion). These anions are generally classified as weakly-coordinating anions (WCAs). Such a combination of solvent and supporting electrolytes are preferred presumably due to their inertness in the oxidation process. Indeed, CH₂Cl₂ is a solvent with one of the lowest donor number values,⁷ thus this solvent would rarely interact with electrochemically generated oxidized species, such as radical cations. X⁻ anion is also known as an anion with a relatively low donor number.⁸ Thus, when using Bu₄NX/CH₂Cl₂ (X = TfO⁻, ClO₄⁻, BF₄⁻, PF₆⁻; the order follows the reported donor number of anions from strong to weak⁸) as an electrolyte, there seemingly are no donor species that can aggressively interact with electrochemically generated (radical) cations.

However, our recent research revealed that the use of Bu₄NX (X = TfO⁻, ClO₄⁻, BF₄⁻, PF₆⁻) as a supporting salt does affect the oxidation process of a conjugated molecule, 2,5-phenyltellurophene (PT).^9,10 It was revealed that even WCAs including TfO⁻, ClO₄⁻, BF₄⁻, and PF₆⁻ coordinate to the tellurium center of oxidized PT to induce a second electron transfer process, affording hypervalent Te(IV) species (Fig. 1a). In other words, TfO⁻, ClO₄⁻, BF₄⁻, and PF₆⁻ are coordinating anions in this case. To avoid the coordination of anion, the use of B(C₆F₅)₄⁻ as an anion was found to be essential. By employing Bu₄NB(C₆F₅)₄/CH₂Cl₂ as an electrolyte, radical cation species, PT^•+, was successfully generated for the first time.⁹

Figure 1.

Concept of this work. (a) A previous report on the electrolyte-dependent oxidation process of PT. (b) Effect of electrolyte-coordination on the electrochemical oxidation of PAHs, and chemical structures of PAHs used in this research.

In this note, we report the effect of the coordination of anions derived from electrolytes on the electrochemical oxidation behavior of PAHs, such as anthracene, 9,10-dimethylanthracene, phenanthrene, pyrene, and perylene, by performing the cyclic voltammetry (CV) measurements in Bu₄NB(C₆F₅)₄/CH₂Cl₂ (as a weakly coordinating electrolyte) and Bu₄NTfO/CH₂Cl₂ (as a coordinating electrolyte), respectively, to highlight how the coordination of the electrolyte affect the electrochemical oxidation behavior of PAHs (Fig. 1b).

2. Experiments

2.1 Materials

All reagents and dehydrated solvents were purchased from commercial sources and used without further purification. Aqueous solution of NaB(C₆F₅)₄ was kindly supplied by Nippon Shokubai Co. Ltd. and NaB(C₆F₅)₄ salt was obtained by evaporating the solvent. Bu₄NB(C₆F₅)₄ was prepared by salt exchange of NaB(C₆F₅)₄ and Bu₄NBr.

2.2 Cyclic voltammetry measurement

Cyclic voltammetry (CV) measurements were performed using potentiostat VSP-3A (Biologic). All CV measurements were carried out in the three-electrode system equipped with a platinum (Pt) disk working electrode (φ = 3.0 mm), a Pt plate counter electrode (20 mm × 20 mm), and Ag/AgNO₃ reference electrode. Ag/AgNO₃ reference electrode was calibrated by ferrocene as a standard material. 0.1 M (= mol L⁻¹) Bu₄NTfO/CH₂Cl₂ or Bu₄NB(C₆F₅)₄/CH₂Cl₂ solution containing 5 mM of PAHs (anthracene, 9,10-dimethylanthracene, phenanthrene, pyrene, and perylene) was used as electrolyte. 0.1 M Bu₄NPF₆/CH₂Cl₂ was also used for the measurement of anthracene (5 mM).

2.3 Square-wave voltammetry measurement

Square wave voltammetry (SWV) measurements were performed at a step potential of 2 mV, an amplitude of 20 mV, and at a frequency of 50 Hz using potentiostat VSP-3A (Biologic). SWV measurements were carried out in the three-electrode system equipped with a platinum (Pt) disk working electrode (φ = 3.0 mm), a Pt plate counter electrode (20 mm × 20 mm), and Ag/AgNO₃ reference electrode. 0.1 M Bu₄NB(C₆F₅)₄/CH₂Cl₂ solution containing 1, 0.5, and 0.1 mM of pyrene was used as an electrolyte. Ag/AgNO₃ reference electrode was calibrated by ferrocene as a standard material.

3. Results and Discussion

First, CV measurements of various PAHs, anthracene, 9,10-dimethylanthracene, phenanthrene, pyrene, and perylene, were performed in Bu₄NB(C₆F₅)₄/CH₂Cl₂ and Bu₄NTfO/CH₂Cl₂, respectively (Fig. 2). Here, B(C₆F₅)₄⁻ was employed as a weakly coordinating electrolyte, whereas TfO⁻ was as a coordinating electrolyte. As for the material of the working electrode, the use of a carbon electrode was avoided, which is known for π-π interactions between the electrode surface and the PAHs, thus platinum disc electrode was employed.¹¹

Figure 2.

CVs of various PAHs (5 mM, I: anthracene, II: 9,10-dimethylanthracene, III: phenanthrene, IV: pyrene, V: perylene) recorded in 0.1 M Bu₄NB(C₆F₅)₄/CH₂Cl₂ (A, B, blue lines in E), 0.1 M Bu₄NTfO/CH₂Cl₂ (C, D, red lines in E). Inlets in column A show linear correlations of peak current value vs. square root of scan rate, suggesting the outer-sphere electron transfer. Measurements in columns A and C were performed at scan rates of 0.1, 0.2, 0.5, and 1.0 V s⁻¹ (from dark to pale), respectively. Measurements in columns B and D were performed in 3 cycles at a scan rate of 0.1 V s⁻¹.

First, we investigated the redox of anthracene, where fully reversible one-electron redox behavior was observed in Bu₄NB(C₆F₅)₄/CH₂Cl₂, and a peak current value showed a linear relationship with the square root of scan rate, suggesting outer-sphere electron transfer (Fig. 2, panels I-A, B). On the contrary, anthracene showed much less reversibility in Bu₄NTfO/CH₂Cl₂ (panels I-C, D). Also, the presence of a coordinating anion, TfO⁻, further electrochemical oxidation proceeded at 1.5 V vs. Ag/AgNO₃. Thus, it is suggested that coordination of TfO⁻ anion induced following chemical and electrochemical processes, resulting in the poor electrochemical reversibly redox process.

The introduction of substituents in PAHs is supposed to increase oxidative tolerance via the removal of oxidatively weak, and thus reactive, C-H bonds.¹² Since 9- and 10-positions are the most reactive in anthracene motif, voltammetric analyses were performed for 9,10-dimethylanthracene as well. Again, 9,10-dimethylanthracene showed a fully reversible response in Bu₄NB(C₆F₅)₄/CH₂Cl₂, whereas the reversibility in Bu₄NTfO/CH₂Cl₂ became slightly better compared to anthracene, but still poor (panels II-A–E).

The same measurements were then performed on phenanthrene, pyrene and perylene, respectively. Phenanthrene showed irreversible responses in all tests, suggesting the instability of the radical cation state of phenanthrene derives from its own structure rather than electrolyte-coordination (panels III-A–D). Pyrene showed drastic differences between the two electrolytes. Reversible oxidation with small bumps was observed in Bu₄NB(C₆F₅)₄/CH₂Cl₂ (panels IV-A, B), whereas irreversibly voltammetry was recorded in Bu₄NTfO/CH₂Cl₂ (panels IV-C, D). A successive scan showed a drastic decrease in current, suggesting the formation of the passivating film (panel IV-D), which was also confirmed by visual observation of the post-measurement electrode (Fig. 3). Perylene showed reversible redox in both electrolytes (Fig. 2, panels V-A–D), although the use of TfO⁻ seems to be less suitable due to the decrease of current in successive scans (panel V-D). Oxidation potentials of PAHs were essentially independent of the nature of the compositions of the electrolyte (panels I–V-E).

Figure 3.

Photographs of Pt disk electrode (a) before and (b) after the oxidation cycle of pyrene in Bu₄NTfO/CH₂Cl₂.

The bumpy reversible redox behavior of pyrene in Bu₄NB(C₆F₅)₄/CH₂Cl₂ caught our attention, thus further investigation was carried out. Similar redox behavior has been reported for substituted pyrene-derivative.^13,14 Square-wave voltammetry (SWV) measurements were performed in Bu₄NB(C₆F₅)₄/CH₂Cl₂ with 1 mM, 0.5 mM, and 0.1 mM of concentrations of pyrene, respectively (Fig. 4a). SWV data with 1 mM of pyrene suggested the presence of two distinct peaks at 1.1 V and 1.3 V. The second oxidation peak at higher potential became smaller with decreasing the concentration of pyrene. Thus, it is concluded that two distinct oxidation is not the two-electron oxidation per pyrene, but rather indicating bimolecular process, such as π-π interaction of neutral and radical cation state of pyrene (Fig. 4b).

Figure 4.

Square-wave voltammetry (SWV) analysis of the oxidation of pyrene. (a) SWV of pyrene (1, 0.5, and 0.1 mM) recorded in 0.1 M Bu₄NB(C₆F₅)₄/CH₂Cl₂. (b) Schematic illustration of bimolecular π-stacking.

Finally, we performed the CV measurement of anthracene with Bu₄NPF₆. As mentioned above, PF₆⁻ is an anion with a donor number in between TfO⁻ and B(C₆F₅)₄⁻, and Bu₄NPF₆ is widely used in the voltammetry study of organic molecules due to its availability, chemical inertness, and stability under redox conditions. Figure 5 represents the overlayed CV of anthracene recorded in Bu₄NB(C₆F₅)₄/CH₂Cl₂, Bu₄NTfO/CH₂Cl₂, and Bu₄NPF₆/CH₂Cl₂, respectively. The reversibility of anthracene is clearly poor in the presence of TfO⁻ and PF₆⁻. Thus, even with PF₆⁻ anion, the coordination to the radical cation state of anthracene is unavoidable, resulting in the following chemical and electrochemical process.

Figure 5.

CVs of anthracene (5 mM) recorded in 0.1 M Bu₄NB(C₆F₅)₄/CH₂Cl₂ (blue), 0.1 M Bu₄NTfO/CH₂Cl (red), and 0.1 M Bu₄NPF₆/CH₂Cl₂ (black), respectively, at a scan rate of 0.1 V s⁻¹.

4. Conclusion

In conclusion, we performed CV measurements of five simple PAHs in Bu₄NB(C₆F₅)₄/CH₂Cl₂ and Bu₄NTfO/CH₂Cl₂, respectively, and investigated the effect of electrolyte-derived anion coordination on the electrochemical oxidation behavior. Despite the lack of heteroatom in the molecules explored here, the use of TfO⁻ tends to afford irreversible voltammetric response, indicating the presence of coordinating anion destabilizing radical cation species to induce further chemical and electrochemical processes. On the other hand, the use of Bu₄NB(C₆F₅)₄/CH₂Cl₂ resulted in a reversible voltammetric response in most cases, suggesting that this electrolyte is enough weakly donating to investigate the inherent oxidation behavior of PAHs tested herein. It is noteworthy that the voltametric redox behavior of anthracene using Bu₄NPF₆, which is probably the most popular supporting electrolyte in CV studies of PAHs, resulted in poor reversibility compared to that recorded in Bu₄NB(C₆F₅)₄/CH₂Cl₂, suggesting the importance of the rational design of electrolyte for the voltammetric analysis of PAHs.

The effect of the coordination of the anions to the electrochemically generated (radical) cation species has been overlooked and rarely discussed. This work will give insights into the rational design of the electrolyte to correctly understand the molecular redox behavior of PAHs, as well as to maximize their potential as functional molecules for the application related to redox chemistry.

CRediT Authorship Contribution Statement

Shohei Yoshinaga: Data curation (Equal), Formal analysis (Equal), Investigation (Lead), Visualization (Equal), Writing – original draft (Equal), Writing – review & editing (Equal)

Mahito Atobe: Project administration (Equal), Resources (Lead), Supervision (Lead), Validation (Equal), Writing – review & editing (Lead)

Naoki Shida: Data curation (Equal), Formal analysis (Lead), Funding acquisition (Lead), Project administration (Lead), Resources (Equal), Supervision (Lead), Validation (Lead), Visualization (Lead), Writing – original draft (Equal), Writing – review & editing (Lead)

Conflict of Interest

The authors declare no conflict of interest.

Funding

Japan Society for the Promotion of Science: 22H02118

Murata Science Foundation

Footnotes

M. Atobe and N. Shida: ECSJ Active Members

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