Online Supplementary Material

Redox-induced solid-solid state transformation of tetrathiafulvalene (TTF)

microcrystals into mixed-valence and π-dimers in the presence of nitrate anions

Shaimaa M. Adeel, Lisandra L. Martin, Alan M. Bond School of Chemistry, Monash University, Clayton, Victoria 3800, Australia

1 Figure S1 Cyclic voltammograms (1st to 50th cycles of potential) obtained over the potential range 200-250 mV at a scan rate of 50 mV s-1 with a drop-cast TTF-modified GC electrode in contact with an aqueous 1.0 M Co(NO3)2 electrolyte.

Influence of the electrode material on Process I

Figure S2 shows cyclic voltammograms obtained (10th cycle of potential) using a TTF- modified (drop cast) at GC (3 mm diameter), Au (1.6 mm diameter), Pt (1.6 mm diameter)

2 and ITO (area 0.09 cm ) electrodes each immersed in 1.0 M aqueous Co(NO3)2 solutions. In each case, the potential is initially scanned in the positive direction from -200 to 250 mV at scan rate of 20 mV s-1. Table S1 summarizes the peak potential parameters of interest derived from Figure S2. The electrode independent nature of these data indicates that the underlying electrode substrate has no significant effect on the voltammetry, thereby suggesting that rate determining nucleation–growth reaction does not include interaction with the electrode.13

2 Figure S2 Cyclic voltammograms obtained at a scan rate of 20 mV s-1 with a drop-cast TTF- modified different electrode materials in contact with an aqueous 1M Co(NO3)2 electrolyte.

Table S1 Voltammetric parameters obtained at a scan rate of 20 mV s-1 when TTF is immobilized by drop casting onto different electrode materials that are then placed in contact with 1 M an aqueous Co(NO3)2 electrolyte.

Electrode Geometric material Area, cm2 mV mV mV mV

GC 0.07 180 2 91 178

Au 0.02 186 2 94 184

Pt 0.02 180 5 91 178

ITO 0.09 180 2 91 178

3 Effect of electrolyte cation on Process I

Cyclic voltammograms (10th cycle) with a TTF-modified GC electrode (drop casting) in contact with 0.1 M aqueous Co(NO3)2, Mg(NO3)2, Ni(NO3)2, Zn(NO3)2, Mn(NO3)2, 0.2 M

KNO3 and 0.2 M NaNO3 are shown in Figure S3. The similarity of the voltammetric responses suggests that the electrolyte cation does not play a significant role in the kinetics or thermodynamics of the solid-solid state transformation. Table S2 summarizes the voltammetric parameters obtained from the data shown in Figure S3. The relatively constant and values implies that the nucleation-growth mechanism only involves incorporation and exclusion of the nitrate anion entering/leaving the lattice of TTF microcrystals. However, from the stability of current viewpoint (the rate of attenuation of voltammetric peak currents with increasing number of cycles of potential) the electrolyte containing the Co2+ cation gave the most persistent response. Therefore, this electrolyte cation was used in electrochemical synthesis studies aimed at characterizing oxidized TTF-NO3 based materials. The substantial differences in peak area is mainly attributable to variability in the amount of TTF attached to the surface when using the drop cast method. Additionally, the solubility of the TTF-NO3 based product apparently varies with the electrolyte counter cation.

4 Figure S3 Cyclic voltammograms (10th cycle) obtained at a scan rate of 20 mV s-1 with a drop-cast TTF-modified GC electrode in contact with 0.1 M an aqueous solution of Co(NO3)2, Mg(NO3)2, Ni(NO3)2, Zn(NO3)2, Mn(NO3)2, 0.2 M KNO3 and 0.2 M NaNO3.

Table S2 Voltammetric parameters obtained at a scan rate of 20 mV s-1 when TTF is immobilized by drop casting onto a GC electrode and then placed in contact with aqueous solution of a nitrate salt.

Cation of Concentration nitrate salt M mV mV mV mV

Zn2+ 0.1 254 26 140 228

Mg2+ 0.1 248 20 134 228

Mn2+ 0.1 224 42 133 182

Co2+ 0.1 257 31 126 196

Ni2+ 0.1 254 16 135 238

K+ 0.2 230 36 133 194

Na+ 0.2 240 28 134 212

5 - Figure S4 Plots of , and versus log [NO3 ] obtained with a TTF-modified GC electrode in

-1 contact with aqueous Co(NO3)2 electrolyte at a scan rate of 20 mV s .

6 Voltammetric evidence for nucleation-growth kinetics for Process I

The scan rate effect

Figure S5 shows voltammograms obtained (10th cycle) with a TTF-modified GC electrode

(drop cast) in contact with a 1.0 M Co(NO3)2 aqueous solution as a function of scan rate, while the voltammetric parameters obtained from these data are summarized in Table S3.

With increasing scan rate, becomes more positive while shifts to more negative potentials.

Thus, increases with scan rate, while is only slightly affected. The scan rate dependence is consistent with a nucleation-growth process9.

Figure S5 Cyclic voltammograms (10th cycle) obtained at the designated scan rates with a drop-cast TTF-modified GC electrode in contact with aqueous 1.0 M Co(NO3)2 electrolyte.

7 Table S3 Voltammetric parameters (10th cycle) obtained with a drop-cast TTF-modified GC electrode in contact with aqueous 1.0 M Co(NO3)2 electrolyte as a function of scan rate.

mV s-1 mV mV mV mV 10 177 5 91 172

20 180 2 91 178

30 185 1 93 184

40 189 -1 94 190

50 192 -3 94 195

60 194 -4 95 198

75 198 -5 96 203 100 208 -9 99 217

8 The effect of switching potential

Figure S6 represents cyclic voltammogram obtained at a scan rate of 10 mV s -1 with drop-cast

TTF-modified GC electrode in contact with 0.1 M aqueous Co(NO3)2 when the potential is switched at 190 mV, which corresponds to the foot of the oxidation process. When the potential is switched shortly after detection of the onset of the oxidation of TTF, a current loop is observed with a zero current crossover potential at 135 mV. This phenomenon is highly characteristic of nucleation-growth kinetics.9, 21

Figure S6 Cyclic voltammogram obtained with a drop-cast TTF-modified GC electrode in -1 contact with aqueous 0.1 M Co(NO3)2 electrolyte at a scan rate of 10 mV s , when the potential was switched at 190 mV.

9 Chronoamperometric evidence for nucleation-growth kinetics for Process I

Double-potential step chronoamperometry (DPSCA) experiments are also useful in establishing the presence of nucleation-growth mechanisms.9,19-21 Figure S7a illustrates the results of a DPSCA experiment for a TTF-modified GC electrode in contact with a 0.1 M aqueous solution of Co(NO3)2 previously subjected to 10 cycles of potential over the range of

-200 - 350 mV vs. Ag/AgCl at a scan rate of 20 mV s-1. The potential was initially held at

-400 mV for 10 seconds, where TTF is electroinactive. The potential was then stepped to 260 mV for 30 seconds to oxidize TTF to TTF+•. The potential was then stepped back to 30 mV for 30 seconds to induce reduction of solid (TTF)2NO3 to solid TTF, accompanied by egress

- of NO3 anions into solution phase. After each potential step, the current–time transient shows the expected response; initial charging current spike, which rapidly decays toward a zero followed by the appearance of well-defined Faradic current. The detection of current maxima in current–time curves (Figure S7) is strong proof for nucleation-growth kinetics in both the oxidation and reduction solid-solid state transformations.2b,3,5 Figure S7b shows the results of

DPSCA experiment under the same potential step conditions as for Figure S7a except that the potential was stepped from -400 mV to designated potentials that introduce oxidation of solid

TTF, while Figure S7c shows the results of DPSCA as in (a) but with the potential being stepped from -400 mV to 260 mV and then returned back to a designated reduction potential.

As expected, for the nucleation–growth mechanism, maxima are observed for oxidation and reduction processes, respectively.

10 Figure S7 Double–potential step chronoamperograms obtained after a drop-cast TTF- modified GC electrode in contact with a 0.1 M aqueous solution of Co(NO3)2 is cycled 10 times over the potential range -400 to 350 mV then: (a) potential initially held at -400 mV for 10 seconds, then stepped to 260 mV for 30 seconds and finally back to 30 mV for 30 seconds, (b) as in (a) but potential stepped from -400 mV to 260, 240, 220, 200, 190, and 180 mV (c) same conditions as in (a), but with the second potential step set to designated potentials.

11 Schematic representation of the mechanism for nucleation-growth

Figure S8 Schematic representation of the nucleation-growth processes associated with oxidation of a TTF-modified electrode in aqueous and reduction of the oxidized solid back to TTF.

12 Voltammetric studies on the second oxidation process (Process II)

Role of electrode material

Voltammetric characteristics obtained from TTF-modified GC, metallic and semiconducting

(ITO) electrodes in contact with 1M aqueous Co(NO3)2 (Figure S9) are almost identical for both processes I and II. Data in Table S4 summarizes voltammetric parameters obtained for both processes and verifies that the underlying electrode material has no significant impact.

Figure S9 Cyclic voltammograms obtained over the potential range of -200 to 450 mV at a scan rate of 20 mV s-1 from a drop-cast TTF-modified ITO, GC, Pt and Au electrodes in contact with aqueous 1.0 M Co(NO3)2 electrolyte.

13 Table S4 Voltammetric parameters obtained when TTF is immobilized by drop casting onto ITO, GC, Pt and Au electrodes which are then placed in contact with 1.0 M aqueous

Co(NO3)2 electrolyte and the potential is then scanned over the potential range of -200 to 450 mV at a rate of 20 mV s-1.

Electrode Geometric material Area, cm2 mV mV mV mV mV mV mV mV

395 300 347 95 GC 0.07 170 -9 80 179 393 300 346 93 Au 0.02 170 -9 80 179 393 300 376 93 Pt 0.02 170 -9 80 179 395 300 347 95 ITO 0.09 172 -10 80 179

14 The effect of the electrolyte cation on the voltammetry of solid TTF (Processes I and II)

Cyclic voltammogram obtained with 0.1 M aqueous Co(NO3)2 Mg(NO3)2, Ni(NO3)2,

Zn(NO3)2, Mn(NO3)2, KNO3 and aqueous electrolytes (Figure S10) and data in Table S5 reveal that there is a constant different between Em values for processes I and II. Thus, the identity of the cation plays only a small role in the solid-solid transformation of either the first or second oxidation processes.

Figure S10 Cyclic voltammograms (10th cycle) obtained with a drop-cast TTF-modified GC electrode in contact with 0.1 M aqueous solutions of Co(NO3)2, Mg(NO3)2, Ni(NO3)2, -1 Zn(NO3)2, 0.2 M KNO3 and 0.2 M NaNO3 at a scan rate of 50 mV s when the potential is scanned over the range of -150 to 580 mV.

15 Table S5 Parameters obtained from voltammograms shown in Figure 11 (main paper) when TTF immobilized by drop casting onto GC electrode and in contact with aqueous solutions of designated nitrate salts.

Cation of Concentration nitrate salt M mV mV mV mV mV mV mV mV

Zn2+ 0.1 258 11 134 247 459 330 394 129

Na+ 0.2 245 72 158 173 456 390 423 66

Co2+ 0.1 212 10 111 202 440 332 386 108

Ni2+ 0.1 228 3 215 225 439 314 376 125

K+ 0.2 228 28 128 200 447 347 397 100

Mg2+ 0.1 264 64 164 200 499 395 447 104

16 Scan rate effect (Process II)

For process II, cyclic voltammograms for a TTF-modified GC in contact with 1.0 M

Co(NO3)2 as the aqueous electrolyte (Figure S11) gave the data in Table S4 by varying the scan rate. The dependence of , and on scan rate was similar to that for process I, while is unaffected.

Figure S11 Cyclic voltammograms obtained at designated scan rates with a drop-cast TTF- modified GC electrode in contact with aqueous 1.0 M Co(NO3)2 electrolyte when the potential is scanned over the range of -200 to 470 mV.

17 Table S6 Voltammetric parameters obtained with a drop-cast TTF-modified GC electrode in contact with aqueous 1.0 M Co(NO3)2 electrolyte as a function of scan rate.

Scan Rate mV s-1 mV mV mV mV mV mV mV mV

10 159 -5 77 161 387 305 346 82

20 177 -10 84 187 395 300 346 95

30 179 -14 82 193 398 295 346 106

40 187 -21 83 208 401 292 346 109

50 191 -23 84 214 402 290 346 111

60 195 -24 85 222 405 287 346 118

75 197 -27 85 224 409 284 346 125

100 209 -34 87 243 414 283 348 131

18 Switching potential effect (Process II)

Evidence for a nucleation and growth mechanism associated with process II was obtained from the cyclic voltammetry via the observation of a current loop with a zero current crossover potential of 375 mV when the potential was switched at the foot of the process

(Figure S12).

Figure S12 Cyclic voltammogram obtained with a drop-cast TTF-modified GC electrode in contact with 0.1 M aqueous Co(NO3)2 electrolyte when the potential was switched at 390 mV at a scan rate of 10 mV s-1.

19 Multiple cycling of potential effect (Processes I and II)

Figure S13 Cyclic voltammograms (1st to 50th cycle) obtained over the potential range of -200

- 450 mV at a scan rate of 50 mV s-1 with a drop-cast TTF-modified GC electrode in contact with an aqueous 1.0 M Co(NO3)2 electrolyte.

20 Effect of electrolyte concentration on Processes I and II

- Figure S14 Plots of ,and versus log [NO3 ] obtained with a drop-cast, TTF-modified GC

-1 electrode in contact with aqueous Co(NO3)2 electrolyte at a scan rate of 20 mV s .

Table S7 Voltammetric parameters obtained with a drop-cast TTF-modified GC electrode in contact with designated concentrations of aqueous Co(NO3)2 electrolyte at a scan rate of 20 mV s-1.

Concentration

M mV mV mV mV mV mV mV mV

1.00 170 -9 80 179 400 300 350 100

0.50 194 6 100 188 431 325 378 106

0.25 220 14 117 206 452 333 392 119

0.10 242 41 141 201 470 363 416 107

0.05 294 74 184 220 500 380 440 120 0.01 339 42 190 297 545 378 461 167

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