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Original article
13 (
1
); 2195-2206
doi:
10.1016/j.arabjc.2018.04.005

Acetocatechol functionalized viologen as polyfunctional material that responds to anion, cation and reductant in aqueous and organic solvents

, , , , ,
 Department of Chemistry, Shanghai University, Shanghai 200444, China

⁎Corresponding authors. xff@shu.edu.cn (Feifei Xing), shourongzhu@shu.edu.cn (Shourong Zhu)

Received: , Accepted: ,
© The Author(s) 2018
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Acetocatechol viologen can response to anion via aceto, metal ion via catechol hydroxyl and reductant via viologen. Its Fe(III) complex decay first- and second-order at RT and 60 °C respectively.

Abstract

Both viologen and catechol have been studied extensively. However, the stability of catechol-Fe(III) without additional oxidant is still not well understood. In this paper, we introduced acetocatechol into viologen to investigate its interactions with anion, cation, and reductant, as well as the stability of its Fe(III) complexes. This acetocatechol functionalized viologen, 1,1′-bis(2-(3,4-dihydroxyphenyl)-2-oxoethyl)- [4,4′-bipyridine]-1,1′-diium chloride (H6V·Cl2) exists in central symmetric ketone cation form in solid state. Viologen cation increased the acidity of the aceto group and deprotonated the enolic proton to form monodeprotonated enolic H5V+ in the presence of anion/base, which had the deepest color in organic solvents. The absorbance maximum of H5V+ increased with the decrease of solvent polarity. It also interacted with B4O72− and MoO42− by forming catechol ester in DMSO solution. The catechol moiety can coordinate to metal ion, especially Fe(III), in both aqueous and DMSO solution. In particular, it coordinated to Fe(III) much more readily in aqueous solution than in DMSO. Green monocatecholato Fe(III) and red-brown bis-catecholato Fe(III) complex also formed in aqueous solution. The monocatecholato Fe(III) complex first-order dimerized in aqueous solution at room temperature but underwent second-order decomposition to Fe(II) complex at 60 °C. The biscatecholato Fe(III) complex also transferred to other Fe(III) complexes at first- and second-order at room-temperature and 60 °C respectively. The t1/2 varied from several hours at room-temperature and several minutes at 60 °C at 10−4 M concentration. The interactions of Fe(III) in DMSO is much more complex than that of acetocatecholate without viologen. Fe(III) can also be reduced to free viologen radicals in the presence of sodium Na2S2O4, but not N2H4. In conclusion, this polyfunctional compound responds to anion via aceto and catechol, metal ion via catechol hydroxyl, while reductant via viologen.

Keywords

Viologen
Catechol
Anion sensor
Fe(III) complex
Stability
1

1 Introduction

1,1′-Disubstituted 4,4′-bipyridinium (referred to as viologen in the rest of this paper) exhibits several reversible two step redox processes: dication (V2+), radical cation (V+•), and neutral (V0) (Adhikari et al., 2017; Palenzuela et al., 2014). The viologen radical cation is colored due to the charge transfer between two oxidation states including pink, purple, and blue (Shi et al., 2015). Thanks to this property it has applications in fields including thermochromics, solvatochromics, and halochromics (Geraskina et al., 2014; Kan et al., 2017), electrochromics and photochromics (Alesanco et al., 2016; Hu et al., 2017; Kan et al., 2017; Sui et al., 2017, Sun et al., 2012), piezochromics (Sui et al., 2017), solar energy storage devices (Amao et al., 2012), fluorescencents (Pramanik et al., 2017; Wang and Tsarevsky, 2016), supramolecular host–guest complexes (Dale et al., 2016; Deligkiozi et al., 2015), and ferroelectrics (Leblanc et al., 2011). We have shown that the chromic effect is much more sensitive upon introduction of acetyl groups (Shi et al., 2015). Some viologen can sense grinding and vapor in solid state. The halochromic effect is much more pronounced in organic solvents than in H2O (Kan et al., 2017). Viologen may respond to anions. For example, the prescence of amines triggers color changes that can be easily monitored by the naked eye (Shi et al., 2015; Swinburne et al., 2010). Due to its electronic deficient nature (Lewis acid), viologen generally has no interaction with metal ions. This can be overcome upon introduction of carboxylic acid, where viologen derivatives can interact with metal ion. Gong et al. (2017) have synthesized several metal-organic frameworks as Li+ storage materials by introducing carboxylate groups to form zwitterionic molecule ligands. Zhang et al. have also reported viologen and cadmium metal-organic framework complexes (Sun et al., 2012). These new complexes show different colors due to different degrees of charge-transfer in complexes. Viologens can be reduced by strong electron-donor hyposulfites to produce a deep colored viologen radical (Shi et al., 2015). Trabolsi et al. have synthesized dendritic viologen compounds as multifunctional redox-tuned materials (Das et al., 2016). Viologen polymer powder turn from yellow to dark blue when treated with Na2S2O4.

Catechol compounds are common and play an important role in nature. It has strong redox, pH response, and significant chelation (Lee et al., 2016), adhesion, biocompatibility, and biodegradability (Bettinger et al., 2009; Guvendiren, Brass, Messersmith, & Shull, 2009; Ku et al., 2010). Catechol compounds have applications in fields including biomedical (Im et al., 2018; Yavvari et al., 2017), sensor, and water treatment (Gao et al., 2013; Song et al., 2013) similar to those of nanoparticles (Huang et al., 2018). The diversity of catechol properties has attracted widespread researcher interest. There has been growing evidence for the critical roles of metal-coordination complexes in defining structural and mechanical properties of unmineralized biological materials, including hardness, toughness, and abrasion resistance. Their dynamic (e.g. pH-responsive, self-healable, reversible) properties inspire promising applications of synthetic materials following this concept. However, the mechanics of these coordination crosslinks that lay the ground for predictive and rational material design have not yet been well addressed (Xu, 2013).

Although there are lots of papers concerning viologen and catechol, viologen response to metal ion and anion (except OH−) remain unexplored. Although it is well known that catechol can form a very stable Fe(III) complex, the complex stability without additional oxidant is still not well understood. We are interested in catechol containing complexes (Gao et al., 2014) and the chromic properties of viologen compounds. We introduced electron-rich catechol moiety to the electron-poor viologen 1,1′-bis(2-(3,4-dihydroxyphenyl)-2-oxoethyl)-[4,4′-bipyridine]-1,1′-diium chloride (abbreviated as H6V·Cl2) to investigate its interactions with OH−, F−, CH3COO−, B4O72−, MoO4−, as well as Fe3+, Cu2+ and Zn2+, and reduction with S2O42− in aqueous solution and N2H4 in DMSO. The stability of mono- and bis-catechol to Fe(III) complex in air without additional oxidant were also explored.

2

2 Experimental section

2.1

2.1 Materials and instruments

All chemicals were of reagent grade quality and used as received without further purification. Catecholate viologen H6V·Cl2 was synthesized by 4,4′-bipyridine and 4-(chloroacetyl)- 1,2-dihydroxybenzene as previously reported (Shi et al., 2015).

The IR spectra were recorded in the 4000–400 cm−1 region using KBr pellets and a Nicolet AVATAR-370 spectrometer. 1H NMR and H-H COSY spectra were measured on a Bruker AV 500 MHz spectrometer. UV–vis absorption spectra were performed on a Puxi TU-1950 with a 1.0 cm quartz cell equipped with a DC-2006 temperature-controlled water bath. Electron paramagnetic resonance (EPR) spectra were recorded by a JES-FA 200 spectrometer fitted with a DICE ENDOR accessory, EN801 resonator, and an ENI A-500 rf power amplifier.

2.2

2.2 Ultraviolet–visible absorption spectroscopy

The cuvettes were rinsed with 1:1 H2O–HCl, double-distilled water, and ethanol, then dried in an oven at 50 °C. Pure water or other organic solvents were used as a reference in all measurements. 150–1500 µL of 1.0 × 10−3 M H6V·Cl2 or 2 -chloro-3′, 4′-dihydroxyacetophenone (CDA) were injected into a 1.0 cm cell with pipette. Appropriate anion/cation/reductant (in 0.075–0.15 M) were mixed and solvents were added until total volume reached 3.0 mL. Spectra was collected immediately after mixing. The working concentration of ligand and metal ion varied from 5.0 × 10−5 to 5.0 × 10−4M. For pH dependent spectra, the solution was poured into the 5 mL beaker after scanning and then adjusted to another pH to scan the spectrum repeatedly. The UV–vis spectra in the presence of Na2B4O7·10H2O were all run at 80 °C for both aqueous and organic solvents to prevent precipitation at room temperature.

Spectra in the presence of reductant were investigated in aqueous and DMSO solution respectively. Stocking solution was 1.0 × 10−4 M in 0.10 M pH 8.0 HEPES buffer. Working concentration was 5 × 10−5 M H6V·Cl2 and 50 mM HEPES buffer. 3 mL of H6V·Cl2 (5 × 10−5 M) were added 1.4 M Na2S2O4 consecutively (1µL, 10 eq each). In DMSO solution, 3 mL of H6V·Cl2 (5 × 10−5 M) were added 0.3 M Na2S2O4 aqueous solution consecutively (0.5 µL, 1 eq. each).

Stability of Fe(III) complex were monitored at room temperature (25 °C) and 60 °C respectively. Time dependent spectra of 2.5–5×10−4 M H6V·Cl2 in the presence of 2 eq. Fe(NO3)3 (0.5 M, 6µL) were collected in both the absence and presence of 5 eq NaOH. The absorbances were monitored at their absorbance maximum for 24 h.

2.3

2.3 Crystallization of (H6V·Cl2)0.5H2O

H6V·Cl2 (13.23 mg, 0.025 mmol) and CdCl2·2.5H2O (11.42 mg, 0.05 mmol) were mixed with 4 mL H2O and 3 mL ethanol, stirring for 10 min to dissolve. The solution was filtered and the filtrate was allowed to stand at room temperature for a week. Red-orange crystals of (H6V·Cl2)0.5H2O settled at 47% yield. Elemental analysis: Calcd. (%) C, 58.990; H, 4.189; N, 5.292; found (%): 59.17; H,4.246; N, 5.073.

2.4

2.4 X-ray crystallography

Well-shaped single crystals were selected for X-ray diffraction studies. Data was collected on a Bruker SMART CCD diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71703 Å) using the φ and ω scan method at 293(2) K. The structures were solved by direct methods with the SHELXS-97 program and refined by full-matrix least-squares on F2 with the SHELXL-97 program (Sheldrick, 1997). All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were located and included at their calculated positions. The crystallographic data and structure refinement results are listed in Tables S1–S5 in the SI.

3

3 Result and discussion

3.1

3.1 Crystal structure of (H6V·Cl2)0.5H2O

This ligand crystallized in the orthorhombic Pbca group in central-symmetric arrangement. The two pyridyl groups were coplanar (Fig. 1). However, the carbonyl was not coplanar to neither pyridyl nor the benzene ring. In solid state, the double-bond characteristic is obvious as the carbonyl C7—O1 was 1.214(3) Å and clearly shorter than that of C7—C8 of 1.523(3) Å. There was no sign of any enolic structure in solid state. Ligand interaction with adjacent ligand via catechol-catechol π-π interaction was evidenced by the parallel and short intermolecular C1⋯C2 distance of 3.427(3) Å. Due to positive charge, no π-π interaction of the pyridyl structure were found. Cl- also formed H-bond with a hydroxyl proton and a water proton (Fig. 1). There was also a weak Cl⋯H—C9 H-bond.

Crystal structure of the catecholate viologen (H6V·Cl2)0.5H2O.
Fig. 1
Crystal structure of the catecholate viologen (H6V·Cl2)0.5H2O.

3.2

3.2 Interaction with anion

3.2.1

3.2.1 Aqueous solution

The interaction of the acetyl catechol with base in aqueous solution has been studied in our previous paper (Gao, Xing, Bai, & Zhu, 2014). 2-chloro-3′,4′-dihydroxy-acetophenone (CDA) without viologen moiety, which was the raw material in the synthesis of the catecholate viologen, was used as a reference. Changes of the UV spectrum of CDA and H6V·Cl2 were quite similar in the presence of NaOH in aqueous solution (Figs. S1 and S2) (Gao et al., 2014; Shi et al., 2015). Fig. S2 shows UV spectra of 5.0 × 10−5 M H6V·Cl2 at different pHs. It can be seen from the figure that there were two absorption peaks at 234 and 285 nm (pH = 4.52) in the absence of NaOH. This pH was obviously lower than that of CDA at pH 4.78. This indicates that Ka1 = 4.61 × 10−5 and 8.26 × 10−6 for H6V·Cl2 and CDA respectively, which shows that the viologen had a stronger electron-withdrawing effect than the Cl atom. This Ka was significantly larger than that of catechol (<10−9). Their absorbance coefficients were 28,000 (234 nm) and 42,000 (285 nm) L·cm−1·mol−1 respectively for H6V·Cl2. The absorption peak at 234 nm and 285 nm weakened and new absorption peaks appeared at 253 nm and 355 nm. These two peak intensities increased gradually with the increase of pH from 4.5–8.4 range (Fig. S2 top). Absorbance coefficients at 253 nm, 355 nm were 26,600, 40,000 L·cm−1·mol−1at pH = 8.4 respectively. These wavelengths all shifted by 5 nm to a longer wavelength compared to the catecholate iminodiacetic acid analog (Gao et al., 2014) and CDA (Fig. S1). The absorption coefficient were roughly double to that of catecholate iminodiacetic acid (Gao et al., 2014). According to previous studies, the absorption peaks at 234, 285 nm were assigned fully protonated acetocatechol H6V2+ while 253 and 355 nm absorption mono-deprotonated catecholate (Shi et al., 2015). Considering the stronger Ka, red-shifted wavelength, and larger absorption coefficient, we attributed the two deprotonations in acidic solution (pH < 6.5) to the methylene group (Fig. S2). The next two deprotonation processes in pH 6.7–7.2 (2–4 eq NaOH, Fig. S2, bottom) were attributed to the catechol moiety. These pKas were also significantly lower than that of catechol at 9.2 and 13.0 (Avdeef et al., 1978), but similar to those for nitrocatechol at 6.7 and 10.8 (Gelb et al., 1989) and acetocatechol analog at 7.08 and 10.65 (Gao et al., 2014). All data indicate that the first two pKa of H6V·Cl2 belong to the CH2 deprotonation while the second pKa in 6.7–7.2 is the pKa of the first hydroxyl of the acetocatechol (H4V to H2V2−). The 355 nm absorbance reached its maximum at pH 8.4. Further increase in pH will decrease absorbances due to deprotonation of the second catechol hydroxyl (V4−).

1.0 × 10−4 M H6V·Cl2 in the presence of 50 eq. base showed a purple blue color with maximum absorbance at 566 nm in aqueous solution (Shi et al., 2015). However, Fig. S3 shows a 548 nm peak upon the addition of Na2B4O7·10H2O at 80 °C. This wavelength clearly differed from that produced from adding NaOH at 566 nm (Shi et al., 2015). According to literature, the 548 nm species is assigned as boronate–catechol complexation (Gennari et al., 2017; He et al., 2011; Yan et al., 2004). To further prove boronate–catechol complexation, d-glucose, a catechol competitive reagent (Zou et al., 2016) was added to the red solution. As shown in Fig. S3 (bottom), the presence of d-glucose decreased the 548 nm absorbance. The 548 nm absorbance still existed even in the presence of 50 eq d-glucose, which indicates that the boronate-glucose complexation is less stable than that of boronate-catechol. This is in good agreement with literature reported (Springsteen and Wang, 2002). It is the strong acidity of the acetocatechol that aids chelation to borax. This catecholate viologen had no interaction with Na2MoO4·2H2O (Fig. S4). Less than 20 eq. F− and CH3COO− had no interaction with H6V·Cl2 in aqueous solution.

3.2.2

3.2.2 Organic solution

5.0 × 10−5 M H6V·Cl2 was essentially colorless in aqueous solution even in the presence of NaOH. However, H6V·Cl2 was pale blue in ethanol (Fig. 2a). Spectral changes in the presence of different eq. NaOH in ethanol is illustrated in Fig. 2a. This catecholate viologen showed the deepest indigo color with absorbance maximum at 582 nm in the presence of 1 eq NaOH in ethanol. The 582 nm absorbance decreased with further increases in NaOH (>1 eq). The absorbance maximum shifted to 563 nm and the peak became broader with further increases in NaOH concentration. The color became purple at NaOH > 2 eq. Further addition of NaOH (>4 eq) did not change the 563 nm absorbance (Fig. 2a inset). Considering Ka1 = 4.61 × 10−5, we attributed absorption at 582 nm to the monodeprotonated H5V+. Clearly, there was an equilibrium between H6V2+ and H5V+ as there were isosbestic points at 340 and 632 nm. The H5V+ had ε582 = 3.1 × 104 L·cm−1·mol−1 in ethanol. However, the H4V, H3V−, and H2V2− have quite similar spectra shape as the 582 nm absorbance of H5V+ slightly shifted to 563 nm upon further adding base. We therefore assign the 563 nm absorbances to the catechol mono-deprotonated H2V2−. The reason for the assignment was that the 359 nm absorbance also reached its maximum, indicating the monodeprotonated catechol species (Gao et al., 2014). The spectra in ethanol were somewhat similar to that in methanol (Fig. S5). The main stable species is H3V− in methanol, with absorbance at 566 (1.4 × 104 L·cm−1·mol−1) and 356 nm (2.3 × 104 L·cm−1·mol−1) evidenced by 3 eq NaOH and the 356 nm catecholate peak. The wavelength is identical to those in H2O except for a much higher absorbance at 566 nm in methanol (Table 1).

UV–vis spectral changes of H6V·Cl2 (5.0 × 10−5 M) upon gradual addition of different equivalents of NaOH in ethanol (a) and DMSO (b). Inset is the photograph of solution color. For comparison, the absorbance with NaOH eq are also plotted. (c) 588 nm absorbance faded over time in the presence of 5.0 eq. of NaOH. (d) 1H NMR of H6V·Cl2 (1.0 × 10−3 M) in the absence or presence of different amounts of NaOD in D6-DMSO.
Fig. 2
UV–vis spectral changes of H6V·Cl2 (5.0 × 10−5 M) upon gradual addition of different equivalents of NaOH in ethanol (a) and DMSO (b). Inset is the photograph of solution color. For comparison, the absorbance with NaOH eq are also plotted. (c) 588 nm absorbance faded over time in the presence of 5.0 eq. of NaOH. (d) 1H NMR of H6V·Cl2 (1.0 × 10−3 M) in the absence or presence of different amounts of NaOD in D6-DMSO.
Table 1 Maximum absorption wavelength λmax (nm) and extinction coefficients ε×10−3 (L·cm−1·mol−1) for H6V·Cl2 and in the presence of different anions in different solvents.
Anion (solvent) Catechol Catechol-Ha Viologen enolic Other
NaOH 285(42) 355(40) 566(4)b
(H2O) ∼315(shoulder)
Na2B4O7·10H2O NAc NA 548(1) ester
(H2O)
NaOH 285(35) 356(23) 566(14)
(methanol) 315(28)
NaOH 287(34) 359(21) 582(31)
(ethanol) 318(28)
NaOH 287(40) 372(32) 589(54)
(DMF) 316(34)
NaOH 288(35) 367(25) 595(16)
(CH3CN) 317(32)
NaOH 288(40) 367(30) 588(59)
(DMSO) 319(34)
Na2B4O7·10H2O 286(38) 372(20) 588 (44) 518(23) ester
(DMSO) 317(34)
Na2MoO4·2H2O 286 (40) 383 (28) 588 (39)
(DMSO) 318 (35)
NaF 287 (40) 372 (30) 588 (52)
(DMSO) 317 (35)
NaAc 288 (40) 375 (33) 588 (60)
(DMSO) 317 (35)
a Catechol-H is the mono-deprotonated catechol.
c Too high concentration to evaluate absorbance coefficient.

DMSO is less polar than ethanol, methanol, and H2O. It was royal blue in DMSO peaking at 588 nm (red shifted compared with polar solvents) in the presence of 1 eq NaOH. The H6V2+ to H5V+ equilibrium was evidenced by the 347 nm isosbestic point (blue line, Fig. 2b inset). We assign the 588 nm (5.9 × 104 L·cm−1·mol−1) to H5V+, which not only red-shifted, but also had a very large absorption coefficient compared to that in ethanol (ε582 = 3.1 × 104 L·cm−1·mol−1), methanol, and H2O. It can be seen that further addition of NaOH will decrease the 588 nm absorbance (Fig. 2b inset). The 588 nm absorbance essentially disappeared in the presence of >4 eq NaOH while ∼512 and 367 nm peaks appeared. H2V2− was the predominate species at 4 eq NaOH or higher (Shi et al., 2015) due to the strongest 367 nm catecholate absorbance. In 4 eq NaOH or higher, the solution was pale-orange in color, which is quite different from that in ethanol and methanol (purple). To further prove this reliability, we also collected spectra in DMF (Fig. S5). The spectra in DMF and DMSO were almost identical.

During the investigation the yellow-orange species peaked at 512 nm (4 eq NaOH). We noticed that adding 4 eq NaOH at once did not give the yellow-orange species. However, it was very dark blue color initially. The dark-blue color faded gradually to give the yellow-orange species. Fig. 2c monitors the fading process of 5.0 × 10−5 M H6V·Cl2 in the presence of 5 eq NaOH. The fading process obeyed first-order kinetics with rate constant k = 1.36 ± 0.02 × 10−2 s−1 (with the correlation coefficient above 99.9%) and t1/2 = 51 s. In our opinion, acid-base reaction is fast. The fading process was possibly due to aggregation. H6V2+ aggregation was neglectable due to its positive charge. When deprotonated to neutral zwitterion H4V, it was much easier to aggregate in solution. Zwitterion H4V ∼ H2V2− had a strong aggregate tendency in less polar solvents, such as DMSO and DMF.

Fig. 2d shows the 1H NMR of H6V·Cl2 in the absence and presence of NaOD. The CH2 group at 6.4 ppm was split into two signals in a 2:1 ratio upon adding 0.5 eq. NaOD (Fig. 2d, yellow-green line). This indicates that one ketone tautomerized into the enolic structure (Danac and Mangalagiu, 2014; Postolachi et al., 2013). This further proves that the 588 nm (blue color) species was the mono-enolic structure. H-H COSY spectra of H6V·Cl2 in the presence of 1 eq NaOD in DMSO showed the two asymmetric pyridyls (Fig. S6). In the presence of 2 eq NaOD, the sample had a very weak NMR signal. In more than 3 eq NaOD, the two pyridyl were identical due to both CH2 deprotonating. EPR spectra were investigated to further identify the species at 588 and 512 nm peaks (Fig. S7). Although H6V·Cl2 at 5.0 × 10−5 M became dark blue upon addition of 1–2 eq. of NaOH, it was essentially EPR silent in DMSO. In the presence of 5 eq. of NaOH, the sample showed a free radical species at g = 2.00 in DMSO (Fig. S7 green line).

We have demonstrated that this catecholate viologen can react with Na2B4O7·10H2O to form red color borate ester (by complexation) in aqueous solution. Catechol have interactions with many anions, such as F− et. al. in organic solution (Miyaji and Sessler, 2001; Winstanley et al., 2006). 2.0 × 10−4 M catechol acetonitrile solution will become blue in color in the presence of excess F−, but other anions will have no observable interactions (Winstanley et al., 2006). Polyphenol (1.0 × 10−4 M) can also interact with some anions to form naked-eye observable species in organic solvents (Miyaji and Sessler, 2001). However, the color change mechanism is still not clear though all evidence suggest H-bond interaction between anion and the catechol hydroxyl. We wondered whether our viologen-catechol could better respond to anions. UV–vis spectral changes of 5.0 × 10−5 M H6V·Cl2 upon gradual addition of Na2B4O7·10H2O, Na2MoO4·2H2O, NaF, and sodium acetate in DMSO are illustrated in Fig. 3. 5.0 × 10−5 M H6V·Cl2 became dark blue upon addition of 0.5 eq. of Na2B4O7·10H2O. The 588 nm absorbance (4.38 × 104 L·cm−1·mol−1) was clear from the mono-deprotonated enolic structure, the same as in the presence of NaOH in DMSO. At this low Na2B4O7·10H2O concentration, Na2B4O7·10H2O acted as a base. This can be explained by the low binding constant between catechol and the Na2B4O7·10H2O. Further addition of Na2B4O7·10H2O bleached the color to pink (3 eq). At high Na2B4O7·10H2O concentration, the solution became red with absorbance maximum at 518 nm (2.34 × 104 L·cm−1 ·mol−1). The 518 nm species was the catechol chelate to the B atom, which is the same as in the aqueous solution discussed above, but obviously shifted to a shorter wavelength compared to 548 nm in aqueous solution. The 518 nm absorbance coefficient in DMSO was much greater than that in aqueous solution at 548 nm. Catechol chelation can also be proven by the 372 nm catecholate in the presence of 2–3 eq Na2B4O7·10H2O shifted to chelate catecholate at 377 nm in the presence of >4 eq Na2B4O7·10H2O (Fig. 3a).

UV–vis spectral changes of 5.0 × 10−5 M H6V·Cl2 upon gradually adding Na2B4O7·10H2O at 80 °C, Na2MoO4·2H2O, NaF and CH3COONa. All experiments are in DMSO.
Fig. 3
UV–vis spectral changes of 5.0 × 10−5 M H6V·Cl2 upon gradually adding Na2B4O7·10H2O at 80 °C, Na2MoO4·2H2O, NaF and CH3COONa. All experiments are in DMSO.

Although 1.0 × 10−4 M H6V·Cl2 had essentially no reaction with Na2MoO4·2H2O in aqueous solution, 5.0 × 10−5 M H6V·Cl2 could clearly interact with Na2MoO4·2H2O in DMSO solution (Fig. 3b). Small amounts of Na2MoO4·2H2O could also act as a weak base that deprotonated enolic protons (1 eq). Further increases in Na2MoO4·2H2O concentration would obviously shift the absorbance to a longer wavelength. The 383 nm catecholate absorbance was the longest wavelength in all our experiments, and clearly longer than that of mono-deprotonate catechol at 367 nm in DMSO (Fig. 2b), indicating chelation between Na2MoO4·2H2O and the catechol moiety.

5.0 × 10−5 M H6V·Cl2 could also change color in the presence of F− to form the enolic structure that had very high absorbance at 588 nm (5.16 × 104 L·cm−1· mol−1). The 588 nm absorbance reached its maximum in 20 eq F−. The 372 nm catecholate absorbance appeared at 20 eq or higher F− with a simultaneous decrease in the 588 nm absorbance. Clearly the 372 nm absorbance in the presence of >20 eq F− was different from that of catecholate at 367 nm in the presence of NaOH in DMSO solution. This indicates H-bond formation between catechol moiety and F−. Because of the higher acidity of the CH2 group compared to that of the catechol hydroxyl, F− formed H-bond with the enolic proton to form a deeply colored enolic species. Only at very high F− concentration would the F− form an H-bond with catechol. The H-bond formation was somewhat similar to deprotonation, therefore, 372 nm absorbance increased.

Acetate anion had basicity and significantly increases the 588 nm absorbance (Fig. 3d). The deepest color appeared in the presence of 3 eq acetate, which was significantly less (higher sensitivity) than that of F− (20 eq). This can be explained by the fact that pKa of acetic was ∼ 4.8, while HF had a pKa of 3.2. The 375 nm catechol-acetate H-bond species was slightly longer than the F−—catechol at 372 nm, but still significantly shorter than the catechol-MoO42− chelate species at 383 nm. Acetate was different from F− in that it had very high absorbance in both 375 and 588 nm in the presence of 3 eq acetate. While in F−, the 372 nm increase would decrease the 588 nm absorbance. The spectra in >5 eq acetate was quite similar to the spectra in the presence of 4 eq NaOH (588 and 512 nm). Although this color change can be explained by the H-bond ring between acetate and the catechol. However, only H-bond could not generate colored species of catechol derivatives. Atmospheric O2 (or CO2, but not H2O) probably played a key role in the colorimetric response (Winstanley et al., 2006). H2O decreased the H-bond ability to form catechol with anions. Therefore, it was much more sensitive in organic solvents than in aqueous solution. For comparison, important visible spectral data for sensor anions are summarized in Table 1.

From Table 1, the acetocatechol had essentially constant wavelength at ∼286 and 316 nm with absorbance coefficient 30–40 × 103 L·cm−1·mol−1), while the mono-deprotonated catecholate absorbance varied between 354 and 383 nm depending on solvents and anions. The very long wavelength in the presence of Na2MoO4·2H2O, Na2B4O7·10H2O and NaAc were possible due to chelation and H-bonding with both hydroxyl of the catechol moiety. The maximum absorbance of the enolic structure was also highly dependent on solvents. Less polar solvents would shift enolic viologen to higher wavelengths. As a whole, this acetocatecholate viologen response to anion was sensitive in organic solvents.

3.3

3.3 Interact with metal ion

3.3.1

3.3.1 Aqueous solution

1.0 × 10−4 M H6V·Cl2 was essentially colorless in aqueous solution (Fig. S8 black line). After adding Fe(III), the solution turned light green immediately and an absorption peak at 570 nm appeared. While increasing the Fe(III) concentration, the absorbance gradually increased and 570 nm absorption peak shifted to 655 nm (Fig. S8). The absorbance did not change after >2.0 eq. of Fe(III). Therefore, each catecholate viologen can coordinate to two Fe(III) correspond to each catechol coordinate to one Fe(III).

NaOH titration experiments of 1.0 × 10−4 M H6V·Cl2 and 2.0 × 10−4 M Fe(III) aqueous mixture are shown in Fig. 4a. 1.0 × 10−4 M H6V·Cl2 had a pH 5.45, which decreased to pH 3.44 after adding 2 eq Fe(III). [H+] = 3.63 mM corresponding to the two catechol hydroxyl both deprotonated to form monocatecholate Fe(III) complex Fe(cat), with formulae of [Fe2(H2V)Cl2(H2O)x]2+, where x is the number of coordinated water. Enolic deprotonation was neglectable in such an acidic solution; the two protons were still bound to CH2. No OH− coordinated to Fe(III) because OH− coordination would have further increased H+ concentration. The 655 nm (4170 L·cm−1·mol−1, pH = 3.44) was between CDA (665 nm, 2030 L·cm−1·mol−1) and IDA functionalized analog (632 nm, 2130 L·cm−1·mol−1) (Gao et al., 2014). Adding 4 eq NaOH (pH 3.89, [H+] = 1.3 × 10−4 M) generated Fe(cat)2 complex with absorbance maximum at ∼566 nm (4000 L·cm−1· mol−1), which is quite normal both in wavelength and absorbance coefficient (Gao et al., 2014). The Fe(cat)2 complex has a formulae [Fe(H2V)(H2O)2]+, its cationic nature was evidenced by the precipitation in the presence of larger anion, such as [Fe(CN)6]3−. [Fe(H2V)(H2O)2]+should be a linear polymeric structure due to the fact that two catechol cannot coordinate to one ion simultaneously.

UV–vis spectral of (a): H6V·Cl2 (1.0 × 10−4 M) and 2.0 × 10−4 M Fe(NO3)3 upon gradual addition of different equivalents of NaOH in aqueous solution. The inset figure is the absorbance at 655 nm at different pHs. Numbers on lines are the eqs of NaOH. (b) 5.0 × 10−5 M H6V·Cl2 and 1.0 × 10−4 M Cu(NO3)2 upon gradual addition of different equivalents of NaOH in aqueous solution. The inset figure is the absorbance at 379 nm at different pHs. (c) 2.0 × 10−4 M H6V·Cl2, 2.0 × 10−4 M Fe(NO3)3 and 2.0 × 10−4 M Cu(NO3)2 with different amounts of NaOH.
Fig. 4
UV–vis spectral of (a): H6V·Cl2 (1.0 × 10−4 M) and 2.0 × 10−4 M Fe(NO3)3 upon gradual addition of different equivalents of NaOH in aqueous solution. The inset figure is the absorbance at 655 nm at different pHs. Numbers on lines are the eqs of NaOH. (b) 5.0 × 10−5 M H6V·Cl2 and 1.0 × 10−4 M Cu(NO3)2 upon gradual addition of different equivalents of NaOH in aqueous solution. The inset figure is the absorbance at 379 nm at different pHs. (c) 2.0 × 10−4 M H6V·Cl2, 2.0 × 10−4 M Fe(NO3)3 and 2.0 × 10−4 M Cu(NO3)2 with different amounts of NaOH.

The monocatecholato Fe(III) complex, [Fe2(H2V)Cl2(H2O)x]2+ (abbreviated as Fe(cat)), and Fe(III)-biscatecholato [Fe(H2V)(H2O)2]+ (abbreviated as Fe(cat)2), in equilibrium as evidenced by the isosbestic at 609 nm. Adding 8 eq NaOH (pH 5.09) generated another species with maximum absorbance at 575 nm (7060 L·cm−1·mol−1, which precipitate soon). Clearly this was not a tricatecholate Fe(cat)3 complex as Fe(cat)3 can only be generated in pH > 7 and has absorbance ∼500 nm (Gao et al., 2014). Higher than 8 eq NaOH will generate precipitation as indicated in the Fig. 4a inset photograph. In the presence of Fe(III), this catecholate viologen differed from free viologen in that it did not show enolic color in aqueous solution because of precipitation at weak acidic/neutral solution. It also differed from other catecholate Fe(III) complex because it could not form Fe(cat)3 complex in aqueous solution. Mono-deprotonated catecholate had an absorbance maximum at 353 nm in its Fe(III) complexes. The doubly deprotonated catechol had absorbance at 337 nm for Fe(III)(cat) and shifted to a longer wavelength at ∼370 nm for Fe(III)(cat)2 as evidenced in Figs. S10 and S11.

Fig. 4b shows UV–vis spectral of H6V·Cl2 (5.0 × 10−5 M) and (1.0 × 10−4 M) Cu(NO3)2 upon gradual addition of different equivalents of NaOH in aqueous solution. Their absorbance maximum vs. pH profile differed from that of free ligand. The catecholate coordinated Cu(II) complex had absorbance at 379 nm, which reached maximum concentration at pH ∼ 5.2. The 379 nm absorbance differed from mono-deprotonated catechol at 355 nm, indicating that catecholate coordinated to Cu(II). The wavelength did not change after further addition of base. Zn(II)-catecholate complex had an absorbance maximum at pH 6.6 (Fig. S9). The pH of maximum monocatecholate complex concentration for Fe(III), Cu(II) and Zn(II) were 3.5, 5.2 and 6.6 (∼4 eq NaOH). We can conclude that in terms of coordination: Fe(III) ≫ Cu(II) > Zn(II).

In the presence of 1:1:1 Fe(III):Cu(II):H6V·Cl2 (2.0 × 10−4 M), 646 nm (1900 L cm−1 mol−1) peaked corresponding to Fe(cat) complex in the absence of base. 0–4 eq NaOH shifted the wavelength to 554 nm, corresponding to Fe(III)Cu(II) heteronuclear complex. The reason that 554 nm was assigned to heteronuclear complex rather than Fe(III)(cat)2 was that the isosbestic was at 690 nm, rather than 609 nm for Fe(III)(cat) and Fe(III)(cat)2.

The 604 nm (5000 L·cm−1 ·mol−1) absorbance in the presence of Fe(III) and Cu(II) reached its maximum at 6 eq. NaOH, which obviously shifted to a longer wavelength than Fe(III) complexes at 575 nm (7100 L cm−1 mol−1). Beyond 6 eq, precipitation was generated. The color of the solution changed from pale green in the presence of Fe(III) and Cu(II) to red wine and yellow–brown upon gradually adding NaOH, similar to those of Fe(III) complexes (Fig. 4a).

3.3.2

3.3.2 Stability of mono- and bis-catecholato Fe(III) at room temperature and 60 °C

Although there are a huge number of studies that deal with iron-catechol complex, the mechanics and stability of these coordination crosslinks, which lay the groundwork for predictive and rational material design, have not yet been well addressed (Xu, 2013). Fe(III)-catecholate stability without additional oxidant in air at different temperatures is less studied as well. The 655 nm absorbance of 5.0 × 10−4 M H6V·Cl2 in the presence of 2 eq Fe(NO3)3 in aqueous solution (pH 2.96) shifted to 636 nm upon standing at room temperature for 24 h (pH 2.95) (Fig. 5a). The absorbance also decreased from 2.2 to 1.5 (Fig. 5a). Time-dependent absorbance indicate that the reaction was first-order with k = 3.81(1) × 10−5 s−1 and t1/2 = 18,200 s (Fig. 5b), where the number in bracket is the error of the last digit. At 60 °C, the maximum absorbance shifted to 672 nm (final pH 2.94) (Fig. 5a). The color fade was much faster than that at room temperature (Fig. 5b). Different from the first-order fade at room temperature, color fade at 60 °C is very good second-order reaction with k2 = 4.176(7) M−1 s−1 and t1/2 = 479 s. The 60 °C faded solution was diluted to 0.125 mM, then added K3[Fe(CN)6] to give very strong Prussian blue absorbance at 706 nm. The Prussian had an absorbance coefficient of 1.87 × 10−4. The fade product of monocatecholato Fe(III) contained at least 65% Fe(II). However, there was no Fe(II) for standing the sample at room temperature as there was no 706 nm Prussian blue absorbance after adding K3[Fe(CN)6]. We think the Fe(III)-catecholate may be dimerized at room temperature; one of the catecholato oxygen could have bridged two Fe(III) (Funabiki et al., 1998; Lavi et al., 2017). The insignificant pH change before and after 24hrs at 60 °C indicate a possible quinone structure rather than a possible muconic acid structure in the presence of oxygen (Wang et al., 2017).

(a) 5.0 × 10−4 M H6V·Cl2 in the presence of 2 eq Fe(NO3)3 in aqueous solution (w/o base, pH 2.96) at room temperature (black line) and 60 °C (purple-red line). Blue dotted line is the 60 °C sample after 24 h after dilution to 1.25 × 10−4 M and addition of 10 eq K3[Fe(CN)6]. The inset photographs are the color changes after 24 h and after adding 10 eq K3[Fe(CN)6]. (b) The time dependent absorbance maximum at room temperature (pH 2.96, black line, monitored at 655 nm) and 60 °C (red line, monitored at 670 nm). The inset is the first order plot at room temperature and second order plot at 60 °C; (c) 5.0 × 10−4 M H6V·Cl2 in the presence of 2 eq Fe(NO3)3 and 5 eq NaOH in aqueous solution (pH 4.3) at room temperature (black line) and 60 °C (purple-red line). Dotted lines are the same materials after 24 h. Inset photograph are color changes after 24 h. (d): The time-depend absorbance maximum at room temperature and 60 °C, both monitored at 566 nm. The inset is the first order plot at room temperature and the second order plot is at 60 °C.
Fig. 5
(a) 5.0 × 10−4 M H6V·Cl2 in the presence of 2 eq Fe(NO3)3 in aqueous solution (w/o base, pH 2.96) at room temperature (black line) and 60 °C (purple-red line). Blue dotted line is the 60 °C sample after 24 h after dilution to 1.25 × 10−4 M and addition of 10 eq K3[Fe(CN)6]. The inset photographs are the color changes after 24 h and after adding 10 eq K3[Fe(CN)6]. (b) The time dependent absorbance maximum at room temperature (pH 2.96, black line, monitored at 655 nm) and 60 °C (red line, monitored at 670 nm). The inset is the first order plot at room temperature and second order plot at 60 °C; (c) 5.0 × 10−4 M H6V·Cl2 in the presence of 2 eq Fe(NO3)3 and 5 eq NaOH in aqueous solution (pH 4.3) at room temperature (black line) and 60 °C (purple-red line). Dotted lines are the same materials after 24 h. Inset photograph are color changes after 24 h. (d): The time-depend absorbance maximum at room temperature and 60 °C, both monitored at 566 nm. The inset is the first order plot at room temperature and the second order plot is at 60 °C.

The Fe(III)(cat)2 complex in the presence of 5 eq NaOH had absorbance at 566 nm, which appeared grey due to wide absorbance in the visible region. Upon standing at room temperature for 24 h, it became red-brown (Fig. 5c, black lines) and peaked at 549 nm. Kinetic trace of 566 nm absorbance is illustrated in Fig. 5d (black line). The reaction is first-order with rate-constant of 1.936(5) × 10−5 s−1. No Fe(II) was detected by adding K3[Fe(CN)6] in the final product. The reaction at 60 °C is second-order with k2 of 4.23(4) M−1 s−1. The half-life is 9.9 h and 15 min at room-temperature and 60 °C respectively. The reaction was not a redox reaction as there was no detectable Fe(II) nor significant pH decrease.

3.3.3

3.3.3 DMSO solution

We already showed that H6V·Cl2 reactions with Fe(III) in aqueous solution can generate species ranging from blue colored Fe(III)-monocatecholato species (655 nm) absent base to red-brown Fe(III)(cat)2 in presence of 3–6 eq. NaOH. Fig. 6a shows the spectral change of 1.0 × 10−4 M H6V·Cl2 in the presence of 2.0 × 10−4 M Fe(NO3)3 upon gradual addition of NaOH in DMSO solution. Without NaOH, the DMSO solution was colorless. The color deepened with the increase of NaOH in 0–6.5 eq range. The absorbance peaked at 615 nm (1.5 × 104 L·cm−1·mol−1). This peak was clearly not the free enolic viologen (peaked at 588 nm, 3.6 × 104 L·cm−1· mol−1, 1 eq NaOH, Fig. 2b).

UV–vis spectral changes of (a): 1.0 × 10−4 M H6V·Cl2 in the presence of 2.0 × 10−4 M Fe(NO3)3 and (b): 1.0 × 10−4 M CDA in the presence of 1.0 × 10−4 M Fe(NO3)3 upon gradual addition of different equivalents of NaOH. Both were in DMSO. The numbers near lines are eqs. Of NaOH.
Fig. 6
UV–vis spectral changes of (a): 1.0 × 10−4 M H6V·Cl2 in the presence of 2.0 × 10−4 M Fe(NO3)3 and (b): 1.0 × 10−4 M CDA in the presence of 1.0 × 10−4 M Fe(NO3)3 upon gradual addition of different equivalents of NaOH. Both were in DMSO. The numbers near lines are eqs. Of NaOH.

To investigate the origin of the 615 nm absorbances, free CDA-Fe(III) complex at the same concentration was investigated in DMSO (Fig. 6b). Free CDA did not seem to coordinate with Fe(III) without NaOH as there was no absorbance in the visible region. Fe-CDA complex formed in the presence of 2NaOH, which peaked at 681 nm (ε1900). This wavelength and absorbance coefficient is comparable with that in aqueous solution (Gao et al., 2014). Further addition of NaOH shifted the absorbance to a shorter wavelength of 568 nm (2300 L·cm−1·mol−1) in the presence of 4 eq NaOH and then Fe(III)(CDA)3 complex in ∼10 eq NaOH (∼465 nm, 4000 L·cm−1·mol−1). The spectral data were quite similar to CDA-Fe(III) in aqueous solution of for mono-, bi-, and tri-catecholato Fe(III) complex at 665(2030), 548(4743) and 455 nm (9635 L·cm−1·mol−1) respectively, except for red-shifted 16–20 nm in DMSO (Gao et al., 2014). The catecholato-Fe(III) was obviously less stable in DMSO than in H2O as 2 eq NaOH was required to coordinate with Fe(III) in DMSO.

UV–vis spectral changes of 1.0 × 10−4 M H6V·Cl2 with 2.0 × 10−4 M Fe(NO3)3 in the presence of NaOH in DMSO were different from that of CDA in DMSO solution. 615 nm (ε615 = 1.5 × 104 L cm−1 mol−1) absorbance at 6.5 eq NaOH was clearly not the Fe(III)-catecholate because this complex formed in 2 eq NaOH as evidenced by CDA (681 nm). Also, the ε1.5 × 104 was far larger than that of the corresponding catecholate Fe(III) complex (∼2000 L·cm−1· mol−1). This could be attributed to the viologen enolic structure (ε3.6 × 104 L·cm−1·mol−1). The 6 eq NaOH clearly indicate that OH− coordinated to Fe(III) before deprotonating the enolic proton. Due to catecholate coordination, the enolic viologen absorbance shifted from free ligand at 588 nm (3.6 × 104 L·cm−1·mol−1) to 615 nm (1.5 × 104 L·cm−1·mol−1). The grey-brown color species in 7–11 eq NaOH was then assigned to the enolic viologen and Fe(III)-biscatecholato complex.

The strength of the viologens' impact on the acetocatechol coordination went far beyond our expectations. To further confirm these phenomena, H6V·Cl2-FeCl3 system (Fig. S12), as well as mono-acetocatecholate analog (Fig. S13) were investigated. In the presence of FeCl3, the spectra were almost identical to those of Fe(NO3)3 except 2 eq. more NaOH were need in the presence of FeCl3. This could be explained by the stronger coordination of Cl− compared with NO3−. The monocatecholate analog interact with Fe(III) could form monocatecholato Fe(III) complex as there was peak at ∼650 nm in ∼1 eq NaOH (Fig. S13). Spectra data further solidified their reliability and complexity.

3.4

3.4 Reaction with reductant

It is well-known that viologen cation can be reduced to a deep colored free-radical species in the presence of reductant such as Na2S2O4 solution (Das et al., 2016). Fig. 7a shows spectral changes of 5.0 × 10−5 M H6V·Cl2 upon gradual addition of Na2S2O4 in hepes buffer solution (pH = 8.0). Colorless H6V·Cl2 solution turned pink and absorbance peaked at 540 nm (8600 L cm−1 mol−1) in the presence of Na2S2O4. The absorption peak of mono-deprotonated catechol at 355 nm (=36,000 L·cm−1·mol−1) gradually decreased upon gradually addition 0–20 eq. of Na2S2O4. New absorption peaks at 368 nm and 400 nm (shoulder) appeared after adding 30.0 eq. of Na2S2O4. The absorption peak of 314 nm (55,000 L cm−1 mol−1) gradually increased and the absorption peaks of 368 nm (27,000 L cm−1 mol−1) and 400 nm gradually weakened with the addition of Na2S2O4. At pH 8.0, this compound existed in the form of H2V2−, the absorption peak at 540 nm was therefore assigned to the radical of H2V3−. Radical monomer absorption peaked at 368 nm and radical dimer absorption peaked at 314 nm in HELPS buffer solution.

UV–vis spectral changes of 5.0 × 10−5 M H6V·Cl2 upon gradual addition of different equivalents of (a) Na2S2O4 in hepes buffer aqueous solution (pH = 8.0). (b) Na2S2O4 in DMSO. The inset graph is the absorbance changes with eqs of Na2S2O4.
Fig. 7
UV–vis spectral changes of 5.0 × 10−5 M H6V·Cl2 upon gradual addition of different equivalents of (a) Na2S2O4 in hepes buffer aqueous solution (pH = 8.0). (b) Na2S2O4 in DMSO. The inset graph is the absorbance changes with eqs of Na2S2O4.

Spectral changes of H6V·Cl2 upon gradual addition of Na2S2O4 in DMSO is shown in Fig. 7b. The blue solution, with a peak at 588 nm (18,800 L·cm−1·mol−1), reached maximum absorbance in the presence of 1.0 eq. of Na2S2O4. This clearly indicated the basicity of Na2S2O4. The 588 nm peak decreased in intensity and shifted to a longer wavelength at 625 nm (5600 L·cm−1·mol−1) as Na2S2O4 concentration increased. The 625 nm peak was attributed to the free radical in DMSO, which was red shifted ∼85 nm compared to that in aqueous solution. At the same time, a ∼515 nm peak appeared. The 515 nm was almost identical to the aggregated form in the presence of >4 eq NaOH.

N2H4 is also a commonly used reductant. It is quite interesting that N2H4 could not reduce this viologen as the spectra was identical to those in the presence of NaOH in DMSO (Fig. S14), although their standard potential quite similar, possibly due to kinetic reason.

4

4 Conclusions

Acetocatecholate viologen has both catechol and viologen properties. Due to strong electron-deficient of the viologen cation, the CH2 adjacent to carbonyl has the highest acidity of all protons in the absence of metal ions. Low concentration anion, B4O72−, MoO42−, CH3COO−, and F− react with CH2 via H-bond to form deep colored species. High concentration anion will react with the catechol hydroxyl, especially B4O72−, MoO42−. The catechol moiety can coordinate to metal ions, especially Fe(III). The viologen moiety can be reduced to radicals in the presence of S2O42− both in aqueous and DMSO solvent, but N2H4 cannot. The mono- and bis-catecholato Fe(III) are relatively stable but will dimerize at room temperature. The dimerization reaction is first order to complex concentration with a half-life of several hours. Intramolecular redox will occur for monocatecholato Fe(III) at 60 °C. Redox reaction does not take place for bis-catecholato Fe(III) complex either at room temperature or 60 °C.

Acknowledgment

This project was supported by the National Natural Science Foundation of China (Grants 21401127 and 21571126). We thank the Instrumental Analysis and Research Center of Shanghai University for measurements.

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Appendix A

Supplementary material

Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.arabjc.2018.04.005.

Appendix A

Supplementary material

Supplementary data 1

Supplementary data 1


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