Incorporating Cobalt Carbonyl Moieties onto Ethynylthiophene-Based

Jun 23, 2014 - Incorporating Cobalt Carbonyl Moieties onto Ethynylthiophene-Based Dithienylcyclopentene Switches. 2. Electro- and Spectroelectrochemic...
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Incorporating Cobalt Carbonyl Moieties onto EthynylthiopheneBased Dithienylcyclopentene Switches. 2. Electro- and Spectroelectrochemical Properties Emma C. Harvey,† Jetsuda Areephong,§ Attilio A. Cafolla,‡ Conor Long,† Wesley R. Browne,§ Ben L. Feringa,§ and Mary T. Pryce*,† †

School of Chemical Sciences and ‡School of Physical Sciences, Dublin City University, Dublin 9, Ireland § Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands S Supporting Information *

ABSTRACT: The redox behavior of dithienyl perhydro- and perfluorocyclopentene photochromic molecular switches, modified with 3-ethynylthiophene and phenyl-3-ethynylthiophene substituents, is explored by cyclic voltammetry and UV/vis-NIR and IR spectroelectrochemistry. The extent of electrochemical oxidation induced cyclization was depedent on whether a perhydro- or perfluorocyclopentene unit was present, with the former favoring ring closure, and on the nature of the substituents on the thienyl ring. The inclusion of a phenyl spacer between the alkynyl and thienyl moieties increased the stability of the molecular switches when addressed electrochemically. Binding of Co2(CO)6 and Co2(CO)4dppm moieties to the alkyne units is shown to destabilize the cationic closed form and, in one example, inhibit oxidative cyclization for the 1,2-bis(5′(4″-phenyl-3‴-ethynylthiophene)-2′-methylthien-3′-yl)perfluorocyclopentene [Co2(CO)6]2 complex (4Fo). However, the electrochemical cyclization observed for the Co2(CO)6 and Co2(CO)4dppm complexes of 1,2-bis(5′-(3″-ethynylthiophene)2′-methylthien-3′-yl)cyclopentene (3Ho and 5Ho, respectively) was induced following oxidation of the cobalt carbonyl moieties (i.e., at lower potentials than oxidation of the open form of the dithienylethene), possibly via an intramolecular electron transfer mechanism and thereby providing an alternative route to control the electrochromic behavior of the switch.

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INTRODUCTION

Scheme 1. Electrochemical Cyclization and Cycloreversion of a Dithienylcyclopentene Switch

Photochromic dithienylcyclopentene switches can undergo cyclization (closing)/cycloreversion (opening) following irradiation with UV and visible light, respectively. Such molecular switches are of considerable interest due to their potential in applications, such as nondestructive optical data recording and storage,1−6 molecular wires,4,5 molecular switches,3−6 filters,1,2 and polarizers.1,2 Although it is well known that ring-opening/ closing can be induced photochemically, it can also be carried out electrochemically. A combination of the photo- and electrochromic properties of the switches has been used for the development of molecular wires3,7 and nondestructive write−read−erase memory devices.8−10 Dithienylethene molecular switches can undergo cyclization or cycloreversion upon oxidation of the dithienylcyclopentene core to a dicationic form, which usually occurs at potentials more positive than 1.0 or 0.5 V, respectively.11 Understanding the factors that determine the direction of electrochemical switching (i.e., the relative stabilities of the oxidized closed and open forms) has attracted considerable interest in recent years. The ability of the bridging cyclopentene moiety to stabilize the dicationic forms has been proposed as the driving force for either the cyclization or cycloreversion to occur.8,11,12 © 2014 American Chemical Society

The nature of the central cyclopentene group (i.e., H vs F) and the substituents attached to the dithienylethene unit determine the direction of oxidatively induced switching, with electron-donating groups favoring cyclization and electronwithdrawing groups promoting cycloreversion.8,11,12 In multicomponent molecular systems, such as those that incorporate dithienylethene switches, ideally the original functionality of each component should be retained, yet at the same time sufficient communication exists between two or more units. This communication allows one moiety to be Received: January 22, 2014 Published: June 23, 2014 3309

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Figure 1. Structures of the dithienylethene molecular switches discussed in the text. The suffix “o” indicates the open form and “c” indicates the closed form (see Scheme 1). The highlighted yellow box shows how the two cobalt atoms bind to the alkyne unit in a bridging fashion.

cobalt carbonyl complexes as a result of the increased electron density at the Co−Co bond and hence increases the lifetime of both the radical cations and anions. In comparison to alkyneCo2(CO)6 complexes, the increased electron density at the Co−Co core allows for oxidation at less positive potentials with greater reversibility and reductions at more negative potentials.23,24,28 Recently, we reported the photochromic properties of dithienylperhydro- and perfluorocyclopentene switches, appended with 3-ethynylthiophene and 4′-phenyl-3-ethynylthiophene groups, and the effect of complexation with Co2(CO)6 and Co2(CO)4dppm moieties, at the alkyne units, on the photochemical properties of the dithienylethene unit.29 We anticipated that coordination of cobalt carbonyl moieties would modify the dithienylethenes’ electrochromic properties in addition to their photochromic behavior. In the present contribution the electrochemical properties of the alkynesubstituted dithienylethene switches (1H/F and 2H/F) and the Co2(CO)6 (3H/F and 4H/F) and Co2(CO)4dppm (5H/F and 6H) complexes are explored (Figure 1). Oxidative cyclization/ cycloreversion was investigated by cyclic voltammetry and UV/ vis-NIR and FTIR spectroelectrochemistry, with a focus on the effects of the cobalt carbonyl moieties on the electrochromic properties of the dithienylethene units.

addressed and thereby tune the photo- and/or electrochemical response of other moieties. In this way the complexity of the response to multiple stimuli can be increased in materials composed of these systems. The key challenge, however, is to balance sufficient interaction to allow each moiety to influence the other’s properties but at the same time limiting the interactions so as not to lose the original functionalities of the unit. Meeting this challenge requires an understanding of how structural variations affect interactions. Recent studies13−20 have described the effects of incorporating organometallic moieties onto dithienylcyclopentene switches, with regard to their influence on the electrochromic properties of the switching unit (i.e., the direction of the switching process) and also in relation to the changes in electronic communication between two metal centers21,22 (on either side of the cyclopentene core) following cyclization/cycloreversion.13 In general, oxidation of alkynyl dicobalt hexacarbonyl complexes (RC2R′Co2(CO)6) is manifested in cyclic voltammetry by an irreversible anodic wave at room temperature, to form the radical cation [RC2R′Co2(CO)6]•+.21,23 One-electron reduction, which is facilitated by the strong π-acceptor CO ligands, generates the radical anion [RC2R′Co2(CO)6]•−, which then undergoes decomposition as a result of metal−metal bond cleavage,23−25 often leading to electrode fouling.23,24,26 Indeed, Co(CO)4−, RC2R′Co(CO)3, the alkyne,24 and metallic Co have been identified as decomposition products. After the oneelectron reduction, the subsequent cycle toward positive potentials reveals oxidation waves assigned to decomposition products, for example, Co(CO)4− at Epc = +0.12 V23,27 and +0.25 V24 and E1/2 = −0.07 V26 and RC2R′Co(CO)3 at −0.56 V or −0.83 V.26 Substitution of some of the CO ligands with phosphine ligands improves the electrochemical reversibility of these

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RESULTS AND DISCUSSION

Compounds 1−6, Figure 1, were available from earlier studies together with data regarding UV/vis and FTIR spectroscopy and their photochemical properties.29 In the present study, the electrochemical properties of the dithienylethenes were studied by cyclic voltammetry and UV/vis-NIR and FTIR spectroelectrochemistry (see Table 5 for an overview of results). 3310

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Cyclic Voltammetry of 1H/F and 2H/F. Cyclic voltammetry was carried out on the open and closed forms of the alkyne-substituted dithienylethene switches in the range −2.0 to +1.6 V (vs SCE), in (0.1 M TBAPF6) CH2Cl2. The open forms of 1Fo and 2Fo did not undergo reduction within the potential window examined; however, two irreversible reduction waves between −0.9 and −1.5 V were observed for the closed forms 1Fc and 2Fc (Table 1). In contrast, neither

by irradiation at 365 nm to a photostationary state (PSS).8,9,11,31 The perfluorocyclopentene-based switch 1Fo11 did not undergo electrochemical cyclization, and the chemically irreversible oxidation of 1Fc indicates that, as for related compounds, electrochemically driven cycloreversion takes place (vide infra).9,32 By contrast the phenyl-substituted perfluorocyclopentene-based 2Fo showed electrochemically driven cyclization, and the oxidation wave of 2Fc was reversible (Figure 2). UV/Vis-NIR Spectroelectrochemistry of 1H/F and 2H/ F. UV/vis-NIR spectroelectrochemistry was employed to confirm the assignment of the processes observed by cyclic voltammetry and to characterize the mono- and dicationic species formed upon oxidation (Table 2).9,11,31 Oxidation of 2Hc, at 0.6 V, resulted in a decrease in absorbance at 326 and 560 nm, with a concomitant increase in absorbance at 442, 781, and >1030 nm, characteristic of the formation of the monocation 2Hc+ (Figure 3). Increasing the potential to 0.9 V resulted in a decrease in the bands at 781 and >1030 nm ascribed to 2Hc+, with a concomitant formation of stronger absorption bands at 467 and 654 nm, and a shoulder at 590 nm, assigned to the dication 2Hc2+. Oxidation of 2Ho at 1.4 V resulted in a decrease in absorbance at 334 nm with an increase in absorbance at 471 and 657 nm and a shoulder at 590 nm (Figure 4), which correspond to the absorption spectrum of 2Hc2+ (Figure 3). Similar spectral changes were observed for 1Hc (Table 2, Figure S1) and 1Ho (Table 2). Oxidation of 2Fo at 1.8 V resulted in the appearance of an absorption band centered at 613 nm, which is similar to the absorption spectrum of the closed form 2Fc, indicating that

Table 1. Redox Data for Open and Closed Forms of 1H/F and 2H/Fa E1/2 [V vs SCE]a 1Ho 2Ho 1Fo 2Fo

1.55 1.23 1.71 1.63

(irr) (irr) (irr) (irr)

E1/2 [V vs SCE]a 1Hc 2Hc 1Fc 2Fc

0.60, 0.90 0.49, 0.81 1.13 (irr), −1.1 (irr), −1.49 (irr) 0.98, 1.12, −0.95 (irr), −1.2 (irr)

ΔE [mV]b 300 320 140

a

In V vs SCE, Epa where irreversible (irr), in 0.1 M TBAPF6/CH2Cl2, at 0.1 V s−1. bΔE = difference between the first and second oxidation processes in the closed state (c/c+/c2+).

1H nor 2H showed reduction processes in either the open or closed forms within the potential window examined, which can be attributed to the ability of the perfluorocyclopentene to stabilize the LUMO of 1Fc and 2Fc to a greater extent than the perhydrocyclopentene substituents.6,11,30 Electrochemical oxidation of 1Ho and 2Ho resulted in cyclization to the closed forms manifested in an irreversible oxidation at >1.2 V, with the appearance of two reversible reduction waves on the return cycle at potentials identical to those of the closed forms 1Hc and 2Hc, respectively, prepared

Figure 2. Cyclic voltammetry of (A) 1H, (B) 2H, (C) 1F, and (D) 2F in the open (o) and closed (c, current offset for clarity) forms, in 0.1 M TBAPF6/CH2Cl2, at a scan rate of 0.1 V s−1. 3311

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Table 2. UV/Vis-NIR Absorption Spectroelectrochemical Data for 1H−6Ha λabs (nm) 1Ho 1Hc 1Hc+ 1Hc2+ 3Ho 3Hc2+

290, 318, 283, 448, 274, 466,

310 540 405, 568, 330, 571,

5Ho 5H2+

263, 346, 430−630 432, 640b

λabs (nm) 2Ho 2Hc 2Hc+ 2Hc2+ 4Ho 4Hcs 4Hcs+ 4Hcs2+ 6Ho 6H+/6H2+

779, >1100 674 450−670 671b

334 326, 442, 467, 267, 262, 328, 332, 276, 456,

560 781, >1030 590(sh), 654 341, 620−650 323, 577c 459, 807, 1030−1620c 567, 661c 351, 430−630 566, 771b

a In 0.1 M TBAPF6/CH2Cl2. bData for 3Hc2+, 5Hc2+, and 6Hc+/2+ were obtained by oxidation of 3Ho, 5Ho, and 6Ho, respectively. cData obtained by oxidation of 4Hcs (prepared by complexation of Co2(CO)6 to the alkyne units of 1Fc or 2Hc, where “s” indicates “synthesised”). See Figure S4 for details.

Figure 3. UV/vis-NIR spectroelectrochemistry of 2Hc, in 0.1 M TBAPF6/CH2Cl2, (A) at 0.60 V, resulting in a decrease in absorbance at 326 and 560 nm, with a concomitant increase at 442, 781, and >1030 nm, due to formation of 2Hc+, and (B) at 0.90 V, resulting in a decrease in absorbance at 781 and >1030 nm, with a concomitant increase at 467 and 654 nm, and a shoulder at 590 nm, ascribed to the formation of 2Hc2+. Spectra were recorded at 4 min increments.

however, they were found to decrease in intensity in the subsequent spectra recorded (Figure 5). The original absorption band at 613 nm was also found to decrease, while an absorption band at 328 nm, coincident with the absorbance of 2Fo, grew in. Such a result can be ascribed to the low stability of the closed cationic species of 2Fc2+ and is in agreement with conclusions drawn from cyclic voltammetric data, whereby the separation (ΔE, Table 1, ca. 140 mV) between the redox waves associated with oxidation of 2Fc to 2Fc+, and subsequently to 2Fc2+, is considerably less than for the perhydro derivatives, 1H and 2H (ca. 300 and 320 mV, respectively). The reduced stability of the oxidized form of 2F is a consequence of the electron-withdrawing fluorine atoms on the central cyclopentene ring, which reduce delocalization of the charge through the entire π-system.11 In all cases, subsequent reduction, at 0.0 V, resulted in a decrease in the bands associated with the cationic closed forms, concomitant with an increase in absorbance in the UV region, associated with the open form. The longer time scale of the UV/vis-NIR spectroelectrochemical experiments (minutes) in comparison to the cyclic voltammetry experiments (seconds) limits the regeneration of the neutral closed species, which is not kinetically competitive with catalytic cycloreversion.11 Cyclic Voltammetry of the Co2(CO)6 Complexes. The cyclic voltammograms of the Co2(CO)6 complexes (3Ho, 3Fo, 4Ho, and 4Fo) showed a single irreversible two-electron21,22 reduction between −1.09 and −1.28 V vs SCE (Table 3). The

Figure 4. UV/vis spectrum of 2Ho, in 0.1 M TBAPF6/CH2Cl2, upon oxidation at 1.4 V. The absorbance at 334 nm decreased, with an increase in absorbance at 471 and 657 nm and a shoulder at 590 nm, due to the formation of 2Hc2+. Spectra were recorded at 4 min increments.

oxidative cyclization occurred. An increase in absorbance in the range 780−1100 nm was also observed, assigned tentatively to the generation of the cationic species of the closed form. The subsequent spectra recorded showed a small increase at 613 nm; however the absorbance in the region 780−1100 nm began to decrease. Furthermore, oxidation (at 1.2 V) of 2Fc resulted in a modest increase in absorbance between ∼780 and 1600 nm. Such spectral features can be attributed to the generation of the monocation 2Fc+ and/or dication 2Fc2+ species; 3312

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Figure 5. UV/vis-NIR spectroelectrochemistry (0.1 M TBAPF6/CH2Cl2) of (A) 2Fo at 1.8 V, resulting in a decrease in absorbance at 328 nm and a small increase at 613 nm, and (B) 2Fc at 1.2 V, resulting in a decrease of the original absorption band at 613 nm, while an absorption band at 328 nm grew in. Inset of (B) shows the same spectrum recorded between 430 and 1500 nm, showing a decrease in the original absorption band at 613 nm (dashed line) and an initial low intensity absorption in the range 780−1600 nm (thick black line). Spectra were recorded at 4 min increments.

Table 3. Redox Data for 1H/F and 2H/F, the Co2(CO)6Based Compounds 3Ho/Fo, 4Ho/Fo, 3Fcs and 4Hcs, and the Co2(CO)4dppm-Based Compounds 5Ho, 6Ho, and 5Foa E1/2 [V]b 1Ho 2Ho 1Fo 2Fo 3Ho 4Ho 4Hcs 3Fo 3Fcs 4Fo 5Ho 6Ho 5Fo

1.55(irr) 1.23(irr) 1.71(irr) 1.63(irr) 0.55, 0.74, 1.21(irr), 1.41(irr) 0.51, 0.80, 1.23(irr), 1.46(irr) 0.47, 0.76, 1.37(irr) 1.23(irr), 1.75(irr) 0.94(qr), 1.73(irr) 1.17(irr), 1.58(irr) 0.21, 0.55, 0.65, 1.35(irr) 0.35, 0.60, 0.89, 1.36(irr) 0.61, 0.71, 1.48(irr)

Epc [V]

−1.28(irr) −1.20(irr) −1.12(irr) −0.35(irr), −1.19(irr) −0.89(irr), −0.96(qr) −1.12(irr)

Figure 6. Reductive cyclic voltammetry of 3Fo (bottom), 3Fcs (middle), and 1Fc (top), in 0.1 M TBAPF6/CH2Cl2, at a scan rate of 0.1 V s−1. The CVs of 3Fcs and 1Fc are offset along the ordinate for clarity.

−1.67(irr)

In V vs SCE, in 0.1 M TBAPF6/CH2Cl2, at 0.1 V s−1. bEp,a where irreversible (irr), qr = quasi-reversible. a

facilitated stronger electronic communication between the two Co2(CO)6 moieties,21,33 compared to 3Fo. Similar results have been reported for other dithienylethene photochromic switches substituted with metal carbonyl and phosphine ligand complexes.13,14,17 A single two-electron irreversible reduction was observed for 4Hcs, which was shifted by 80 mV, to less negative potentials, in comparison to its open isomer 4Ho. This difference is attributed to the extended π-conjugation of the system in 4Hcs, which helps to stabilize the radical anion of the Co2(CO)6 units. The fact that a single two-electron reduction wave was observed for 4Hcs demonstrates that ring-closure of 4Ho did not change the extent of electronic interaction between the metal centers and is consistent with the presence of the phenyl rings, which increase the distance between the Co2(CO)6 moieties.27 The Co2(CO)6 moieties of 3Ho/Fo and 4Ho/Fo underwent oxidation between 0.9 and 1.71 V (Table 3). All of the open compounds displayed a single irreversible two-electron oxidation, i.e., one-electron oxidation of each of the two Co2(CO)6 moieties. Despite the presence of the Co2(CO)6 moieties, oxidative electrochemical cyclization of the perhydrocyclopentene-based compounds 3Ho and 4Ho was evident from their cyclic voltammograms (Figure 7). In the case of 4Ho, the oxidation at 1.23 V (vs SCE) was followed by two redox waves at E1/2 = 0.51 and 0.80 V in subsequent cycles,

redox wave corresponds to one-electron reduction of each of the two Co2(CO)6 units present. Reduction of the Co2(CO)6 units results in rapid decomposition manifested in the appearance of additional oxidation waves on the return cycles (Table 3).21−23,26 The hexafluorocyclopentene-based switches, 3Fo and 4Fo, underwent reduction at less negative potentials in comparison to their corresponding perhydro analogues, 3Ho and 4Ho (Table 3). These shifts indicate that the electronwithdrawing effect is communicated even with a phenyl spacer present, i.e., 3Fo vs 4Fo (vide supra), albeit with the reduction of 4Fo and 4Ho occurring at more similar potentials, consistent with a decrease in the influence of the hexahydro and hexafluoro cyclopentenes (Table 3). The reduction of the Co2(CO)6 complexes of the closed compounds 3Fcs and 4Hcs (prepared by complexation of Co2(CO)6 to the alkyne units of 1Fc or 2Hc, where “s” indicates “synthesised”)29 was also investigated. 3Fcs underwent reduction at a less negative potential (230 mV) compared to 3Fo (Figure 6). Furthermore, in contrast to 3Fo, two oneelectron21−23 reduction waves were observed for 3Fcs at Epc = −0.89 and −0.96 V. The reduction at −1.01 V was followed by a corresponding oxidation at −0.90 V. These data suggest that the π-conjugated backbone of the closed isomer, 3Fcs, 3313

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Figure 7. Cyclic voltammetry of (A) 3Ho and (B) 4Ho (bottom) and 4Hcs (top, offset along the ordinate for clarity) in 0.1 M TBAPF6/CH2Cl2, at a scan rate of 0.1 V s−1.

0.94 V (ipc/ipa ≈ 0.75, Figure 8), which is similar to that observed for 1Fc (E1/2 = 1.01 V) and hence is assigned to the

characteristic of the oxidative ring-closing. At more positive potentials, a second irreversible oxidation was observed at 1.46 V, which was assigned to the oxidation of the Co2(CO)6 moieties. The cyclic voltammetry of the Co2(CO)6 complex of the closed form 4Hcs showed two redox waves at E1/2 = 0.47 and 0.76 V (vs SCE, ΔE ≈ 280 mV) assigned to oxidation to the mono- and, subsequently, dicationic state. The separation between the first and second redox waves is similar to that of 2Hc (ΔE ≈ 320 mV), indicating that delocalization across the cobalt moieties in the closed form is not substantial. At more positive potentials, an irreversible oxidation was observed at 1.37 V, assigned to oxidation of the Co2(CO)6 units (Figure 7). In comparison to the open isomer, the potential at which the cobalt carbonyl moieties were oxidized was shifted by 90 mV, to less positive potentials, ascribed to destabilization by the extended π-conjugated system of the closed form. In the case of 3Ho, the proximity of the Co2(CO)6 moieties to the dithienylethene unit has a substantial effect on the electrochemical behavior observed in comparison with that of 4Ho, in which a phenyl group separates the two moieties. Irreversible oxidation was observed at 1.21 V, for 3Ho, followed by a second oxidation at 1.41 V. The oxidation at 1.21 V is assigned to oxidation of both the Co2(CO)6 moieties and the oxidation of the dithienylethene core; however the process is likely to involve several distinct steps.11,12 Initially the Co2(CO)6 moieties are oxidized followed by rapid intramolecular electron transfer coupled with cyclization to form 3Hc. In subsequent cycles two new redox waves at El/2 = 0.55 and 0.74 V (Figure 7) are observed, which are consistent with the formation of the closed form 3Hc. The most prominent effect of the Co2(CO)6 moieties on the electrochemical behavior of this dithienylethene was the decrease in the potential at which the dithienylethene unit was oxidized, in comparison to 1Ho (ΔE = 140 mV). The decrease in the separation between the first and second oxidation waves of 3Hc (ΔE = 190 mV) compared with 1Hc (ΔE = 300 mV) indicates delocalization of charge onto the Co2(CO)6 moieties in contrast to the phenyl-spaced 4Hc. The perfluoro analogue 3Fo showed two irreversible oxidation waves at 1.23 and 1.75 V (vs SCE), ascribed to a two-electron oxidation of the Co2(CO)6 moieties (i.e., oneelectron oxidation of each of the two Co2(CO)6 units) and a two-electron oxidation of the dithienylethene unit, respectively. Oxidatively driven cyclization was not observed, which is consistent with data obtained for 1Fo (vide infra). The closed form, 3Fcs, underwent a quasi-reversible oxidation at E1/2 =

Figure 8. Cyclic voltammetry of 3Fcs (bottom) and 1Fc (top), in 0.1 M TBAPF6/CH2Cl2, at 0.1 V s−l. The CV of 1Fc is offset along the ordinate for clarity.

oxidation of the dithienylethene moiety. The increased reversibility for 3Fc compared with 1Fc reflects the presence of the Co2(CO)6 moieties, which stabilize the oxidized closed isomer and retards ring-opening in the oxidized state. 3Fcs underwent irreversible oxidation at 1.71 V, assigned to oxidation of the Co2(CO)6 units. 4Fo underwent irreversible oxidation at 1.17 and 1.58 V (vs SCE), assigned to oxidation of the Co2(CO)6 moieties and the dithienylethene unit, respectively. However, oxidatively driven cyclization was not evident for 4Fo, in contrast to 2Fo. This difference is ascribed to the oxidation of the Co2(CO)6 moieties occurring at less positive potentials than for the dithienylethene unit of 4Fo, and hence cyclization is inhibited since the oxidation potential of the closed dithienylethene unit (2Fc) is ca. 1.13 V. Hence, in contrast to 4Ho, the driving force for intramolecular electron transfer is negligible. Similar behavior has been reported for other organometallic dithienylethene switches,16 whereby the electrochromic properties of the switching unit were dependent on the relative potentials at which the metals were oxidized. Oxidation of the Co2(CO)6 moieties increases their electron-withdrawing character, and therefore, together with the electron-withdrawing perfluorocyclopentene ring, the open dicationic species is more stable than the closed dication, thus preventing ringclosure. Finally, it should be noted that after a number of cycles, following oxidation of the Co2(CO)6 moieties, fouling of the 3314

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electrode occurred in each case, due to decomposition of the cobalt carbonyl units.21,23,28 UV/Vis-NIR Spectroelectrochemistry of the Co2(CO)6 Complexes. New absorption bands were observed in the UV− vis/NIR spectrum upon oxidation of the Co2(CO)6 complex 4Ho, at 1.0 V (Table 2, Figure 9), which are attributed to the

Table 4. FTIR Spectral Data of the Co2(CO)6 and Co2(CO)4dppm Complexes, before (Parent Bands) and after Oxidation (New Bands)a parent bands, ν(CO) cm−1

a

3Ho 4Ho 4Hcs

2089, 2055, 2024 2089, 2055, 2024 2090, 2055, 2026

3Fo 3Fcs 4Fo 5Ho 6Ho 5Fo

2092, 2093, 2090, 2022, 2021, 2025,

2058, 2063, 2055, 1997, 1995, 2000,

2028 2035 2026 1969 1968 1972

new bands, ν(CO) cm−1 − − 2093, 2096, − − − 2065, 2067, 2069,

2061, 2037 2067, 2043

2043, 2022 2044, 2021 2047, 2025

In 0.1 M TBAPF6/CH2Cl2. − indicates no new IR bands appeared.

ocyclopentene unit. In the case of the closed isomer 3Fcs, the carbonyl bands were present at marginally higher wavenumbers (2093, 2063, and 2035 cm−1) in comparison to its open form 3Fo (2092, 2058, and 2028 cm−1). For 3Ho/Fo, 4Ho/Fo, and 3Fcs, oxidation resulted in a decrease in intensity of the three carbonyl stretching bands between 2100 and 2020 cm−1; however, new bands were not observed (Table 4 and shown for 3Fo in Figure S6). Subsequent reduction at 0.0 V resulted in the reappearance of the original bands, albeit only to 60% of their original absorbance, indicating that the irreversibility is due to relatively slow chemical reactions subsequent to electrochemical oxidation. The absence of new bands is also indicative of adsorption of the oxidized species to the working electrode. Oxidation of 4Hcs at 0.2 V resulted in bleaching of the parent bands at 2090, 2055, and 2026 cm−1 and the concomitant appearance of three new bands at 2093, 2061, and 2037 cm−1 (Figure 10), assigned to the monocationic species 4Hc+. Oxidation at 0.6 to 0.8 V resulted in depletion of these new bands and the appearance of a further three new bands at still higher wavenumbers (2096, 2067, and 2043 cm−1). The absorption bands are, tentatively, assigned to the formation of

Figure 9. UV/vis absorption spectrum of the open Co2(CO)6 complex, 4Ho, following oxidation at 1.0 V: The initial absorbance spectrum (black line) with λmax = 267 and 341 nm; the initial increase in absorbance at 481, 655, and 778 nm (blue lines); further increase in absorbance with λmax = 481 and 655 nm (red lines).

formation of the mono- and dication species of the closed isomer, by comparison with spectra obtained for the corresponding closed complex (4Hcs, Figure S4) and the ligand (2Hc) (vide supra). These data confirm that 4Ho undergoes oxidatively driven cyclization. A similar result was also obtained for 3Ho (Figure S3), following bulk electrolysis at 1.3 V (Table S2). In the case of the perfluoro derivatives, 3Fo and 4Fo, cyclic voltammetry indicated that oxidative cyclization does not occur, which was confirmed by UV/vis-NIR spectroelectrochemistry, as new spectral features were not observed in the visible or NIR regions of the spectra, following bulk electrolysis, in either case. Furthermore, oxidation of the closed Co2(CO)6 complex, 3Fcs, at 0.8 up to 1.4 V, resulted in a decrease in the absorption in the visible region (Figure S5), indicating that 3Fcs undergoes oxidative cycloreversion to the open-ring form. In all cases, subsequent reduction at 0.0 V resulted in a decrease in the intensity of the spectral features present in the vis-NIR regions, and the original absorption bands in the UV regions did not return to their initial intensities. Furthermore, there was an increase in absorbance in the UV region, at wavelengths associated with the absorptions of the corresponding compounds 1−2Ho/Fo. These data indicate that decomposition of the Co2(CO)6 moieties was taking place and possibly cleavage of the metal−metal bond from the alkynyl unit, thus releasing the ligand. This result can be assigned to the instability of the cationic form of the switch and the cobalt carbonyl moieties over the time scale of the bulk electrolysis experiments (min).11 FTIR Spectroelectrochemistry of the Co2(CO)6 Complexes. The νCO bands for the perfluorocyclopentene derivatives were shifted to higher wavenumbers than for the perhydrocyclopentene analogues due to the electron-withdrawing effect of the fluorine atoms (Table 4). This effect is more pronounced for 3Fo, in comparison to 4Fo, which is consistent with the presence of the phenyl ring in 4Fo that separates the Co2(CO)6 moieties from the dithienyl-hexafluor-

Figure 10. FTIR difference spectra for 4Hcs, in 0.1 M TBAPF6/ CH2Cl2, following oxidation at 0.2 V (green line) resulting in bleaching of the parent bands at 2090, 2055, and 2026 cm−1 and the concomitant appearance of three new bands at 2093, 2061, and 2037 cm−1 (assigned to 4Hc+); at 0.6 V (red) and 0.8 V (blue), showing the depletion of these new bands and the appearance of a further three new bands at 2096, 2067, and 2043 cm−1 (assigned to 4Hc2+); and subsequent reduction at 0.0 V (black), resulting in a decrease in the new absorption bands and some recovery of the absorption bands of 4Hc. 3315

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the dicationic species 4Hc2+. Subsequent reduction at 0.0 V leads to a decrease in the new absorption bands and recovery of the absorption bands of 4Hc, albeit to only approximately 80% of the initial absorbance. Cyclic Voltammetry and UV/Vis-NIR Spectroelectrochemistry of the Co2(CO)4dppm Complexes. In contrast to the Co2(CO)6-based compounds 3Ho and 4Ho discussed above, the perhydro Co2(CO)4dppm complexes, 5Ho and 6Ho, did not undergo reduction at potentials more positive than −2.0 V, reflecting the electron-donating strength of 1,2bis(diphenylphosphine)methane (dppm).21−23,27,28 A single irreversible two-electron reduction was observed at −1.67 V for 5Fo with two oxidations at −0.36 and −0.24 V on the subsequent oxidative sweep. The irreversibility of the reductions was unexpected due to the tendency of phosphine ligands to impart reversibility, which was reported for related cobalt carbonyl complexes.23,24,28 The introduction of electron-donating dppm ligands onto Co2(CO)6 moieties results in increased electron density at the cobalt redox centers, thus facilitating oxidation of the Co2(CO)4dppm units at less positive potentials and stabilizing the oxidation products.21−23,27,28 For [(1-(3-thienylethynyl)-4bromobenzene)Co2(CO)4dppm] (Figure 11) a quasi-reversible

Figure 12. Cyclic voltammetry of 6Ho, between 0.1 and 0.96 V (top) and 1.37 V (bottom), in 0.1 M TBAPF6/CH2Cl2 at 0.1 V s−1. The upper voltammogram is offset along the ordinate for clarity.

unambiguously demonstrate that oxidative cyclization occurs for 6Ho. UV/vis-NIR spectroelectrochemistry, however, supports cyclization. Following oxidation of 6Ho, absorption bands appeared in the visible region (456, 566, and 771 nm), which are associated with the formation of cationic species of the closed form (Table 2, Figure 13). However, these

Figure 11. Cyclic voltammetry of [(1-(3-thienylethynyl)-4bromobenzene)Co2(CO)4dppm] in 0.1 M TBAPF6/CH2Cl2 at 0.1 V s−1.

Figure 13. UV/vis absorption spectrum of 6Ho, before oxidation (black line), with λmax = 276 and 351 nm, and after oxidation at 1.4 V (broken red line), resulting in a decrease of the original absorbance bands and the appearance of new absorbances in the visible region at 456, 566, and 771 nm, associated with the formation of cationic species of the closed form.

oxidation is observed at El/2 = 0.66 V (ipc/ipa ≈ 0.6) together with an irreversible oxidation at 1.49 V (vs SCE), corresponding to the oxidation of the Co2(CO)4dppm moiety and the thienyl ring, respectively. Similar effects of the dppm ligand were observed for 5Ho, 5Fo, and 6Ho (Table 3). For 6Ho a quasi-reversible oxidation at El/2 = 0.60 V (ipc/ipa ≈ 0.7) was observed, assigned to two one-electron oxidations centered on each of the Co2(CO)4dppm moieties. At higher potentials, an irreversible two-electron oxidation of the dithienylethene unit was observed at 1.36 V. The subsequent negative sweep displayed two new reversible redox waves at El/2 = 0.35 and 0.89 V (vs SCE, Figure 12). These redox processes are absent in the voltammetry of [(1-(3-thienylethynyl)-4bromobenzene)Co2(CO)4dppm], Figure 11, and indicate oxidatively driven ring-closing of the dithienylethene unit. However, the ΔE value between the two new redox waves (ca. 690 mV) does not correspond to the ΔE values found for the closed cationic species of 2Ho (ca. 320 mV) and its Co2(CO)6 complex 4Ho (ca. 280 mV). The presence of the redox wave at El/2 = 0.60 V may obscure a further redox wave associated with the closed form. Therefore, cyclic voltammetry cannot

absorption bands were weak in comparison to those of the uncomplexed switch (2Hc) and the Co2(CO)6 complex (4Hc), and as bulk electrolysis continued, they began to decrease again, thus indicating that the presence of the Co2(CO)4dppm moieties reduces the stability of the closed cation species over the time scale of the experiment. Oxidation of the Co2(CO)4dppm moieties in 6Ho is evident by a single redox wave, and for 5Fo two quasireversible oneelectron oxidation waves at E1/2 = 0.61 and 0.71 V (ΔE = 100 mV, Figure S7) were observed, indicating that the two dicobalt centers communicate, either through-bond or else electrostatically. Oxidation of the dithienylethene occurred at 1.48 V, although no new redox waves were observed in the subsequent sweeps. Similarly, in the case of 5Ho, two quasi-reversible oneelectron redox processes for the Co2(CO)4dppm units were observed, but at less positive potentials (El/2 = 0.55 and 0.65 V, Figure 14). However, following the first oxidation of the cobalt carbonyl moieties, a new redox wave was observed in the 3316

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cm−l from the slower rate of decay compared with the bands at 2000 and 1972 cm−1. These changes in the FTIR absorption spectrum are ascribed to conversion of 5Fo to a dicationic species. The loss of an electron from each of the two Co2(CO)4dppm moieties results in a decrease in electron density at the metal centers, reducing back-bonding and hence shifting the carbonyl bands to higher wavenumbers. Depletion of the new bands at 2069, 2047, and 2025 cm−l occurred following subsequent reduction at 0.0 V, with the parent bands recovering to approximately 85% of their initial absorbance values. This reversibility is therefore considerably better than observed for the analogous Co2(CO)6 complex 3Fo (∼60%, Figure S6). Prolonged oxidation, and oxidation at more positive potentials, resulted in an overall decrease in the absorbance of the carbonyl bands. Therefore, the FTIR spectroscopic data correlate well with the quasi-reversibility of the oxidation observed by cyclic voltammetry and the low reversibility observed by UV/vis absorption spectroelectrochemical studies. Similar changes were observed for the perhydro derivatives 5Ho (Table 4) and 6Ho (Table 4, Figure 16).

Figure 14. Cyclic voltammetry of 5Ho between −0.2 and 0.9 V (top) and between −0.2 and 1.4 V (bottom), in 0.1 M TBAPF6/CH2Cl2 at 0.1 V s−1. The upper voltammogram is offset along the ordinate for clarity.

subsequent sweeps at El/2 = 0.23 V (Figure 14). Oxidation at higher potentials (at 1.4 V) revealed an irreversible oxidation at 1.35 V, attributed to a two-electron oxidation of the dithienylethene unit. In the subsequent sweeps, the relative intensity of the redox wave at El/2 = 0.23 V increased (Figure 14). These data indicate that cyclization of 5Ho begins following oxidation of the Co2(CO)4dppm moieties. However, only one new redox wave was found for 5Ho, rather than the expected two redox processes normally observed following cyclization to the closed form, possibly due to overlap with the redox waves related to the oxidized Co2(CO)4dppm units. The changes observed in the UV/vis-NIR spectrum of 5Ho (Table 2, Figure 15) are consistent with electrochemical ring-closing

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CONCLUSION In agreement with earlier studies,12 the direction of electrochemically induced switching of the dithienylethene compounds (i.e., cyclization/cycloreversion) is mainly controlled by the cyclopentene ring, with the presence of hydrogen atoms promoting oxidative cyclization (1−6Ho, Table 5). The driving force for the oxidatively induced cyclization is attributed to the electron-donating ability of the perhydrocyclopentene ring. In the case of the perfluorocyclopentene-based switch, 1Fc, oxidative ring-opening was observed. However, the ability to tune the electrochromic properties, via the substituents attached to the thienyl rings of the dithienylethene unit, was highlighted by the electrochromic behavior of 2Fo. Incorporating an electron-rich phenyl group onto the perfluorinated switch, 2Fo, changed the direction of the switching process, from the open to the closed form, in comparison to 1F. Overall, the effects of the substituents (H vs F, phenyl group) can be attributed to the ability of the substituents to stabilize the dicationic states of either the open or closed forms of the dithienylethene unit.8,11,12 Incorporating cobalt carbonyl moieties onto the perhydro switch 2H served to reduce the stability of the closed cationic/ dicationic forms of 4Hc and 6Hc. Such a result was evident from UV/vis-NIR spectroelectrochemistry, as the intensity of the absorbance bands recorded for the cobalt carbonyl complexes (4Hc and 6Hc) were significantly reduced in comparison to the absorbances of the corresponding free-ligand cations (2Hc). In the case of the perfluoro derivative 2Fo, introducing Co2(CO)6 moieties (4Fo) completely inhibited cyclization of the switch. Such results can be assigned to an effect of the metal fragments, which destabilize the oxidized cyclized form of the switch. On the other hand, the proximity of the cobalt carbonyl moieties to the dithienyl switching unit in 3Ho and 5Ho was found to have a substantial effect on the electrochemical behavior, as oxidation of the Co2(CO)6 and Co2(CO)4dppm moieties on 3Ho and 5Ho, respectively, resulted in cyclization. The mechanism for this is tentatively assigned as electron transfer between the oxidized substituents and the dithienylethene unit.12 Therefore, the ability to utilize the substituents attached to the dithienylethene unit as a “remote control” to induce opening/closing processes, at lower potentials than that required to oxidize the open dithienyle-

Figure 15. UV/vis absorption spectrum of 5Ho (thick black line), upon oxidation at 0.8 V, resulting in a decrease in the bands at 263 and 346 nm, along with an increase in absorbance in the visible region, with λmax = 432 and 640 nm. Spectra were recorded at 4 min intervals.

(λmax = 432 and 640 nm), following oxidation at 0.8 V (i.e., the potential at which the Co2(CO)4dppm moieties undergo oxidation). This observation supports a model in which oxidation of the Co2(CO)4dppm units induces cyclization via an intramolecular electron transfer mechanism, as the open form of the switching unit undergoes oxidation at a much higher potential (1.35 V). FTIR Spectroelectrochemistry of the Co2(CO)4dppm Complexes. The bands of 5Fo, at 2025, 2000, and 1972 cm−l, decreased in absorbance concomitant with the appearance of two new bands at 2069 and 2047 cm−l upon electrochemical oxidation (between 0.6 and 0.8 V) (Table 4, Figure 16). An additional band was apparent beneath the parent band at 2025 3317

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Figure 16. FTIR difference spectra of the (μ2-dithienylcyclopentene)Co2(CO)4dppm complexes following oxidation at 0.6 V, in 0.1 M TBAPF6/ CH2Cl2: (A) 5Fo, showing depletion of the parent bands at 2025, 2000, and 1972 cm−l concomitant with the appearance of new absorbance bands at 2069, 2047, and 2025 cm−l, and (B) 6Ho, showing depletion of the parent bands at 2021, 1995, and 1968 cm−l concomitant with the appearance of new absorbance bands at 2067, 2044, and 2021 cm−l. Spectra were recorded at ∼15 s intervals.

Table 5. Summary of Electrochromic Behavior of the Switchesa

− indicates oxidative cyclization does not occur. * indicates cyclization induced following oxidation of the cobalt carbonyl moieties; # indicates that it undergoes cycloreversion.

a

With regard to applications, it should be noted that electrochemical switching offers advantages not available through photochemical switching in terms of speed and efficiency when immobilized within a material (i.e., at an electrode). In the present study the focus has been on solution studies, and hence assessing the efficiency of electrochemical switching is challenging, in particular due to the disproportionation-driven ring-opening of the closed form that occurs in solution during bulk oxidation/reduction cycles as originally noted by Peters and Branda11a and later explored in depth by several groups.8−10 It should be noted though that in earlier studies of electrochemical switching of dithienylethenes when immobilized as monolayers on gold36 or ITO37 or as polymers on surfaces,38 the electrochromic efficiency has been found to be exceedingly high, with full ring-closure with a single oxidation/reduction cycle.

thene units, has been established. Such a phenomenon was reported previously by Browne et al.,12 when a methoxy-phenyl substituent was employed; however, this has not been reported in the literature so far for organometallic-DTE complexes. Furthermore, we have highlighted the potential to utilize infrared spectroscopy to monitor the oxidation processes of the closed Co2(CO)6 complex 4Hcs and the Co2(CO)4dppm complexes without inducing switching behavior. On a separate note, the influence of the bridging unit on the degree of electronic communication between the cobalt moieties is also apparent with the single carbon−carbon bond, allowing for the extended π-conjugated closed systems to promote electronic interaction between the metal centers. Therefore, the cobalt carbonyl switches described here have the potential to be used in the development of molecular wires. Switching between the open and closed isomers has the prospect to control the electronic communication between the ON (closed) and OFF (open) state. 3318

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

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The synthesis of (1-(3-thienylethynyl)-4-bromobenzene)Co2(CO)4dppm is provided in the SI. All other compounds were available from earlier studies.29 Solvents used for analytical experiments were of spectroscopic grade (Sigma-Aldrich). The tetrabutylammonium hexafluorophosphate, decamethylferrocene, and d6acetone were obtained from Sigma-Aldrich and used as received. Solutions were argon purged prior to photo- and electrochemical experiments. Cyclic voltammetry and bulk electrolysis were carried out using a CH Instruments CHI600a potentiostat. A glassy carbon (working), silver wire (reference), and platinum wire (counter) electrode were employed for cyclic voltammetry. The reference electrode was calibrated versus the decamethylferrocene redox couple (Fc*+/Fc*), which has a formal potential E1/2 = −0.07 V versus SCE.34 UV/vis-NIR spectroelectrochemistry was performed using a JASCO V-670 spectrophotometer, in a custom-made quartz cuvette (2 mm path length) equipped with a solvent reservoir. A platinum gauze (working), a silver wire (reference), and a platinum wire (counter) electrode were employed. Infrared spectroelectrochemistry was carried out using a PerkinElmer Spectrum 65 FTIR spectrometer with an OTTLE cell35 with a platinum gauze working and counter electrode and a Ag/AgCl reference electrode and CaF2 windows. The closed forms of the dithienylethene photochromic switches were generated in deuterated acetone in a sealed NMR tube by irradiation at 313 or 365 nm (using a 200 W Hg lamp, Oriel Instruments, model no. 68911 with a band-pass filter) to a photostationary state, monitored by 1H NMR spectroscopy. The duration of the irradiation was limited to avoid buildup of photochemical byproducts. 1H NMR spectra were recorded on a Bruker AC 400 MHz spectrometer. S Supporting Information *

The synthetic method for (1-(3-thienylethynyl)-4bromobenzene)Co2(CO)4dppm, cyclic voltammograms, and UV/vis-NIR and IR spectroelectrochemical data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +353 1 7008005. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Science Foundation Ireland Research Frontiers Program (Grant No. 06/RFP/PHY082, E.C.H., A.A.C., M.T.P.), TRIF DCU (E.C.H., M.P.), the University of Groningen (Ubbo Emmius Scholarship, J.A.), and the European Research Council (ERC AdvGrt, 227897, B.L.F.) are acknowledged for financial support.

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