Bis(3-methylthio-1-azulenyl)phenylmethyl Cations and Dications Connected by a 1,4-Phenylene Spacer: Synthesis and Their Electrochemical Properties

Abstract

The preparation of bis(3-methylthio-1-azulenyl)phenylmethyl cations and 1,4-phenylenebis[bis(3,6-di-tert-butyl-1-azulenyl)methyl] dications was accomplished by the hydride abstraction of the corresponding hydride derivatives, which were synthesized by the acid-catalyzed condensation of 1-azulenyl methyl sulfide with benzaldehyde and terephthalaldehyde with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. The intramolecular charge transfer among the azulene ring and the methylium moieties of these cations and dications was investigated by UV–Vis spectroscopy and electrochemical analyses. The pK_R⁺ values of the cations were examined for their thermodynamic stability spectrophotometrically. The voltammetry experiments of these cations revealed their reversible reduction waves on their cyclic voltammograms. Moreover, a notable spectral change of cations was observed by spectroelectrochemistry during electrochemical reduction conditions.

1. Introduction

Azulene and its derivatives have attracted interest in terms of their specific optical properties [1,2,3,4,5,6] and pharmacological activity [7,8,9,10]. The azulene system significantly stabilizes cationic as well as anionic states through the contribution of tropylium and cyclopentadienide substructures. Utilizing this characteristic property, the synthesis of redox-active chromophores composed by the azulene derivatives has been employed in our group due to the creation of stabilized electrochromic materials [11,12,13,14,15,16,17,18,19,20,21]. As a part of this research, a high thermodynamic stability and reversible redox behavior under the electrochemical conditions were observed in bis(1-azulenyl)phenylmethyl cations and dications connected by a 1,4-phenylene spacer, namely, 1⁺ and 2²⁺ (Figure 1) [11,12,22]. However, in spectroelectrochemical measurements, 1⁺ and 2²⁺ showed significant decomposition under electrochemical reduction conditions.

Previously, we have described the effective synthetic procedure of several 1-azulenyl sulfides, as well as their unique reactivity and properties [23,24,25,26,27,28]. In these studies, we found that 1-azulenyl sulfides exhibit remarkable redox stability toward the electrochemical reactions. Considering these results, cations and dications incorporating the 1-methylthioazulene moieties should improve electrochemical stability in the electrochromic systems, i.e., having high reversibility in the redox behavior, in addition to a large thermodynamic stability in the ionic states.

We describe herein the preparation of bis(3-methylthio-1-azulenyl)phenylmethyl cations 3a,b⁺·PF₆⁻ and dications 4a,b²⁺·2PF₆⁻ connected by a 1,4-phenylene spacer. The thermodynamic stability of cations 3a,b⁺·PF₆⁻ and dications 4a,b²⁺·2PF₆⁻ were measured spectrophotometrically. Their redox behavior, as examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV), were also discussed. The spectroelectrochemistry of these cations was also examined, which showed notable spectral changes in the visible region in different redox states.

2. Results and Discussion

Synthesis: The strategy of reacting the aldehydes with azulene derivatives under acidic conditions to obtain the condensation product has been reported previously by several researchers, including our group [29,30,31,32,33,34,35]. The synthesis of bis(3-methylthio-1-azulenyl)methylbenzenes 6a,b was accomplished by the condensation of 5a,b [23] with benzaldehyde in acetic acid (AcOH), followed by the usual workup process in 93% and 100% yields, respectively (Figure 2). The higher reaction yields are considered to be due to the reaction control by the methyl sulfide group at the 1-position of the azulene ring, which suppresses the undesired oligomerization reactions occurring at both the 1- and 3-positions [36,37]. Compounds 7a,b were obtained in 87% and 95% yields by the reaction of 5a,b with terephthalaldehyde in a similar manner (Figure 2).

The synthesis of 3a,b⁺·PF₆⁻ and 4a,b²⁺·2PF₆⁻ was established by the reaction of 6a,b, and 7a,b with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), as shown in Figure 3. The reactions of 6a and 6b with DDQ in dichloromethane (CH₂Cl₂), followed by the treatment with 60% HPF₆ solution, produced 3a⁺·PF₆⁻ and 3b⁺·PF₆⁻ in 99% and 91% yield, respectively (Figure 3). Similar to the synthesis of 3a,b⁺·PF₆⁻, the hydride abstraction of 7a,b with a two-fold amount of DDQ gave the dications 4a,b²⁺·2PF₆⁻ in 91% and 99% yields, after the addition of aq. HPF₆ (Figure 3).

Spectroscopic properties: Cations 3a,b⁺·PF₆⁻ and 4a,b²⁺·2PF₆⁻ were fully characterized by spectroscopic data, as appears in the “Materials and Methods” section. High-resolution mass spectra (HRMS) revealed the correct molecular ion peaks. The chemical shifts in ¹H NMR spectra of the azulene moiety for 3a,b⁺·PF₆⁻ and 4a,b²⁺·2PF₆⁻ revealed low-field shifts compared with those of 6a,b and 7a,b, attributing to the electron-withdrawing property of the methylium ion attached.

As expected by their cationic structures, UV–Vis spectra of 3a,b⁺·PF₆⁻ and 4a,b²⁺·2PF₆⁻ displayed the characteristic strong absorption band in the visible region (

Table 1. The longest absorption maxima [nm] and their coefficients (log ε) of 3a,b⁺·PF₆⁻ and 4a,b²⁺·2PF₆⁻ in several solvents, and those of 1⁺·PF₆⁻ and 2²⁺·2PF₆⁻ in acetonitrile (MeCN) as a reference.

		λmax, nm (log ε)
Compound	CH2Cl2	MeCN	Hexane a
3a+·PF6−	755 (4.44)	740 (4.44)	732 (4.42)
3b+·PF6−	740 (4.40)	738 (4.44)	723 (4.38)
4a2+·2PF6−	785 (4.55)	773 (4.55)	− b
4b2+·2PF6−	777 (4.54)	766 (4.62)	766 (4.44)
1+·PF6− [22]	−	681 (4.61)	−
22+·2PF6− [22]	−	703 (4.85)	−

^a 10% CH₂Cl₂ was added to maintain the solubility. ^b The longest absorption maxima in this solvent could not be determined since the absorption band was broadened into the near-infrared region.

). For instance, the UV–Vis spectra of 3a,b⁺·PF₆⁻ and 4a,b²⁺·2PF₆⁻ in MeCN showed an absorption band at λ_max = 740 nm (3a⁺·PF₆⁻), 738 nm (3b⁺·PF₆⁻), 773 nm (4a²⁺·2PF₆⁻), and 766 nm (4b²⁺·2PF₆⁻), which spread into the near-infrared region. The absorption maxima of 4a,b²⁺·2PF₆⁻ showed a modest red shift compared with those of 3a,b⁺·PF₆⁻, implying the expansion of the π-electron system through the 1,4-phenylene spacer. The molar absorption coefficients of 4a,b²⁺·2PF₆⁻ are approximately twice as large as those of 3a,b⁺·PF₆⁻, owing to the two bis(1-azulenyl)methylium units substituted. The strong and broad absorption bands of these cations can be ascribed to intramolecular charge transfer (ICT) across the two azulene rings, as illustrated by the resonance structure (Figure 4).

Solvatochromism is one of the characteristic features of dipolar molecules [38,39,40]. The solvent dependence of the UV–Vis spectra in the visible region of 3a,b⁺·PF₆⁻ and 4a,b²⁺·2PF₆⁻ implied the ICT character of the absorption band. A strong absorption band of 3a⁺·PF₆⁻ at λ_max = 755 nm in CH₂Cl₂ showed a hypsochromic shift to λ_max = 732 nm in the less-polar solvent, i.e., in 10% CH₂Cl₂/hexane-mixed solvent (Figure 5). Similar solvent effects observed in CH₂Cl₂ and 10% CH₂Cl₂/hexane in 3a⁺·PF₆⁻ can also be observed in 3b⁺·PF₆⁻ and 4a,b²⁺·2PF₆⁻.

The pK_R⁺ values of 3a,b⁺·PF₆⁻ and 4a,b²⁺·2PF₆⁻ were measured spectrophotometrically to investigate the thermodynamic stability of the cations (

Table 2. pK_R⁺ Values ^a of 3a,b⁺·PF₆⁻, 4a,b²⁺·2PF₆⁻, 1⁺·PF₆⁻, and 2²⁺·2PF₆⁻.

Sample	pKR+	Sample	pKR+
3a+·PF6−	9.9 ± 0.1	4a2+·2PF6−	9.8 ± 0.1
3b+·PF6−	11.6 ± 0.1	4b2+·2PF6−	11.7 ± 0.1
1+·PF6− [22]	12.4	22+·2PF6− [22]	12.1 ± 0.2

^a The pK_R⁺ values were determined spectrophotometrically in a buffer solution prepared in 50% aqueous MeCN.

). The pK_R⁺ value of 3b⁺·PF₆⁻ (pK_R⁺ = 11.6 ± 0.1) was higher than that of 3a⁺·PF₆⁻ (pK_R⁺ = 9.9 ± 0.1). These outcomes could be ascribed to the stabilization by the hyperconjugation of the tert-butyl group at the seven-membered moiety of the azulene ring [23]. The 4a²⁺·2PF₆⁻ (pK_R⁺ = 9.8 ± 0.1) and 4b²⁺·2PF₆⁻ (pK_R⁺ = 11.7 ± 0.1) with two cation units were neutralized simultaneously. Thus, 4a²⁺·2PF₆⁻ and 4b²⁺·2PF₆⁻ exhibit high stability, as similar to those of 3a⁺·PF₆⁻ and 3b⁺·PF₆⁻. The pK_R⁺ values of 3a⁺·PF₆⁻ and 3b⁺·PF₆⁻ are lower than that of 1⁺·PF₆⁻ (pK_R⁺ = 12.4). These outcomes indicate that the stabilization by the methylthio group is less effective than the tert-butyl group, which is most likely attributed to the ineffective electron-donating ability of the sulfur atom.

The redox behaviors of 3a,b⁺·PF₆⁻ and 4a,b²⁺·2PF₆⁻ were investigated by CV and DPV, in order to clarify their electrochemical properties (

Table 3. Redox Potentials ^a of 3a,b⁺·PF₆⁻, 4a,b²⁺·2PF₆⁻, 1⁺·PF₆⁻, and 2²⁺·2PF₆⁻ in benzonitrile.

Sample	Method	E1red [V]	E2red [V]	E1ox [V]	E2ox [V]
3a+·PF6−	CV	–0.60
	(DPV)	(–0.59)	(–1.71)	(+0.65)	(+0.96)
3b+·PF6−	CV	–0.68
	(DPV)	(–0.66)	(–1.45)	(+0.64)	(+0.97)
4a2+·2PF6−	CV	–0.35
	(DPV)	(–0.33)	(–1.65)	(+0.67)
4b2+·2PF6−	CV	–0.42		+0.65
	(DPV)	(–0.40)	(–1.74)	(+0.63)
1+·PF6− [23] b	CV b	–0.78		+0.88
22+·2PF6− [23] b	CV b	–0.55		+0.87

^a Redox potentials were measured by CV and DPV [V vs. Ag/AgNO₃, 1 mM in benzonitrile containing Et₄NClO₄ (0.1 M), Pt electrode (i.d.: 1.6 mm), scan rate = 100 mV s^–1, and Fc/Fc⁺ = +0.15 V]. In the case of reversible waves, half-wave potentials measured by CV are presented. The peak potentials measured by DPV are shown in parentheses. ^b Measured in MeCN containing Et₄NClO₄ (0.1 M) and Fc/Fc⁺ = +0.07 V.

). Cation 3a⁺·PF₆⁻ revealed a reversible reduction wave at E₁^red = −0.60 V on the CV, indicating the generation of a neutral radical species (Figure 6). The half-wave potential of E₁^red = –0.68 V on the CV was observed in the electrochemical reduction of 3b⁺·PF₆⁻. The E₁^red value of 3b⁺·PF₆⁻ showed a slight cathodic shift compared with that of 3a⁺·PF₆⁻, indicating the stabilization of the carbocation by the tert-butyl groups, as expected from the results of the pK_R⁺ measurements.

Although the pK_R⁺ values are almost equal each other, the E₁^red of 4a²⁺·2PF₆⁻ (E₁^red = –0.35 V) displayed a more anodic shift than that of 3a⁺·PF₆⁻ (E₁^red = –0.60 V). The anodic shift of 4a²⁺·2PF₆⁻ may be ascribed to the electrochemical instability resulting from the electrostatic repulsion between the cations through the benzene ring connected. The reversible reduction waves of 4a,b²⁺·2PF₆⁻ may be attributable to the one-step reduction of the two cation units forming the closed-shell quinoidal structures 8a and 8b, as shown in Figure 7 [41].

Electrochromism is a phenomenon that displays a significant color change in different redox states and is observed in molecules that show reversible redox behavior. Since high redox stability is required for the application to electrochromic materials, durability toward the redox cycle is an important factor to construct such materials [42,43,44]. The concept of violene–cyanine hybrids was proposed by Hünig et al. as a guideline for the creation of electrochromic materials [45,46,47,48,49]. The hybrid consists of a violene substructure involving delocalized, closed-shell polymethine (e.g., cyanine) dyes as terminal groups. Thus, in a redox system with a hybrid structure, the color of the cyanine dye could be observed by overall two-electron transfer (Figure 8). In contrast to the violene-type redox system, hybrid structures should improve the stability of electrochromic systems because both colored and discolored species comprise a closed shell structure.

Therefore, spectroelectrochemical measurements of 3a,b⁺·PF₆⁻ and 4a,b²⁺·2PF₆⁻ were investigated under an electrochemical reaction. In the electrochemical reduction, the longest absorption band of λ_max = 740 nm for 3a⁺·PF₆⁻ gradually decreased, at which time the color of the solution changed from green to yellow. However, despite the good reversibility observed on the CV, the reverse oxidation of the yellow-colored solution did not reproduce the original spectrum of 3a⁺·PF₆⁻ (27% recovery, Figure 9). The less reversible color change might be explained by the generation of unstable neutral radical species under the measurement conditions. The absorption band at λ_max = 738 nm of 3b⁺·PF₆⁻ was also reduced upon the electrochemical reduction, along with the green-colored solution turning to yellow (see Supplementary Materials). Similar to 3a⁺·PF₆⁻-, the visible spectrum of 3b⁺·PF₆⁻ was incompletely recovered when the reduced solution was reversely oxidized (12% recovery).

The color change of 4a,b²⁺·2PF₆⁻ was also observed under the electrochemical reduction. In the absorption spectrum of 4a,b²⁺·2PF₆⁻, an absorption band was developed at around λ_max = 600 nm during the electrolytic reduction, accompanied by the disappearance of the absorption band centered at λ_max = 773 nm. The original absorption spectrum of 4a²⁺·2PF₆⁻ was regenerated by the reverse oxidation of the reducing species (51% recovery) (see Supplementary Materials). This reversible spectral change was also confirmed by the electrochemical reduction of 4b²⁺·2PF₆⁻, with a gradual development of the absorption band at around λ_max = 610 nm, resulting in the green color of the solution turning blue. Reverse oxidation resulted in the disappearance of the newly generated absorption band, together with the regeneration of the original absorption of 4b²⁺·2PF₆⁻ (64% recovery, Figure 10). The higher reversibility of 4a,b²⁺·2PF₆⁻, compared to that of 3a,b⁺·PF₆⁻, could be explained by the generation of closed-shell species, i.e., quinoidal species 8a,b, by electrochemical reduction.

Dications 4a,b²⁺·2PF₆⁻ contain two bis(3-methylthio-1-azulenyl)methylium units as the end groups, which are regarded as cyanine-type substructures to form a delocalized closed-shell system in the violene–cyanine hybrid structure. Thus, the electrochemical reduction of 4a,b²⁺·2PF₆⁻ could generate the other closed-shell systems 8a,b by two-electron transfer. As seen from the electrochemical behavior of 4a,b²⁺·2PF₆⁻, the radical cationic species are not important in the redox systems, so the color change in the absorption spectra is mainly due to these two closed-shell species throughout. Since the dications 4a,b²⁺·2PF₆⁻ displayed distinct spectral changes at the different redox states, the electrochromic behavior of these dications should serve as the violene–cyanine hybrid, in which the four end groups X and Y in Figure 8 are azulene rings to form a quinoidal substructure in their reduced form.

3. Materials and Methods

Melting points were measured with a Yanagimoto MPS3 micro melting apparatus. High-resolution mass spectra were obtained with a Bruker APEX II instrument. IR spectra were measured with a JASCO FT/IR-4100 spectrophotometer (JASCO Corporation, Tokyo, Japan). The UV–Vis spectra were recorded with a Shimadzu UV-2550 spectrophotometer (Shimadzu Corporation, Kyoto, Japan). ¹H and ¹³C NMR spectra were recorded in CDCl₃ with a Bruker Avance 400 at 400 MHz and 100 MHz (Bruker BioSpin, Rheinstetten, Germany), respectively.

Bis(3-methylthio-1-azulenyl)phenylmethane (6a): a mixture of 5a (273 mg, 1.57 mmol) and benzaldehyde (55 mg, 0.52 mmol) in acetic acid (5 mL) was stirred at room temperature for 24 h. The reaction mixture was diluted with CH₂Cl₂. The organic layer was washed with a 5% NaHCO₃ solution and water, dried over MgSO₄, and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel with CH₂Cl₂ to afford 6a (212 mg, 93%) as greenish-blue crystals. M.p. 186.0–188.0 °C (CH₂Cl₂); IR (KBr disk): ν_max = 3021 (w), 2915 (w), 1574 (s), 1493 (m), 1451 (w), 1399 (s), 1381 (m), 1333 (w), 1248 (w), 1129 (w), 1075 (w), 1030 (w), 945 (w), 914 (w), 878 (w), 862 (w), 737 (s), 706 (m), 573 (w), 561 (w) cm⁻¹; UV–Vis (CH₂Cl₂): λ_max (log ε) = 240 (4.60), 296 sh (4.75), 299 (4.76), 366 sh (4.02), 374 (4.06), 619 (2.67) nm; ¹H NMR (400 MHz, CDCl₃): δ = 8.44 (d, 2H, J = 9.6 Hz, H-4), 8.04 (d, 2H, J = 9.6 Hz, H-8), 7.36 (s, 2H, H-2), 7.32 (t, 2H, J = 9.6 Hz, H-6), 7.14–7.02 (m, 5H, Ph), 6.98 (t, 2H, J = 9.6 Hz, H-5), 6.80 (d, 2H, J = 9.6 Hz, H-7), 6.54 (s, 1H, CH), 2.22 ppm (s, 6H, SCH₃); ¹³C NMR (100 MHz, CDCl₃): δ = 145.37, 141.33, 140.10, 138.90, 137.16, 135.93, 134.48, 132.67, 129.27, 128.92, 126.74, 123.45, 123.42, 120.85, 43.14, 20.99 ppm; HRMS (ESI): Calculated for C₂₉H₂₄S₂ + Na⁺ [M + Na]⁺ 459.1212; found: 459.1211.

Bis(6-tert-butyl-3-methylthio-1-azulenyl)phenylmethane (6b): the procedure used for the preparation of 6a was adopted here. Reaction of 5b (1.00 g, 4.34 mmol) and benzaldehyde (185 mg, 1.74 mmol) in acetic acid (15 mL) at room temperature for 24 h, followed by column chromatography on silica gel with CH₂Cl₂ to afford 6b (660 mg, 100%) as a blue solid. M.p. 152.0–155.0 °C (CH₂Cl₂); IR (KBr disk): ν_max = 2965 (m), 2869 (w), 1580 (s), 1493 (w), 1408 (w), 1397 (m), 1364 (w), 1306 (w), 1254 (w), 1067 (w), 963 (w), 839 (w), 716 (w), 698 (w), 677 (w), 581 (w) cm⁻¹; UV–Vis (CH₂Cl₂): λ_max (log ε) = 241 (4.60), 305 (4.91), 360 (4.07), 376 (4.08), 600 (2.87) nm; ¹H NMR (400 MHz, CDCl₃): δ = 8.51 (d, 2H, J = 10.4 Hz, H-8), 8.12 (d, 2H, J = 10.4 Hz, H-4), 7.38 (s, 2H, H-2), 7.34 (dd, 2H, J = 10.4, 1.6 Hz, H-7), 7.17 (dd, 2H, J = 10.4, 1.6 Hz, H-5), 7.3–7.1 (m, 5H, Ph), 6.60 (s, 1H, CH), 2.35 (s, 6H, SCH₃), 1.40 ppm (s, 18H, tBu); ¹³C NMR (100 MHz, CDCl₃): δ = 162.56, 145.55, 140.69, 138.87, 135.89, 134.70, 133.50, 132.34, 129.24, 128.75, 126.48, 121.59, 121.45, 119.87, 42.86, 38.98, 32.22, 21.16 ppm; HRMS (ESI): Calculated for C₃₇H₄₀S₂ + Na⁺ [M + Na]⁺ 571.2464; found: 571.2462.

1,4-Bis[bis(3-methylthio-1-azulenyl)methyl]benzene (7a): the procedure used for the preparation of 6a was adopted here. Reaction of 5a (527 mg, 3.03 mmol) and terephthalaldehyde (67 mg, 0.50 mmol) in acetic acid (15 mL) at 50 °C for 24 h, followed by column chromatography on silica gel with CH₂Cl₂ to afford 7a (347 mg, 87%) as dark green crystals. M.p. 183.5–187.0 °C (CH₂Cl₂); IR (KBr disk): ν_max = 3019 (w), 2919 (w), 1572 (s), 1504 (m), 1489 (w), 1449 (w), 1418 (w), 1395 (m), 1377 (m), 1333 (w), 1312 (w), 1217 (w), 1022 (w), 961 (w), 943 (w), 772 (w), 749 (m), 729 (s), 559 (w) cm⁻¹; UV–Vis (CH₂Cl₂): λ_max (log ε) = 242 (4.89), 291 (5.04), 299 (5.03), 321 sh (4.61), 363 sh (4.31), 374 (4.34), 614 (3.08), 647 sh (3.07) nm; ¹H NMR (400 MHz, CDCl₃): δ = 8.55 (d, 4H, J = 9.6 Hz, H-8), 8.16 (d, 4H, J = 9.6 Hz, H-4), 7.54 (t, 4H, J = 9.6 Hz, H-6), 7.45 (s, 4H, H-2), 7.15 (t, 4H, J = 9.6 Hz, H-7), 7.05 (s, 4H, Ph), 7.00 (t, 4H, J = 9.6 Hz, H-5), 6.62 (s, 2H, CH), 2.36 (s, 12H, SCH₃) ppm; Low solubility hampered the measurement of ¹³C NMR. HRMS (ESI): Calculated for C₅₂H₄₂S₄ + Na⁺ [M + Na]⁺ 817.2062; found: 817.2056.

1,4-Bis[bis(6-tert-butyl-3-methylthio-1-azulenyl)methyl]benzene (7b): the procedure used for the preparation of 6a was adopted here. Reaction of 5b (1.98 g, 8.59 mmol) and terephthalaldehyde (196 mg, 1.46 mmol) in acetic acid (20 mL) and CH₂Cl₂ (2 mL) at room temperature for 24 h, followed by column chromatography on silica gel with CH₂Cl₂ to afford 7b (1.42 g, 95%) as green crystals. M.p. > 300 °C (CH₂Cl₂); IR (KBr disk): ν_max = 2963 (m), 2917 (w), 2869 (w), 1578 (s), 1549 (w), 1505 (w), 1408 (w), 1362 (w), 1337 (w), 1308 (w), 1252 (w), 1067 (w), 1021 (w), 965 (w), 835 (m), 822 (w), 675 (w), 588 (w), 450 (w) cm⁻¹; UV–Vis (CH₂Cl₂): λ_max (log ε) = 241 (4.89), 296 sh (5.15), 305 (5.18), 360 (4.36), 376 (4.36), 601 (3.15) nm; ¹H NMR (400 MHz, CDCl₃): δ = 8.48 (d, 4H, J = 10.0 Hz, H-4), 8.13 (d, 4H, J = 10.0 Hz, H-8), 7.39 (s, 4H, H-2), 7.33 (dd, 4H, J = 10.0, 1.2 Hz, H-5), 7.17 (dd, 4H, J = 10.0, 1.2 Hz, H-7), 7.06 (s, 4H, Ph), 6.58 (s, 2H, CH), 2.35 (s, 12H, SCH₃), 1.39 (s, 36H, tBu) ppm; ¹³C NMR (100 MHz, CDCl₃): δ = 145.37, 141.33, 140.10, 138.90, 137.16, 135.93, 134.48, 132.67, 129.27, 128.92, 126.74, 123.45, 123.42, 120.85, 43.14, 20.99, 15.78 ppm; HRMS (ESI): Calculated for C₆₈H₇₄S₄ + Na⁺ [M + Na]⁺ 1041.4566; found: 1041.4560.

General Procedure for the Preparation of Hexafluorophosphates 3a,b⁺∙PF₆⁻ and 4a,b²⁺∙2PF₆⁻: DDQ was added to a solution of 6a, 6b, 7a, and 7b in CH₂Cl₂ and the solution was stirred at room temperature for 30 min. 60% HPF₆ solution was added to the mixture and stirred for an additional 15 min. Water was added to the mixture and the resulting suspension was collected by filtration. The filtrate was also extracted with CH₂Cl₂, washed with water, dried with MgSO₄, and concentrated under reduced pressure. The residue was crystallized from CH₂Cl₂ and Et₂O. The precipitated crystals were collected by filtration and washed with Et₂O to give the corresponding cations 3a⁺, 3b⁺, 4b²⁺, and 4b²⁺ as hexafluorophosphates.

Bis(3-methylthio-1-azulenyl)phenylmethylium Hexafluorophosphate (3a⁺∙PF₆⁻): the general procedure was followed using DDQ (65 mg, 0.29 mmol), 6a (104 mg, 0.24 mmol), and 60% HPF₆ (5 mL) in CH₂Cl₂ (25 mL). Recrystallization from CH₂Cl₂/ether gave 3a⁺∙PF₆⁻ (131 mg, 99%) as dark green crystals. M.p. > 300 °C (decomp.); IR (KBr disk): ν_max = 2923 (w), 1592 (w), 1570 (w), 1538 (w), 1470 (s), 1437 (m), 1408 (s), 1325 (s), 1310 (s), 1277 (s), 1227 (m), 1200 (w), 1090 (w), 999 (w), 968 (w), 922 (w), 878 (m), 839 (s), 739 (w), 693 (w), 596 (w), 558 (m), 486 (w), 453 (w), 434 (w) cm⁻¹; UV–Vis (CH₃CN): λ_max (log ε) = 235 (4.68), 254 sh (4.62), 295 (4.60), 370 (4.37), 422 (4.17), 518 sh (3.78), 740 (4.44) nm; UV–Vis (CH₂Cl₂): λ_max (log ε) = 237 (4.60), 256 sh (4.58), 300 (4.54), 376 (4.36), 428 (4.18), 521 sh (3.75), 755 (4.44) nm; UV–Vis (Hexane): λ_max (log ε) = 266 sh (4.54), 300 (4.49), 367 (4.35), 427 (4.21), 671 sh (4.30), 733 (4.42) nm; ¹H NMR (400 MHz, CD₃CN): δ = 8.80 (dd, 2H, J = 10.0, 1.2 Hz, H-4), 8.16 (t, 2H, J = 10.0 Hz, H-6), 8.05 (ddd, 2H, J = 10.0, 0.8, 0.8 Hz, H-5), 7.93 (dd, 2H, J = 10.0, 0.8 Hz, H-8), 7.84 (s, 2H, H-2), 7.83 (tt, 1H, J = 8.0, 1.2 Hz, p-Ph), 7.64 (ddd, 2H, J = 8.0, 1.2, 1.2 Hz, m-Ph), 7.57 (ddd, 2H, J = 10.0, 0.8, 0.8 Hz, H-7), 7.50 (dd, 2H, J = 8.0, 1.2 Hz, o-Ph), 2.61 ppm (s, 6H, SCH₃) ppm; ¹³C NMR (100 MHz, CD₃CN): δ = 160.30, 151.35, 150.45, 145.75, 143.72, 143.13, 141.49, 140.26, 136.56, 136.46, 136.44, 136.38, 134.71, 134.53, 130.52, 17.69 ppm; HRMS (ESI): Calculated for C₂₉H₂₃S₂⁺ [M − PF₆]⁺ 435.1241; found: 435.1241.

Bis(6-tert-butyl-3-methylthio-1-azulenyl)phenylmethylium Hexafluorophosphate (3b⁺∙PF₆⁻): the general procedure was followed by using DDQ (272 mg, 1.20 mmol), 6b (549 mg, 1.00 mmol), and 60% HPF₆ (10 mL) in CH₂Cl₂ (50 mL). Recrystallization from CH₂Cl₂/ether gave 3b⁺∙PF₆⁻ (630 mg, 91%) as dark green crystals. M.p. 168.0–175.0 °C (decomp.); IR (KBr disk): ν_max = 2963 (w), 2872 (w), 1572 (w), 1497 (w), 1470 (s), 1441 (m), 1416 (s), 1370 (w), 1347 (m), 1323 (s), 1294 (s), 1244 (s), 1192 (m), 1111 (m), 1075 (m), 870 (w), 839 (s), 733 (w), 704 (w), 558 (m) cm⁻¹; UV–Vis (CH₃CN): λ_max (log ε) = 258 (4.57), 304 (4.55), 370 (4.31), 426 (4.13), 515 sh (3.74), 740 (4.40) nm; UV–Vis (CH₂Cl₂): λ_max (log ε) = 261 (4.55), 306 (4.54), 376 (4.29), 429 (4.11), 515 sh (3.73), 738 (4.40) nm; UV-Vis (Hexane): λ_max (log ε) = 262 (4.55), 306 (4.52), 362 (4.29), 424 (4.02), 672 sh (4.30), 723 (4.38) nm; ¹H NMR (400 MHz, CD₃CN): δ = 8.69 (d, 2H, J = 10.4 Hz, H-4), 8.23 (dd, 2H, J = 10.4, 0.8 Hz, H-5), 7.80 (t, 1H, J = 7.6 Hz, p-Ph), 7.78 (d, 2H, J = 10.8 Hz, H-8), 7.71 (dd, 2H, J = 10.4, 0.8 Hz, H-7), 7.70 (s, 2H, H-2), 7.63 (t, 2H, J = 7.6 Hz, m-Ph), 7.50 (d, 2H, J = 7.6 Hz, o-Ph), 2.58 (s, 6H, SCH₃), 1.40 (s, 18H, 6-tBu) ppm; ¹³C NMR (100 MHz, CD₃CN): δ = 171.37, 159.23, 150.07, 148.90, 143.10, 142.78, 140.46, 138.96, 136.23, 135.64, 134.95, 134.46, 134.34, 134.18, 130.42, 40.78, 32.08, 17.86 ppm; HRMS (ESI): Calculated for C₃₇H₃₉S₂⁺ [M − PF₆]⁺ 547.2488; found: 547.2488.

1,4-Phenylenebis[bis(3-methylthio-1-azulenyl)methylium] Bis(hexafluorophosphate) (4a²⁺∙2PF₆⁻): the general procedure was followed by using DDQ (36 mg, 0.16 mmol), 7a (52 mg, 0.065 mmol), and 60% HPF₆ (4 mL) in CH₂Cl₂ (30 mL). Recrystallization from CH₂Cl₂/ether gave 4a²⁺∙2PF₆⁻ (65 mg, 91%) as dark green crystals. M.p. > 300 °C (decomp.); IR (KBr disk): ν_max = 2960 (w), 1472 (m), 1437 (w), 1408 (m), 1325 (s), 1308 (s), 1277 (s), 1229 (m), 1086 (w), 968 (w), 926 (w), 878 (w), 837 (s), 760 (w), 739 (w), 691 (w), 558 (m), 509 (w), 455 (w) cm⁻¹; UV-Vis (CH₃CN): λ_max (log ε) = 236 (4.82), 255 (4.82), 298 (4.76), 376 (4.46), 423 (4.49), 607 (4.29), 773 (4.55) nm; UV–Vis (CH₂Cl₂): λ_max (log ε) = 260 (4.83), 302 (4.77), 382 (4.51), 428 (4.50), 612 sh (4.31), 785 (4.55) nm; UV–Vis (Hexane): λ_max (log ε) = 259 (4.60), 306 (4.52), 384 (4.39), 442 (4.32), 634 sh (4.19) nm; ¹H NMR (400 MHz, CD₃CN): δ = 8.80 (d, 4H, J = 9.6 Hz, H-4), 8.22 (t, 4H, J = 9.6 Hz, H-6), 8.11 (d, 4H, J = 9.6 Hz, H-8), 8.09 (t, 4H, J = 9.6 Hz, H-5), 7.99 (s, 4H, H-2), 7.70 (s, 4H, Ph), 7.66 (t, 4H, J = 9.6 Hz, H-7), 2.67 (s, 12H, SCH₃) ppm; ¹³C NMR (100 MHz, CD₃CN): δ = 156.92, 151.52, 150.56, 146.92, 146.03, 143.09, 141.73, 140.37, 137.33, 137.02, 136.92, 136.72, 134.98, 17.58 ppm; HRMS (ESI): Calculated for C₅₂H₄₀S₄²⁺ [M − 2PF₆]²⁺ 396.1001; found: 396.1001.

1,4-Phenylenebis[bis(6-tert-butyl-3-methylthio-1-azulenyl)methylium] Bis(hexafluorophosphate) (4b²⁺∙2PF₆⁻): the general procedure was followed by using DDQ (60 mg, 0.26 mmol), 7b (109 mg, 0.11 mmol), and 60% HPF₆ (5 mL) in CH₂Cl₂ (25 mL). Recrystallization from CH₂Cl₂/ether gave 4b²⁺∙2PF₆⁻ (128 mg, 99%) as dark green crystals. M.p. 252.0–257.0 °C (CH₂Cl₂/ether); IR (KBr disk): ν_max = 2965 (w), 2872 (w), 1574 (m), 1497 (m), 1472 (s), 1443 (s), 1416 (s), 1370 (m), 1320 (s), 1296 (s), 1246 (s), 1194 (m), 1111 (m), 1076 (m), 1044 (w), 972 (w), 924 (w), 839 (s), 737 (w), 700 (w), 558 (m), 507 (w), 469 (w) cm⁻¹; UV-Vis (CH₃CN): λ_max (log ε) = 236 (4.85), 264 (4.88), 303 (4.89), 386 sh (4.55), 416 (4.57), 604 (4.33), 766 (4.62) nm; UV–Vis (CH₂Cl₂): λ_max (log ε) = 266 (4.79), 307 (4.80), 385 (4.50), 424 (4.50), 612 sh (4.31), 777 (4.54) nm; UV–Vis (Hexane): λ_max (log ε) = 265 (4.65), 306 (4.67), 424 (4.41), 766 (4.44) nm; ¹H NMR (400 MHz, CD₃CN): δ = 8.73 (d, 4H, J = 10.8 Hz, H-4), 8.28 (dd, 4H, J = 10.8, 2.0 Hz, H-5), 7.92 (s, 4H, H-2), 7.88 (d, 4H, J = 10.8 Hz, H-8), 7.76 (dd, 4H, J = 10.8, 2.0 Hz, H-7), 7.75 (s, 4H, Ph), 2.66 (s, 12H, SCH₃), 1.42 (s, 36H, t-Bu) ppm; ¹³C NMR (100 MHz, CD₃CN): δ = 171.67, 155.99, 150.23, 148.98, 146.87, 142.16, 140.56, 139.08, 136.48, 136.36, 135.31, 134.95, 134.60, 40.88, 32.09, 17.82; HRMS (ESI): Calculated for C₆₃H₇₂S₄²⁺ [M − 2PF₆]²⁺ 508.2253; found: 508.2253.

4. Conclusions

In summary, we have prepared cations 3a,b⁺·PF₆⁻ and dications 4a,b²⁺·2PF₆⁻ and clarified their properties. Although the pK_R⁺ values of 3a,b⁺·PF₆⁻ and 4a,b²⁺·2PF₆⁻ indicated a lower thermodynamic stability compared with 1⁺·PF₆⁻ and dications 2²⁺·2PF₆⁻, these cations still exhibited high pK_R⁺ values, indicating high thermodynamic stability. These observations indicate the less effective stabilization of the methylthio moiety compared with that of the tert-butyl groups at the same position. The CV experiments showed that 3a,b⁺·PF₆⁻ and 4a,b²⁺·2PF₆⁻ exhibited a reversible reduction wave. Furthermore, a noticeable color change was observed during the electrochemical reduction of 3a,b⁺·PF₆⁻ and 4a,b²⁺·2PF₆⁻. In particular, 4a,b²⁺·2PF₆⁻ displayed a remarkable color change resulting from the formation of the quinoidal structures 8a,b. These facts indicate that 4a,b²⁺·2PF₆⁻ served as the violene–cyanine hybrid in terms of a one-step two-electron reduction to generate a quinoidal form.